• Author / Uploaded
• pham thang

Citation preview

Application Guide for High Voltage Accessories 2nd edition

Application Guide for High Voltage Accessories Creation:

Brugg Cables

Authors:

Hansjörg Gramespacher, Ruben Vogelsang, Matthias Freilinger

Copyright:

All Rights Reserved. The use, reproduction and distribution of these information is authorized only for a non-commercial purpose on condition that the source is explicitly quoted.

A publication of the Brugg Cables Academy 2nd edition, 2015 Printed in Switzerland by Effingerhof, Brugg ISBN: 978-3-033-04936-9

Preface For modern societies all over the world, electric

High voltage cable accessories have to connect

energy is one of the key factors for industrial

two cable lengths together or, often, one cable

growth and individual prosperity. This admit-

to other high voltage interfaces, such as gas-

tedly simplified but true statement nevertheless

insulated switchgears, oil-insulated transform-

assumes a high level of availability of electric

ers or outdoor overhead lines. Illustratively

energy, ensuring uninterruptible manufacturing

speaking, high voltage accessories have to con-

processes for heavy industries, as well as a reli-

nect two worlds, which can be quite different in

able power supply for the IT infrastructure for

terms of their geometry, material and behav-

commercial business services, which both rely

iour. From the material point of view, electro-

on electric energy.

chemical aspects have to be considered, when

Besides single and spatially concentrated high

joining an aluminium conductor to a copper

voltage equipment of major importance, such

conductor, for instance. Also physical aspects

as turbo generators or large power transform-

have to be taken into account, such as different

ers, the high voltage network represents the most important asset for a utility and its customers in terms of size, value and pertinence. For many reasons, for example their environmental impact, route consumption and the resulting acceptance of the affected population,

conductor diameters or materials with different thermal coefficients of expansion. With regard to the insulation materials, commonly silicone rubber, polyethylene and oil-impregnated paper, interactions also have to be considered. And finally, pointing out the major challenge of

high voltage lines are increasingly making use

accessories, the ability to intransigently restore

of high voltage cable technology. Already very

the electrical insulation of two separate parts,

common in urban regions for the medium volt-

electrical field distribution has to be actively

age range, high voltage cable systems are also

controlled using proper materials and designs,

used successfully in the power transmission

this being the most important secret of every

sector, particularly as the knowledge and expe-

manufacturer of high voltage accessories.

rience of the major cable manufacturers is con-

All these technical aspects have to be taken into

stantly increasing, and as a result reliable prod-

account to design a reliable high voltage cable

ucts are available on the market.

accessory that meets the specified require-

As AC high voltage cables with production

ments. Economic aspects have to be considered

lengths of some hundred metres up to several

as well. To ensure a sufficient level of practical

kilometres, with a symmetrical cylindrical de-

usability, the design of the accessories also has

sign, and well-known electrical field stress appear to be state-of-the-art for system voltages of several hundred kilovolts, and therefore uncritical, cable accessories such as terminations and joints have to be discussed in more detail.

to tolerate a minimum level of faulty assembly works, making them practicable for the “real world” for a long operating period. Taking all these factors into consideration, a customer will of course choose high- quality high voltage accessories, as the accessories used ultimately

define the quality of a complete cable system, and even major parts of a cable network. This “Application Guide for High Voltage Accessories” will enable the reader to choose, and define in advance the requirements for the accessories needed for his specific task, by understanding design principles, manufacturing processes and test criteria in accordance with international standards for quality assurance.

The City of Kiel, Germany, April 2015

Professor Kay Rethmeier

Director of the Institute of Electrical Power Engineering and High Voltage Laboratories of the University of Applied Science Kiel

Foreword After the appreciation from our customers and our industry for the 1st edition of our “Application Guide for High Voltage Accessories”, we are tremendously proud to present this 2nd edition. It has been completely reworked and summarises the complex topics relevant to the reliable operation of high voltage accessories in a fundamental way.

Working in the field of high voltage accessories, we are constantly being asked a variety of questions concerning the application of accessories for high voltage cable systems. These vary from simple questions, such as the difference between composite and porcelain insulators for terminations to the more complex, such as how to design an appropriate earthing layout for cable systems. Although diverse, most of the questions have one thing in common: They are related to the application of accessories for high voltage cable systems.

High voltage cable systems are only as good as the installed high voltage accessories. Therefore we aim to supply high voltage accessories that are safe and reliable. We at Brugg Cables constantly expand our know-how and expertise on the application of high voltage accessories and are pleased to share our knowledge with you.

We wish you happy reading and a lot of useful benefits in your practical work!

Roger Braun

Aman Sapra

Head of Business Unit Power Accessories & Cables

Head of Marketing Power Accessories & Cables

Contents PREFACE FOREWORD

CONTENTS 1

FUNDAMENTALS .................................................................................................................................... 1

1.1 Fundamental electric relations ...................................................................................3 1.1.1 Electric charges, current, electric field, voltage and potential ....................................................... 3 1.1.2 Ohm’s law and Kirchhoff’s laws....................................................................................................... 5 1.1.3 Terms of electric power .................................................................................................................... 5 1.1.4 Electric resistivity, conductivity, insulators and semiconductivity................................................ 7 1.1.5 Magnetic field .................................................................................................................................... 8 1.1.6 Electromagnetic induction ................................................................................................................ 9 1.2 Electric field..............................................................................................................10 1.2.1 Electric field and field lines ............................................................................................................. 10 1.2.2 Electric field in technical objects .................................................................................................... 11 1.2.3 Capacity ............................................................................................................................................ 13 1.3 Insulating materials in high voltage technology.......................................................14 1.3.1 Solid materials ................................................................................................................................. 14 1.3.2 Liquids .............................................................................................................................................. 15 1.3.3 Gases ................................................................................................................................................ 15 1.4 Power transmission ..................................................................................................15 1.4.1 Basics of electric power transmission systems ............................................................................ 15 1.4.2 Overhead lines ................................................................................................................................. 17 1.4.3 Power cables .................................................................................................................................... 17 1.4.4 Transmission capability .................................................................................................................. 18 1.4.5 Power transmission and environment........................................................................................... 19 1.5 Terms and definitions ...............................................................................................21 1.5.1 Definition of voltage values for cable systems ............................................................................. 21 1.5.2 Definition of terms for terminations and cable systems .............................................................. 22 2

AGEING AND LIFE EXPECTANCY....................................................................................................... 23

2.1 Ageing in polymers ...................................................................................................25 2.1.1 Theory of electric lifetime law ........................................................................................................ 25 2.1.2 Practical experiences with electric lifetime law and electric ageing ........................................... 26 2.1.3 Thermal ageing................................................................................................................................ 27 2.2 Volume effect of real polymeric arrangements ........................................................27 2.3 Life expectancy .........................................................................................................28 2.3.1 Basic failure behaviour.................................................................................................................... 28 2.3.2 Short-term failures .......................................................................................................................... 28 2.3.3 Occasional failures .......................................................................................................................... 29 2.3.4 Failures due to ageing..................................................................................................................... 30

3

TESTS AND STANDARDS .................................................................................................................... 31

3.1 Tests .........................................................................................................................33 3.1.1 Basic idea of testing ........................................................................................................................ 33 3.1.2 Development tests........................................................................................................................... 33 3.1.3 Type tests ......................................................................................................................................... 33 3.1.4 Prequalification tests ....................................................................................................................... 37 3.1.5 Requalification tests ........................................................................................................................ 38 3.1.6 Routine tests .................................................................................................................................... 39 3.1.7 After installation tests ..................................................................................................................... 40 3.1.8 Alternative methods for after installation tests............................................................................. 42 3.2 Standards ..................................................................................................................43 3.2.1 Introduction...................................................................................................................................... 43 3.2.2 Main differences between IEC and IEEE standards ...................................................................... 43 3.2.3 Relevant IEC standards ................................................................................................................... 45 3.2.4 Relevant IEEE, AEIC, ANSI and ICEA standards............................................................................ 46 4

HIGH VOLTAGE XLPE CABLES .......................................................................................................... 47

4.1 Design and types of high voltage XLPE cables.........................................................49 4.1.1 Cable design..................................................................................................................................... 49 4.1.2 Types of high voltage XLPE cables ................................................................................................ 51 4.2 Cable layout and system design ...............................................................................53 4.2.1 General ............................................................................................................................................. 53 4.2.2 Electric field, capacity and charging current ................................................................................. 54 4.2.3 Inductive values of the cable .......................................................................................................... 55 4.2.4 Cable losses ..................................................................................................................................... 56 4.2.5 Dynamic forces ................................................................................................................................ 58 4.3 Laying of high voltage cables ...................................................................................58 4.3.1 Laying arrangements ...................................................................................................................... 58 4.3.2 Current carrying capacity and temperature calculation ............................................................... 61 4.3.3 Reduction of magnetic field............................................................................................................ 62 4.4 Cable selection process ............................................................................................62 5

HIGH VOLTAGE ACCESSORIES FOR POLYMER CABLES .............................................................. 63

5.1 Introduction ..............................................................................................................65 5.2 Technologies for slip-on elements ............................................................................66 5.2.1 Control of the electric field.............................................................................................................. 66 5.2.2 Semiconducting parts ..................................................................................................................... 68 5.2.3 Comparison of main materials ....................................................................................................... 68 5.2.4 Cold shrink elements....................................................................................................................... 70 5.2.5 Three-piece silicone rubber slip-on elements ............................................................................... 71 5.2.6 One-piece silicone rubber slip-on elements.................................................................................. 72 5.2.7 One-piece EPDM/EPR slip-on elements ......................................................................................... 73 5.2.8 Lapped technology .......................................................................................................................... 74 5.2.9 Final comparisons and conclusions............................................................................................... 74 5.3 Terminations .............................................................................................................75 5.3.1 Basic design ..................................................................................................................................... 75 5.3.2 Outdoor terminations...................................................................................................................... 76 5.3.3 Explosion resistant terminations.................................................................................................... 78 5.3.4 Classic SF6 and transformer terminations ..................................................................................... 79 5.3.5 Dry-type plug-in terminations ........................................................................................................ 80

5.4 Joints ........................................................................................................................82 5.4.1 Basic design ..................................................................................................................................... 82 5.4.2 Conductor connections ................................................................................................................... 82 5.4.3 Moisture and mechanical protection of joints .............................................................................. 84 5.4.4 Application of joints with different protection degrees................................................................ 85 5.4.5 Grounding connections................................................................................................................... 86 6

ADDITIONAL ACCESSORIES............................................................................................................... 89

6.1 Cable clamps.............................................................................................................91 6.1.1 Main requirements .......................................................................................................................... 91 6.1.2 Forces in a cable system ................................................................................................................. 91 6.1.3 Types of cable clamps..................................................................................................................... 93 6.1.4 Cable clamps at joints ..................................................................................................................... 94 6.1.5 Cable clamps at terminations ......................................................................................................... 94 6.1.6 Cable clamps for cable laying......................................................................................................... 95 6.2 Surge arresters .........................................................................................................98 6.2.1 Fundamentals .................................................................................................................................. 98 6.2.2 Application of sheath voltage limiters in cable systems.............................................................. 98 6.2.3 Dimensioning of sheath voltage limiters....................................................................................... 99 6.3 Earthing devices for joints and terminations..........................................................100 6.3.1 Fundamentals ................................................................................................................................ 100 6.3.2 IP and NEMA protection classes .................................................................................................. 100 6.3.3 Earthing boxes ............................................................................................................................... 102 6.3.4 Cross-bonding boxes .................................................................................................................... 103 6.3.5 Earthing clamps for terminations................................................................................................. 103 7

INSTALLATION AND OPERATION .................................................................................................... 105

7.1 Installation of accessories ......................................................................................107 7.1.1 Basics.............................................................................................................................................. 107 7.1.2 Installation of terminations........................................................................................................... 108 7.1.3 Installation of joints ....................................................................................................................... 114 7.2 Earthing ..................................................................................................................120 7.2.1 Background of earthing................................................................................................................. 120 7.2.2 Induced voltages at cable screen ................................................................................................. 120 7.2.3 Principles of earthing systems ..................................................................................................... 120 7.2.4 Earthing of terminations ............................................................................................................... 122 7.2.5 Earthing of joints ........................................................................................................................... 123 7.3 Operation ................................................................................................................123 7.3.1 Terminations in non-vertical position.......................................................................................... 123 7.3.2 Terminations on high voltage towers.......................................................................................... 124 7.3.3 Wind load for terminations........................................................................................................... 124 7.3.4 Seismic calculations ...................................................................................................................... 125 8

MEASUREMENTS, MONITORING AND DIAGNOSTICS ................................................................... 127

8.1 Introduction and basic definitions ..........................................................................129 8.1.1 Introduction.................................................................................................................................... 129 8.1.2 Basic definitions............................................................................................................................. 129 8.2 Possible PD phenomena in high voltage cables, terminations or joints.................130 8.3 Measurements of PD...............................................................................................131 8.3.1 Introduction.................................................................................................................................... 131

8.3.2 8.3.3 8.3.4 8.3.5 8.3.6

Challenges of on-site PD measurements..................................................................................... 132 Measurement methods ................................................................................................................. 132 Sensor types established for on-site measurements ................................................................. 136 PD pattern recognition .................................................................................................................. 138 PD measurement and monitoring system design ...................................................................... 139

8.4 Temperature measurements and monitoring of cables ..........................................141 8.4.1 Basics.............................................................................................................................................. 141 8.4.2 Applications ................................................................................................................................... 142 8.5 Other measurement and monitoring methods ........................................................143 8.5.1 Infrared temperature measurements for terminations .............................................................. 143 8.5.2 Water monitoring for cables......................................................................................................... 143 8.6 Other diagnostic methods ......................................................................................144 8.6.1 Oil analysis for terminations......................................................................................................... 144 8.6.2 Diagnostics based on loss-factor and polarisation-depolarisation measurements................. 144 9

TENDENCIES AND FUTURE DEVELOPMENTS................................................................................ 145

10 REFERENCES ..................................................................................................................................... 151 11 SYMBOLS AND ABBREVIATIONS..................................................................................................... 157 12 APPENDIX............................................................................................................................................ 161 12.1 SI units and SI prefixes ..........................................................................................163 12.2 Conversion table to the metric system ..................................................................164 BRUGG CABLES

1. Fundamentals

Chapter 1

Fundamentals

1

2

1. Fundamentals

1.1 Fundamental electric relations 1.1.1 Electric charges, current, elec-

3 is always proportional to the charge q. Therefore an electric field 𝐸 can be defined between the charges. Like the force 𝐹 the electric field 𝐸 is a vector. Consequently, 𝐸 is defined by:

tric field, voltage and potential Electric charges All electric phenomena are based on the flow or

𝐸=

accumulation of electric charges. The electric charges can be positive or negative and are always related to atoms. The atoms themselves consist of three different types of particles: protons, electrons and neutrons. Together, the heavy neutrons and protons form the atomic nucleus of the atoms. The much lighter electrons form the atomic shell.

𝐹 𝑞

(Eq. 1-1)

The basic unit of the electric field is newton per coulomb (N/C). In practice, however, the unit volt per metre (V/m) is much more frequently used. Common units for 𝐸 are:

𝐸 =

𝑁 𝑉 𝑘𝑉 = = 10!! 𝐶 𝑚 𝑚𝑚

While neutrons are electrically neutral particles, hence the name “neutron”, the protons and electrons possess an electric charge. The charge “e” of these particles is a fundamental parameter in our universe and has a value of e = 1.602⋅10-19 C. Protons possess the positive charge e and electrons

Since in most technical expressions, only the absolute value of the electric field is considered, in the following the electric field is given as “E”, without the vector.

the negative charge e. The electric charge is given as “q” or “Q”. The unit of electric charge q is coulomb (C), named after the French engineer and physicist August Coulomb. An attractive force occurs between positively and negatively charged particles. As atoms in general have the same number of electrons and protons, the forces are usually not noticed in our daily life. However, if an object A has more electrons (negatively charged) than protons, whilst another object

Electric field and force of two charged objects; Left: Objects have charges with different polarity and hence attracting force; Right: Both objects have same positive charges and hence repellent force

B has fewer electrons than protons (positively charged), then an attractive force occurs between the two objects. This force is given by the expres-

Electric current

sion 𝐹. The arrow above the letter indicates that the

If there is an electric conductor between the pairs

force is a vector, which means that both the abso-

of charged objects, a movement of the charged

lute value and the direction of the force have to be

particles takes place and an electric current flows.

considered.

The electric current “I” is defined as “the amount of charge that moves through a cross section of an object per unit of time” [Lindner 93]. Therefore, the

Electric field

definition of the current I is given as:

The force 𝐹, which accelerates the positive charge q towards the negative object, is proportional to the absolute value of the charge q. In general, this force is different at each point. However, the force

𝐼=

𝑐ℎ𝑎𝑟𝑔𝑒 𝑄 = 𝑡 𝑡𝑖𝑚𝑒

(Eq. 1-2)

4 The basic unit of the current is coulomb per second

Here, the difference between two potentials Φ1 and

(C/s). A much more frequently used unit for the cur-

Φ2 is called voltage “U” (sometimes written as

rent is ampere (A), named after the French physicist André Marie Ampère. It is described as:

𝐶 𝐼 = =𝐴 𝑠

voltage V). Consequently, the voltage U is described as the difference between two potentials. The unit of the voltage is Volt (V), named after the Italian physicist Alessandro Volta.

Electric potential and voltage

DC and AC current, frequency

If a charge Q in an electric field is moved from a

The term “DC” (sometimes given as “d.c.” or “dc”)

reference point P0 to another point P1, then a cer-

stands for “direct current”. The term “AC” (some-

tain amount of energy has to be applied. According

times given as a.c. or ac) stands for “alternating

to equation 1-1, this energy not only depends on

current”.

the position of point P1 but also on the charge Q. If the same charge is moved to a second point P2, this energy changes. Similar to the definition of the electrical field, these energies can be divided by the charge Q. The result of this mathematical relation is a parameter, which is related to the electric field. This parameter is called potential “Φ“ of the elec-

A current is called DC if it does not change its amplitude and direction over time. A current is called AC if it changes direction and if its value (magnitude) changes on a regular basis. The sum of the positive and negative values integrated over time is zero.

tric field 𝐸 . All points on a line that have the same

1

potential are placed on a so-called equipotential line. To move a charge along an equipotential line, However, energy is necessary for moving a charge Q from a point P1 with potential Φ1 to a point P2 with potential Φ2. This energy Welectric is [Lindner 93]:

Sinusoidal current (voltage)

0.6

dc current (voltage)

0.4

Amplitude

no energy is necessary.

0.8

0.2 0 -0.2 -0.4 -0.6 -0.8 -1

Time t

Amplitude of DC and AC currents

𝑊%#%&!"$& = 𝑄 ∙ Φ! − Φ! = 𝑄 ∙ 𝑈

(Eq. 1-3) Although the terms DC and AC refer to “currents”, they are also used to describe voltages. Consequently, DC voltage stands for a non-changing voltage and ac voltage stands for a voltage that changes its value (magnitude) on a regular basis. The number of turns of an AC current per unit of time is referred to as the frequency “f” of that current. The unit of the frequency f is given in 1/s or “Hz” (Hertz), named after the German physicist Heinrich Hertz. In ac power supply systems, the standard frequency is 50 Hz for most European and Asian states and 60 Hz for most American states. In the case of railway systems, lower frequency

Electric field lines (black) and equipotential lines (violet) in between differently charged objects

values, such as 16 ⅔ Hz, are often used.

1. Fundamentals

5

1.1.2 Ohm’s law and Kirchhoff’s

connection of resistors, the currents split in each line of the circuit.

laws Ohm’s law When a conductor is present between two potentials in an electrical field, an electric current will flow. The current can be calculated as follows:

Second Kirchhoff’s law: Mesh rule The second law of Kirchhoff says that the sum of all voltages around any closed loop (electric circuit) is zero. The second law of Kirchhoff is defined as [Lindner 93]:

𝐼=

𝑈 𝑅

(Eq. 1-4) 𝑈=0

Consequently, the resistance “R” of the conductor is given as:

(Eq. 1-7)

This law states that the sum of all voltage sources is equal to the sum of all voltage users – in other words: the electric charges in an electric circuit re-

𝑅=

𝑈 = 𝑐𝑜𝑛𝑠𝑡 𝐼

(Eq. 1-5)

main in the circuit itself. This means that in an electric circuit with a series connection of resistors, the current is the same in each part of the circuit.

This equation is called Ohm’s law, named after the German physicist, Georg Simon Ohm. The unit of

Both laws are named after the German physicist Gustav Robert Kirchhoff.

the resistance is “Ω” (Ohm) and is given as:

𝑅 =

𝑉 =Ω 𝐴

1.1.3 Terms of electric power Effective power P

Ohm’s law reveals a distinct linear relation between voltage and current. This means that with a constant electric resistance at a voltage level of 50%, only 50% of the current flows.

The effective electric power P is the product of electric current and voltage and is given as:

𝑃 =𝑈∙𝐼

(Eq. 1-8)

First Kirchhoff’s law: Nodal rule The first law of Kirchhoff considers a node in an electric circuit. It describes the principle of the con-

When applying Ohm’s law (Eq. 1-5), the electric power can also be given as:

servation of electric charge. The law says that at any node (or electric junction) the sum of all currents flowing into that node is equal to the sum of

𝑃 = 𝐼! ∙ 𝑅

(Eq. 1-9)

all currents flowing out of that node. The first law of Kirchhoff can be expressed as [Lindner 93]:

or:

𝑃= 𝐼"! =

𝐼#!"

(Eq. 1-6)

𝑈! 𝑅

(Eq. 1-10)

Equations 1-9 and 1-10 show that the electric power has a quadratic relation to voltage and current. This If incoming currents are taken as positive and outgoing currents are taken as negative, the first law describes the sum of all currents in a knot as zero. This means that in an electric circuit with a parallel

means that doubling the current (or voltage) and keeping the resistance constant means increasing the power by a factor of four.

6 The effective power P is the electric power that can

Power factor

be directly transferred into other forms of power,

The presence of reactive power leads to a phase

such as light, mechanical, thermal or chemical power. A useful example of such a transfer is electric heating. An unwanted example is the ohmic losses in the conductor of a cable during current flow. The unit of the effective power is Watt (W), named after the Scottish inventor James Watt, and is defined as:

shift between the current I and the voltage V. The angle of this phase shift is called “ϕ“ (phi). Usually this phase shift is described by the cosines of the corresponding angle ϕ and is named as the “power factor“. The greater the reactive power, the higher is the phase shift. In most electrical power equipment, the power factor cos ϕ is between 0.82 and 0.93.

𝑃 =𝑉∙𝐴 =𝑊

Considering Equation 1-11 and taking the power factor into account, the effective and reactive power in a three-phase system with voltage U can be

Reactive power Q In contrast to the effective power P, the reactive

given as:

power “Q” cannot be transferred directly into any other terms of power. Reactive power is needed to create electric and/or magnetic fields. When transferring electric energy in a cable or an overhead line, not only the effective power P (in the

𝑃 = 𝑈 ∙ 𝐼 ∙ 3 ∙ cos 𝜑 𝑄 = 𝑈 ∙ 𝐼 ∙ 3 ∙ sin 𝜑

(Eq. 1-12)

(Eq. 1-13)

conductor), but also the reactive power Q (in the insulation and in the magnetic field around the line) is transferred. Since reactive power cannot be di-

Example 1-1:

rectly used, it usually contributes to losses and

Æ A 30 MVA transformer and a 132 kV polymer cable connecting it

must therefore be limited as much as possible (as

 Electric current in the cable

far as practical application for cable systems are

P According to equation 1-11 together with 1-12 and 1-13 the current can be calculated as:

concerned). To differentiate the types of power from each other, the unit of reactive power is given in “var” (volt

𝐼=

𝑆 𝑈∙ 3

=

30 000 𝑘𝑉𝐴 = 131 𝐴 132 𝑘𝑉 ∙ 1.73

ampere reactive). Example 1-2:

Apparent power S The geometric sum of effective power P and reactive power Q is the apparent power “S”. The apparent power can be calculated by using the law of Pythagoras and is given as:

𝑆=

𝑃! + 𝑄!

Æ A 30 MW generator, a power factor of cos ϕ = 0.85 and a 132 kV polymer cable connecting it  Electric current in the cable P According to equation 1-12 the current can be calculated as:

𝐼=

𝑃 𝑈 ∙ 3 ∙ cos 𝜑

=

30 𝑀𝑊 = 155 𝐴 132 𝑘𝑉 ∙ 1.73 ∙ 0.85

(Eq. 1-11) Electric energy

To differentiate between the different types of

Electric energy is defined as the product of (usable)

power, the unit of apparent power S is given in

effective power in a certain unit of time. It is there-

“VA” (Volt Ampere). Since in almost all electric

fore given as:

equipment effective and reactive power are consumed (e.g. in a cable: effective power in the con-

𝑊%#%&!"$& = 𝑃 ∙ 𝑡

(Eq. 1-14)

ductor, reactive power in the magnetic and the electric field), apparent power is usually given to

The unit of energy is joule (J or Ws), named after

describe the power capacity of the equipment.

the British physicist James Prescott Joule.

1. Fundamentals

7

1.1.4 Electric resistivity, conductivity, insulators and semi-

conductor decreases and with conductors of a greater length, the resistance increases.

conductivity Change of resistance with temperature

Resistivity and resistance The current flow through a metallic conductor is a transport of electrons. These electrons interact with the lattice structure of the metals. The electrons lose kinetic energy when interacting with the lattice structure. On a macroscopic scale this loss of kinetic energy is described by the electrical resistivity (ρ). As the degree of interaction of electrons with the lattice structure differs according to the type of

A rise of temperature causes the lattice in the material to oscillate at higher amplitude, resulting in a stronger interaction of the electrons with the lattice. This process leads to a higher loss of energy and, ultimately, to a rise in temperature of the conductor. Most metals including all common conductor materials, such as copper or aluminium, show an increase in the resistance when subjected to a rise in temperature. Such materials are called positive

metal, the degree of electric resistivity also varies.

temperature coefficient materials. The relation of

The electric resistivity of each material can be

resistance and temperature is given by [Lindner

found in appropriate literature. Typical values of

93]:

metals are given in the table below. Electric resistivity of common materials at 20 °C [Lide 03], [Friedrich 93] Material

Electric resistivity

Silver

0.015 ⋅ 10-6 Ωm

Copper

0.017 ⋅ 10-6 Ωm

Gold

0.021 ⋅ 10-6 Ωm

Aluminium

0.027 ⋅ 10-6 Ωm

Brass

0.064 ⋅ 10-6 Ωm

Iron

0.10 ⋅ 10-6 Ωm

Lead

0.21 ⋅ 10-6 Ωm

Stainless steel

0.71 ⋅ 10-6 Ωm

𝑅 𝑇 = 𝑅!" ∙ (1 + 𝛼!" 𝑇 − 𝑇!" )

(Eq. 1-16)

In which: R (T):

Resistance at a certain temperature T

R20:

Resistance at 20°C

α20:

Temperature coefficient of the material at 20°C; αAl = 16.8⋅10-6 1/K, αCu = 23.9⋅10-6

T:

Temperature

T20:

Temperature of 20°C

Conductivity The opposite of electric resistivity is electric conductivity. The electric conductivity “κ” (Kappa) is defined as:

The resistance of a conductor is given by the material specific values and its geometry. It is defined as [Lindner 93]:

𝑅 =𝜌∙

𝑙 𝐴

(Eq. 1-15)

𝜅=

1 𝜌

(Eq. 1-17)

The unit for κ is siemens per metre (S/m). The unit S (Siemens) is named after the German engineer and inventor Werner v. Siemens.

In which: R: Resistance of the conductor ρ:

Resistivity of the conductor

l:

Length of the conductor

A:

Cross-section area of the conductor

Equation 1-15 shows that with higher cross-section values the resistance (and therefore losses) of a

Electric conduction As stated in Section 1.4, electric current is the flow of electrons. As all metals allow a very high flow of electrons, they are referred to as “electric conductors” or merely “conductors”.

8 Ionic conduction In addition to the flow of electrons, entire atoms can also move through a material. The charged atoms (negatively or positively) are called ions. Ions typically move through liquids. Positively charged ions are called “cations”, while negatively charged

1.1.5 Magnetic field Magnetic field and magnetic field strength A magnetic field occurs between the poles of permanent magnets and in the surroundings of current carrying conductors [Lindner 93].

ions are called “anions”. Whilst the transport of matter does not occur during electron flow, it does occur during the flow of ions – a phenomenon that can be seen in batteries.

N Electric insulators As a general rule, electric insulators are materials that do not allow a significant amount of electric current to flow. Electric insulators are air, oil, ce-

S

ramics, epoxy resin, glass, polymers, rubber or wood. Since they are very important for the equipment of high voltage cable systems, more details are given in a separate section (Chapter 5). Magnetic field lines of a permanent magnet

Electric semiconductivity (as applied to cable systems) Although the application of semiconducting materials in electronic components, such as doped silicon, is not covered in this book, it is worth mentioning that the expression “semiconductive (or semiconducting) material” frequently occurs in the field of high voltage technology. This expression is usually used for materials with a level of resistivity that is between the high conductivity of common metals and the low conductivity of insulators. In most cases, these semiconducting materials consist of carbon black filled polymers. The polymers can be made of thermoplastic materials, such as polyethylene, or elastomers, such as silicone rubber or EPR (ethylene propylene rubber). Carbon black filled polyethylene material is used for the inner and outer semiconducting layer of polymer cables, while carbon black filled elastomers are used for the deflectors and middle electrodes of high voltage accessories. Typical resistivities of these materials are around 10 Ωcm.

Magnetic field around a current carrying conductor

Like electric field lines, magnetic field lines represent the magnetic field of a material. In contrast to electric field lines, magnetic field lines are always “closed”, having neither a beginning nor an end. In a permanent magnet, the magnetic field lines travel outside the magnet from north- to south pole. Since magnetic field lines are always closed, they go inside the magnetic material from south- to north pole. In a current carrying conductor, mag-

1. Fundamentals

9

netic field lines occur concentrically around the conductor. The strength and direction of the magnetic field is given by the magnetic field strength “H”. Similar to the electric field E, the magnetic field strength H is a vectorial parameter and always rectangular to the

Φ=𝐵∙𝐴

(Eq. 1-19)

The unit of the magnetic flux is volt multiplied by second (Vs) or Weber (Wb), named after the German physicist Wilhelm Eduard Weber.

causing current I [Lindner 93]. For the sake of simplicity, the vector is not used in the following. The

Inductivity

unit of the magnetic field strength is ampere per

The inductivity L of a coil is given as [Lindner 93]:

metre (A/m):

𝐻 =

𝐴 𝑚

𝐿=

𝑁∙Φ 𝐼

Magnetic flux density

In which:

The magnetic flux density (sometimes referred to

N:

(Eq. 1-20)

Number of turns in a coil

as magnetic induction) “B” provides the link be-

Φ:

Magnetic flux

tween the magnetic field strength H and material

I:

Current

properties. The magnetic flux density B is given as [Lindner 93]: The unit for the inductivity is Henry (H), named af-

𝐵 = 𝜇! ∙ 𝜇! ∙ 𝐻

(Eq. 1-18)

ter the US-American physicist Joseph Henry, and can be given as:

𝐿 =𝐻=

In which: µ0 :

Absolute permeability = 1.257⋅10-6 H/m

µr:

Relative permeability

H:

Magnetic field strength

For most conductors the relative permeability µr is

𝑉∙𝑠 𝐴

1.1.6 Electromagnetic induction Electromagnetic induction is one of the fundamen-

close to one, only for strong magnetic materials -

tal findings within the field of electricity. All kinetic

the so called ferrite materials - µr is much larger

energy generated by electric energy (and vice ver-

than one. The unit of the magnetic flux density is

sa), such as that found in electric motors and gen-

tesla (T), named after the Serbian engineer and in-

erators, is based on this relation.

ventor Nikola Tesla.

Faraday found that the electromotive force (EMF)

𝐵 =𝑇=

𝑉∙𝑠 𝑚!

Magnetic flux

produced around a closed path is proportional to the rate of change of the magnetic flux through any surface bounded by that path. The electromagnetic induction (or simply “induction”) is defined as the creation of an electric volt-

The magnetic flux “Φ” is the magnetic equivalent

age in an electric conductor caused by the change

of the electric current I. It is driven by a magnetic

of the magnetic flux in the area of that conductor

field (comparable to the current I which is driven by

[Lindner 93]. The induced voltage according to the

the voltage) and describes the magnetic flux

law of magnetic induction by Faraday can be ex-

through a material. For a homogeneous field with

pressed as:

the magnetic flux density B and the area A, the magnetic flux is given as [Lindner 93]:

10

𝑉"!# = −𝑁

𝑑Φ 𝑑𝑡

(Eq. 1-21)

ture in order to explain the basic relation between the expressions. In technical objects, such as cable insulations, a very large number of electric charges occur, creat-

In which: Vind:

Induced voltage

ing the electric field in the insulation materials. In

N:

Number of turns of a coil

order to visualise the electric field, electric field

dΦ/dt:

Rate of change of magnetic flux

lines are applied. Electric field lines describe the way positive charg-

The sign “–“ indicates that the induced voltage Vind has the opposite direction to that of the change of the flux that produces this voltage. The relation becomes clearer when applying equation 1-18 and 119 to equation 1-21:

es move in the insulating material. They only occur in insulating materials, not in conductors. In contrast to magnetic field lines, electric field lines always have a start and an end point, which are always on the surface of a conductor. To prevent confusion between the terms “conductor” and “current carrying conductor” (such as the copper

𝑉"!#

𝑑𝐻 = −𝑁 ∙ 𝐴 ∙ 𝜇! ∙ 𝜇! ∙ 𝑑𝑡

(Eq. 1-22)

conductor of a cable), conductors hosting an electric field are referred to as “electrodes”. Electric field lines always enter and leave the electrodes

An electric voltage is induced in any conductor in a

perpendicular to their surfaces.

changing magnetic field, whether the field itself

If electrodes are metallic plates, lying parallel to

changes or the conductor is moved within the

each other, the direction of all field lines between

magnetic field. When the conductor in a changing

the plates is the same and the absolute values are

magnetic field is a closed loop, a current can flow

equal. In this case the electric field is homogenous.

in that loop. By applying Ohm’s law, the current flow in that loop can be determined. It is worth mentioning that only voltage can be in-

+Q

-Q

+ + + + + + + +

-

duced, not current. Current flow is always a result of the induction of voltage together with the resistivity of the material. Electromagnetic induction is particularly relevant to cables. The current flow through the (inner) conductor of a cable induces voltage into the outer ground wires of the cable. This induced voltage is the reason why the outer ground wires of the cable have to be earthed and/or cross-bonded. Since this

d

topic is highly relevant to cable design and accessory application, it is described in detail in Chapter 4.

1.2 Electric field 1.2.1 Electric field and field lines Electric field lines Section 1.1.1 showed that an electric force acts between two charges, resulting in the creation of an electric field. This explanation is of a physical na-

Schematic drawing of the homogenous electric field between two parallel plates, charged with positive and negative charge Q

Electric field strength When two parallel plates are charged with the voltage U, the correlation between the electric field and the voltage is given by the following expressions [Küchler 96]:

1. Fundamentals

𝐸=

𝑈 𝑑

11 (Eq. 1-23)

In which: E:

Electric field

V:

Voltage between the two plates

d:

Distance between the two plates

The electric field between two plates is a simple arrangement and can be expressed easily. More complex arrangements, such as the electrical field in joint bodies, are calculated with the help of computer simulations. These calculations are done with finite element programs.

1.2.2 Electric field in technical objects Electric field in insulation bodies of joints When designing electric equipment, knowledge of

FEM calculation of the electric field in the stress cone of a termination and in the basement of the insulator (red = highest electric field; dark blue = no electric field)

Electric field in cylindrical cable insulations The electric field in a cable can be calculated with an approximation of two coaxial cylinders. Thus, the electric field E(x) at the position x in the cable insulations is given as [Küchler 96]:

the electric field in and around the equipment is essential. The shape of technical equipment, such as electrodes in an insulation body of a polymer joint, are usually so complex that modern computer simulation methods (so called Finite Element Method –

𝐸 𝑥 =

𝑈 𝑥 ∙ ln

(Eq. 1-24)

𝑅 𝑟

In which:

or just FEM) are necessary to calculate the electrical

E(x):

Electric field at point x

field at each point of the arrangement. The different

U:

Applied voltage

colours represent different values of the electric

R

Radius of the outer conductive layer

field strength.

r

Radius of the inner conductive layer

x:

Position

R r FEM calculation of the electric field in the insulation body of a polymer joint (red = highest electric field; dark blue = no electric field)

E Electric field in terminations Similar to the electric field in joint bodies, the electric field in stress cones for terminations as well as in the whole arrangement of the terminations themselves is also calculated with the help of numerical FEM tools.

E(r) x Schematic drawing of the insulation between the inner and outer semiconducting screen of a cable and the radial electric field in the insulation

12 Equation 1-24 shows that the highest electrical field

is considered. The smaller the ratio, the smaller the

in the insulation of a cable occurs at the inner sem-

factor by which the electric field is increased.

iconducting layer. The design of the cable and the manufacturing process must guarantee that the maximum electric field during the different tests remains below the dielectric strength of the insulating polymer. While the maximum electric field of medium voltage cables ranges from 2 to 4 kV/mm, those of high

Ellipsoidal shaped protrusion

voltage cables are much larger and range from 6 to 14 kV/mm. Inclusions with different permittivity in the Electric field in cable insulations with inclusions at the semiconducting layer Equation 1-24 also shows that the shape of the electrodes determines the distribution of the electric field. In order to use the insulation material as effectively as possible, the electric field should re-

insulating material If two different materials A and B with a relative permittivity of εA and εB are between two parallel electrodes, the electric field EA and EB in the two materials is given by the following expression [Küchler 96]:

main uniform, otherwise considerable insulation material is needed, making the cable expensive and heavy.

𝐸! 𝜀! = 𝐸! 𝜀!

(Eq. 1-25)

To avoid unnecessary high electric fields, it is essential that the electrodes are plane and rounded.

Equation 1-25 shows that in the material with the

Sharp edges or peaks increase the electric field. For

higher relative permittivity, the electric field is low-

this reason, it is very important that high voltage

er.

cables and cable accessories are manufactured in clean surroundings. Even very small conductive

+Q

particles of 50 to 100 µm on the electrode surface will increase the electric field dramatically [Weis-

+ + + + + + + +

senberg 86], [Weissenberg 09].

Electric field lines

High voltage

Ground

-Q

ε1=1

ε2=2

Ε1=2

Ε2=1

d/2

d/2

-

Influence of a sharp edged particle on the electric field distribution

Schematic drawing of electric fields in two different materials with different relative permittivity (the density in the field lines reflects the electrical field strength)

The factor by which the electric field is locally in-

If an air or gas inclusion occurs in the insulating

creased depends on the exact shape of the conduc-

polymeric material, the electric field in that bubble

tive particles or protrusions. Therefore, the ratio of

can be calculated by using Equation 1-25 as fol-

height to width of an ellipsoidal shaped protrusion

lows:

1. Fundamentals

13 be seen to cause electric discharges and, hence,

𝐸#"! =

𝜀%#!'$&(%"# ∙ 𝐸%#!'$&(%"# 𝜀#"!

(Eq. 1-26)

insulation failure. Although the above calculations were applied to inclusions of air in the insulation material of XLPE,

Equation 1-26 shows that the electric field in air in-

they are also applicable and relevant to all insula-

clusions is higher than in the surrounding polymer.

tion materials.

Taking into account that the dielectric strength of air is also much lower than that of the polymer, the danger of air inclusions in polymeric materials becomes obvious.

1.2.3 Capacity Every

Polymer material

arrangement

of

a

“conductor-insulator-

conductor” is able to store electrical charges. The capability of such an object to store charges is given by the capacity “C”. The capacity of such an object depends only on the insulating material and the geometrical arrangement. The capacity of two

Gas

parallel plates can be calculated as [Küchler 96]:

𝐶 = 𝜀! ∙ 𝜀! ∙ Schematic drawing of the electric field in an air-filled void within a polymeric insulation (the density in the field lines reflects the electric field strength) (idealised situation)

𝐴 𝑑

(Eq. 1-27)

In which: C:

Capacity

ε0 :

Absolute permittivity or electric field constant; ε0= 8.854 ⋅ 10-12 F/m

εr :

Relative permittivity

Example 1-3:

A:

Area of the plates

Æ The relative permittivity εr of XLPE = 2.3, the electric field in the insulation of Einsulation = 10 kV/mm and the dielectric strength of air is: Ebreakd. air = 2.5 kV/mm

d:

Distance between the plates

 Electric field of air inclusion compared to dielectric strength of air P Taking the relative permittivity of air as εair = 1 and calculating according to Equation 1-26, the electric field in the air bubble can be calculated as:

𝐸#"! =

!.! !

∙ 10

!" !!

= 23

!" !!

.

9 The electric field strength of the air bubble is about 10 times higher than the dielectric strength of air. If such an air bubble were in the insulation, a local breakdown (partial discharges) in the bubble would occur, thus damaging the cable insulation.

The unit of the capacity is farad (F), named after the English physicist Michael Faraday. It is defined as:

𝐶 =𝐹=

𝐴∙𝑠 𝑉

The relation between the stored charge Q and the capacity C is given by:

𝐶=

𝑄 𝑈

(Eq. 1-28)

Example 1-3 illustrates why small air or gas filled

The relative permittivity εr of the material reflects

voids are so dangerous in insulating materials. In

the polarity of the atoms or molecules in the mate-

these voids, the electric field is higher than in the

rial and is expressed with a dimensionless number.

surrounding insulating material and the dielectric

In other words, this material constant describes

strength of air is much lower than that of the solid

how well electrical charges can be held in an insu-

insulation. Thus, voids in an insulation material can

lating material. The higher the relative permittivity,

14 the higher the polarity of the material. The higher the polarity of the material, the more charges can

1.3 Insulating materials in high voltage technology

be stored in the insulation – hence the capacity of the material is higher. Typical values of the relative permittivity are given in the table below.

1.3.1 Solid materials There are two general types of solid insulating ma-

Relative permittivity εr of different insulating materials, used in high voltage technology (at 20°C and 50 Hz); [ABB 92], [Küchler 96]

terials in high voltage technology, ceramic materials and polymers. The most common type of ceramic material is porcelain, used for insulators and the supporting bodies of high voltage terminations.

Material

Relative permittivity

Cross-linked polyethylene (XLPE)

2.3

Polyvinylchloride (PVC)

3.3 – 7.0

Polyurethane (PUR)

3.0

Silicone Rubber (SIR)

2.7

Epoxy Resin

3.0 – 4.2

Air

1

Silicone Oil

2.8

Transformer Oil

2.8

Porcelain

2–6

prevents the polymer from becoming liquid when

SF6

1

temperatures exceed melting point. High density

Water

80

For polymers, a considerable number of different materials occur. In general, these fall into the following groups: - Thermoplastic polymers - Thermoset materials - Elastomer materials

The most common thermoplastic material is crosslinked polyethylene (XLPE), which is used for cable insulation. The cross linking of the polyethylene

polyethylene (HDPE) and polyvinylchloride (PVC) two other thermoplastic materials - are used for the cable sheaths of high voltage cables. Thermoset materials are used as insulating bodies

Example 1-4:

for high voltage cable terminations, particularly for

Æ Area of capacitor = 1 m2, thickness of the capaci-

GIS and transformer terminations. The most com-

tor = 1 mm by considering the two materials εXLPE =

mon thermoset material for such applications is

2.3 for XLPE and εWater = 80 for water

epoxy resin.

 Capacity of the arrangement

Elastomer materials, such as silicone rubber (SiR)

P Calculating the capacity according to equation 127; the results are:

or ethylene propylene rubber (EPR), are widely

𝐹 1 𝑚! ∙ 2.3 ∙ = 0.02 𝜇𝐹 𝑚 0.001 𝑚

advantage of elastomers is their flexibility and the

𝐶 = 8.85 ∙ 10

!"!

𝐹 1 𝑚! 𝐶 = 8.85 ∙ 10!"! ∙ 80 ∙ = 0.71 𝜇𝐹 𝑚 0.001 𝑚

used for stress cones and joint bodies. The main fact that they can be elongated considerably, especially the material of silicone rubber. Thus, these properties enable certain sizes of stress cones to be used with a wide range of different cable diameters. Silicone rubber compounds are also used as material for outdoor terminations. Particularly in heavily polluted environments, to which their hydrophobicity is excellently suited.

1. Fundamentals

1.3.2 Liquids The majority of liquids used in high voltage technology are mineral and synthetic oils. These oils are used in oil-filled cables and transformers.

15

1.4 Power transmission 1.4.1 Basics of electric power transmission systems

Another insulating liquid is silicone oil, which is

According to a general definition, electric power

used in terminations for polymer cables. For these

transmission is the bulk transfer of electrical ener-

applications it is important that the silicone oils ful-

gy, a process involving the delivery of electricity to

fil certain electric requirements. The most im-

consumers [Wikipedia 09-2].

portant ones are the dielectric strength and the loss

A power transmission network typically connects

factor – both of which are influenced by the humidi-

power plants to multiple substations near a popu-

ty of the oil.

lated or industrial area. The wiring from the power plants to the substations is referred to as electrical

1.3.3 Gases The most important gas in high voltage technology – aside from air – is sulphur hexafluoride (SF6). This gas is used in gas insulated switchgears (GIS) and has a good dielectric strength. In addition, SF6 is a gas with electro negativity properties, which makes it very suitable for use in switching chambers of circuit breakers [ABB 92]. In general, the breakdown voltage of gases also depends on the pressure and distances between the electrodes. This relation is described by the “Paschen law”, named after the German physicist Friedrich Paschen. The following diagram shows the “Paschen law” for air. The breakdown voltage is a function of the air pressure times the distance between the electrodes.

transmission. The wiring from substations to consumers is referred to as electrical distribution. The electric power transmission allows distant energy sources to be connected to consumers in population centres. The energy sources can be traditional energy sources, such as coal or hydroelectric power plants, or renewable energy sources, such as wind farms or solar plants. The power transmission network is referred to as the “grid”. Multiple and redundant lines between points of the network are made in order that the power can be routed from any power plant in the grid to any load centre through a variety of lines. This is done to provide redundancy in order to enhance security of energy supplies. Recently, power transmission has come to be greatly influenced by the economics of the transmission path. Thus, the cost of power has become just as important as the redundancy of the system.

Relation between the breakdown voltage and the product of the gas pressure and the distance between the electrodes for air.

16

Coal plants

Nuclear plants Extra high voltage

275 kV – 1000 kV (mostly AC, some HVDC)

Large wind farms

High voltage 110 kV and higher Medium sized power plants

Transmission Grid Industrial Power plants Distribution Grid

City power plants

Medium voltage

Industrial customers

Private solar farms

Low voltage

Private consumers

Wind farms Extra high High Medium Low voltage

Transformer

Solar farms

Structure of a power transmission and distribution system

Transmission lines usually use a three-phase alter-

enables a maximum transmission capacity with a

nating current (ac). In order to reduce losses during

minimum of losses.

transmission, electricity is transmitted at high volt-

High voltage direct current (HVDC) is used for long

ages. In terms of transmission lines, “high voltage” means voltage levels above or equal 110 kV. In general, the higher the transmission distance, the higher the transmission voltage should be. This

distance transmission or in long oversea cables.

1. Fundamentals

17

1.4.2 Overhead lines Overhead lines are metal-wired conductors with no insulation cover. The conductor material is usually aluminium alloy consisting of several strands, often reinforced with steel strands. Copper in overhead lines is only used for very specific applications, such as railway lines. Conductor sizes range from those of 10 mm2 to those of 1000 mm2, with varying resistance and current-carrying capacity. Thicker wires are more expensive, leading only to a relatively small increase in current carrying capacity due to the skin

Optical ground wire – OPGW

effect (for more information, see Chapter 4). Overhead lines contain a tower and insulators to separate the voltage-carrying conductors from the

1.4.3 Power cables

ground. Most high voltage overhead lines have an

Power cables are metal-wired conductors covered

earth wire on top, which conducts earthing current

by a solid insulation. Their conductors consist of

at an asymmetrical phase shift of the three phases

either copper or aluminium. Their solid insulation

and protects the high voltage overhead line from

consists of either a polymer or of an oil-filled or

lightning strokes. Such earth wires are usually

mass impregnated paper.

made with integrated fibre optics, the “optical ground wire” (OPGW). OPGW are used for additional data transmission. Since overhead transmission lines are insulated by air, their design requires a minimum of clearances to the ground for maintaining the required safety.

Advantages of underground power cables in comparison to overhead lines are: - Far less subject to damage from severe weather conditions

Adverse weather conditions – such as heavy winds

- Less required space for transmission path

or storms, ice loads, or even extremely high tem-

- Can be used to cross large lakes or seas

peratures – can affect the performance and have to

- Invisible to the public

be considered in their design and operation.

- Less required material for insulators and towers - Lower probability of external failures, such as tree-falling or bird collision - No danger to flying aircrafts

Disadvantages of cables in comparison to overhead lines are: - Higher costs - Lower transmission capacity due to lower heat dissipation - More difficult to repair failures in the system Tower of an overhead line with wires, insulators and OPGW on top of the tower

- High capacitive charging current for the operation of the system

18 Example 1-5: Depending on the voltage level, cables only require approximately 1 – 10 metres for installation, whereas overhead lines require a surrounding strip of approximately 20 – 200 metres, which must be kept permanently clear for safety, maintenance and repair.

tem, detailed information on these components is given in Chapter 5. For an efficient cable system further accessories are necessary; these are: cross-link boxes, earthlinking boxes, surge voltage limiters or cable clamps. More information about these additional accessories is given in Chapter 6.

Electric power cables have their ingredient part in the electric power transmission. They are used for the transmission of power in and through: - Densely populated areas - Areas where land is unavailable for overhead lines because the consent for planning of overhead lines may not be given

1.4.4 Transmission capability The amount of power that can be transmitted over transmission lines, be it overhead lines or cables, is limited. The reasons for such limits are related to the length of the line. In the case of short lines,

- Rivers and other natural obstacles

losses caused by current flow through the conduc-

- Territory with natural or environmental herit-

tors produce a thermal limit. If too much current

age - Areas of significant or prestigious infrastructural development - Territory which needs to maintain its value for future urban expansion or other developments

flows through a cable, the insulation of the equipment may be damaged irreversibly. The insulation of cables makes the thermal limits lower than those of overhead lines. For lines of intermediate or long lengths, the voltage drop of the lines sets the limit. As a general rule, the economic distance of an ac overhead line

Before the 1970s, underground power cables were

in km is approx. equal to the voltage level in kV. For

insulated with oil-paper. The oil was held under

example; when transmitting power over a distance

pressure in order to prevent formation of voids,

of 350 km, a voltage level of 380 kV is typically

which would lead to partial discharges in the insu-

used.

lation and finally cause a breakdown of the system. Throughout the world, many of today’s power grids still make use of oil-paper insulated cables. However, since that technology is not state of the art and several literatures are available on the market covering that topic, high voltage oil-filled cables

Despite urban areas, higher voltages are usually transmitted through overhead lines, whilst cables usually transmit lower voltages. The ratio of AC overhead lines to cables in the German grid is shown below [Kirchner 09], [Henningsen 09].

in this book. Nowadays most of the high voltage underground cables are insulated by cross-linked polyethylene (more information is given in Chapter 4). To connect the cable to other electrical equipment, such as substations, transformers or overhead lines, terminations are used. To achieve greater transmission length, joints are used to connect two cable segments together. Terminations and joints for high voltage cables are referred to as “high voltage accessories”. Since high voltage accessories are vital for the functioning of the cable sys-

% of overhead lines and cables

and their associated accessories are not considered 100

Overhead lines

90

79.7

80

99.7

93.8

Cables

70

64.4

60 50 40 30 20

35.6 20.3 6.2

10 0

≤ 1 kV

>1 - 60 kV

110 kV

0.3

220 kV

Voltage level

Rate of overhead lines to cables in the German grid [Kirchner 09], [Henningsen 09]

1. Fundamentals Until a few years ago, it was difficult to predict temperature distributions along a cable route. As a

19 Typical electric fields for different situations

result, the maximum applicable current load was

Description

Electric field

usually a compromise between an estimation of

Natural electric field on earth (without thunderstorm) [Wikipedia 10-1]

About 130 V/m = 0.13 kV/m

Typical value in houses (due to electric home equipment and home power supply) [TU-Graz 10]

5 – 40 V/m = 0.005 – 0.04 kV/m

todays and future operation conditions and also reflected the desire to reduce the risk of thermal failures to a minimum. Today, with the availability of industrial distributed temperature sensing systems, the monitoring of cables is easier. Together with intelligent software, it enables the operator to predict the thermal load of the system. For more

50/60 Hz 220 kV overhead line [TU-Graz 10]

detailed information on the distributed temperature

- directly under the line

2.5 – 6 kV/m

system of cables, see Chapter 8.

- with 20 m distance of the line

1 – 2 kV/m

During use of an electric blanket (50/60 Hz) [TU-Graz 10]

4.5 kV/m

1.4.5 Power transmission and environment Electric field

The values of electric field limits are different in certain countries or regions. A selection of limits for the ac electric field is given in the table below.

The topic of power transmission and environment, especially the issue of the influence of the electro-

Selected limits for electric ac fields at 50 Hz

magnetic field on surrounding individuals, has become increasingly prominent over the past years.

Source

As this topic is also related to cable systems, the most important issues are discussed here. However, due to the complexity of this topic, additional details from the relevant literature should be taken into account. When generating, transmitting or using electrical power, electromagnetic fields (EMF) occur. In the case of overhead lines, EMF passes in an unfiltered manner out to the external environment. In contrast, EMF in cables is considerably lower due to shielding by the outer metallic shield of the cable, as well as the soil around the cables themselves.

Limits of the electric field

26th BImSchV (German regulation for electromagnetic pollution) [BImSchV 97]

5000 V/m = 5 kV/m

International Commission on Non-Ionizing Radiation Protection (ICNIRP) World Health Organisation (WHO) [ICNIRP 10]

5000 V/m = 5 kV/m

DIN/VDE 0848 (German Electrotechnical Standard) for average populated area [DIN/VDE 0848]

7000 V/m = 7 kV/m

DIN/VDE 0848 (German Electrotechnical Standard) for working space area [DIN/VDE 0848]

20000 V/m = 20 kV/m

Although there are no shields for the electromagnetic field of overhead lines, this must not necessarily be a cause for concern, as they are usually located at a greater distance from the ground or

The values in the above tables are for 50 Hz. However, they can also be assumed for 60 Hz. The val-

surroundings. The highest value for the electric

ues for 16 ⅔ Hz, typically occurring in railway

field of high voltage overhead lines occurs directly

overhead lines, may slightly differ. In the 26th BIm-

under the line. The electric field in terms of envi-

SchV (German regulation for electromagnetic pol-

ronmental influence is usually given in V/m or

lution), the limit for a 16 ⅔ Hz electric field is

kV/m. Typical electric field levels in overhead lines

10 kV/m (instead of 5 kV/mm for 50 Hz fields) [BIm-

located in various environments are shown in the

SchV 97]. A similar IEC standard to that of the

table below.

DIN/VDE 0848 is the IEC 62226 [IEC 62226].

20 Magnetic field Overhead lines and cables not only emit electric fields, but also magnetic fields. It is hardly possible to give a number for the value for underground cables, as this is very much depends on the laying conditions of the cable system and the current flow. When laying cables close together in the ground, the magnetic field on the ground surface is

Vacuum cleaner at a distance of 30 cm [BfS 10]

2 – 20 µT

Drill machine at a distance of 30 cm [BfS 10]

2 – 3.5 µT

Electric cooking oven at a distance of 30 cm [BfS 10]

0.15 – 0.5 µT

Fluorescent lamp at a distance of 30 cm [BfS 10]

0.5 – 2 µT

almost degraded and is thus very low. When laying cables somewhat apart (e.g. at a distance of 60 – 80

* It should be mentioned that the natural magnetic field

cm, and at a depth of 1 – 4 m), similar values to

on the surface of the earth is approximately 30 µT at the

those occurring in high voltage overhead lines can

equator, and 60 µT at the poles. While the earth’s magnet-

be seen. Typical magnetic field levels of overhead

ic field is static, the magnetic field emitted by overhead

lines and cables in various environments are

lines varies according to line frequency, and is thus

shown in the table below.

16 ⅔ Hz, 50 or 60 Hz.

Typical magnetic fields for low and intermediate frequency (similar to 50/60 Hz) for different situations

e.g. by means of a faraday cage, it requires special

Description Natural magnetic field [Wikipedia 10-2] Typical house values due to domestic electric equipment and power supply) [TU-Graz 10] High voltage overhead line with a current flow of 1000 A [TUGraz 10] - directly under the line - with 50 m distance of the line At certain working space areas (close to transformer stations, switching stations or inductive heating ovens) [TU-Graz 10]

Magnetic field

While electric fields are relatively easy to shield, engineering techniques or designs to reduce their magnetic fields. The values for limits of the magnetic field differ be-

about 50 µT*

tween certain countries or regions. A selection of limits for alternating magnetic fields of low to in-

0.05 – 0.1 µT

termediate frequency - such as those caused by electric power supply equipment is given in the table below.

8 – 16 µT 1 – 3 µT

Selected limits for magnetic alternating fields at 50 Hz Source

up to few 1000 µT

Medium value in a German city (due to electric equipment in the city) [BfS 10]

0.06 µT

Medium value in a German family home (due to electric home equipment) [BfS 10]

0.06 µT

Medium value in a German highrise building (due to electric equipment in the building) [BfS 10]

0.076 µT

Medium value in a German office (due to electric office equipment) [BfS 10]

0.05 µT

Hairdryer at a distance of 30 cm [BfS 10]

0.01 – 7 µT

Limits of the magnetic field

26th BImSchV (German regulation for electromagnetic pollution) [BImSchV 97]

100 µT

International Commission on Non-Ionizing Radiation Protection (ICNIRP) World Health Organisation (WHO) [ICNIRP 10]

100 µT

DIN/VDE 0848 (German Electrotechnical Standard) exposition area 2, areas, such as living areas, sport or leisure areas [DIN/VDE 0848]

424 µT

Depending on the country or region as well as on certain areas where people can be exposed to magnetic radiation, the limits may vary significantly.

1. Fundamentals

21

The values given in the tables above are for 50 Hz. However, they can also be assumed for 60 Hz.

1.5 Terms and definitions 1.5.1 Definition of voltage values for cable systems

The values for 16 ⅔ Hz, typically occurring in railway overhead lines, may slightly differ. In the 26th

To understand and participate in discussions on

BImSchV (German regulation for electromagnetic

technical issues, certain knowledge of the terms

pollution), the limit for a 16 ⅔ Hz electric field is

used in the particular field is necessary. The follow-

300 µT instead of 100 µT for 50 Hz fields [BIm-

ing section provides definitions of the main terms,

SchV 97].

sion, state-of-the-art and standard terms are used

A comment on the effects of EMF on health and the environment The effects of EMF on the environment, particularly in terms of human health, have been the topic of a large number of studies. The studies can be classified as laboratory studies, such as studies on cells and

epidemiological

which occur in this book. To facilitate comprehen-

investigations.

Laboratory

as much as possible.

The voltages for cable systems are given in different values. The most common are U0, U, Um and Up. According to [IEC 60183], these values are defined as follows: U0

studies focus on changes at the molecular or cellu-

sheath for which cables and accessories

lar level after exposure of material to different val-

are designed

ues of EMF. Epidemiological investigations focus on the occurrence and distribution of diseases,

U

The rated r.m.s. power-frequency voltage between any two conductors for which ca-

such as cancer in human populations.

bles and accessories are designed

In general, the results of the studies and investigations showed that the effect of EMF on human

The rated r.m.s. power-frequency voltage between each conductor and screen or

Um

The

maximum

r.m.s.

power-frequency

health depends on the frequency of the EMF, on

voltage between any two conductors for

the length of exposure to the EMF and on the

which cables and accessories are designed. It is the highest voltage that can be

strength of the electric and magnetic fields.

sustained under normal operating condi-

Further results of these studies show considerable

tions at any time and at any point in a sys-

variation and are too complex to be taken up in the

tem. It excludes temporary voltage varia-

framework of this book.

tions due to fault conditions and the sud-

The World Health Organization (WHO), as well as an independent scientific organisation, The International Commission on Non-Ionizing Radiation Protection (ICNIRP), have both published guidelines for limiting exposure to EMF up to 300 GHz [ICNIRP 10].

den disconnection of large loads Up

The peak value of the lightning impulse withstand voltage between each conductor and screen or sheath for which cables and accessories are designed

U0

U0 U

U0 U

Definition of voltage values in a cable system according to [IEC 60183]

22 Relation between U0, U and Um according to [IEC 60183] Rated voltage of cables and accessories

1.5.2 Definition of terms for terminations and cable systems

Nominal system voltage

Highest voltage for the equipment

U / kV

Um / kV

20

24

Cable-termination Equipment fitted to the end of a cable to ensure

U0 / kV 12 18

30

33

36

26

45

47

52

36

60

64

66

The main terms used for terminations in cable systems are defined in [IEC 62271].

electric connection with other parts of the system and to maintain the insulation up to the point of

69

72.5

110

115

123

76

132

138

145

87

150

161

170

127

220

230

245

Cable termination, comprised of a separating insulating barrier between the cable insulation and the

160

275

287

300

190

330

345

362

220

380

400

420

290

500

525

550

430

700

750

765

connection. Two types are described in this standard [IEC 62271].

Fluid-filled cable-termination

gas insulation of switchgear. The cable-termination includes an insulating fluid as part of the cable connection assembly [IEC 62271].

Dry-type cable-termination Cable termination, comprised of an elastomeric

According to [IEC 62271] another type of voltage is

electric stress control component in intimate con-

defined as:

tact with a separating insulating barrier (insulator)

Ur

The rated voltage for the equipment of the

between the cable insulation and the gas insulation

cable connection is equal to the lowest of

of the switchgear. The cable-termination does not

the values for the cable and the gas-

require any insulating fluid [IEC 62271].

insulated metal-enclosed switchgear Fluid and insulating fluid The rated voltage Ur shall be selected from the fol-

The term “fluid” means a liquid or a gas for insula-

lowing standard values:

tion purposes [IEC 62271].

72.5 kV – 100 kV – 123 kV – 145 kV – 170 kV – 245 kV – 300 kV – 362 kV – 420 kV – 550 kV

Cable system

In the case of cables, the rated voltage Ur corre-

accessories”

sponds to the highest voltage for equipment Um

[IEC 62067].

A cable system is defined as a “cable with installed

[IEC 62271]. According to [IEC 62271], “the equipment” is the gas-insulated metal-enclosed switchgear. However, when a cable is connected to other power equipment, the terms related to the cable can considered to be similar.

[IEC

62271],

[IEC

60840]

and

Ageing and Life Expectancy

Chapter 2

Ageing and Life Expectancy

23

24

2. Ageing and Life Expectancy

25

2.1 Ageing in polymers 2.1.1 Theory of electric lifetime law

quire polyethylene materials of maximum purity and an extremely clean manufacturing process. Electric field ageing, ageing without the presence of partial discharges, is described by the lifetime

Basics When operational loads are applied, the electric

law as follows:

lifetime of polymers, such as cross-linked polyethylene (XLPE) or silicone rubber (SiR), is determined by internal and external influences. With applied voltage, two electric ageing processes tend to occur: partial discharge (PD) ageing and field ageing [Weissenberg 86],

[Peschke 99],

[Olshausen 01],

[Weissenberg 04-1], [Weissenberg 09].

PD ageing in polymers PD ageing due to discharge processes in cavities of the polymeric material or at any interface of the insulation system, such as the interface from the cable to the silicone slip-on element, leads to a rapid breakdown of the insulation. It is therefore vital that

𝐸 ! ∙ 𝑡 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡

(Eq. 2-1)

In which: E:

Electric field

n:

Lifetime exponent

t:

Time

The lifetime law provides a mathematical relation for the physical fact that material properties, the electric field and the time to breakdown are related. The lifetime law means that the lower the electrical field, the longer the time to breakdown of a material and vice versa.

both the cable and accessories of high voltage cacharges. Because of this, PD measurements are a standard in routine testing as a post-production quality control for cables and silicone parts of accessories [Weissenberg 04-1], [Weissenberg 09], [IEC 60840], [IEC 62067]. In addition, PD tests have become increasingly popular for additional measurements during after

Log. el. field strength E

ble systems are free of any internal partial disEbd1

n>40 (Silicone) n=20 (EPR)

Ebd2 Emax tbd1

tbd2

tbdmax

Log. time to breakdown tbd

Basic relation of lifetime law for polymeric materials

installation tests (for more information see Chapter 8).

In principle, the electric lifetime law is valid for a constant temperature level only. The materials usually used for high voltage cable insulation –

Electric field ageing for polymers Even in the absence of PD, polymeric insulation is subjected to ageing once an electric field is present. This process is called electric field ageing. Microscopic spurs and occlusions in the polymeric material cause the electric field ageing. At these areas, the electric field is elevated and the electric ageing process takes place more intensively. These spurs and occlusions are normal for polymeric materials and do not indicate a poor quality. However, the ageing process at these occlusions means that high and extra high voltage cable insulations re-

XLPE or ethylene propylene rubber (EPR) for cables and SiR or EPR for slip-on bodies of accessories – are very stable in the temperature range of their application. The influence of temperature on the electric field ageing is therefore negligible. Pure thermal ageing is, however, quite a different issue. This process is described in Section 2.1.3. The lifetime law is used to determine the expected lifetime of the insulation. For this purpose, breakdown tests with different levels of the electric field are carried out. The advantage of that is that the lifetime law is well-known and the estimated values are reliable. The disadvantage is that breakdown

26

2.1.2 Practical experiences with

values for real arrangements, such as polymeric cables, require a considerable effort to be deter-

electric lifetime law and electric

mined. If reliable values of the lifetime law are to

ageing

be obtained, breakdown values with a low electric field must be taken. This may take a long time and

In detailed investigations on breakdown of XLPE

is very expensive to carry out.

cables, it was found that XLPE has a lifetime expo-

Log. el. field strength E

nent of around 12. These investigations were made Short-term tests Ebd1 Ebd2

on cable samples, representing the real arrangement of a high voltage cable [Weissenberg 86],

Long-term tests

[Peschke 99], [Olshausen 01], [Weissenberg 04-1], [Weissenberg 09].

Designed lifetime

Ebd3 Ebd4 Emax

Max. electric field in insulation

Minutes

Months Hours Log. time to breakdown

Breakdown

50 years

b = 21 - 49 mm d = 12 - 31 mm

Different types of tests to determine expected lifetime of polymeric insulations

b

The design of the insulation is constructed in such

d

a way that the maximum electric field in the obCable dimensions used for ageing investigations

jects, such as cables or accessories, is lower than the residual dielectric strength after a period of about 50 years.

In detailed investigations on breakdown of SiR, it

Although not covered in this book, it should be

was found that SiR has a lifetime exponent above

mentioned that the ageing of oil-filled cables (and

that of 40. For SiR, investigations were made on

their accessories) can not be described with this

material samples as well as on real arrangements

lifetime law.

[Oesterheld 96].

100

n

Elektrical field strenght [kV/mm] Electric field strength [kV/mm]

E •t = constant n ≥ 12...17

10

Withstood values Breakdown values

Routine tests

Type test

Successfully in service

PQ test

1 0.1

1

10

100

1000

10000

100000

Time [h]

Breakdown values and electric ageing for XLPE cables according to [Weissenberg 86], [Peschke 99], [Olshausen 01], [Weissenberg 04-1] and values of practical experiences of Brugg Cables

2. Ageing and Life Expectancy

27

Test values and values of practical experiences of silicone elastomer based cable accessories of Brugg Cables

2.1.3 Thermal ageing

2.2 Volume effect of real polymeric arrangements

Chemical reactions, such as oxidation or creation of radicals, are influenced by temperature. They occur faster at higher temperatures. In the case of insulat-

The dielectric strength of a material also depends

ing materials, these reactions can lead to a change

on the volume at which the electric field is applied.

in the dielectric properties of the materials and can

This relation is called volume effect. The reason for

start after a relatively short time if the temperature

the volume effect is that the breakdown is initiated

continues to increase. An example of this is the fact

and propagates along small irregularities and weak

that the loss factor of an insulating material will in-

points of the material. With increasing volume, the

crease with the creation of polar molecules.

probability of having more weak points in the ma-

To reduce these kinds of reactions in polymers, small amounts of stabilisers and anti-oxidants are

terial also increases. This is a simple statistical effect.

added. However, even with a package of different

If the dielectric strength at a given volume VA is

stabilisers, it is not possible to stop these reactions

measured and the corresponding Weibull distribu-

entirely. Consequently, it is essential to determine

tion with the slope parameter b is known, the die-

the electric lifetime curve of the insulating materi-

lectric strength of a sample with volume VB can be

als as used in high voltage cables and accessories.

determined according to the following equation

Thermal ageing of materials is also used to deter-

[Küchler 96]:

mine the long-term performance of polymer properties. Polymer samples are stored in ovens at a higher temperature than that of the actual application temperature and remain there for several

𝑉! 𝐸#%"$! 𝑉! = 𝐸#%"$! 𝑉! 𝑉!

! !

(Eq. 2-2)

weeks or months. Material properties are then measured on samples that were stored at different

In which:

temperatures. From the results, the ultimate elon-

Ebreak:

electric field at breakdown

gation after several years at application tempera-

V:

Volume

b:

slope parameter in Weibull distribution

ture can be extrapolated.

The relation of the volume effect is of considerable practical relevance for cable systems. It is observed that companies with new products in the market tend to increase the thickness of the insulation in order to be on “the safe side”. But although in-

Failure probability

28

Failures due to ageing

Short-term failures

creased insulation thickness for a given voltage

Occasional failures

level brings a lower electric field, this is only half of the issue. The volume effect causes a higher number of potential weak points in the material, causing in turn a higher probability of breakdown processes and consequently a shorter lifetime.

Time

Bathtub curve representing failure occurrence in technical systems (such as cables and accessories)

Depending on the quality of the material, the advantage of a lower electric field with increased insulation thickness may almost be compensated by the volume effect in the material.

2.3.2 Short-term failures Short-term failures are often listed under a group of failures called “teething problems”. In general, they result from: - Improper design

2.3 Life expectancy 2.3.1 Basic failure behaviour

- Improper materials - Production failures - Installation failures

Life expectancy of the cable system is difficult to estimate. Electric and thermal ageing processes are known and lifetime expectancy can be estimated to a certain extent.

These types of failures usually occur during the first few weeks of the application. To avoid shortterm failures, a great variety of tests are done be-

However, knowledge of electric and thermal ageing

fore the product is delivered to the customer. A

is only one aspect in the process of making lifetime

number of different development tests as well as

predictions of high voltage power cables and their

type and prequalification (PQ) tests should prevent

accessories. In practice many other factors also

(or at least limit) the occurrence of short-term fail-

need to be considered. The sum of the factors in-

ures caused by improper design or improper mate-

fluencing the lifetime of the cable system lead to a

rials. Because of this, type and PQ tests have to be

failure curve that is high at the beginning, low in

repeated once a design or materials have changed

the middle and increases at the end of the lifetime.

significantly.

This behaviour is not only valid for cable systems, it is also typical for most technical systems (including cars, electronic items etc.). Due to its shape, the curve is referred to as “bathtub curve”.

The occurrence of production failures should be prevented by routine tests. To exclude production failures of accessories, pre-tested slip-on bodies are recommended. Pre-testing gives the customer

Factors influencing the lifetime of technical sys-

the proof that the equipment on-site has not suf-

tems, such as a cable system, can be distinguished

fered from production failures.

in: - Short-term failures - Occasional failures and - Failures due to long time ageing

The occurrence of installation failures should be limited by after installation tests. For additional security, additional partial discharge (PD) measurements may be used during after installation tests.

2. Ageing and Life Expectancy More information on tests for cable systems is given in Chapter 3. More information on PD tests for cable systems is given in Chapter 8.

Routine test of a cable drum after production

2.3.3 Occasional failures

29 Possible occasional failures in a cable systems and suggested measures of prevention Type of occasional failure

Suggested measure of prevention

Digging into the cable

Laying the cable in a tunnel or concrete duct or covering the cable system with concrete elements

Terrorist attacks or other human induced violence

In general very difficult; “hiding” the cable in underground concrete ducts, inserting terminations in strong housings

Shooting of insulators

Using grey insulators that are less visible than brown insulators, inserting terminations in strong housings

Bird picking of composite insulators

Using porcelain insulators in areas with a large number of birds that are potentially known for birds picking; using special composites

Earthquakes

Strengthening the fundament of the terminations, appropriate fixing of the cable and accessories to the fundament

Lightning strokes

Insulation coordination with surge voltage limiters for the systems and use of deflectors of solid materials

Once the cable system does not suffer from shortterm failures, the basis for a long lifetime is laid. From then on, only occasional failures can impair the performance of the cable system. Occasional failures mainly result from external influences. They may be: - Induced damage of the cable system by humans, such as digging into a cable - Human induced violence to the system, such as terrorist attacks or shooting on insulators - Systematic naturally induced destruction of the system, such as bird picking of composite insulators - Occasional types of naturally induced destruction, such as earthquakes or wind storms

In general, it is possible to prevent most occasional failures. However, this requires a certain effort and the owner of the system must consider whether such an effort and expenditure is in their interest or not. Possible measures to prevent or reduce the occurrence of occasional failures are listed in the table below.

Concrete ducts to protect the cable

30 than 40 years can be expected to be achieved, with a potential of an even longer lifetime being possible. To achieve a long lifetime of the cable system, the following occurrences must be particularly avoided or kept to a minimum: - Unnecessary high number of transient overvoltages, such as lightning switching impulses - Very high short circuits - Unnecessary high mechanical stress Protected transformer terminations for the power supply of a football stadium; installed in a building for protection (and other reasons)

- Constant high water pressure, especially in joints and terminations - Rodents and termites in the vicinity

Example 2-1:

Æ Cable system  General rules of operation for organic materials, such as XLPE P An increase of the operating temperature by 8 to 10°C reduces the service life by half. P An increase of the operating voltage by 8 to 10% reduces the service life by half.

Terminations with grey porcelain insulators for less visibility

2.3.4 Failures due to ageing Since high voltage cable systems have been in operation for decades, a wealth of experience already exists on the subject of long-term behaviour and ageing of cable systems. Factors that influence the ageing are: - Electric field - Temperature - Mechanical load - Moisture

Once short-term and occasional failures have been excluded, the above-mentioned factors run within the specified limits and an unnecessary overload of the cable system can be excluded, lifetime of more

Chapter 3: Tests and Standards

Chapter 3

Tests and Standards

31

32

3. Tests and Standards

3.1 Tests

33 In general, all materials and designs must undergo extensive development tests before considered for

3.1.1 Basic idea of testing Testing reveals the technical limits of products and systems. Other reasons for testing include quality

use in commercial products. A typical development test is to investigate the mechanical properties of polymeric materials.

control of the material, checking the design, simulating ageing processes or minimising production and installation failures. As a result, tests for cable systems can be typically categorised into different types as listed in the table below.

Overview of typical tests for cable systems Type of test Development test

Type test

Prequalification (PQ) test

Purpose of application To reveal the technical limits of the material and/or the system To stimulate ageing processes To examine material and design of single elements, such as joints, terminations and cables

Determination of the mechanical strength of a polymeric material in the application laboratory at Brugg Cables

To examine material and design of whole cable systems To stimulate first ageing processes

3.1.3 Type tests

Routine test

To detect production failures

Overview

After installation test

To detect installation failures

After a product has been developed, a type test must be carried out. According to IEC 60840, type tests are “tests made before supplying on a general commercial basis a type of cable system or cable or

3.1.2 Development tests

accessory covered by this standard, in order to demonstrate satisfactory performance characteris-

Development tests are carried out to investigate the

tics to meet the intended application. Once suc-

limits of the applied materials and the chosen de-

cessfully completed, these tests need not be re-

sign variants. Development tests are usually de-

peated, unless changes are made in the cable or

structive tests. This means that the material or the

accessory materials, or design or manufacturing

component, such as high voltage polymer cables or

process which might change the performance

high voltage accessories, are tested until break-

characteristics” [IEC 60840].

down occurs.

Put more simply, the standard says that the type

The results of such tests show the security margin

test determines the right dimensioning of the mate-

of the material and the component. Development

rial and design of the components that were devel-

tests must consider all possible factors that may

oped. A type test must be passed if the product is

influence the cable system throughout its lifetime.

to be sold.

These are electric, mechanic and thermal loads, the

IEC standard type tests for high voltage cables and

influence of moisture and all combinations of the

accessories

above-mentioned factors. The results of develop-

IEC 62067.

ment tests are a matter of the company and are subject to strict secrecy policy.

are

described

in

IEC 60840

and

34 IEC 60840 and IEC 62067 not only use different

equipment being tested but also from laboratory

voltage levels, they also employ different proce-

use and installation. To reduce the costs, as many

dures. IEC 60840 allows a separate testing of acces-

devices as possible should be tested at the same

sories, while IEC 62067 considers the testing of the

time.

whole “cable system”, that is, cables and accesso-

The standard design of a type test for a cable sys-

ries in the same sequence.

tem contains the following elements:

Relevant IEC type test standards for high voltage cables and accessories Standard

Voltage range

IEC 60502

1 kV ≤ Um ≤ 36 kV

IEC 60840

36 kV < Um ≤ 170 kV

IEC 62067

170 kV < Um ≤ 550 kV

- Cable with at least one segment of a minimum length of 10 m between the accessories (other segments must have a minimum length of 5 m between the accessories) - Outdoor termination with porcelain insulator - Outdoor termination with composite insulator - Joint - Back-to-back joint (consisting of an SF6 termination and a transformer termination)

Layout of type test The tested equipment is subject to very high stress, making it unable for commercial use afterwards. In addition, type tests are time intensive and expensive. Various costs accumulate, not only from the

Type tests are conducted in high voltage test laboratories, either at the cable or accessory manufacturer or at an independent test laboratory.

Outdoor terminations (1 composite & 1 porcelain insulator)

(Cross-bonding) Joint Cable (> 5m)

Back-to-back joint (SF6 & transformer termination) Cable (> 5m)

High voltage test transformer Cable (> 10m)

High current transformer (for thermal heating) Typical layout of a type test

3. Tests and Standards

35 d)

PD tests at ambient and high temperature

e)

Switching impulse voltage test (required for cable systems with Um ≥ 300 kV)

f)

Lightning impulse voltage test with 10 positive and 10 negative impulses followed by a power frequency voltage test

g)

PD tests, if not previously carried out in d) above

h)

Tests of outer protection for buried joints, thus containing a water immersion and heat cycling test and a separate different

Type test of a 550 kV cable system at an independent test laboratory

voltage test at the joint i)

Examination of cable system with cable and accessories after completion of the

Test sequence

tests

According to IEC 60840, the type test on accessories for voltages 36 kV < Um ≤ 170 kV shall be subjected to the following sequence: a) b)

Partial discharge (PD) test at ambient tem-

Besides these tests on the cable systems a type test according to the standards IEC 60840 and IEC 62067 includes also tests on the material of the cable.

perature

These material tests include the measurement of

Heating cycle voltage test with 20 cycles of

the resistivity of the conductor and insulation

an 8 h heating period and a 16 h cooling

screen and the mechanical properties of the cable

period at a voltage of 2U0

insulation material.

c)

PD tests at ambient and high temperature

d)

Lightning impulse voltage test with 10 pos-

Electric field in the equipment

itive and 10 negative impulses followed by

In both standards, the type test must be carried out

a power frequency voltage test

on the part of the equipment in which the highest

e)

PD tests, if not previously carried out in c)

electric field occurs. The electric field E(x) in the ca-

f)

Tests of outer protection for buried joints,

ble insulation is given by:

thus containing a water immersion and heat cycling test and a separate different voltage test at the joint g)

Examination of the accessories after completion of the tests above.

According to IEC 62067, the type test on cable systems for voltages 170 kV < Um ≤ 550 kV shall be subjected to the following sequence: a)

𝐸 𝑥 =

Bending test on the cable followed by installation of accessories and a partial dis-

𝑉 𝑥 ∙ 𝑙𝑛

𝑅 𝑟

(Eq. 3-1)

Eq. 3-1 shows four important facts for the electric field distribution in cables and accessories. These are: 1.

The highest field in the cable occurs at the inner diameter of the cable insulation.

2.

The highest field in a slip-on element of accessories occurs at the outer diameter of the

charge test at ambient temperature

cable insulation.

b)

Measurement of tan δ

c)

Heating cycle voltage test with 20 cycles of

highest value of the electric field is higher

an 8 h heating period and a 16 h cooling

than that which occurs at a larger conductor

period at a voltage of 2U0

3.

At a smaller conductor cross-section, the

36

4.

cross-section (assuming that the insulation

that for a cable with a large conductor cross-section

thickness is the same).

the thickness of the insulation is smaller than for a

At the outer semiconducting layer, the high-

cable with a smaller conductor cross-section.

est value of the electric field which occurs at

Consequently, type tests for high voltage accesso-

a larger conductor cross-section is higher

ries and the cables are typically carried out at a

than that which occurs at a smaller conduc-

conductor cross-section of 2500 mm2. Usually with

tor cross-section (assuming that the insula-

such a cable the requirements for the type test of the cable and that for the accessories are fulfilled.

tion thickness is the same).

Lower values of the cable cross-sections are covered by the standard, the electric field in the accessories being lower.

R1

Example 3-1:

r1

E

Æ A type test with the following values:

E(r2) E(r1) x

-

Cable of manufacturer A

-

Accessories of manufacturer A

-

U0 = 76 kV

-

Conductor cross-section = 2500 mm2

-

Diameter over inner conductor = 63 mm

-

Insulation thickness = 12.3 mm

 Is the accessories type test valid for a cable with the following details:

R2 r2

Schematic distribution of electric field in a cable with small (orange) and large (grey) conductor crosssection

-

Cable of manufacturer B

-

Accessories of manufacturer A

-

U0 = 76 kV

-

Conductor cross-section = 630 mm2

-

Diameter over inner conductor = 34 mm

-

Insulation thickness = 17.3 mm

P According to Equation 3-1, the electric field at the type tested cable and accessories is:

Taking the different factors of the electric field dis-

-

at inner cond. of the cable = 7.3 kV/mm

tribution and demands of the standards into ac-

-

at outer cond. of the cable = 5.3 kV/mm

count, one could think that cables and accessories need to undergo two different sorts of type test. One test should be made “for the cable” with the lowest possible conductor cross-section (as the electric field is highest at the inner semiconducting layer). Another test should be made “for the accessories” with the highest possible conductor crosssection (as the electric field is highest at the outer semiconducting layer of the cable, which is the same as the inner part of the joint body). However in reality the assumption to have the same thickness of the insulation independent of the cross section of the conductor is not correct. Cable manufacturer usually design the cable in such a way, that for a certain voltage level, the electrical field strength at the conductor is the same, independent of the conductor cross section. This means

According to Equation 3-1, the electric field at the cable, which shall be applied, and the accessories is: -

at inner cond. of the cable = 6.4 kV/mm

-

at outer cond. of the cable = 3.2 kV/mm

The calculated values show that for the accessories of manufacturer A, which shall be applied with a cable of manufacturer B, the electric field is lower at the outer semiconducting layer than in the type test. According to IEC 60840, the cable and accessories can be from different manufacturers.

9 Considering both facts, it can be concluded that the type test (of the accessories) is valid and the accessories of manufacturer A can be applied with that type of cable of manufacturer B.

3. Tests and Standards

37

Duration of type test The longest part of the test is the heating cycle voltage test with a duration of 20 days. The test itself lasts approx. 30 days including all other electric test sequences and the test of the outer protection for the buried joint. The installation of the equipment together with the time required for preparing the laboratory means that the full duration of a type test typically lasts about two months. To ensure that the test parameters meet the standards, the type tests are either carried out at an independent type test laboratory or are witnessed by a representative of an independent test institute.

Type test certificate Once the type test is passed, a type test certificate is given. This certificate lists all equipment tested, the various test procedures and the result of the test. With a type test certificate, the commercial

Example for cable (above) and accessory (bottom) data in a type test report

sale of the product can be made.

3.1.4 Prequalification tests Layout of tests A type test must be successfully completed before a prequalification (PQ) test is started. While a type test demonstrates satisfactory performance characteristics of the cable or the accessories (joints and terminations), the PQ tests examines the compatibility of the cable with the accessories. PQ tests are only required for cables systems if the calculated nominal electrical stresses at the conductor screen will be higher than 8 kV/mm or at the insulation screen higher than 4 kV/mm. According to IEC 62067, PQ tests are “tests made before supplying on a general commercial basis a type of cable system covered by this standard, in order to demonstrate satisfactory long term performance of the complete cable system. The prequalification test needed only be carried out once unless there is a substantial change in the cable system with respect to material, manufacturing process, design and design levels” [IEC 62067]. PQ tests are time intensive and expensive, being carried out for 365 days. Laboratory costs make the tests

particularly

expensive.

Consequently,

as

38 many devices as possible should be tested at the

a)

same time.

Heating cycle voltage test with a test period of 8760 h (1 year) at a voltage of 1.7U0. Parallel to the voltage load, 180 heating

The layout of a PQ test is similar to that of a type

cycles have to be made. These heating cy-

test, it typically contains:

cles must be at least 8 h, whereby the con-

- Full sized cable with a total length of approxi-

ductor temperature shall be maintained

mately 100 metres

within the stated temperature limits for at

- Outdoor termination with porcelain insulator

least 2 h of each heating point. The heating

- Outdoor termination with composite insulator

cycle shall be followed by a cooling period

- Joint

of at least 16 h*

- Back-to-back joint (consisting of an SF6 termi-

b)

Lightning impulse voltage test with 10 positive and 10 negative impulses

nation and a transformer termination) c)

Examination of the cable system after completion of the above tests

* According to the standard, the minimum time for the heating cycles is (24 h x 180 = 180 days). Once the heating cycles are finished, the heating of the cable can be stopped. However, the voltage test of 1.7U0 for one year must still be finished. The heating and cooling cycles usually occur over a period of one year, e.g. 16 h heating and 32 h cooling.

PQ test certificate Similar to the type test, the PQ test is carried out PQ test of a 245 kV cable system at an independent test institute

Test arrangement According to IEC 62067, “the test arrangement shall be representative of the installation design conditions e.g. rigidly fixed, flexible and transition arrangements, underground and in air.” [IEC 62067]. This means that the 100 m test cable length should be typical of the application, which is usually buried directly, in air or in a (concrete) tunnel.

either at an independent test institute or is witnessed by a representative of an independent test institute. Once passed, a PQ test certificate is given. In general, the commercial sale of the cable system is made with the PQ test certificate. Since the PQ test takes a long time, it is not untypical of installations with cables and accessories to be made in the field while the PQ test is still running. Such procedure has to be agreed between the cable manufacturer and the final customer. The PQ test certificate has to be delivered as soon as the test was passed.

It is further stated in IEC 62067 that “Ambient conditions may vary between installations and during the test and are not considered to have any major

3.1.5 Requalification tests

influence” [IEC 62067]. Taking into consideration

Layout of tests

that the test conditions in Mexico City in summer may be significantly different throughout the whole year than in St. Petersburg in winter, the detailed layout of the test should be discussed between the cable manufacturer and the customer in detail.

“Tests for the extension of the prequalification of a cable system”, often called “Requalification tests” is a new test possibility in the latest version of IEC 62067. This type of test has been introduced into the standard to consider changes in the material or production process without the necessity of

Test sequence

carrying out a complete (extensive and expensive)

According to [IEC 62067], the PQ test on the cable

PQ test. Such changes may be a new production

system shall be subjected to the sequence as:

facility (e.g. extrusion line) on a proven cable de-

3. Tests and Standards sign and cable material. The requalification test re-

39 h)

quires a valid prequalification test. Similar to the PQ test, the requalification test exam-

Lightning impulse voltage test followed by a power frequency voltage test

i)

ines the compatibility of the cable with the accesso-

PD tests, if not previously carried out in f) above

ries.

j)

According to [IEC 62067], the standard design of a

k)

Tests of outer protection for buried joints Examination of the cable system with ca-

requalification test is similar to that of a type or PQ

ble and accessories shall be carried out af-

test. It usually contains:

ter completion of the tests above

- Full sized cable with a total length of approximately 20 metres - Outdoor termination with porcelain insulator - Outdoor termination with composite insulator - Joint - Back-to-back joint (consisting of an SF6 termination and a transformer termination)

Test arrangement The test arrangement for a requalification test is similar to that of a type test with the exception that the minimum total cable length should be 20 m [IEC 62067].

l)

The resistivity of semi-conducting screens shall be measured on a separate sample

* According to the standard, the heating shall be applied for at least 8 hours with 2 hours of stated conductor temperature. The cooling period shall be 16 hours. The cycle of heating and cooling shall be carried out 60 times [IEC 62067]. ** According to the standard, the minimum number of heating cycles with voltage of 2.0 U0 shall be 20. The heating cycles can be interrupted.

Requalification test certificate Similar to the type and PQ test, the requalification test is carried out either at an independent test institute or is witnessed by a representative of an independent test institute.

Test sequence According to [IEC 62067], the requalification test on the cable system shall be subjected to the following sequence: a)

Since failures in the production never can be ex-

Bending test without final partial discharge

cluded, routine tests are carried out to detect pro-

test followed by installation of the accesso-

duction failures before the product (e.g. such as a

ries that are part of the tests for the exten-

cable drum) is delivered to the customer.

sion of the prequalification b)

3.1.6 Routine tests

According to IEC 60840, routine tests are “tests

A partial discharge test is applied after the

made by the manufacturer on each manufactured

bending test to check the quality of the in-

component (length of cable or accessory) to check

stalled accessories

that the component meets the specific require-

c)

Heating cycle test without voltage*

ments” [IEC 60840].

d)

Measurements of tan δ

In a routine test, high voltage tests and partial dis-

e)

Heating cycle voltage test**

f)

PD tests at ambient temperature and high temperature. This test shall be carried out after the final cycle of item e) above or, alternatively, after the lightning impulse voltage test in item h) below

g)

Switching impulse test (required for Um ≥ 300 kV)

charge (PD) measurements are carried out. In order to avoid unnecessary electric ageing effects, the test values for routine tests are less than those for type tests. The outline of the routine test, the requirements and the procedures for cables and accessories are dependent on the voltage level and listed in the relevant standards. After each test, a test certificate is given. It shows the equipment that was tested, the voltage range, the PD level, the date of test and the test person.

40

3.1.7 After installation tests Tests according to IEC After installation tests are made to detect failures that have occurred during the installation process, especially during the installation of accessories. According to IEC 60840, electric tests after installation are “tests made to demonstrate the integrity of the cable system as installed”. They are tests on new installations and “are carried out when the installation of the cable and its accessories has been completed” [IEC 60840].

Routine test of a slip-on element for high voltage joints in the fully screened test laboratory of Brugg Cables

Since cables and accessories are tested during the routine test in the factory, failures in the cable system can only occur during the transport. Damage of accessories, particularly the sealed slip-on ele-

!?.@ 6?+-A> 8A)?A ;-&

):=#TER+#R!!C ' S37H4 9((( S&:

#37?/> 29+?

:7237L ,N46GI26 82I4 =K%* "G5% >;U$ :/J4 P!!#AT#RDA

;-& #.,3? 2?@> 0-3>.=?

DAE@!T@ RR!#RED O9 T0R!!! MMR PTA O9 P! M2L

"?>.73$

'

#-11?/>$

'

".>?%47=/.>