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„

T&T Subsection Local IAS Chapter GROUNDING-DESIGN AND CONSTRUCTION Saturday 12th November 2005

„

Presented by

„

Eng. Sahadeo Latchmepersad, MIEEE, MAPETT, R. Eng.

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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ACKNOWLEDGEMENT „The

Trinidad and Tobago Electricity Commission Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Range of Large Industrial Customers „

Rate D3 -

4-25 MVA at 33, 66 or 132 kV

„

Rate E >

25 MVA at 66 or 132kV Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Transmission „

B.C/Union Estate Schematic

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Objectives of Grounding „

To provide means to carry electric currents to earth under normal and fault conductors without exceeding any operating and equipment limits or adversely affecting continuity of service.

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Objectives of Grounding„

To ensure that a person in the vicinity of grounded facilities is not exposed to the danger of critical electric shock.

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Basic Shock Situations

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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BRIGHTON 66kV SUBSTATION

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Ventricular Fibrillation „

The most dangerous consequence of electric current passing through the vital parts of a human body is a heart condition – ventricular fibrillation – immediate arrest of blood circulation.

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Design Algorithm

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Measuring Soil Resistivity „

Procedure depends on the instrument

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Soil Resistivity Calculation

„ρ

= 2 x Pi x a x R Where R is Maximum measured Value Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Tolerable Step Voltage Step Voltage Limit 50 kg E Step 50 = (1000 + 6 Cs ρs) 0.116 „

√ts

E

Step 70

= (1000 + 6 Cs ρs) 0.157 √ts

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Tolerable Touch Voltage Limit „

E

touch50

„

E

touch70

= (1000 + 1.5 Cs ρs) 0.116 √ts = (1000 + 1.5 Cs ρs) 0.157 √ts

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Constants „

„ „ „

Cs = 1 – [0.09 (1 - ρ/ ρ s)] [2 hs + 0.09] ρ = soil resitivity ρ s = surface layer resistivity Ts = duration of shock current in seconds including clearance time of circuit breaker plus relay Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Conductor Sizing „

I = Amm2 [TCAP.10-4]

Ln Ko + Tm

[tc αr ρr]

Substation Grounding- Sahadeo Latchmepersad, T&TEC

Ko + Ta

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Grid Resistance 1 1 1   Rg = ρ  + 1 +  20 A  1 + h 20 / A   LT Whereρ is the soil resistivity LT is the total length of conductor h is the depth of burial Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Calculated Step Voltage „

Es = ρKs Ki IG Ls

„

Ls = 0.75Lc + 0.85 LR

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Calculated Mesh Voltage Em = ρ Km Ki IG Where LM „ Lm = Lc + 1.55 +1.22 „

Lr

LR

LX2 + Ly2 LR = total length of ground rods „ Lc = length of conductor „

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Earthing Designs „

Nitrogen 2000 66kV Substation

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Output file „

Nitrogen 2000 substation

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Exothermic Joints „

Step 1 Position clean, dry cables in mold and close clamp.

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Insert disk, insert pour weld metal and starter material into crucible, and ignite to form the weld.

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Remove finished connection. Clean mold

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FENCE EARTHING 1. Inclusion of the fence within the ground grid area. Under this philosophy, the perimeter conductor of the grid will parallel the fence at a short distance of 1.0m outside. The crushed rock covering will also be required outside the fence. Substation Grounding- Sahadeo Latchmepersad, T&TEC

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FENCE EARTHING 2. Placement of the fence outside the ground grid area with close electric coupling between fence and adjacent earth along its length, but with no electrical tie between the fence and the main station grid

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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ANSI STANDARD 80-2000

Date: Date:

November 11, 2005

T&TEC Grounding Program INPUT DATA Line-to-line Voltage V Maximum fault curent HV side Maximum Fault Current LV side Soil resistivity ρ Substation Length L Substation Width W Spacing D Number of ground rods Length of ground rod Depth of grid burial h Fault Duration t f

66000 Volts 20000 Amps 20000 Amps 15 34 30 4 28 2.44 0.5 0.1

Current Division Factor Sf X/R Ratio

1 7.20

Crushed Rock Resistivity ρs

2500

Ωm metre metre metre Ea metres metres s

Thickness of crushed rock hs Grid shape (Square=S, Rect.=R, L- shaped=L) Substation L-shape Length l Substation L-shape width w

1792 Volts 5678 Volts

Touch voltage limit E touch 50 Step voltage limit E step 50

1324 Volts 4195 Volts

GPR = Em= Es=

4944 Volts 606 Volts 619 Volts

Length of 4/0 Copper Required

574 metres

Size of Copper d = No. of Copper Rods

0.01 metres 28 Ea

Grid Resistance

0.23

Ωm

o 40 C

Ambient temp Ta

RESULTS Touch voltage limit E touch 70 Step voltage limit E step 70

Ω

0.102 metres r 0 metre 0 metre

50kg Design O.K 70kg Design O.K Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Development Program

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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AutoGrid Pro

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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Connection to Customer’s Substation Fence eathing „ Number of Tails „ Increased area-lower Rg „ Increased amount of copper „

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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The End „

Questions?

„

Thank You

Substation Grounding- Sahadeo Latchmepersad, T&TEC

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