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IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

IEEE Power and Energy Society

Sponsored by the Transformers Committee

IEEE 3 Park Avenue New York, NY 10016-5997 USA

IEEE Std C57.147™-2018

(Revision of IEEE Std C57.147-2008)

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IEEE Std C57.147™-2018

(Revision of IEEE Std C57.147-2008)

IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers Sponsor

Transformers Committee

of the

IEEE Power and Energy Society Approved 15 February 2018

IEEE-SA Standards Board

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Abstract: Assistance to equipment manufacturers and service companies to evaluate the suitability of unused natural ester insulating liquids being received from suppliers is provided in this guide. Information for transformer operators in evaluating and maintaining natural ester insulating liquids in serviceable condition is also provided. Keywords: dielectric coolant, high fire point liquid, IEEE C57.147™, insulating liquid, lessflammable liquid, natural ester liquid, transformer, vegetable oil

The Institute of Electrical and Electronics Engineers, Inc. 3 Park Avenue, New York, NY 10016-5997, USA Copyright © 2018 by The Institute of Electrical and Electronics Engineers, Inc. All rights reserved. Published 19 July 2018. Printed in the United States of America. IEEE is a registered trademark in the U.S. Patent & Trademark Office, owned by The Institute of Electrical and Electronics Engineers, Incorporated. PDF: Print:

ISBN 978-1-5044-4919-9 ISBN 978-1-5044-4920-5

STD23135 STDPD23135

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Participants At the time this IEEE guide was completed, the Natural Ester Fluids Working Group had the following membership: C. Patrick McShane, Chair C. Clair Claiborne, Vice Chair James Graham, Secretary Roberto Asano Derek Baranowski Claude Beauchemin Julio Caldeira Juan Castellanos Luis Cheim Donald Cherry Jermaine Clonts Valery Davydov Stephanie Denzer

George Frimpong Rainer Frotscher Eduardo Garcia James Gardner David Hanson Jesse Inkpen Gael Kennedy Robert Kinner Libin Mao Susan McNelly

Vinay Mehrota Nicholas Perjanik Jimmy Rasco Scott Reed Gregory Stem Craig Stiegemeier Roger Wicks Deanna Woods

The following members of the individual balloting committee voted on this guide. Balloters may have voted for approval, disapproval, or abstention. Donald Ayers Peter Balma Thomas Barnes Barry Beaster Enrique Betancourt Wallace Binder Thomas Bishop Thomas Blackburn William Bloethe W. Boettger Paul Boman Stephan Brauer Paul Cardinal Juan Castellanos Donald Cherry C. Clair Claiborne John Crouse Willaim Darovny Dieter Dohnal Gary Donner Don Duckett Donald Dunn Jorge Fernandez Daher Joseph Foldi Bruce Forsyth Michael Franchek Fredric Friend George Frimpong Ramsis Girgis James Graham William Griesacker Randall Groves Ajit Gwal Attila Gyore

John Harley Roger Hayes Werner Hoelzl Gary Hoffman Jill Holmes Hali Jackson Richard Jackson John John Laszlo Kadar Gael Kennedy Sheldon Kennedy Gary King James Kinney Zan Kiparizoski Axel Kraemer Krzysztof Kulasek Jim Kulchisky John Lackey Benjamin Lanz Thomas La Rose Aleksandr Levin Thomas Lundquist Richard Marek J. Dennis Marlow Omar Mazzoni William McDermid Mark McNally Susan McNelly C. Patrick McShane C. Michael Miller Daleep Mohla Charles Morgan Daniel Mulkey Jerry Murphy

Ryan Musgrove Ali Naderian Jahromi K. R. M. Nair Michael Newman Joe Nims Lorraine Padden Dwight Parkinson Luke Parthemore Bansi Patel George Payerle Brian Penny Howard Penrose Branimir Petosic Christopher Petrola Alvaro Portillo Kevin Rapp Jimmy Rasco Robert Rasor Jean-Christophe Riboud John Roach Oleg Roizman Zoltan Roman Thomas Rozek Daniel Sauer Bartien Sayogo Stephen Shull Hyeong Sim Richard Simonelli Jeremy Smith David Stankes Juan Thierry James Thompson Roger Verdolin John Vergis

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Jane Verner Sukhdev Walia David Wallach

Joe Watson Lee Welch Kenneth White

Roger Wicks Deanna Woods Jennifer Yu

When the IEEE-SA Standards Board approved this guide on 15 February 2018, it had the following membership: Jean-Philippe Faure, Chair Vacant Position, Vice Chair John D. Kulick, Past Chair Konstantinos Karachalios, Secretary Ted Burse Guido R. Hiertz Christel Hunter Joseph L. Koepfinger* Thomas Koshy Hung Ling Dong Liu

Xiaohui Liu Kevin Lu Daleep Mohla Andrew Myles Paul Nikolich Ronald C. Petersen Annette D. Reilly

Robby Robson Dorothy Stanley Mehmet Ulema Phil Wennblom Philip Winston Howard Wolfman Jingyi Zhou

*Member Emeritus

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Introduction This introduction is not part of IEEE Std C57.147-2018, IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers.

This guide was prepared by the Insulating Fluids Subcommittee of the Transformers Committee of the IEEE Power and Energy Society. The purpose of this guide is to identify standards for acceptance and maintenance of natural ester insulating liquid in transformers. This guide is the first revision of the initial guide published in 2008. Overall, the guide was substantially updated to include the current state of the art for natural ester insulating liquids. This revision includes updates to follow the current IEEE SA policies, style, and terms appropriate for a Guide. In this revision, where appropriate, the term “fluid” has been replaced with “liquid” to be more descriptive of the application and for consistency of terminology within C57 transformer standards. The normative references have been updated and expanded. Based on available data from testing samples from operating transformers, the normative and informative information on limits for continued service have been expanded. The bibliography has been expanded to provide additional background for the user. Two additional informative annexes were added: Annex C provides discussion on fire safety, environmental and sustainability factors of natural ester insulating liquids; Annex D discusses considerations when applying natural esters to load tap changers (LTCs).

Acknowledgments Table B.1 and Figure B.1 were modified with permission from Doble Engineering Company, Lewand, L. R., “Laboratory evaluation of several synthetic and agricultural-based dielectric liquids,” Proceedings of the 86th Annual International Conference of Doble Clients, Doble Engineering Company, Watertown, MA, USA, 2001 [B13]. Figure B.1 was reprinted from IEEE Std C57.106™-2002 [B26]. Figure B.2, Figure B.3, and Figure B.4 were reprinted from McShane, C. P., J. Luksich, and K. J. Rapp, “Retrofilling aging transformer with natural ester based dielectric coolant for safety and life extension,” Proceedings of the IEEE IAS/PCA Cement Industry Technical Conference, Dallas, TX, USA, May 2003 [B35]. Table C.1 was reprinted with permission from CIGRE Technical Brochure 436, Experiences in service with new insulating liquids, © 2010 [B9].

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Contents 1. Overview��������������������������������������������������������������������������������������������������������������������������������������������������� 11 1.1 Scope�������������������������������������������������������������������������������������������������������������������������������������������������� 11 1.2 Purpose����������������������������������������������������������������������������������������������������������������������������������������������� 11 1.3  System design������������������������������������������������������������������������������������������������������������������������������������� 11 1.4  Background information on mixtures of natural ester liquids with other dielectric liquids���������������� 12 2. Normative references�������������������������������������������������������������������������������������������������������������������������������� 12 3. Acronyms and abbreviations��������������������������������������������������������������������������������������������������������������������� 14 4. Liquid tests and the significance of each test��������������������������������������������������������������������������������������������� 15 4.1 General����������������������������������������������������������������������������������������������������������������������������������������������� 15 4.2  Practices for sampling (ASTM D923)������������������������������������������������������������������������������������������������ 16 4.3  Acid number (ASTM D664 and ASTM D974)����������������������������������������������������������������������������������� 16 4.4  Dielectric breakdown voltage (ASTM D1816)����������������������������������������������������������������������������������� 17 4.5  Dielectric breakdown voltage—impulse conditions (ASTM D3300)������������������������������������������������ 17 4.6  AC loss characteristics—dissipation factor and relative permittivity (ASTM D924)������������������������� 18 4.7  Interfacial tension (ASTM D971)������������������������������������������������������������������������������������������������������� 18 4.8  Color (ASTM D1500)������������������������������������������������������������������������������������������������������������������������ 18 4.9  Kinematic viscosity (ASTM D445)���������������������������������������������������������������������������������������������������� 19 4.10  Flash point and fire point—Cleveland Open Cup Method (ASTM D92)������������������������������������������ 19 4.11  Relative density (ASTM D1298)������������������������������������������������������������������������������������������������������ 19 4.12  Pour point (ASTM D97, ASTM D5949 [B5], and ASTM D5950 [B6])������������������������������������������� 19 4.13  Volume resistivity (ASTM D1169)��������������������������������������������������������������������������������������������������� 20 4.14  Gas analysis (ASTM D3284, D3612)����������������������������������������������������������������������������������������������� 20 4.15  Oxidation stability���������������������������������������������������������������������������������������������������������������������������� 21 4.16  Water content—Karl Fischer Method (ASTM D1533)��������������������������������������������������������������������� 22 4.17  Visual examination of used liquids (ASTM D1524)������������������������������������������������������������������������� 22 4.18  Gassing of insulating liquids under electrical stress and ionization (ASTM D2300)������������������������ 22 4.19  Corrosive sulfur test (ASTM D1275)����������������������������������������������������������������������������������������������� 23 4.20  Polychlorinated biphenyls (PCBs) (ASTM D4059)������������������������������������������������������������������������� 23 4.21  Furanic compounds (ASTM D5837)������������������������������������������������������������������������������������������������ 23 5. Liquid compatibility with transformer materials��������������������������������������������������������������������������������������� 23 6. Handling and evaluation of natural ester liquids for use in filling transformers at the installation site������ 23 6.1 General����������������������������������������������������������������������������������������������������������������������������������������������� 23 6.2  Shipping containers���������������������������������������������������������������������������������������������������������������������������� 23 6.3  Check tests on receipt������������������������������������������������������������������������������������������������������������������������� 24 6.4  Handling of the liquid by the user and placing the liquid in storage���������������������������������������������������� 24 6.5  Handling and testing of natural ester liquids for installation into apparatus���������������������������������������� 26 7. Evaluation of natural ester liquids received in new equipment and after filling apparatus on-site������������� 27 8. Maintenance of natural ester liquids���������������������������������������������������������������������������������������������������������� 28 8.1  Field screening����������������������������������������������������������������������������������������������������������������������������������� 28 8.2  Laboratory screening�������������������������������������������������������������������������������������������������������������������������� 29 8.3  Test limits for in-service natural ester liquids������������������������������������������������������������������������������������� 29 8.4 Reconditioning����������������������������������������������������������������������������������������������������������������������������������� 30 8.5 Reclaiming����������������������������������������������������������������������������������������������������������������������������������������� 31 8.6  Mixtures of different types of insulating liquids��������������������������������������������������������������������������������� 32 9. General environmental procedures������������������������������������������������������������������������������������������������������������ 32 9.1 General����������������������������������������������������������������������������������������������������������������������������������������������� 32

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9.2  Leaks�������������������������������������������������������������������������������������������������������������������������������������������������� 32 9.3  Minor spills���������������������������������������������������������������������������������������������������������������������������������������� 32 9.4  Spills on soil��������������������������������������������������������������������������������������������������������������������������������������� 33 9.5  Spills on water������������������������������������������������������������������������������������������������������������������������������������ 33 Annex A (informative) Bibliography�������������������������������������������������������������������������������������������������������������� 34 Annex B (informative) Additional technical information������������������������������������������������������������������������������� 38 Annex C (informative) Additional information on fire, safety, environmental, and sustainable properties of natural esters�������������������������������������������������������������������������������������������������������������������������������������������������� 42 Annex D (informative) Discussion regarding natural ester immersed load tap changers (LTCs)������������������� 44

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IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers 1. Overview 1.1 Scope This guide recommends tests and evaluation procedures, as well as criteria and methods of maintenance, for natural ester-based (e.g., vegetable oil) insulating liquids. Methods of reconditioning, field applications, and diagnostics of natural ester-based insulating liquids are also described.

1.2 Purpose This guide recommends standard tests and evaluation procedures of natural ester insulating liquids for application in distribution and power transformers and other liquid-filled electrical equipment. Natural ester insulating liquids are also being applied in retrofilling existing liquid-filled equipment. This guide provides recommendations for new and retrofill field applications including: field testing of equipment filled with natural ester insulating liquids, methods of reconditioning and reclaiming natural ester insulating liquids, and the analysis results at which reprocessing or replacement of the insulating liquid is necessary.

1.3  System design The reliable performance of natural ester liquids in an insulation system depends upon certain basic liquid characteristics that can affect overall apparatus characteristics. Such liquid characteristics are integral to equipment design for which the manufacturer should have final responsibility. The reliable operation of the equipment in service, for which the transformer operator should have final responsibility, also depends on maintaining certain basic liquid characteristics. Adherence to the recommended liquid characteristics can assist in obtaining the desired equipment characteristics. Other tests or verification of the integrity of the insulation system may be necessary. The essential properties of insulating liquids used in transformers should be maintained if the liquid is to perform its multiple roles as electrical insulation and heat transfer agent. It should have adequate dielectric strength to withstand the normal range of electric stresses imposed in service. It should have a certain combination of thermal conductivity, specific heat, and viscosity so that its ability to transfer heat is sufficient for the particular equipment. It should have a sufficiently high flash point and fire point to meet safety requirements. The natural ester liquid should not be allowed to become so deteriorated or contaminated that it adversely affects other materials in the apparatus.

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IEEE Std C57.147-2018 IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

1.4 Background information on mixtures of natural ester liquids with other dielectric liquids Mineral Oil Insulating Liquids: Natural ester insulating liquids are typically miscible and compatible with mineral oil insulating liquids, as well as with halogenated hydrocarbon insulating liquids. Mixing unused insulating natural esters with mineral oil may or may not significantly impact their typical property values or impact performance. For the property values that do change, the change may or may not be proportional to the ratio of the content of the liquids. See B.3 for additional details. NOTE—If the purpose of using a natural ester liquid is to comply with Article 450.23 of the National Electrical Code® (NEC®) (NFPA 70®, 2017 Edition), (for example, to use a transformer indoors without a vault), that article requires that less-flammable transformer liquids have an ASTM D92 fire point of not less than 300 °C and that the installation complies with all restrictions provided for in the product listing of the liquid (see Article 100 of the 2017 NEC).1 Also, too much mineral oil contamination in a natural ester may impact meeting the requirements of the National Electrical Safety Code® (NESC®) (Accredited Standards Committee C2). Contact the natural ester manufacturer to determine the maximum mineral oil content range to prevent the open cup fire point from dropping below the 300C minimum requirement. Typically a maximum of 7% mineral oil contamination is acceptable.2

Less and Non-Flammable Insulating Liquids: Although in many cases different types of less-flammable liquids [e.g., synthetic esters, synthetic hydrocarbons, and high molecular weight hydrocarbons (HMWHs)] are miscible, such mixtures should generally be avoided in transformers and liquid processing equipment as practical, unless such mixtures are done purposely for certain applications or to achieve certain properties. Silicone insulating liquids are typically not miscible with natural ester dielectric liquids, so any cross contamination should be avoided. Typically natural esters are miscible with non-flammable halogenated hydrocarbon insulating liquids, such as polychlorinated biphenyls (PCBs). Although natural esters would not normally be mixed with halogenated hydrocarbons, this could occur after retrofilling older transformers containing such insulating liquids. Consult with the manufacturer of each insulating liquid for advice if mixing has occurred in situ inadvertently or purposefully.

2.  Normative references The following referenced documents are indispensable for the application of this document (i.e., they must be understood and used, so each referenced document is cited in text and its relationship to this document is explained). For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments or corrigenda) applies. Accredited Standards Committee C2, National Electrical Safety Code®(NESC®).3,4 Accredited Standards Committee C2-2017, National Electrical Safety Code® (NESC®). ASTM D92, Standard Test Method for Flash and Fire Points by Cleveland Open Cup Tester.5 ASTM D97, Standard Test Method for Pour Point of Petroleum Products. ASTM D445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and Calculation of Dynamic Viscosity).

Information on references can be found in Clause 2. Notes in text, tables, and figures of a standard are given for information only and do not contain requirements needed to implement this standard. 3 The NESC is available from the Institute of Electrical and Electronics Engineers (http://​standards​.ieee​.org/​). 4 National Electrical Safety Code and NESC are registered trademarks and service marks of the Institute of Electrical and Electronics Engineers, Incorporated. 5 ASTM publications are available from the American Society for Testing and Materials (http://​www​.astm​.org/​). 1 2

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IEEE Std C57.147-2018 IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

ASTM D664, Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration. ASTM D923, Standard Practices for Sampling Electrical Insulating Liquids. ASTM D924, Standard Test Method for Dissipation Factor (or Power Factor) and Relative Permittivity (Dielectric Constant) of Electrical Insulating Liquids. ASTM D971, Standard Test Method for Interfacial Tension of Oil Against Water by the Ring Method. ASTM D974, Standard Test Method for Acid and Base Number by Color-Indicator Titration. ASTM D1169, Standard Test Method for Specific Resistance (Resistivity) of Electrical Insulating Liquids. ASTM D1275, Standard Test Method for Corrosive Sulfur in Electrical Insulating Liquids. ASTM D1298, Standard Test Method for Density, Relative Density, or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method. ASTM Dl500, Standard Test Method for ASTM Color of Petroleum Products (ASTM Color Scale). ASTM D1524, Standard Test Method for Visual Examination of Used Electrical Insulating Liquids in the Field. ASTM D1533, Standard Test Method for Water in Insulating Liquids by Coulometric Karl Fischer Titration. ASTM D1816, Standard Test Method for Dielectric Breakdown Voltage of Insulating Liquids Using VDE Electrodes. ASTM D2300, Standard Test Method for Gassing of Electrical Insulating Liquids Under Electrical Stress and Ionization (Modified Pirelli Method). ASTM D3284, Standard Practice for Combustible Gases in the Gas Space of Electrical Apparatus Using Portable Meters. ASTM D3300, Standard Test Method for Dielectric Breakdown Voltage of Insulating Oils of Petroleum Origin Under Impulse Conditions. ASTM D3455, Standard Test Methods for Compatibility of Construction Material with Electrical Insulating Oil of Petroleum Origin. ASTM D3612, Standard Test Method for Analysis of Gases Dissolved in Electrical Insulating Oil by Gas Chromatography. ASTM D4059, Standard Test Method for Analysis of Polychlorinated Biphenyls in Insulating Liquids by Gas Chromatography. ASTM D5837, Standard Test Method for Furanic Compounds in Electrical Insulating Liquids by HighPerformance Liquid Chromatography (HPLC). ASTM D6871, Standard Specification for Natural (Vegetable Oil) Ester Fluids Used in Electrical Apparatus.

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IEEE Std C57.147-2018 IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

Code of Federal Regulations Title 40 Part 112 (40 CFR 112), Protection of Environment—Oil Pollution Prevention.6 IEC 61039 Classification of insulating liquids.7 IEC 61125, Insulating liquids – Test methods for oxidation stability – Test method for evaluating the oxidation stability of insulating liquids in the delivered state. IEC 62770, Fluids for electrotechnical applications – Unused natural esters for transformers and similar electrical equipment. IEEE Std 637™, IEEE Guide for the Reclamation of Mineral Insulating Oil and Criteria for Its Use.8,9 IEEE Std 980™, IEEE Guide for Containment and Control of Oil Spills in Substations. IEEE Std C57.104™, IEEE Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers. IEEE Std C57.106™, IEEE Guide for Acceptance and Maintenance of Insulating Mineral Oil in Electrical Equipment. IEEE Std C57.152™, IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors. IEEE Std C57.155™, IEEE Guide for Interpretation of Gases Generated in Natural Ester and Synthetic EsterImmersed Transformers. NFPA 70®, National Electrical Code®(NEC®).10,11 NFPA 70®, 2017 Edition, National Electrical Code® (NEC®).

3. Acronyms and abbreviations AOCS

American Oil Chemists Society

BEES

Building for Environmental and Economic Sustainability

CFR

Code of Federal Regulations

cSt

centistokes (units of measurement for kinematic viscosity)

DGA

dissolved gas analysis

EPA

Environmental Protection Agency

HMWH

high molecular weight hydrocarbon

HPLC

high-performance liquid chromatography

IEC

International Electrotechnical Commission

CFR publications are available from the U.S. Government Publishing Office (http://www.ecfr.gov/). IEC publications are available from the International Electrotechnical Commission (http://www.iec.ch) and the American National Standards Institute (http://www.ansi.org/). 8 The IEEE standards or products referred to in Clause 2 are trademarks owned by the Institute of Electrical and Electronics Engineers, Incorporated. 9 IEEE publications are available from the Institute of Electrical and Electronics Engineers (http://standards.ieee.org/). 10 The NEC is published by the National Fire Protection Association (http://www.nfpa.org/). Copies are also available from the Institute of Electrical and Electronics Engineers (http://standards.ieee.org/). 11 National Electrical Code, NEC, and NFPA 70 are registered trademarks of the National Fire Protection Association. 6 7

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IEEE Std C57.147-2018 IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

LTC

load tap changer

NCP

National Contingency Plan

NEC

National Electrical Code

NESC

National Electrical Safety Code

PCB

polychlorinated biphenyl

SDS

safety data sheet

SIC

specific inductive capacity

SPCC

Spill Prevention, Control, and Countermeasure

VDE

Verband Deutscher Elektrotechniker

4.  Liquid tests and the significance of each test 4.1 General The ASTM tests included within the scope of Committee D27 pertain to electrical insulating liquids and gases. Initially focused on insulating liquids of petroleum origin, its scope has expanded to include alternative liquids and gases for many years. While many of the more recent revisions now include natural esters and other alternative insulating liquids, some remain specific to mineral oil. The following information reviews the most commonly applied property tests of ASTM and provides guidance on how each test method can be applied to natural ester insulating liquids. Due to the inherent differences of chemical, electrical, and physical properties between natural ester and mineral oil insulating liquids, some of the ASTM methods that have not been updated for natural esters require some clarification. For example, some typical values and value limits for unused and used natural ester insulating liquid can be significantly different from those established for mineral oil. For unused liquid acceptance value limits, see ASTM D6871 for natural ester and see ASTM D3487 [B4] for mineral oil. Some methods may require slight modifications in test protocol as discussed below. The list of tests is offered for classification purposes in Table 1 below. Discussion of the significance of each test and how they apply to natural ester insulating liquids follows Table 1. See Clause 2 for ASTM standards referenced. Table 1—Insulating liquid tests suitable for natural ester-based dielectric liquids Significance (subclause)

ASTM or IEC method number

Test

4.2

Practices for sampling

D923

4.3

Acid number (neutralization)

D664, D974

4.4

Dielectric breakdown voltage

D1816

4.5

Dielectric breakdown voltage—impulse conditions

D3300

4.6

AC loss characteristics—dissipation factor and relative permittivity

D924

4.7

Interfacial tension

D971

4.8

Color

D1500

4.9

Kinematic viscosity

D445

4.10

Flash point and fire point—Cleveland Open Cup Method

D92

4.11

Relative density (specific gravity)

D1298

4.12

Pour point

D97, D5949 [B5], D5950 [B6]

4.13

Volume resistivity

D1169

a

Table continues

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IEEE Std C57.147-2018 IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

Table 1—Insulating liquid tests suitable for natural ester-based dielectric liquids (continued) Significance (subclause)

ASTM or IEC method number

Test

4.14

Gas analysis

D3284, D3612

4.15

Oxidation stability

IEC 61125, Method Cb

4.16

Water content—Karl Fischer Methodc

D1533

4.17

Visual examination of used liquids

D1524

4.18

Gassing under electrical stress and ionization

D2300

4.19

Corrosive sulfur test

D1275

4.20

Polychlorinated biphenyls (PCBs)

D4059

4.21

Furanic compounds

D5837

NE insulating liquids tinted with dye by manufacturer should not impact test beyond the 1.0 limit. As modified in IEC 62770, Annex A (reducing test duration from 164 h to 48 h). c Alternate reagents as listed in ASTM D1533 for natural esters as modified in IEC 62770, Annex A (reducing test duration from 164  h to 48 h). a

b

4.2  Practices for sampling (ASTM D923) Accurate sampling, whether of the complete contents or only part thereof, is extremely important from the standpoint of evaluation of the quality of the product sampled. Careless sampling procedures or contamination in the sampling equipment may result in a sample that is not truly representative, leading to erroneous conclusions concerning quality. The appropriate procedures and precautions outlined in ASTM D923 should be followed.

4.3  Acid number (ASTM D664 and ASTM D974) The acid (neutralization) number for unused and service-aged insulating liquids is, in general, a measure of the amount of acidic constituents in the liquid. The formation of acidic components is commonly associated with oxidation of mineral oil. The acidic components formed from mineral oil are considered undesirable as they can contribute to solid insulation aging and sludge formation. The acid number has been used as a general guide for determining when mineral oil should be replaced or reclaimed. In natural ester liquids, acidic components arise from diverse processes and may not be associated with adverse effects. The source of these components, as well as their magnitude, should be considered when determining what action should be taken. The acidic components are produced mainly from hydrolysis, pyrolysis, and oxidation of natural ester. Hydrolysis of natural ester bonds releases fatty acids. The free fatty acids introduced by this process are long chain organic acids. Most of these acids are 18 carbons in length, C18, while a few are 16 carbon sequences, C16. The strength of organic acids is inversely related to the chain length. Short chain acids such as acetic acid, C2, are strong and, in sufficient amount, can be detrimental to the condition of other materials in contact with the insulating liquid. Long chain acids such as stearic acid, C18, are weak and have not been associated with any detrimental effects. The presence of dissolved water in the liquid facilitates the hydrolysis reaction. It is common for the acid number to significantly increase during the first months of the transformer operation, attributable to the hydrolysis reaction with the initial moisture from the insulating paper. Pyrolysis of the ester bonds also yields fatty acids. The heat that causes the release of the fatty acids also causes some of them to break down further. Consequently, increases in acid number from this process directly correspond to increases in dissolved hydrocarbon and carbon oxide gases. As with hydrolysis, the fatty acids introduced by this process are long chain organic acids and are not considered to be detrimental.

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IEEE Std C57.147-2018 IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

Oxidation of the ester liquids results in the production of shorter chain acids. The most easily oxidized sites in natural ester liquids produce acids with chain lengths in the range of C7 to C11. Acids with even shorter chain lengths may also be produced. The acidic components produced by oxidation are the components of greatest concern. As with any insulating liquid, acidic constituents can also arise from contamination or reactions of materials other than the liquid itself. Significant changes in acid number that are not attributable to hydrolysis, pyrolysis, or oxidation should be investigated for these causes. ASTM D664 is the preferred method when testing dielectric liquids that have become discolored, because it uses a potentiometric endpoint rather than a colorimetric endpoint as used in ASTM D974.

4.4  Dielectric breakdown voltage (ASTM D1816) The dielectric breakdown voltage of an insulating liquid is of importance as a measure of its ability to withstand electrical stress. It is the voltage at which breakdown occurs between two electrodes under prescribed test conditions. ASTM D1816 prescribes the use of spherically capped electrodes of the Verband Deutscher Elektrotechniker (VDE) type. It serves primarily to indicate the presence of contaminating agents (e.g., water, dirt, and conducting particles in the liquid), one or more of which may be present when low dielectric breakdown values are found by test. Care should be taken when filling the test cell with natural ester liquids to guard against trapping air bubbles that can lead to misleading, low breakdown voltages. Due to their higher viscosity, a longer sample rest time (equal to or greater than 15 min at room temperature) is recommended for natural ester liquids than for mineral oils to allow air bubbles to escape. NOTE—ASTM dielectric breakdown voltage test D877 is in the process of being eliminated in the ASTM insulating liquid standards and has been eliminated in other IEEE standards and guides.

4.5  Dielectric breakdown voltage—impulse conditions (ASTM D3300) Insulating liquids used in transformers are subjected to transient voltage stresses while being subjected to steady-state voltage stresses associated with continuous operation of the apparatus at commercial power frequencies. The ability of the insulating liquid to withstand transient voltage stresses has become increasingly important to the designers of transformers. Transient voltages may be either negative or positive in polarity. Although polarity of the voltage wave has little or no effect on the breakdown strength of a liquid in uniform (essentially symmetrical) fields, polarity does have a marked effect on the breakdown voltage of a liquid in non-uniform (asymmetrical) electric fields. Transient voltages can also vary over a wide range in both the time to reach crest value and the time to decay to half-crest or to zero magnitude. The standard impulse test, ASTM D3300, specifies a 1.2 × 50 μs negative polarity wave. The standard wave shape for switching surge tests on transformers is 100 μs to crest and equal to or greater than 1000 μs to zero. The purchaser of an impulse generator may want to specify the necessary features to make switching surge tests possible. Consideration may be given to other electrode configurations such as VDE electrodes, which are similar to those used in ASTM D1816, because it may be desirable to obtain the ratio between power frequency and impulse breakdown under similar conditions. Care should be taken when filling the test cell with natural ester liquid to guard against trapping air bubbles that may result in misleading, low breakdown voltages.

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IEEE Std C57.147-2018 IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

Due to their higher viscosity, a longer sample rest time (equal to or greater than 15 min at room temperature) is recommended prior to impulse testing natural ester liquid than for mineral oils to allow air bubbles to escape.

4.6 AC loss characteristics—dissipation factor and relative permittivity (ASTM D924) This method describes the determination of dissipation factor and relative permittivity of unused electrical insulating liquids, as well as liquids in service or subsequent to service in transformers. Dissipation factor (power factor) is a measure of the dielectric losses in an electrical insulating liquid in an alternating electric field and of the energy dissipated as heat. A low dissipation factor indicates low dielectric losses. Losses due to dissipation factor should not be confused with transformer load and excitation losses, which are indicative of the transformer’s energy efficiency. The losses associated with dissipation factor are several orders of magnitude lower than the load and excitation losses. Unused natural ester liquids have inherently higher dissipation factors than mineral oils. Field data indicates a higher rate of increase in the dissipation factors under normal operating conditions relative to mineral oils. However, additional analysis is required prior to setting limits for continued service. Relative permittivity, often referred to as dielectric constant and occasionally as specific inductive capacity (SIC), is the ratio of the capacitance of a capacitor using the material to be measured as the dielectric to the capacitance of a capacitor with vacuum as the dielectric, both having identical electrodes. The relative permittivity of materials in contact with each other can affect the local voltage stress distribution. Unused natural esters have inherently higher relative permittivity than unused mineral oils, which is closer to that of cellulose insulation, leading to an improvement in electrical stress distribution.

4.7  Interfacial tension (ASTM D971) This method covers the measurement, under non-equilibrium conditions, of the interfacial tension of insulating liquids against water. The interfacial tension between electrical insulating liquids and water is an indirect measure of the surfactant content of the insulating liquid that migrates under charge attraction into the water at the interface. The surfactants are polar or ionic soluble-contamination or liquid-deterioration products that decrease the interfacial tension value. Water molecules are strongly attracted to one another and require a certain force to break their interfacial tension. Surfactant species are attracted by the polar charges on water molecules. As surfactants in the oil are attracted across the oil-water interface, they obstruct some of the water-to-water attractions that weaken the tensile forces of the water interface. The amount and type of surfactants determines the amount of weakening of these forces. This weakening is measured as a decrease in the interfacial tension value. Interfacial tension is measured in millinewtons per meter (mN/m). ASTM has not published an acceptance value limit for interfacial tension of unused natural ester liquids. Natural ester liquids have inherently lower interfacial tension than unused mineral oils. Until ASTM has published a limit for interfacial tension of unused natural ester liquids, this guide cannot include such a limit value. Additional field data is required before limits for field-aged liquid can be established for this guide. However, further investigation should be done when there is more than a 40% decrease in the interfacial tension value from initial transformer samples taken prior to energization.

4.8  Color (ASTM D1500) A low color number of mineral insulating oil is desirable to permit inspection of assembled apparatus in a tank. Unused natural ester liquids may initially be slightly darker in color (typically a slight amber appearance) than highly refined unused mineral oil. While an increase in color number during service is an indicator of oil

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IEEE Std C57.147-2018 IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

deterioration or contamination in mineral oil, this may not be the case for natural ester liquids. Other tests (such as dissipation factor and neutralization number) are better measures of liquid deterioration or contamination. Note that natural ester liquid manufacturers may add clear colorants for identification purposes. Such tints should not impact the ASTM color and visual examinations beyond the 1.0 limit.

4.9  Kinematic viscosity (ASTM D445) The viscosity of dielectric coolants within the range of normal operating temperatures is important because it can impact both the cooling and performance of some other transformer components such as load tap changers (LTCs), which are immersed with the same insulating liquid. Viscosity is the measure of the resistance of a liquid to flow. Kinematic viscosity is the ratio of the dynamic viscosity of a liquid to its density. Dynamic viscosity is the ratio between the applied shear stress and rate of shear of a material. The viscosity of mineral insulating oil and natural ester liquids is usually measured by the time of flow of a given quantity of liquid under controlled conditions of temperature and pressure. The viscosity at the operating temperatures of electrical insulating liquids influences their heat transfer properties in natural and forced (pumped) convective flow and, consequently, the temperature rise of operating transformers containing them. Natural esters typically have higher viscosity than mineral oils. An increase in viscosity over time can indicate excessive polymerization of natural esters from oxidation, typically due to abnormal exposure to air and heat.

4.10  Flash point and fire point—Cleveland Open Cup Method (ASTM D92) The flash point of a flammable liquid is the lowest temperature at which the vapor pressure is sufficient to form a flammable mixture with air near the surface of the liquid. The fire point is the lowest temperature at which a liquid is heated in an open container, and attains sufficient combustible vapors to ignite and sustain a fire for 5 s. Low values of either flash or fire points are an indication of contamination with lower flash and fire point materials, such as conventional mineral oil. This method should only be used to measure and describe the properties of liquids in response to heat and flame under controlled laboratory conditions and should not be used to describe or appraise the fire hazard or fire risk of liquids under actual fire conditions. Results of this test can be used as an element of a fire risk assessment that takes into account other factors that are pertinent to an assessment of the fire hazard of a particular end use. Natural esters have significantly higher flash and fire points than that of conventional mineral oil. ASTM D6871 requires a minimum 275 °C flash point and 300 °C fire point per method D92 for unused natural ester insulating liquid as received. See Annex C for more information.

4.11  Relative density (ASTM D1298) The relative density (specific gravity) of an insulating liquid is the ratio of the weights of equal volumes of liquid and water at 15 °C (60 °F). Relative density is not significant in determining the quality of a liquid; it may be pertinent in determining suitability for use in specific applications. In certain cold climates, ice may form in equipment exposed to subzero (30 °C) or high humidity locations (unless desiccants are available and maintained). Existing storage tanks that have been used for mineral oil can be used for natural ester liquids if the following conditions are met: — Transfer pumps and lines are of adequate capacity to pump the more viscous liquid. If the tank and transfer system are situated so that the liquid may have to be moved while it is cold, use of electric- or steam-line tracing and tank-heating apparatus may be necessary. — The tank is thoroughly cleaned and inspected for rusting conditions or leakage. — The tank should be thoroughly drained and flushed with 60 °C to 80 °C natural ester liquid before being filled to help avoid contamination.

WARNING It is important that pumps and lines are properly grounded during liquid transfer to prevent the build-up of a static electric charge.

6.4.2 Venting Equipping a tank with a proper pressure/vacuum vent valve and a desiccant-type vent dryer lowers the dew point of the air in the tank to help prevent water condensation. Each vent dryer should have a color indicator to show when maintenance is required. Changes should be planned in advance of the anticipated indicator change. Dehumidification of the air in the storage tank can also be used. A tank vent filter between the desiccant canister and the tank is also recommended. Filtering incoming air to the tank may prevent introduction of airborne particulate material into the liquid during storage.

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IEEE Std C57.147-2018 IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

6.4.3 Pumps So that pump suppliers can specify the correct pump size, they should be made aware of the liquid viscosity and the required pumping rate, suction lift, and discharge head. The following factors should be considered: — Capacity. Because the viscosity of natural ester liquids is generally higher than conventional mineral oil, care should be used in selecting a pump with the horsepower and capacity required. First, determine the maximum flow rate required and then select a pump and motor that can handle this flow rate at the lowest temperature (highest liquid viscosity) that could be encountered. — Type. The most commonly recommended pump for natural ester liquids is the positive displacement gear pump. A standard iron pump with either mechanical seal or stuffing box is also satisfactory. For capacities up to 4.8 L/s (75.8 gpm), direct-driven pumps have proven to be satisfactory. For higher pumping rates, a reduction-gear or belt-driven pump may be required. Other pumps that have been used successfully are the air-operated diaphragm pump, progressive cavity pump, and flexible impeller pump.

WARNING Insulating liquids, including natural ester liquids, passing through filter papers or ungrounded or unbonded hoses can acquire an electrostatic charge that could be transferred to the transformer windings as the transformer is filled. Under some conditions, the electrostatic voltage on the winding may be hazardous to personnel or equipment. To avoid this possibility, all externally accessible transformer bushing terminals, as well as the tank and liquid filtering equipment including hoses, should be properly grounded during filling.

6.5  Handling and testing of natural ester liquids for installation into apparatus The preferred method of filling transformers is under vacuum conditions. Additional vacuum processing of the natural ester liquid is recommended to sufficiently degas the liquid, prior to filling the transformer tank, to help avoid excessive foaming. Where instructions given by the transformer manufacturer differ from recommendations made in this guide, the manufacturer’s instructions should be given preference.

CAUTION Do not exceed the transformer tank vacuum limits (see nameplate or contact the transformer manufacturer for information) or tank damage may result.

Commercial dehydration and degassing units are available that can process insulating liquids to acceptable levels of dissolved water and dissolved air. The degassing of natural ester liquid should be carried out at temperatures somewhat different than those required for mineral oil. The processing temperature should be obtained from the insulating liquid manufacturer. Proper processing temperature helps ensure sufficient degasification and dehydration of the natural ester liquid prior to introduction into the transformer. After the natural ester liquid is processed through the degasifier and particulate filter, it should be introduced directly into the transformer under vacuum. If in doubt as to the filling procedure to be followed, seek guidance

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IEEE Std C57.147-2018 IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

from the transformer manufacturer. If guidance from the transformer manufacturer cannot be obtained, a storage tank that can maintain a vacuum equal to or greater than the vacuum maintained during transformer filling should be utilized. If the recommended vacuum cannot be achieved, the natural ester liquid may exhibit excessive foaming in the transformer tank during the vacuum filling operation, depending on the amount of dissolved gases in the natural ester liquid. In instances where the transformer is required to be filled on-site without the use of vacuum impregnation, consult the liquid’s manufacturer for instructions concerning fill rate and characteristics of the liquid after filling. NOTE 1—Natural ester liquids are miscible and compatible with typical mineral oil insulating liquids. If the purpose of using a natural ester liquid is to comply with Article 450.23 of the 2017 NEC the requirements should be followed, including that less-flammable transformer liquids have an ASTM D92 fire point of not less than 300 °C and that the installation complies with all restrictions provided for in the product listing of the liquid. The 2017 NESC (Accredited Standards Committee C2-2017) also recognizes less-flammable insulating liquids as a means to minimize fire hazard, for both outdoor installations (in Rule 152A), and for indoor installations (in Rule 152B). A high fire point, >300 °C, is the requirement of IEC 61039 to be classified as a K Class insulating liquid. For such installations, avoid contamination of natural ester liquid with mineral oil to prevent lowering the flash and fire points, and for possible environmental regulations purposes. Dedicated equipment is the preferred means to help prevent excessive contamination. Otherwise, it is recommended to flush the equipment with an appropriate volume of at least 5% natural ester liquid. NOTE 2—Some natural ester power transformer manufacturers, when using vapor phasing processing, might require initially filling the transformer with mineral oil. In such cases, it is strongly recommended that the unit be retrofilled with the natural ester prior to running the factory tests. NOTE 3—Medium and large power transformer tanks may be filled with a natural ester in the plant for testing, then drained and filled with dry gas prior to shipping. Dry nitrogen is technically preferred to help avoid the potential for thin film oxidation to occur between the time of the drainage at the factory and the field filling. However, that option may not be available due to local safety regulations (to help prevent possible asphyxiation). If dry air fill is required, contact the natural ester manufacturer for guidance.

7. Evaluation of natural ester liquids received in new equipment and after filling apparatus on-site In sampling liquid that is contained in apparatus, use care to obtain a representative sample. The sampling methods described in ASTM D923 should be followed. Natural ester liquids exhibiting the characteristics presented in Table 3 are considered acceptable. After the filling is completed and the standing time is also completed, tests on the natural ester liquid should be made before energization of the transformer (see Table 3). Table 3—Test limits for unused natural ester liquid received in new equipment, prior to energization Test and ASTM method

Value for voltage class ≤ 69 kV

> 69 kV < 230 kV

≥ 230 kV

Dielectric strength, ASTM D1816, kV, minimum 1 mm gap 2 mm gap

25 45

30 55

35 60

Dissipation factor, ASTM D924, %, maximum, 25 °C

0.5

0.5

0.5

L1.0

L1.0

L1.0

Color, ASTM D1500, ASTM units, maximum

Table continues

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IEEE Std C57.147-2018 IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

Table 3—Test limits for unused natural ester liquid received in new equipment, prior to energization (continued) Value for voltage class

Test and ASTM method

≤ 69 kV

> 69 kV < 230 kV

≥ 230 kV

Bright and clear

Bright and clear

Bright and clear

Neutralization number (acidity), ASTM D974, mg KOH/g, maximum

0.06

0.06

0.06

Water content, ASTM D1533, mg/kg, maximum

300

150

100

Fire point, ASTM D92, °C

300

300

300

Kinematic viscosity, ASTM D445, mm2/s (cSt) at 40 °C, maximum

50

50

50

—

0.5% or per manufacturer’s requirementsb

Visual examination, ASTM D1524

Total dissolved gas, ASTM D3612, %, maximum

—

The test limits shown in this table apply to natural ester liquid as a class. Due to differences in their chemistry, certain values are significantly different than the limits for mineral oil. See Clause 4 for details. Specific typical values for each brand of liquid should be obtained from each liquid manufacturer. If test results, while in compliance with this table, are significantly different from published typical values, it is recommended that the liquid manufacturer be contacted. b This value should be obtained from a sample collected 24 h to 48 h after the transformer is filled, and applies only to transformers with diaphragm conservator systems. The maximum of 0.5% (5000 ppm) is used for total dissolved gases, which is the sum of the individual gases including atmospheric gases. a

Most transformer manufacturers have found it advisable to allow natural ester liquid-immersed transformers to wait longer than normally allowed for mineral oil after filling and breaking vacuum before energizing or high-voltage testing. A conservative option is to wait until the transformer has cooled to room temperature. The additional wait time is recommended because natural esters generally take significantly longer to impregnate cellulose insulation than mineral oil under the same conditions. Transformers with heavy thicknesses of pressboard insulation require standing times that are adequate to allow the required impregnation. The impregnation rate of natural ester liquid is a function of liquid temperature and the thickness of the cellulose material to be saturated. Contact transformer, insulating paper/pressboard manufacturers, and insulating liquid manufacturers for guidance concerning impregnation rates. Recommended minimum standing times vary depending on the type of pressboard, thickness, initial liquid temperature, ambient temperature, voltage class, etc. If such guidance is unavailable, for distribution transformers consider a minimum of an 8 h wait time, with 24 h being preferred. For power transformers, consider a minimum wait time of at least 24 h or more, depending on the variables mentioned above. Liquid circulating pumps, if any, should be operated for at least several hours of the standing time. Most transformer manufacturers have specific written procedures for this standing time and pump operation, and they should be consulted for their recommendations.

8.  Maintenance of natural ester liquids 8.1  Field screening Field screening of natural ester liquids should follow the procedures now being used for mineral oil. Experience in this matter indicates that visual condition and dielectric breakdown voltage are the most applicable screening methods. A sample should be drawn in a clean clear glass or high-density polyethylene jar and allowed to reach room temperature prior to testing. The sample jar should be sealed as soon as practical to avoid moisture contamination from the air. Aluminum or tin-plated steel cans can also be used as containers for the samples. The liquid should then be checked for clarity, color, odor, and viscosity (relative to an unused sample). Dielectric breakdown voltage should then be measured. Portable dielectric test sets have

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been available for some time and have proven quite satisfactory in determining whether additional laboratory screening is necessary.

8.2  Laboratory screening Natural ester liquids that have unsatisfactory appearance and dielectric values should be further evaluated. The following tests are adequate for classifying in-service natural ester liquids: — Visual condition (ASTM D1524) — Color (ASTM D1500) — Dielectric breakdown voltage (ASTM D1816) — Water content (ASTM D1533) — AC loss characteristic (dissipation factor) (ASTM D924) — Fire point (ASTM D92) — Viscosity (ASTM D445) NOTE 1—For DGA see IEEE Std C57.155, IEEE Guide for Interpretation of Gases Generated in Natural Ester and Synthetic Ester-Immersed Transformers.

The following ASTM tests are usually not required for classifying service-aged liquids. However, they can be useful, in more completely characterizing the condition of insulating liquids. — Interfacial tension (ASTM D971) — Relative density (ASTM D1298) — Pour point (ASTM D97) — Volume resistivity (ASTM D1169) — Neutralization number (ASTM D664 and ASTM D974) NOTE 2—The American Oil Chemists Society (AOCS) Official Method, Cd 18-90, p-Anisidine Value, developed for edible oils, may also prove to be a useful indicator of the condition of aged natural ester liquids used in transformers. The method determines the amount of aldehydes (principally 2-alkenals and 2,4-dienals) in natural esters. The aldehydes are formed as byproducts during oxidation of the vegetable oil base. Additional field data collection and evaluation are suggested to determine recommended values for transformer application.

8.3  Test limits for in-service natural ester liquids Acceptable limits for in-service natural ester liquids are shown in Table 4. These should be used as a guide in the absence of the manufacturer’s recommendations.

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Table 4—Suggested limits for continued use of in-service natural ester liquids (grouped by voltage class) (see also IEEE Std C57.152™) IEEE value for voltage class

Test and ASTM method

≤ 69 kV

> 69 kV < 230 kV

≥ 230 kVb

23 40

28 47

30 50

(see footnote c)

(see footnote c)

(see footnote c)

Dielectric strength , ASTM D1816, kV, minimum 1 mm gap 2 mm gap c

Dissipation factor (power factor), ASTM D924, %, maximum 25 °C 100 °C Water content: ASTM D1533, mg/kg, maximum

450

350

200

Optionald Fire point, ASTM D92, °C, minimum

300

300

300

Kinematic viscosity increase from initial value

≥10%

≥10%

≥0%

a The test limits shown in this table apply to natural ester insulating liquids as a class. Due to differences in their chemistry, certain values are significantly different than the limits for mineral oil. See Clause 4 for details. Specific typical values for each brand of insulating liquid should be obtained from each insulating liquid manufacturer. If test results, while in compliance with this table, are significantly different from published typical values, it is recommended that the insulating liquid manufacturer be contacted. b Provisional for ≥ 230 kV. c At the time this guide was written, there was insufficient field data analysis available to provide reliable recommended limit values for dissipation factor. Users are encouraged to forward data to the IEEE Transformer Committee Insulating Fluids Subcommittee for possible future use. See B.2 for non-normative guidance for threshold values indicating possible abnormalities or normal aging limit reached. d Most outdoor transformer installations do not require the NEC “Less-Flammable” designation. After retrofilling with unused natural esters a reduction in the fire point of the ester liquid due to residual mineral oil. For transformers requiring a less-flammable rating, or otherwise known to require its insulating liquid to meet a minimum 300 °C fire point, any sample testing with a fire point below 300 °C indicates excessive residual mineral oil and should be investigated. For more information, please refer to Figure B.4 and C.1.

8.4 Reconditioning 8.4.1 General For the purposes of this guide, reconditioning is defined as “the removal of water and solid materials by mechanical means,” while reclaiming is defined as “the removal of acidic and colloidal contaminants and oxidized matter by chemical and adsorbent means.” The mechanical means that are used for removing water and solids from liquids include several types of filters, centrifuges, and vacuum dehydrators. In general, water removal filters and vacuum dehydrators should be placed before the final particulate removal filters. 8.4.2 Water removal 8.4.2.1 General If, during the transport or storage of the liquid, water is introduced into the liquid above a limit that would not permit the liquid to be introduced into a transformer, additional treatment is required. 8.4.2.2 Free water removal If the water is in the form of free water, filter elements utilizing blotter paper have been used effectively. Filter cartridges packed with moisture absorbing media are recommended to help achieve desired dryness. Proper care and storage of the water absorbing filters is essential to help ensure they do not absorb moisture before use. The manufacturer’s temperature rating of the elements should not be exceeded.

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Most types of filters now being used on mineral oil can be used for natural ester liquids. The cartridge-type filter is well suited for this service. It is available in various nominal pore size ranges, and sizes for either low- or high-flow rates. Filters of the adsorption type, such as activated Fuller’s earth, can be used; however, certain pour point depressant and antioxidant additives can be removed from the liquid by these filters if the vacuum is too high. The manufacturer of the liquid should be consulted to determine whether the possibility of additive removal is a concern. Just as when selecting pumps, care should be taken in selecting a filter for natural ester liquids. Because natural ester liquid viscosities are higher than those for mineral oil, larger filters or higher liquid temperature may be required to achieve the same flow rate. If filters for mineral oil are used, a decrease in flow rate may be necessary unless steps are taken to decrease the liquid viscosity by heating. 8.4.2.3  Dissolved water removal If the dissolved water content should be lowered, a high vacuum dehydration system may be required. The vacuum dehydrator is an efficient means of reducing the gas and water content of an insulating liquid to a very low value. Two types of vacuum dehydrators are in general use today. The operating principle of both is the same, i.e., the liquid is exposed to a high vacuum and heat for a short interval of time. In one method, the exposure of the liquid is accomplished by spraying the liquid through a nozzle into a vacuum chamber. In the other type of vacuum dehydrator, the liquid is allowed to flow over a series of baffles inside a vacuum chamber, thus forming thin films so that a large surface is exposed to the vacuum. If the liquid contains solid matter, it is advisable to filter it before processing it in the vacuum dehydrator because solid contaminants can plug the nozzle of one type of dehydrator or pass through either type without being removed from the liquid. In addition to removing water, processing natural ester insulating liquid with a vacuum dehydrator also extracts dissolved gases, normally removing any volatile acids. However, most remaining dissolved acids are relatively unaffected by the process. Thus, it is doubtful the overall acidity of the liquid can be significantly reduced by using the vacuum dehydration method. In either type of dehydrator, some means of automatically recirculating a very wet liquid should be provided to prevent excessive water content in the outgoing liquid. Molecular sieve filters have been found effective for removing dissolved water from natural ester insulating liquid. Activated grade 3A or 4A molecular sieves are recommended for water removal from natural ester liquids and are effective over a broad temperature range, provided adequate care is taken in filter selection to help ensure sufficient residence time in the filter and provided a particulate filter is used downstream of the molecular sieve filter (to catch fine particulates from the sieves). An in-line moisture hygrometer with alarm calibrated for use in natural ester insulating liquid should be installed downstream of any moisture absorbing filter or molecular sieve to help ensure that the moisture removing media has not become saturated, potentially causing the release of moisture into the liquid.

8.5 Reclaiming The removal of deterioration products from natural ester insulating liquid is usually accomplished by the use of reclaiming processes involving Fuller’s earth. Other absorbents that should also be effective include magnesium silicate, magnesium aluminum phyllosilicate, activated alumina, and bauxite. Mixtures of absorbents may improve removal performance.

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The manufacturer of the liquid should be consulted for recommendations regarding reclaiming, as the recommended treatments may vary from those outlined in IEEE Std 637™. Some additives provided with unused liquid can be removed by the reclamation process and may need to be added back to the reclaimed liquid.

8.6  Mixtures of different types of insulating liquids Although in most cases different types of insulating liquids are miscible (with the exception of silicone liquid), such mixtures should typically be avoided in transformers and liquid processing equipment when possible and practical. This is due to potential negative impact on key environmental, performance, and fire safety characteristics. Obviously, some low percentage contamination cannot be avoided when retrofilling, particularly transformers and other equipment with impregnated cellulose material. Transformers containing mineral oil and HMWH have been retrofilled with natural ester. Consult the manufacturers of the insulating liquids or the transformer for advice if mixing different types of insulating liquids is permissible or has occurred. Refer to B.3 for mixture information, including an example of the impact on a few key properties using a natural ester with different ratios of mineral oil content.

9.  General environmental procedures 9.1 General Typically, natural esters covered by this guide have been formulated to minimize health and environmental hazards. Although no known hazard is involved in the normal handling and use of natural ester liquids, additives to the base vegetable oils may differ. Users should obtain a safety data sheet (SDS) for each natural ester liquid in use. Where manufacturer’s instructions differ from recommendations made in this guide, the manufacturer’s instructions should be followed. Personnel should avoid eye/liquid contact and inhalation of spray mists, and take appropriate steps if such incidents occur. SDSs should provide appropriate guidelines with respect to handling these liquids. Although not listed as a hazardous substance or waste by any U.S. federal agency, disposal of natural ester liquids may require certain precautions, depending on regulatory jurisdiction, as environmental regulations can vary by country, province, state, county, and local community. Currently, the U.S. EPA Spill Prevention, Control, and Countermeasure (SPCC) regulation (40 CFR 112) makes no practical distinction between mineral oils and vegetable oils, except for possible reduction in spill remediation requirements. For natural esters that contain mineral oil, such as natural ester liquid used as a flushing liquid for retrofilling transformers originally containing mineral oil, follow the same disposal requirements for mineral oil provided by the authority having jurisdiction.

9.2 Leaks During scheduled routine maintenance, or a regular equipment maintenance schedule, checks should be made for leaks. For units with pressure gauges, a constant periodic reading of zero-gauge pressure is a strong indication of a head space leak or some other problem that should be investigated. Areas to check and repair should include valves, bushings, gauges, tap changers, welds, sample ports, manhole covers, pipe fittings, and pressure relief valves. If a leak does not involve a replaceable seal, welding or epoxy sealing kits can typically be used to seal it. Proper care should be taken to protect the integrity of the natural ester liquid and equipment insulation if leak repair requires lowering the liquid level. Clean and dry temporary liquid storage containers should be used. Testing of the liquid before returning it to the equipment is recommended. The recommendations on sampling, testing, and filling of transformers presented in this guide should be followed.

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9.3  Minor spills Minor spills, such as those occurring in the manufacture or repair of transformers and in testing natural ester liquid, can be cleaned using absorbent rags. Using suitable cleaners facilitates the cleanup. Many common solvents suitable for use with petroleum liquids may not be effective with natural esters. Common household detergents are recommended. If thin films of natural esters have partially or completely polymerized, household detergents may not be effective. The surface area should be saturated with a suitable cleaner (water-based, biodegradable, non-flammable, non-conductive cleaner/degreaser) and then steam or hot water spray should be applied. Contact the natural ester liquid manufacturer for recommended cleaners. See IEEE Std 980™ for additional information.

9.4  Spills on soil States typically have jurisdiction for spills onto soils. Many states currently do not list natural esters as soil spill-regulated material. However, state and local regulations should be consulted to enable compliance with any applicable regulations. Soil acts as an absorbent and typically offers excellent conditions for natural biodegradation. If the presence of natural ester liquid is objectionable to the authorities having jurisdiction, the soil can be treated with a bioremediation catalyst to maximize the biodegradation rate. The two most frequently used technologies for oil spill cleanups in the United States are fertilization and seeding. Fertilization helps speed up the biodegradation process by adding nutrients to stimulate the growth of microorganisms. Seeding adds additional microorganisms to assist the native organisms in the degradation process. In terms of potential physical hazard (e.g., slippage), natural ester liquids in spill situations behave much the same as motor oil or hydrocarbons of comparable viscosity. The same cleanup requirements may be applicable. See IEEE Std 980 for more information.

9.5  Spills on water Because natural ester liquids float on water, a spill can be contained by floating booms or dikes. If containment equipment is unavailable or impractical, the natural ester liquid can be treated by applying surface-active dispersant chemicals, also known as detergents, designed to remove the oil from the surface of the water and into the water column. Only chemical dispersants that are listed on the U.S. National Contingency Plan (NCP) Product Schedule should be used to treat oil spills. For spills into water surfaces, check with the authorities having jurisdiction for reporting and remediation requirements. Once the natural ester liquid has been concentrated, it can be removed from the water surface by systems normally used for vegetable oil spills. These systems include pumps, skimmers, and physical absorbents. When collected, the liquid can be reclaimed or incinerated in a suitable burner. See IEEE Std 980.

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Annex A (informative)

Bibliography Bibliographical references are resources that provide additional or helpful material but do not need to be understood or used to implement this standard. Reference to these resources is made for informational use only. [B1] ASTM D117, Standard Guide for Sampling, Test Methods, and Specifications for Electrical Insulating Oils of Petroleum Origin.13 [B2] ASTM D2112, Standard Test Method for Oxidation Stability of Inhibited Mineral Insulating Oil by Pressure Vessel. [B3] ASTM D2440, Standard Test Method for Oxidation Stability of Mineral Insulating Oil. [B4] ASTM D3487, Standard Specification for Mineral Insulating Oil Used in Electrical Apparatus. [B5] ASTM D5949, Standard Test Method for Pour Point of Petroleum Products (Automatic Pressure Pulsing Method). [B6] ASTM D5950, Standard Test Method for Pour Point of Petroleum Products (Automatic Tilt Method). [B7] ASTM D6866, Standard Test Methods for Determining the Biobased Content of Solid, Liquid, and Gaseous Samples Using Radiocarbon Analysis. [B8] Boss, P., Insulating fluids for power transformers, prepared by P. Boss in the name of CIGRE SC A2. [B9] CIGRE Technical Brochure 436, Experiences in service with new insulating liquids, CIGRE WG A2-35, 2010.14 [B10] Claiborne, C. C., T. V. Oommen, H. D. Le, E. J. Walsh, and J. P. Baker, “Enhanced cellulosic insulation life evaluation in a high oleic vegetable oil dielectric fluid,” Paper 3C, Doble Conference, Apr. 2002. [B11] Code of Federal Regulations Title 40 Part 761.1 (40 CFR 761.1), Protection of Environment— Polychlorinated Biphenyls (PCBs) Manufacturing, Processing, Distribution in Commerce, and Use Prohibitions—Applicability.15 [B12] Davydov, V. G., “New Natural Ester Dielectric Liquids for Transformers and Other HV Apparatus,” Proceedings of TechCon 2014 Asia-Pacific, pp. 181–200, Sydney, Australia 2014. [B13] Doble Engineering Company, Lewand, L. R., “Laboratory evaluation of several synthetic and agricultural-based dielectric liquids,” Proceedings of the 86th Annual International Conference of Doble Clients, Watertown, MA, USA, 2001. [B14] Duy, C. T., O. Lesaint, A. Denat, and N. Bonifaci, “Streamer propagation and breakdown in natural ester at high voltage,” IEEE Transactions on Dielectrics and Electrical Insulation, vol. 18, pp. 285–294, 2011.

ASTM publications are available from the American Society for Testing and Materials (http://​www​.astm​.org/​). CIGRE publications are available from the Council on Large Electric Systems (http://​www​.e​-cigre​.org/​). 15 CFR publications are available from the U.S. Government Publishing Office (http://​www​.ecfr​.gov/​). 13 14

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[B15] EPA OPPTS 835.3100, Fate, Transport and Transformation Test Guidelines: Aerobic Aquatic Degradation.16 [B16] EPA OPPTS 835.3110, Fate, Transport and Transformation Test Guidelines: Ready Biodegradability. [B17] Eriksson, A., R. Liu, and C. Törnkvist, “Differences in Streamer Initiation and Propagation in Ester Fluids and Mineral Oil,” IEEE ICDL Conference 2011, File no. 40, http://​dx​.doi​.org/​10​.1109/​ICDL​.2011​ .6015421. [B18] Factory Mutual Global, FM Approval Guide—Electrical Equipment, 2006.17 [B19] Fritsche, R., U. Rimmele, F. Trautmann, and M. Schafer, “Prototype 420 Kv Power Transformer using Natural Ester Dielectric Fluid, Proceedings of TechCon 2014, Albuquerque, New Mexico, USA. [B20] FM Approvals 3990, FM Approval Standard for Less or Nonflammable Liquid-Insulated Transformers, January 2018.18 [B21] Frotscher, R., D. Vuković, M. Jovalekic, S. Tenbohlen, J. Harthun, M. Schäfer, and C. Perrier, Behaviour of Ester Liquids under Dielectric and Thermal Stress – From Laboratory Testing to Practical Use, CIGRE Conference 2012, Paris, Paper D1-105. [B22] IEC 61099, Insulating liquids – Specifications for unused synthetic organic esters for electrical purposes.19 [B23] IEC 61203:1992, Synthetic organic esters for electrical purposes—Guide for maintenance of transformer esters in equipment. [B24] IEC 60156, Insulating liquids – Determination of the breakdown voltage at power frequency – Test method. [B25] IEEE Power Distribution & Regulating Transformers Collection: VuSpec™ (2014).20,21 [B26] IEEE Std C57.106™-2002, IEEE Guide for Acceptance and Maintenance of Insulating Mineral Oil in Electrical Equipment. [B27] Insulect Energy Blog, Guide for Considering Natural Ester Fluids for Retrofilling, November 24, 2016.22 [B28] Lewand, L. R., C. C. Claiborne, and D. B. Cherry, “Oxidation and Oxidation Stability of Natural Ester Dielectric Liquids,” Paper IM-2, Doble Conference, Mar. 2010. [B29] Lippiatt, B.C., BEES 4.0: Building for Environmental and Economic Sustainability. Technical Manual and User Guide. NIST Interagency/Internal Report (NISTIR) – 7423, August 1, 2007. [B30] Liu, Q. and Z. D. Wang, “Streamer Characteristic and Breakdown in Synthetic and Natural Ester Transformer Liquids under Standard Lightning Impulse Voltage,” IEEE Transactions on Dielectrics and Electrical Insulation, vol. 16, pp. 1582–1594, 2009. EPA publications are available from the Environmental Protection Agency (https://​www​.regulations​.gov/​). FM publications are available from Factory Mutual Global (https://​www​.fmapprovals​.com/​). 18 Documents are available from FM Approvals (http://​www​.fmapprovals​.com/​). 19 IEC publications are available from the International Electrotechnical Commission (http://​www​.iec​.ch) and the American National Standards Institute (http://​www​.ansi​.org/​). 20 IEEE publications are available from the Institute of Electrical and Electronics Engineers (http://​standards​.ieee​.org/​). 21 The IEEE standards or products referred to in Annex A are trademarks owned by the Institute of Electrical and Electronics Engineers, Incorporated. 22 The blog is available at http://​insulect​.com/​energy​-blog. 16 17

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[B31] Lu, W., Q. Liu, and Z. D. Wang, “Combined Effect of Cellulose Particles and Moisture on Lightning Impulse Breakdown Voltages of Ester Transformer Liquids,” 2013 International Symposium on High Engineering (ISH): 1326–1331. eSchojlarID: 258292. [B32] Luksich, J. and K. Rapp, “Review of Kraft paper/natural ester fluid insulation system aging,” IEEE ICDL Conference 2011, File no.110. [B33] McShane, C. P., “New safety dielectric coolants for distribution and power transformers,” IEEE Industry Applications Magazine, vol. 6, no. 3, pp. 24–32, May/June 2000, http://​dx​.doi​.org/​10​.1109/​2943​.838037. [B34] McShane, C. P., K. J. Rapp, J. L. Corkran, G. A. Gauger, and J. Luksich, “Aging of paper insulation in natural ester dielectric fluid,” IEEE/PES Transmission and Distribution Conference and Exposition, Oct. 28– Nov. 2, 2001, http://​dx​.doi​.org/​10​.1109/​TDC​.2001​.971319. [B35] McShane, C. P., J. Luksich, and K. J. Rapp, “Retrofilling aging transformer with natural ester based dielectric coolant for safety and life extension,” Proceedings of the IEEE-IAS/PCA Cement Industry Technical Conference, Dallas, TX, USA, May 2003, http://​dx​.doi​.org/​10​.1109/​CITCON​.2003​.1204715. [B36] OECD 203, OECD Guidelines for the Testing of Chemicals, Section 2, Test Number 203: Fish, Acute Toxicity Test.23 [B37] OECD 420, OECD Guidelines for the Testing of Chemicals, Section 4, Test Number 420: Acute Oral Toxicity – Fixed Dose Procedure. [B38] Official Methods and Recommended Practices of the AOCS, American Oil Chemists Society. [B39] Oommen, T. V., C. C. Claiborne, E. J. Walsh, and J. P. Baker, “A new vegetable oil based transformer fluid: development and verification,” IEEE Conference on Electrical Insulation and Dielectric Phenomena, Vancouver, BC, Canada, Oct. 2000, pp. 308–312, http://​dx​.doi​.org/​10​.1109/​CEIDP​.2000​.885288. [B40] Paella, T., C. Perrier, Y. Zelu, G. Morin, and M. Saravolac, “Study on flow electrification hazards with ester oils,” IEEE ICDL Conference 2011, File no. 13. [B41] Rapp, K., J. Luksich, and A. Sbravati, “Application of Natural Ester Insulating Liquids in Power Transformers,” My Transf 2014, Turin, Italy, November 18–19, 2014. [B42] Singha, S., R. Asano Jr., G. Frimpong, C. C. Claiborne, and D. Cherry, “Comparative Aging Characteristics between a High Oleic Natural Ester Dielectric Liquid and Mineral Oil,” IEEE Transactions on Dielectrics and Electrical Insulation, vol. 21, no. 1, pp. 149–158, February 2014, http://​dx​.doi​.org/​10​.1109/​ TDEI​.2013​.003713. [B43] Underwriters Laboratories Gas and Oil Equipment Directory, 2006.24 [B44] Unge, M., S. Singha, N. V. Dung, D. Linhjell, S. Ingebrigtsen, and L. E. Lundgaard, “Enhancements in the lightning impulse breakdown characteristics of natural ester dielectric liquids,” Applied Physics Letters, vol. 102, no. 17, p.  172905, 2013, http:/​/​dx​​.doi​​.org/​10​​.1063/​1​​.4803710. [B45] U.S. Environmental Protection Agency, Environmental Technology Verification, Joint Verification Statement for Vegetable Oil-Based Insulation Dielectric Fluid, Statement VS-R-02-02, June 2002, and VS-R02-03, June 2002.

23 24

OECD publications are available from the Organisation for Economic Cooperation and Development (https://​www​.oecd​-ilibrary​.org/​). UL publications are available from Underwriters Laboratories (http://​www​.ul​.com/​).

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[B46] U.S. Environmental Protection Agency, Environmental Technologies Verification (ETV) Reports, EPA 600/R-02/042 and EPA 600/R-02/043, Office of Research and Development, National Center for Environmental Research, Washington, DC. [B47] U.S. Environmental Protection Agency, Test Method OPPTS 835.3110, Office of Prevention, Pesticides, and Toxic Substances Test Guidelines, Washington, DC, Sept. 1995. [B48] Viertel, J., K. Ohlsson, and S. Singha, “Thermal aging and degradation of thin films of natural ester dielectric liquids,” IEEE ICDL Conference 2011, File no. 128, http:/​/​dx​​.doi​​.org/​10​​.1109/​ICDL​​.2011​​.6015478. [B49] Vukovic, D., S. Tenbohlen, J. Harthun, C. Perrier, and H. Fink, “Breakdown Strength of Vegetablebased Oils Under AC and Lightning Impulse Voltages,” IEEE ICDL Conference 2011, File no. 116, http:/​/​dx​​ .doi​​.org/​10​​.1109/​ICDL​​.2011​​.6015468.

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IEEE Std C57.147-2018 IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

Annex B (informative)

Additional technical information B.1  Relative moisture saturation of natural esters compared to conventional mineral oil Natural ester water solubility is expressed by the following equation: Log y = −A / K + B where A B K y

is a constant of a given liquid (contact insulating liquid supplier for the values) is a constant of a given liquid (contact insulating liquid supplier for the values) is 273.1 + °C is milliliters/kilogram (mL/kg) (ppm)

Table B.1—Calculated water saturation values (mg/kg) for natural ester liquids Typical mineral oil

Natural ester (average of 3 types)

22

658

10

36

814

20

55

994

Temperature (°C) 0

30

83

1198

40

121

1427

50

173

1681

60

242

1962

70

332

2269

80

447

2604

90

593

2965

100

773

3354

Modified with permission from Doble Engineering Company, Lewand, L. R., “Laboratory evaluation of several synthetic and agricultural-based dielectric liquids,” Proceedings of the 86th Annual International Conference of Doble Clients, Doble Engineering Company, Watertown, MA, USA, 2001 [B13].

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Modified with permission from Doble Engineering Company, Lewand, L. R., “Laboratory evaluation of several synthetic and agricultural-based dielectric liquids,” Proceedings of the 86th Annual International Conference of Doble Clients, Doble Engineering Company, Watertown, MA, USA, 2001 [B13].

a

b

Source: IEEE Std C57.106™-2002, pages 8–10 [B26].

Figure B.1—Natural ester liquid versus mineral oil saturation curves

B.2  Service-aged natural ester liquids Table B.2—Provisional guidelines for key property values of service-aged natural ester liquids for triggering prompt additional investigation IEEE value for voltage class

Test and ASTM method

≤ 69 kV

≥ 69 kV < 230 kV

≥ 230 kV

≥3

≥3

≥3

≥ 10

≥ 10

≥ 10

Dissipation factor (power factor), ASTM D924, at 25 °C, % Viscosity increase from value at time of initial energization, ASTM D445, at 40 °C, % Acid number, ASTM D974, mg KOH/gm

≥ 0.5

≥ 0.3

≥ 0.3

Flash point, ASTM D92, °C

≤ 275

≤ 275

≤ 275

c

Color, ASTM D1500

≥ 1.5

≥ 1.5

≥ 1.5

Interfacial tension

≤ 10

≤ 12

≤ 14

Inhibitor content

(see footnote d)

(see footnote d)

(see footnote d)

Applies only for transformers originally designed for and filled with unused natural ester liquid. These values are based on limited accelerated aging and field samples taken over a period of up to 20 years. As statistically significant data is developed for each property, the property listing should be moved to Table 4. c For samples with significant discoloration, ASTM D664 may be the preferred test method. d Contact the manufacturer of the specific liquid for recommended inhibitor test method and trigger point values. a

b

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IEEE Std C57.147-2018 IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

B.3 Examples of key properties of mixtures of an unused natural ester with unused mineral oil

Source: McShane, C. P., J. Luksich, and K. J. Rapp, “Retrofilling aging transformer with natural ester based dielectric coolant for safety and life extension,” Proceedings of the IEEE IAS/ PCA Cement Industry Technical Conference, Dallas, TX, USA, May 2003 [B35].

Figure B.2—Effect on the viscosity of a natural ester liquid mixed with mineral oil

Source: “Retrofilling aging transformer with natural ester based dielectric coolant for safety and life extension,” Proceedings of the IEEE IAS/PCA Cement Industry Technical Conference, Dallas, TX, USA, May 2003 [B35].

Figure B.3—Effect on the pour point of a natural ester liquid mixed with mineral oil

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IEEE Std C57.147-2018 IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

Source: “Retrofilling aging transformer with natural ester based dielectric coolant for safety and life extension,” Proceedings of the IEEE IAS/PCA Cement Industry Technical Conference, Dallas, TX, USA, May 2003 [B35].

Figure B.4—Effect on the fire point and flash point of a natural ester liquid mixed with mineral oil

B.4  Relative cooling performance properties Four cooling performance properties of the dielectric coolant are often used for transformer design. These include viscosity, coefficient of expansion, thermal conductivity, and heat capacity. As viscosity is the most significant, limits are listed within the main clauses of this guide. However, for thermal design optimization, it is useful to have the values of the other three properties. Specific typical values of thermal properties for each brand of liquid should be obtained from the liquid manufacturer.

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IEEE Std C57.147-2018 IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

Annex C (informative)

Additional information on fire, safety, environmental, and sustainable properties of natural esters C.1  Fire safety considerations The first “less-flammable” insulating liquids were introduced in the 1970s as the use of polychlorinated biphenyl (PBC) based insulating liquids was being regulated, and production banned in 1979 in the USA. Several non-flammable and less-flammable liquids were introduced as PCB alternatives, but HMWH and silicone liquids became dominant, followed by synthetic esters. Since 1984, listing as a “less-flammable” insulating liquid per the National Fire Protection Association (2017 NEC, Article 50.23), the transformer insulating liquids have had the following two key requirements: — The fire point shall be 300 °C or higher per ASTM test method D92. — The liquid shall be listed “less-flammable” by one or more nationally recognized safety testing laboratories. Most commercially available less-flammable insulating liquids are listed as UL Classified, FM Global Approved, or both. Less-flammable insulating liquids are also recognized as a fire safeguard in Section 15 of the 2017 NESC for distribution and generation substations. Listings and classifications are also available for less-flammable liquid immersed transformers. For UL, transformer listings are per XPLH as ANSI-compliant. Within the UL XPLH listing is the option for additional certification markings to Article 450.23 of the 2017 NEC as UL Classified for use as less-flammable liquid insulated transformer. The FM Global requirements for an FM Approved Transformer insulated with lessflammable insulating liquids are found in FM Standard 3990 [B20]. Natural ester insulating liquids were initially developed in the 1990s as an additional askarel (PCB) alternative. Because less-flammable insulating liquids have inherently higher flash and fire points than that of conventional mineral oil, ASTM D6871 requires a minimum 275 °C flash point and a minimum fire point of 300 °C as received from the natural ester manufacturer. If unused natural ester tests below 300 °C fire point, it should be assumed that it has been contaminated. IEC 61039 designates a class K for insulating liquids with a fire point above 300  °C. This International Standard defines a system for classifying insulating liquids according to fire-point and net calorific value. The characteristics on which the system is based are given together with limiting values in Table C.1 below. Table C.1—Fire classification of liquids Class

Fire point

 

Class

Net calorific value

O

≤ 300 °C

 

1

≥ 42 MJ/kg

K

> 300 °C

 

2

< 42 MJ/kg and ≥ 32 MJ/kg

L

No measurable fire point

 

3

< 32 MJ/kg

Reprinted with permission from CIGRE Technical Brochure 436, Experiences in service with new insulating liquids, CIGRE WG A2-35, 2010 [B9].

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IEEE Std C57.147-2018 IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

Due to increased focus on environmental and sustainable practices by the electrical power industry, and the resulting improved cellulose insulation life when impregnated with natural esters, the industry began applying natural esters for new and retrofilled transformers, including installations not requiring less-flammable liquids. For mineral oil transformers retrofilled with natural esters, due to residual mineral oil in the impregnated cellulose insulation, the flash point can fall below the ASTM minimum value for unused liquid as received. With proper retrofill procedures, the fire point should remain above the 300 °C for natural esters with >350 °C fire point. However, a fire point falling below 300 °C should not impact the functionality of the transformer.

C.2  Environmental and health safety considerations Most currently available natural ester insulating liquids in the USA are edible oil based. Edible vegetable base oils have relatively fast biodegradation rate, and are naturally non-toxic. While the base vegetable oil typically constitutes a minimum 95% of the content of the insulating liquid, there are additives applied, generally for improved pour point and oxidation stability. Because the health and environmental properties of additives can impact the overall safety of the insulating liquid, it may be important to users to be able to determine the overall environmental and health properties. At a minimum, users should request a current SDS from the manufacturer. Currently, the ASTM Guide D6871 does not list minimum requirements for environmental and health related properties. However, there are common tests used for environmental and health assays. These include the following: — Aerobic aquatic biodegradation by test guideline EPA OPPTS 835.3100 [B15] — Ready biodegradability by test guidelines EPA OPPTS 835.3110, Method 301b [B16] — Acute aquatic toxicity by test method OECD 203 [B36] — Acute oral toxicity by test method OECD 420 [B37] — Total life cycle greenhouse gas determination per NIST Building for Environmental and Economic Sustainability (BEES) software evaluation method [B29]

C.3  Sustainability considerations As the electrical power industry increasingly recognizes the need supply chain sustainability, it may be important to factor in the relative sustainability of insulating liquids. To be designated biobased for natural ester based insulating liquids, a U.S. EPA program requires a minimum biobased content of 95%. The method used to determine bio-based content is ASTM D6866 [B7].

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IEEE Std C57.147-2018 IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

Annex D (informative)

Discussion regarding natural ester immersed load tap changers (LTCs) Regulated power transformers are equipped with LTCs to change their ratio and, subsequently, adapt the transformer output voltage to the respective conditions. This enables the power supply network to be kept stable under changing load conditions. Tap changers are complex electro-mechanical devices that also should adapt to high-voltage conditions. This combination makes them unique components in energy supply technology. While the insulating liquid in a transformer has dual tasks of cooling and (in combination with the solid insulation) electrically insulating windings and bushings against high voltage, a suitable insulating liquid for tap changers should also fulfill the following recommendations: — The switching arcs should be cooled and quenched by the surrounding liquid. — All mechanically moving parts (gears, selector contacts, etc.) should be sufficiently lubricated in order to reach a high mechanical life, which correlates to the lifespan of the transformer (>30 years). — The spring-driven switches should be able to help ensure a proper switching sequence of the contact system within the entire permissible insulating liquid temperature range (typically −25 °C to +125 °C). A diverter switch is an example of a spring-driven switch. — A large variety of different high-tech materials may be used inside a tap changer to achieve high electrical and mechanical functionality and a long working life; all of those should be compatible with the insulating liquid used. Thus the dielectric, mechanical, chemical, and thermal properties of the insulating liquids should be evaluated. Historically, tap changers have been optimized to work reliably in mineral oil. If the same tap changer design is used for natural esters, some limitations may apply, due to the differences in certain properties (Frotscher et al. [B21]). For example, the higher viscosity can limit tap changer operation in cold insulating liquids. A tap changer has a limited amount of spring force to operate the diverter mechanism. If the insulating liquid is too viscous, the switching sequence slows down. If, in the worst case, the switching operation begins but cannot be completed, a failure could occur. Viscosity also affects the arc-quenching behavior and can lead to a reduced switching capacity. Natural esters also show different breakdown behavior in long gaps in highly non-uniform electrode configurations such as needle to plane and needle to sphere. This is due to a different streamer propagation mechanism that exists at voltages higher than the partial discharge inception level (Duy et al. [B14] and Liu and Wang [B30]). Fast streamers can develop at significantly lower voltages than in mineral oil. They have a long stopping length so there is a significant possibility they can bridge long insulating liquid gaps and cause a breakdown. This can occur under impulse voltage at inhomogeneous electrode configurations with uncoated electrodes, as commonly applied on tap changers. The geometric shape of tap changer electrodes and contacts is the result of a compromise between mechanical function, mechanical endurance, number of taps, required load current, and electrical insulation. The established compromises for mineral oil cannot be assumed to be optimal for other types of insulating liquids. When electrical insulation is the only function of the insulating liquid, the electrode shape can be optimized for minimum field stress. Thus, the same dielectric performance is achievable. In contrast, non-homogeneous electrode configurations used in tap selectors can result in significantly reduced withstand voltages, depending on the degree of non-homogeneity and insulating

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IEEE Std C57.147-2018 IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

liquid gap length. Such insulating distances have to be designed carefully and adjustments may be necessary to limit the test voltage levels to acceptable values and, consequently, derating. Because the voltage level deratings are specific for different tap changer models, the tap changer manufacturer should be consulted to determine permissible test voltage levels for ac and LI. Distribution class transformers equipped with step voltage regulators probably do not need any deratings relative to immersion in mineral oil because of lower electrical stress conditions. Natural esters require prevention from continuous contact with ambient air (sealed tank design) to avoid oxidation and subsequent increase in the viscosity of the insulating liquid due to polymerization of the ester molecules. Accordingly, for LTCs immersed in natural esters, it may be preferable to use vacuum interrupters where the switching arcs are encapsulated inside vacuum cells. Arc-breaking-in-insulating liquid tap changers can also be used if the conservator is fitted with a rubber bag and the LTC is equipped with a venting one-way breather. A nitrogen filled cycle purged LTC is yet another option. The following are some suggested design guidelines for the application of natural ester insulating liquids in on-load tap changers: — Vacuum LTC models preferably may be used. However, arc-breaking LTC models could be used, depending on the breaking capacity and construction of the preservation system. Discuss the application with the manufacturer. — Avoid highly non-homogeneous fields wherever possible. Consider adding shielding or coatings on uncoated electrodes to avoid partial discharge inception, and to prevent streamers from formation and propagation. Alternatively, consider selecting a higher voltage class (longer insulating liquid gaps) than required by the application for mineral oil. — For electrode setups where constructive measures are needed but not possible (conventional tap selector design), lower withstand voltages for test voltage levels for applied voltage and lightning impulse tests (IEC: AC and LI) than in mineral oil may be required. — Contact the manufacturer regarding low temperature limits for the operation of the tap changer. The tap changer should not be operated if the temperature of the insulating liquid surrounding the moving parts is lower than the specified low temperature limit. — Mitigate prolonged arcing times on the reversing or coarse tap switch by reducing the admissible switching capacity, if necessary. The tap changer manufacturer should take appropriate measures to compensate for this reduction.

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