Revised IEEE Guide for Acceptance and Maintenance of Natural Ester Fluids in Transformers

Presented By:
C. Patrick McShane
Kevin Rapp
Cargill Inc.
TechCon 2019


The IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers (Guide) recommends tests and evaluation procedures as well as criteria and methods of maintenance for natural ester based insulating liquids. Also, this IEEE Guide describes methods of reconditioning, field applications, and diagnostics. IEEE first published its guide for natural ester insulating liquids in 2008, approximately 11 years after the initial utility trial transformers were put in service. In February 2018, the IEEE SA Standard Board approved the first revision, IEEE Std. C57.148-2018. The focus of this paper is to: 1) highlight the most significant of the nearly two hundred technical and editorial revisions; 2) briefly summarize other relevant standards involving natural ester; and 3)provide a brief discussion of a new IEEE SA PAR for consolidation of IEEE insulating liquid guides into a single guide.


Conventional wisdom infers that standard development lags technical advancements. That is certainly true in the development of IEEE‘s guide for natural ester insulating liquid because IEEE guides for insulating liquids utilize ASTM standards for the base for determining limits for new, unused liquids. ASTM first established its Standard Specification for Natural (Vegetable Oil) Ester Fluids Used in Electrical Apparatus in 2003, later revised in 2008 and again in 2017.

ASTM’s 2017 version forms the basis of the acceptable values for receipt of shipments of unused natural ester fluid in the revised IEEE Guide. Because 10 years had spanned between the original IEEE Guide and the revised Guide, important field technical performance data was gathered and incorporated into the revision. In addition, with increased applications in large power transformers, the scope of ratings was increased. Finally, as with most standard revisions, improved wording and formatting was incorporated, which indeed led to the greatest number of changes.

Significant additions for the revised Guide are found in its Annex. New to the Guide is Annex C, which provides additional information on fire and environmental safety, and on sustainable properties of natural esters. Also new is Annex D that discusses natural ester immersed load tap changers.

This paper focuses mainly on technical changes as these are typically more significant to Guide users than the more numerous editorial changes. Thus, only the most important editorial revisions are included here.

This paper concludes with: 1) an overview of other important natural ester standards of ASTM and IEC that are being expanded and developed as a result of rapidly growing application and research; and 2) a brief discussion of a current IEEE SA PAR for the consolidation of various insulating liquid guides into a single all-encompassing Guide.

Part 1: Changes in the first revision of C57.147

Listed below are the key revisions that appear in the 2008 Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers. This listing is organized by section number and sub-number.

Key Revisions of Section 1: Overview

1.2 Purpose: The purpose of the original Guide was expanded from “small distribution class to medium power class transformers” to application “in distribution and power transformers and other liquid-filled equipment”.

1.3 System design: A statement was added to advise that if the purpose of selecting a natural ester insulating liquid is for the explicit purpose to comply with the National Electrical Code, then the fire point must be maintained not less than 300ºC.

1.4 Background information on mixtures of natural ester fluids with other insulating liquids: The information in the note of the original Guide was essentially transferred to Section 1.3 in the revised Guide.

Key revisions of Section 2: Normative references

  • Eliminated or moved to Annex
    • ASTM D2112 (Oxidation stability inhibited Mineral Oil [MO])
    • ASTM D2440 (Oxidation stability MO)
    • ASTM D2945 (Gas Content)
    • IEEE Std. 62 (Field Testing)
    • AOCS Methods and Practices
  • Added
    • IEC 61039 (Classification of insulating liquids)
    • IEC 61125 (Oxidation stability)
    • IEC 62770 (Unused natural ester limits)
    • IEEE C57.106 (MO guide)
    • IEEE C57.152 (Equipment diagnostic field testing)
    • IEEE C57.155 (DGA esters)

Key revisions of Section 3: Acronyms and abbreviations

  • Revised
    • MSDS changed to SDS
  • Added

Key revision to Section 4: Liquid tests and their significance

4.1 General

  • Added
    • Two paragraphs were added to this sub-section. One discusses the progress of ASTM D27 of expanding its standards to include alternative insulating liquids to its historical standards based on petroleum insulating liquid. The other paragraph explains why there may be differences in acceptance value limits between esters and MO and that some of the ASTM test methods may require modifications.

4.1 Table 1 – List of suitable ASTM or IEC test methods for natural esters

  • Omitted
    • Sampling practice D3305
    • Gas analysis D2945
    • Detectable PCB D4050
  • Added
    • Pour point D5949, D5950
    • Oxidation Stability IEC 61125, Method C
    • Furanic compounds D5837

4.2 Sampling Practice

  • Omitted
    • D3305

4.3 Acid number

  • Added
    • Significant discussion was added to this section with five new paragraphs that discuss the important differences between natural esters and MO for acid formation and acid type and impact. Briefly, vegetable oil molecules naturally contain “fatty acids.” Additional acids produced due to aging are typically “long chain” acids that are much less reactive than short chain acids produced in aging MO.

4.4 Dielectric Breakdown

  • Added
    • A note was added advising that the D877 method is in the process of being eliminated in the ASTM and IEEE standards and guides. Thus, D1816 is the preferred method.

4.5 Impulse breakdown

  • Added
    • Discussion of impact of symmetry of voltage stress fields and polarity.

4.6 AC loss characteristics: No change.

4.7 Interfacial Tension

  • Added
    • Discussion of the impact of surfactants.

4.8 Color

  • Added
    • Discussion of differences between color changes in natural esters vs. MO.

4.9 Viscosity: Editorial changes only.

4.10 Flash point and fire point

  • Omitted
    • Discussion on NEC moved to another section.
  • Added
    • Discussion of details on fire point testing, values relative to MO, and related ASTM minimum value requirements. A reference to Annex C is included.

4.11 Density: No change.

4.12 Pour point

  • Added
    • Included ASTM D5949 and D 5950 as acceptable test methods.

4.13 Volume resistivity: No change.

4.14 Gas analysis

  • Added
    • Includes ASTM D3284 as an acceptable test method. This section is revised due to significant developments in DGA of esters since the 2008 Guide and to improve its readability. It refers to IEEE C57.155 (DGA guide for esters) for additional details.

4.15 Oxidation stability

  • Added
    • Three paragraphs on the issue of oxidation stability of natural esters relative to MO focus on the improved understanding of the oxidation mechanism, different esters relative stability, prevention, and differences in by-products of oxidation between natural esters and MO. Also mentioned is that IEC 61125 Method C is now included in IEC 62770 as a method for measuring the oxidation stability of natural esters insulating liquids.

4.16 Water content

  • Added
    • Discussion on reagents that are suitable for mineral oil may not be suitable for natural esters as stated in ASTM D1533 Annex A1 “alternative solvent systems.”

4.17 Visual examination field test (ASTM D1524)

  • Omitted
    • ASTM D1500 as a preferred method.

4.18 Gassing due to electrical stress (ASTM D2300): No change.

4.19 Corrosive sulfur (ASTM D1275): No change.

4.20 PCBs (ASTM D4059): No change.

4.21 Furanic compounds (ASTM D5837)

  • Added
    • The sub-section is new to the revised Guide.

Key Revisions to Section 5: No significant change.

Key Revisions to Section 6: Handling and Evaluation

6.1 General: No change.

6.2 Shipping containers: No change.

6.3 Check tests on receipt

  • Omitted
    • Table 2: Eliminated water content limits differences for different types of shipping containers. 
    • Table 2: Eliminated dielectric strength differences for different types of shipping containers. 
  • Added
    • Table 2: Included two methods for testing pour point, D5949 and D5950.

6.4 Handling and Storage

6.4.1 General

  • Added
    • Underground tanks added as an alternate preferred storage method. • Warning box that highlights the need for grounding pumps and lines to prevent static charge build-up. 
  • Changed
    • Reduced the recommended temperature limit for localized heater surface temperature from 180 ºC to 100 ºC. 

6.4.2 Venting: No change.

6.4.3 Pumps:

  • Omitted
    • Redundant statement on determining correct pump size. 
  • Added
    • Warning box that highlights issue of grounding hoses, filter systems, accessible transformer bushings and equipment to prevent static charge build-up during the filling process. 

6.5 Recommended transformer filling procedures

  • Omitted
    • Eliminated specific range of processing temperatures 
  • Added
    • Note 1 now contains discussion of the National Electrical Safety Code and IEC  Standard 61039, both related to fire point requirements. 
    • New Note 2 discusses the potential impact on flashpoint due to contamination of residual kerosene from improper vapor phasing process or due to residual mineral oil if used during the vapor phasing process for initial impregnation. 
    • New Note 3 discusses procedures for factory units that drain natural esters for transportation.  

Key revisions of Section 7: Evaluation of natural ester in new equipment

7.0 Table 3 Test limits for key properties

  • Changed
    • Changed from 2 from 2 to 3 voltage class columns to include values for >230 kV.
    • Changed voltage limit for >69 kV ≤ 230 kV from 52 to 55 kV. 
  • Added
    • Total dissolved gas, ASTM D2612 limit for >230 kV class. 
    • Second note providing information on sampling and evaluation of D2612.
    • Added information regarding recommendations on wait times (in hours) for proper impregnation for distribution and power transformers.

Key revisions of Section 8: Maintenance of in-service natural ester liquid

8.2 Laboratory Screening for classifying

  • Changed
    • Switched some tests between required and helpful but not required categories.  Viscosity is now listed as required and neutralization and IFT are now listed  useful but not required. 
  • Omitted
    • Furanic compounds. 
  • Added
    • Note that for DGA refer to IEEE C57.155. 

8.3 Table of test limits for in-service natural ester liquid

  • Changed
    • Table number now Table 4 (was 5
  • Omitted
    • Removed Note D regarding water content and relative saturation 
  • Added
    • Water content limits established (prior was only a note, without numeric values listed). 
    • Minor editorial revision to Note C. 
    • New Note D, information on designation of “less-flammable” and fire point, as related to possible indication of mineral oil contamination in the field. 

8.4 Reconditioning

  • Now includes recommendation to use a downstream in-line moisture hygrometer with an alarm calibrated for use in natural esters to indicate when the moisture-removing material has become saturated. 

8.5 Reclaiming

  • Now includes four additional types of suitable materials for moisture removal and suggests that mixtures of absorbing materials can be beneficial. 

8.6 Mixtures of different types of insulating liquids

  • Now includes a reference to Annex B.3 for additional information. 

Key revisions of Section 9: General environmental procedures

9.1 General

  • Omitted
    • Eliminated last sentence in first paragraph related to the EPA Environmental  Technologies Verification (ETV) program, which ended operation in early 2014.
  • Added 
    • Now includes statement that for natural esters contaminated with mineral oil,  such as flushing residual mineral oil prior to retro filling, to use the same disposal requirements for mineral oil.

9.4 Spills on soil

  • Omitted
    • Eliminated references to EPA Test Method OPPTS 835.3110 regarding “ultimate  biodegradation.” 

Informative (non-normative) Annexes

Key revisions of Annex A – Bibliography

  • Reformatted
    • Removed the differentiation between A.1 Sources, and A.2 Reference papers and sources.  
  • Omitted
    • ASTM D88 on Saybolt viscosity. 
  • Added
    • 30 additional bibliographical references to the original 18. 

Key revisions of Annex B – Additional technical information

  • Omitted
    • Original clause B.1 on alternative oxidation stability tests removed (now covered by main guide clauses). 
    • Original B.3 on DGA data (now covered by IEEE C57.155). 

Revised (Old B.3, now B.1) Moisture Saturation

  • The original clause was specific to the data based on a specific natural ester base vegetable oil to a general formula that can use specific constants to 3  different available types of two additional base oils available during the revision development. 
  • Revised the water stability formula to allow using specific constants of different types of natural esters insulating liquids. 
  • Table B.1, 3rd column now shows values using an average of 3 natural ester liquids using 3 different types of base vegetable oils, all having nearly a quantum higher value than mineral oil at 50 ºC. 

Revised (Old B.4, now B.2) – Test values of service-aged samples for triggering additional investigation

  • Omitted
    • Water content (now covered in normative section) 
  • Changed
    • Note b, removed the word “very” before “limited” regarding availability of field samples showing significant changes and changed the collection period from 10  to 20 years. 
  • Added
    • Interfacial tension limits. 
    • Note c, regarding using ASTM D664 rather than D974 to test for acid number when the samples have significant discoloration.

Annex C (New) – Additional information on fire, safety, environmental, and sustainable properties of natural esters

C.1 – Fire safety considerations: Provides more details regarding “less-flammable” classification of high fire point insulating liquids that became available due to the demise of PCB liquids. Also includes a discussion of fire classification of insulating liquids by the IEC.

C.2: Environmental and health safety considerations: Because environmental and health properties of insulating liquids base oils and additives can impact the overall safety of the insulating liquid, this section provides a listing of available tests suitable to help determine the overall related properties. The tests listed include those of EPA and OCED. For relative life cycle greenhouse gas determination, a NIST method is listed.

C.3: As utilities increasingly recognize and factor in sustainability of purchased equipment, ASTM D6855 method to determine biobased content. Also discussed is the EPA BIOBASED designation and its required minimum biobased content for insulating liquids.

Annex D (New) – Discussion regarding natural ester immersed LTCs. This section is a bit out of place for an insulating liquid guide as it relates more to equipment design guidelines than insulating liquid properties. One major manufacturer convinced the WG that certain dielectric, mechanical, and chemical properties’ differences between natural esters and mineral oil should be considered in the design of LTCs. This section reviews the significant differences in properties, particularly related to viscosity and non-uniform fields. This informative annex lists suggested LTC design guidelines.

Part 2: Related ASTM, IEEE, and IEC Standards Status 

ASTM Natural Ester Standard Specification D 6871 

The ASTM D6871 natural ester (NE) standard specification was re-issued at the end of 2017 without significant revision. Presently, there are several editorial and technical revisions desired to satisfy D27 members before the next mandatory revision in 2024. Some of the more significant possible future revisions are as follows:

  • Change term of NE fluids to NE liquids. 
  • Color test as a Table 1 requirement was removed to account for dyed NE liquids.
  • Saybolt viscosity was removed as an allowed test as it is no longer used.
  • Automated pour point methods were added. 
  • The standard shall conform to the ASTM Form & Style manual. 
  • Supplemental design information was changed to SI units. 
  • Pour point and cold storage statements were revised to include the possibility of additives to enhance these properties. 
  • Unused NE liquid was removed from the scope. 
  • NE liquids can be obtained from a 3rd party supplier so supplier or manufacturer can supply the NE liquid. 
  • Additives should or shall be declared with types and amounts.
  • Inclusion of an oxidation stability test method and limit value when they are developed within D27. 

ASTM Synthetic Ester Liquid (Under Development)

ASTM D27 is developing a standard for synthetic ester liquids as work item WK 46195, presently in the 2nd balloting.

IEC Unused Natural Ester Liquids Standard 62770

The initial IEC proposal to standardize NE liquids was initiated by the USNC in July 2005 and ratified at the October 2005 Plenary meeting. The final IEC 62770 standard for unused natural ester liquid for transformers was approved in 2013. The main difference is that the IEC standard uses ISO test methods vs. ASTM test methods.

Natural Ester Use and Maintenance Guide IEC 62975 (Under Development)

With nearly one million transformers operating with natural ester liquids by 2013, end users requested that IEC guidance was needed for effective maintenance practices. At the November 2014 IEC Plenary meeting, a new WG was formed to develop a user’s maintenance guide for natural ester insulating liquids. The development of a new standard identified as IEC 62975 is in process. The WG used IEC 60422 for MO as the template for the new use and maintenance standard.

A database of natural ester liquid properties from thousands of operating transformers was collected for diagnosing good, fair, and poor insulating liquid and transformer condition. The 62975 standard is in the 2nd committee draft (CD) stage, to be circulated in 2019 for approval to elevate the project to Committee Draft for Vote (CDV).

IEC TC 14 Functional Requirements of Insulating Liquids Used in Transformers

An IEC group (AHG-1) was formed in 2016 to develop the functional requirements for liquids used in power transformers. A document containing numerous functional requirements was approved at the 2018 IEC Plenary meeting. At that meeting, the delegates recommended that an IEC Technical Brochure be prepared for use in both IEC TC 10 and TC 14. As of this writing, the Technical Brochure draft is being prepared and will be circulated to the national committees of IEC.

IEEE PAR PC57.166 for the Consolidation of IEEE Insulating Liquid Guides

Within the Insulating Fluids Sub-Committee of the IEEE PES Transformer Committee, the WG is working on the development of a single guide to encompass the current existing guides for mineral oil, natural esters, silicone, and HMWH. The IEEE SA PAR is active with the designation PC57.166 – Acceptance and Maintenance of Insulating Liquids in Transformers and Related Equipment.


1. IEEE Guide for Acceptance and Maintenance of Natural ester Insuolating Liquid in Transformers, IEEE Std C57.147TM – 2018.
2. IEEE Guide for Acceptance and Maintenance of Natural Ester Fluids in Transformers, IEEE Std C57.147TM – 2008.
3. ASTM D6871 Standard Specification for Natural (Vegetable Oil) Ester Fluids Used in Electrical Apparatus.
4. IEC 662770 Ed/ 1. 2013 Fluids for electrotechnical applications – Unused natural esters for transformer and similar electrical equipment, International Electrotechnical Commission.

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