Electrical Testing Reference

Liquid-filled transformers

Review status: Agent reviewed

First published
Not yet published
Last reviewed
Not yet reviewed
Last substantive change
2026-10-13
Revision
1
Editor of record
Editor of record not yet appointed
Reviewing engineer
Not yet engineer-reviewed

A liquid-filled transformer is a transformer whose core and windings are immersed in an insulating liquid that both insulates and cools them. [1] IEEE C57.12.00-2021 sets general electrical and mechanical requirements for the liquid-immersed class, which takes in distribution, power and regulating units, and field testing of the class is covered by section 7.2.2 of ANSI/NETA ATS-2025 at acceptance and by section 7.2.2 of ANSI/NETA MTS-2023 in maintenance. [2] [3] [4] [5]

Description

Because the liquid forms part of the insulation system, testing of the class extends to the liquid as well as to the windings and the solid insulation, with sampling and laboratory quality tests of the liquid among the listed tests. [1] [6] [7] IEEE C57.12.00-2021 applies to units whose highest-voltage winding is rated above a stated threshold, and it lists the transformer types it excludes. [2]

Tests at acceptance

Acceptance testing under section 7.2.2 of ANSI/NETA ATS-2025 starts with visual and mechanical inspection. [8] [4]

The insulation system is examined by insulation resistance, insulation power factor or dissipation factor, bushing power factor and capacitance, dielectric frequency response and core insulation resistance tests. [9] [10] [11] [12] [13] The windings and the magnetic circuit are checked by turns ratio, winding resistance and dynamic winding resistance, excitation current, leakage reactance and sweep frequency response analysis. [14] [15] [16] [17] [18] [19]

The insulating liquid is sampled for laboratory quality tests and dissolved gas analysis, and the oxygen content of any gas blanket is measured. [6] [7] [20] [21] Connections, accessories and controls are covered by bolted connection resistance, tests of surge arresters, resistance of resistors and coils and verification of controls and functions. [22] [23] [24] [25]

Tests in maintenance

ANSI/NETA MTS-2023 applies a parallel set of tests to transformers in service at section 7.2.2. [5] On the insulation side these include insulation resistance and insulation power factor or dissipation factor, and on the winding side turns ratio and winding resistance. [26] [27] [28] [29] Insulating liquid quality tests and dissolved gas analysis follow the condition of the liquid over the service life of the unit. [30] [31]

Governing standards

Evaluation of the results draws on tables referenced from section 7.2.2 of ANSI/NETA ATS-2025: Table 100.3 for dissipation factor or power factor, Table 100.4 for insulating fluid limits and Table 100.5 for transformer insulation resistance. [32] General requirements for the class come from IEEE C57.12.00-2021, which the IEEE catalog lists as an active standard. [33] [34]

Notes

  1. Electrical Testing Reference editorial taxonomy, 2026-10 edition. A liquid-filled transformer is a transformer whose core and windings are immersed in an insulating liquid that serves both as electrical insulation and as a cooling medium. Datum equip.liquid-filled-transformers.definition
  2. IEEE C57.12.00-2021. The standard sets general electrical and mechanical requirements for liquid-immersed distribution, power, autotransformer and regulating transformers whose highest-voltage winding is above the low-voltage class stated in the datum value, and it lists the transformer types it excludes.Value: 601 V. Datum std.ieee-c57-12-00.scope
  3. IEEE C57.12.00-2021. IEEE C57.12.00-2021 is the 2021 edition of the IEEE document titled IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers, published on 2022-01-21.Value: 2021. Datum std.ieee-c57-12-00.edition.2021
  4. ANSI/NETA ATS-2025, clause 7.2.2. Section 7.2.2 of ANSI/NETA ATS-2025 covers the acceptance testing of liquid-filled transformers. Datum equip.liquid-filled-transformers.ats-section
  5. ANSI/NETA MTS-2023, clause 7.2.2. Section 7.2.2 of ANSI/NETA MTS-2023 covers the maintenance testing of liquid-filled transformers. Datum equip.liquid-filled-transformers.mts-section
  6. ANSI/NETA ATS-2025, clause 7.2.2.B.14. Insulating Liquid Sampling is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.B.14 of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.insulating-liquid-sampling.equipment.transformer-fluid.acceptance(provenance pending)
  7. ANSI/NETA ATS-2025, clause 7.2.2.B.14. Insulating Liquid Quality Tests is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.B.14 of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.insulating-liquid-quality-tests.equipment.transformer-fluid.acceptance(provenance pending)
  8. ANSI/NETA ATS-2025, clause 7.2.2.A. Visual and Mechanical Inspection is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.A of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.visual-and-mechanical-inspection.equipment.transformer-fluid.acceptance(provenance pending)
  9. ANSI/NETA ATS-2025, clause 7.2.2.B.2. Insulation Resistance is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.B.2 of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.insulation-resistance.equipment.transformer-fluid.acceptance(provenance pending)
  10. ANSI/NETA ATS-2025, clause 7.2.2.B.4. Insulation Power Factor Dissipation Factor is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.B.4 of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.insulation-power-factor-dissipation-factor.equipment.transformer-fluid.acceptance(provenance pending)
  11. ANSI/NETA ATS-2025, clause 7.2.2.B.5. Bushing Power Factor and Capacitance is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.B.5 of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.bushing-power-factor-and-capacitance.equipment.transformer-fluid.acceptance(provenance pending)
  12. ANSI/NETA ATS-2025, clause 7.2.2.B.13. Dielectric Frequency Response is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.B.13 of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.dielectric-frequency-response.equipment.transformer-fluid.acceptance(provenance pending)
  13. ANSI/NETA ATS-2025, clause 7.2.2.B.11. Transformer Core Insulation Resistance is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.B.11 of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.transformer-core-insulation-resistance.equipment.transformer-fluid.acceptance(provenance pending)
  14. ANSI/NETA ATS-2025, clause 7.2.2.B.3. Transformer Turns Ratio is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.B.3 of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.transformer-turns-ratio.equipment.transformer-fluid.acceptance(provenance pending)
  15. ANSI/NETA ATS-2025, clause 7.2.2.B.8. Winding Resistance is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.B.8 of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.winding-resistance.equipment.transformer-fluid.acceptance(provenance pending)
  16. ANSI/NETA ATS-2025, clause 7.2.2.B.9. Dynamic Winding Resistance is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.B.9 of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.dynamic-winding-resistance.equipment.transformer-fluid.acceptance(provenance pending)
  17. ANSI/NETA ATS-2025, clause 7.2.2.B.6. Transformer Excitation Current is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.B.6 of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.transformer-excitation-current.equipment.transformer-fluid.acceptance(provenance pending)
  18. ANSI/NETA ATS-2025, clause 7.2.2.B.10. Leakage Reactance is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.B.10 of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.leakage-reactance.equipment.transformer-fluid.acceptance(provenance pending)
  19. ANSI/NETA ATS-2025, clause 7.2.2.B.7. Sweep Frequency Response Analysis is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.B.7 of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.sweep-frequency-response-analysis.equipment.transformer-fluid.acceptance(provenance pending)
  20. ANSI/NETA ATS-2025, clause 7.2.2.B.15. Dissolved Gas Analysis is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.B.15 of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.dissolved-gas-analysis.equipment.transformer-fluid.acceptance(provenance pending)
  21. ANSI/NETA ATS-2025, clause 7.2.2.B.12. Gas Blanket Oxygen Content is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.B.12 of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.gas-blanket-oxygen-content.equipment.transformer-fluid.acceptance(provenance pending)
  22. ANSI/NETA ATS-2025, clause 7.2.2.B.1. Bolted Connection Resistance is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.B.1 of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.bolted-connection-resistance.equipment.transformer-fluid.acceptance(provenance pending)
  23. ANSI/NETA ATS-2025, clause 7.2.2.B.17. Surge Arrester and SPD Tests is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.B.17 of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.surge-arrester-and-spd-tests.equipment.transformer-fluid.acceptance(provenance pending)
  24. ANSI/NETA ATS-2025, clause 7.2.2.B.18. Resistor and Coil Resistance is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.B.18 of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.resistor-and-coil-resistance.equipment.transformer-fluid.acceptance(provenance pending)
  25. ANSI/NETA ATS-2025, clause 7.2.2.B.19. Control and Functional Verification is specified for liquid-filled transformers in acceptance testing at clause 7.2.2.B.19 of ANSI/NETA ATS-2025.Value: liquid-filled transformers. Datum tests.control-and-functional-verification.equipment.transformer-fluid.acceptance(provenance pending)
  26. ANSI/NETA MTS-2023, clause 7.2.2.B.2. Insulation Resistance is specified for liquid-filled transformers in maintenance testing at clause 7.2.2.B.2 of ANSI/NETA MTS-2023.Value: liquid-filled transformers. Datum tests.insulation-resistance.equipment.transformer-fluid.maintenance(provenance pending)
  27. ANSI/NETA MTS-2023, clause 7.2.2.B.4. Insulation Power Factor Dissipation Factor is specified for liquid-filled transformers in maintenance testing at clause 7.2.2.B.4 of ANSI/NETA MTS-2023.Value: liquid-filled transformers. Datum tests.insulation-power-factor-dissipation-factor.equipment.transformer-fluid.maintenance(provenance pending)
  28. ANSI/NETA MTS-2023, clause 7.2.2.B.3. Transformer Turns Ratio is specified for liquid-filled transformers in maintenance testing at clause 7.2.2.B.3 of ANSI/NETA MTS-2023.Value: liquid-filled transformers. Datum tests.transformer-turns-ratio.equipment.transformer-fluid.maintenance(provenance pending)
  29. ANSI/NETA MTS-2023, clause 7.2.2.B.8. Winding Resistance is specified for liquid-filled transformers in maintenance testing at clause 7.2.2.B.8 of ANSI/NETA MTS-2023.Value: liquid-filled transformers. Datum tests.winding-resistance.equipment.transformer-fluid.maintenance(provenance pending)
  30. ANSI/NETA MTS-2023, clause 7.2.2.B.14. Insulating Liquid Quality Tests is specified for liquid-filled transformers in maintenance testing at clause 7.2.2.B.14 of ANSI/NETA MTS-2023.Value: liquid-filled transformers. Datum tests.insulating-liquid-quality-tests.equipment.transformer-fluid.maintenance(provenance pending)
  31. ANSI/NETA MTS-2023, clause 7.2.2.B.15. Dissolved Gas Analysis is specified for liquid-filled transformers in maintenance testing at clause 7.2.2.B.15 of ANSI/NETA MTS-2023.Value: liquid-filled transformers. Datum tests.dissolved-gas-analysis.equipment.transformer-fluid.maintenance(provenance pending)
  32. ANSI/NETA ATS-2025, clause 7.2.2. Section 7.2.2 of ANSI/NETA ATS-2025 refers to Table 100.3 for dissipation factor or power factor, to Table 100.4 for insulating fluid limits and to Table 100.5 for transformer insulation resistance. Datum equip.liquid-filled-transformers.ats-tables
  33. IEEE C57.12.00-2021. The publisher catalog gives the full title of IEEE C57.12.00-2021 as IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers.Value: IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers. Datum std.ieee-c57-12-00.title
  34. IEEE C57.12.00-2021. The IEEE catalog lists IEEE C57.12.00-2021 with the status Active Standard.Value: Active Standard. Datum std.ieee-c57-12-00.status

Data records

  • Section 7.2.2 of ANSI/NETA ATS-2025 covers the acceptance testing of liquid-filled transformers. Source: ANSI/NETA ATS-2025, clause 7.2.2. Verified 2026-10-13. Review status: Agent reviewed. Record equip.liquid-filled-transformers.ats-section
  • Section 7.2.2 of ANSI/NETA ATS-2025 refers to Table 100.3 for dissipation factor or power factor, to Table 100.4 for insulating fluid limits and to Table 100.5 for transformer insulation resistance. Source: ANSI/NETA ATS-2025, clause 7.2.2. Verified 2026-10-13. Review status: Agent reviewed. Record equip.liquid-filled-transformers.ats-tables
  • A liquid-filled transformer is a transformer whose core and windings are immersed in an insulating liquid that serves both as electrical insulation and as a cooling medium. Source: Electrical Testing Reference editorial taxonomy, 2026-10 edition. Verified 2026-10-13. Review status: Agent reviewed. Record equip.liquid-filled-transformers.definition
  • Section 7.2.2 of ANSI/NETA MTS-2023 covers the maintenance testing of liquid-filled transformers. Source: ANSI/NETA MTS-2023, clause 7.2.2. Verified 2026-10-13. Review status: Agent reviewed. Record equip.liquid-filled-transformers.mts-section

Related entries

Sources

  • ANSI/NETA ATS-2025, Acceptance Testing Specifications for Electrical Power Equipment and Systems, 2025 edition, NETA. Cited: clause 7.2.2; clause 7.2.2.B.14; clause 7.2.2.A; clause 7.2.2.B.2; clause 7.2.2.B.4; clause 7.2.2.B.5; clause 7.2.2.B.13; clause 7.2.2.B.11; clause 7.2.2.B.3; clause 7.2.2.B.8; clause 7.2.2.B.9; clause 7.2.2.B.6; clause 7.2.2.B.10; clause 7.2.2.B.7; clause 7.2.2.B.15; clause 7.2.2.B.12; clause 7.2.2.B.1; clause 7.2.2.B.17; clause 7.2.2.B.18; clause 7.2.2.B.19. Held as: Purchased single-user copy. Publisher page
  • ANSI/NETA MTS-2023, Maintenance Testing Specifications for Electrical Power Equipment and Systems, 2023 edition, NETA. Cited: clause 7.2.2; clause 7.2.2.B.2; clause 7.2.2.B.4; clause 7.2.2.B.3; clause 7.2.2.B.8; clause 7.2.2.B.14; clause 7.2.2.B.15. Held as: Purchased single-user copy. Publisher page
  • Electrical Testing Reference editorial taxonomy, Electrical Testing Reference editorial taxonomy of test methods, 2026-10 edition, Electrical Testing Reference. Held as: Own observation.
  • IEEE C57.12.00-2021, IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers, 2021 edition, IEEE. Held as: Public page. Publisher page

Obtaining the documents

Changelog (47 entries)
DateRevisionChangeReasonBy
2026-10-13created (equip.liquid-filled-transformers.ats-section)method fact restated from the licensed-copy digest (Task 10 exemplars)agent
2026-10-13provenance noted (equip.liquid-filled-transformers.ats-section)locator from the NETA index table of contents (section and table numbers, titles and tables_referenced only); no body text usedagent
2026-10-13reviewed (equip.liquid-filled-transformers.ats-section)fresh-context review task-10-exemplars-review.md + fix round 1agent
2026-10-13created (equip.liquid-filled-transformers.ats-tables)method fact restated from the licensed-copy digest (Task 10 exemplars)agent
2026-10-13provenance noted (equip.liquid-filled-transformers.ats-tables)locator from the NETA index table of contents (section and table numbers, titles and tables_referenced only); no body text usedagent
2026-10-13reviewed (equip.liquid-filled-transformers.ats-tables)fresh-context review task-10-exemplars-review.md + fix round 1agent
2026-10-13created (equip.liquid-filled-transformers.definition)editorial definition of general physics (Task 10 exemplars)agent
2026-10-13reviewed (equip.liquid-filled-transformers.definition)fresh-context review task-10-exemplars-review.md + fix round 1agent
2026-10-13created (equip.liquid-filled-transformers.mts-section)method fact restated from the licensed-copy digest (Task 10 exemplars)agent
2026-10-13provenance noted (equip.liquid-filled-transformers.mts-section)locator from the NETA index table of contents (section and table numbers, titles and tables_referenced only); no body text usedagent
2026-10-13reviewed (equip.liquid-filled-transformers.mts-section)fresh-context review task-10-exemplars-review.md + fix round 1agent
2026-10-131created hand-written exemplar for the article generator (Task 10)agent
2026-10-13created (std.ieee-c57-12-00.edition.2021)extracted from public pageagent
2026-10-13reviewed (std.ieee-c57-12-00.edition.2021)fresh-context agent review, research-review.mdagent
2026-10-13created (std.ieee-c57-12-00.scope)extracted from public pageagent
2026-10-13reviewed (std.ieee-c57-12-00.scope)fresh-context agent review, research-review.mdagent
2026-10-13edited (std.ieee-c57-12-00.scope)statement reworded; figure moved to value (prose rule 7)agent
2026-10-13created (std.ieee-c57-12-00.status)extracted from public pageagent
2026-10-13reviewed (std.ieee-c57-12-00.status)fresh-context agent review, research-review.mdagent
2026-10-13created (std.ieee-c57-12-00.title)extracted from public pageagent
2026-10-13reviewed (std.ieee-c57-12-00.title)fresh-context agent review, research-review.mdagent
2026-10-13created (tests.bolted-connection-resistance.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.bushing-power-factor-and-capacitance.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.control-and-functional-verification.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.dielectric-frequency-response.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.dissolved-gas-analysis.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.dissolved-gas-analysis.equipment.transformer-fluid.maintenance)seeded from the test taxonomyagent
2026-10-13created (tests.dynamic-winding-resistance.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.gas-blanket-oxygen-content.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.insulating-liquid-quality-tests.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.insulating-liquid-quality-tests.equipment.transformer-fluid.maintenance)seeded from the test taxonomyagent
2026-10-13created (tests.insulating-liquid-sampling.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.insulation-power-factor-dissipation-factor.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.insulation-power-factor-dissipation-factor.equipment.transformer-fluid.maintenance)seeded from the test taxonomyagent
2026-10-13created (tests.insulation-resistance.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.insulation-resistance.equipment.transformer-fluid.maintenance)seeded from the test taxonomyagent
2026-10-13created (tests.leakage-reactance.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.resistor-and-coil-resistance.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.surge-arrester-and-spd-tests.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.sweep-frequency-response-analysis.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.transformer-core-insulation-resistance.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.transformer-excitation-current.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.transformer-turns-ratio.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.transformer-turns-ratio.equipment.transformer-fluid.maintenance)seeded from the test taxonomyagent
2026-10-13created (tests.visual-and-mechanical-inspection.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.winding-resistance.equipment.transformer-fluid.acceptance)seeded from the test taxonomyagent
2026-10-13created (tests.winding-resistance.equipment.transformer-fluid.maintenance)seeded from the test taxonomyagent

Cite this page

Cite the governing standard for the fact itself; cite this page as the finding aid.

Permanent revision URL: https://www.electricaltestingreference.com/equipment/liquid-filled-transformers/rev/1/. Current version: https://www.electricaltestingreference.com/equipment/liquid-filled-transformers/.

Plain

"Liquid-filled transformers." Electrical Testing Reference. Revision 1, 2026-10-13. https://www.electricaltestingreference.com/equipment/liquid-filled-transformers/rev/1/.

Legal (Bluebook, internet source)

Electrical Testing Reference, Liquid-filled transformers (rev. 1, Oct. 13, 2026), https://www.electricaltestingreference.com/equipment/liquid-filled-transformers/rev/1/.

BibTeX

@misc{etr-equipment-liquid-filled-transformers-rev1,
  title = {Liquid-filled transformers},
  howpublished = {\url{https://www.electricaltestingreference.com/equipment/liquid-filled-transformers/rev/1/}},
  organization = {Electrical Testing Reference},
  publisher = {Electrical Testing Reference},
  year = {2026},
  note = {Revision 1; last substantive change 2026-10-13}
}

Wikipedia

{{cite web |title=Liquid-filled transformers |website=Electrical Testing Reference |publisher=Electrical Testing Reference |url=https://www.electricaltestingreference.com/equipment/liquid-filled-transformers/rev/1/ |date=2026-10-13 |access-date= |version=1}}