Power transformer commissioning verifies on site that the unit arrived undamaged from transport, was properly assembled and is ready to be energized. Turns ratio, winding resistance, insulation resistance, power factor, excitation current and, as a reference fingerprint, frequency response are measured; the oil is analyzed, accessories and transformer protection devices are verified, and the unit is energized under observation. The results are compared with the factory test report and become the maintenance baseline.
What transformer commissioning is looking for
A power transformer is tested three times in its life, each with a different purpose. At the factory, the routine, type and special tests of IEC 60076-1 (IEEE C57.12.00 / C57.12.90) prove that the design and the unit meet the specification; they are explained in transformer factory tests. In maintenance, tests look for deterioration trends. At commissioning the question is different: did it arrive sound, was it assembled correctly and is it ready to be energized? It is one of the stages of substation commissioning.
Between the factory and energization there are risks that do not exist on the test floor: impacts during transport, moisture during assembly of bushings and radiators, a poorly executed oil filling, swapped connections. That is why site tests are always compared with the factory test report, and the results become the baseline against which the transformer will be measured for the rest of its life. The reference guide for field testing of liquid-filled transformers is IEEE C57.152.
Electrical tests before energization
| Test | What it detects at commissioning |
|---|---|
| Turns ratio, polarity and vector group | Wrong internal connections, shorted turns, tap changer in the wrong position |
| Winding resistance | Loose connections at bushings and tap changer, high-resistance tap changer contacts |
| Insulation resistance and polarization index | Moisture or contamination in the insulation, unintended core grounds |
| Power factor and tan delta | Condition of winding and bushing insulation, compared with the factory |
| Excitation current | Core problems, shorted turns, residual magnetism |
| Short-circuit impedance | Winding displacement relative to the nameplate |
| Frequency response analysis (FRA) | Mechanical deformation from transport; leaves the reference fingerprint |
| Tap changer dynamic resistance | Interruptions or abnormal timing during on-load switching |
Order matters. DC tests such as winding resistance leave residual magnetism in the core, which alters the excitation current and the low-frequency part of the frequency response. That is why FRA and excitation are usually measured first, or the core is demagnetized after the DC tests.
Temperature correction. Insulation resistance and power factor depend strongly on temperature. To compare them with the factory they must be corrected to a reference temperature; you can do it with our free insulation temperature correction tool.
Oil and active part
Before energizing, an oil sample is taken to verify that filling did not introduce moisture or particles: dielectric breakdown voltage, water content and, as a reference, dissolved gas analysis. IEC 60422 guides the supervision of mineral oil in service and gas interpretation relies on IEC 60599 and IEEE C57.104; the details are in the dielectric oil analysis guide. A second gas sample after an initial period of operation rules out abnormal gas generation caused by energization.
Accessories and transformer protection devices
The transformer comes with its own mechanical and thermal protection devices, and every one of them must reach the trip:
- Buchholz (gas) relay and the tap changer oil surge relay: alarm and trip wired to the right relay and breaker.
- Pressure relief device and sudden pressure relay (63).
- Oil and winding temperature indicators (26/49): calibration, alarm and trip thresholds, cooling control.
- Oil level of the tank and the conservator.
- On-load tap changer: local and remote operation, limits, position indication and, if transformers run in parallel, the voltage regulator.
- Cooling: fans and pumps by groups, with their supplies and alarms.
Each signal is verified from the sensor to the control cabinet, the relay and SCADA. The parts of the cabinet are described in the guide on the transformer control cabinet.
Transformer protection
The 87T differential, backup overcurrent and restricted earth fault protection are tested by secondary injection with the approved settings, as described in protective relay commissioning. There are two critical points at commissioning:
- Vector group compensation and CT ratio. An error here does not show up until the transformer takes load. You can check the criterion with the 87T vector group compensation tool.
- Inrush stability. The differential must block with the energization current. The phenomenon is explained in transformer tripping on energization and the function in transformer differential protection 87T.
What changes depending on the transformer
The scope is not the same for every unit:
- High and extra-high voltage transformers. They usually arrive without oil or with the bushings removed, and on-site assembly takes long. Verification of the transport impact recorder, control of drying and vacuum filling and, often, the power factor and capacitance of each bushing are added.
- Oil-filled medium-voltage transformers. They arrive assembled and filled. Tests focus on turns ratio, winding resistance, insulation and the transformer protection devices.
- Dry-type transformers. There is no oil to analyze; inspection of cleanliness, clearances, the temperature sensors of each winding and ventilation gain weight. Their later maintenance is covered in dry-type transformer.
- Autotransformers and transformers with a tertiary winding. More ratio and impedance combinations, and a more demanding differential.
Energization and on-load checks
The first energization is done at no load, with all protection enabled and personnel out of the area. Inrush current, noise, temperature and any alarm are observed. Then, under load, phase sequence, the residual differential current of the 87T, consistency of the measurements and operation of the cooling are verified. It is the only way to confirm that the differential compensation and CT polarity are correct. The complete substation sequence is in first energization of a substation.
We commission your transformer
Our team commissions medium, high and extra-high voltage power transformers, in substations and industrial plants, in Colombia, or elsewhere on request, and delivers test records compared against the factory that serve as a baseline. For on-site tests, see power transformer testing or write to us.
Related guides
- Power transformer
- Transformer factory tests
- How to request a quote for power transformer testing
- Frequency response analysis (FRA)
- Transformer differential protection 87T
- On-load tap changer
- Substation primary equipment tests
- Commissioning dossier: test records, reports and as-built
- Dielectric oil analysis: sampling, DGA and furans
- Transformer control cabinet: signals and wiring
References
- IEC 60076-1:2011, Power transformers – Part 1: General.
- IEC 60076-18, Power transformers – Part 18: Measurement of frequency response.
- IEEE Std C57.152-2025, Guide for Diagnostic Field Testing of Liquid-Filled Power Transformers, Regulators, and Reactors.
- IEEE Std C57.149-2024, guide for the application and interpretation of frequency response analysis.
- IEC 60422:2024, Mineral insulating oils in electrical equipment – Supervision and maintenance guidance.
- IEC 60599:2022, interpretation of the analysis of dissolved and free gases.
- IEEE Std C57.104-2019, Guide for the Interpretation of Gases Generated in Mineral Oil-Immersed Transformers.
- IEEE C37.91-2021, Guide for Protecting Power Transformers.
- ANSI/NETA ATS-2025.
Frequently asked questions
Which tests are performed on a transformer before energizing it?
Turns ratio and vector group, winding resistance, insulation resistance, power factor of windings and bushings, excitation current, short-circuit impedance, reference frequency response, oil analysis and verification of accessories and transformer protection devices. The exact scope is set by the client's specification and the manufacturer.
Why repeat on site tests that were already done at the factory?
Because between the factory and energization come transport, assembly of bushings and radiators, oil filling and connection. Site tests confirm that nothing changed and, compared with the factory test report, detect damage that cannot be seen from the outside.
Is it normal for the differential to trip when energizing a new transformer?
It should not be. Inrush current has a second-harmonic content that the differential uses to block itself. If it trips, check whether it was poorly restrained inrush, a settings or CT connection error, or a real internal fault, before energizing again.