Before energizing a power transformer, the check does not repeat the tests: it confirms they are done, that they were corrected to the right temperature and that each result was compared with something, whether the manufacturer's data, earlier tests or the other phases. This guide explains which results are requested, what they are compared with and which findings stop the energization.
What this check is and what it is not
There is already a guide on power transformer commissioning covering the tests and accessories. This is the check that comes after: the hour before energization, when someone has to say “the transformer is ready” and back that sentence up with paperwork. For the acceptance tests of the auxiliary services transformer, see basic tests of the auxiliary services transformer.
No test is repeated. You check that the results exist, that they are comparable and that nothing is left open. The test procedures we consulted to verify it are used as a map of topics; only the criterion is set out here.
The six sets of results
The acceptance procedures for oil-filled transformers that we consulted group their tests into these sets. For each one, the value, the temperature, the date and the equipment used are requested, with its calibration up to date.
1. Insulation resistance. Measured between high and low, between high and ground and between low and ground, with the winding not under test connected to ground and at the voltage the manufacturer specifies. The procedures record the reading for ten minutes, so the polarization index comes out of the same measurement. The result is corrected to the temperature of the manufacturer’s tests and compared with its values. Test details in winding insulation resistance.
2. Power factor of the insulation and of the bushings. This is a different insulation test from the previous one: it measures dielectric losses and capacitance. On capacitive bushings each one is measured, with the analyzer in different modes for the main capacitance and for the test-tap capacitance. Values are corrected for temperature and compared with the manufacturer’s. See power factor and tan delta.
3. Winding resistance. Measured with direct current on each winding and compared with the manufacturer’s data and between phases. IEC 60076-1 requires recording the resistance of each winding, the terminals it was measured between and its temperature. In an oil-filled transformer, the winding temperature is taken as the oil temperature once it has been at least three hours without excitation. See winding resistance.
4. Turns ratio and polarity. Repeated at each tap of the changer and compared with the nameplate and the manufacturer’s data, to also confirm the vector group. See turns ratio.
5. Excitation current. Measured with single-phase voltage on each phase and at each tap-changer position. Compared with earlier tests or with identical transformers and, in a three-phase unit, between its phases. See no-load excitation test.
6. Oil. The procedures call for samples for physicochemical tests, gas chromatography, furanic compounds and dielectric strength. The first three are done by a certified external laboratory. Dielectric strength is measured on site with the method given by the test standard. The sample is taken carefully so as not to introduce air or contamination, and is transported in the dark. See oil sampling and dielectric oil analysis.
What each result is compared with
A number on its own says nothing. In the check, each result must have something to be compared against:
| Result | Compared with |
|---|---|
| Insulation and power factor | Manufacturer’s value, corrected for temperature; failing that, the table in the test standard |
| Winding resistance | Manufacturer’s data and between phases |
| Ratio and polarity | Nameplate and tap by tap |
| Excitation current | Earlier tests, twin transformers and between phases |
| Oil | Laboratory limits and the applicable oil standard |
If there is no reference at all, the result is stored as a baseline for the future and the check says so.
Findings that stop the energization
Without giving values, which depend on each unit and its manufacturer, these cases stop the switching until they are clarified:
- A test with no recorded temperature, or corrected with a table that does not apply.
- A result outside the manufacturer’s data without explanation, or different between phases when they should be equal.
- Gases in the oil that have not been interpreted. See elevated dissolved gases in the transformer.
- Measuring equipment without up-to-date calibration.
- A winding that was left without a ground connection or a test jumper that was not removed.
What is confirmed besides the tests
The check includes what is not a measurement: that the transformer is disconnected from the tests and connected to its service, that the associated current cores are in their correct state, and that its differential protection is set and enabled. The general picture for the substation is in what is checked before energizing a substation. For the test sequence for a voltage transformer installed in a cubicle, see acceptance tests of a voltage transformer in switchgear.
Common mistakes
- Accepting an insulation reading without noting the temperature: it can be neither compared nor corrected.
- Measuring winding resistance with the transformer just de-energized: the standard asks for at least three hours without excitation to take the oil temperature as the winding temperature.
- Keeping only “passed” without the value: the number is the baseline for all later maintenance.
- Leaving the oil for later: its results take time, because an external laboratory produces them, and they must be in before energizing.
What is not published here
We do not include the step-by-step procedure or our formats. The limit values are defined by the manufacturer and the test standards; this guide does not replace them.
We test and check your transformer
At Tiza Group we carry out acceptance tests and the pre-energization check of power transformers. See power transformer testing or write to us through contact.
Related guides
- Power transformer commissioning
- Transformer maintenance
- Low insulation resistance in the transformer
- First energization of a substation
- Voltage transformer in switchgear: acceptance tests
- Auxiliary services transformer: basic tests
References
- IEC 60076-1, Power transformers – General: list of routine tests (clause 10.1.1), measurement of winding resistance (10.2) and reference temperature.
- Acceptance procedures for oil-filled power transformers (reviewed only as a map of tests; not reproduced).
Frequently asked questions
Which results should you have at hand before energizing a transformer?
Insulation resistance, power factor of the insulation and of the bushings, winding resistance, turns ratio and polarity, excitation current and oil analysis. Each one with its temperature and the measuring equipment used.
What temperature are the results corrected to?
IEC 60076-1 sets 75 °C as the reference temperature for oil-immersed transformers. For dry-type transformers it refers to the general test requirements of the applicable standard. Insulation results are corrected with the table given by the manufacturer or the test standard.
Who decides whether a doubtful result allows energizing?
The technical person in charge of commissioning, together with the owner and, if the contract requires it, the manufacturer. A value outside what is expected, without explanation, stops the energization until the test is repeated or the cause is clarified.