Auxiliary services transformer: basic tests

Why it deserves its own protocol

The auxiliary services transformer feeds the battery chargers, the control panels and the protection. If it fails, the substation loses its services even though the power side is healthy. It is small, and that is why it gets tested little and late. A short protocol, done well and filed, prevents that oversight.

This guide describes the content of a basic protocol, taken from an auxiliary services transformer test form that we consulted as a map of topics. We do not reproduce the form or its values.

What the protocol contains

Equipment data. Manufacturer, type, cooling, rating in kVA, medium and low voltages, current, phases, frequency, insulation level of each side, weight, altitude, year of manufacture, standard, vector group and serial number. With them you can later check that the transformer that was tested is the one that was installed.

Test equipment. Each item with its designation, manufacturer, model, serial number and calibration date. Without current calibration the result is not valid as evidence. In the form we saw there are a megohmmeter and a primary test unit.

Tests. The two that follow.

Remarks, conclusions and annexes. This is where you write what the numbers do not say, and where the equipment reports are attached.

Test 1: insulation resistance

It is measured with a megohmmeter in three connections: medium voltage to ground, low voltage to ground and medium to low voltage. The form asks you to note, for each, the test voltage, the leakage current in nanoamperes, the measured resistance in gigaohms and an assessment. Ambient temperature and humidity are also requested.

What is worth knowing:

• The test voltage is defined by the manufacturer or the test standard; choosing it at random can damage a small winding or make the test meaningless.
• Humidity and temperature affect the reading. A low resistance on a rainy day does not mean the same as on a dry day.
• It is compared with the manufacturer’s value, with the other readings of the same transformer and with an earlier measurement. To interpret a low value see low insulation resistance in the transformer and insulation resistance and polarization index.

Test 2: winding resistance

The DC resistance of the medium-voltage and low-voltage windings is measured by injecting a current and reading the resulting voltage. The form records the current read, the voltage read in millivolts, the resistance measured at the temperature of the moment and the resistance corrected to a reference temperature, in addition to the assessment.

Two precautions stated by the transformer standard:

• IEC 60076-1 requires recording the resistance of each winding, the terminals it was measured between and the winding temperature, using direct current and taking care to minimize the effects of self-inductance.
• The reference temperature for oil-filled transformers is 75 °C. For dry-type units the standard refers to the applicable general test requirements. The form we saw corrected to 75 °C; if your transformer is dry-type or uses another insulation, confirm the reference temperature in the standard for your equipment.

In an oil-filled transformer, the standard indicates that the winding is taken at the average oil temperature after at least three hours without excitation. In a dry-type unit, the equipment must rest at constant ambient temperature for at least three hours. See winding resistance.

The resistance is compared with the nameplate or factory values and between phases. A phase that differs from the others is a finding that is explained before energizing.

What the basic protocol does not cover

If the transformer is oil-filled, the basic protocol does not include oil analysis. If the connection needs to be confirmed, it does not include the turns ratio and vector group either. And it does not cover the transformer’s own protections. The full list for a power transformer is in power transformer: pre-energization check; for an auxiliary services unit, it is decided according to its size and insulation.

Common mistakes

• Testing without noting temperature and humidity: the insulation reading cannot be interpreted afterwards.
• Measuring winding resistance before the transformer has stabilized at ambient temperature, when the standard asks for hours of rest.
• Not removing the test jumpers or leaving a winding without a ground connection when finished.
• Filing only the conclusion and not the values: without them there is no baseline for maintenance.

After the tests

Before connecting, the transformer must be left in its service state: tightened connections, ground connected and medium-voltage protection set. At the first transfer, the phase sequence is verified and that the services get the expected voltage. For the substation picture see what is checked before energizing a substation. More on the test sequence for a voltage transformer installed in a cubicle in acceptance tests of a voltage transformer in switchgear.

We help with your auxiliary services

At Tiza Group we test and commission auxiliary services transformers together with the rest of the substation. See power transformer testing or write to us through contact.

Related guides

• Auxiliary services transformer: what it is and how it is sized
• Metrel MI 3201 and MI 3210 megohmmeter
• Dry-type transformer maintenance
• Power transformer commissioning
• Voltage transformer in switchgear: acceptance tests

References

• IEC 60076-1, Power transformers – General: measurement of winding resistance (10.2) and reference temperature.
• Test protocol form for an auxiliary services transformer (consulted as a map of topics; not reproduced).

Frequently asked questions