Voltage transformer in switchgear: acceptance tests

What an acceptance test in a cubicle is

A medium-voltage voltage transformer installed inside a metal-enclosed cubicle, for example at a 34.5 kV level, is tested before commissioning to check that it arrived and was installed as specified. What is checked is short: physical condition, insulation, windings, ratio and polarity.

The general aspects of the equipment are in voltage transformer. This guide describes the testing day, based on an acceptance form for switchgear voltage transformers that we consulted as a map of topics. The form is not reproduced.

Safety first, then the test

The form we reviewed treats preparation as part of the test, with a sequence that works as a model:

1. De-energize the transformer.
2. Verify the absence of voltage with a detector whose operation is checked beforehand.
3. Prepare the area: padlock on the equipment’s operating mechanism, work tag, area marked off and grounds applied.
4. Short safety talk with the whole team, five minutes in the form.
5. Disconnect the high- and low-voltage conductors.
6. Inspect the terminals and clean the transformer.
7. Carry out the electrical tests. In each one, personnel move away from the test points and the terminals are discharged as soon as it ends.
8. Reconnect the transformer.

The main risk of the tests is electric shock. All the preventive actions point to the same thing: nobody near the terminals while test voltage is applied and everything discharged at the end. Personal protective equipment: safety footwear, helmet, glasses and dielectric gloves; dust mask if the environment calls for it. More on the acceptance tests of the auxiliary services transformer in basic tests of the auxiliary services transformer.

Visual inspection

Checked per phase: external condition of the transformer, primary-side, secondary-side and, if present, tertiary terminals, junction box, insulators, oil level if oil-filled, and the ground connection. Manufacturer, model, voltages, type (dry or oil), burden rating, rated current, year, frequency, vector group, standard and class are noted. With these data you know what was tested.

Insulation resistance

Measured between primary and ground, between primary and secondary plus ground, and between secondary and ground. The form records the reading over ten minutes, with a test voltage stated for each connection, and from it the polarization index. Readings are compared with the manufacturer’s data and between phases. Details in insulation test of voltage transformers.

Winding resistance

The DC resistance of the primary and the secondary is measured, at each position of the changer if there is one. Voltage, current and resistance are noted. On the secondary side the values are small, in the order of milliohms, which is why the form uses millivolts and milliohms. It is compared with the manufacturer’s data and between phases. See winding resistance of a voltage transformer.

Turns ratio and polarity

The ratio of each phase is measured at each position, with equipment that also gives the polarity. In the form we saw, polarity is reported separately, with the equipment report attached. The ratio is compared with the nameplate, and the polarity with the terminal marking. IEC 61869-3 covers terminal marking: terminals with the corresponding letter, such as A and a, have the same polarity. See turns ratio.

Capacitance and loss factor

For oil-filled transformers, the form includes capacitance and loss factor between high and low and between each winding and ground, with the instrument in different measurement modes. The result is corrected to 20 °C and the oil temperature and relative humidity are noted. See power factor and tan delta.

What is attached

A good acceptance report includes, for each test, the equipment used with its model, serial number and calibration date, the date and time of the test, the measured values and the equipment reports. With that the test can be repeated or audited.

Common mistakes

• Testing without removing the high- or low-voltage conductors: the measurement includes the cable or other equipment.
• Not discharging the terminals after an insulation test.
• Mixing up the primary polarity with the secondary one when reconnecting. Terminals A and a correspond.
• Closing the cubicle with the secondary miniature circuit breaker open or with a terminal block link left unclosed.

Before reconnecting

After reconnecting the conductors, tightness and continuity are checked. The secondary miniature circuit breaker must be left closed and without a short circuit, because a short circuit on the secondary of a voltage transformer is a fault. See short circuit on the secondary of a voltage transformer and what is checked before energizing a substation. More on what is checked before energizing a power transformer in pre-energization check of a power transformer.

We test your voltage transformers

At Tiza Group we carry out acceptance tests of voltage and current transformers. See testing of current and voltage transformers or write to us through contact.

Related guides

• Voltage transformer
• Inductive voltage transformer
• Omicron CPC 100: CT and PT testing
• Test blocks and bay isolation
• Auxiliary services transformer: basic tests
• Power transformer: pre-energization check

References

• IEC 61869-3, Instrument transformers – Part 3: Additional requirements for inductive voltage transformers: terminal marking (6.13.301) and routine tests (7.3).
• Acceptance form for 34.5 kV voltage transformers in switchgear (consulted as a map of topics; not reproduced).

Frequently asked questions