Feb 1, 2016

Voltage Transformer Test - Star Winding


iring Test - The Star Winding! How does it work? Today we present an efficient procedure for the practical verification of the secondary transformer circuit wiring of a star winding.

To carry out the wiring check, a test voltage is injected on the primary side of the voltage transformer. All work on the primary and secondary parts of the plant must be complete and the plant must be ready for operation.

We apply a symmetrical three-phase AC voltage system to the primary side. To simplify matters, we recommend using a voltage that will produce the smoothest possible value on the secondary side of the transformer.

The following drawing shows the layout of the test circuit:

Modern 3-phase test tools can easily handle 300 V AC. The test voltages shown in the table have stood the test of time:

First, we check the operational measured values on the protection devices or use a multimeter to check whether the expected secondary voltage is present.

A rotary field measuring device is employed to confirm that the rotary field is OK. If no rotary field measuring device is available, or if the measuring device we used did not detect the low secondary voltage, we can use the negative sequence system detection (symmetrical components) of one of the connected digital protection devices to achieve our goal. If the rotary field is correct, the negative sequence system must show 0 V

The voltage across the Udelta windings of the delta winding is also checked. This must measure about 0 V as well.

To check that the phases are allocated properly, we can now modify the amplitudes of our 3 test voltages. We do this by increasing the amplitude of P1 by about 10 % while at the same time reducing the voltage of P3 by approx. 10 %. This change must of course be reflected in the secondary phase-to-ground measuring loops.

Does everything look OK? If so, we can then move on to the open delta winding.

This article is from the chapter "voltage transformer test" from my current technical book: "The Bible of Generator Protection Engineering". The test of voltage transformers is carried out in the 7 steps:

🌐 Visual check
🌐 Checking the grounding
🌐 Insulation test of secondary loop wiring

🌐 Checking the wiring of the star winding

🌐 Checking the wiring of the broken delta

🌐 Transformation test
🌐 Measurement of rated burden

The expert working of these tests are described in detail in my specialist book "The Bible of Generator Protection Engineering".

HEARTfelt Greetings Alexander Muth

Safety Notice

Failure to observe the following points can result in death, serious injury or material damage!

Hazardous voltages may be present when carrying out the tests and checks described in this manual. The safety rules and regulations regarding electrical systems must be strictly observed at all times. The generator must always be shut down when working on the primary system; appropriate grounding and short-circuiting facilities are to be provided at the respective workplaces. When carrying out a primary check on a turbine set, take care to ensure that no overheating of the turbine occurs.

The work described in this manual may only be carried out by qualified personnel, who must be conversant with the relevant safety regulations and safety measures as well as the warnings in the manuals provided by the suppliers of the various components. The contents of this manual 10 must not construed as work instructions. All statements in this manual must be carefully considered in light of the safety rules and regulations. The information presented in this manual does not claim to be complete.