Dec 10, 2015

Voltage Transformer - Broken Delta?


iring check of the broken-delta winding In our today's article, we explain you how you check the proper connection of the "broken delta winding" of a voltage transformer. To do this, we use a very simple but effective trick:

We short-circuit the 3 phases on the primary side of the transformer and apply a common test voltage. The effect of this is to generate three test voltages on the secondary side that have exactly the same amplitudes and phasing. This parallel connection of the primary windings means that the phase voltages in the delta winding are not extinguished, but are added vectorially with their full amplitude.

The diagram below illustrates the test connection configuration:

The test voltages that we recommend are shown in the following table. When using these test voltages, the result of the measurement on the broken delta winding must always be around 1.73 V:

The phase-to-phase and phase-to-ground voltages of the star winding during this test must be 0 V and approx. 1 V respectively.

If there are any problems with the Udelta winding, it's always a good idea to take a look at the partial Udelta voltages. All we need to do is to measure the individual voltage drops across the windings. If they're OK, then the only possible answer is a wiring error in the windings of the secondary circuit. Finding this wiring error is what makes all this so much fun.

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.