Oct 10, 2015

Current Transformer - Rated Burden


easurement of rated burden, but how ? The purpose behind the measurement of the rated burden is to demonstrate that the burden of the connected secondary circuit does not exceed the rated burden of the current transformer. In other words, we want to exclude the possibility of “overburdening” the current transformer, as only then is it safe to assume that the transformer can handle what its rating plate would have us believe.

This is done by injecting the secondary nominal current of the current transformer, usually 1 A or 5 A, into the phase-to-ground loops of the terminal block in the transformer's terminal box and measuring the voltage drop on the terminals with a multimeter. If it proves impossible to carry out the measurement directly on the terminal board, we'll simply move to the next terminal block, e.g., the one in the on-site control cubicle. We can make an intelligent guess for the section we've had to skip over and add this figure to the result of the measurement. The table below provides a quick guide to the values for the section we haven't been able to measure and also helps us carry out a plausibility check on the measured values and the overall results. The “protection boxes” themselves don't add much to the result, as digital relays usually have internal burdens of less than 100 mΩ.

For all those out-of-the-ordinary system configurations that are not to be found in the table, and for circuits between Earth and the planet Zog, the following equation will provide a rough estimate:

For length l, the simple line length is used, the calculated result naturally corresponds to the operating baton of the entire conductor loop including the outgoing and return conductor.

After the wiring tests have been completed, the terminals opened at the beginning of the test are shifted and the short circuit to the converter is removed again.

This article is an excerpt from chapter 1 "Current Transformer Test" from my book "The Bible of Generator Protection". The "IBS Bible" has been published this month and is now available. For more information about the book simply click on the following picture with the book:

Kind regards 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.