Nov 11, 2015

Current Transformer - Nominal Power and Rated Accuracy Limit Factor


urrent Transformer Test - Here we go! Normally, two things have to be tested when commissioning a current transformer:

- the secondary circuit wiring included the connected equipment

- the transformer itself

A very important test, which is relevant to the transformer itself, is the verification of the nominal transformer power and the rated accuracy limit factor. The best way of checking the nominal transformer current and rated accuracy limit factor in practice is to measure the knee point voltage according to British Standard 3938. The knee point voltage is roughly the secondary voltage above which a 10% increase in voltage causes a 50% increase in current. To determine this saturation point, we use the famed magnetization characteristic.

We apply an alternating voltage to the core we want to measure at the first accessible terminal block and measure the resulting current. It's a good idea to enter the results in an Excel spreadsheet and take the knee point voltage directly from the characteristic curve according to the above definition.

Computations are carried out to check that the following conditions relating to the transformer nominal power and overcurrent factor are satisfied:

Nominal accuracy limit factor

The mysterious factor of 1.25 is there to allow for the fact that the underlying saturation voltage for both equations always refers to the total error of the current and not just to the rise in the magnetization characteristic. This limit-EMF is in fact about 25 % higher than the measured knee point voltage.

As an extra to the method described above, the use of the Omicron© CPC100© test tool enables the transformer nominal burden, the nominal accuracy limit factor and the phase-angle error to be measured very easily, efficiently and to a high degree of accuracy.

This article is an excerpt from my recently published book "The Bible of Generator Protection Engineering".

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.