Nov 11, 2015
in
CT&VT

# Current Transformer - Nominal Power and Rated Accuracy Limit Factor

C

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.