How to measure mag curves of CT's

[rainfire]

Hi, can anyone please advise on how to measure Mag curnves on class X protection CT's. what is physically required. Do you require isolation of the cable or can it be done by secondary injection testing?

Thanks

 

[ScottyUK]

You need the primary side to be open circuit - no earths or the like forming closed loops. The secondary side should disconnected from the relay panel etc.

Connect a variable AC source to the secondary terminals of the CT with a sensitive ammeter in series to measure the magnetising current drawn by the CT. Start with the voltage at zero. Measure the current drawn at nominal increments of applied voltage - say every 10V - until the current starts to rise noticeably. At this point reduce the increments to a smaller value - say 2 volts. If you plot the results on a graph you will see the characteristic magnetising curve: as the CT core starts to approach saturation the current begins to increase rapidly. The point at which a 10% increase in voltage causes a 50% increase in current is the 'kneepoint'.

For a Class X CT you will need a high voltage variable source - perhaps up to 1000V. Be careful!


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I don't suffer from insanity. I enjoy it...

 

[rainfire]

So for an 11Kv 3ph distribution ring, with all cables protected with translay SDP 101, I could:

Open relevent circuit breaker at my end, open remote CB at other end of cable to create open circuit status (hence isolating the cable but it will be floating, and not earthed)

Inject AC volts onto secondary side of CT and measure the current drawn from the CT. Increase Voltage in increments untill the knee point is reached.

 

[ScottyUK]

Yes, that is essentially correct. You could have one circuit main earth 'on' at the primary side, just make sure you don't earth both ends of the primary conductor and thus form a closed loop.

I've tend to use a standard variac driving a 415/110V control transformer in reverse to get the required voltage if it is a one-off test and I'm without proper test gear. If you need more than 415V - which is quite likely for a Class X CT - use two transformers and connect the 415V windings in series to reach 830V. Some instruments don't like to be floated too high above ground - something to consider for your ammeter.

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I don't suffer from insanity. I enjoy it...