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Installing low voltage CTs on medium voltage system

Installing low voltage CTs on medium voltage system

Installing low voltage CTs on medium voltage system

(OP)
Dear all,

We have an application where we are installing low voltage CTs on a medium voltage system.

The CTs will be installed on insulated cables on the secondary of a transformer, that is operating at 5kV.

As you are aware, it is fine to use LV CTs on MV systems as long as the system has proper insulation and the CTs are not close to any exposed terminals.

Here’s the precaution that the manufacturer recommends:

“Where a low voltage window type CT is intended for use in a higher voltage application, it is the purchaser’s responsibility to ensure the operating conditions are met and the necessary precautions are taken. This is usually verified by performing insulation tests at an appropriate level of system voltage with the low voltage current transformers installed. “

I wanted to seek your opinion on what you think would be an appropriate ‘insulation test’ for this configuration.

Would a megger test on the secondary of the transformer with test voltage defined as per NETA requirements be adequate? Or perhaps a DC or AC hipot test? My concern with the AC hipot test is that it would be damaging to the transformer, as I recall based on a conversation I had once with a transformer manufacturer.

Would appreciate your input.

Thank you

RE: Installing low voltage CTs on medium voltage system

According to NETA WORLD
The Official Publication of the InterNational Electrical Testing Association Fall – Winter 2004 USA
Current Transformer Testing
"Voltage Class: The CT voltage class determines the maximum volt¬age with which a CT can be in direct contact. For example, a 600-volt window CT cannot be installed on or around a bare 2400-volt conductor. However, a 600-volt window CT can be installed around a 2400-volt cable if the CT is installed around the insulated portion of the cable and the insulation is rated correctly".
That means you have to test the cable not the CT.
You have to check the distances from any point of ct enclosure to the live parts.
See:
http://www.google.co.il/url?sa=t&rct=j&q=w...

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