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Topic Title: AC test voltage for a switchgear circuit
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Created On: 31 January 2014 06:42 AM
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 31 January 2014 06:42 AM
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In my company, a 6.6kV/415V transformer is supplied from a 6.6kV switch board through a Air Ciruict Breaker(ACB) and a cable.

Recently, 2CTs(current tranformers) installed in a transformer Air Circuit Breaker cubicle(ACB) (High Votlage side) was found burnt and flash over occurred. After replacement of the CTs (two new CTs), I want to perform a High Voltage AC test on the system (including ACB sprout in the switchgear cubicle, the CTs and the cable) with ccable disconnected from the transformer to confirm the insulation integrity of the system before energisation.

The question is: What is the test voltage to be used and the duration of the test?

Thank you.
 01 February 2014 08:55 PM
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In my opinion, for safety, you need expert help. I would seek advice from the Circuit Breaker Manufacturer if you haven't done that already.

In my experience an ACB is not the usual means of switching/isolation/protection for supplying a 6.6kV/415V Transformer, normally you have a fully fault rated Vacuum Circuit Breaker, SF6 Circuit Breaker or oil switch and fuses.

If you have had the flash over damage you state then the carbon deposits etc left in the device could cause further flash overs if you energise it.
 02 February 2014 06:50 AM
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Are you sure its ACB, because ACB is normally not used at Medium Voltage systems. This should be Vacuum CB , SF6 or in some old system Air blast CB or Magnetic CB. Now coming to your question : There are standard procedures for testing , i think test voltage should be 1.5 times of the operating one.

Tariq iqbal
Manager Operations
Albario Engg. (Pvt) Ltd
Habibullah Coastal Power
Complex Quetta.
 02 February 2014 10:42 AM
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There is a minimum and a maximum test voltage, and it will depend on your company procedures and what they deem necessary. It all depends on what condition you suspect the cable to be in, how long it has been de-energised.

For example, for an 11 kV cable (no transformers attached), you might specifiy a maximum of 18kV DC for 5 minutes or a minimum of 1 kV for 1 minute. You would certainly want to do this later test once the transformer is re-connected. However, if you are replacing the CTs and want to test them through, you should also isolate the circuit and inject primary current to test the protection.

This sort of testing is not trivial - it is very dangerous and potentially lethal.

Unless you have a rigourous up-to-date written procedure and lots of experience of HV testing including the issue of a sanction-for-test then you should stop and get expert on-site advice from a suitably authorised person, and get them to do the testing.

Edited: 02 February 2014 at 02:45 PM by Zuiko
 24 February 2014 02:21 AM
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In the 60's Whipp & Bourne made both 3.3 and 6.6KV ACB's rated at either 1600 or 2400A. They were rated for motor duty but also had a down rated transformer duty.

Its a few years ago, but I'm sure I used 5KV DC to test our 3.3KV by then somewhat venerable units. I condemned one. The commissioning test voltage was 7.8KV.

I was unlucky enough to close one on to a fault, it was frightening. The entire switch room lit up!

As for working on them you needed the incredible hulk as a mate. The trucks weighed in at a modest ¾ ton.

Could there be a better way?

In theory yes, but in practice?

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