DC Hi-Pot testing a Transformer & Cables

[SparkyLarks]

I need to test the insulation resistance of a 3-Core cable, but one end is still attatched a 3.3kV/400V 1.5MVA oil filled trandsformer.

I'm planning to connecct 5kV to all three cables and the earth to the Cable Armour, which is connected in turn to the Transfomer frame.

That will check the insulation of all three cores to the steel armour.

I can't check from a single core to the other two cores earting one of the cores would mean path from the live cable through the transformer windings to earth, but is ther any other tests availible to me?

 

[Zogzog]

DC hipot testing of MV cables >5 years old is not recommended by IEEE, ANSI, NETA, IECA, or any other standard, it is considered a destructive test and not very effective anyways besides finding gross installation errors. VLF, PD, and Tan Delta are all acceptabel tests that each have thier pros and cons.

This has been discussed here many times in depth, you can catch up on the discussions by searching the forum for previous threads on the subject.

 

[Marmite]

You are stuck with testing all three phases to earth unless you disconnect the cable from the transformer winding. If the HV termination is dry then you should be able to unbolt the cable cores and bend them away from the transformer connections to do the test. If its compound filled then you would need to take the lid off the transformer to disconnect the winding from the cable. This situation is common in the utility environment I work in, and the testing is usually in reality a combined cable/transformer test to earth, rather than going to the effort of disconnecting the cable. The use of DC overvoltage is common, but utilities are generally interested in go/no go test prior to making live, rather than condition assessment. There is a school of thought that in this situation the connections should be left as they are and not disturbed to do testing, as you may inadvertently reconnect something wrongly, or even leave a spanner somewhere you shouldn't.
Regards
Marmite

 

[SparkyLarks]

Marmite
The cable terminations are in a compound filled cable box.
and it is a Utility customer. WE discussed removing the compound but a concern was raised that heating the compound may result in damage to the Transformer bushings.

It is exactly a go/no go test that is wanted prior to energising.
it's a 3.3kV Transfomer. Is testing at 5KV too high for an existing cable & transformer. Normally each cable or winding would only see a voltage of 1.9kV above earth. Voltage of 3kV to ewarth be more appropiate>

 

[k1y9l8e7]

Page 13 on the attached document suggests a 3.3kV system should be tested at 5 kV phase to phase (if your cables arn't screened) and 3 kV phase to earth/armour, although this is for new cables. Hope this is of help.

Regards

Kyle

 

[Zogzog]

You won't find a recommended test voltage for DC on service aged cables in any standard because all standards agree you should not DC hipot service aged cables.

 

[Zogzog]

I suggest you read this thread

http://www.eng-tips.com/viewthread.cfm?qid=248906

 

[dpmac]

Assuming it;s strictly a GO/No Go test to insure no grounds exist. Assuming un-screened cables rated 3.3 kV a simple 1000V insulation resistance (Megger) test will tell you its ok to energize or not.

And you get the added bonus of using a Megger test set at 1000VDC instead of a DC hipot at 1000VDC which will prevent the DC Hipot Police from coming to your house late one night.