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Topic Title: VLF testing
Topic Summary: Is there an industry accepted standard?
Created On: 22 February 2013 04:04 PM
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 22 February 2013 04:04 PM
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GitaJ

Posts: 1
Joined: 16 December 2008

Is there a standard/technical reccomendations which specify what voltage and frequency you should VLF HV cables say 33kV? Is it 2*Vo?

Also, in a cable/transformer network I know you should isolate the transformer first. What risks does a VLF test pose to transformers?
 24 February 2013 10:12 PM
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JonathanHill

Posts: 225
Joined: 09 September 2002

When in Client Engineer role, for a new installation, I would generally look for 2 x Uo rms for 45 minutes.

From memory, BS & IEC standards on this are a bit thin on the ground - I think the above figures come from an IEEE standars or draft consultation. Indeed, I think BS 6622 (and possibly IEC 60502) still refer to dc pressure testing, which I would strongly challenge due to a raft of anecdotal reports pointing to this promoting premature failures. Some DNOs are still using dc pressure tests as standard (driven mainly, I believe by the greater portability and lower price of dc pressure test sets).

Frequency is commonly determined by the cable capacitance and the drive capability of the test set.

Re transformer testing - I have included transformers in tests in the past - always with the manufacturer's blessing. This obviously requires that all 3-phases are tested simultaneously (which may present a greater cable capacitance than the test set can accommodate). I believe that more recently, transformer manufacturers have become reluctant to accept this.

Provided that you have high confidence in the protection, a 24-hr soak test subsequent to a 5kV Megger Test of the entire circuit may suffice (and this approach is allowed in (most) cable standards).

Hope this is helpful.

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Jonno
 25 February 2013 10:06 PM
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PaulTusonInlec

Posts: 8
Joined: 25 February 2013

Thanks Jonno

That is an excellent answer.

Unfortunately there is a lot of confusion regarding specifications for a VLF test. Most people often are not aware that the rating for the majority of VLF sets is based on the peak voltage they achieve rather than the RMS output that the test should be based on.

Therefore what is called a VLF 60 is producing a 62KV peak but only 44KV RMS.

It is essential to make sure that you choose the right tool for the job.

As far as frequency is concerned ideally you would use 0.1Hz however the lower the frequency you use the higher the capacitance you can overcome.

We are finding more and more people turning to VLF as an alternative to using more bulky 50Hz Test sets. Vlf is far more powerful than an equivalent sized power frequency set.


Paul Tuson

-------------------------
Regards
Paul Tuson
Inlec Test Equipment Hire


Edited: 25 February 2013 at 11:57 PM by PaulTusonInlec
 25 February 2013 10:36 PM
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JonathanHill

Posts: 225
Joined: 09 September 2002

Thank you, Paul,

Your additional comments are very helpful. However, my understanding is that these discussion forums are not intended as vehicles for explicit blatant advertising and removal of your last 2 paras would respect this ethic (perhaps before the moderator does this for you).

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Jonno
 26 February 2013 12:00 AM
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PaulTusonInlec

Posts: 8
Joined: 25 February 2013

You are right and thank you for pointing it out.

I responded to that post before reading the guidelines. The offending lines have now been removed although I had thought i was not being overly blatant.

On rereading I can see the point however.

-------------------------
Regards
Paul Tuson
Inlec Test Equipment Hire
IET » Energy » VLF testing

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