Hi Pot testing of cables

[byjuvc]

Friends,
I am working in a Breakdown gang. Can anyone provide me with an authoritative document on Hi Pot testing of 11 KV cables. Also can anyone explain the following observations:
Usually we are supposed to do a hipot test on a cable with both ends isolated. But when it comes to a long overhead line from which a number of cables are connected at various spans it is much time consuming. So we apply the test potential (12 KV DC) to the cable with the transformers connected. The leakage current will be higher but still we get to know that the cable Insulation resistance is ok. What I am unable to understand is that I get the results in a certain o/h feeder with a number of transformers but I am unable to get the results in another line with just two transformers in line.With the cable still fine. So I am forced to open the cable from the other end and then do a hipot test.
Is there any advanced method of determining cable faults ? In our utility we use fault passage indicators but when there is a Overhead line with a nuber of cables at different intervals and no breakers in between it becomes a cumbersome job to locate the fault.

Thanks in anticipation

Byju
"Principle without practice is useless and practice without principle is dangerous"

[jersey hostage]

dont want to sound stupid but sometimes its the stupid things.Is there any spacer cable or open wire branched off?Because if its all cable that doesnt make any sense.Usually when we thump cable we clear everything and it is time consuming but if it works sometimes and not others something must be different.Different systems are hard to figure out so I dont want to confuse things more.

[WFO]

If you're testing with transformers attached, what type are they, single or double bushing?
I'm thinking that you get resonable results when the pots are double bushing (i.e., no ground reference) and poor results when the pots are single bushing (where the H2 end of the winding is solidly grounded internally).
Also, some hi-pots use a radar technology now for locating faults (Von makes one). They work on URD....don't know about overhead.

[byjuvc]

If you're testing with transformers attached, what type are they, single or double bushing?
I'm thinking that you get resonable results when the pots are double bushing (i.e., no ground reference) and poor results when the pots are single bushing (where the H2 end of the winding is solidly grounded internally).
Also, some hi-pots use a radar technology now for locating faults (Von makes one). They work on URD....don't know about overhead.

We have three phase Transformers DYN11 type. 11KV / 0.44KV that is the primary with 11 KV and secondary with 440 Volts. Also the Primary is Delta connected and the secondary is star connected with the Neutral point earthed. But I am applying the test potential on the Primary side which is delta connected only with no earthing.

Regards,

Byju
"Principle without practice is useless and practice without principle is dangerous"

[WFO]

With an ungrounded delta primary connection you're backfeeding into the other two phases so theoretically your reading would be 3 times what you would expect.
Also, I'm assuming your "11 Kv" system is the phase to phase voltage (correct me if I'm wrong). If so, and you have surge protection in the 9 Kv range, you may be getting arresters breaking down randomly depending on the make, style, etc.

[byjuvc]

With an ungrounded delta primary connection you're backfeeding into the other two phases so theoretically your reading would be 3 times what you would expect.
Also, I'm assuming your "11 Kv" system is the phase to phase voltage (correct me if I'm wrong). If so, and you have surge protection in the 9 Kv range, you may be getting arresters breaking down randomly depending on the make, style, etc.

I am able to understand the first line. Following is what I understand from the second line, pls correct me if I am wrong.
Yes it is 11 KV Phase to phase so it comes out to be 11/1.73 phase to earth . Again we are using the 9KV Surge arrestors.So when I apply 12 KV line to earth test voltage, the Surge arrestors start conducting and hence the high leakage current !
We had widespread cases of these Porcelain surge arrestors rupturing / bursting leading to feeder tripping . So as an interim measure at various important feeders the surge arrestors have been by passed and they are up on the pole only for show. Presently the company is in the process of replacing these porcelain Arrestors with the Polymeric Arrestors. I remember our maintenance team had undertaken a "surge arrestor by pass exercise" in the feeder section in question. I will patrol the line today and confirm.

Thanks and regards

Byju
"Principle without practice is useless and practice without principle is dangerous"