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Bellsam
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Join Date: Jul 2010
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Continued 33kV Cable Fault

07/09/2010 3:53 AM

Have a 33kV 1C 300mm XLPE cable which failed on blue phase. Completed VLF test on all three phases at 52Kv Peak. Yellow & Red Phase passed. Located and repaired fault to Blue phase and did VLF test at 35kV peak (reduced) only due to cable age.Re-energised cable. Breaker tripped after 36 hours on yellow phase directional protection

Located fault on yellow phase repaired . Insulation resistance test after splicing was 20G Ohms ( slight rain ) did VLF test at 35kV peak O/K. Insulation test after VLF 116G Ohms (in dry conditions)

Placed feeder back in service tripped after 59 minutes at this stage phase unknown

Cable length +- 750 meters long, about 35years old . Sheath tests are not good. Screens to earth at one end only

Concern that cable was VLF ed at 52kV peak ( yellow phase ) and faulted after initial repair. Any theories on continued failure
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PWSlack
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#1

Re: Continued 33kV cable fault

07/09/2010 5:42 AM

<...Any theories on continued failure...>

One of them must be a failure to renew it.

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Le_Noble
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#2

Re: Continued 33kV Cable Fault

07/09/2010 8:27 AM

Any theories on continued failure

Theory of lifetime, 35 years is much more than the expected life time of such cable, be happy that failure occur at 35 years not before

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PeterT
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Join Date: May 2010
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#3

Re: Continued 33kV Cable Fault

07/09/2010 12:00 PM

XLPE insulation on MV cable of the age reported (35 yrs) has a history of insulation breakdown by "treeing," especially when buried or in wet / outdoor environments. VLF testing will help prevent failure due to testing of the cables, but if they are at the end of their lifetime, they will fail anyway when energized.

I agree that it's time to retire these cables and install new ones.

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mountk2
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#4
In reply to #3

Re: Continued 33kV Cable Fault

07/09/2010 6:19 PM

I do agree.

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energygod
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#5

Re: Continued 33kV Cable Fault

07/10/2010 12:39 AM

In total agreement with 2,3 and 4-

Recommendation- when you replace cables, use THHN insulation and install in a PVC conduit (at least one size larger than required by code) and have both ends terminate above grade.

Somewhere along the routing, install a tee with a vertical pipe that terminates at least 8 feet above grade with a double 90 looking down. The riser will create a slight but constant airflow through the conduit to keep cables cool (underground bury has cool exterior but is also an insulator). Heat from cables will cause air in conduit to warm up, creating chimney effect at riser and the resulting negative air pressure will draw in cooling air.

Since you will be facing a reasonable expensive rework, a little more cost will virtually guarantee that this will be the LAST time the cables need to be replaced.

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PeterT
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#7
In reply to #5

Re: Continued 33kV Cable Fault

07/10/2010 8:17 PM

At 33kV, you are not going to find THHN insulation (that's 600V class). Insulation of medium voltage cable will typically be XLPE (Cross-linked Polyethylene) or EPR (Ethylene Propylene Rubber).

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energygod
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#8
In reply to #7

Re: Continued 33kV Cable Fault

07/10/2010 10:35 PM

Sorry- you are totally correct.

I "forgot" that part of the initial discussion, but the rest of the "solution" still works.

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Anonymous Poster
#6

Re: Continued 33kV Cable Fault

07/10/2010 7:04 AM

No specific theory behind this, the cable is OLD and So... Lay a New cable. Due to aging and various other factors like moisture ingress etc.. the resistance value of the cable will suffer. Check the cable meggar value and compare with the desired cable meggar value.
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BB Raina
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Join Date: Jun 2010
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#9

Re: Continued 33kV Cable Fault

07/12/2010 6:40 AM

Hi!

The cable is suffering from the phenomena of "treeing". The process of treeing was happening on the 35 year old XLPE cable which has been accentuated by VLF test post repair at 52KV (p). This was too harsh for the cable. Result: Yellow phase gave way. I am sure it must be Blue phase or yellow phase again when it tripped within one hour. Nonethess, I am happy to know that an XLPE cable has lasted 35 years without such failures. Any pssibility of water ingression into the cable on its route?

BB Raina
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Anonymous Poster
#10

Re: Continued 33kV Cable Fault

02/21/2011 11:28 AM

What kind of failure was it, joint failure or cable insulation failure. what is the status of the insulation screen
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Previous in Forum: How to Charge the Spare Single Phase Main Transformer   Next in Forum: Harmful Voltage

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