Mv cable testing blow cable

[ohmhead]

General old cable question 5kv 500 mcm 8 years old testing cable ?

Most effective way safe way ?

We had a track hoe hit a cable blow out 4160 v cable is 750 feet long .

To repair this a splice in a manhole was made one end replaced new and old end remained thur a manhole to the MDP switch to save cost requested by owner .

The old cable end was tested with a meg test showed good meaning the old part of that run left in it was asked by engineer not to hi pot this old cable due its age .

The new cable now spliced was high potted before splice was made tested good .

When we turned on MDP sw we had voltage issues the transformer it was feeding was a step down trans 4160 v to 480 v 3 phase its secondary output was 435v p/p and went down under load p/p 160 volts we shut her down fast .

Took off ABC terminations at MDP SW took ohm meter to each phase there was a dead short to its ground foil ground inside cable one[ A ]phase cable .

So the megger did not detect this before hand but the engineer would not let us use the hi pot .

Now would a VLF test been better to use or a discharge test your answer please ?

What would have been the most effective way i wanted to replace the entire run but was stopped by others .

Now there wishing it was done the fuses in MDP did not trip out but the shield inside cable is shorted to the core why or how can this be not effecting the trip .

ZOG were are you !

 

[brian john]

What voltage did you megger at?

What were the readings?

 

[ohmhead]

This time we had a testing company come out to test so can not give what voltage they used during the megger or what readings .

Were just wonder why it did not trip out how could a cable with a short on one phase do this and would a VLF or another test pick it up the first time .

This is strange so i thought someone might have seen this once in there life time .

 

[wiigelec]

Is the 5kV side an ungrounded delta?

 

[bob]

What voltage did you have in mind with the hi pot test? 5kv or less?

 

[ohmhead]

What voltage did you have in mind with the hi pot test? 5kv or less?

Well what were asking is not to use a hi pot on the old cable but would a VLF or maybe a discharge test be better to locate what we missed the first time out .

This cable was around 8 years old and its already shorted now it needs to be replaced now

What would have been a better choice is what iam meaning to ask for a more effective test to locate a faulted cable .

 

[ohmhead]

Is the 5kV side an ungrounded delta?

Let me ask you if it was a ungrounded delta and a track hoe hit the cable underground would that make it better or worst .

 

[wiigelec]

Were just wonder why it did not trip out how could a cable with a short on one phase do this

If the configuration is an ungrounded delta the short to your shield (ground) would in effect corner ground the A-phase of the delta and with no path for the fault current not trip your breaker. The next "ground fault" on phase B would then represent a phase-phase fault (complete circuit to A-phase through the ground) and you would be in big trouble. This could also cause the voltages you were seeing...

"IN AN UNGROUNDED SYSTEM (DELTA OR UNGROUNDED Y), THE FIRST GROUND FAULT CAUSES NO SHORT CIRCUIT CURRENT TO FLOW."
http://www.lehmanengineering.com/quiz/quiz1sol.html

 

[LarryFine]

"IN AN UNGROUNDED SYSTEM (DELTA OR UNGROUNDED Y), THE FIRST GROUND FAULT CAUSES NO SHORT CIRCUIT CURRENT TO FLOW."
http://www.lehmanengineering.com/quiz/quiz1sol.html

Funny you should post that. Lo Lehman is one of my best friends.

 

[bob]

Well what were asking is not to use a hi pot on the old cable but would a VLF or maybe a discharge test be better to locate what we missed the first time out .

This cable was around 8 years old and its already shorted now it needs to be replaced now

What would have been a better choice is what iam meaning to ask for a more effective test to locate a faulted cable .

I know that Hi Pot test can be destructive. If the cable was new, a Hi Pot test might put approximately 15 kv on the cable. My thought was to Hi Pot the cable but only use 5KV. A cable that is only 8 yrs old should be able to handle that type of test and voltage high enough to locate the fault. What is your thoughts on this?
Are you familiar with this type of test?

 

[ohmhead]

If the configuration is an ungrounded delta the short to your shield (ground) would in effect corner ground the A-phase of the delta and with no path for the fault current not trip your breaker. The next "ground fault" on phase B would then represent a phase-phase fault (complete circuit to A-phase through the ground) and you would be in big trouble. This could also cause the voltages you were seeing...

"IN AN UNGROUNDED SYSTEM (DELTA OR UNGROUNDED Y), THE FIRST GROUND FAULT CAUSES NO SHORT CIRCUIT CURRENT TO FLOW."
http://www.lehmanengineering.com/quiz/quiz1sol.html

This is very real and most likely you are correct makes sense .

 

[ohmhead]

I know that Hi Pot test can be destructive. If the cable was new, a Hi Pot test might put approximately 15 kv on the cable. My thought was to Hi Pot the cable but only use 5KV. A cable that is only 8 yrs old should be able to handle that type of test and voltage high enough to locate the fault. What is your thoughts on this?
Are you familiar with this type of test?

Let me re state the original post old cable was hit by track hoe therefore we cut two phases bang !! It blew out fuses in MDP 4160 v this run was going thur a manhole 750 feet so the owner says lets save a dollar and just splice the bad part . Now half is new cable and half is old cable we wanted all new but they didnt .


So we had one new cable that we tested with dc hi pot and passed and one old cable which the factory says no hi pot after 5 years so we megged it and passed .

Now we have a internal short in that old cable on one phase only and the megger did not find it during the test on that old cable so my question was is there a better way to test a old cable better than a megger but be safe and not damage the cable .

 

[zog]

There has been a lot of talk of late about DC hipot being destructive on MV cables, sure it is, all overpotential tests are, but the real concern is on XLPE cables.

But the real issue is, and always has been, is a DC hipot test is not a very effective test for MV cables. Some sort of AC test is required, the problem is for cable testing a power frequency AC hipot needs to be very large for long runs due to the capacitie reactance, and most companies do not have one that large so they use a cheap portable DC hipot that is uneffected by the Xc.

Here is a summary of what all of the current standards say about DC testing of MV cables.

NETA MTS:
-Does not specify what testing but rather refers to several other specifications (IEEE and ICEA)
-Allows for Direct Current, Alternating Current, Partial Discharge or VLF Testing
-Provides the "standard" DC High Potential Test warning for Cables over 5 years in service
-Provides guidelines for maintenance frequencies based on criticality and equipment condition.

ICEA:
-Allows for testing at reduced voltages for first 5 years
-Will only discover "gross" problems
-Not expected to reveal deterioration
-DC Testing after 5 years is "not recommended"
-Evidence that DC Testing can lead to early failures

IEEE 576:
-Allows for DC testing at reduced voltages for first 5 years
-Does not provide for testing after 5 years in service

IEEE 400
-Provides an overview of techniques for performing electrical tests in the field on shielded power cable systems from 5 – 500kV
-Provides a summary and warnings about DC high potential testing.
-States "even massive insulation defects cannot be detected with DC at the recommended voltage levels"

 

[zog]

Ohmhead, what type of insulation is the cable? Are the new and old sections the same type?

Sounds like an imporper termination was made, I can't believe the engineer would nto let you do anything more tthan a Megger test, why not at least a VLF?

 

[ohmhead]

Zog for this one i just pulled out the old cable and repaired conduit and pull in new cable on one end my scope .
Our company subbed out the terminations and testing for this project had to be done fast .

When owner turned the cable on it did not blow the fuses the 4160v step down to 480 v/277 v was putting out 434v on one phase and 160 v neutral to phase voltages were up and down and as load was increased it started to fall off .

The owner shut down we had Ge service tech come out to test breakers/ transformers as this project is a mess old equipment did megger on transformer tested fuses tested switch ratio test insulation test complete look at system Ge was not happy .

When i took off the cable terminations for Ge i took a ohmmeter did a loop resistance test on cables each to ground and to each cable shield to each i found a dead short from A phase to itself .

The owner did not want the whole run replaced to save cost hummmm !!!!

So its the old cable or like you said it could be the new termination but i really think its the old cable .

Heres why when they turned it on the first time the HV contactor in the fused MDP of 4160 gear chatter was a issue it did not pull in this went on two or three times but the plant electrician who is as unsafe as one can be .

No hot suit no protection i walked out of the room when he operated the switch could this not making contact blow a old cable? .

Did the megger not detect a issue in the old cable ? yes

Should we just have pulled in the whole new complete cable ? yes

Did our sub splice it bad ? maybe yes

Cant tell you Zog about the type of cable or if it matched other just a MV 5KV dont remember the insulation of it sorry this was all done at 2 am ya know all night job .

This place needs total repair of there breakers and switches per GE service rep hes disgusted with them .

No pm no one knows whats going on its bad news .

The real reason they did not want us to pull a whole new cable was there MDP switch board is in bad shape and they didnt want us to see it or tamper with it .

Thanks for the info Zog ill let ya know what happens .

 

[zog]

I don't think the whole cable needed to be replaced, but the terms were obviously done wrong and proper testing should have been done prior to energizing.

 

[LarryFine]

When owner turned the cable on it did not blow the fuses the 4160v step down to 480 v/277 v was putting out 434v on one phase and 160 v neutral to phase voltages were up and down and as load was increased it started to fall off .

With an ungrounded system, the voltages to ground are meaningless.

 

[ohmhead]

Update zog

Update zog

Up date were going to pull a new cable in to replace the old one and hook up a portable generator back up as there running on a tie breaker now if it fails well ya know thats not good .

The testing company sub told me Zog today that he did a hi pot on the new cable only and then megged the old cable at 4800 volts and megged it before and after splice he thinks the chatter of the contacts in MDP or just the old age blew it out ?


We found out that the relay control circuit in the switchboards of this MV gear is sharing one control circuit thanks to our fine plant crew that thought it was better to save a dollar on a few control transformers and blow fuses .


It happens to be on A Phase ! All relays of each switch in this MDP GE says this is bad news what do ya think Zog .

The transformer feeding the MDP is a wye to wye 4160 v 2400 amps secondary. Has a pri grounded and a secondary grounded feeds MDP then goes to MV sw down stream with a step down transformer to 480 v 277 v .

Were going with a generator that can run all normal power while we refit this gear totally and clean it up like new GE is going to do that .

Theres 8 MDP switches all need repair and rework bad and lots of other things not to go on to much .

 

[zog]

Sounds like a mess. So the testing company did a DC hipot on the new cable? Why, because they don;t have any real test equipment? His excuse for the cable failure is hilarous, can't they just admit they messed up the termination?

 

[mkgrady]

Sounds like a mess. So the testing company did a DC hipot on the new cable? Why, because they don;t have any real test equipment? His excuse for the cable failure is hilarous, can't they just admit they messed up the termination?

I agree. I doubt the cable had a post termination with a good megger reading and then failed once it was energized.