Overheating Cable Armor

[Cloud]

It's a 36kv 20MW system, and we have previous experiences that when the ground was disconnected on the other end, the fault disappear. The problem now is the specs of the surge voltage limiters to be installed. That is why we are requesting a shield voltage study.

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[Haji]

Interesting. The fault energy that is capable of melting down metallic armoring of the cable is proposed to be absorbed by surge voltage limiters. But it seems the surge limiters only have limited capacity to absorb energy............

 

[Sahib]

It's a 36kv 20MW system, and we have previous experiences that when the ground was disconnected on the other end, the fault disappear. The problem now is the specs of the surge voltage limiters to be installed. That is why we are requesting a shield voltage study.

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If the armor is bonded at both ends, surge voltage limiters and shield voltage study may not be required.

 

[Cloud]

As far as I know, solidly bonded system is notorious for heating up. How can we prevent this if not with svl? Is there any other method? It's the 3rd time now, same manifestations and fault locations on cables. That is why we are thinking that this is not phase to ground fault alone.

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[Cloud]

And the other plant that removed their bond on the other end is running for 8 mos now, without fail.

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[Cloud]

Interesting. The fault energy that is capable of melting down metallic armoring of the cable is proposed to be absorbed by surge voltage limiters. But it seems the surge limiters only have limited capacity to absorb energy............

We think that the fault is accumulated as the system can run for about 2 weeks before a fault could happen. And it only happens when the cables reach about 80% capacity at 16+MW.

 

[Sahib]

As far as I know, solidly bonded system is notorious for heating up. How can we prevent this if not with svl? Is there any other method? It's the 3rd time now, same manifestations and fault locations on cables. That is why we are thinking that this is not phase to ground fault alone.
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svl is used only when it is desired to prevent energy losses in the cable. What is more, the energy losses in the cable would not be high enough to do damage to the cable; also the transient surge fault energy in the cable sheath would not damage the cable, if it is bonded at both the ends. So one reasonable course of action you may take is:

Next time try to measure the circulating current with a suitable meter.

 

[Cloud]

Now the system is running smoothly for 3 weeks without fault. We disconnected the other end of the grounding temporarily while waiting for the SVL to installed.

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[Tony S]

So the contractor was correct.

There’s a surprise ;-)

 

[Cloud]

The problem comes with the design overlooking this circulating currents on mv cables which depends on load and distance. Now we will install SVL for safety purposes since there will be high current on the disconnected end.

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[Tony S]

The same problem occurs with LV cables. Voltage plays no part in it, current is the problem.

MV will break the insulation down quicker, the result is the same, a big bang.

 

[Cloud]

But yes. The higher the load, the higher the circulating currents the insulation cannot handle. We have another feeder, only 4 towers connected without any problem. This thing only occurs on 6 towers (12MW) above with distance of 2km+.

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[Tony S]

That’s interesting, I would have thought it would be the other way around.

Time for a think. The problems still the same but………..?

 

[iwire]

But yes. The higher the load, the higher the circulating currents the insulation cannot handle. We have another feeder, only 4 towers connected without any problem. This thing only occurs on 6 towers (12MW) above with distance of 2km+.

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Capacitance coupling perhaps?

I am asking more than suggesting as my first hand experience is all under 600 volts

 

[Cloud]

There should be link boxes on longer lengths of cable as suggested before. Pros and cons of different grounding configuration explained: http://www.arresterworks.com/arrest...esterFacts_032_The_Sheath_Voltage_Limiter.pdf

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[Cloud]

Update: disconnected 1 grounding end on every tower interconnect, installed a 3kV SVL on that end to discharge the excess current if ever a fault should happen. Plant is running continously for 3 mos now without fail and the client will have their ROI in 5 years.

 

[jumper]

Update: disconnected 1 grounding end on every tower interconnect, installed a 3kV SVL on that end to discharge the excess current if ever a fault should happen. Plant is running continously for 3 mos now without fail and the client will have their ROI in 5 years.

Good job. Have a cookie.

 

[gersh]

Overheating cable

Overheating cable

If the shield is being used as a neutral, that would explain the heat if there was significant imbalance for an extended period. I'm not familiar with jacketed armored underground cable. Is there armor in addition to a concentric conductor? You said grounded at both ends. Does that mean a grounded conductor (such as a neutral), a separate ground wire or the armor? Do you use a ground CT and associated overcurrent relay for ground fault protection or just imbalance such as that on a three phase recloser control? Could the ground fault tripping have been blocked at the control panel?

Why would you need a Neutral on High Voltage Line....

 

[Michael M Perez]

Overheated cable

Overheated cable

It could be due to loose connection. Poor connections often result in overheatingand generate a high wattage over a small area for a long period of time. The oxide formed at the contact area cause power dissipation.

 

[Cloud]

Except it's not. We have performed torque test on all the connection. As what I've said previously it was due to the grounding being it solidly bonded and the circulating currents. Now it is single bonded, the problem is corrected. And with 32 plus terminations it only happened on the end line of the longest circuit, where thr highest production is thus high circulating currents as well. It's been producing continously for a year now.

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