Overheating Cable Armor

Cloud

Member
Location
PH/VN
Hello.
We have installed AL armored cable 18/36 kV. All is running fine until recently (3 months operation), the armor of the cables overheated melting the outer insulation and even the XLPE causing a ground fault trip.

What can cause this armor to overheat? We are grounded on both ends of the installation.

Thanks in advance.
 

big john

Senior Member
Location
Portland, ME
What makes you believe the armor is overheating instead of the more obvious choice that the internal conductors are overheating?

The short answer is you have excessive current flow, either due to severe circuit overloads or else due to very serious ground return current flowing over the cable armoring. Ground-return would be easy to verify as the zero sequence component could be seen by clamping an ammeter around the cable.

On a 36kV system I expect your protection would give you a lot of clues about possible causes. What relays are in place and what cuurents are they recording?
 

Cloud

Member
Location
PH/VN
Because of this? The heated armor is spiraling until 1 meter away from the termination point. We only have the ground fault because it already melted the XLPE exposing the core.
I don't know really whether to say it is the core or the armor, that's why I ask for the opinions of the seniors here.

DSCN2564.JPG

Thanks for the reply.
 

Cloud

Member
Location
PH/VN
I have not yet checked the readings on the protection panel at the substation as it is beyond my scope. My team is only responsible for doing the termination on the switchgears. the cables are of 630mm2, 20MW of power at 36 kV at full operation. we are grounded at both ends of the switchgear- the contractor is telling us that we should ground at one end only as they have experienced this before. is that correct?

Thanks!
 
You need to do a shield voltage calculation and will probably need link boxes and shield voltage limiting arresters at one end of cable or possibly in multiple spots. Contact a licensed electrical engineer.
 
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Cloud

Member
Location
PH/VN
I have some info from operation that there was phase imbalance before tripping. Now, I'm thinking of the setting of the protection panel in substation.
 

meternerd

Senior Member
Location
Athol, ID
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?
 
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Cloud

Member
Location
PH/VN
This is a windfarm. The setup is like a series connection with cables interconnecting switchgears. There's a separate grounding bare copper cable and is connected with the tower body and switchgear (general) . We have used the cable armor as the ground. If it is being used as a neutral, I'm not sure. The substation only needs the line voltage to step up to 69kv for distribution. But we have installed all necessary protection for imbalance, overcurrent, SC etc. It tripped and sense the fault, but is late and the cables burnt already. Also, I don't have any clue what causes the imbalance which is the root cause anyway.

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Cloud

Member
Location
PH/VN
What I mean from end to end is the armor is grounded at both ends of the switchgear. The switchgear in return is interlink with the system ground bare copper that runs parallel with the Mv cable in the trench. Does it have any effect on inductive voltage?

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What type of grounding do you have on the medium voltage system? High resistance, low resistance, solidly grounded, etc.?
 

Cloud

Member
Location
PH/VN
29 A. But that was after line 3 and ground short circuit which was caused by overheating armor. The cause of overheat we need to solve. We only have 2km of cable so link boxes is not included in the design.

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Tony S

Senior Member
I have not yet checked the readings on the protection panel at the substation as it is beyond my scope. My team is only responsible for doing the termination on the switchgears. the cables are of 630mm2, 20MW of power at 36 kV at full operation. we are grounded at both ends of the switchgear- the contractor is telling us that we should ground at one end only as they have experienced this before. is that correct?

Thanks!
Correct.

Grounded at both ends you will get circulating currents in the armour.
 
Was a shield voltage study performed? Distance alone does not decide whether you need links boxes and SVL's. Shield type, circuit ampacity, installed configuration, system grounding, etc. all factor in.
 

Cloud

Member
Location
PH/VN
No. I think this is the first time for the designer to connect this amount of towers, with this distance in a single feeder line.

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Sahib

Senior Member
Location
India
You need to examine any ground fault occurred in the vicinity, because such a ground fault current or part of it, on return to source, could have passed through the armoring of the cable as the armor is grounded at the two ends and that earth fault current might be high enough to do the damage you reported.
 

Cloud

Member
Location
PH/VN
What's happening actually is, the armor overheats first, melting the xlpe insulation, then triggering ground fault. From our observation, it only happens when the production is high, thus circulating and induced voltage is high as well and generated heat is enough to damage the insulation.

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Cloud

Member
Location
PH/VN
We need the shield voltage study from the designer, as epimetheus said.

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Sahib

Senior Member
Location
India
What's happening actually is, the armor overheats first, melting the xlpe insulation, then triggering ground fault.
Hmm. I doubt the induced circulating current could be of such a magnitude as to damage the cable....... it, of course, tends to neutralize electromagnetic interference with any nearby communication cables, an advantage of two ends bonding......

From our observation, it only happens when the production is high, thus circulating and induced voltage is high as well and generated heat is enough to damage the insulation.
Next time try to measure the circulating current with a suitable meter.
 
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