Will the load contribute to the SC damage damage of the cable 1

[Beengineer]

As it's said, the degree of the short circuit damage depends on the impendance. If the fault location is close to the current source (substation), then the damage will be more severe.

My question is, if the cable is 100% loaded and has a fault, compare to the cable that is 20% loaded under same fault condition, will the damage be the same?

Thanks

 

[rcw retired EE]

The short circuit cable damage curves are drawn based on some assumptions:

1. Cable operating at design temperature (60C, 75C, 90C) prior to short circuit. (XLPE insulation = 90C)
2. All heat generated by short cicuit current stays inside the cable. (Not enough time for the heat to dissipate.)
3.Cable is OK if the resulting temperature is less than the cable insualtion damage temperature. (XLPE = 250C)

These assumptions or similar statements should be noted on the chart.

By loading the cable to only 20%, assumption #1 is probably violated and the starting temperature is less. The cable can take more heat before it is damaged. Check the Buff Book or Okonite website for the temperature damage equation, estimate your starting temperature, and calculate the allowable short circut time.

 

[rbulsara]

For all practical purposes, loading of the cable has no or negligible effect on the cable damage due to a fault. In fact the load drops to zero when the fault occurs. The "type" of load may contribute somewhat like contribution from motor.

This assumes that the fault itself was not caused by the overloading (overheating).

Rafiq Bulsara

http://www.srengineersct.com

 

[electricpete]

But as rcwilson has pointed out, the initial temperature of the fully-loaded cable will be higher than the 20%-loaded cable (assuming initial steady state conditions).

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[rbulsara]

Short circuit faults could and normally do vaporize copper. Copper melts at 1083 deg C. Whether you reach there from 30 deg C or 90, would not make any difference.

Also holds true for cable insulation overheating which is normally caused by loose connections. A loose connection will eventually cause enough heat to damage the insulation, whether it is fully loaded or partially.

Rafiq Bulsara

http://www.srengineersct.com

 

[pwrtran]

In most cases cable damage refers to the damage of insulation layers not the current carrying metal either copper or aluminum. Operating in hotter temperature does accelerate the degradation of the insulation over the time and it can lead thermal breakdown at weakest spots. If strictly talking about copper melting, a good example is the ground electrodes, by comparing a 3/0 copper vs 1000MCM copper cable you can see the difference in terms of the ampacity.

 

[racobb]

I would also say that your assumption that if the fault is closer to the sub then the damage will be more is not necessarily the case.

Depends on how much closer and the protection at the sub.

Closer to the sub should mean higher fault currents and hopefully faster clearing times, given proper protection.

Who said "As it's said"?

Alan

 

[Beengineer]

Alan,

When the engineer performs the GPR study, the distant from sustation is the main factor to consider, then the fault clearing time (and other factors of course). Doesn't matter how the fault is closer to the substation, the clearing time would not be less than 5 cycles for most of utilities, due to the speed of circuit breaker.

Same thing applies to the cable splice fault.

WOuld you tell why some damage caused by cable splice failures (fault) were hardly visible except the substation circuit breaker was tripped. But others caused the explosion even even vault fire?

 

[waross]

The effects of prior loading on initial temperature have been noted.
What do you mean by damage? Cable damage, damage at the site location or some other damage.
A solid ground fault will be fed from the source and may be fed by regenerating motors. Regenerated current may increase the damage at the site of the fault, but may not be seen by the cable ahead of the fault.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Beengineer]

Damage caused by fault, such as conductor liquefying, insulation & splice sleeve ripped open. And in the worse case, damaged the site or even caused vault fire.

THanks

 

[waross]

There are many types of failures. Few of them are solid phase to phase or phase to ground. They may be phase to phase with different impedances that change but not equaly or in proportion as the failure develops. Add the that the complications when one or more phases goes to ground with differing and changing impedance.
To see the effects of loading, you must consider the existence of regenerating loads and then analyse the failure circuit to see if the regenerating current adds to the supply current, has no effect or subtracts from the supply current.
In some instances an open may do much more damage at the failure location than a short or partial short. This may be even more so in the event of a far out failure where the open is not immediately detected or the open arcs for a time before going to ground and initiating ground fault protection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Marmite]

A fault can have many different outcomes. A cable box fault could cause a trip with no visible damage, or it could blow the roof and doors off the substation. Fault calculations assume a theoretical worst case fault with no impedance. Real faults have impedance which can vary widely over the duration of the fault. It is not possible to reliably predict the physical outcome of any given fault. In my view you can not directly correlate physical damage with fault level and it is not correct to say that the damage will be worse if the fault is closer to the substation. It is correct only to say that the fault level will be higher closer to the substation.
Regards
Marmite

 

[cherry2000]

One factor to remember is that if it is a motor load, a running motor will convert the running kinetic energy to electrical energy and feed power back to the fault. A thumbrule for motor contribution to a line fault is 4 times the FLA of the motor. Thus besides the initial temp of the cable (which is usually taken as 75degC which is the highest temp that the cable can attain based on the limitation posed by 75degC terminations), the motor contribution can also have an effect on the cable. Definitely a cable with 20% loading, will result in far lower temp., 4% of the 75degC since temp rise is approx proportional to square of the current. Thus you now have a much better cushion on which you can tolerate a short circuit current for a longer period (i.e higher i2t let through energy).