MV Cable application on an HRG system 1

[nightfox1925]

One of my colleagues is working on a 4.16kV MCC connected to a 4.16kV HRG system. My colleague suggested to use a higher rated voltage (like 15kV) instead of using a 5kV cable. I ask as to why he is doing that and told me that he is treating the system like an ungrounded system such that a line to ground fault on a phase will cause overvoltage on the other phases in such an amount that it may exceed 5kV. I am presently looking on the concept behind this but an assistance from our experts here will gear me up.

Thanks

 

[dpc]

With a HRG system, the maximum voltage to ground is line-to-line voltage. So 5 kV cable is adequate. You can specify 133% insulation or 8 kV insulation. 15 kV cable is serious overkill.

HRG systems are NOT ungrounded. That is the whole point. There is enough of a ground path to keep the neutral point voltage under control and eliminate repetitive restrike problems that cause overvoltages in ungrounded systems.

 

[nightfox1925]

Thanks dpc, I also found this thread: thread238-154874 and I Quote:

"On the other hand, for medium voltage systems above 5kV, the Insulated Power Cable Engineers Association (IPCEA) have requirements in which conductor insulation thickness for a particular voltage is determined by the length of time that a phase-to-ground fault is allowed to persist. Three thickness sizes are specified and are related by the terms 100%, 133% and 173% levels to be applied as follows:
a) 100% level, where the clearing time will not exceed 1 minute.
b) 133% level, where the clearing time exceeds 1 minute, but does not exceed 1 hour.
c) 173% level, where the clearing time exceeds 1 hour.

Obviously the 100% level can be used on any system whether solidly or resistance grounded, providing phase-to-ground faults are cleared in the specified time. This will almost inevitably require fault relaying.
The 133% and 173% levels will apply mainly to ungrounded and high resistance grounded systems, since other forms of grounding will most probably involve ground fault currents that could not be tolerated even for the time permitted. Selection between the 133% and 173% level of insulation will be determined by the time required, after
identification of the faulted feeder, to perform an orderly shutdown of the process being served."

Does the above mentioned standard stating the use of a 173% insulation for a high resistance grounded system mandatory (contrary to 133%)?

 

[dpc]

That will depend on how the system is operated. If you are going to operate indefinitely with a ground fault, you will definitely want the higher insulation. You could get 8 kV with 133% insulation if you really were concerned.

Most systems will trip for a ground fault, even with HRG, so duration will be less than a minute.

You can check with a cable manufacturer such as Okonite and get their recommendation for your specific application.

Cheers,

Dave