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Voltage grade of cables
- Thread starter 2919lina
- Start date
Since the transformer on the upstream side (132/11kV) is high resistance grounded, you need to have same precautions as when the network 11kV were isolated. Hence, you must use 11kV/11KV grade cables.
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.
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.
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