Cable Insulation Hi-Resistance Ground 1

[EP007]

The 4.16 KV system has an NGR, 5A continuous. For a ground fault in the zone of service transfomer secondary and 4.16 KV SWGR main breaker, that breaker will open. The transformer primary has only a fuse, which will not open for the small current. The fault will then persist and be alarmed, but will only be cleared eventually by pulling the primary fuse on the pole. The elevated phase to ground voltage on two phases would be 173%. I believe the 100% 5 KV cable is not proper and the next higher available 8 KV insulation level is required, between the service transformer and SWGR. Cable would be 3/c, armoured, jacketted, probably unshielded. Does anyone have similar experience?

 

[dpc]

8 kV cable would be better. I'd check with cable manufacturer.

 

[burnt2x]

173% of the Line to neutral voltage = 2.4*1.732 = 4.16 kV! You are still within range if you calculated 173%! However, if you were ungrounded, the line to neutral voltage could be very high (capacitive charging of lines)!

 

[rockman7892]

I agree that the L-N voltage would only still be 4.16kV on the two ungrounded phases and the cable would be within spec.

I dont understand however if ungrounded how the capacitive charging would charge this voltage to be higher?

 

[dpc]

Ungrounded systems can experience voltages much greater than line-to-line voltage during ground faults due to "repetitive restrike". Restriking arcs can pump up the voltage to ground by charging the capacitance to ground in the circuit faster than it can be discharged. High-resistance grounded systems provide just enough resistance to get the time constant down low enough to avoid this.

 

[burnt2x]

dpc,
great explanation! short and precise!star to you!

rockman,
Consider the line insulation as a capacitor. When an intermittent ground strikes, this "capacitor" charges; discharges on the negative cycle through the grounding resistor. If the charge was not drained (high resistance grounding), another "restrike" will pump up the charge; hence the voltage increases. Could be more than the line to line voltage if not interrrupted in time.

 

[EP007]

Cable spec of 5 KV 100% is a phase to phase nomenclature. It is 2.887 KV phase to ground cable rating. On the continuous L-G fault, we have 4.16 KV L-G on two of the phases, which is 144% of the 2.887 cable rating.

If we require a cable of 4.16 L-G, that means 1.73x4.16 = 7.2 KV phase-phase min. Next avaialble size is 8 KV. A 5 KV 173 % would also work, but I don't see any available.

We can also refer to IEEE 242-2001, Protection and Coordination of Industrial and commercial Power Systems, Chap 9, Conductor Protection, p 286, which lists the insulation level requirements 100%, 133%, 173%, and calls for the 173% rating if the ground fault persists past one hour, 133% for up to one hour, and 100% for up to one minute. Mine could persist much longer than one hour.

 

[EP007]

This is in addition to my previous post. Consider the situation if we intentionally left the transformer neutral ungrounded, but instead solidly grounded one of the phases. And operated continuously like that, supplying only 3 phase loads. What would the required cable insulation level be then?

I also see, my old SouthWire Power Cable Manual gives the same 100/133/173% insulation requirements as Ieee 242.