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6.9KV System powered by Emergency Generator w/High Resistance Grounded

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jliu1

Electrical
Jul 12, 2002
23
We have 6.9KV system powered by Emergency Generator with high resistance grounded. It is a requirement that the system will stay in operation for first ground fault, considering its emergency functions.

The questions are, under the condition:

1. Will the 8KV cable be specified with 173% insulation since the 133% insulation is not enough to cover the 6.9KV*1.73=11.95KV? Or 15KV cable needs to be used?

2. What about MV motor winding insulation?

3. What about Switchgear?

Thanks for any input.

 
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Some of us would consider this to be an 11.95 kV system and use 15 kV cable.

2. What about MV motor winding insulation?

3. What about Switchgear?

If this is a grounded system it may not be wise to run high resistance grounded during emergencies.

It may be wiser to re-engineer the system as an HRG system so that there is no change in available ground fault current whether on normal power or emergency generator power.
Rather than the HRG giving you extra dependability during an outage it may cause an unexpected failure during an emergency.
Switching from a low impedance ground to an HRG may cause unreliable operation of protective relays as well as possible voltage stress failures.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
IEEE Std 141-1993[Red Book] ch. 12.4.1 Voltage rating [Cable Systems] states:
"On ungrounded systems where the clearing time of the 100% level category cannot be met, and yet there is adequate assurance that the faulted section will be cleared within 1 h, 133% insulation level cables are required.
On systems where the time required to de-energize a grounded section is indefinite, a 173% insulation level is used."
So if you could limit the clearing fault time to maximum 1 h you may use a 6.9*1.33=10 kV cable.
[According to IEC standard 60502 the 6/10 kV [maximum 12 kV] is a standard cable.]
According to the Red Book table 3-1 the maximum voltage for 6.9 kV system it is 7240 V.
If the switchgear was tested for this maximum voltage no damage could happened.
Surge protector has to be provided anyway.
The induction motor rated voltage usually is 6600 V and 110% [7260 V] is still permitted.
In order to protect induction motor against transients a surge protector should be provided. See, for instance:
 
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