High Voltage Distribution Network Discrimination

[11KvSAP]

Please give me your opinion, We have an 11Kv HV network connected to the national grid. We have 4x(1200Amp) Incomers onto a Bus Bar and 800Amp Feeders feeding radial circuits. There are large 11Kv 3MVA Motors on some circuits, This particular Feeder relay over current protection is by IDMT set at 900Amp pick up. The equipment (Cable and Circuit Breakers) are rated at 800Amps. Are there situations on HV networks such as this where the over current IDMT protection can be set above the rating of the equipment, or should this never be the case and protection always to be set below the equipment rating?

 

[Marmite]

It depends what you are trying to protect. If it is feeder protection with a standard inverse curve then the relay is primarily protecting for phase to phase faults, not overload. Typical settings might be upto 150%. A 900A setting doesn't mean it operates at 900A.
Regards
Marmite

 

[11KvSAP]

The motor protection is down stream protected by additional relays, but what i was questioning was the fact that the (equipment as in circuit breaker and cable) are only rated at 800amp and the only over current protecting this circuit breaker is set at 900amp. I would expect this to be below the 800amp rating? as with LV equipment you must have your protection set below the current ratings and current carrying capacities.

With the set up as is currently, would it be possible to have a 3 phase fault that could draw a load above the equipment rated 800amp but below the 900amp IDMT setting that could allow continuous over current flow through the 800amp breaker and cable?

 

[veman]

We protect our feeders with a three phase overcurrent pickup of 720A. Our feeder equipment rating is 600A. Our station breakers may have a higher rating but I'm not sure. For feeders like this, we have 24 hour control room monitoring so if the load approaches 5 or 600 A then we would off load the feeder onto other less loaded feeders or stations. For distribution feeders, this is normal protection around here. Ground protection elements are set at 480A. For is, the amps pickup is the setting at which the breaker would trip...
Veman.

 

[jghrist]

Overcurrent protection on utility feeders is to isolate faulted feeders, not to protect against overload. The settings must be high enough to allow emergency overloading of the feeders, but otherwise are not necessarily related to the equipment load ratings.

 

[shocker9]

I agree with jghrist. Overload is independant of fault factors.
However they can be configured to operate simultaneously.
One overcurrent/earth fault relay cannot perform as a fault and an overload protection device.