Ground fault coordination

[mull982]

Does anyone know a good resource for ground fault protection coordinaiton specifically on resistance grounded system. I am looking at a resistance grounded 4.16kV secondary limited to 400A and wanted to understand the best way to coordinate ground fault protection devices between secondary main and feeder breakers.

The secondary 4.16kV side has a CT on the transformer neutral bushing. The secondary of the transformer feeds an MV switchgear lineup with a main and feeder breakers. Should I simply set the feeder ground fault protection under the ground fault protection that is on the neutral of the transformer that trips the main breaker?

 

[jim dungar]

The secondary 4.16kV side has a CT on the transformer neutral bushing. The secondary of the transformer feeds an MV switchgear lineup with a main and feeder breakers. Should I simply set the feeder ground fault protection under the ground fault protection that is on the neutral of the transformer that trips the main breaker?

Yes.
As simple as it seems, this is what coordination is: the downstream device clears the fault prior to the upstream device.

 

[templdl]

A lot of the coordination is accomplished through time delays, the GF device closest to the fault set faster than thise upstream. Pick-up values can also be coordinated but I is of my belief that timedelays provide the best coordination where the upsream device waits just enough to allow the down stream device to clear the fault.
In some applications the devices actually have the ability to communicate with one another.
But all of these options cost which some are not willing to pay for.

 

[brian john]

You could specify your GFP relays with Zone Selectivity. Where the GFP relay closest to the fault sends a restraint signal to the upstream GFP relay. Should the GFP relay closet to the fault not contain the fault the upstream device can do it?s thing.

 

[templdl]

Yes, I overlooked the GFP which is another great option when trying to coordinate GF protection. Shold you have exsisting breakers it could be addedmbut the existing breakers would have to have shunt trips already in them or they would have to be added to them.

 

[jim dungar]

The OP was about GF on a medium voltage LRG system.

 

[templdl]

You're right Jim,
I got lead down the proverbial rabbit trail and lost sight of how it all started.

 

[rcwilson]

The grounding resistor limits fault currents to 400A maximum. At 4.16 kV, there is not much limiting of the fault current by cable impedance or arc voltages so trying to coordinate strictly by the pickup values is difficult. Coordinate using definite time trip characteristics with staggered time delays. Pickups are usually 10-50% of maximum fault current (40-200 Amps). I prefer all pickups to be at 10%.

Feeder breakers or motor starters with GF functions are set the fastest with minimal time delay to ride through nuisance trips on inrush currents. Then the Tie Breakers, and the Main Breaker.

With modern multi-function relays it is usually possible to have a second ground fault function monitoring the neutral current. One of our clients insisted that we needed to protect the resistor from a long term low level fault. A 400 A resistor is usually rated for 10 seconds and has a 40 A continuous rating. Setting the transformer neutral overcurrent with an inverse time function that picks up at 45 amps and trips in 10 seconds at 400 amps provided that protection and backed up the other main breaker functions.

Note that an internal transformer fault on the 4.16 kV winding can cause current flow through the neutral CT. The usual main breaker relay will only trip the Main and leave the transformer energized. If possible have the backup ground fault trip the transformer high voltage breaker.

 

[jim dungar]

Note that an internal transformer fault on the 4.16 kV winding can cause current flow through the neutral CT. The usual main breaker relay will only trip the Main and leave the transformer energized. If possible have the backup ground fault trip the transformer high voltage breaker.

Been there, got the pictures.

It was good that someone was outside on a smoke break, and happened to looked up at the roof.