Coordination with Utility and generator

[SELEC]

I am doing a design for a compressor station. After I did calculaton, I found out the short circuit level when it is connected to utility side is much higher than the short circuit level when it is connected to the on site generator side.The main CB on SWGR is controled by SR750 Relay. I am not sure how to setup the OC. Which S-C I should base on? My understanding is, we should protect the system based on normal running scenaio.We should allow the system running under emergency state without proyection.I know there is another way of solving the problem in SR750. That is, to set up two setting groups by control fuction of Realy which are controled by the aux contacts from the breakers of both sides respectively.I argued with my superviser about this. He insited on protecting Geneartor side so that the utility side can be protected as well. The problem is, the settings are too low, causing unecessry trippings sometimes.

 

[wbd]

I would argue for having two settings contollable by which source is connected. Here's another justification - arc flash hazard. Having the setting set for the utility Isc would be higher than the Isc for the generator. This will lead to long trip times under arc flash scenarios. It's entirely possible that with the utility connected, an arc flash hazard on a bus may be Category 0 but with the generator connected the arc flash hazard, on the same bus, may be Category 4 or higher.

 

[cuky2000]

Current limiting reactor may be an option to balance the SC level when operated connected to the utility and disconnected otherwise.

 

[davidbeach]

Settings groups or reactor would both work. Reactor would make for lower fault currents but would have losses that you (or your client) would be paying for forever. I think that settings groups would be your best option.

 

[rcw retired EE]

We have had problems using setting group changes with the 750 when changing "on-the-fly." Look at using different overcurrent elements in the relay and ading some internal logic based on the generator breaker status to block and/or allow the differnt overcurrent elements to operate. I believe teh 750 has enough overcurrent elements to do that.

Changing setting groups works, but I advise investigating how long the relay is out of service during a setting group change.

 

[davidbeach]

There is at least one brand of relay that does a settings group change without going off out of service; others can take up wards of seconds to make a complete settings group change.

 

[alehman]

I agree that you need protection for both sources. Sometimes it is much simpler to use separate relays. Which protection element is causing the false trip?

 

[SELEC]

Instaneous protection element is causing the false trip. Possibly we set this value toolow based on Isc on Generator side.

Thank you for these valuable inputs.

 

[alehman]

Is it tripping when switching from utility to generator? Generator to utility? If the switch is open transition (break before make)? the tripping is probably due to load inrush. What is the set point relative to Isc for each source? What is the load?

 

[SELEC]

The tripping wa due to load inrush. the set point was based on Isc for Generator side.