instanneous settings and short circuit levels 1

[victa1288]

Hi,

I have a tricky question. When we do the realy coordiantion, we always expect the protective device can operate at less than the minimum short circuit, like L-L faul.

In my case, I am using an extremely inverse curve to protect against Minimum fault to max. fault, when the fault increases to a higher level, I want the instaneous element to operate.But this setting should be less than the available maxmum fault current and higher than the max. fault at the downstream.

The problem is, the fault level at the downstream is slightly less than the fault level. It looks like, the only thing I can do to achieve the coordiantion is to delay the ins. tripping of upstream device. Is it Ok when the fault level is close to max. fault level?

I checked the cable damage curve, the cable can be protected at the max.fault level.

I can't set the Ins. too low, because there would be a miscordiantion with downstream fuse.


Thank you for your valuable inputs.

 

[victa1288]

02101972,

You're right, the breaking capacity of the contactor is not high enough to interupt the bolted fault. But the bolted fault is not high enough to trip the fuse before the contactor can trip. That's why I am thinking of allowing 469 to open contactor at lower fault level before it deveops to bolted fault to trip conatactor at higher fault level.

 

[nightfox1925]

Hi victa1288, this is to wrap everything up.

For the "facility (pump station)" you were referring to:

You have a medium voltage MCC with several feeder starters having fuse-contactor arrangements with SR 469 MPRs. This medium voltage MCC is connected to an upstream medium voltage switchgear via outgoing feeder circuit breaker. I assume that the upstream switchgear is double ended with both incomers and bus ties. I also assume that the bus tie is closed. I also assume that the medium voltage switchgear and mendium voltage MCC are in one substation and hence there is no main disconnecting device on the medium voltage MCC.

Is this the scenario at hand?

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata