Co Generation system

[Krishnans]

Hi
Please review the following and clarify
? Two Co gen sets 1500kVA @ 415V 50HZ supply operate in parallel with 3 x 1500kVA Utility power supply system.
? Utility supply is received at LV switchboard designed for a symmetrical short circuit current 53kA @ 1 sec.
? The two Co gen sets when running in parallel have a three phase symmetrical short circuit current of 40kA
? Utilty requires restricting the fault level at their switchboard to 53kA @ 1sec when operating in parallel

Question
Under parallel operation If there is a fault in Utility LV switchboard, the fault current contribution from the two Co gen sets has to be limited so that the magnitude of 53kA @ 1sec is not exceeded.
Please suggest a solution
Krishnan S

 

[eric9822]

This needs to be looked at by a PE. There are a lot of variables involved.

 

[dbuckley]

The classical answer is a reactor, which is almost a standard solution on cogen plants when the fault currents get silly.

But.... 53KA sounds low for a switchboard of this size, and my guess would be that the utility supply on its own could exceed that. Thus it appears that the margin for error here is very tight, maybe too tight for reactors to be the solution.

An expensive solution is static switches between the cogen gensets and the switchboard, but, fast as static switches are, you could still have almost a complete cycle of current delivered through the switches before they open (on the phase zero cross) and for that most-cycle the fault current could be the worst case of cogens + utility.

So yes, I agree professional advice should be sought, this is too close for cookie cutter solutions.

 

[Mayimbe]

Question
Under parallel operation If there is a fault in Utility LV switchboard, the fault current contribution from the two Co gen sets has to be limited so that the magnitude of 53kA @ 1sec is not exceeded.
Please suggest a solution

Two overcurrents relays at the outputs of the co gen. Adjusted at a trip time < 1 sec. Previously you have to do a shortcircuit analysis on both co gen, its going to take you time and effort. Just guessing