Plant Performance Requirements in Connection Conditions

[rezaa]

I would be very grateful for one sending to me the reason for "why the short circuit ratio of Generating units should not be less than 0.5 as a
connection condition." and, may be something regarding the technical aspects of subject.

 

[fieldserveng]

This requirement is to limit transient overvoltages (caused by generators interconnected to a utility's system) during fault conditions which could damage the utility's and/or your own equipment. For a more specific and detailed description you need to refer to IEEE C62.92.2-1989 (Guide for the application of neutral grounding in electrical utility systems, Part 2 - Grounding of synchronous generating systems)

 

[RRaghunath]

Rezaa,
The SCR of a generator decides what current the generator can contribute in the event of faults (symmetric 3phase faults) in the system. This is important for the health of the network as the fault detection and clearance depend on overall fault current seen by the relays. Secondly, the voltage available at the system bus bars, during fault conditions, is a function of the fault current available.

Having said the above, the requirement of higher SCR is becoming less and less relevant with modern, fast acting AVRs that include field forcing capability during faults to maintain the fault current. For larger generators, IEC 60034 stipulates SCR of 0.45 (hope my memory is correct).

 

[rezaa]

Thanks, for reply but I want to emphasis that the Short Circuit Ratio doesn't any relation to the over voltage,as mentioned in the above post, it is just a measure for a Xd and X'd,…. And a factor for a transient stability of generator,and ofcourse fault level as you mentioned in this mail, the smaller vales cause the cost of construction would be cheaper and in the IEC60034-1 it is standardized as 0.5(I guess) or 0.45 as you refered. My question is how one has reached this 0.5 as an optimum value.