Synchronous Generator - Short Circuit Ratio

[bahram]

Can anybody help me to answer the below question:

Our contractor has proposed a hydro generator (2000kW, 750rpm, 6kV, pf=0.9) with the short circuit ratio of 0.64
but the original tender document asked for SCR=0.9

The question is:"How is the price impact if we choose generator with SCR=0.64 instead of 0.9? is it getting cheaper or more expensive?
It is obvious that any change in SCR has affect on Xd and consequently the electromagnetic circuit.

Is there any rule-of-thumb to calculate the percentage of price differnce?

 

[RRaghunath]

Higher the SCR for generator better it is from the point of view of feeding motor starting inrush currents (down stream loads)and maintaining the currents / generator terminal voltage during faults in the system.

IEC 34 stipulates minimum value as o.5 (from memory, I may be slightly off the mark) for smaller machines.

I thik with modern AVRs which can act pretty fast to boost the excitation to about 10times the rated (for a short time, ofcourse)when required and are reliable, the stipulation on higher SCR is not much significant.

Price - I guess the generator with higher SCR should be costing high. May be, it is more to do with the design difficulty and standardisation of designs - some one with generator design background can clear.

 

[rbulsara]

Pleading my ignorance..

what is SCR of a generator? definition? must be an IEC term, not heard of it in the USA.

 

[ScottyUK]

rbulsara,

SCR - Short Circuit Current: the ratio of the field current required to develop rated open-circuit voltage to the field current to circulate rated short-circuit current.

We have generators of US origin - their specs include SCR, so I think it might be worldwide.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[rbulsara]

thanks, scottyUK.

I should say I never had to deal with that term in specifying a generator of that range. I only specify X"d range or max. short current to suite the requirements.

Just a side note: Watch out for competeing goals. Lowering the cost is only one of them. Reactance and hence the available SCC also depends on the pitch factor of the windings. At least in one case I know of, while insisting on certain reactance and SCC values, the manufacturers (and engineers)overlooked the specified pitch factor requirement which resulted in unacceptable circuitlating currets while parallleling with the utility and six (6) 2000kW alternator had to be replaced (by the mfr.).