Available MVA rating

[Danilo917]

Gents,

Could you advise please if its normal to assume that parameters of positive phase sequence is equal to negative phase sequence at single line to ground fault? Is it possible to derive from positive and zero phase sequences reactive MVA ratings the source available MVA ? Grateful if you could show simple calcs.

Regards,
Danilo

 

[7anoter4]

1)From ABB Protection Application Handbook ch.4.1 STEP BY STEP INSTRUCTIONS - SHORT CIRCUIT CALCULATIONS :
"Negative sequence components for overhead lines are always equal to the positive sequence components."
2) If with "available MVA" you meant "initial symmetrical three-phase short-circuit apparent power" then, in my opinion, you could derive from zero phase [single phase-to-ground fault] short-circuit current if you would know the positive and zero system impedance-considering negative sequence equal to positive sequence. But you have to check the rest of the circuit-transformers and generators. Usually if generators are far from the fault location the positive
and negative impedance are close.
3)For simple calculations see ABB above mentioned handbook [or IEC 60909-0 standard].

 

[magoo2]

It depends how far away from a generating station the fault location is. A generator has a positive sequence impedance that is generally different from its negative sequence impedance. Most other impedances - transformers and lines - have their positive sequence impedance equal to their negative sequence impedance.

A transmission system is largely interconnected so we don't see that difference when we deal with distribution faults, for example. I'm sure that's what 7anoter4 had in mind with his comments and I agree with his conclusion too.

 

[davidbeach]

For all lines and transformers the positive and negative sequence impedance are identical. For generators, the negative sequence impedance is comparable to the subtransient impedance, but without the time constant. Near generation that matters, deep into a system where most of the generation is remote the positive and negative sequence components of the source impedance are nearly (but not exactly) identical. If you've got to do the fault calculations by hand, then sure, call the negative sequence impedance equal to the positive sequence. But who does that? Any real system of any size is modeled in software that easily tracks all three sequence impedances throughout the system.