Short circuit level.

[Mlmlzx]

Dear sir,

a) For a 27.3 MVA, 11 kV generator, with 750 rpm, what is the range of value of subtransient reactance? . Do we have to take unsaturated value of d-quadrature subtransient reactance? Value of short circuit MVA is then given by (27.3*1000000/(subtransient reactance?)

I found out by graph that for around 30 MVA and 750 rpm, rated voltage per unit subtransient reactance is around 0.15.

b) If it is an underground powerhouse with GIS and transformer cavern also underground, can excitation system be selected as brushless excitation as it is a 70 MW generator (system is of 1500 MW).

c) Can we take metal enclosed bus duct as segregated bus duct as the rated current of generator is only 1432 amps, as this is economical. But it appears that isolated phase bus duct are practically free from three phase short circuit?

d) For this rating, how to calculate the neutral isolating switch rated current? (I found 400 amps as the NIS rating for 14 MW alternator, which clearly has rated current of 735 Amps.

Optimistically waiting!!

 

[7anoter4]

If it is about a salient poles generator of less than 16 poles [this is 8 poles generator] and it is provided with a copper or brass damper winding then [for German units] x"d=0.14 up to 0.23
The initial symmetrical short circuit current, I”k=c*Un/sqrt(3)/Zk
Ik"=U1/Xd" symmetrical three phase short circuit current at the generator terminals [neglecting stator resistance]
Un=phase-to-phase rated voltage c=1.1 [see IEC 60909-0]
Xd"=Un^2/Sn*xd"
I don't have enough data to answer the other points. Sure, I.Ph.Busduct is safer but more expensive.

 

[7anoter4]

In connection with item d) the problem it is not so simple since there are many grounding systems. If it is solidly grounding system then the generator power-and mainly the generator impedance [xd,xo,x2 ] are important. Usually, it is about of low resistance grounding where a series resistance is introduced between the grounding grid and the generator neutral terminal.
The power dissipated in this resistance is very high depending on supply voltage-11/sqrt(3) for instance in this case-2.54 MW. However, the standard resistance rated power it could be 0.254 MW but will withstand only 10 seconds. However, like the solidly grounded neutral system, the circuit must be shut down after the first ground fault.
Low resistance grounding resistors are typically rated 400 amps for 10 seconds, and are commonly found on medium and high voltage systems
So, 400 A depends only on manufacturer standard resistance and the rated voltage.