Alternator impedance

[electromott]

I am struggling with a fault calculation for a generator installation.

I have a 1500kVA 400V 3ph generator with instantaneous If 16655A, short term If 12028A for 0.135s and sustained If 5,400A. But what do I imput for the external impedance; is it simply 230/5400=0.04?

 

[mpparent]

Which reactance do you want? Sub-transient? Transient? Steady State? What exactly are you trying to do? These values change over time of an event.

Mike

 

[electromott]

I have the reactances from the manufacturer and want to ensure the final circuits are clearing correctly i.e 5s max. Should I use the sustained If with impedance of 0.04 or a combination or something else?

Xd 2.96
X'd 0.18
X''d 0.13

All at 1550kVA base 400/230V.

 

[cuky2000]

Check if the enclose information related with rotating machine could help you.

http://www.arcadvisor.com/reference.html#RotatingMachine

 

[mpparent]

The best thing is to get the capability curve from the alternator manufacturer, and then overlay that with your protective device.

Mike

 

[daniele039]

The base impedance is :

z_base = Un^2/An = 0.106 ;

The xd , or sincronous reactance (steady state) , in per units
is equal to 2.96 .

xd [ohm] = 2.96*0.106 = 0.316

that is close to 0.4 .

It is correct ?

 

[alehman]

mpparent has the correct answer. The manufacturer should provide what is called a "decrement curve" for the generator. It is difficult to calculate this curve analytically because besides for the machine impedances, it depends on rotating inertia, voltage regulator, exciter and speed control performance. Many small genreators have what is called "field forcing" which increases the excitation current to maintain a specified fault current level for a period of time to assure predictable operation of overcurrent protection. 3x rated current for 10s is a common requirement.

The initial fault current can be determined from the sub-transient reactance, but after a few cycles, all of the other factors mentioned above come in to play.