Fault Current Calculation at E.G. and 1st downstream panel

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anbm

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Hi,

I have to use point-to-point method to calculate fault current current and submit to Florida state. How to we calculate fault current at emergency generator and its first downstream panel?

I mean I have standby 250kW, 480V/3PH, emergency generator EG1 which feeds emergency panel 'EDPH' and panel 'EDPH' will feeds ATS1 and ATS2.

They are looking for fault current at EG1 and panel 'EDPH'. ATS1 and ATS2 are also fed out of normal power source which I knew how to calculate the fault current at these points.


Thanks!

anbm:cool:
 
anbm said:
Hi,

I have to use point-to-point method to calculate fault current current and submit to Florida state. How to we calculate fault current at emergency generator and its first downstream panel?

I mean I have standby 250kW, 480V/3PH, emergency generator EG1 which feeds emergency panel 'EDPH' and panel 'EDPH' will feeds ATS1 and ATS2.

They are looking for fault current at EG1 and panel 'EDPH'. ATS1 and ATS2 are also fed out of normal power source which I knew how to calculate the fault current at these points.


Thanks!

anbm:cool:
Click to expand...

You need a value for the subtransient reactance (Xd") of the generator. Xd" is used similar to the %Z of a transformer. In most cases (i.e. unless the generator was custom built for you) you can only get Xd" from the manufacturer.
 
One way would be to get the specs on the generator, and calculate the available fault current. (I don't know the exact formula's or exactly what specs you need off the top of my head.)

Another way might be to get the available fault current from teh generator rep.

A third way might be to get the AIC rating off the generator main, and start with that.

If all else fails, generators typically can put out about 8 times their rated current. So, if it were me, I would probably consider using something like 15x or 20x the rated current. Thats probably stil a lot less than the utilities available fault current.
 
It depends on what you are trying to do. The Xd" is the Subtransient reluctance that determines the fault current during the first cycle of the fault. In about 0.10 seconds the reactance increases to Xd' Transient reactance which determines the fault for about 1/2 to 3 seconds. Xd Synchronous reactance is the value that determines the fault current after several seconds has passed. The fault interrupting devices, fuses and breakers, usually will operate before this steady state
condition is reached thus Xd Syn is seldom used in fault calculations.

To determine if the MCCB is going to trip on its instantaneous element, you would use the subtransient reactance.

For determining operation of short-time delay elements (3-10 cycles) , Xd' the transient reactance is more accurate.


Check here http://www.aptinc.net/info_center_papers/Generator_X_to_R_ratio1.pdf
 
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