Fault current through generator cables

[davva]

If you have 4 generators in parallel each capable of supplying 10kA fault current, could you able the argument:-

With all generators running and for a fault on the cables of one generator, would the maximum fault current seen by that cable be 3 x 10kA, rather than 4 x 10kA? (obviously approx. 4 x 10kA flows into the fault with 3 parts being supplied by 3 sets and 1 part being supplied by the faulted generator). This argument is being put forward to me as part of a protection study which includes I2t protection of the cables.

All sounds good in terms of Kirchoffs but I've always worked on the basis that the switchboards and cables would all be rated on maximum fault level.

 

[cuky2000]

The total SC fault current is sum of all generators, motors or any other source connected in the system minus the current dampened in the cable and any other impedance connected in series with the generators circuit.

If this is an isolated system from the utility and the cable runs is short, probably the dampening may be neglected. There is a possibility that the fault is larger than the 4 generators contribution if there are large motor load.

 

[davidbeach]

davva, for a fault in a cable between a generator and the board where the output of the four generators is paralleled, the fault in the cable will draw 40kA, of which 10kA will flow from the generator, through the breaker at the generator, and the other 30kA will flow through the breaker at the board, back toward the generator. A fault on a feeder between the board and any load will draw the full 40kA through the breaker at the board. Somewhat simplified by eliminating fault current reduction due to cable impedance and any increase in fault current due to motor contribution. If you don't have breakers at the generators, you should consider high resistance grounding of the generators to drastically reduce the possible phase-to-ground fault.

 

[rbulsara]

davva:

Technically the argument you stated can be made and is correct "only if" there is no motor contribution and the generators are not paralleled with the utility source. It is also only true for the cable between the generator and the paralleling swithgear.

 

[dpc]

It is true that for a fault at a particular location, not all equipment "sees" the same fault current, especially when there are multiple sources. This fact can be used in evaluating equipment short circuit interrupting and withstand ratings as long as you are sure you have the worst case covered.

Obviously, if you use maximum bus fault current, you will always be safe.

The analysis software we use actually caculates the maximum fault current through each breaker (in either direction) and uses that value to compare to the equipment rating.

 

[nightfox1925]

It is a normal practice to have directional overcurrent relays provided for switchboards having multiple incoming feeds. Normally, your generators may have separate bus bars interconnected via bus tie circuit breakers. Assuming you will have similar set-up, there should be proper coordination between the incomer directional relays, bus ties and upstream protection for both incoming line cable fault and bus bar faults. The fault current values will vary depending on the location of the fault in all the cases.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata