Fault Condition Voltage Dip and Step Load Increase

[jbond]

Scenario:
Say there is a small power station being fed by 10 diesel gen-sets on a common bus running at 11kV and being stepped up off the bus feeder to 22kV for distribution and somewhere on the network 22kV there is a fault to earth.
The fault current will be accompanied by a voltage dip at the gen-sets, correct?

Question:
The fault current and the impedance of the path to earth will appear as a step load to the gen-sets and cause them to get bogged down and stall, however to calculate if that fault will be cleared in time without stalling them i need to look at the protection curves on that feeder. If after doing that, i can ascertain that the fault should be cleared in 0.6s by the CB, i now need to see what happens to that gen-set within 0.6s of the fault conditions. How do i calculate what that step load will be? Do I need to know the voltage dip? How do I work that out?


It seems to me that the calculations that I did assuming the system to be at 22kV to determine the fault current may be wrong given that the voltage will dip below that. I seem to have used a static-type calculation that doesn't take the transient nature of the voltage into account.

Any and all help is appreciated !

Cheers,
Matt.

 

[ijl]

1) 0.6 s is a long time. The fault should be cleared much more quickly. Thus, this problem may be a bit theoretical.

2) With a proper calculation method (sequence networks, subtransient reactances etc) for the calculation of the fault current there is no need to worry about the effect of the voltage drop on the current. The fault current and the voltage drop are both consequences of the fault.

3) The voltage drop at the generators can be calculated similarly as the fault current itself, in principle, at least. There are few programs that do it, or display the voltages. One such a program can be downloaded at

http://pp.kpnet.fi/ijl

Note that the currents, voltage drops and thus the "step load" are unsymmetrical.

4) It may be best to study this problem with a transient simulator, such as the ATP, EMTP or similar.