how to calculate and limit fault currents in paralleled systems
how to calculate and limit fault currents in paralleled systems
(OP)
We need to design/manufacture a distribution system with a total capacity of 12000kVA of generated power.
This involves 6 off 2000kVA diesel generating sets paralleled onto a common busbar with each set having a 3200A air circuit breaker in line to feed the busbar.
Apart from the obvious; to break the busbars into manageable sections as suggested by us; what other magical means are available to handle the high prospective fault
currents on the busbars and to protect existing switchgear with " fault limiting inductors or other devices " as suggested by the consulting engineers office.
This involves 6 off 2000kVA diesel generating sets paralleled onto a common busbar with each set having a 3200A air circuit breaker in line to feed the busbar.
Apart from the obvious; to break the busbars into manageable sections as suggested by us; what other magical means are available to handle the high prospective fault
currents on the busbars and to protect existing switchgear with " fault limiting inductors or other devices " as suggested by the consulting engineers office.






RE: how to calculate and limit fault currents in paralleled systems
Can you give a bit more of a description of your system. In particular:
operating voltage
number of feeds required from the "common busbar"
loads of each feed
nature of loads to be fed (existing MCC's maybe??)
If existing MCC's, their fault rating?
Have you calculated the prospective fault currents at the "common busbar", three-phase and phase to earth? what are they?
I assume you are looking at generating and distributing at LV, there must be a good reason for this. The sort of generating capacity you have suggests a smaller number of HV machines and an HV distribution system may have been better, with HV/LV transformers feeding the LV system so why not use an HV approach?
If you have to go LV, it is possible to obtain type tested 100kA LV switchboards, 100kA (150kA even) ACB's and 100kA MCCB's.
Your suggesation (splitting) is a very good one. Is it practical for your installation? Other possibilities include:
feed via fuses (may stuff up downstream discrimination)
feed via hi capacity current limiting MCCB's
feed via reactors (not sure if practical at LV)
Also, take into account the fault current attenuation that LV feeder cables provide.
Depending on the equipment you will be feeding, a combination of the above (with splitting) may well get you there.
Have you spoken to a switchboard or switchgear manufacturer?
Regards
RE: how to calculate and limit fault currents in paralleled systems
They are essentially fast acting fuses that are shunted in normal operation by a bridge. When the controls sense the inception of a fault current that will exceed the rated capacity of the protected switchgear, an explosive charge is fired that opens the shunt bridge. The current limiting fuse then interrupts the current before it reaches the first peak.
Sounds horrendous, but they do work. Check the ABB website for details.
I do prefer the passive current limiting reactor approach, if your system can accommodate the voltage drop. In fact, looking back at the above, I fully agree with Bigamp that it would be a far better idea to generate at medium voltage (say 2.4 kV, 3.3 kV or 4.16 kV depending on your location and standard voltages) and provide a single step down transformer to the existing LV switchgear. The transformer provides the current limiting reactance required.
RE: how to calculate and limit fault currents in paralleled systems
RE: how to calculate and limit fault currents in paralleled systems
Once the fault is cleared the system operates through the reactor until the current limiter is replaced. You can also play with current limiters in startegic locations such as bus ties, main incomming, etc.
RE: how to calculate and limit fault currents in paralleled systems
http://www.gwelec.com/clip.htm
RE: how to calculate and limit fault currents in paralleled systems
RE: how to calculate and limit fault currents in paralleled systems
Bung
Just a suggestion!
RE: how to calculate and limit fault currents in paralleled systems
Here's a question that's arisen from jwerthman's reply to this post:
What sort of problems do you anticipate on a system using current limiters? I am working on a project looking to install at least 3 current-limiters (possibly ABB Is-limiters) separating 6 off 40MW generators (11kV system). We are using it as gordonl described in his reply. Thank you!
Etrix
RE: how to calculate and limit fault currents in paralleled systems
Be sure to watch your line to ground fault values. They can be considerably higher than a 3-phase fault. Many low voltage generators have extremely low zero sequence reactances. This is largely a function of the winding pitch. I've been caught by this a time or two. You may need a neutral grounding reactor to reduce SLG fault values.
MCCB's (insulated case) work well to protect for high fault currents, but they don't coordinate well due to their instantaneous trip. MV is the way to go if you can.
RE: how to calculate and limit fault currents in paralleled systems