Fault Current and OCPD
Fault Current and OCPD
(OP)
I have (5) generators in parallel which results in 140kAIC and on that board we will have some UPS'S and VFD's that will only be rated for 100kAIC. My client will not use fuses. Can I use a power circuit breaker and change the instantaneous setting to protect that equipment. Is there any other methods to minimize fault current.






RE: Fault Current and OCPD
RE: Fault Current and OCPD
By the way, you did not mention- what is the board's rating? Power circuit breakers without fuses are normally not rated more than 100kAIC.
Rafiq Bulsara
http://www.srengineersct.com
RE: Fault Current and OCPD
RE: Fault Current and OCPD
I have a related question. I think that it's alright but what do others think?
If reactors are installed in the generator conductors, can the AVR sense connections be made past the reactors so that the AVR compensates for the voltage drop across the reactors?
Any issues? Any serious issues with the quadrature circuit?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Fault Current and OCPD
The breakers in the board does not help the rating of the loads (UPS and VFD), how are they handled?
Rafiq Bulsara
http://www.srengineersct.com
RE: Fault Current and OCPD
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Fault Current and OCPD
Don't know of any reason why the AVR voltage sensing couldn't be downstream of reactors in the generator connection to the bus.
RE: Fault Current and OCPD
The application was adding larger generators to an older plant, but it might work for your installation if you can divide the loads onto separate busses and insert the reactors at the tie points. With clever distribution of load, you could minimize reactor losses and voltage drop, maintain a lot of redundancy and drop the fault current level at each bus.
With five generators, split the load onto five busses in a ring, connecting the busses with the center tapped reactors. If space or cost limitations come in, look at three busses with three interconnecting reactors. Not all of the generators would need to be feeding through reactors, just enough units to drop the current to manageable levels. Note that a two bus, two reactor setup doesn't provide any benefit.
The arc flash study will be interesting. Do all maintenance with robots?
Found the article - IEEE Industrial Applications, March-April 2006, Author: Paul Hamer
RE: Fault Current and OCPD
I assume this is a 480/277V system.
Z(140)= 277/140000 = 0.00198 ohms
Z(100)= 277/100000 = 0.00277 ohms
delta = 0.00079.
This is about 20' of 750MCM cable, if you can ignore phaser addition of impedance issues that David brings up.
The tiniest of errors in your impedance measurements will make a lot of difference in the available fault current.
RE: Fault Current and OCPD
Extra cable length would be cheaper and easier to fit into a limited space.
RE: Fault Current and OCPD
Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.
RE: Fault Current and OCPD
"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)