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Fault Current and OCPD

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

Reactors are about the only thing you can do.  Time to interruption doesn't help.

RE: Fault Current and OCPD

For smaller loads feeder cables also helps so perform an actual SCC calcs and see what you get, unless all the loads are close to the board.

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

(OP)
Yeah I found some 200kAIC breakers and all the loads are right next to the board.  I thought about running extra cable around the room a few times to choke it down but thought that was kinda silly.

RE: Fault Current and OCPD

In support of Rafiq's suggestion, consider the impedance of the conductors from the generators to the board.

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

Keeps getting more interesting..Found some breakers?.... What about the board's bus bracing?

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

Been there, done that. Specs on a job called for a minimum length of cables to feed the unit subs and PDCs from the 13kV switch lineup. The close in units had more than half the feed cable looped down and back the cable tray. Cable is usually cheaper per Ohm impedance than reactors and a lot cheaper than higher rated breakers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Fault Current and OCPD

Cable gives mostly resistive impedance while reactor give mostly reactive impedance, so for a given amount of fault current reduction you will have more continuous losses with added cable than with a reactor.  The other thing that works wonders is a transformer, even one with the same voltage in and out.

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

I recall an article on a three-terminal, center-tapped reactor with a source (generator) in the middle and a load bus attached to each end. (Can't locate it now). The two halves of the reactor were wound with opposing coils to create high impedance for current flowing from one bus to the other but minimal impedance to current out of the generator to each bus.  (Sketch the current flow, in one half of the reactor the generator load current is opposite the current flowing from the other bus.)

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

It does not take much impedance to drop from 140kA to 100kA.
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

The correct term for the reactor is a duplex reactor.

Extra cable length would be cheaper and easier to fit into a limited space.

RE: Fault Current and OCPD

Also be sure to check the phase-to-ground fault current if the gens are solidly grounded. That can be significantly higher than 3-phase.

Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.

RE: Fault Current and OCPD

Regarding the 200 kA circuit breakers - you should verify that these do not use integral current-limiting fuses to achieve this rating.  

 

"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)

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