Why don't XFMR's Mag current or Shunt Capacitors contribute to SC? 2

[majesus]

I'm puzzled, I did a search on this for a while, and I couldn't find a direct answer.

I'm studing short circuit analysis and in practice, when we (as electrical power engineers) do short circuit calculations we take in account the contribution of induction motors because this machine has stored energy as magnetizing flux. However, how come we don't take into account the energy stored in a transformer (ie, its magnetizing flux?) Also I am curious why capacitor banks are omited?

On page 109-110 of the IEEE Redbook (Std 141-1993) it says:
"Charged power capacitors can also produce extremely high transient short-circuit discharge currents, but they are of natural frequency much higher than power frequency and
usually of such short duration that the calculated power frequency short-circuit duty current is not signifcantly increased by adding the capacitor discharge."

How can we assume that? Can't this high short circuit discharge cause equipment damage? Aren't we looking for the largest magnitude when we do a short circuit analysis? I understand, it's the energy level that is important, the I^2t, but the current can be big! Also, what about the impedance of the capacitor's discharge path? It would effect the capacitor's discharge rate, how can we simply ignore this? IE, suppose we have a huge capacitor bank that is far from short circuit, the cable impedance will limit the rate of dischage.

 

[RRaghunath]

Majesus,
Theoretically speaking, shunt capacitors do contribute to the fault. But, I see the effect would be very less, just like ignoring the resistance of the system and assuming the fault impedance to be totally lagging.
With regard to Transformer, I don't see how it can contribute. The magnetising current of transformer is well below 5% and hence the stored energy will be consequently low, unlike that of motor (which could be about 30% of motor rated current).
I don't think I have answered you completely, but it is just a small effort.

 

[davidbeach]

The magnetizing branch of the transformer does not store energy that can be released into the fault, all of the current into the magnetizing branch feeds the transformer no load or "iron" losses and is dissipated as heat rather than stored for release later. Remove power from a transformer and the flux at that moment remains as residual flux.

Capacitors do not, in general, need to be considered as, in general, their contribution is small and very short in duration. Sometimes, though, it is necessary to consider the capacitor contribution and that complicates things considerably. A typical fault current calculation program can't do capacitors, you need an EMTP type of program, such as ATP, and a sufficiently detailed model to know what is happening during the capacitor discharge. If you find the right people to talk to at the breaker manufacturer you can find out what the capacitor interrupting ratings are for the breaker. Then you are dealing with separate capacitor and fundamental currents, plus they will want to know the frequency of the capacitor current as there will be a maximum frequency the breaker can deal with.

A distribution feeder with up to 3MVAr or so of caps I wouldn't worry about. A substation ring bus with over 50MVAr around the ring is a whole different animal and many special studies do need to be performed to determine if the breakers are adequate to interrupt the fault current. It may also be necessary to delay the breaker operation if the currents are such that a closed breaker could withstand them but not safely interrupt the currents.

 

[majesus]

Thank you gentleman, I appreciate the help. I couldn't find any info in the books I had at home. I plan on getting A Practical Guide to Short Circuit Calculations... I heard from DPC it is extremely good. Plus it is in the thread on recommended books to get.

 

[Area]

Majesus,
I might be wrong, but I think that the contributionof motors (LV and MV) is dur to the fact, that you have stored mechanical (rotation) energy; so the motor acts as a generator driven by the inertia of the driven machine.
That's how I understood it.

 

[cranky108]

What you are seeing in a typical fault study is a study that does not include the transiet responce of the network connected devices. Therefore all devices in the study have a longer contribution time period. Short contribution devices are not included in this type of study.
In order to see the contribution of devices like induction motors and capacitor banks you need to be completing a transiet responce study.

Also on a typical fault study, grounded wye capacitor banks will make a zero sequence contribution with an angle of 180 degrees from that of inductive sources. The net effect will be to reduce the ground fault currens on most power systems.

 

[cheleng]

Full explanation in Chapter 7 "Capacitor contributions to short-circuit currents" IEEE Std.551-2006