Generator Steady State Fault Current 1

[KJvR]

This might be a dump question but I get lost every time I have to do something on a generator. I must calculate the steady state fault current (energy of fault) of a bus being fed by both the utility and two small 13 MW MV generators. I have the AVR settings. It states: "Current Limiter: Machine current limiter I thermal: 100%". Does this mean that the AVR will control the fault current at it's nominal current rating after the transient period. I have used 300% in previous calculations but the "manufacturer's decremental curves" were available and was easier to interperate.

 

[powerjunx]

kjvr,
it is better, you thoroughly read the manual.

May i knew the maker of that AVR?

 

[dpc]

The steady state fault current is a function of the synchronous reactance of the machine. In most cases, the synchronous reactance is greater than 1.0 per unit meaning the fault current is less than the full load current.

However, this is based on the assumption that the generator excitation remains constant. This will probably not be true. On generator/exciter/regulator systems that some type of "current boost" or fault current support, the steady state fault current is typically around 300% of full load amps.

What value to use will depend on the purpose of your calculations. If you are concerned about breaker interrupting ratings (unlikely since you are talking about a sustained fault), I would use the 300% value. If you are concerned about a minimum fault value for setting backup relaying, you might want to assume the excitation remains constant as a worst-case.

If you're doing arc-flash calculations, you will need to do it both ways and determine which is worst.

It's pretty unlikely that a fault will ever last this long on a 13 MW generator on a well-implemented system, barring relay or breaker failure.

 

[KJvR]

It is a ABB - UNITROL-F AVR. The purpose of the calculations is for the substation ceiling hight design. The manufacturer provide certain minimum ceiling hights for specific fault currents (busbar arcing faults) and fault duration. I want to calculate the I2t during the first 0.5 sec or 1 sec of the fault to determine an equivalent current to apply to the tables provided by the switchgear manufacturer.

 

[MineGuy]

Hi
Just out of curosity, after reading this thread, what ceiling height of a substation has to to with fault current. Another thing, I am a jr electrical engg and I also have been involved in short circuit calc. but I dont recall ever considering a role of AVR in that. Could you kindly advise what I am missing here.
Thanks

 

[KJvR]

Hi Mineguy, the particular switchgear that we will be using specify various ceilling heights for various fault currents for Internal Arcing Current Certification - the busbars is very high in comparrison to other MV switchgear. IEC 62271-200 or IEEE 37.20.7-2001 provides more detail for IAC tests.

The generator steady state fault current will be very small if the excitation is kept constant. I do not have a lot of experience on generators (therefore this threat) but the AVR will increase the excitation after 0.3 sec (?) to ensure a large enough current (it seems typical 300%) to detect a fault. The AVR will have no influence for calculating initial and 3 cycle break currents.

 

[dpc]

For this particular problem, I'd use the 300% value, although it would be even better to get a real number from the generator supplier.

MineGuy: Generator fault contributions are normally broken down by time into "subtransient", "transient", and "steady-state" or synchronous. As KJvR mentions, for the subtransient (first cycle or two), and transient (maybe 3-5 cycles) the excitation doesn't have much impact since it hasn't had time to react.

But for a close-in fault, the generator terminal voltage will drop to nearly zero and the AVR will increase excitation to the maximum limit to try to get the voltage up to the setpoint. If the AVR has a source of power separate from the generator, this action can boost the steady-state fault current beyond the "intrinsic" value determined by the synchronous reactance.

BTW, the ANSI/IEEE standard is actually C37 ("C" was missing).

 

[DanDel]

The manufacurer's decrement curve for the generator will also give you the steady-state fault current.