Generators available short circuit current

[sceepe]

I have been looking through manufacturers literature and found very little info as to the available short circuit produced by the generator.

Only thing I have been able to find is 250-300% of rated current (for 3 phase fault) and 700-800% of rated current (for 1 phase faults) depending on alternator type and excitation

Is this consistent with the numbers you use when doing short circuit calcs?

 

[ron]

Re: Generators available short circuit current

It is dependant on the % impedance of the generator, similar to a transformer. What is is % impedance from the spec.

 

[Guest]

Re: Generators available short circuit current

When a generator reaches its current rating, the voltage will drop as more load is added until the exciter is dumped.

Available fault current is the same as maximum current rating.

 

[templdl]

Re: Generators available short circuit current

This is an area that most people don't give any consideration to in that they assume that a breaker will provide fault and overload protection.
It is taken for granted that the generator is capable of producing enough fault current to trip the common thermal magnetic breaker instantaneously and protect the generator.
Also, is the generator capable of tripping the breaker thermally as a result of an overload, that is will it produce enough current for a long enough period of time.
In both cases the breaker will most likely never trip.
Unlike the power grid of a power company distribution system the fault and overload current is limited to what that sole generator is capable of producing.
I have normally advised people that breaker should be tripped by an internal shunt trip using the protective features as my be available on the generator control panel.

 

[Ed MacLaren]

Re: Generators available short circuit current

Available fault current is the same as maximum current rating.

I can't see why a generator would be any different than a transformer, with regard to available fault current.

In either case we are talking about the short time before any protective devices operate, including exciter disconnection.

Literature that I have seen recently referring to protection of conductors supplied by a generator state that "some generators, especially those with series-boost systems, can supply very large ground-fault or short-circuit currents."

Here is another quote, this from the website of the equipment manufacturer, ABB.

"Generator circuit-breakers are the ?big boys? of the breaker world; they have to be.
A fault of up to 200 kA somewhere in the field is bad enough, but close to a generator it can lead to an event of literally seismic proportions ? the fault current can be of such a magnitude that the induced magnetic forces cause solid steel shafts to bend and crash."

Ed

 

[steve66]

Re: Generators available short circuit current

8 times the generator current is what I use for your basic diesel or natural gas generator.

It's not often I would challange Bennie (or even Bennie2) , but I like Ed's comment about a generator not being much different from a transformer. I was thinking I don't know why a generator would be much different than a motor. And I've always used 6 times the fault current as an approx. for the motor fault current.

Steve

 

[Guest]

Re: Generators available short circuit current

A transformer has a primary system that transfers power to the secondary. The capacity of the primary source is extremely large.

A generator receives it power transfer from a 125 volt exciter at approx. 35 Amps. Plus the power to drive the rotor by the prime mover.

Now figure the possible current flow in a transformer against that of a generator.

I don't see any comparison.

 

[ron]

Re: Generators available short circuit current

You are correct in that the SUSTAINED short circuit current from a synchronous generator may be a relatively low value. This is because the effective reactance in this steady state time frame would be the machine synchronous reactance, Xd, which has a high value - typically around 2.0 per unit or higher.

The value of reactance that is considered when evaluating short circuit conditions is the subtransient reactance, X"d, which typically will be in the region of 0.09 pu for a 2-pole generator. This determines the maximum value of short circuit current at the instant that a fault occurs. For a machine operating with 1.0 pu terminal voltage and X"d=0.09, the maximum Isc = (1.0/0.09) = 11.1 pu RMS symmetrical.

Incidentally, the value of X"d for induction motors is typically around 0.17 pu, which gives the 6x typical value for motor short circuit contribution. There is no steady state contribution from induction motors.

A bit more info at http://xnet.rrc.mb.ca/janaj/1_3_1_transient_circuit_model.htm

An example of data I requested on the earlier post is shown as an example http://www.generac.com/PublicPDFs/0168800SBY.pdf on the second page.

[ April 07, 2004, 08:49 PM: Message edited by: ron ]

 

[Guest]

Re: Generators available short circuit current

The method for drying a generator, after a storm, is to short all three phases together.
run the generator for about an hour, the circulating current, based on impedance variation of the windings, will produce enough heat to thorougly dry the stator and rotor.

The no voltage output will make the exciter ineffective.

Don't try this on a transformer.

 

[kkp]

Re: Generators available short circuit current

Caterpillar published literature indicates "Caterpillar Generators typically produce eight times rated current on a three-phase fault."
Caterpillar also indicates following:
Three phase 1st cycle, symmetrical, RMS short circuit current developed by a generator is:

Isc = Ir/X"d

Isc = Avg. symmetrical amp, 3 ph, 1st cycle
Ir = rated amps
X"d = direct axis subtransient reactance per unit value.

 

[steve66]

Re: Generators available short circuit current

Cat could have been nice and included a table of X"d for their different generator models. But I bet you can't find a single spec for one. That doesn't make the formula very useful.

On the other hand, their rule of thumb for 8x rated current is easy (and I bet I got it out of the same book as you have)

Steve