Calculating generator fault current from given generator values

[mull982]

I'm trying to double check a generator avaliable fault current to that which my software program is giving me. I have the generator specs which gives the generator voltage, kVA, p.f. and all of the impedance paramaters.

For the impedance paramaters it gives the synchronous reactance, neg sequence reactance, zero sequence reactance, transient reactance, subtransient reactance, and quadrature reactance.

My first question is which of these values I would use to calculate the generator avaliable fault current. I guess this would depend on at what length of time I wanted the fault current for, 1/2 cycle, 5 cycles etc... I'm using skm powertools and I believe it lists the fault current as a 5 cycle current so I'd be interested in finding the 5 cycle fault current. Would I use the transient or synchronous reactance values for this?

My second question is the calculation method. All the values listed above are given in p.u. So could I simply take the voltage of the generator to be 1pu and divide one of the above p.u. impedances into the the 1pu voltage to come up with the p.u. current? I could then take this p.u. current and compare it to the base or rated current of the generator to determine the avaliable fault current? Does this sound correct?

This generator is listed as a 240/480V generator so I dont know how this effects the way we look at these paramaters or carry out the p.u. calculations?

 

[jim dungar]

How accurate are you trying to get?
Why are you doing this?

For the most part, the subtransient reactance (X"d) can be treated just like the %Z of a transformer, for example a .20pu X"d will yield a fault current of about 5 times FLA.

Of course if you are trying to develop a generator protection scheme, it is much more complicated.

 

[mull982]

How accurate are you trying to get?
Why are you doing this?

For the most part, the subtransient reactance (X"d) can be treated just like the %Z of a transformer, for example a .20pu X"d will yield a fault current of about 5 times FLA.

Of course if you are trying to develop a generator protection scheme, it is much more complicated.

I'm just trying to match a calculated fault current value to that produced by SKM. For the generator source in SKM I entered all of the generator information and the fault report gave me fault currents at the generator bus for 1/2 cycle, 2 cycles, 3 cycles, 5 cycles, and 8 cycles. I want my hand calculation with the given information to match one of the results produced by SKM just so I understand how it was calculated.

 

[kingpb]

Unless you plan on using the exact same method/calculation, take into consideration all the same parameters, i.e. dc offset, and have a accuracy tolerance equal to SKM computing power, I doubt you will get the same answer. However, you should be able to get something reasonably close by simply taking the Rated KVA/MVA of the generator, divide by x"d, then divide by voltage at sqrt 3.

Because of the inertia in the generator, it is going to be valid to use the subtransient reactance for the the 5 cycle fault current.

 

[mull982]

Unless you plan on using the exact same method/calculation, take into consideration all the same parameters, i.e. dc offset, and have a accuracy tolerance equal to SKM computing power, I doubt you will get the same answer. However, you should be able to get something reasonably close by simply taking the Rated KVA/MVA of the generator, divide by x"d, then divide by voltage at sqrt 3.

Because of the inertia in the generator, it is going to be valid to use the subtransient reactance for the the 5 cycle fault current.

Generator is rated 312kVA and subtransient reactance is .093pu.

So taking 312kVA/.093 = 3354.8kVA

Then taking 3354.8kVA/480V/1.73=4040A.

I am getting 4040A where SKM is giving me about 2499A. Why would there be about a 1000A difference? With the subtransient reactance wouldn't I be calculating the 1/2 cycle fault current? Is it possible that I'm calculating the 1/2 cycle and SKM is reporting the 3,5 or 8 cycle?

Also for a synchronous with a .8 power factor, wouldn't this power factor be leading since it is supplying as opposed to being lagging?

 

[jim dungar]

Generator is rated 312kVA and subtransient reactance is .093pu.

So taking 312kVA/.093 = 3354.8kVA

Then taking 3354.8kVA/480V/1.73=4040A.

I am getting 4040A where SKM is giving me about 2499A. Why would there be about a 1000A difference? With the subtransient reactance wouldn't I be calculating the 1/2 cycle fault current? Is it possible that I'm calculating the 1/2 cycle and SKM is reporting the 3,5 or 8 cycle?

Also for a synchronous with a .8 power factor, wouldn't this power factor be leading since it is supplying as opposed to being lagging?

X"d = .093pu seems pretty low.
With a FLA of about 375A and a more 'typical/default' X"d = .15pu you would have the SKM result. Did you change the X"d in the proper places in SKM (i.e. ANSI vs IEC)?

 

[RoberteFuhr]

Verify that the kVA listed on the Generator Data sheet matches the kVA that you are entering into SKM. Many Generator Data Sheets list the impedance values for a specific kVA rating of the generator. The sheet will list other kVA ratings but the impedance is always calculated for a base kVA rating.

If you are using a different kVA rating in SKM than on the data sheet, then you'll have to convert the impedance from the data sheet using the new (SKM) base.

 

[kingpb]

Using ETAP I get and answer that is within 0.2 of the calculated by hand value, The default data FOR A 312KVA Generator was 0.19pu.

I change the X"d to 9.3pu and still get consistent results between hand and computer.

Double check to make sure you have input the 9.3pu correctly. I see SKM actually wants it input as 0.093 on the ANSI contribution screen.

I developed a little simple model with a 312KVA gen and used the 0.093 on the first screen (ANSI contribution) where it says subtransient reactance; positive negative, and zero sequence.

The SC analysis calculated 4028A, symmetrical which is within 0.2 of the hand method.

It would seem you have probably entered your data incorrectly. BTW: the pf should be lagging. If it is leading that means you would be producing Watts but taking in Vars.

 

[mull982]

X"d = .093pu seems pretty low.
With a FLA of about 375A and a more 'typical/default' X"d = .15pu you would have the SKM result. Did you change the X"d in the proper places in SKM (i.e. ANSI vs IEC)?

I am entering the generator impedance data in the Generator "Decrement Curve" component of the generator. I have entered the X, X', X", T", T', and Ta values under the decrement curve box.

I am looking am also looking at a 160kVA generator which is about 1/2 the size that has an X" listed as .17 which is in line with what you mentioned. I haven't ran the fault study yet but I will in the next day or so and will compare these results.

 

[mull982]

Verify that the kVA listed on the Generator Data sheet matches the kVA that you are entering into SKM. Many Generator Data Sheets list the impedance values for a specific kVA rating of the generator. The sheet will list other kVA ratings but the impedance is always calculated for a base kVA rating.

If you are using a different kVA rating in SKM than on the data sheet, then you'll have to convert the impedance from the data sheet using the new (SKM) base.

The generator datasheet says:

Submittal Data: 240/480V, 312kVA, 1800RPM, 60Hz 3phase

And then goes on below to list the different impedance values, so I am assuming that they are all referenced to the 312kVA base which I have entered in SKM. The given X"d value is given as .093pu.

Using ETAP I get and answer that is within 0.2 of the calculated by hand value, The default data FOR A 312KVA Generator was 0.19pu.

I change the X"d to 9.3pu and still get consistent results between hand and computer.

Double check to make sure you have input the 9.3pu correctly. I see SKM actually wants it input as 0.093 on the ANSI contribution screen.

I developed a little simple model with a 312KVA gen and used the 0.093 on the first screen (ANSI contribution) where it says subtransient reactance; positive negative, and zero sequence.

The SC analysis calculated 4028A, symmetrical which is within 0.2 of the hand method.

It would seem you have probably entered your data incorrectly.

What voltage are you using? Can you show me how you are doing the hand calculation and what values you are using to get what results? Are you getting results around 4300A with the hand calculation using 240V? My SKM values match what you are getting but I cant get them to match using 312kVA and .093 xd" for the hand calc.

QUOTE=kingpb;1274916]BTW: the pf should be lagging. If it is leading that means you would be producing Watts but taking in Vars.[/QUOTE]

I thought a motor which is taking in Vars has a lagging power factor but a generator which is producing VARS would have a leading power factor.

 

[steve66]

Generator is rated 312kVA and subtransient reactance is .093pu.

So taking 312kVA/.093 = 3354.8kVA

Then taking 3354.8kVA/480V/1.73=4040A.

I am getting 4040A where SKM is giving me about 2499A. Why would there be about a 1000A difference? With the subtransient reactance wouldn't I be calculating the 1/2 cycle fault current? Is it possible that I'm calculating the 1/2 cycle and SKM is reporting the 3,5 or 8 cycle?

Also for a synchronous with a .8 power factor, wouldn't this power factor be leading since it is supplying as opposed to being lagging?

With SKM and the typical generator settings, I get just under 2500 amps (i used a very short piece of cable between the generator and the buss).

When I change x" to 0.093, I get about 4028 amps.

I changed x" on the ANSI tab for positive, negative and zero sequence. Don't hit the typical button, or it will reset those values to typical.

Then rerun the study.

Steve

 

[kingpb]

BTW: the pf should be lagging. If it is leading that means you would be producing Watts but taking in Vars.

I thought a motor which is taking in Vars has a lagging power factor but a generator which is producing VARS would have a leading power factor.

Take a look at a generator reactive capability curve, they are helpful in visualizing the VAR capability as the PF changes.

You are correct that for a motor the reactive power is absorbed, i.e. I lags V (lagging PF); a generator however delivers power to the system, and the generator action therefore supplies reactive power, I lags V, i.e. lagging PF.