MVAsc0 Calcultation of Generator Set

[ezekiel gatus]

Good Day To All,

Can anyone help me? We have two equal generator sets: 2500 kVA, 0.8 pf, 60 Hz, 13800Y/7967 Volts, Xd-1.63 pu, X2-0.237 pu, X0-0.026 pu, X'd-0.178 pu, X''d-0.165 pu.

How do I calculate the MVAsc0 for one generator set running? And How do I calculate the MVAsc0 for two generator set in parallel? The supplied transformer is Delta/Wye Connection 13800-400Y/230 Volts. Thank you.

 

[ver]

Hi Ezekiel,

Thanks for being the first to post in this forum.

To be able to answer your query properly, let me just be clear in what you are trying to ask. Do you want to calculate the Zero Sequence fault MVA of your generators at the generator terminal?

Thank you

 

[ezekiel gatus]

Hi Sir Ver,

Thanks for the fast reply.

Yes, I want to calculate the Zero Sequence fault MVA of our generators at the generator terminal.

Thank you

 

[ver]

As per the data of the generator you provided, the zero sequence impedance is much lower than the positive or negative sequence impedance. This would only suggest that you have an impedance or resistance grounding of your generator to limit maximum ground fault to at least the same level as your maximum three phase fault. If not, then you have an ungrounded system which does not have zero sequence fault current.

To calculate the zero sequence fault current:

$$I_O = {1 \over (X_1 + X_2 + X_0 + 3Z_n)}$$
$$I_N = {3 \over (X_1 + X_2 + X_0 + 3Z_n)}$$

Where:
$I_O=I_1=I_2$
$I_N=3I_O$
$X_1$ = positive sequence impedance
$X_2$ = negative sequence impedance
$X_0$ = zero sequence impedance
$Z_n$ = grounded impedance (if any)

From $I_O$, you will be able to calculate the zero-sequence MVA of your generator.

For your second question, the total MVA of identical generators will be twice that of one unit.

You also have a delta-wye supply transformer. I assume that the delta connected winding is the primary. This means that on the secondary of your supply transformer, there will be no generator contribution to your zero-sequence fault current.

I hope this helps.

 

[ezekiel gatus]

Sir,

For additional information each generator set has a Neutral Grounding Resistor (NGR) of 26.6 ohm. Kindly provide me the calculated value of Zero Sequence fault MVA of our generators at the generator terminal, so that I can compare it to my calculated value.

Thanks a lot,

 

[ver]

There was a slight typo in my previous formula. I updated it.

Sir,

For additional information each generator set has a Neutral Grounding Resistor (NGR) of 26.6 ohm. Kindly provide me the calculated value of Zero Sequence fault MVA of our generators at the generator terminal, so that I can compare it to my calculated value.

Thanks a lot,

You need to convert the ohmic values to per unit.
$Z_{base}={(kV_{LL})^2 \over MVA_{3\phi}}$
$Z_{base}=76.176\Omega$

$Z_N={26.6 \over 76.176}=0.349pu$

$I_O=0.341pu=35.65A$
$I_N=3 \times I_O=1.023pu=106.96A$

 

[ezekiel gatus]

Thank you very much, This help me a lot.

 

[ver]

Don't mention it!