Question on the short-circuit current from the Utility

[shamsdebout]

Based on the values from the utility engineer the bolted 3-phase fault for a 500 KVA. 208Y/120V pad-mounted transformer is equal to the phase to neutral fault (phase to neutral fault less than bolted phase to phase). I am not an expert but with the little I know, this doesn't seem correct. We normally make the assuption that the bolted 3-phase fault will cause the maximum short-circuit current. When the engineer was asked if this is common or seems right, he said this is how it worked out this time.
Has anyone ever gotten similar information from the utility or have verified by calculation that this can happen?

 

[Hameedulla-Ekhlas]

Based on the values from the utility engineer the bolted 3-phase fault for a 500 KVA. 208Y/120V pad-mounted transformer is equal to the phase to neutral fault (phase to neutral fault less than bolted phase to phase). I am not an expert but with the little I know, this doesn't seem correct. We normally make the assuption that the bolted 3-phase fault will cause the maximum short-circuit current. When the engineer was asked if this is common or seems right, he said this is how it worked out this time.
Has anyone ever gotten similar information from the utility or have verified by calculation that this can happen?

yes, he is right
Short-circuit kVA is the power available at a bolted three phase fault. Bolted means all three phases connected together with no added impedance. X/R is the ratio of reactance to resistance in the supply. Short-circuit kVA and X/R may need to be derived from other data.

Short-circuit current (ISC) is sometimes supplied by the power company rather than short-circuit kVA. This current is the current in one phase of a three phase bolted fault. The short-circuit kVA can be calculated from the short-circuit current using the following equation.

kVAsc = 1.73*Ic*VLL/1000

 

[don_resqcapt19]

I thought the line to neutral short circuit current was always less than the line to line short circuit current on the same transformer. Is that not correct?

 

[jim dungar]

I thought the line to neutral short circuit current was always less than the line to line short circuit current on the same transformer. Is that not correct?

It is not correct.

There are situations where the secondary conductors are short, as well as with some generator connections, that can have SLG faults higher than 3-phase bolted.

For those of you that like math, it will take a discussion of symmetrical components to fully explain these conditions.

 

[ptonsparky]

I just recieved three studies from our local POCO and one had higher L-G fault currents than L-L-L. 17583 vs 17191.

 

[Hameedulla-Ekhlas]

It is not correct.

There are situations where the secondary conductors are short, as well as with some generator connections, that can have SLG faults higher than 3-phase bolted.

For those of you that like math, it will take a discussion of symmetrical components to fully explain these conditions.

now
For generator we know
A single line to ground at the terminals of a generator with solidaly grounded neutral is more severe than a 3-phase fault at the same location.

For transmision line:
3 phase fault currrent is more than the current for a single line to ground fault.

for Transformer?

Suppose the transformer is far away from the generator. then See the attached file for fault at the terminal of a transformer( line to line neutral bolted fault).

That is why I said it he is right. He may have seen also the same thing like me. Hope to make it clear for me.

 

[Hameedulla-Ekhlas]

I thought the line to neutral short circuit current was always less than the line to line short circuit current on the same transformer. Is that not correct?

If we consider just at the terminal of transformer and regardless of generator or far away from generator, line to neutral is more that line to line. please see the attached photo.

 

[kellytshort]

For a transformer doesn't the positve, negative and zero sequences all equal eachother x1=x2=x0? Because I know on a generator you are supposed to have the 3X the zero sequence to equal the positive sequence which in turns limits the SLG fault current.

So wouldn't the transformer limit the fault current on a l-g fault more because the zero sequence equals the positve and the negative?

 

[shamsdebout]

Thanks for the input, I will do some more research, I find this an interesting topic.
@ Jim: Can you recommenda source that explores this occurence?
@ Ham: You attached a document, what source is that from?

 

[Hameedulla-Ekhlas]

Thanks for the input, I will do some more research, I find this an interesting topic.
@ Jim: Can you recommenda source that explores this occurence?
@ Ham: You attached a document, what source is that from?

shams it is "Copyright 2002 by Cooper Bussmann?" and you can find in google or even in this forum.

 

[kingpb]

For a transformer doesn't the positve, negative and zero sequences all equal eachother x1=x2=x0? Because I know on a generator you are supposed to have the 3X the zero sequence to equal the positive sequence which in turns limits the SLG fault current.

So wouldn't the transformer limit the fault current on a l-g fault more because the zero sequence equals the positve and the negative?

Generators and motors have internal voltages of positive sequence only. Positive and negative sequence impedances are the same. The SLG is limited due to a resistor or reactor typically used between the neutral and ground.

For transformers, it depends on the connection as to whether zero sequence current will flow. It is only in the Y-Gnd / Y-Gnd and Y-Gnd / Delta trnasformer connections that zero sequence can flow, and in the second case only in the Y-Gnd side; not the Delta.

When unbalanced voltages, and unbalanced currents exist in an otherwise symmetrical system, which is going to be all the time, negative and zero sequence networks must be used to analyze the system. Faults create unsymmetrical voltages and currents and are therefore important to the analysis. The many factors involved in the sequence network analysis related to system configuration or how the fault occurs, can often lead to SLG fault values greater then 3phase faults.