Considering Voltage Regulator Impedance for For Substation short-circuit calculations

[Pitt03]

I am performing some short-circuit calculations on a 13.8kV distribution substation which has 267kVA Step-Type Voltage regulators on the feeder circuits leaving the substation. I was wondering weather or not the impedance of these regulators needs to be considered for any aspect of the short-circuit calculations?

Each regulator is a single phase regulator with a bypass switch so it appears that worst case fault currents on feeder equipment would be when regulators are bypassed and thus not really a need to consider regulator impedance for worst case? The only reason I can think of to consider regulator impedance is to come up with minimum fault currents at downstream locations on feeder to make sure that feeder re-closers (located on line side of regulators)will see minimum fault current downstream?

If I need to include regulators I don't have the capability to model an autotransformer in analysis software so I was going to model as 2-winding transformer and convert the autotransformer impedance to a 2-winding impedance. Any issues with this? I cannot seem to find any published impedance data for specific regulators and only find a reference to impedances in the Westinghouse T&D Reference book. Are these suitable for use if needed?

Do the regulators themselves have a fault withstand rating that needs to be considered? I didn't see anything on the datasheet I am looking at but thought I saw a reference elsewhere that required them to withstand faults of 25x rating?

Regulators are located on delta winding of the substation transformer with a grounding transformer located on the delta side. Any special wiring configuration (dela vs wye)for these regulators being applied on this delta system?

 

[cranky108]

It depends.

Maximum short circuit is with the regulators bypassed.

 

[Pitt03]

Cranky108

I'd be curious to hear more on situations where "it depends". Would it just be cases where minimum fault current needed to be considered for protection and coordination?

 

[bacon4life]

Theoretically, assuming the transmission voltage and load are constant, moving the regulator to the extreme lower position would increase fault current at the substation bus. Moving the regulators to raise would reduce the turns ratio and hence reduce fault current at the substation bus. Faults down the line are a different story. When the distribution feeder impedance is large compared to the substation transformer impedance, moving the regulator to the extreme lower position leads to voltage drop on the feeder that outweighs the change in turns ratio, resulting in lower fault current at the end of line.

Practically, the regulators on our system usually don't go anywhere near the extreme limits of tap position. Prior to having a powerflow model of our system, it was assumed all transformers at their nominal ratios. Now that we have a model that has both powerflow and short circuit capabilities, I typically run faults with regulators/LTCs at the same position they are in for the system peak load.

We do not have manufacturer data for most substation transformers as to how the impedance changes as the LTC or DETC moves. Depending on the winding configuration, the middle tap might the minimum, middle or maximum impedance.

 

[cranky108]

For cases where you would want to calculate voltage drop for a motor start, you would use maximum regulator impedance.

I'm not so sure that the regulator can add current for a fault, because it is an impedance. It dos not generate any energy. Besides as the voltage drops it's ability to support voltage also drops.

For customer arc-flash, or maximum short circuit cases, I would not give them anything except the step-down transformer impedance at the pole, or intertie, and state they should assume a zero impedance source on the high side, as this would remain correct no matter what system changes you make on the utility side.

 

[bacon4life]

Yes, in terms of fault MVA (fault current * prefault voltage), the change in current is offset by the change in voltage, so the additional regulator impedance results in a smaller MVA value. However, in terms of fault current, a change turns ratio has a direct relation fault current. The additional impedance of the regulator my be rather small compared to the impedance of the rest of the circuit.

 

[jghrist]

For customer arc-flash, or maximum short circuit cases, I would not give them anything except the step-down transformer impedance at the pole, or intertie, and state they should assume a zero impedance source on the high side, as this would remain correct no matter what system changes you make on the utility side.

There are cases where arc flash energy is higher when the fault current is lower. This generally would occur where a lower fault current is below a short-time breaker trip pickup and results in a significantly larger clearing time. Arc hazard studies need to take into account both maximum and minimum fault currents.

 

[wbd]

See attached from a GE VR manual

 

[cranky108]

What is minimum fault current? That would be zero, as the minimum.

How weak can I make the system, and still have it function is the question, and most of us can't give a good answer. The person calculating the arc-flash needs to have some thought on this, and not be asking how weak the power system can be. If I were asked for a weaker feed impedance, I might be inclined to give an answer.