Considering Voltage Regulator Impedance for For Substation short-circuit calculations
Considering Voltage Regulator Impedance for For Substation short-circuit calculations
(OP)
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?
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?






RE: Considering Voltage Regulator Impedance for For Substation short-circuit calculations
Maximum short circuit is with the regulators bypassed.
RE: Considering Voltage Regulator Impedance for For Substation short-circuit calculations
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?
RE: Considering Voltage Regulator Impedance for For Substation short-circuit calculations
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.
RE: Considering Voltage Regulator Impedance for For Substation short-circuit calculations
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.
RE: Considering Voltage Regulator Impedance for For Substation short-circuit calculations
RE: Considering Voltage Regulator Impedance for For Substation short-circuit calculations
RE: Considering Voltage Regulator Impedance for For Substation short-circuit calculations
RE: Considering Voltage Regulator Impedance for For Substation short-circuit calculations
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.