HRG on a Delta-Delta transformer
- Thread starter eengr
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- Illinois
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- retired electrician
You have to create some type of neutral to use the HRG. The following is from one manufacturer.
JoeStillman
Senior Member
- Location
- West Chester, PA
Here's what the Code says...
JoeStillman
Senior Member
- Location
- West Chester, PA
Sorry, that reference should say 250.36(A)
wwhitney
Senior Member
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- Berkeley, CA
- Occupation
- Retired
Naive question: is there a technical reason that HRG corner grounding is prohibited?
Cheers, Wayne
Cheers, Wayne
Thank you, Don! So, do I need to connect a zig-zag transformer to the secondary of the Delta-Delta and then connect the zig-zag to the HRG unit? Also, may I know what is the real purpose of connecting a Delta-Delta to an HRG unit as we will not have any SLG faults on the transformer, correct?
Thanks for the reference Joe!
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
I was going to ask the same thing.
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
Tradition!
Seriously, I can think of two technical issues that would make a neutral HRG system better than a corner HRG system, but I don't see why they would be deal breakers.
1) The neutral system has lower line-ground voltages
2) In a corner grounded HRG system, detecting ground faults in the grounded circuit conductor would be more difficult. Such a ground fault converts the HRG system to a sort of solidly grounded system (depending upon how solid the fault is) and you would want to reliably detect it.
On point 2, I note that HRG systems are not permitted to serve L-N loads, even though the neutral is present.
A related question: is the 'neutral' of a high leg delta system neutral enough to qualify under 250.36(A)??
-Jon
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
Sorry, how is that different from the grounded conductor in a HRG wye system? Seems like the same issue is present.
Or are you suggesting that for 3 wire 3 phase loads, there's a particular benefit to having the HRG grounded conductor not being a circuit conductor?
Cheers, Wayne
winnie
Senior Member
- Location
- Springfield, MA, USA
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- Electric motor research
I added this in an edit, but HRG wye systems are not permitted to serve L-N loads. So the neutral is only present at the source of the system or at the grounding transformer output. Not distributed through the system supplying loads.
-Jon
- Occupation
- Licensed Electrician
Wouldn't a resistor that limits the current to about one amp on one phase of a delta system screw up any loads on that phase during a ground fault?
- Location
- Illinois
- Occupation
- retired electrician
It only limits line to ground currents and should have no effect on line to line loads. The loads would be connected on the transformer side of the grounding resistor.
- Occupation
- Licensed Electrician
Aha. I see now. I was fixated on the idea that in a wye connected HRG there are no line to neutral loads.
synchro
Senior Member
- Location
- Chicago, IL
- Occupation
- EE
One important reason for HRG systems vs. ungrounded systems is to significantly reduce the transient over voltages that can occur with re-striking ground-fault arcing. Such over voltage can result from successive charging of line-to-ground capacitance, and also resonances of the capacitance with the line inductance (more significant with longer lines). The grounding resistor acts to dampen these events by absorbing energy that might otherwise be accumulated in the reactive elements (capacitance or inductance).
When a grounding transformer like a zig-zag or wye-delta is used with a grounding resistor, a minimal current should normally flow through this resistor, because of the currents through the line-ground capacitances will be substantially equal in magnitude and 120° from each other. That would not be the case if a resistor was used for corner grounding, because the capacitance to ground on the resistively grounded line would have a minimal voltage across it, and therefore minimal current flowing through it. The resulting current imbalance would cause a higher nominal current through a grounding resistor than with a grounding transformer approach. As Jon mentioned above, this could make accurate ground fault detection more problematic. Also, a given resistance would have to continuously dissipate more power due to the unbalanced capacitive currents, even in the absence of any ground fault.
When a grounding transformer like a zig-zag or wye-delta is used with a grounding resistor, a minimal current should normally flow through this resistor, because of the currents through the line-ground capacitances will be substantially equal in magnitude and 120° from each other. That would not be the case if a resistor was used for corner grounding, because the capacitance to ground on the resistively grounded line would have a minimal voltage across it, and therefore minimal current flowing through it. The resulting current imbalance would cause a higher nominal current through a grounding resistor than with a grounding transformer approach. As Jon mentioned above, this could make accurate ground fault detection more problematic. Also, a given resistance would have to continuously dissipate more power due to the unbalanced capacitive currents, even in the absence of any ground fault.
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