Ground Fault Detection on secondary of wye-wye transformer
Ground Fault Detection on secondary of wye-wye transformer
(OP)
In looking at the zero sequence network of a ungrounded-wye / grounded-wye transformer it appears that this transformer would not produce zero sequence current on the secondary wye side even though it is solidly grounded.
Am I looking at the zero sequence circuit correctly in that the secondary of this transformer will not produce zero sequence current and will be the same as an ungrounded transformer?
How is ground fault current from this transformer usually detected? Similar to methods used for detecting ground fault on ungrounded systems?
I have seen several utilities use this configuration at the service entrance? Is there any particular reason for using this configuration?
Thanks
Am I looking at the zero sequence circuit correctly in that the secondary of this transformer will not produce zero sequence current and will be the same as an ungrounded transformer?
How is ground fault current from this transformer usually detected? Similar to methods used for detecting ground fault on ungrounded systems?
I have seen several utilities use this configuration at the service entrance? Is there any particular reason for using this configuration?
Thanks






RE: Ground Fault Detection on secondary of wye-wye transformer
RE: Ground Fault Detection on secondary of wye-wye transformer
RE: Ground Fault Detection on secondary of wye-wye transformer
I first noticed this configuration when I was reviewing a short circuit study completed by others. I noticed that the fault report was showing any L-G fault and when I looked closer I noticed that the utility transformer was modeled in this configuration.
As I go back now and look into the response letter from the utility I notice that they said the transformer is a "12.47kV:480/277 Wye-Wye with a Solid Neutral-ground Connection". Reading this I would have to assume that both the primary and secondary wye points are solidly grounded perhaps with the H0 and X0 bonded internally as mentioned. So based on this new information it would appear to me that both the primary and secondary wye's are grounded and the engineer completing the study made an error by modeling the primary of the transformer as an ungrounded wye.
From the zero sequence circuit it appears that with both the primary and secondary wye points grounded the transformer will pass zero sequence current from primary to secondary?
If for some reason the primary of the wye was ungrounded (not sure why anyone would want this as mentioned above) then this is essentially the same as having an ungrounded system on the secondary. Is this correct?
Thanks for the help!
RE: Ground Fault Detection on secondary of wye-wye transformer
1) I donot think any transformer manufacturer will deliver his transformer with H0 & X0 bonded & grounded internally.
It has to be done by the user after the transformer is installed.
2) If you do a simple system sudy by modeling Y(GND)/y(gnd) you will find that it is equivalent to a DELTA/wye(gnd) connection
for ground faults.
RE: Ground Fault Detection on secondary of wye-wye transformer
RE: Ground Fault Detection on secondary of wye-wye transformer
RE: Ground Fault Detection on secondary of wye-wye transformer
Regards
RE: Ground Fault Detection on secondary of wye-wye transformer
No,you don't have the required ampere turns on the primary to balance the ampere turns on the secondary during a secondary phase to ground fault
-SB
RE: Ground Fault Detection on secondary of wye-wye transformer
I don't recall seeing transformer sequence diagrams that include the phantom delta, but it's a recognized phenomena.
RE: Ground Fault Detection on secondary of wye-wye transformer
Regards
Marmite
RE: Ground Fault Detection on secondary of wye-wye transformer
RE: Ground Fault Detection on secondary of wye-wye transformer
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Ground Fault Detection on secondary of wye-wye transformer
Anybody else seen this?
-SB
RE: Ground Fault Detection on secondary of wye-wye transformer
HV energised with LV open
HV energised with LV sorted
LV energised with HV open and
LV energised with HV shorted
Based on these result you can compute zero sequence impedance of the transformer.
Hope this helps.
Regards,
RE: Ground Fault Detection on secondary of wye-wye transformer
The only way to test this should be by applying 3-phase voltage on the primary terminals with the primary neutral unconnected and one secondary phase shorted to neutral, the other secondary terminals left open.
The "use" of this effect can't be much more than the annoying need to add zero sequence filtering in the transformer differential protection?
Funny though, I've never actually paid this phenomenon much of a thought, less having to deal with it's negative side effects.
Anyhow, it's always nice to learn new things.
-SB
RE: Ground Fault Detection on secondary of wye-wye transformer
RE: Ground Fault Detection on secondary of wye-wye transformer
You will have (secondary) zero-sequence current flowing in any transformer with a grounded secondary.
The best way to detect zero-sequence current is with a zero-sequence CT and relay.
Any reason why you are not considering a neutral-ground resistor for limiting ground-fault current on the secondary side? Obviously a NGR system cannot facilitate any phase-to-neutral loads. I like NGRs a lot and would never consider not installing one, but then I'm tainted.
BTW, what size of transformer are we talking about here?