autotransformer with impedence ground
autotransformer with impedence ground
(OP)
I have a 750kva autotransformer 480v/1000v, it has a 15a 600v resistor connected to the xo/ho terminal. The autotransformer feeds 2 400a/1000v breakers equipped with shunt trips and ground monitoring. I have experienced 3 faults with damage to a phase conductor in the trailing cable and the ground fault monitor has not tripped. I don't believe this is the proper application for an autotransformer. What really bothers me is even if the fault went all the way to our distribution transformer why didn't the ground fault monitor trip? its looking for 5a.






RE: autotransformer with impedence ground
RE: autotransformer with impedence ground
Zo%=((N-1)*Zps%/N)+3Zn%*((N-1)/N)^2
(ref- old copy of Westinghouse T&D book)
Zn is the per unit value of the grounding resistor.
If a '15a 600V' resistor translates to 600/15 ohm, Zn is 3000% (100%*40/(1000^2/750000))
This suggests a very low earth fault current (<14A, or 1/30-th of 750kVA at 1000V by my reckoning), probably way below pickup of your ground monitoring.
Bung
Life is non-linear...
RE: autotransformer with impedence ground
If this is an installation that is subject to U.S. National Electrical Code, it is an illegal installation. The supply neutral can really only be used to supply revenue meters and power monitoring devices that need a true Blondel configuration for accuracy purposes.
RE: autotransformer with impedence ground
RE: autotransformer with impedence ground
RE: autotransformer with impedence ground
1] source is Y connected, neutral (star point) solidly grounded.
2] autotransformer has the star point connected to ground through a resistor - please clarify its resistance
3] ground fault is occurring on the load side of the autotransformer.
4] autotransformer has no delta winding. Is it in a steel tank?
If this is how it is, then the grounding resistor will very definitely have a marked effect. Also, the higher the value of the resistor, the greater the (relative) effect of the "tank delta".
Where the current is "going back to" is only half the story. It also has to come from somewhere, and that somewhere is the phases. It will result in a zero phase sequence current flow which your core balance CT arrangement will pick up if there is any ground fault current flow.
Bung
Life is non-linear...
RE: autotransformer with impedence ground
Bung,
I'll do the best I can, thanks for the input so far.
1. correct
2. correct, the nameplate on the resistor says 15A,600V only.
3. correct
4. correct, it is a dry-type winding.
I understand that the current is coming from the phases, I fail to see what role the grounding resistor plays in limiting GF current when the phases are not isolated in the autotransformer. The ground fault monitor is setup to trip between 3-5 amps flowing through the ct, when i inject 5amps through the ct it trips like clockwork. It may be that I am not getting enough fault current during a fault, or my relay should be setup for .25A . just a thought.
still pondering,
RE: autotransformer with impedence ground
The 15A refernece on the resistor is ambiguous.Presumably it is intended to limit the earth fault current to 15A, with a maximum working voltage of 600V (ie maximum dissipation of 9kW).
Your ground fault setting sounds a bit on the high side, as it is only 1/3 of the maximum ground fault current (which I am assuming is the 15A from the resistor spec). I would be more inclined to look for a 1/6 - 1/10 ratio. But don't try changing it without proper, competent analysis by somebody who has access to all the facts! I can only see what you have told us here!
Bung
Life is non-linear...
RE: autotransformer with impedence ground
By definition of an autotransformer, the primary and seconday WILL NOT be electrically isolated. Because the two windings are electrically connected, there will be zero sequence currents flowing through the transformer winding from primary to secondary. Absent a delta tertiary the zero sequence network does not have a connection to neutral, looking instead much more like the zero sequence network of a grounded-wye grounded-wye two winding transformer.
RE: autotransformer with impedence ground
RE: autotransformer with impedence ground
1. install a 277V 15A resistor on the xo of the supply transformer,as there are no 277V loads on this transformer.
2. change out existing zero sequence monitors with trip setting of 3-5A, to an adjustable trip setting from .25 to 2A.
3. float xo at autotransformer.
4. set GF on 2000A breaker in service disconnect to 5A.
Thanks for all the help everyone. badass