Distorted current waveform on one phase

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vectormechanic said:
I apologize for the confusion. In the one line I posted, yes, I did depict a wye/delta transformer. That is one of our more typical plants, not the one in question. I attached it because I am in disbelief that the generator neutral should be directly tied to the transformer neutral as Besoeker states (bypassing the HRG setup). It is not a representation of the plant that we were originally having the problem with. I have attached a one line of the plant in question.
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I agree the gennie's and transformer primary's neutrals should be tied together directly, i.e not through the HRG xfmr. Additionally, this connection should should not be solidly grounded at any point. The xfmr frame should be tied to the ground grid.


vectormechanic said:
Our utility does not (?) provides us with a delta connection (i.e. no neutral). Why a wye/wye transformer was chosen is beyond me. To be frank, 20 years ago when the plant was built, the company probably got a smoking deal on the transformer and decided to use it.
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I believe you mean your utility is 3?, 3-wire only... i.e. essentially delta.

Strictly speaking, 3? line-to-line voltages can be wye-supplied without the neutral or delta supplied, with no difference in transmission or usage (as long as they are in sync :grin:).
 
Smart $ said:
I agree the gennie's and transformer primary's neutrals should be tied together directly, i.e not through the HRG xfmr. Additionally, this connection should should not be solidly grounded at any point.
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Why shouldn't it?
Just asking.
 
Besoeker said:
Why shouldn't it?
Just asking.
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I agree with Smart $, tie the primary neu together but do not ground. Grounding would bypass the HRG system (GF indication and/or protection.)
Vectormechanic: Is the HRG used to indicate GF condition only or does it trip a breaker?
Tkx

Tony
 
vectormechanic said:
I apologize for the confusion. In the one line I posted, yes, I did depict a wye/delta transformer. That is one of our more typical plants, not the one in question. I attached it because I am in disbelief that the generator neutral should be directly tied to the transformer neutral as Besoeker states (bypassing the HRG setup). It is not a representation of the plant that we were originally having the problem with. I have attached a one line of the plant in question.

As for why the B phase opened above the cutout, we don't know. I had to leave the site, and when the utility's lineman repaired it, he simply stated that it had "burned" off, and no one from our company asked for any more details.

Our utility does not provides us with a delta connection (i.e. no neutral). Why a wye/wye transformer was chosen is beyond me. To be frank, 20 years ago when the plant was built, the company probably got a smoking deal on the transformer and decided to use it.

3216320368_79ed78f4bf_b.jpg
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It should be noted here that a HRG scheme is usually used to indicate a GF (phase to gnd condition) and send an alarm on an ungrounded system without shutting down the system (rail systems and manufacturing plants use this.) The operator would then quickly have to locate and remove the fault before it develops into a phase to phase fault. Although in rare cases I have seen it trip breakers.

Tony
 
tonytonon said:
I agree with Smart $, tie the primary neu together
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Yes. I made the same point in several posts from #3 onwards so no argument on that score.
tonytonon said:
but do not ground. Grounding would bypass the HRG system (GF indication and/or protection.)
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My choice would be to ground it solidly at the generator star point (only) and use a core-balance CT for earth leakage and ground fault detection and protection.
More than one way to skin a cat..........:wink:
 
vectormechanic said:
<snip>(a month ago when we first restarted the plant and noticed the problem, we called the utility who sent a lineman that claimed everything was fine on their side...)
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After seeing the pics you posted, that lineman obviously never even got his behind out of the truck....that burned off jumper is pretty obvious.

As for why it burned off, I'd bet that clamp on the cutout was never tightened properly.

Finally the cables going to your plant in that conduit look like they're under a lot of downward stress. I wouldn't be surprised to see an outage in the future when they either pull loose or short out to the shield from the stress.
 
Besoeker said:
My choice would be to ground it solidly at the generator star point (only) and use a core-balance CT for earth leakage and ground fault detection and protection.
More than one way to skin a cat..........:wink:
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With the HRG scheme, the primary system can float on a ground fault and has a higher probability of not interrupting output. Not so with a solidly grounded connection, or at best a lower probability of uninterrupted output.
 
I'm still having problems with a wye/wye tranny being connected to backfeed a delta supply.

I cant explain why, exactly, but it just seems wrong. Surely without the centre point connected to something related to the three phase wires, there will be the backwards version of a lost neutral...?

Just to add - another good reason for using HRG (ie avoiding high current faults) in this sort of setup is that fault currents can get really big when one has generators onlline feeding the utility, as under fault conditions both the utility and the generators are current sources for the fault.
 
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Smart $ said:
With the HRG scheme, the primary system can float on a ground fault and has a higher probability of not interrupting output. Not so with a solidly grounded connection, or at best a lower probability of uninterrupted output.
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Yes I can see the merit in that, but does mean that it can float at any voltage.
 
Wow, I take a three day weekend and look what happens!

There was a bit of a typo in one of my responses - the utility does provide us with a standard 3?, 3W service.

I agree that there is more than one way to skin a cat when it comes to ground fault detection schemes. As dbuckley mentioned, we use the HRG to limit the fault current - some of our busses have a 1000MVA fault rating due to the contribution of our generators. Just for info, the 1252MVA units two and three at San Onofre Nuclear Generating Station in California also use the exact same setup.

As for the cables under stress, that's all the utility's equipment, so we are at their mercy.

In our GF detection scheme, we do trip our generator breakers in the event of a GF. We have seen trips due to a grounded stator winding (GF caught it before differential), and where a cable failed in the line cubicle of the generator (13.8kV units).
 
vectormechanic said:
Wow, I take a three day weekend and look what happens!

There was a bit of a typo in one of my responses - the utility does provide us with a standard 3?, 3W service.

I agree that there is more than one way to skin a cat when it comes to ground fault detection schemes. As dbuckley mentioned, we use the HRG to limit the fault current - some of our busses have a 1000MVA fault rating due to the contribution of our generators. Just for info, the 1252MVA units two and three at San Onofre Nuclear Generating Station in California also use the exact same setup.

As for the cables under stress, that's all the utility's equipment, so we are at their mercy.
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:smile:
But you still need to connect the generator star point to the transformer star point.
Grounding and ground fault detection and protection are a different matter.
High resistance grounding is a method we have often applied on DC systems. Fault current limitation is just the resistive component whereas on AC systems its impedance and mainly the reactive component that limits fault current.
 
Back to the beginning, I don't see how you measured current (even distorted current) in all three phases when one of the primary phases was open. In a wye-wye transformer, the current in the secondary phase will be zero if the current in the primary is zero.

Why does the generator neutral have to be connected to the transformer neutral? If the utility service is a 3-wire service, then there cannot be any neutral flow in the transformer. Primary phase currents must sum to zero if there is no primary neutral. If the primary phase currents sum to zero, then so will the secondary phase currents, which are equal to the primary phase currents multiplied by the turns ratio.
 
We are somewhat baffled as to how we had voltage on all three phases on the LV side of the transformer given that the HV side was single phasing. The only explanation that we can come up with (as well as two engineering/testing firms that I spoke with) is that the transformer acted in an open wye configuration.
 
vectormechanic said:
We are somewhat baffled as to how we had voltage on all three phases on the LV side of the transformer given that the HV side was single phasing. The only explanation that we can come up with (as well as two engineering/testing firms that I spoke with) is that the transformer acted in an open wye configuration.
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You have voltage on the transformer from the generators. There will also be voltage on the primary side induced by the generator voltage. What I don't understand is how you have current, except for the parasitic load, because you can't have current in the transformer secondary winding.
 
jghrist said:
...What I don't understand is how you have current, except for the parasitic load, because you can't have current in the transformer secondary winding.
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It's just the opposite of single phase line-to-line loading on a 3? wye.
 
Even with the generator offline, pulling plant parasitic load from grid through the transformer, we had voltage on all three phases, albeit unbalanced. We were able to run motor loads, etc. The difference was about 25 volts from the high phase to the low phase.
 
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