Circulating Currents / Gen / DeltaWye
Circulating Currents / Gen / DeltaWye
(OP)
The setup that is currently existing involves a pair of 480V generators that feed transformers that step the voltage up to 13.8kV , and in turn feed a piece of switchgear at this voltage. Utility feeds into switchgear, and switchgear feeds a couple buildings, presumably being dropped down to 480 or other distribution level voltage closer to the loads.
The question I have been asked and didn't have a good answer to involves explaining the tradeoffs in selecting a delta or wye winding on the generator sides of those transformers. It seems like on the switchgear side, delta will be fine, and as I understand it slightly cheaper for both the transformer and the fewer conductors that are pulled (a N/G instead of a separate N and then G). But on the generator side, what goes into the selection of whether the generator needs to supply a neutral to this transformer?
I realize this is a subject that is very deep but I need to develop a much better pros/cons kind of engineering tradeoff understanding of this decision.
Thanks in advance to you smart ones-
The question I have been asked and didn't have a good answer to involves explaining the tradeoffs in selecting a delta or wye winding on the generator sides of those transformers. It seems like on the switchgear side, delta will be fine, and as I understand it slightly cheaper for both the transformer and the fewer conductors that are pulled (a N/G instead of a separate N and then G). But on the generator side, what goes into the selection of whether the generator needs to supply a neutral to this transformer?
I realize this is a subject that is very deep but I need to develop a much better pros/cons kind of engineering tradeoff understanding of this decision.
Thanks in advance to you smart ones-






RE: Circulating Currents / Gen / DeltaWye
RE: Circulating Currents / Gen / DeltaWye
Delta at 480V with a ground fault detection system would be best for the generator connections, as David said, if there are not too many other 480V loads on that bus.
Feeders from the 13.8kV wye transformer only have to be three wire. A neutral wire is not needed. The step down transformers at the loads would be the typical delta wye with delta on the 13.8 kV.
RE: Circulating Currents / Gen / DeltaWye
The relative cost tradeoffs between delta and wye windings and the costs for a neutral conductor are trumped by the fundamental protection and isolation issues associated with a utility interface to a local generation system.
The serving utility will want to have a major input into the transformer configuration since it will impact their system protection and relaying.
RE: Circulating Currents / Gen / DeltaWye
Bill
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"Why not the best?"
Jimmy Carter
RE: Circulating Currents / Gen / DeltaWye
If your generators are big enough to justify stepping up to 13kV and you have no 480V loads on the generator bus you might want to look at 4160V for the generators. Your circuit between generator and will need only 11.5% the ampacity at 4160V as it will at 480V. 2MW requires a 2770A circuit at 480V but only 320A at 4160V. One or two conductors per phase vs. nine per phase.
RE: Circulating Currents / Gen / DeltaWye
But I have seen star/star GT where generator is used to supply the plant also to simultaneously pump in to grid at times of low demand in plant
As far as transformer is concerned,from economic point both delta and star are the same for this voltage level.
RE: Circulating Currents / Gen / DeltaWye
Using a delta LV winding and HRG on the generator neutral limits the damage that a stator earth fault will cause. With an HRG system limited to about 10A you might get a small burn on the core and need some remedial work to avoid a hot spot. With solid grounding and a fault level of 50kA or so you will probably need a new core or major rebuild plus a full rewind.
Are you confusing the effects of stator zero sequence currents with rotor negative sequence currents? The latter are highly undesirable, and more so on larger machines.
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If we learn from our mistakes I'm getting a great education!
RE: Circulating Currents / Gen / DeltaWye
Davidbeach
I'm not following why you would need the transformer to be a grounded wye on the primary if L-N single phase loads were needed to be supplied directly off of the grounded wye generator?
RE: Circulating Currents / Gen / DeltaWye
Grounded wye on the high side makes the generation appear effectively grounded as seen by the utility. To be effectively grounded, the low (generator) side of the transformer needs to be either a grounded wye connected to solidly grounded generators or a delta and any generator grounding scheme. No probably with solidly grounded generator connected to a delta winding, but if you do that you won't be able to serve the L-N connected load from the utility when the generators are off line.
RE: Circulating Currents / Gen / DeltaWye
Also please explain how the wye on the 13.8 side in this referenced example would be the better option for ground fault protection.
Can someone please describe how ground fault protection would work on a 3W system?
I'm used to in the simplest case, when a 4W system is being used, installing a CT on the bond between the nuetral and ground bus bars (solidly grounded system). I understand this not to work for more 'complicated' setups, we ran into this last week actually.
RE: Circulating Currents / Gen / DeltaWye
The most significant benefit of the 13.8kV being effectively grounded is for voltage stability during ground faults. That's why the utility will probably require effective grounding.
RE: Circulating Currents / Gen / DeltaWye
Regarding the 13.8 kV side - look at it from the utility perspective. If there is a ground fault on their 13.8 kV feeder, they need to clear the fault. They also do not want to end up tripping their source and leaving your generators supplying their customers. Without a ground source at your generators, it is harder to detect a 13.8 kV ground fault at your facility. With the grounded wye, there will be plenty of ground fault current provided by the delta-wye transformer and they can trip you off the line nice and fast.
RE: Circulating Currents / Gen / DeltaWye
What would be the issue is the low side of the transformer was a solidly grounded wye and the generator was connected as something other than a solidly grounded wye such as a delta or resistance grounded
RE: Circulating Currents / Gen / DeltaWye
If these 480 V generators are directly serving 480/277 system with 277 V loads, then they need to be solidly grounded. Otherwise, it's much better to use some type of resistance grounding.
RE: Circulating Currents / Gen / DeltaWye
The first is that the low side system would look very different fed only from the utility vs. fed only from the generator.
The second is that when seen from the utility side a grounded wye-grounded wye transformer with the generator not solidly grounded would not present an effectively grounded source to the system. The grounded wye-grounded wye transformer is not a zero-sequence source the way that a delta (low side)-grounded wye (high side) would be and the generator grounding is the zero-sequence source, so it now matters.
RE: Circulating Currents / Gen / DeltaWye
The delat-wye transformer connection is used for connecting generators to transmission systems because of two very important reasons.
First generators are provided with sensitive ground fault relay protection.The delta -wye trf is a source of ground currents for loads and faults on the transmission system,yet the generatorground fault protection is completely isolated from ground currents on the primary side of trfs.
Second rotating machines can literally be shaken apart by mechanical forces resulting from zero sequence currents.The delta connected winding blocks zero sequence currents on the transmission system from the generator.
Unquote
I wish to get your expert opinion on the above view.Do you agree.
I wish to correct myself.When export /import of power is involved I have seen star/delta generator trfs feeding to grid-not star/star with which it will be impossible.
RE: Circulating Currents / Gen / DeltaWye
No argument that heavy imbalance on the loading can cause problems. The reason is magnetic assymetry which occurs in the air gap due to the interaction of unevely loaded phase conductors in the stator, which leads to potentially very powerful forces acting to move the rotor out of its normal rotational path. Those forces ultimately act on the machine bearings and will show up as a vibration signature. To be honest it's not something I have spent a vast amount of time looking at because large machines always have a delta wound GSU transformer with an HRG scheme and it has not been of huge practical significance to the utility class machines I work with, certainly not in the way that negative sequence currents can affect our machines. The link below might be useful - I bookmarked it a while ago as 'interesting' and it has some explanatory narrative in the first few pages.
http
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If we learn from our mistakes I'm getting a great education!
RE: Circulating Currents / Gen / DeltaWye
RE: Circulating Currents / Gen / DeltaWye
To summarize what I do think I understand; Ground fault is easy to detect when a 3W (delta) feed from a generator is used, because ground fault current = sum of phase currents which will equal 0 in a no fault condition.
Now comes the 87 (differential current) detection. How does this tie in with this discussion? I understand this to be a CT on the ground conductor at the generator, and this current is compared to the ground current in whatever the generator is feeding. But in the case we have been discussing where there is a transformer in between, would the 87 relay function be useful? How is this related, and am I off base in trying to relate this to the selection of the grounding method of the generator.
Thanks in advance for access to your wealth of knowledge.
RE: Circulating Currents / Gen / DeltaWye
Please don't mix 87G generator differential protection against phase to phase faults and 87N restricted earth/ground fault protection or 87U block transformer-generator differntial protection.
For the your OP David wrote full and correct answer.
Delta on the generator side.
star (wye) on the 13.8kV side.
Best Regards.
Slava
RE: Circulating Currents / Gen / DeltaWye
Thanks,
RE: Circulating Currents / Gen / DeltaWye
You don't understood me right, sorry for my not clear post.
Of course, we can continue with protection issue.
87 relay operation isn't related to your OP, but 87N, yes relayted. If you will use NER ( high impedance or low-impedance), it's very recommended use 87N ( REF) protection.
Best Regards.
Slava