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08-22-2013 10:33 PM

help! 35KV SWG in project

Power on 35kv provided by 2 Systems with 220/35kv transformers "YNynd(d1)" configurations and few 11KV Generators with 11/35KV transformers marked "Dyn11" on single line diagram, but showing that "D" from 11KV and "yn" from 35KV sides. If "Dyn" marks for transformer, it is usually mean that "D" from high side transformer and "yn" from low side. This is my question. Please tell me, what is right for my project: "Dyn11", "YNd1" or "YNd11" and what rotation will be for the generators-ACB or ABC?
08-22-2013 10:34 PM
Top #2
08-22-2013 10:34 PM
The basic connection groups are:

A) 0° (zero) phase displacement (Yy0, Dd0, Dz0)
B) 180° phase displacement (Yy6, Dd6, Dz6)
C) -30° phase displacement (Yd1, Dy1, Yz1)
D) +30° phase displacement (Yd11, Dy11, Yz11)

The transformers within the same group can be paralleled, for example u can parallel transformers of the group 4 Yd11 and Dy11.

The transformers of the group 1 & 2 can be paralleled only with transformers of their own group and transformers of the groups 3 and 4 can be paralleled only by reversing the phase sequence of one of them.

Your main transformer is primary Y, secondary Y with tertiary, so the phase displacement between primary (wye) and secondary (wye) is 0° guessing transformer is Yy0 and not Yy6.

Now reviewing basic groups we can see that Yy0 is within group 1 and can be paralleled with a Dd0 or Dz0 but not with transformers of the group 4 (Dyn11).

Of course u can try to use different groups but need to check the equalizing current, which value depends on the difference between shift angles.

A word of caution: For example if u connect in parallel a Yy0 and Yy6 transformer with the same impedance and the same capacity the circulating current will be 20 the rated current..!!! a lot of current.

Regarding to use ACB or ABC you should ask the utility.
08-22-2013 10:36 PM
Top #3
08-22-2013 10:36 PM
Your single line diagram is correct.

Incoming Wye-wye withndelta tertiary, solidly grounded wye. these types of tranformer are typical for grid tie and likely you may have local HV ring bus and tap off elsewhere fro that station, if not its OK. Therefore designation in correct YNynd(d1.

Generator Delta low voltage side and Wye HV side solidly grounded, designation is also correct Dyn11. High side leads low side by 30 degrees.

The Delta is required to clean up the third harmonic if both line are out of service for the step up transformer.

The Delta is also required to clean up the third harmonic on the Wye-Wye or else it will fron in the neutral/ground path in case the generator is out of service.

The 35 KV bus need to be reference to ground since it is utility supply, so solidly grounded is a requirement for utility so as to prevent neutral point voltage rise.

08-22-2013 10:37 PM
Top #4
08-22-2013 10:37 PM
Alec has 2 questions:
1. What should be vector group for the generator 11/35kV step-up transformer, Dyn11, YNd1 or YNd11?

2. What should be phase rotation of connection from generator to the step-up transformer, ACB or ABC?

1. The vector group for the generator step up transformer must be either YNd1 or YNd11. You may have a mixture of two different vector groups transformer for the multiple generators, but I doubt you will do that in a single project. You should be purchasing all the generator step-up transformers with the same vector group. The 35kV side of each generator step-up transformers must have a neutral for grounding which is needed in the event of disconnection from the grid, therefore it must be of YN winding.

2. The phase rotation of generator connection must be ABC. I don't see you have any other choice and it must be the same for all generators. Phase shift due to choice of different transformer vector groups (1 or 11 o'clock) has no relationship to phase rotation ABC or ACB.

If this is an existing installation and the question is whether the information on the drawing is correct, then the 11kV winding should be marked "d" and 35kV winding should be marked "YN". Verify this at site from the transformer nameplate.
08-22-2013 10:38 PM
Top #5
08-22-2013 10:38 PM
Last two comments could be right only if your generators work isolated from utility, otherwise both last comments are wrong because if utility and generators are in parallel transformer connection group shall be as previously noted.

By the way, delta in tertiary and delta in generator transformer are not for clean up the 3th harmonic, this is just an additional benefit of such connection the main purpose is other.
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