Transformer Vector group

[alansam]

1.I think Dyn11 vector group is the most commonly used vector group in distribution transformers.This means the low voltage supply lags behind high voltage by 30 degrees.As per my understanding this is merely done to compensate for the Phase lead provided at the generation side by Yd1 step up transformer.

So y do we use Yd1 transfromer at the generator side at all?Cant we use Yy or Dd transformer at the gen side so that we can use a similar kind in the distribution side too??I mean the requirement is to just step up the voltage so that it can be transmitted over long distances.Why introduce a phase diff at the gen side?

I know this might include a lot of explanation and maths..I would be glad if any1 could give me an explanaion and a link for the math(i mean network analysis)

Regards,

Alan

 

[Besoeker]

.I think Dyn11 vector group is the most commonly used vector group in distribution transformers.This means the low voltage supply lags behind high voltage by 30 degrees.As per my understanding this is merely done to compensate for the Phase lead provided at the generation side by Yd1 step up transformer.

The arrangement is not to provide a phase shift for for compensation.
The delta primary requires three supply conductors.
The star secondary provides line to line for three phase loads and line to neutral for single phase 240V loads.

 

[alansam]

That does not explain the phase difference.This is the HV Side or the Switchyard side of the Generator Transformer is connected in Delta and the LV Side or the generator side of the GT is connected in Star, with the Star side neutral brought out.Thus the LV side voltage will ?lag? the HV side voltage by 30 degrees.

 

[Besoeker]

That does not explain the phase difference.

It's a bit misleading to think of it as a phase shift.

Here's a diagram.

The voltages of the secondary windings are in phase with those of the corresponding primary windings. So the line to neutral voltages would be in phase with the supply. But if you take secondary line to line voltage it is made up of two different line to neutral voltages and its vector is their resultant. For example, L1 - L2 comes from windings B and C.

 

[shieldcracker]

Alansam, there is indeed a phase shift. This phase shift appears as a result of connecting the transformer for three phase. There are standards that define the phase shift angle, but I don't remember which one. For each type of three phase transformer connection (DD, YY, DY,YY) there are not one but two possible angle displacements. DY and YD can have 30 or 210 degrees in their corresponding current and voltage primary quantities. DD and YY have 0 and 180 degrees. In north America standards dictate that the connections be made to get 0 and 30 phase shift. Again the phase displacement depends on how you make the connections on the transformers.

 

[mivey]

As per my understanding this is merely done to compensate for the Phase lead provided at the generation side by Yd1 step up transformer.

Not really. Unless we are going to tie distribution systems together, the phase shift is a don't care scenario.

So y do we use Yd1 transfromer at the generator side at all?Cant we use Yy or Dd transformer at the gen side so that we can use a similar kind in the distribution side too??I mean the requirement is to just step up the voltage so that it can be transmitted over long distances.Why introduce a phase diff at the gen side?

Some main reasons:
1) To block zero sequence currents.
2) So we can insert a grounding resistor to limit the ground fault current for the generator.
3) We usually transmit at the HV level using a delta configuration. The wye usually appears at the MV level (and not even then for some systems).

 

[alansam]

Not really. Unless we are going to tie distribution systems together, the phase shift is a don't care scenario.

Some main reasons:
1) To block zero sequence currents.
2) So we can insert a grounding resistor to limit the ground fault current for the generator.
3) We usually transmit at the HV level using a delta configuration. The wye usually appears at the MV level (and not even then for some systems).

Thanks Mivey. Tie distribution systems together means to "parallel distribution transformers"? And if phase shift is a dont care scenario in non paralleled distribution systems,can I use a dyn5 vector group transformer in a grid that normally uses Dyn11 transformers for dedicated distribution systems?

 

[mivey]

Thanks Mivey. Tie distribution systems together means to "parallel distribution transformers"?

No. It is usually paralleling two substation transformers with miles of feeder impedance in the tie. A bus-tie at a substation bus would not have all that impedance in the tie.

And if phase shift is a dont care scenario in non paralleled distribution systems,can I use a dyn5 vector group transformer in a grid that normally uses Dyn11 transformers for dedicated distribution systems?

Not sure what you mean by "a grid that normally uses Dyn11 transformers". For an isolated system, it really does not matter electrically since it is isolated. If there are any ties with a different system, you need to coordinate.

FWIW, it is wise to use a standard rotation convention in case an alternate feed is ever needed.

 

[alansam]

No. It is usually paralleling two substation transformers with miles of feeder impedance in the tie. A bus-tie at a substation bus would not have all that impedance in the tie.

Not sure what you mean by "a grid that normally uses Dyn11 transformers". For an isolated system, it really does not matter electrically since it is isolated. If there are any ties with a different system, you need to coordinate.

FWIW, it is wise to use a standard rotation convention in case an alternate feed is ever needed.

Only the load is connected on the LV side of the transformer.So I think that is what you call an isolated system.?

Between thanks for all information.

 

[mivey]

Only the load is connected on the LV side of the transformer.So I think that is what you call an isolated system.?

If all the loads are on this one transformer, and do not tie with loads from a different transformer, then you are isolated.

thanks for all information.

You are welcome.