Everybody Know Transformer Vector Group, But..

A1) It is not usual to run a neutral conductor in high voltage transmission lines.

A2) A neutral conductor is needed in low voltage distribution so that single phase loads can be connected.

A3) -

<...Everybody...> How abstruse.

I think I have answered this in an earlier post here. Well! Let me try recalling the same.

i) I hope you understand what is Ynd11. 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. The LV side voltage will "lead" the HV side voltage by 30 degrees. Isn't it?

As the generator is connected in Star, it is advisable to connect the Generator side winding of the GT also in Star so that there is a closed path for earth fault currents. The switchyard side is normally connected to long transmission lines and it is advisable to go for Delta Connection here to save on an extra conductor for the entire length of the transmission system. (And, I hope you are aware that Delta is three-phase, three-wire and Star is three-phase, four-wire). Now, as to the phase displacement, going by the 11 O' clock vector connection, the LV Voltagge would lead the HV Voltage by 30 degrees. But, since it is a generating station, we should view this form the Generator side. Thus, viewing from the generator side, if the LV would "lead" the HV by 30 degrees, it is also true that the HV would "lag" behind the LV bu 30 degrees. Thus, in a GT the HV side voltage of the GT would "lag" behind the LV side voltage by 30 degrees. Thus, in a generating station we create a 30 degrees lagging voltage for transmission, with respect to the generator voltage.

ii) As we have created a 30 degrees lagging connection in the generating station, it is advisable to create a 30 degrees leading connection in distribution so that the user voltage is "in phase" with the generated volatge. And, as the transmission side is Delta and the user might need three-phase, four-wire in the LV side for his single phase loads, the distribution transformer is chosen as Dyn11.

iii) Beyond my comprehension; will try to go thro this and answer soon.

thank you,

but why lagging and leading needed?

what is its advantages?

what happen if we are using transformer with out phase shift every where

eg: YNd0,Dyn0

Dear Yavadkv,

YNd0 and Dyn0 are not physically possible.

The windings of primary and secondary [for one phase] are wound over each other or end to end on the same iron limb.

For a perfect transformer they share the same magnetic flux and the same induced voltage. secondary can be in-phase with primary or 180 degrees away.

For Y-d connection, a line to line voltage on the d side must be in phase with a line to star point voltage on the Y side.

Hence a line to line voltage on Y side cannot be in phase with line to line voltage on D side.

So there must always be a phase shift in a Y-d transformer.

It is essential you apply physical principals in your thinking, or you will not understand anything.

Please refer to the web reference I gave in post #6. Search for "transformer".

67model

Searching the web for "3 phase transformer" will help.

http://en.wikipedia.org/wiki/Main_Page

- is a good place to search, since it deals in facts and information, instead of getting droves of advertisements for transformers, which is what google wants you to get!

Delta winding connections are useful to reduce harmonics in the power system. Try a search for "power system harmonics".

Replying to your questions....

1. Transformers can be used for almost any purpose in a power station and Ynd11 is not limited to power stations. As PWS wrote, if a supply is mainly for motors (particularly over 415/240V), there is no benefit in running a neutral conductor. Ynd11 is usual for generators themselves, because a neutral earth on the High Voltage HV side is needed somewhere and a generating plant is a convenient point (note: Y, D indicates HV side, while y, d are used for LV side). One factor is that the insulation can be thinner at the earth end of the Y winding - big generator transfos usually go up to Grid voltages over 132 kV. A "d" winding at LV cancels some harmonics, which would cause more generator heating and reduce its rating.
2. LV [400/230V] supplies at 50 Hz are usually 3 phase, earthed neutral, so a "yn" LV winding is needed. A reason for using Yd between e.g. 132 and 33 kV, then Dy from 33 to 11 kV is that their phase shifts can cancel out - it is then possible to parallel a 132/11 kV YY transfo, at 11 kV, with the 33/11 kV [a YY transformer often has a third, delta, winding to reduce harmonics]. If one went Dy11 - Dy11 from 132 to 11 kV, there would be a 60 degree shift, which is not possible in one transfo. The "standard" transfo groups in distribution avoid that kind of limitation, as a result of thought and experience leading to lowest cost over many years.
3. A Dy5 is the same transfo as a Dy11, but with the y windings connected together at the a2, b2, c2 end of the LV windings, instead of a1, a2, a3 -- a 180 degree phase change moving from 11 o'clock to 5 o'clock. I cannot think of any special requirement for excitation transfos, since their LV is not paralleled to anything else. It could be just an economy reason - shorter and easier physical connection of out-going LV conductors if connected to a1, b1, c1. Physically, it may be just moving a shorting bar to different terminals - but it alters the transfo rating plate vector/connection diagram and other documentation.
   

So your suggestions of "nullify" [of harmonics] and "compensate" [of phase] were in the right area.