vector group

Analyze this diagram and study your system. If you need further help, better consult with an expert engineer.

o.k. dear friends,

the above graph clarifies the question, that in this case lv voltage will lead by 150 deg with dyn7. but the question is what will be effect in the system as compared to vector group dyn11.

thanks and regards.

V R Rao

Please read the previous threads:

http://cr4.globalspec.com/thread/36484/Vector-Groups-for-Transformers

and

http://cr4.globalspec.com/thread/35957#comment374937

-MS

I can't add any thing useful at this stage, but I will be interested in the replies.

Other than simple Star/Delta, I never could make sense (during school lessons) of the theory behind the primary/secondary winding connections, other than it had an effect on the way various harmonics were handled/generated.

Going off at a tangent a bit, what would be the output voltage if three secondary windings were connected in series to provide a single phase supply.

This will create imbalance currents in your system and circulating currents will flow during parallel operation

As previously pointed out, as long as you do not connect the secondary side in parallel with another source derived from a transformer secondary with a different phase angle difference, nothing will happen.

If you do, there will be circulating currents on each phase, additional losses, etc.

This is the reason when several transformers are mounted in parallel to serve the same network, one condition is also that all shall have the same vector group (the same phase angle between the corresponding primary and secondary phase).

yes friend, nice answer but could you please explain what are the advantages or disadvantages if connected 2 transformers of dyn7 in parallel, in place of 2 transformers of dyn11. perhaps no problem both of same vector group, but against dyn11 to dyn7.

and what are the factors needed to choose a particular vector group.

although mostly choosen is dyn11, for 30 deg lead only, why!

Ramchandrarao,

Each transformer converts a voltage. Both voltages ( primary as well as secondary) are AC ( sinusoidal). If the secondary is connected as yn7 respectively yn11, it means that at any time there will be phase difference of (11 - 7) x 30 deg = 120 deg for each phase, that translates into a voltage difference (relative) for each phase ( phase A for yn7 and phase A for yn11) of between -1.5 and +1.5, i.e. for 230V it would be between -345V and +345V.

This difference will cause a circulating current between the 2 points, and, depending on the specific time moment, the direction of the current alternates between yn7 -> <- yn11, each of the transformers sees this current as an additional load, etc..

If the transformers belong to the same vector group, this difference is 0 at any time, so no circulating currents appear.

Tomad,

i think u not understand the question....

question is what happen when transformer with Dyn11 replaced with Dyn7.....

forget it's paralleling.

I hope you have some background in Electrical Engineering, and so you can not say 'forget it's paralleling' to get the answer. It has been stated many times that the term 'Vector Group' is very important for parallel operation.

The answer has been given few times before directly or indirectly. Here it is again:

In your system, is the Dyn11 transformer is running parallel with any other transformers somehow?

If the answer is 'Yes', you can not do it because there will be heavy circulating current flowing among the transformers (due to the large voltage difference as indicated by 'tomad' or see the vector diagram posted by me) and the parallel transfomers will be damaged.

If the answer is 'No', you can replace it.

If you post the one (single) line diagram of your system, then it can be answered directly.

-MS

Dear lasam,

I have some Electrical background & i know about it's paralleing,but i think u also not understand or read my question carefully.

If we parallel Transformer with vector group Dyn7 with other vector group then it will damage,but my question is what is effect on our system when we replace Dyn11 with Dyn7.............

& u say No effect on system then why Dyn11 is almost used everyware?

Iasam,

Nothing happens. The only difference is a different phase difference between primary and secondary.

exactly the same question was mine,

o.k. it will lead by 150 deg. and earlier replies says no problem in the system.

i agree.

but again i repeat the same Q- with all the previous repliers that why mostly choosen is dyn11. and why not dyn7.

and thats again what is the advantage of dyn11 over dyn7. i mean why 30 deg lead is always prefffered, any particular reason!!!!.

The way the secondary windings (assumed vertical) can be connected to achieve a Y (star) connection, keeping all the other characteristics the same is either to create the neutral point at one part of the winding (bottom) or at the other (top). In one case it results the yn11 and in the other case the yn7 vector group.

The default neutral connection (technologically) is on the bottom, so that should be yn11.

7 would represent an angle difference of 7 x 30 = 210 deg while 11 x 30 = 330 deg toward the same phase in the primary part. These are the 2 basic phase differences (with an angle difference of 120 deg between them). Other combinations can be obtained by selecting the secondary phases (for instance - new secondary phase A as old secondary phase B), that would lead to yn3, etc.

It would be the same voltage, but related to another secondary winding, whose voltage is at an angle difference of 120 deg. Under normal circumstances nothing changes, except that the windings considered for the same phase (a) are no longer placed on the same leg.

The only thing that is different is for the case of unbalanced load ( as an example: 2 phases full load and one phase zero) , where the secondary phase A is at no-load, which would translate into another phase (B) on the primary side, carrying the lowest load.

The identification/designation of the 3 phases as A,B,C (R,S,T) is arbitrarily selected anyway, but that should not be a problem, since the secondary system is considered as a separately derived system.

Excellent explanation.

Nice and thanks Mr Tomad

There is magnetic coupling between HT and LT.When the load side(LT) suffers some dip the LT current try to go out of phase with HT current, so 30 degree phase shift in Dyn-11 keeps the two currents in phase when there is dip.

nice and thanks.