POWER TRANSFORMER PARALLEL RUN & VECTOR GROUP 4

[11basonia]

WHAT ARE MOST IMPORTANT THINGS TO REMEMBER ABOUT VECTOR GROUP OF TRANSFORMERS,WHEN WE NEED TO RUN THEM IN PARALLEL? E.G DY11 TRF AND DY1 TRF.I HEARD IF VECTORS GROUPING OF TRANSFORMERS IS IGNORED, THERE IS A RISK OF CREATING SHORT CIRCUIT.

 

[Marmite]

If vector grouping is ignored there is an absolute certainty of creating a short circuit if you try and parallel a DY1 and a DY11.
To operate transformers in parallel they need:-
1. The same voltage ratio.
2. The same percent impedance.
3. The same polarity.
4. The same phase sequence.
5. The same inherent phase angle difference between primary and secondary terminals.

You can only parallel Dy1 and Dy11 by crossing two incoming phases and the same two outgoing phases on one of the transformers, so if you have a DY11 transformer you can cross B&C phases on the primary and secondary to change the +30 degree phase shift into a -30 degree shift which will parallel with the Dy1, assuming all the other points above are satisfied.
Regards
Marmite

 

[stevenal]

faq238-1154

 

[waross]

If you parallel transformers with equal or similar per-unit impedances they will mostly share the load in the ration of their KVA ratings.
I say mostly because it is possible to have two transformers with equal per-unit impedances but different X/R ratios. In this case the line current will be less than the sum of the transformer currents and the combined capacity will be reduced acordingly.
A rigorous calculation of load sharing between two transformers would entail determining the resistance and reactance of each transformer.
For each transformer the voltage drop due to resistance is calculated.
For each transformer the voltage drop due to reactance is then calculated.
These values may then be converted to per-unit values to help to determine the sharing of both the active current and the reactive current.
This is seldom done and I am not suggesting that you do it, but if you get unexpected results from paralleled transformers this is an avenue to explore.
Usually go for equal per-unit impedances.
respectfully

 

[RalphChristie]

Just an additional point:

Any star/delta transformer can be connected in parallel with another star/delta transformer if the criteria in "Marmite's" post is obeyed. Either by doing phase changes external on one side of the transformer or changes on both sides of the transformer (depending on the two phasor-groups) any of these transfomers may be connected in parallel:
"Yd1, Yd3, Yd5, Yd7, Yd9 and Yd11"

The same is true for delta/star transformers.
Dy1, Dy3, Dy5, Dy7, Dy9 and Dy11.

For delta/delta transformers just certain vector-groups can be connected in parallel by external phase-changes:
Dd0, Dd4, and Dd8
or
Dd6, Dd2, and Dd10
thus
a Dd0 trsf and a Dd6 trsf can't be put in parallel without doing changes internally to a transformer.(or by opening the transformer)

The same is true for star/star transformers.
Yy0, Yy4, and Yy8
or
Yy6, Yy2, and Yy10

Regards
Ralph

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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[11basonia]

1.On power transformer vector group of DY11(-+30 DEGREE PHASE SHIFT),BETWEEN WHICH QUANTITIES DOES THIS PHASE SHIFT EXIST? OR IS IT BETWEEN THE PRIMARY AND THE SECONDARY VOLTAGES OF THE TRANSFORMER? CAN YOU CLEARIFY THAT FOR ME?

 

[RalphChristie]

Between your primary A-phase and the secondary a-phase.

A-phase is always your reference voltage.
According to international adopted convention, phase rotation is is always anti-clockwise.

Thus, for a Dy11 transformer:
Secondary a-phase is leading Primary A-phase with 30°

See also:
faq238-1090

Regards
Ralph

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376