paralleling of power autotransformers

[johny66]

Good afternoon,

Please if you can, help me with a information.
If I have an autotransformer of 250 MVA; 400/132/38.5 kV(tertiary), impedance voltage 10.2% , with tap changer located in neutral, +- 8 steps, +9%-10%, voltage regulation on 132kV side.
Please let me know if exist a possibility to put this autotransformer in parallel with other 2 similar autotransfomers, but which have different value of impedance voltage on the ends of domain of voltage regulation.
The rest of data are similar.
My autotransfomer have the impedance voltage on the ends of the domain of voltage regulation: for max ratio: 7.84% and for min ratio: 15.07%
The other two autotransfomers have the values of these voltage impedances: for max ratio: 9.9% and for min ratio 10.3%

Thank you very much for your information.

 

[cranky108]

It's always possible, just be aware of circulating current.

The different impedance will mean that the lower impedance transformer will take more of the load.

 

[slavag]

is not so good, difference more than 10%.
you must work with minimum circulating current paralleling system.

 

[stevenal]

Your text to link here...

 

[stevenal]

Let me try that again

 

[FPelec]

jonny66,
you also should check voltage ratio on each tap.

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[johny66]

Thank You very much for your answers.
Does anyone know to calculate the maximum possible power transfer for the three autotransformers (of 250 MVA) in parallel, in this case?

 

[waross]

The first step is to normalize the % impedances by re-rating the KVA or MVA bases.
EG We want to connect a 1000 KVA transformer in parallel with a transformer with 4% impedance voltage.
The 1000 KVA transformer has 8% impedance voltage. 1000 KVA / 8% = 125 KVA
125 KVA x 4% = 500 KVA. When this transformer is supplying 500 KVA the 4% impedance transformer will be 100% loaded.
Note that if the second (4% impedance) transformer is rated at less than 500 KVA the total allowable load may be less than the 1000 KVA transformer alone.

Bill
--------------------
"Why not the best?"
Jimmy Carter