# help me explain the conditions of 2 transformer to operate parallel?

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• #10551
duclampv
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could you explain to me the conditons of 2 transformers operate parallel?

#12325
Keymaster

duclampv said:

could you explain to me the conditons of 2 transformers operate parallel?

## Parallel Operation of Transformers

The essential conditions for successful parallel operation of transformers are:

I.   Transformation or turn-ratios and voltage ratings should be the same.

II.  Polarities of the transformers should be the same.

III. Percent impedances of the transformers should be the same.

IV. Ratios of resistance to reactance should be the same.

V.  Phase displacement between primary and secondary windings of the transformers should be the same.

VI. Phase sequences or vector group of the transformers should be the same.

#12338
Spir Georges GHALI
Participant

duclampv said:

could you explain to me the conditons of 2 transformers operate parallel?

Dear ;

To connect 2 Transformers on parallel, we should take into consideration the following points for the Transformers and the Panelboards and the installed Circuit Breakers :

1- Transformers :

– The specifications ” Power, Usc, Wtr, Vector Group, Primary & Secondary connection, V1, V2, … ” of these 2 transformers should be the same.

– It's better that are produced by the same manufactures.

– The ” Tap-Off ” changer ( primary side ) of both should be connected to the same point.

– We should pay attention of the connection of LV sides ” L1(Tr1)  →  L1(Tr2),  L2(Tr1)  → L2(Tr2),  L3(Tr1)  →  L3(Tr2),  N(Tr1)  →  N(Tr2) “

– If we use Cables or Busbars between the transformers and the MDB it's be better to be the same kind and the same length.

– As there's no fine adjustment for the Output Voltage of transformers, it's better that are connected to the same Feeder of Medium Voltage.

2- Panelboards :

– If the Panelboards are ” Totally Type Tested Assemblies “, the test's Short Circuit Withstand Current for the distribution's Busbars should be ” > 2 x Isc3max “ ( depending on the values of ” Isc3max ” in the inatallation palces ).

3- Circuit Breakers :

The most important points about the installed Circuit Breakers are :

A- The Breaking Capacities ” B.C. ” :

As the value of the maximum short circuit current ” Isc3max ” is same for the 2 transformers, so :

– For main C.B. of MDB : Equal or Bigger than the maximum short circuit current for 1 transformer ” B.C.  ≥  Isc3max “.

– For other installed C.B. : Equal or Bigger than the maximum short circuit current for 2 transformer ” B.C.  ≥  2 x Isc3max “ ( 2 transformers are connected ).

B- The Magnetic Protection ” Im ” :

As the value of the minimum short circuit current ” Isc1min ” is same for the 2 transformers, and also the earth fault current ” If ” is same for both, and as the Magnetic Protection ” Im ” should trip due to ” Short Circuit  &  Earth ” faults if thers's no Earth Fault Protection, so :

– For all C.B.  : The ” Im ” should be Smaller than the smallest value of ” Isc1min  or  If ” for 1 transformer ( as only 1 transformer is connected ).

But if there's also an Earth Faults Protection that should trip due to an ” Earth ” faults, the adjustments will be :

– For all C.B.  : The ” Im ” should be Smaller than ” Isc1min ” for 1 transformer ( as only 1 transformer is connected ).

– For all C.B.  : The ” Ig ” should be Smaller than ” If ” for 1 transformer ( as only 1  transformer is connected ).

Rem. : The values of ” Isc1min  &  If ” should be calculated for each C.B.

Regards.

#12345
rohit katore
Participant

thanks

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