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

Home Electrical Engineering Forum General Discussion help me explain the conditions of 2 transformer to operate parallel? Re: help me explain the conditions of 2 transformer to operate parallel?

#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.