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