Application of Busbar Trunking System between Transformer Secondary End to LT Switch gear and Its Short Circuit Capacity
Application of BBT Between Transformer to LT Switchgear
Nowadays busbar trunking system (BBT) are commonly used to connect between the transformer and LT panel rather than cable. In that case, it is very important to know what will be the short circuit capacity of this BBT because in the time of a fault, a huge amount of current flowing into the circuit. So to withstand this current limit, we have to know its short circuit current value which is known as the breaking capacity of this BBT.
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| BBT FROM XMER TO PANEL/remondas.blogspot.com |
Breaking Capacity
The Breaking Capacity of the BBT will be equal to the short circuit current at the secondary of MV/LV Transformer. Now we learn how to calculate the short circuit current at the secondary of MV/LV Transformer.
In Time of One Transformer:
In a simplified approach, the impedance of the MV system is assumed to be negligibly small, so that:
Where,
S = kVA rating of the transformer
U20 = phase-to-phase secondary no-load voltage
In = rated current of the transformer
Isc = short-circuit current in amps
Usc = short-circuit impedance voltage of the transformer in %.
U20 = phase-to-phase secondary no-load voltage
In = rated current of the transformer
Isc = short-circuit current in amps
Usc = short-circuit impedance voltage of the transformer in %.
Typical values of Usc for distribution transformers are given in below table:
Transformer rating (kVA)
|
Usc in %
|
|
Oil-immersed
|
Cast-resin dry type
|
|
50 to 750
|
4
|
6
|
800 to 3,200
|
6
|
6
|
Example:
400 kVA transformer, 420 V at no load
Usc = 4%
That means, the breaking capacity of the BBT from Transformer secondary to LT panel should be more than 14kA. In practice, here 25kA or 30kA BBT will use.
In the time of more than One Transformer:
ISC= ISC1 +ISC2 +ISC3
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