ABB UAA326A04 After sales service is provided for the control card of the power supply device

Description

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The transformer protection devices must also guarantee that the transformer
cannot operate above the point of maximum thermal overload under shortcircuit conditions; this point is defined on the time-current diagram by the value
of short-circuit current which can pass through the transformer and by a time
equal to 2 s, as stated by Standard IEC 60076-5. The short-circuit current (Ik
)
flowing for a fault with low impedance at the LV terminals of the transformer is
calculated by using the following formula:
where:
• Ur
is the rated voltage of the transformer [V];
• ZNet is the short-circuit impedance of the network [Ω];
• Zt
is the short-circuit impedance of the transformer; from the rated power of
the transformer (Sr
[VA]) and the percentage short-circuit voltage (uk
%) it is
equal to:
Considering the upstream short-circuit power of the network to be infinite
(ZNet=0), formula (1) becomes:
The diagram below shows the inrush current curve for a 20/0.4 kV of 400 kVA
transformer (uk
% = 4 %) and the point referred to the thermal ability to withstand
the short-circuit current (Ik; 2 sec.).

In summary: for the correct protection of the transformer and to avoid unwanted
trips, the trip curve of the protection device must be above the inrush current
curve and below the thermal overload point under short circuit conditions.
Choice of the circuit-breakers on the primary of a LV/LV transformer
These types of transformer are mainly used to supply control and switch auxiliary
circuits, since they are often supplied at lower voltages in comparison with
those for power distribution; another application example can be represented
by the need of changing the neutral system according to the installation
requirements.
As regards choice and settings of the circuit-breaker on the supply side of
the primary, it is necessary to take into consideration both the “inrush current”
phenomenon as well as the transformer maximum short-time thermal load
capability, as described in the previous pages.
The following figure shows the possible positioning of the trip curve of a circuitbreaker on the primary of a 250kVA transformer at 690/400 with uk
=4%.