Impedance voltage, also known as short-circuit voltage, is an important parameter in a transformer. It is determined as follows:The secondary winding of the transformer is short-circuited, and the voltage of the primary winding is slowly increased, and when the secondary current reaches the rated current, the ratio percentage of the voltage applied by the primary winding (short-circuit voltage) to the rated voltage。Typically, the impedance voltage of a transformer is between 5% and 10%.
The impedance factors affecting the transformer mainly depend on the capacity of the transformer, copper loss, iron loss, coil material and structure, etc., which reflect the quality of the above parameters.
The transformer with the same capacity has low impedance voltage, low cost, high efficiency, low efficiency, low voltage, small voltage drop and voltage change rate during operation, and the voltage quality is easy to control and guarantee. In order to ensure the current limiting conditions of the transformer, it is hoped that the impedance voltage is larger, so as to avoid damage to the electrical equipment (such as circuit breakers, disconnectors, cables, etc.) that cannot withstand the effect of short-circuit current during operation.
In general, the higher the voltage level, the greater the required impedance voltage. When multiple transformers are connected in parallel, their impedance voltages need to be the same to avoid overload of transformers with smaller impedance in transformer groups.
In practice, the impedance voltage can be used to calculate some important parameters.
1.The impedance voltage is used to calculate the maximum short-circuit current that a transformer can withstand
For example, the capacity of the transformer is 1250kva, the rated voltage of the low-voltage side is 400V, the impedance voltage is 6%, and the short-circuit current is
i (n)= s (n)/ √3 u (n)= 1250 / 1.732 x 0.4 = 1840 a
i (k) = i (n)/u (k) = 1.804 / 0.06 = 30.1ka
2.The reactance of the transformer is calculated based on the impedance voltage
x (t) = u (k)% / 100/s (nt) *u²(*
where u (k)% is the transformer short-circuit voltage, u (* is the calculated voltage at the short-circuit calculation point, and s (nt) is the transformer rated capacity.
It can be seen that the impedance voltage of the transformer is a more intuitive indicator to understand the performance of the transformer. The selection of the appropriate impedance voltage can give full play to the maximum efficiency of the transformer grid system, which is often overlooked.
Picture: Lei Lang Electric.
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