Certain considerations were determined while designing the impedance value of the power transformer. Impedance for the transformer is due to the inductance and resistance present in the transformer.
During short circuit in the power systems short circuit current in the power system is limited by the presence of impedance drop due to transformers, transmission lines and cables. These impedance drops in the power system reduces the short circuit level. This the advantage of having high impedance values for the transformer.
However large transformer impedance cause large drop in the power system during the normal operation. This will cause a significant impact on the starting of the large motors (when large motors are started during low voltage condition other loads connected to the bus will trip due to huge drop in voltage at the bus) and voltage regulation of the power system
Hence transformer impedance is designed for a optimum value considering the above conditions.
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