Determining the opposition a transformer presents to alternating current flow involves quantifying both resistance and reactance. This quantification typically results in a percentage or per-unit value referenced to the transformer’s voltage and kVA rating. For example, a transformer with a stated value of 5% indicates that 5% of the rated voltage is required to circulate full load current when the secondary terminals are short-circuited.
Accurate determination of this value is essential for several reasons. It plays a crucial role in determining the fault current levels that the transformer can withstand and that protective devices must interrupt. Furthermore, this value influences voltage regulation under varying load conditions and impacts the parallel operation of transformers, ensuring proper load sharing and preventing circulating currents. Historically, this value was determined through short-circuit testing, a method still widely employed for its accuracy.
