What are impedance voltage and load loss tests?

"The voltage required to circulate the rated current under short-circuit conditions when connected on the rated voltage tap is the impedance voltage. Rated current is circulated through the windings with the secondary short-circuited. The impedance voltage and load loss is measured. They are corrected to rise +20xC reference temperature. (Note: This is a standard test only on units over 500kVA. It will only be carried out on lower kVA units when specifically requested.) "