Abstract: A portable apparatus for voltage transformation is capable of temporarily augmenting a power transformer. A transformer with a first medium voltage primary winding, a second medium voltage primary winding and a low voltage secondary winding is selectably coupled to a plurality of medium voltage electrical power input couplings capable of temporarily coupling with medium voltage power on a plant. An interlocked switch selectably couples the medium voltage electrical power input couplings to the medium voltage primary windings of the transformer such that only one medium voltage primary winding of the transformer is coupled at a time to the medium voltage electrical power input couplings. The interlocked switch can use a captive key system to prevent more than one secondary winding from being simultaneously connected. Medium voltage circuit protection devices such as fuses are included.