An apparatus and a method for reactive power compensation of a test transformer with an inductor and a stepped capacitor bank and a generator. The generator is used for generating a generator current and a generator voltage with a predeterminable test frequency. The inductance of the inductor can be set and can be interconnected with a stepped capacitor bank in such a way that the generator can be operated on virtually exclusive active power. Within the test circuit, the generator is therefore almost always set at an optimum operating point for testing of the test transformer and does not need to be dimensioned to be any larger than required. As a result, the power of the generator can be used in optimum fashion for generating the generator voltage and, as a result of the reduction in the component part sizes which is carried out on the essential components in the test circuit, enables the mobile use of the entire apparatus for in-situ testing of the power transformer to be tested.