This article outlines the advantages of the resonant circuit of an autonomous inverter when using its high-voltage sinusoidal output for testing interturn insulation of coils of high-voltage power transformers and reactors. The article demonstrates the possibility to test the coils before their mounting on the core by means of applying high-frequency voltage. The results of the computer modeling of transient processes in inverter circuit with the tested coil as a load prove the considerable influence of the turn insulation fault of the part of the winding on the nature of such processes, including output voltage. The article justifies the advantage of inverters implemented on the basis of fully controllable electronic switches, i.e. MOSFET or IGBT power transistors.