The objective of this paper is to propose and to validate the behavior of an impulse current transformer with a nanocrystalline alloy toroidal core. The purpose of this transformer is to measure a current square waveform with a fundamental harmonic frequency of 25 kHz. Current ratio 1:1 and the real burden are assumed, and the transformer will operate in an electric power system at voltage levels up to 25 kV. These special requirements are typical for applications such as photovoltaic power plants, where the currents in high voltage inverters need to be measured. These inverters usually use the principle of pulse width modulation (PWM), and are able to convert from a low voltage direct current to high voltage alternating current. In this way, the electric power can be supplied directly to the high voltage distribution grid without the use of a power transformer.