High precision AC/AC power supply unit based on DSP controlled PWM inverter

The paper introduces a precision switching mode AC/AC power supply unit, that has been developed for calibration of current transformers. The device is fed by the 220 V 50/60 Hz mains, the output power is 3 kVA, and the output frequency is 50 or 60 Hz, which can be selected from the program. Large current control ratio, low level harmonic content of the output current, high accuracy, high resolution, high precision are the most important demands. Data transfer interface between the device and a supervisory computer is also necessary. Due to the large control ratio a two-stage controller is used. The controllable rectifier utilises changeable DC voltage level for the single-phase controlled PWM inverter. The PWM inverter carries out the DC/AC transformation and the fine adjustment of the output voltage. The harmonics contents of the inverter output current are reduced by an LC lowpass filter. The output voltage signal is perfect sinusoidal, because the PWM waveform is generated by a high performance, high accuracy 32-bit floating-point DSP.