Frequency / duty cycle control of LCC resonant converter supplying high voltage very low frequency test systems

Very low frequency (VLF) high-voltage (HV) sinusoidal test waveforms are suitable for testing characteristics and insulation qualities of long buried power cables. Unlike as with conventional PWM converters, a newly developed LCC resonant inverter combined with a symmetrical three-stage voltage multiplier rectifier is introduced in this paper to generate VLF HV sinusoidal waves. Due to the wide ranges of output voltage and output load, both the switching frequency and the duty cycle are adopted as control variables. Based on a small-signal equivalent circuit model and a reduced-order transfer function, an average mode current controller is developed for the zero voltage switched LCC resonant converter generating up to 85 kV (RMS) at 0.1 Hz as test voltage. The theoretical analysis and design are verified by simulations and experimental measurements with a prototype setup.