Small-signal model and control design of LCC resonant converter with a capacitive load applied in very low frequency high voltage test system

Very low frequency (VLF) high voltage (HV) sinusoidal waveforms are suitable for testing characteristics and insulation qualities of long buried cables. Such a test generator should provide a sinusoidal voltage from some tens to hundreds of kV. A LCC resonant inverter with a three-stage symmetrical voltage multiplier rectifier is developed for this application. In this context, the contribution deals with the derivation of a smallsignal model of the LCC resonant converter with a capacitive load as a basis for the subsequent controller design. For that purpose, the concepts of generalized averaging, extended describing functions and order reduction are adopted in this paper. As a particular feature of the designed current controller, both the converter switching frequency and the duty cycle are utilized as actuating variables in order to cope with large ranges of output voltage and load. The small-signal model and the controller design are verified by measurements on prototypes.