Resonant Inverters for Photovoltaic Array to Utility Interface

A number of high-frequency (hf) link power converter schemes suitable for interconnecting photovoltaic (PV) array to the utility line using resonant inverters are presented. Use of hf resonant inverters result in reduced size, weight and cost. The schemes presented are (i) hf resonant inverter (hfri)-cycloconverter, (ii) hfri-rectifier-pulse width modulated voltage source inverter, (iii) hfri-rectifier-line commutated inverter working with a fixed maximum possible inverting firing angle, and (iv) hfri-rectifier-line connected inverter (with line current modulation). These schemes are compared and based on the comparison, scheme (iii) was selected for initial study and the working principle of this scheme is explained. Although both 1-Ø and 3-Ø systems are easy to implement with this scheme, they generate line frequency current harmonics. Also, line pf is less than unity. To overcome these problems, line current is modulated sinusoidally in scheme (iv). In this scheme, hfri is controlled appropriately to obtain a rectified sine wave of 120 Hz in the dc link and the function of line connected inverter is to unfold this waveform (α = 180°) on alternate half cycles to obtain a sinusoidal line current which is in phase with the line voltage. Methods of commutation of line connected inverter are explained. A 3-Ø system is realized using three identical 1-Ø systems. Experimental results obtained from a 1-Ø prototype system for scheme (iv) are presented.