Application of Power Transistors to Residential and Intermediate Rating Photovoltaic Array Power Conditioners

An investigation into power conditioners that interface with photovoltaic arrays and utilities has been recently completed. The ratings for this investigation include residential systems (5-30 kW) that interface with a 240-V single-phase utility connection and intermediate systems (30-200 kW) that interface with a 480-V three- phase utility connection. Both systems mandated that an isolation transformer be provided between the array and the utility interface. A trade-off study was performed for many transistor and thyristor circuits and configurations. The weighting criteria included full- and part-load efficiency, size, weight, reliability, ease of control, injected harmonics, reactive power requirements, and parts cost. As the result of this study, a 10-kW high-frequency PWM transistor inverter feeding a high-frequency isolation transformer with a sinusoidally shaped current wave was selected. The output of the transformer is rectified with a diode bridge rectifier. Four thyristors, used as 60-Hz switches, reverse the polarity of the rectified current on every other half-cycle of the utility voltage. This reversal is accomplished slightly before the natural zero crossing of the voltage, thereby providing commutation for the thyristors. The results in the intermediate rating study targeted on a 100-kW design using power transistors in a full-bridge circuit connected to a 60-Hz three-phase transformer. Three bridge circuits are provided to interface with the polyphase utility. The switching strategy for the power transistors is based on a sine wave shape of current with a controlled deadband.