Energy control for long lifetime photovoltaic ac module inverter

Photovoltaic power generation is spreading all around the globe with increasing support from the public and governments. Among the obstacles yet to overcome are higher cell efficiency, lower cost, and longer lifetime. The latter is the focus of the research presented here as it provides an alternative photovoltaic ac module inverter with long lifetime. Four commercial inverters analyzed in our laboratory contained electrolytic capacitors whose lifetimes are limited to 12000 hours. In the proposed solution these components are obviated by using an energy control scheme that stores all the energy fluctuation in the dc-link polypropylene capacitor. The energy control strategy has been applied in a prototype power conditioning unit that comprises a phase-shift full-bridge dc-ac-dc converter connected in series with a current source inverter. The grid current magnitude is controlled using a proportional derivative compensator that maintains the average value of the dc-link capacitor at a voltage level of 475V, while allowing oscillations with a peak value of 125V. Simultaneously, a pulse-width-modulation integrated circuit controller maintains a steady dc voltage at the photovoltaic panel's terminals, hence enhancing the performance of the maximum power point tracking algorithm. The sinusoidal shape of the current injection is attained using current-mode-control. It is shown that the converter operates at high efficiency and that the energy fluctuation is absorbed at the dc-link capacitor. In addition, it is shown that the perturb and observe maximum power point tracking algorithm performs well over a day of variable irradiance conditions.