Grid Connected Photovoltaic (PV) Inverter with Robust Phase-Locked Loop (PLL)

An array of solar panels is connected to the mains through a three-phase active voltage-source inverter and a step-up transformer. The inverter synchronizes to the grid by means of a robust phase-locked loop (PLL), using input´s quadrate method, and a multi-variable filter removes voltage harmonics caused by unbalance and distortion. The PWM active inverter utilizes a voltage oriented control (VOC) control structure, using a Park transform, to convert the measured ac currents to synchronous reference properties (d-q reference). Since both the PLL and the VOC are sharing some common properties (Park transform, reference angle), they are combined in one circuit. A field programmable gate array (FPGA) is used for the purpose. Cumbersome algorithms like sine and cosine calculations are replaced by look-up tables. This provides a fast and reliable tool, and no other control circuits are needed for inverter operation. Equipment for maximum power point tracking and monitoring and protecting schemes are beyond the scope of this article. Simulations and experimental results are presented