Frequency-adaptive current controller for grid-connected renewable energy systems

An adaptive proportional-resonant (APR) controller for current controller of grid-connected DC/AC systems for renewable energy applications is presented. The increase in renewable energy on the electricity grid poses stability risks to the network as the frequency and amplitude are not as tightly regulated as in previous years. The proposed APR controller is able to maintain high quality current at frequencies well beyond the IEEE1547 standard. The controller is constructed in two parts, a phase-locked loop (PLL) for synchronization and identification of the grid frequency, and a current controller based on the form of the well-known PR controller. The current controller is able to dynamically adjust to the shifting frequency of grid which ensures high quality, unity power-factor across the 59.3-60.5Hz spectrum set by IEEE1547, and beyond. The PLL implemented is an amplitude-decoupled adaptive notch filter (AANF) which provides a very fast, an accurate solution for frequency estimation as well as reference signal generation. The AANF and the APR controller are implemented digitally on an FPGA platform and verified experimentally on a 1kW DC/AC prototype.