High-performance and cost-effective multiple feedback control strategy for standalone operation of grid-connected inverter

This paper presents a multi-loop control strategy with capacitor voltage differential and low pass filter feedback as inner loop, with P controller and voltage reference feedforward as outer loop for voltage-source grid-connected inverters standalone operation. A multi-loop controller with capacitor current feedback can effectively eliminate the inherent high-resonant peak of the output LC-filter to increase system stability margin. It also has better disturbance rejection capability compared to inductor current feedback. Capacitor voltage differential feedback has the same function as capacitor current feedback while also has its own merits that it can save a highly accurate and fast responded current sensor. However, the differential function will enlarge some small disturbance in the output voltage, especially in the high frequency range so as to cause stability problem. Hence a low pass filter (LPF) is introduced to use with differential feedback to reject the high frequency disturbance. In addition, in order to improve the dynamic response, eliminate the tracking phase error caused by PI controller and also maintain a continuous output voltage waveform in the transition from grid-connected to standalone, a voltage reference feedforward has been added to the outer loop. Simulation and experimental results are given in this paper to verify that the system, with proposed control strategy, possesses very high stability, low THD and fast dynamic response when supplying linear and nonlinear loads.