A New Current Feedback PR Control Strategy for Grid-Connected VSI with an LCL Filter

Proportional Resonant (PR) controller with harmonic compensators has extremely higher gain at the desired frequencies to reduce the harmonic distortions in the alternative voltage or current waves. However, the harmonic compensators of the PR controller are limited to several low-order current harmonics, due to the system instability when the compensated frequency is out of the bandwidth of the system control loop, especially for the grid-connected converters with the use of an LCL-filter and conventional current feedback methods. In this paper, a new current feedback method and PR control strategy for grid-connected voltage source inverters (VSI) with an LCL-filter is proposed. The weighted average value of the currents flowing through the two inductors of the LCL-filter is used as the feedback to the current PR regulator. Consequently, the control system with the LCL-filter is degraded from a third-order function to a first-order one. In this way, a large proportional control loop gain can be chose to obtain a wide control loop bandwidth that is required by the harmonics compensations of the PR controller, the system can be optimized easily for minimum current harmonic distortions as well as the system stability. The characteristics of the inverter system with the proposed controller are investigated and compared with those using traditional control methods. Experimental results on a 5 kW fuel cell inverter are provided, and the new current control strategy has been verified.