A Harmonic Control Strategy for Three-phase Grid-connected Inverter with LCL Filter Under Distorted Grid Conditions

In order to improve the efficiency of three-phase LCL grid-connected inverter, a flexible harmonic current control strategy is proposed in this paper by introducing the functionality of harmonic compensation into the inverter. The proposed control strategy consists of two loops: the outer grid current loop and the inner capacitive current loop. Moreover, the outer current loop consists of two parallel control branches: the fundamental current control branch and the harmonic current control branch, adopting different control methods in sychronous rotating frame to achieve decoupling control for fundamental and harmonic component of the inverter's output current, respectively. In addition, the harmonic current control branch can operate in two modes: the harmonic suppression mode and the harmonic compensation mode. The grid-connected inverter can flexibly switch the modes from one to another according to its own capacity and control objectives. Therefore, the proposed strategy can realize effective compensation for local harmonic current and weaken the negative impact of background harmonic voltage on output current without harmonic detection. This paper deduces the stationary frame equivalent universal expression of the controller in synchronous rotating frame and analyzes the control performance of the proposed strategy by using frequency-domain method. The simulation and experimental results verify the feasibility and effectiveness of the strategy.