Research On Control Technique For Grid-Connected Inverter Under Distorted Three-Phase Systems

Renewable energy sources, particularly solar energy, are seen as a reliable alternative to thetraditional energy sources such as fossil fuel which caused environmental problems. Photovoltaic (PV)grid generation, which is an effective and popular use of solar energy, has made great progresses inrecent years. And the controllability of grid-side inverters as a vital role in PV grid generation systemneeds to be more and further researched. Especially when the grid voltage appears unbalancing andharmonics, the control of the grid-side inverter is critical, as otherwise the inverter could suffer frompoor performation or even instability.A control strategy under voltage unbalancing and harmonics is discussed in the paper, which canbe divided into two aspects: the phase-locked loop (PLL) and the current loop. The former providesthe angle information and the latter regulates the grid current.First of all, the paper introduces a digital synchronization method based on variable samplingperiod (VSP_PLL). In order to reject disturbances that caused by the negative sequence voltage andthe harmonics, a filter based on the Sliding Goertzel Transform is used in the method. Therefore, thePLL is capable of locking to the phase and frequency of the positive sequence fundamental voltage.The mathematical model of three-phase inverter is derived subsequently and a positive and negativesequence current control strategy is discussed based on the model. Combing with PLL, the totalcontrol strategy is given in the paper. Simulations are done to verify the control strategy.At last, a5KW experimental prototype is designed and the detailed hardware and softwaredesign are given in the paper. The proposed control strategy under the distorted three-phase systemhas been verified by experiments based on this prototype.