(1. School of Electrical Engineering and Automation, Hefei University of Technology, Hefei 230009, China; 2. Sungrow Power Supply Co. Ltd., Hefei 230088, China; 3. China Electric Power Research Institute(Nanjing), Nanjing 210003, China)

Three-level LC-filter type string inverters are widely used in photovoltaic power systems. whose non-isolated characteristic may cause differential-mode harmonic circulating current. To analyse the influencing factors and specific forms of the harmonic circulating current, a differential-mode harmonic circulating current model is built in ideal situations. The relations between harmonic circulating current and power device switching transient as well as relations between peak-peak value of harmonic circulating current and carrier interleaving angle and grid vector angle are obtained. Afterward, the effects of stray parameters on high frequency harmonic circulating current in non-ideal situations are analyzed. On this basis, the effects of three typical harmonic circulating current reduction methods on harmonic circulating current in the above model are analyzed, and the pros and cons as well as the scope of application of those methods are worked out. Finally, the analysis of harmonic circulating current and the reduction methods are verified through simulations and experiments. This work is supported by National Natural Science Foundation of China(No. 51277051)and State Grid Corporation of China.