Research On Circulating Current Suppression Of Modular Photovoltaic Grid-connected Inverter

The current trend in the development of new energy technologies is photovoltaic power generation. The power level of the converter restrict the development of the photovoltaic power. Parallel converter technology is good at increasing the system power levels, in the meanwhile it can contribute to the harmonic suppression. Parallel converter technology is also able to meet the requirements of photovoltaic power generation converter device for modular design. But circulating current will be harmful to efficiency and stability of system as for using parallel converter.PWM modulation technology is the basis for inverter control technology. This paper analyzes the sinusoidal pulse width modulation and space vector modulation strategy and unified the relationship between the two modulation strategy. On this basis, this paper analyzes the different output voltage harmonic spectrum by mathematical analysis and it is the foundation for the analysis of circulating current harmonic spectrum.Net current control strategy under different coordinate systems would be impact on the design for circulation control. The circulating current is divided into differential mode loop and zero sequence current and is builded in the equivalent model under different coordinate systems for analyzing the reason of generating the circulating and the form of circulating current. The result of analyzing in the stationary coordinate system and the synchronous coordinate system is the filter inductor asymmetry parameter will cause zero sequence current and differential mode coupling loop and zero sequence current will affect each other. This conclusion is a theoretical foundation for designing the circulating control strategy.Carrier Phase Shifting technology using in modular parallel system can improve the quality of the current in net, but it will creat circulating current. System output currents will be eliminated by the analyzing of mathematical analysis and simulation.By analyzing the the effects of different phase shift angle for common-mode voltage, we can note that the carrier phase-shifting technology to improve the quality of system output current harmonics is based on introducing the circulating current.To solve the problem reduction of stabilize caused by the circulating current, firstly a new scheme of LCL filter based on sharing neutral point of capacitances is introduced to suppress high frequency common-mode circulating current, because it improve the order of the zero-sequence circulating current transfer function. Then, an appropriate harmonic compensator is designed to reduce the low frequency components of the zero- sequence circulating current. A composite strategy of suppressing circulating current is proposed working with the new sheme of LCL and the harmonic compensator. Finally, a simulation is constructed. Results of simulation show that the result of theoretical analysis on the circulating current and the proposed control strategy are correct.