Research On Circulation Current Reduction Of Three-level Inverters Parallel System

Three-level photovoltaic grid-connected inverters are widely used in the field of high-voltage and high-power, the three-level parallel system obtained by parallel technology can make the system capacity doubled, and has advantages of high power density, high reliability and redundancy, which has broad application prospects. But there is problem of zero-sequence circulating current (ZSCC) between the parallel three-level inverters, ZSCC lead to unbalanced current stress of inverter, increase the system loss and reduce the system efficiency, so the research on ZSCC reduction of the three-level photovoltaic grid-connected inverters parallel system is of great significance.In this paper, the research background and the diode Neutral-Point-Clamped three-level inverter topology and its basic work principles were introduced firstly, then the fundamental principles of the widely used space vector pulse width modulation (SVPWM) were expounded, and the intrinsic link between the vector modulation and carrier modulation was analyzed, on the basis of the above analysis, the vector thought and carrier modulation implementation method of three typical three-level modulation strategies were studied are analyzed in detail.According to the problem that ZSCC existed in parallel system of three-level photovoltaic grid-connected inverters, the equivalent model of ZSCC was proposed based on the loop analysis in this paper, ZSCC was divided into three types according to different exciting sources:conduction ZSCC, switching ZSCC and hybrid ZSCC. A parallel scheme sharing neutral buses was proposed to reduce the conduction ZSCC, and a parallel scheme based on improved LCL filter was proposed to reduce high frequency component of ZSCC.The ZSCC controller is used to reduce the low frequency component of ZSCC. The suppression results of conventional proportional (P) regulator, proportional integral (PI) regulator and proportional resonant (PR) regulator are analyzed and compared. The regulator saturation characteristic caused by sample deviation and amplitude limit is analyzed. On this basis, a ZSCC controller based on the proportion quasi resonant controller is proposed. The relationship between the ZSCC suppression and neutral point balance control is also analyzed, and the coordination control method is proposed.Finally, a 10KW three-level converter experimental platform is builted, which use the TMS320F28335 DSP as the control core and CPLD as an auxiliary control. The experiment results verify the correctness of theoretical analysis and the validity of ZSCC suppression ability of the proposed quasi-PR regulator.