Analysis And Control Of Zero-sequence Circulating Current In Parallel Three-Level NPC Inverters

Parallel technology is one of important aspects in increasing the capacity of inverters. It has advantages in increasing the capacity of inverters, easy expandability,high efficiency, and high reliability. For its great application prospect, it has got intensive studies. Compared with the conventional two-level inverter, three-level inverter has many attractive features in the area of medium and high-voltage high-power area, such as lower withstand voltage level on power semiconductors, lower distortion of output voltage and lower switching losses, therefore, the parallel of three-level inverters has great significance.In this paper, the background, the common three-level inverter topologies and the modulation methods focusing on the widely used space vector pulse width modulation (SVPWM) were introduced firstly. This paper adopted a parallel system where two three-level NPC inverters connected both ac and dc sides directly, an averaged model of the circulating current was built, then the influence factors of circulating currents was analyzed in detail.The SVPWM modulation, which is a carrier-based PWM (CB-PWM)strategy with zero-sequence voltage injection, doesn’t require complex zoning and computing, easy to achieve on engineering, and it is associated with the zero-sequence circulating current, thus,studied the method of realizing seven-segment SVPWM, known as the nearest three vector space vector modulation(NTV-SVM), using the CB-PWM.13vector space vector modulation(13-SVM), a modulation method, which was said having automatically neutral-point balance capacity, was introduced. On the basis of NTV-SVM and13-SVM, a medium vectors being synthesized space vector modulation (MVS-SVM) strategy and its CB-PWM was introduced. The influences of different modulation strategies on zero-sequence circulating current were compared and analysis, the introduction of zero-sequence circulation current control (ZSCC) loop is effective.Neutral-point voltage balancing problem is an inherent problem in the three-level NPC inverter, the law of control was analyzed from the vector and the current which is related with mid-point. A balance factor control method, which was applied in the NTV-SVM, was presented. The paper put forward to a unified midpoint balance control algorithm, not limited to the NTV-SVM, and adapted to the situation with zero-sequence circulating current, achieved by injecting zero-sequence voltage on the CB-PWM. Finally, a zero-sequence circulation current control taking into account the mid-point balance was proposed.Based on the theoretical analysis, simulations were done by Matlab/Simulink about SPWM and several SVPWM algorithms, the unified midpoint balance control algorithm on one inverter, and the zero-sequence circulation control with the mid-point balance. DSP+CPLD experimental platform had been built, the algorithms and balancing methods were applied in the experiments for verification and comparison. The simulation and experiment results verified the feasibility and effectiveness of the algorithms.