Zero-Sequence Circulating Current Suppression Strategy of Neutral-Point-Clamped Three-Level Inverter Parallel System
Xu Chang1, Gong Jinwu1, Zhang Guoqin2, Dai Kemin1, Zha Xiaoming1
1. School of Electrical Engineering and Automation Wuhan University Wuhan 430072 China; 2. School of Electronics and Electrical Engineering Wuhan Textile University Wuhan 430200 China
Abstract：Parallel operation of inverters is the most common and direct means to improve power capacity and power level. However, in parallel inverter system, zero-sequence circulating current (ZSCC) occurs because of the common AC and DC bus. This will lead to distortion of output current and additional device loss, which seriously threatens the stability of the parallel system. The ZSCC of neutral-point-clamped (NPC) three-level inverter parallel system studied in this paper is more complex than that of the two-level parallel inverters, and it is necessary to take both hardware and software measures to suppress it. Most of the existing studies focused on software control strategies to suppress ZSCC. For the use of hardware measures, following the traditional scheme was a common choice. Few studies have been done to improve the traditional hardware measures. This paper optimizes the connecting pattern of the improved LCL, which is a traditional hardware measure commonly used to suppress ZSCC, and forms a module shared capacitance scheme under carrier phase-shifting control. Compared with the traditional improved LCL, the added connection cable has smaller current ripple. Firstly, the loop of circulating circuit is analyzed according to the switch state of the bridge-leg, and the mathematical and circuit model of ZSCC are established. The circulating current is also quantified by Fourier decomposition. Then, a switch function is introduced to further refine the category of ZSCC. For different types of ZSCC, the generation mechanism is explained, and hardware measures of sharing DC side midline and improved LCL and software measures of quasi-proportional-integral-resonance controller are proposed to suppress the corresponding ZSCC. Simulations and experiments show the validity of the proposed measures. At the same time, experiments show that the proposed methods are also feasible when hardware parameters of each inverter are inconsistent and the inverter modules are put in or removed. In addition, it should be noted that there is a large high frequency current on the connection cable between the AC filter capacitor midpoint and the DC side capacitor midpoint in the traditional improved LCL. Its overall amplitude is around 100A during the simulation verification, which contains both odd switching frequency ripple with the amplitude of 80A and even switching frequency ripple with the amplitude of 20A. Therefore, an optimization scheme of module common filter capacitance under carrier phase-shifting control is proposed, which transfers the high frequency ripple at odd switching frequency to the common capacitance with no extra circuit. Since the current ripple at odd switching frequency is opposite under carrier phase-shifting control, the auto-cancellation of current ripple component at odd switching frequency is achieved and current ripple component at even switching frequency is doubled, but in general the total current on the connection cable is reduced by nearly 50% to about 50A. The following conclusions can be drawn from the analysis: (1) For different types of ZSCC, the combined use of hardware and software measures mentioned in this paper can effectively and reliably suppress the ZSCC in the parallel system of NPC three-level inverters. (2) The module shared filter capacitance scheme under carrier phase-shifting control has smaller high frequency ripple on the connection cable and better electromagnetic performance of the device than traditional improved LCL hardware measures while the original effect of ZSCC suppression is maintained. (3) The module shared filter capacitance scheme under carrier phase-shifting control saves the consumption of capacitive components, reduces the current stress of components, which provides the possibility for the high power density design of the prototype.
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2023, Vol. 38 
