In this article, an optimized model predictive control (MPC) method with fixed switching frequency is proposed to eliminate the common-mode voltage (CMV) and balance the capacitor voltage. First, the zero CMV vector diagram is divided by finite control set MPC (FCS-MPC) to obtain the candidate vectors effectively. Then, in order to control the capacitor voltage and avoid the great computational burden originated from the redundant switching combinations, a piecewise control scheme is proposed to determine the required ones directly. The sequence generated by the selected vectors is further optimized for the purpose of saving switching transitions according to the internal and external parts of the vector diagram. Finally, the duty ratios are evaluated based on the results of the FCS-MPC in dividing sectors so as to avoid any extra complex calculation. The experimental evaluation has been carried out to validate the feasibility and effectiveness of the proposed method.

This article addresses the common-mode (CM) resonance problem in parallel inverters with separate dc inputs and improvedLCLfilters. Unlike single inverter system, parallel system presents a more challenging picture. Especially when the grid impedance and parasitic capacitance are considered, the parallel inverter interactions introduce more complex coupling relationships. In this situation, intricate CM resonances are excited at various frequencies, which is different from the resonance characteristics of a single inverter. Therefore, the control methods based on the single inverter system is insufficient in the parallel system and cannot behave as expected. To solve this issue, the CM resonance suppression problem in parallel system with separate inputs and parasitic parameters is being considered and analyzed for the first time. Firstly, the Thevenin's CM equivalent model of the parallel system is established. Compared with a single inverter model, the increase of CM loops triggers another two CM resonance peaks, which further deteriorates the system stability. Then, the internal and external CM excitations between each inverter are analyzed. The results show that the CM current of each inverter is coupled with the CM excitation sources of all inverters and the resonance characteristics are affected by the number of inverters. Furthermore, the influence of grid impedance and parasitic capacitance on the CM resonance characteristics of the system is analyzed. Accordingly, a CM resonance controller and a control parameter design method are proposed. By damping the CM excitation sources, the CM resonance suppression and CM current attenuation can be achieved simultaneously.

In parallel T-type three-level rectifiers system (PT23LRs), when the open-circuit fault (OCF) of neutral-point (NP) switches occurs, the existing zero-sequence circulating current (ZSCC) suppression methods are not capable of suppressing the ZSCC any more since the ZSCC model of PT23LRs is changed. Significantly, the ZSCC suppression issue of PT23LRs under the OCF of NP switches has not been researched in previous literatures. To overcome this limitation, a novel ZSCC suppression method for PT23LRs with the OCF of NP switches is proposed. First of all, according to the principle of area equivalence, the fault-tolerant control (FTC) is accomplished by reconstructing the switching sequence. Second, a novel ZSCC model with FTC is established. Then, according to the novel ZSCC model, the ZSCC control method based on “PI + feedforward” is proposed. This method is realized by modifying the turn-ontime of three-phase of fault rectifier in PT23LRs. Consequently, the method proposed in this article effectively restrains the ZSCC of PT23LRs with the OCF of NP switches. Finally, the simulation and experiment results verified the correctness of proposed method.

In transformerless three-level photovoltaic inverter systems, the modifiedLCfilter, which directly connects the dc-side neutral point to the common point of filter capacitors, is considered to be an ideal scheme to suppress the troublesome leakage current. However, a new common-mode (CM) resonant current is emerged, which leads to inverter-side current distortion and even system instability. Therefore, a second-order passivity-based controller in the time domain featuring excellent system stability is proposed for the CM resonant current mitigation. First, the reason for the CM resonant current generation with the modifiedLCfilter is analyzed in detail. Then, the CM current model is derived, which reveals that it is a second-order system. The passivity of this second-order model is strictly proved. Moreover, based on the passive control theory, zero state detectable theory, and Lyapunov stability theorem, a second-order passivity-based controller is designed step by step to suppress the CM resonant current, and the system global asymptotic stability is also proved. The proposed controller is based on the time domain, which is more intuitive and avoids the error amplification in the conversion from frequency domain to time domain. Finally, the effectiveness and the correctness of the theoretical derivation of the passivity-based control method are validated by experiments. Compared with the passive damping resistor method, the proposed method not only has better performance but also avoids the use of real resistors and reduces the cost by using software instead of hardware.

In high-power applications, selective harmonic elimination pulsewidth modulation (SHEPWM) method is widely used to eliminate low-order harmonics in three-level inverters with low power losses. However, it faces the challenges that the conventional optimization methods for solving harmonic elimination equations in SHEPWM are easy to be trapped in local optimum. Moreover, the neutral point (NP) voltage of three-level inverter cannot be balanced under low-voltage ride-through (LVRT) condition with conventional SHEPWM method. To address these problems, a SHEPWM scheme is proposed, which is on the basis of an improved particle swarm optimization (IPSO) algorithm to solve the harmonic elimination equations and an enhanced NP balance control method in LVRT operation. First, an IPSO algorithm is proposed to calculate the switching angles of the transcendental equations for harmonic elimination. The IPSO algorithm defines a nonlinear negative exponential inertia weight considering the current and optimal fitness value, which can dynamically adjust the local and global searching speed and step size according to the actual situation, greatly improving the convergence speed and solution accuracy and avoiding falling into the local optimum. Then, by increasing the degree of freedom of the output current direction and combining with the NP voltage deviation, an enhanced NP voltage balance control method of SHEPWM in LVRT operation is proposed. The validity of the proposed method is proved by simulation and experimental results.

Modified LCL-type filter is adopted in parallel three-level inverters for high-frequency zero circulating current mitigation. However, it will generate internal resonance circulating current (IRCC) and external resonance circulating current (ERCC), which causes the output current unstable unavoidably. To address this issue, in this article, the model of IRCC and ERCC is established and analyzed. It reveals that the IRCC and ERCC are highly related to inverter-side zero sequence circulating current (ISZSCC) and common-mode voltage. Therefore, IRCC and ERCC can be suppressed by controlling the ISZSCC. To suppress the resonance currents, the ISZSCC model is first set up. Furthermore, a controller of ISZSCC is proposed and the parameters designing process of closed-loop controller are elaborated to stabilize the system based on the established model. Moreover, the resonance current suppression is achieved by adjusting the duty times of common-mode small vectors in space vector modulation. Thus, the current waveform quality and system stability are greatly improved after applying the proposed method. Finally, simulation and experiments results prove the effectiveness of theoretical analyses and proposed control strategy.

The three-level inverter has been widely researched and adopted in industry. This article further proposes a carrier-based modulation method to reduce the common-mode voltage (CMV) and obtain high-quality output currents for three-level inverter with unbalanced neutral-point voltage conditions. In this method, the zero-sequence voltage is calculated by three-phase modulation waves, three-phase output currents, and voltages across the dc-link capacitors. The limitations of the zero-sequence component are carefully considered in order not to generate the basic voltage vector with high CMV magnitudes. In addition, a deadbeat control scheme for controlling the dc-link voltages asymmetrically with the proposed pulsewidth modulation method is presented, and asymmetrical control of dc-link voltages can be realized with fast response. Compared with the conventional space vector modulation (SVM) and the virtual SVM methods, the proposed scheme can reduce the CMV by half. Simulated and experimental results validate the effectiveness of the proposed method.

The Z-source neutral point clamped (NPC) inverters can realize buck-boost energy conversion with all satisfying advantages of the three-level inverters. A significant problem related to this topology is the fluctuation of the neutral-point potential (NP). However, the previous modulation techniques for this topology cannot achieve the considerate stable voltage. In this paper, based on the analysis of the relationship among the shoot-through duty cycle and the NP voltage, the average NP current, switching frequency capacitor value, a new feed-forward compensator by modifying the shoot-through duty cycle which can improve to balance the NP voltage balancing is proposed. Meanwhile, in consideration of a low voltage gain caused by the NP voltage balance method, a new control method based on a maximum constant boost control method is proposed, which brings the benefits in both high voltage gain and low-frequency ripple. The considerate stable NP voltage and the high voltage gain have been proven in simulation and through experimental results from a prototype converter.