Jun Mei

Modular Multilevel Inverter with New Modulation Method and Its Application to Photovoltaic Grid-Connected Generator

This paper proposed an improved phase disposition pulse width modulation (PDPWM) for a modular multilevel inverter which is used for Photovoltaic grid connection. This new modulation method is based on selective virtual loop mapping, to achieve dynamic capacitor voltage balance without the help of an extra compensation signal. The concept of virtual submodule (VSM) is first established, and by changing the loop mapping relationships between the VSMs and the real submodules, the voltages of the upper/lower arms capacitors can be well balanced. This method does not requiring sorting voltages from highest to lowest, and just identifies the MIN and MAX capacitor voltages index which makes it suitable for a modular multilevel converter with a large number of submodules in one arm. Compared to carrier phase-shifted PWM (CPSPWM), this method is more easily to be realized in field-programmable gate array and has much stronger dynamic regulation ability, and is conducive to the control of circulating current. Its feasibility and validity have been verified by simulations and experiments.

Modular Cascaded H-Bridge Multilevel PV Inverter With Distributed MPPT for Grid-Connected Applications

This paper presents a modular cascaded H-bridge multilevel photovoltaic (PV) inverter for single- or three-phase grid-connected applications. The modular cascaded multilevel topology helps to improve the efficiency and flexibility of PV systems. To realize better utilization of PV modules and maximize the solar energy extraction, a distributed maximum power point tracking control scheme is applied to both single- and three-phase multilevel inverters, which allows independent control of each dc-link voltage. For three-phase grid-connected applications, PV mismatches may introduce unbalanced supplied power, leading to unbalanced grid current. To solve this issue, a control scheme with modulation compensation is also proposed. An experimental three-phase seven-level cascaded H-bridge inverter has been built utilizing nine H-bridge modules (three modules per phase). Each H-bridge module is connected to a 185-W solar panel. Simulation and experimental results are presented to verify the feasibility of the proposed approach.

A New Selective Loop Bias Mapping Phase Disposition PWM With Dynamic Voltage Balance Capability for Modular Multilevel Converter

This paper presents an improved phase disposition pulsewidth modulation (PWM) (PDPWM) for the modular multilevel converter (MMC) which is based on the selective loop bias mapping (SLBM) method. Its main idea is to change the bias of the PDPWM carrier wave cycling according to the balance situation of the system. This new modulation method can operate at symmetric condition to generate an output voltage with as many as 2N + 1 levels, and by SLBM, the voltages of the upper/lower arm capacitors can be well balanced. Compared to carrier phase-shifted PWM, this method is more easily to be realized and has much stronger dynamic regulation ability. Specially, this method has no issues of sorting, which makes it suitable for MMC with a large number of submodules in one leg. With simulation and experiments, the validity of the proposed method has been shown.

Elimination of Harmonics in a Modular Multilevel Converter Using Particle Swarm Optimization-Based Staircase Modulation Strategy

A particle swarm optimization (PSO) algorithm-based staircase modulation strategy for modular multilevel converters (MMC) is proposed. To reduce switching losses and device stress, the staircase modulation method has been adopted in high-voltage and high-power energy conversion applications. In particular, the selection of the appropriate iterative initial values of switching angles is a significant step to realize a staircase modulated MMC. The proposed method is able to find the optimum initial values of switching angles, while it has the advantages of global optimization and quadratic convergence, which benefit from the PSO algorithm and Newton method, respectively. The paper presents analytical discussion of the voltage balancing control approach with rotation of switching angles. The main benefit of the efficient switching patterns is that the MMC has lower switching losses and minimum dV/dt stress. Simulation and experimental results are presented to verify the practical feasibility of the proposed scheme for the MMC.

Control of cascaded H-bridge multilevel inverter with individual MPPT for grid-connected photovoltaic generators

A single-phase cascaded H-bridge multilevel inverter for a grid-connected photovoltaic (PV) system with nonactive power compensation is presented in this paper. To maximize the solar energy extraction of each PV string, an individual maximum power point tracking (MPPT) control scheme is applied, which allows the independent control of each dc-link voltage. A generalized nonactive power theory is applied to generate the nonactive current reference. Within the inverters capability, the local consumption of nonactive power is provided to realize power factor correction. A single-phase modular cascaded multilevel inverter prototype has been built. Each H-bridge is connected to a 195 W solar panel. Simulation and experimental results are presented to validate the proposed ideas.

A modulation reconfiguration based fault-tolerant control scheme for modular multilevel converters

A modulation reconfiguration based fault-tolerant method is proposed for the modular multilevel converter (MMC) system. A practical method to reconfigure the reference sine wave is proposed from the point of view that the output line-to-line voltages should be guaranteed to be well regulated and not vary. Voltage balancing for sub-module capacitors is achieved while using carrier rotation algorithm based multicarrier pulse width modulation. Experimental results obtained from the laboratory system verified the validity and reliability of the proposed fault-tolerant control algorithm in the MMC application.

Quasi-Fixed-Frequency Hysteresis Current Tracking Control Strategy for Modular Multilevel Converters

This study proposes a quasi-fixed-frequency hysteresis current tracking control strategy for modular multilevel converters (MMCs) on the basis of voltage partition principle. First, by monitoring the grid voltage and the deviation between the output and reference currents, the output voltage is determined, thus prompting the output current to quickly and efficiently track the given current. Second, the voltages of the upper/lower capacitor of the arm and the voltages between the upper and lower arms are balanced by combining these arms with virtual loop mapping and arm voltage balance control, respectively. In particular, the proposed method is designed for any level and number of sub-modules. The validity of the proposed method is verified by simulations and experimental results of a five-level MMC prototype.

Novel Switched-Inductor Quasi-Z-source Inverter

A novel switched-inductor quasi-Z-source inverter is proposed in this study. Compared with classic topologies, the boost ability of the proposed topology is strengthened. The voltage stress of the capacitors, diodes, and power devices are reduced, and the current ripple of the DC voltage source is suppressed. Conversion efficiency is also improved. The operation principle of the proposed topology is analyzed in detail and compared with that of similar topologies. The feasibility of the proposed topology is verified by simulations and experiments on a laboratory prototype.

Balancing Control Schemes for Modular Multilevel Converters Using Virtual Loop Mapping With Fault Tolerance Capabilities

A novel complementary phase disposition pulsewidth modulation method for the modular multilevel converter (MMC) with a limited number of submodules is proposed in this paper. Compared with the previous works, more practical issues have been considered, and the corresponding solutions have been provided. First, a new preset PWM signal mapping sequence and an improved virtual loop mapping (VLM) are used in the modulation. This method not only achieves a favorable capacitor voltage balance in the symmetry system but lowers the switching frequency of MMCs devices. Second, to further reduce the switching frequency, the unbalance margin of capacitor voltages is adopted to achieve the switching between the VLM and the selective VLM (SVLM). Moreover, the system robustness has been improved by considering the situation of symmetry losing, like cell fails; in these cases, the capacitor voltages of the upper and lower arms are balanced through the collaboration of the SVLM and the reconfigurable VLM. Finally, the effectiveness of the proposed method has been verified with simulation and experimental studies.

Control Method for Fault-Tolerant Active Power Filters

New direct and indirect current control methods for a fault-tolerant active power filter topology are presented in this paper. Since a three-phase four-switch topology has a phase bridge current which cannot be directly controlled, a hysteresis control method in the α-β plane which controls the three-phase current in the two-phase stationary coordinate system is proposed. The improved SVPWM algorithm is able to eliminate the operation of the trigonometric functions in the traditional algorithm by rotating the α-β coordinates and alternating the sequence of the output vectors, which in turn simplifies the algorithm and reduces the switching frequency. The selection of the DC-side reference voltage and DC-side capacitor equalization strategy are also discussed. Simulation and experiments demonstrate that the proposed control method is correct and feasible.