Sixing Du

A Flying-Capacitor Modular Multilevel Converter for Medium-Voltage Motor Drive

This paper presents a flying-capacitor modular multilevel converter (FC-MMC) based on series-connected submodules. It is intended for completely improving the performance of a medium-voltage motor drive system in the entire speed range especially at zero/low speed under rated torque condition. The proposed FC-MMC circuit is characterized by the cross connection of upper and lower arm middle taps through a flying capacitor in per phase leg. By properly controlling the ac current flowing through the flying capacitor, the power balance between upper and lower arms is achieved, leading to very small voltage ripples on submodule dc capacitors in the entire speed range from standstill to rated speed even under the rated torque condition. Meanwhile, no common-mode voltage is injected. Simulation results obtained from a 4160-V 1-MW model show that the proposed FC-MMC along with the proposed control method performances satisfactorily in dynamic and static state even when operated at zero/low speed. Experiments on a downscaled prototype also prove the effectiveness of the proposal.

An Active Cross-Connected Modular Multilevel Converter (AC-MMC) for a Medium-Voltage Motor Drive

This paper presents an active cross-connected modular multilevel converter (AC-MMC) based on series-connected half-bridge modules. It is intended for completely enhancing the performance of a medium-voltage motor drive system in the full speed range from standstill to rated speed under all load conditions. The proposed AC-MMC circuit is characterized by the cross connection of upper and lower arm middle taps through a branch of series-connected half-bridge converters, which have an identical voltage and current rating with the submodules in the upper and lower arms. This cross-connected branch provides a physical power transfer channel for the upper and lower arms. By properly controlling the amount of high-frequency current flowing through the cross-connected branch, the power balance between the upper and lower arms is achieved even at a zero/low motor speed under constant torque condition. Meanwhile, no common-mode voltage is introduced in the whole speed range. A control strategy with focus on submodule capacitor voltage control is also proposed in this paper to guarantee the normal converter operation. Simulation results obtained from a 4160-V, 1-MW model verify the feasibility of the proposal. Experiments on a downscaled prototype also confirm the validity of the novel circuit and the associated control strategy.

A Novel Medium-Voltage Modular Multilevel DC–DC Converter

This paper proposes a novel modular multilevel dc-dc converter intended for transforming dc voltage and interconnecting dc grids for medium-voltage networks. The converter is composed by two strings of submodules, each of which consists of an upper arm and a lower arm with their middle points crossly connected through a dc capacitor. With assistant of the cross-connected capacitors, dc and ac power loops are formed for the dc-dc converter, leading to the power balance between primary and secondary sides, as well as between upper and lower arms. The avoidance of transformer brings the favorable features of low cost and light weight to the proposed dc-dc converter. In order to guarantee the normal operation of the dc-dc converter, a control strategy with focus on converter power balance control is presented. Simulation performed in MATLAB/SIMULINK validates the operation principle of the dc-dc converter. Experimental results obtained from a 300-V 3.6-kW downscaled laboratory prototype also prove the effectiveness of the proposal.

A Transformerless Bipolar Multistring DC–DC Converter Based on Series-Connected Modules

This paper proposes a transformerless bipolar multistring dc-dc converter based on series-connected submodules. It is intended for converting dc voltage and managing power flow for medium-voltage high-power dc networks. This new converter has a flexible structure of multistrings and each string consists of four valves with their middle points crossly connected through two cost-effect dc capacitors to form power balancing channels between valves. Its transformerless solution and single-stage configuration greatly reduce the converter volume, weight, cost, and power loss. In this paper, voltage ratio range and its influence on submodule design are analyzed. Moreover, a control strategy with focus on current sharing control and submodule capacitor voltage control is proposed. A simulation model rated at ±10 kV 2 MW verifies the feasibility of the proposed dc-dc converter. Experimental results obtained from a 100-V 1.5-kW laboratory setup also confirm the validation of the proposal.

A Simple and Cost-effective Precharge Method for Modular Multilevel Converters by Using a Low-Voltage DC Source

A simple and cost-effective precharge method for modular multilevel converter (MMC) by using a low-voltage direct current (dc) source is proposed in this paper. The submodule (SM) capacitors in MMC are required to be precharged to their nominal voltage values at start-up to ensure MMC work normally. Conventional methods perform the precharging either through grid side or dc side by using costly high-voltage bypass breakers, charging resistors or a high-voltage dc source. Voltage of the high-voltage dc source must be equal to SM capacitor voltage and could be as high as thousands of volts. By contrast, the proposed method employs a very low voltage dc source on dc bus through a series-connected blocking diode, and requires no charging resistor and bypass breakers. It takes advantages of the existing power devices, arm inductors, and SM capacitors in MMC, and configures them into boost circuit for the precharging. The proposed method is simple and cost-effective. Moreover, the characteristics of the boost circuit render the proposed method flexible. For instance, the voltage selection of the dc source is flexible and not restricted to a fixed value. The SM capacitors can be charged in different groups. In addition, the method is applicable to different MMC SMs, including SM with different capacitor voltages. The effectiveness of the proposed method is verified through simulation and experiment.

A Star-Channel Modular Multilevel Converter for Zero/Low-Fundamental-Frequency Operation Without Injecting Common-Mode Voltage

This paper proposes a star-channel modular multilevel converter (MMC). It overcomes the zero/low-fundamental-frequency operation issues of conventional MMC while not injecting the common-mode voltage (CMV). The proposed converter is characterized by the cross connections of upper three arms and lower three arms through two sets of star-channel branches. These star-channel branches rebalance the power among three phase legs, resulting in minimized submodule capacitor voltage ripples even at zero/low-fundamental-frequency operation without CMV injection. This feature enables the proposed converter to interface medium/high-voltage DC and variable-frequency AC systems. A 4160-V 1-MW simulation model is performed in MATLAB/Simulink, and a 208-V 5-kW experimental setup is constructed in the lab. Both of the simulation and experimental results verify the feasibility of the proposal.

A Startup Method for Flying-Capacitor Modular Multilevel Converter (FC-MMC) With Effective Damping of LC Oscillations

The flying-capacitor modular multilevel converter (FC-MMC) overcomes the low-/zero-speed operation issue of conventional MMC in medium-voltage motor drive application. However, the introduced flying capacitor (FC) in each phase leg brings great difficulties for circuit startup: 1) precharging of FC to nominal voltage is necessary for normal operation; and 2) the oscillation between FC and inner buffer inductors need to be mitigated in the precharging process. In this paper, a startup method with simple control algorithm is proposed for solving the above two issues. The proposed method operates the switches of submodules (SMs) to make the FC connected in parallel with SM capacitors for charging up. Meanwhile, the oscillation caused by the equivalent LC circuit is effectively mitigated by properly implementing the damping modes. The suppressed oscillation prevents the FC from any negative voltage, which guarantees the safety of FC, especially when electrolytic capacitor is utilized. The validation of the proposal is proved by simulations and experiments.

A Transformerless Bipolar Modular Multilevel DC–DC Converter With Wide Voltage Ratios

This paper presents a transformerless bipolar dc–dc converter based on series-connected submodules. It is intended for transforming dc voltage and managing power flow for medium/high-voltage dc grids. The dc–dc converter is composed of positive-pole and negative-pole subsystems, each of which consists of six arms/branches. Every two arms and one branch are constructed into a T-type circuit with the outer terminals interface to primary- and secondary-side dc buses. The two T-type circuits in one subsystem are connected in parallel to transmit power between primary and secondary sides. The transformerless structure (none bulky coupled inductors as well) solves the challengeable insulation and cooling issues of the extremely high-power magnetics in prior arts, while not sacrificing the power efficiency and system reliability. A

Common-Mode Voltage Elimination for Variable-Speed Motor Drive Based on Flying-Capacitor Modular Multilevel Converter

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A Delta-Channel Modular Multilevel Converter for Zero/Low-Fundamental-Frequency Operation

10-kV 2-MW simulation model performed in MATLAB/Simulink verifies the feasibility of the bipolar dc–dc converter. Experimental results obtained from a laboratory setup also confirm the validation of the proposal.