This paper gives an overview of medium-voltage (MV) multilevel converters with a focus on achieving minimum harmonic distortion and high efficiency at low switching frequency operation. Increasing the power rating by minimizing switching frequency while still maintaining reasonable power quality is an important requirement and a persistent challenge for the industry. Existing solutions are discussed and analyzed based on their topologies, limitations, and control techniques. As a preferred option for future research and application, an inverter configuration based on three-level building blocks to generate five-level voltage waveforms is suggested. This paper shows that such an inverter may be operated at a very low switching frequency to achieve minimum on-state and dynamic device losses for highly efficient MV drive applications while maintaining low harmonic distortion.

Medium-Voltage Multilevel Converters—State of the Art, Challenges, and Requirements in Industrial Applications

A novel multilevel inverter with a small number of switching devices is proposed. It consists of an H-bridge and an inverter which outputs multilevel voltage by switching the dc voltage sources in series and in parallel. The proposed inverter can output more numbers of voltage levels in the same number of switching devices by using this conversion. The number of gate driving circuits is reduced, which leads to the reduction of the size and power consumption in the driving circuits. The total harmonic of the output waveform is also reduced. The proposed inverter is driven by the hybrid modulation method. In this paper, the circuit configuration, theoretical operation, Fourier analysis, simulation results with MATLAB/SIMULINK, and experimental results are shown. The experimental results accorded with the simulation results.

A Single-Phase Multilevel Inverter Using Switched Series/Parallel DC Voltage Sources

A novel switched-capacitor inverter is proposed. The proposed inverter outputs larger voltage than the input voltage by switching the capacitors in series and in parallel. The maximum output voltage is determined by the number of the capacitors. The proposed inverter, which does not need any inductors, can be smaller than a conventional two-stage unit which consists of a boost converter and an inverter bridge. Its output harmonics are reduced compared to a conventional voltage source single phase full bridge inverter. In this paper, the circuit configuration, the theoretical operation, the simulation results with MATLAB/SIMULINK, and the experimental results are shown. The experimental results accorded with the theoretical calculation and the simulation results.

A Switched-Capacitor Inverter Using Series/Parallel Conversion With Inductive Load

The contribution of this paper has been focused on investigating a new microgrid architecture that integrates the solid-state transformer with zonal dc microgrids. By utilizing the dc and ac links of the solid-state transformer, both ac and dc networks can access the distribution system, which renders the coordinate management of the power and guarantees high power supply reliability. In addition, the presented system together with the proposed power management strategy can minimize the effect of newly established zonal dc microgrid to the existing power grid, of which promises a better stability. To this end, this paper takes the photovoltaic and battery as the typical renewable energy resource and energy storage device, and develops a simulation platform for system study. Simulation results are shown to demonstrate the feasibility of the proposal.

On Integration of Solid-State Transformer With Zonal DC Microgrid

In this paper, a new three-phase, five-level inverter topology with a single-dc source is presented. The proposed topology is obtained by cascading a three-level flying capacitor inverter with a flying H-bridge power cell in each phase. This topology has redundant switching states for generating different pole voltages. By selecting appropriate switching states, the capacitor voltages can be balanced instantaneously (as compared to the fundamental) in any direction of the current, irrespective of the load power factor. Another important feature of this topology is that if any H-bridge fails, it can be bypassed and the configuration can still operate as a three-level inverter at its full power rating. This feature improves the reliability of the circuit. A 3-kW induction motor is run with the proposed topology for the full modulation range. The effectiveness of the capacitor balancing algorithm is tested for the full range of speed and during the sudden acceleration of the motor.

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A Five-Level Inverter Topology with Single-DC Supply by Cascading a Flying Capacitor Inverter and an H-Bridge

This paper explores the mathematic model and control method of neutral-point voltage balancing problem in three-level NPC inverter. A comprehensive mathematic model of three-level inverter is established. Then, based on the space vector PWM strategy, a simple neutral-point voltage balancing control method, which depends on the direction information detection of each phase leg current and neutral-point voltage ripple, is proposed in this paper. According to the direction information, the control method selects appropriate switching states sequences to drive three-level NPC inverter. As a result, the neutral-point imbalance is restrained in a small bound, while the switching loss and the dv/dt of output voltage will not increase. The experimental results verify the correctness and feasibility of the proposed control scheme

Modeling and Control of Neutral-point Voltage Balancing Problem in Three-level NPC PWM Inverters

Traditional multilevel inverters in high voltage high power applications have some problems associated with their low frequency switching which produces high harmonic distortion. To solve the problem, this paper presents a topology of a modified multilevel cascade voltage source inverter (VSI) with different DC voltage sources. Also, the modified multilevel cascade VSI using H-bridge is modeled and analyzed with circuit DQ transform in detail. To implement the novel multilevel Inverter, a hybrid modulation strategy is presented through theoretical analysis. The simulation and experimental results are given to verify the feasibility.

Study on a modified multilevel cascade inverter with hybrid modulation

This paper presents a kind of AC electronic load for testing AC power source with energy recycling capability. The proposed structure is based on back to back single-phase PWM rectifiers of the voltage-source type. The operation principle of the model is described in detail to research the power matching problem and DC bus harmonic phenomenon. To achieve power balance and good performance, an independent control strategy for the two PWM rectifiers is proposed. In addition, a novel control technique of compensating second-harmonic voltage of dc bus is put forward. The research is proved to be effective in power recovery by the prototype experiments.

Research on AC electronic load based on back to back single-phase PWM rectifiers

A novel power electronic load (PEL) is introduced in this paper. This equipment can simulate R,L,C load as well as non-linear load. Furthermore, it can recycle the energy back to grid. Topology for single phase AC PEL, three-phase AC PEL, and DC PEL are presented firstly. Based on disadvantages of repetitive controller, an improved repetitive controller is presented to improve the dynamic performance of the system. Relative problems in back to back system are analyzed and mean filter is adopted to eliminate the harmonics in DC loop. The whole efficiency can be as high as 80%-90%. Simulation and experimental results are carried out to validate the effectiveness of the proposed control strategy.

Research on Power Electronic Load: Topology, Modeling, and Control

The conventional modulation strategies for diode-clamped three-phase three-level inverter have some disadvantages, such as switching loss of switching device This paper presents a novel space vector pulse width modulation (SVPWM) scheme to reduce switching loss Using this novel modulation strategy, the changing of switch states cause only one single phase voltage change every time In this way, the inverter generates similar output voltage as that under the common strategies, whereas the switching loss is reduced The proposed scheme has been digitally implemented by using TMS320F240 and its feasibility has been verified by the experimental results

Yunping Zou

Papers

Modeling and Control of Neutral-point Voltage Balancing Problem in Three-level NPC PWM Inverters

Study on a modified multilevel cascade inverter with hybrid modulation

Research on AC electronic load based on back to back single-phase PWM rectifiers

Research on Power Electronic Load: Topology, Modeling, and Control

Digital implementation of diode-clamped three-phase three-level SVPWM inverter