http://ieeexplore.ieee.org/iel5/5610941/5624686/05624745.pdf

Power electronics

The modular multilevel converter (MMC) has been a subject of increasing importance for medium/high-power energy conversion systems. Over the past few years, significant research has been done to address the technical challenges associated with the operation and control of the MMC. In this paper, a general overview of the basics of operation of the MMC along with its control challenges are discussed, and a review of state-of-the-art control strategies and trends is presented. Finally, the applications of the MMC and their challenges are highlighted.

Operation, Control, and Applications of the Modular Multilevel Converter: A Review

Modular multilevel converters have several attractive features such as a modular structure, the capability of transformer-less operation, easy scalability in terms of voltage and current, low expense for redundancy and fault tolerant operation, high availability, utilization of standard components, and excellent quality of the output waveforms. These features have increased the interest of industry and research in this topology, resulting in the development of new circuit configurations, converter models, control schemes, and modulation strategies. This paper presents a review of the latest achievements of modular multilevel converters regarding the mentioned research topics, new applications, and future trends.

Circuit topologies, modeling, control schemes, and applications of modular multilevel converters

In this paper, the principle of modularity is used to derive the different multilevel voltage and current source converter topologies. The paper is primarily focused on high-power applications and specifically on high-voltage dc systems. The derived converter cells are treated as building blocks and are contributing to the modularity of the system. By combining the different building blocks, i.e., the converter cells, a variety of voltage and current source modular multilevel converter topologies are derived and thoroughly discussed. Furthermore, by applying the modularity principle at the system level, various types of high-power converters are introduced. The modularity of the multilevel converters is studied in depth, and the challenges as well as the opportunities for high-power applications are illustrated.

Modular Multilevel Converters for HVDC Applications: Review on Converter Cells and Functionalities

In order to address the load sharing problem in islanding microgrids, this paper proposes an enhanced distributed generation (DG) unit virtual impedance control approach. The proposed method can realize accurate regulation of DG unit equivalent impedance at both fundamental and selected harmonic frequencies. In contrast to conventional virtual impedance control methods, where only a line current feed-forward term is added to the DG voltage reference, the proposed virtual impedance at fundamental and harmonic frequencies is regulated using DG line current and point of common coupling (PCC) voltage feed-forward terms, respectively. With this modification, the impacts of mismatched physical feeder impedances are compensated. Thus, better reactive and harmonic power sharing can be realized. Additionally, this paper also demonstrates that PCC harmonic voltages can be mitigated by reducing the magnitude of DG unit equivalent harmonic impedance. Finally, in order to alleviate the computing load at DG unit local controller, this paper further exploits the band-pass capability of conventionally resonant controllers. With the implementation of proposed resonant controller, accurate power sharing and PCC harmonic voltage compensation are achieved without using any fundamental and harmonic components extractions. Experimental results from a scaled single-phase microgrid prototype are provided to validate the feasibility of the proposed virtual impedance control approach.

An Islanding Microgrid Power Sharing Approach Using Enhanced Virtual Impedance Control Scheme

Ideally, the inner (the upper or lower arm) current of a modular multilevel converter (MMC) is assumed to be the sum of a dc component and an ac component of the fundamental frequency. However, this current is usually distorted and the peak/RMS value of it is increased compared with the theoretical result. This is because ac current flows through the submodule (SM) capacitors and the capacitor voltages fluctuate with time. The increased currents will increase power losses and may threaten the safe operation of the power devices and capacitors. This paper proposes a novel close-loop method for suppression of the inner current in MMC. This method is very simple and is implemented in stationary frame, and no harmonic extraction algorithm is needed. Hence, it can be applied to single-phase or three-phase MMC. What is more important, this method does not influence the balancing of the SM capacitor voltages. Simulation and experimental results show that the proposed method can suppress the peak and RMS values of the inner currents dramatically. Meanwhile, the harmonic contents in the output current can also be suppressed satisfactorily even when the SM capacitor voltage ripple factor is as large as about ±19%. Therefore, the proposed method can also be adopted to reduce the SM capacitance requirement.

A novel inner current suppressing method for modular multilevel converters

In this paper, an improved pulse width modulation (PWM) method for chopper-cell (or half-bridge)based modular multilevel converters (MMCs) is proposed. This method can generate an output voltage with maximally 2N+1 (where N is the number of submodules in the upper or lower arm of MMC) levels, which is as great as that of the carrier-phase-shifted PWM (CPSPWM) method. However, no phase-shifted carrier is needed. Compared with the existing submodule unified pulse width modulated (SUPWM) method, the level number of the output voltage is almost doubled and the height of the step in the staircase voltage is reduced by 50%. Meanwhile, the equivalent switching frequency in the output voltage is twice that of the conventional SUPWM method. All these features lead to much reduced harmonic content in the output voltage. What is more, the voltages of the submodule capacitors can be well balanced without any close-loop voltage balancing controllers which are mandatory in the CPSPWM schemes. Simulation and experimental results on a MMC-based inverter show validity of the proposed method.

An Improved Pulse Width Modulation Method for Chopper-Cell-Based Modular Multilevel Converters

In this paper, the influence of one-sample delay for sampling and computation in digital control on the bandwidth of the inner current loop of a 400-Hz ground power unit (GPU) is analyzed first. The results show that it is difficult and even impossible for high-power 400-Hz GPUs to maintain low total harmonic distortion content in the output voltage with the conventional proportional-integral-based double-loop control. To improve the performance, resonant controllers with parallel structure which are widely used in active power filters are applied to the single-loop control of the 400-Hz GPU. The parameter design criterion for the parallel resonant controllers is discussed in the discrete time domain. Meanwhile, adoption of proportional gain in the single-loop control is investigated. The results show that it can improve the performance little and may cause instability problems. Comparisons between different control methods for the 400-Hz GPU are also made, and the single-loop control method in this paper seems to be the most suitable one in terms of simplicity and performance. Experiments on a 16-b fixed-point DSP-controlled 90-kVA 400-Hz GPU prototype show satisfactory results of the single-loop method feeding linear/nonlinear and balanced/unbalanced loads.

Single-Loop Digital Control of High-Power 400-Hz Ground Power Unit for Airplanes

In the last decade, many methods for control of three-phase boost-type pulsewidth modulation (PWM) rectifier under unbalanced input voltage conditions have been studied and presented. These methods use the sequential components of input voltages, pole voltages, and input currents of the rectifier to analyze and control the instantaneous powers, of which dual (positive-sequence and negative-sequence rotating) frame control is the most common structure. Anyway, this paper analyzes the instantaneous powers of the PWM rectifier in two-phase stationary frame. Based on this analysis, the input-power-control, input-output-power-control, and output-power-control methods for PWM rectifier under unbalanced voltage conditions are proposed in single stationary frame. Compared with the existing methods, simplicity may be the major advantage of the method in this paper. Rotating transformation and phase detection of the input voltages are both eliminated. Moreover, sequential component extraction of the control variables is also not necessary, leading to much reduced time delay to the control system. Experimental results on a 9-kVA PWM rectifier show validity and effectiveness of the proposed methods.

Control of Three-Phase Boost-Type PWM Rectifier in Stationary Frame Under Unbalanced Input Voltage

In this paper, a power electronic transformer (PET) topology for distribution grid is presented based on modular multilevel converter (MMC). The presented topology is of three-phase type and have two dc voltage terminals, i.e. the high-voltage dc terminal and the low-voltage dc terminal. So, it can be connected directly to high-voltage direct current (HVDC) systems and renewable energy systems simultaneously. This feature makes it possible to perform as an “energy router” in smart grids. What is more important, the proposed PET can substantially reduce (by 44.4% for the studied case) the number of medium frequency transformers (MFTs), compared with the existing solutions. Computer simulation and experimental results on a 10 kV prototype show validity of this PET.

Haibin Zhu

Papers

A novel inner current suppressing method for modular multilevel converters

An Improved Pulse Width Modulation Method for Chopper-Cell-Based Modular Multilevel Converters

Single-Loop Digital Control of High-Power 400-Hz Ground Power Unit for Airplanes

Control of Three-Phase Boost-Type PWM Rectifier in Stationary Frame Under Unbalanced Input Voltage

A three-phase 10 kVAC-750 VDC power electronic transformer for smart distribution grid