Multilevel inverters (MLIs) are being used in wide range of power electronic applications. These converters have attracted a lot of attention during recent years and exist in different topologies with similar basic concepts. This paper presents five main submodules (SMs) to be used as the basic structures of MLIs. The paper reviews the common MLI topologies from the structural point of view. The topologies are divided into the different SMs to show conventional MLI configurations and future topologies that can be created from the main SMs. A comparative study between different topologies is performed in detail. The MLIs are categorized and investigated with from different perspectives such as the number of components, the ability to create inherent negative voltage, working in regeneration mode and using single dc source.

A General Review of Multilevel Inverters Based on Main Submodules: Structural Point of View

The wind power generation is a rapidly growing grid integrated renewable energy (RE) technology with an installed capacity of 539.291 GW. The capability of the wind energy conversion system (WECS) to remain integrated into the utility network in the case of low voltage events is called low-voltage ride-through (LVRT) capability. This paper offers a comprehensive overview of improvement techniques of the LVRT capability in WECS to increase the wind energy penetration level in the utility grid. Exhibited portrait manifests a broad spectrum of 1) wind turbines, 2) electrical generators used for wind power applications, 3) international grid codes applicable for grid integration of WECS, 4) LVRT fundamentals in WECS, 5) wind turbines LVRT methods by doubly fed induction generator (DFIG), 6) wind turbines LVRT methods by permanent magnet synchronous generators (PMSG), and 7) LVRT methods of wind turbines using squirrel cage induction generator (SCIG). This ready-reckoner paper critically reviews and classifies more than 190 research papers on LVRT issues, practices, and available technologies for grid integration in wind energy systems, and it aims to be a quick reference for the researchers, designers, manufacturers, and engineers working in the same field.

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Comprehensive Overview of Low Voltage Ride Through Methods of Grid Integrated Wind Generator

The multilevel converter, (MC) had been recommended for high- and medium-level power applications for decades. It is one of the most popular converters due to its ability to decrease the harmonic distortion in the output waveform without decreasing the converter power output. This paper presents the basic concept of four different types of multilevel converter: the neutral-point clamped, flying capacitor, cascaded H-bridge and modular. This paper also reviews five different modulation techniques for multilevel converters: pulse width modulation, space vector pulse width modulation, phase shifted carrier pulse width modulation, sinusoidal pulse width modulation and selective harmonic elimination pulse width modulation. In addition, recent publications regarding developments and improvements of fundamental concern to MCs and modulation techniques are reviewed in this paper. The implementation of MCs within renewable energy systems is also discussed here.

Reviews on multilevel converter and modulation techniques

This paper aims to present a novel control strategy for modular multilevel converters (MMC) based on differential flatness theory, in which instantaneous active and reactive power values are considered as the flat outputs. To this purpose, a mathematical model of the MMC taking into account dynamics of the ac-side current and the dc-side voltage of the converter is derived in a d-q reference frame. Using this model, the flat outputs-based dynamic model of MMC is obtained to reach the initial value of the proposed controller inputs. In order to mitigate the negative effects of the input disturbance, model errors, and system uncertainties on the operating performance of the MMC, the integral-proportional terms of the flat output errors are added to the initial inputs. This can be achieved through defining a control Lyapunov function which can ensure the stability of the MMC under various operating points. Moreover, the small-signal linearization method is applied to the proposed flat output-based model to separately evaluate the variation effects of controller inputs on flat outputs. The proficiency of the proposed method is researched via MATLAB simulation. Simulation results highlight the capability of the proposed controller in both steady-state and transient conditions in maintaining MMC currents and voltages, through managing active and reactive power.

Novel Control Strategy for Modular Multilevel Converters Based on Differential Flatness Theory

Multilevel inverters (MLIs) are playing a pivotal role in the power sector with potential applications, such as interfacing renewable energy sources with the grid and several industrial drive applications. MLIs with a smaller number of switching devices are more promising due to their compact size, reduced cost, and higher efficiency compared with their traditional counterparts. This paper, therefore, presents a new three-phase seven-level inverter. This topology is a combination of two cascade-connected two-level voltage-source inverters (VSIs) and H-bridge cells with flying capacitors (FCs). This paper presents the operating principle and the balancing technique for the dc-link capacitors and FCs. The generation of various output voltage levels and the limitation of the sinusoidal pulsewidth modulation control for FC voltage balancing is also presented. The number of components in the proposed circuit configuration and their voltage ratings are considerably lower compared with the recently proposed topologies. The behavior of the proposed circuit configuration is first assessed with simulation studies and is then tested with a laboratory prototype. The simulation and experimental results validate the effectiveness of the proposed topology and the voltage balancing technique.

A Seven-Level VSI With a Front-End Cascaded Three-Level Inverter and Flying-Capacitor-Fed H-Bridge

Traditional low voltage high power wind energy conversion system (WECS) requires heavy cables and large step-up grid interfacing transformers. In recent years, power rating of offshore wind turbines already exceeds 10 MW, and medium voltage (MV) converters have become strongly demanded. Modular multilevel converter (MMC) is suitable for MV high power applications. However, there are two technical difficulties for MMC when applied in MV permanent magnet synchronous generator (PMSG) WECS. The first one is the large sub module (SM) voltage fluctuation caused by low frequency and high amplitude PMSG phase current. The other one is the low voltage ride through (LVRT) problem. Overall control strategy based on carrier-phase-shifted -sinusoidal pulse width modulation technique of the MMC WECS is proposed. For the first difficulty mentioned, a second-order circulating current injection method based on closed-loop control is proposed. For the LVRT problem, a distributed braking chopper solution is provided. The effectiveness of the injection method and the distributed braking chopper solution are validated by simulation of a 5 MW/3.3 kV MMC WECS. Influences of the current injection amplitude on SM voltages and arm currents are discussed and a trade-off method is given to optimise system design, which is also verified by simulation.

Application of modular multilevel converter in medium voltage high power permanent magnet synchronous generator wind energy conversion systems

Aiming at the bad performance of EMI filter in the high frequency band, the non-ideal factors of common-mode chokes and the limitations of the existing high-frequency model are summarized. Considering the frequency characteristics of core material and the influence of parasitic capacitance, an improved cascaded Foster network model is established, and then the common-mode and differential-mode high-frequency equivalent circuits of EMI filter in lS0kHz-30MHz are developed. Furthermore, an EMI filter design method based on the optimization of the impedance characteristics of common- mode chokes is proposed. An experiment is carried out to verify the method by an 1.2SkW Boost PFC prototype. Results indicate that the device with the proposed EMI filter can pass the conducted EMI test successfully, and the design scheme can provide effective guidance for the remediation of conducted electromagnetic interference.

EMI Filter Design Based on High-Frequency Modeling of Common-mode Chokes

Due to the low switching frequency and excellent harmonic performance, selective harmonic eliminated pulse width modulation (SHEPWM) is very suitable for high power grid-connected converters. However, most of the research on SHEPWM so far is based on an ideal grid, which ignores the influences of the unbalanced and harmonic grid voltages that turn out to be rather significant and have limited the engineering application of SHEPWM. Based on a three-level grid connected converter topology, this paper studies the characteristics of SHEPWM under a non-ideal grid. An improved modulation algorithm and a novel filter design method are proposed to solve the problems caused by the unbalanced grid and harmonically distorted grid conditions, respectively. Simulation results confirmed the effectiveness of the method proposed.

An improved SHE algorithm and filter design method for high power grid-connected converter under unbalanced and harmonic distorted grid

In this paper, a modified fast repetitive control strategy is proposed, to shorten the inherent time-delay and storage space of the traditional repetitive control. The design method of an attenuation function of modified m multiple fast repetitive control and its internal model were adopted, and this design can make all the control frequencies be maintained, meanwhile, it adds a feedforward channel, so that between the two resonant peaks, an inverse resonant peak is added to reduce the interference. And the designed method allows the system to achieve a fast dynamic response, and the program occupies minimal storage space. A proportional-integral(PI) regulator is also added to the current control loop, to ensure system stability under severe harmonic distortion conditions. Dynamic and steady state performance of the proposed controller have been compared with the traditional repetitive control. The excellent performances of the control strategy are verified by simulation and experiment results.

Analysis and design of fast-transient strategy based on modified fast repetitive control for shunt active power filter

By the way of establishment of mathematical model for three-phase four-wire T-type three-level Active Power Filter (APF), the relation of DC-link voltage difference, the neutral current and the duty-ratio of each bridge arms are revealed. Consequently, a control strategy of DC-link voltage balancing is proposed with the controllable neutral current injection, which is easier to be realized and more accurate compared with conventional ones. To deal with the unbalanced load operation, a separated-phase control strategy is also proposed. Results of simulation verify that by using the proposed control strategy the capacitors voltage difference can be suppressed well, while the compensation effect of APF is almost not affected compared with the popular ones and it is suitable for unbalanced load condition.

Yaowei Hu

Papers

Application of modular multilevel converter in medium voltage high power permanent magnet synchronous generator wind energy conversion systems

EMI Filter Design Based on High-Frequency Modeling of Common-mode Chokes

An improved SHE algorithm and filter design method for high power grid-connected converter under unbalanced and harmonic distorted grid

Analysis and design of fast-transient strategy based on modified fast repetitive control for shunt active power filter

A DC-link voltage balancing strategy based on controllable neutral current injected for T-type three-level active power filter