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

This paper describes a new approach to online forecasting of power production from PV systems. The method is suited to online forecasting in many applications and in this paper it is used to predict hourly values of solar power for horizons of up to 36 h. The data used is 15-min observations of solar power from 21 PV systems located on rooftops in a small village in Denmark. The suggested method is a two-stage method where first a statistical normalization of the solar power is obtained using a clear sky model. The clear sky model is found using statistical smoothing techniques. Then forecasts of the normalized solar power are calculated using adaptive linear time series models. Both autoregressive (AR) and AR with exogenous input (ARX) models are evaluated, where the latter takes numerical weather predictions (NWPs) as input. The results indicate that for forecasts up to 2 h ahead the most important input is the available observations of solar power, while for longer horizons NWPs are the most important input. A root mean square error improvement of around 35% is achieved by the ARX model compared to a proposed reference model.

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Online Short-term Solar Power Forecasting

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.

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

With wide-spread application of power electronic converters in high power systems, there has been a growing interest in system reliability analysis and fault-tolerant capabilities. This paper presents a comprehensive review of conventional fault-tolerant techniques regarding power electronic converters in case of power semiconductor device failures. These techniques can be classified into four categories based on the type of hardware redundancy unit: switch-level, leg-level, module-level, and system-level. Also, various fault-tolerant methods are assessed according to cost, complexity, performance, etc. The intent of this review is to provide a detailed picture regarding the current landscape of research in power electronic fault-handling mechanisms.

Survey on Fault-Tolerant Techniques for Power Electronic Converters

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 arm's capacitors can be well balanced. This method does not requiring sorting voltages from highest to lowest, and just identifies the MIN and MAX capacitor voltage's 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 Multilevel Inverter with New Modulation Method and Its Application to Photovoltaic Grid-Connected Generator

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.

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

Microgrid optimal scheduling with chance-constrained islanding capability

This study proposes a robust optimisation-based optimal scheduling model for microgrid operation considering constraints of islanding capability. The objective is to minimise the total operation cost, including generation cost and spinning reserve cost of local resources as well as purchasing cost of energy from the main grid. To ensure the resiliency of a microgrid and improve the reliability of the local electricity supply, the microgrid is required to maintain enough spinning reserve (both up and down) to meet local demand and accommodate local renewable generation when the supply of power from the main grid is interrupted suddenly, i.e. microgrid transitions from grid-connected into islanded mode. Prevailing operational uncertainties in renewable energy resources and load are considered and captured using a robust optimisation method. With proper robust level, the solution of the proposed scheduling model ensures successful islanding of the microgrid with minimum load curtailment and guarantees robustness against all possible realisations of the modelled operational uncertainties. Numerical simulations on a microgrid consisting of a wind turbine, a PV panel, a fuel cell, a micro-turbine, a diesel generator and a battery demonstrate the effectiveness of the proposed scheduling model.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-gtd.2016.1699

Bailu Xiao

Papers

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

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

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

Microgrid optimal scheduling with chance-constrained islanding capability

Robust optimisation-based microgrid scheduling with islanding constraints