The significant benefits associated with microgrids have led to vast efforts to expand their penetration in electric power systems. Although their deployment is rapidly growing, there are still many challenges to efficiently design, control, and operate microgrids when connected to the grid, and also when in islanded mode, where extensive research activities are underway to tackle these issues. It is necessary to have an across-the-board view of the microgrid integration in power systems. This paper presents a review of issues concerning microgrids and provides an account of research in areas related to microgrids, including distributed generation, microgrid value propositions, applications of power electronics, economic issues, microgrid operation and control, microgrid clusters, and protection and communications issues.

State of the Art in Research on Microgrids: A Review

Photovoltaic (PV) energy has grown at an average annual rate of 60% in the last five years, surpassing one third of the cumulative wind energy installed capacity, and is quickly becoming an important part of the energy mix in some regions and power systems. This has been driven by a reduction in the cost of PV modules. This growth has also triggered the evolution of classic PV power converters from conventional singlephase grid-tied inverters to more complex topologies to increase efficiency, power extraction from the modules, and reliability without impacting the cost. This article presents an overview of the existing PV energy conversion systems, addressing the system configuration of different PV plants and the PV converter topologies that have found practical applications for grid-connected systems. In addition, the recent research and emerging PV converter technology are discussed, highlighting their possible advantages compared with the present technology. Solar PV energy conversion systems have had a huge growth from an accumulative total power equal to approximately 1.2 GW in 1992 to 136 GW in 2013 (36 GW during 2013) [1]. This phenomenon has been possible because of several factors all working together to push the PV energy to cope with one important position today (and potentially a fundamental position in the near future). Among these factors are the cost reduction and increase in efficiency of the PV modules, the search for alternative clean energy sources (not based on fossil fuels), increased environmental awareness, and favorable political regulations from local governments (establishing feed-in tariffs designed to accelerate investment in renewable energy technologies). It has become usual to see PV systems installed on the roofs of houses or PV farms next to the roads in the countryside. Grid-connected PV systems account for more than 99% of the PV installed capacity compared to

An Overview of Recent Research and Emerging PV Converter Technology

Photovoltaic (PV) energy has grown at an average annual rate of 60% in the last five years, surpassing one third of the cumulative wind energy installed capacity, and is quickly becoming an important part of the energy mix in some regions and power systems. This has been driven by a reduction in the cost of PV modules. This growth has also triggered the evolution of classic PV power converters from conventional single-phase grid-tied inverters to more complex topologies to increase efficiency, power extraction from the modules, and reliability without impacting the cost. This article presents an overview of the existing PV energy conversion systems, addressing the system configuration of different PV plants and the PV converter topologies that have found practical applications for grid-connected systems. In addition, the recent research and emerging PV converter technology are discussed, highlighting their possible advantages compared with the present technology.

Grid-Connected Photovoltaic Systems: An Overview of Recent Research and Emerging PV Converter Technology

Today, conventional power systems are evolving to modern smart grids, where interconnected microgrids may dominate the distribution system with high penetration of renewable energy and energy storage systems. The hybrid ac/dc systems with dc and ac sources/loads are considered to be the most possible future distribution or even transmission structures. For such hybrid ac/dc microgrids, power management strategies are one of the most critical operation aspects. This paper presents an overview of power management strategies for a hybrid ac/dc microgrid system, which includes different system structures (ac-coupled, dc-coupled, and ac–dc-coupled hybrid microgrids), different operation modes, a thorough study of various power management and control schemes in both steady state and transient conditions, and examples of power management and control strategies. Finally, discussion and recommendations of power management strategies for the further research are presented.

Overview of Power Management Strategies of Hybrid AC/DC Microgrid

Impedance networks cover the entire of electric power conversion from dc (converter, rectifier), ac (inverter), to phase and frequency conversion (ac-ac) in a wide range of applications. Various converter topologies have been reported in the literature to overcome the limitations and problems of the traditional voltage source, current source as well as various classical buck-boost, unidirectional, and bidirectional converter topologies. Proper implementation of the impedance-source network with appropriate switching configurations and topologies reduces the number of power conversion stages in the system power chain, which may improve the reliability and performance of the power system. The first part of this paper provides a comprehensive review of the various impedance-source-networks-based power converters and discusses the main topologies from an application point of view. This review paper is the first of its kind with the aim of providing a “one-stop” information source and a selection guide on impedance-source networks for power conversion for researchers, designers, and application engineers. A comprehensive review of various modeling, control, and modulation techniques for the impedance-source converters/inverters will be presented in Part II.

Impedance-Source Networks for Electric Power Conversion Part I: A Topological Review

This paper discusses islanding control of power electronics interfaced distributed generation (DG) systems in microgrids. Particularly, the following topics will be addressed: microgrid sytem configurations and features, grid-connected system control mode, transient analysis during power outage, islanding detection and load shedding, standalone system control mode, synchronization in reconnection. Some simulation and experimental results are also shown in this paper to illustrate the above mentioned issues.

Islanding control of DG in microgrids

Intentional islanding describes the condition in which a microgrid, which consists of a load and a distributed generation (DG) system, is isolated from the remainder of the utility system. In this situation, it is important for the microgrid to continue to provide adequate power to the load. Under normal operation, each DG inverter system in the microgrid usually works in constant current (or constant power) control mode in order to provide a pre-set power to the main grid. When the microgrid is cut off from the main grid, each DG inverter system must detect this islanding situation and switch to a voltage control mode. In this mode, the microgrid will provide a constant voltage to the local load. This paper describes a control strategy to implement intentional islanding operation of microgrids. The described method proposes two control algorithms, one for grid-connected operation and the other for intentional islanding operation.

Intelligent control for intentional islanding operation of microgrids

This paper introduces a novel control strategy “PWAM” method for HEV/EV motor drive system. This modulation method is quite different from other PWM methods that have been well researched or commonly used for the inverter in HEV/EV system. By using this method, only one phase leg of the inverter is doing switching action for every PWM-carrier period. Therefore, the inverter total switching time is reduced to one third comparing with conventional SPWM method and the switching loss can be reduced by at least 50% or even more depending on the load power factor value. In addition, the inverter dc-link requires much smaller capacitor when PWAM method is applied, which makes the system smaller size and lighter weight. Theoretical analysis, simulation and experimental results are provided to verify the inverter efficiency improvement.

Boost converter — Inverter system using PWAM for HEV/EV motor drive

This paper presents a novel current-fed quasi-Z-Source Inverter (qZSI) used to replace the two stages of bidirectional DC-DC converter and conventional pulsewidth modulation inverter to control the power to the motor, and the state of charge (SOC) of the battery in hybrid vehicles. Besides the advantages inheriting from the ZSIs, the qZSIs also have their own merits. This paper presents a comprehensive study on the new features of the current-fed qZSI, including the advantageous buck-boost function, improved reliability, reduced passive component ratings, and unique regeneration capability. The current-fed qZSI are bidirectional with an additional diode, unlike the voltage-fed Z-source inverter than a switch to achieve bidirectional power flow. A reverse blocking IGBT (RB-IGBT) based current-fed qZSI prototype is developed in laboratory. Both simulation and experimental results are shown to verify the theoretical analysis.

Application of current-fed quasi-Z-Source Inverter for traction drive of hybrid electric vehicles

This paper presents two modulation methods for current source inverter both in SVPWM point of view: Discontinuous-PWM (DPWM) and Continuous-SVPWM (CSVPWM), and proposed the third method Pulse-Width-Amplitude-Modulation (PWAM), also based on SVPWM theory. Different switching sequences of DPWM are proposed and compared. The Double Fourier Series equations and integration limits for both DPWM and PWAM are given. A THD and switching loss comparison has been conducted between all methods. Simulation results are given for all methods to demonstrate the theory analysis.

https://www.egr.msu.edu/~bingsen/files_publications/C-12_ECCE.pdf

Qin Lei

Papers

Islanding control of DG in microgrids

Intelligent control for intentional islanding operation of microgrids

Boost converter — Inverter system using PWAM for HEV/EV motor drive

Application of current-fed quasi-Z-Source Inverter for traction drive of hybrid electric vehicles

Unified space vector PWM control for current source inverter