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

The recently proposed current-fed QZSI[1] can buck and boost voltage and provide bidirectional power flow[2–6]. A crucial criterion for the size of three phase PWM converters is the cooling of the power semiconductors and thus determination of power dissipation at certain operating points. In the modified SVPWM control method for this circuit, different PWM sequences can lead to different switching loss, current ripple, total harmonic distortion, and also the voltage spike on the switching devices. In order to select the best PWM sequence to obtain the best performance and efficiency, A complete analysis and calculation for the four mentioned criterions are presented for different sequences. For each criterion, the best sequences are selected to obtain better performance. Finally, the optimized PWM sequence in the modified SVPWM control for this circuit is selected with the optimized hardware design together to achieve the best efficiency at full operation range. The optimized hardware design has been built in the lab, including the optimal design of the coupled inductor, optimal design and selection of Z-source capacitors and output capacitors, the selection of Z-source diode, and the final layout of the hardware. In order to bring the prototype into real application in hybrid electrical vehicle[3], the inverter efficiency is measured according to the motor P-V curve. The estimated efficiency curve and experiment results are plot and compared. The best efficiency at 15kW reaches 97.6% at unity voltage gain.

Novel SVPWM switching pattern for high efficiency 15KW current-fed quasi-Z-source inverter in HEV motor drive application

LLC and CLLC resonant converters are good candidates for the isolated dc-dc stage in EV on-board chargers due to their capability of achieving zero-voltage-switching (ZVS) at full load range. The synchronous rectifier (SR) is usually utilized to reduce the conduction loss and improve the system efficiency compared with the conventional diode bridge rectifier. In this paper, a high-dv/dt-immune, fine-controlled and parameter-adaptive gate driving scheme is presented for GaN-based synchronous rectifier in EV on-board chargers. A novel self-driven SR drain-to-source voltage sensing circuit is proposed. The circuit provides a low-impedance bypassing path for the displacement current induced by the high dv/dt, which addresses the over-voltage and oscillation issues for the controller input. The detailed operating principles and the design considerations of the novel sensing circuit are discussed as well. Moreover, the adaptive SR on-time tuning algorithm is implemented, which avoids the influence from the loop stray inductance and the propagation delay in the path and reaches the SR zero current turn-off moment with fine accuracy. A 3.3 kW, 500 kHz CLLC resonant converter prototype is built to validate the proposed SR gate driving scheme. With the employment of the proposed gate driving scheme, the SR almost achieves zero current turn-off for the whole operating frequency range. The prototype demonstrates the peak efficiency of 97.6% and the power density of 130 W/inch3.

High-dv/dt-Immune Fine-Controlled Parameter-Adaptive Synchronous Gate Driving for GaN-Based Secondary Rectifier in EV On-Board Charger

This paper proposes a pulse-width-amplitude-modulation (PWAM) method for a boost-converter-inverter system in electrical vehicle motor drive application. A 1kW prototype has been built and tested. By using this method, a 87% switching loss reduction, a two times power density increase, and 30% cost reduction can be achieved. It is also demonstrated that it posses less harmonic distortion than SPWM at only 1/3 switching frequency of the latter one. The maximum overall system efficiency reaches 96.7% at 1kW. The whole system power density is 2.3kW/L or 0.5kW/b. As a result, a high efficiency, high power density, high temperature and low cost boost-converter-inverter system can be achieved to use in electrical vehicle motor drive.

PWAM boost-converter-inverter system for EV engine starter/alternator

The conventional voltage source inverter is only a buck converter and conventional current source inverter is only a boost converter. By adding a Z-Source network [1], the buck-boost capability can be achieved. The recently proposed Quasi-Z-Source inverter (qZSI) [2] is an important improvement to Z-Source inverter (ZSI), which greatly reduces the passive component stress. The current-fed qZSI can buck-boost voltage and achieve bidirectional power flow without replacing the diode with an active switch [3]. It has lower current stress on inductor compared to current-fed ZSI. However, the analysis and control methods proposed in [3] are based on the assumptions that the capacitor voltage is almost constant and equal to the input voltage. These assumptions become invalid when the capacitor is very small or the lower power factor is low in some applications that the volume is a very crucial factor. The capacitor voltage has high ripple or even becomes discontinuous. In these cases, the circuit has two new operations modes except for the normal three modes, which is called discontinuous operation modes. This paper analyzes the characteristics of the discontinuous operation modes, and derives the critical conditions for these new modes under different control strategies. Simulation and experiment results are given to verify the theoretical analysis.

Discontinuous operation modes of current-fed Quasi-Z-Source inverter

SiC MOSFET has superior switching performance over Si IGBT in terms of power loss and temperature characteristics In order to significantly improve the efficiency and power density of medium voltage drive and high-power converters, this paper proposes a current source gate driver for series connected SiC MOSFETs, forming a universal block of series connected SiC devices with higher voltage rating Proposed current source gate driver has better gate voltage synchronization performance because of its constant gate current and novel gate driver structure By implementing synchronized gate voltages, the snubber circuit is designed only for power loop difference, gate displacement current difference, and the snubber can be minimized or even be eliminated The proposed block has been verified by LTSPICE simulations and multi-pulse test experiments under 200kHz, 2kV, 0~60A condition using three 12kV/60A C2M0040120D SiC MOSFETs connected in series

Qin Lei

Papers

Novel SVPWM switching pattern for high efficiency 15KW current-fed quasi-Z-source inverter in HEV motor drive application

High-dv/dt-Immune Fine-Controlled Parameter-Adaptive Synchronous Gate Driving for GaN-Based Secondary Rectifier in EV On-Board Charger

PWAM boost-converter-inverter system for EV engine starter/alternator

Discontinuous operation modes of current-fed Quasi-Z-Source inverter

A Universal Block of Series Connected SiC MOSFETs Using Current Source Gate Driver