This paper presents the latest comprehensive literature review of AC and DC microgrid (MG) systems in connection with distributed generation (DG) units using renewable energy sources (RESs), energy storage systems (ESS) and loads. A survey on the alternative DG units' configurations in the low voltage AC (LVAC) and DC (LVDC) distribution networks with several applications of microgrid systems in the viewpoint of the current and the future consumer equipments energy market is extensively discussed. Based on the economical, technical and environmental benefits of the renewable energy related DG units, a thorough comparison between the two types of microgrid systems is provided. The paper also investigates the feasibility, control and energy management strategies of the two microgrid systems relying on the most current research works. Finally, the generalized relay tripping currents are derived and the protection strategies in microgrid systems are addressed in detail. From this literature survey, it can be revealed that the AC and DC microgrid systems with multiconverter devices are intrinsically potential for the future energy systems to achieve reliability, efficiency and quality power supply.

AC-microgrids versus DC-microgrids with distributed energy resources: A review

This paper presents a literature review of the recent developments and trends pertaining to Grid-Connected Photovoltaic Systems (GCPVS). In countries with high penetration of Distributed Generation (DG) resources, GCPVS have been shown to cause inadvertent stress on the electrical grid. A review of the existing and future standards that addresses the technical challenges associated with the growing number of GCPVS is presented. Maximum Power Point Tracking (MPPT), Solar Tracking (ST) and the use of transformless inverters can all lead to high efficiency gains of Photovoltaic (PV) systems while ensuring minimal interference with the grid. Inverters that support ancillary services like reactive power control, frequency regulation and energy storage are critical for mitigating the challenges caused by the growing adoption of GCPVS.

Trends and Challenges of Grid-Connected Photovoltaic Systems – A Review

One of the most promising power electronics converter topologies is the Z-source inverter (ZSI). The ZSI is an emerging topology for power electronics dc?ac converters with interesting properties such as buck-boost characteristics and single-stage conversion. A two-port network, composed of two capacitors and two inductors connected in an X shape, is employed to provide an impedance source (Z-source) network, coupling the inverter main circuit to the dc input source. The ZSI advantageously uses the shoot-through (ST) state to boost the input voltage, which improves the inverter reliability and enlarges its application fields. In comparison with other power electronics converters, it provides an attractive single stage dc?ac conversion with buck-boost capability with reduced cost, reduced volume, and higher efficiency due to a lower component number. For emerging power-generation technologies, such as fuel cells, photovoltaic (PV) arrays, and wind turbines, and new power electronic applications such as electric and hybrid vehicles, the ZSI is a very promising and competitive topology [1]-[4].

Z-Source Inverter: Topology Improvements Review

A new family of single-stage high-step-up boost voltage inverters based on transformers is proposed, called TZ-source inverters. By replacing two inductors in the classical Z-source inverter with two transformers, the proposed inverter produces a very high boost voltage gain when the turn ratio of the transformers is larger than 1. Compared to the trans-Z-source inverters, the proposed TZ-source inverter uses a lower transformer turn ratio, which reduces the transformer's size and weight while producing the same output voltage gain. The extension of the proposed inverter topology to dc-link-type and embedded-type topologies is presented. The operating principles, analysis, and simulation results are shown. To verify the performance of the proposed TZ-source inverter, a laboratory prototype was constructed and operated with 60-V dc input. The simulation and experimental results verified that the converter has high-step-up inversion ability.

TZ-Source Inverters

In this paper, a fault-tolerant single-phase five-level inverter configuration is proposed for photovoltaic (PV) generation systems. Conventional two-level inverters are popularly used in PV applications, but these inverters provide the output voltage with considerable harmonic content. One of the efficient ways to improve the power quality of PV generation systems is to replace a two-level inverter with a multilevel inverter. Conventional multilevel inverters reduce total harmonic distortion and filter requirements effectively, but it has limitations in terms of reliability due to increased device count and capacitor voltage balancing issues. Therefore, a fault-tolerant single-phase five-level inverter is presented, which is constructed by using a half-bridge two-level inverter, a three-level diode clamp inverter, and a bidirectional switch. The proposed inverter topology can tolerate the system faults due to failure of the source and/or switching devices with least modification in the switching combinations. It has less number of switching devices compared to conventional five-level inverters. The topology also has the energy-balancing capability between sources which helps in reducing uneven charge of batteries in case of partial shading or hotspots on one side of the PV panels. The proposed system under normal and faulty condition is simulated in MATLAB/Simulink environment, and results are verified with a laboratory prototype.

A Fault-Tolerant Single-Phase Five-Level Inverter for Grid-Independent PV Systems

A new single inductor dual buck full-bridge inverter is proposed in this paper. The inverter is improved on the basis of a dual buck inverter, and it retains the advantage of a no reverse recovery of body diode. The dc voltage utilization rate can be increased by adopting the voltage commutation bridge instead of the voltage-dividing capacitors in the inverter. In addition, comparing with other dual buck inverters, the inverter has just one filter inductor, which can make the volume and weight of the system decreased observably and improve the integration. The controlling method, combining the high- and low-frequency modulation, is adopted to weaken the complexity of control and improve the reliability and stability of the system. The operating principle, characteristic analysis, simulation and experiment are provided in this paper. A 1-kW prototype is established and tested, and the results of simulation and experiment verify the correctness of the theoretical analysis.

Single Inductor Dual Buck Full-Bridge Inverter

Renewable power systems as distributed generation units often experience big changes in the inverter input voltage due to fluctuations of energy resources. Often, a front-end boost converter is added to step up the dc voltage when the energy resources are at a weak point. However, when a very high boost gain is demanded, the duty cycle may come to its extreme, and large duty cycles causes serious reverse-recovery problems. This paper proposes a novel single-stage boost-type inverter with coupled inductor. By introducing impedance network, including coupled inductor, into the three-phase bridge inverter, and adjusting the previously forbidden shoot-through zero state, the converter can realize a high boost gain and output a stable ac voltage. The single-stage operation of the converter may lead to improved reliability and higher efficiency. Theoretical analysis, simulation, and experimental results are presented to verify good performance.

Single-Stage Boost Inverter With Coupled Inductor

This letter presents a modulation technique for the modified coupled-inductor single-stage boost inverter (CL-SSBI)-based grid-connected photovoltaic (PV) system. This technique can reduce the system leakage current in a great deal and can meet the VDE0126-1-1 standard. To maintain the advantages of the impedance network, only a diode is added in the front of the original topology, to block the leakage current loop during the active vectors and open-zero vectors. On the other hand, the near-state pulse width modulation (NSPWM) technique is applied with one-leg shoot-through zero vectors in order to reduce the leakage current through the conduction path in the duration of changing from and to open-zero vectors. Simultaneously, the leakage current caused by other transitions can also be reduced due to the fact that the magnitude of common-mode voltages is reduced. Simulation results of the transformerless PV system are presented in two cases: modified CL-SSBI modulated by maximum constant boost (MCB) control method and NSPWM. Experimental results for both CL-SSBI topology modulated by the MCB control method and modified CL-SSBI topology modulated by NSPWM are also obtained to verify the accurateness of theoretical and simulation models.

A Transformerless Grid-Connected Photovoltaic System Based on the Coupled Inductor Single-Stage Boost Three-Phase Inverter

A novel interleaved dual buck full-bridge three-level inverter (IDBFTI) is proposed for the grid-connected system in this paper It retains the advantages of interleaved parallel technology for inverter as follows: reducing the output ripple current and the total harmonic distortion of the output current, increasing the power density of system, and reducing the current stress and thermal stress of power devices Besides, it can solve the problem of zero sequence circulation current (ZSCC) in interleaved inverter system by the circuit structure, and it also has the advantages of no shoot-through problem, no reverse recovery of the body diode, and three-level output The interleaved inverter circuits share two power-frequency switches, which are resized for each specific application, and it is easier to extend the system by improving current capability of the two switches The working principle of this system is introduced in detail, and a 2-kW experimental prototype has been established and tested Test results verify the principle and the excellent performance of IDBFTI

Interleaved Dual Buck Full-Bridge Three-Level Inverter

This paper presents a single-phase to three-phase adjustable-speed drive (ASD) system, which consists of a diode rectifier and a single-stage boost inverter without electrolytic capacitors (E-caps). The system has no shoot-through issues and gains high reliability due to the shoot-through zero-state regulation method. By using electrolytic capacitors-less topology, the lifetime can be greatly increased. By properly designing the tapping position of the inductor, the system has a high boost-inversion gain and can ride through grid voltage sags. By using the proposed harmonic injection method, the system can realize high input power factor and small dc-link ripple voltage, simultaneously. Experimental results of the electrolytic capacitors-less single-stage boost inverter (E-caps-less SSBI) based single-phase to three-phase ASD system are obtained to verify the actual performances.

Yufei Zhou

Papers

Single Inductor Dual Buck Full-Bridge Inverter

Single-Stage Boost Inverter With Coupled Inductor

A Transformerless Grid-Connected Photovoltaic System Based on the Coupled Inductor Single-Stage Boost Three-Phase Inverter

Interleaved Dual Buck Full-Bridge Three-Level Inverter

Single-Phase Input Variable-Speed AC Motor System Based on an Electrolytic Capacitor-Less Single-Stage Boost Three-Phase Inverter