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Energy consumption is increasing rapidly; hence, the energy demand cannot be fulfilled using traditional power resources only. Power systems based on renewable energy, including solar and wind, are effective and friendly for the environment. Islanded hybrid microgrid systems (IHMS) are relatively new in this industry and combine two or more sustainable sources, such as wind turbines, solar photovoltaic (PV), and other renewable alternatives, ocean, wave, and geothermal energy, etc. While sustainable, long-lasting power sources are the best choice to satisfy the growing energy demands, they are still not yet ready to be used on a large scale due to their stochastic characteristics. Furthermore, integrating these sources into the existing energy system can cause high technical difficulties, due to the stochastic nature of solar and wind in the conventional grid system and common stand-alone framework. A review of research and applications of the effective hybridization of renewable energy sources is therefore essential to address those technical and economic issues and ensure system stability, reliability, and cost-effectiveness. This article discusses the challenges that might arise when a PV plant and a wind power station are combined to produce power for the conventional main grid or in a stand-alone system. In addition, this analysis provides light on optimization approaches for improving power quality and cost-effectiveness in a solar and wind integrated IHMS. Voltage fluctuation, frequency deviation, and the uncertain nature of solar irradiation and wind sources are significant challenges for both grid-connected and standalone hybrid systems. This study then provides an overview of the control strategies which might help enhance the integration of the IHMS in producing electricity for distribution to the grid-connected load and the islanded load. In this study, the possible issues that can hinder the smooth integration of these renewable sources have been discussed. Finally, this study discusses the recent platforms being used in IHMS as well as the potential of dispatch strategies on solar and wind-integrated IHMS. [ABSTRACT FROM AUTHOR]

The recently stipulated grid codes require wind generators to re-initiate normal power production after grid voltage sag. This paper presents a comparative performance of two commonly employed variable speed wind turbines in today's electricity market, the doubly fed induction generator (DFIG) and the permanent magnet synchronous generator (PMSG) wind turbines. The evaluation of both wind turbines was performed for weak, normal and strong grids, considering the same machine ratings of the wind turbines. Because of the critical situations of the wind turbines during faulty conditions in the weak grids, an analysis was done considering the use of effective series dynamic braking resistor (SDBR) for both wind turbines. The grid voltage variable was employed as the signal for switching the SDBR in both wind turbines during transient state. Additionally, an overvoltage protection system was considered for both wind turbines using the DC chopper in the DC-link excitation circuitry of both wind turbines. Furthermore, a combination of the SDBR over-voltage protection scheme (OVPS) was employed in both wind turbines at weak grid condition in order to improve the performance of the variable speed wind turbines and keep the operation of the power converters within their permissible limits. Furthermore, the performance of the DFIG and PMSG wind turbines in weak grids were further investigated, considering the combination of 75% and 50% effectively sized SDBR and OVPS. It was observed that, even with a 50% reduction in SDBR or OVPS, the performance of both wind turbines is still satisfactory with faulty conditions. Therefore, it is recommended to use a combination of the SDBR and OVPS with DFIG- or PMSG-based variable speed wind turbines to achieve a superior fault ride through performance, especially in weak grids. The system performance was evaluated using the power system computer design and electromagnetic transient including DC (PSCAD/EMTDC) platform. [ABSTRACT FROM AUTHOR]

Summary: With the recent proliferation and penetration of wind farms into existing power grids, it is paramount to conduct numerous studies to counter grid disturbances based on operational grid codes. One of the ways of generating electric power from wind energy is by employing the promising technology of the permanent magnet synchronous generator (PMSG) wind turbine. In order to solve the transient stability intricacies posed by the stochastic nature of wind energy during grid faults, this paper proposes a fault current limiter for a PMSG‐based wind turbine control. The fault current limiter used in this study is a series dynamic braking resistor (SDBR). The best location to place the SDBR on the machine‐side and grid‐side converters (GSCs) of the wind generator was investigated, considering the grid voltage as its switching signal during transient state. Efforts have been given to find a suitable sizing of the SDBR. The performance of the SDBR has been investigated at various network strengths in weak and strong grids. A severe three‐line‐to‐ground fault and asymmetrical faults were used to test the robustness and rigidity of the controllers of the wind generator. The system performance was evaluated using power system computer aided design and electromagnetic transient including DC (PSCAD/EMTDC) platform. The same conditions of operation were used in investigating the various scenarios considered in this study, for effective comparison. [ABSTRACT FROM AUTHOR]

This article examines the dynamic and transient performances of a battery energy storage system (BESS) connected with the output of a wind energy conversion system to smoothen the short-term fluctuations in the output power. A low-power experimental real-time testbed, using battery storage and representative power converters, is interfaced with a real-time digital simulator for the reduced-scale power hardware-in-the-loop simulation. Different configurations, where the energy storage system is integrated on the ac side via static synchronous compensator and directly at the dc link of the frequency converter using back–to–back converters, are examined under both constant and variable references. For each case, the resulting error in the net power flow, the operation of the battery, and the conduction losses in the power conversion stage are examined. The performances using the constant and variable reference of the storage device with full and reduced-order hardware stage of a storage element are demonstrated. The low-voltage transient performance of the BESS is also tested and the results are presented. The novel approach of the real-time low-power BESS testbed opens the door for in-lab testing of the energy storage devices for large wind or solar photovoltaic farms to save time and money. [ABSTRACT FROM AUTHOR]

Throughout the past several years, the renewable energy contribution and particularly the contribution of wind energy to electrical grid systems increased significantly, along with the problem of keeping the systems stable. This article presents a new optimization technique entitled the Archimedes optimization algorithm (AOA) that enhances the wind energy conversion system's stability, integrated with a superconducting magnetic energy storage (SMES) system that uses a proportional integral (PI) controller. The AOA is a modern population technique based on Archimedes' law of physics. The SMES system has a big impact in integrating wind generators with the electrical grid by regulating the output of wind generators and strengthening the power system's performance. In this study, the AOA was employed to determine the optimum conditions of the PI controller that regulates the charging and discharging of the SMES system. The simulation outcomes of the AOA, the genetic algorithm (GA), and particle swarm optimization (PSO) were compared to ensure the efficacy of the introduced optimization algorithm. The simulation results showed the effectiveness of the optimally controlled SMES system, using the AOA in smoothing the output power variations and increasing the stability of the system under various operating conditions. [ABSTRACT FROM AUTHOR]

This paper proposes a control strategy of doubly fed induction generators (DFIGs) with new protection schemes for enhancing fault ride through capability of wind farms composed of DFIGs and induction generators (IGs). Since the DFIGs will be stressed or overloaded in the process of stabilizing the wind farm during a grid fault, it is paramount to consider a protection scheme for the DFIG, in order to protect its power converters. Two schemes, the dc-link chopper-controlled braking resistor with the supplementary rotor current (SRC) control of the rotor side converter of the DFIG and series dynamic braking resistor (SDBR) connected to the stator of the DFIG, are proposed and compared. Merits and drawbacks of both schemes are highlighted as well. The simulation results in power system computer aided design and electromagnetic transient including DC (PSCAD/EMTDC) show that the two proposed schemes can eliminate the need for an expensive crowbar switch in the rotor circuit, because both could limit the rotor current of the DFIG within its nominal value during a grid fault. Finally, considering the overall system performance, the latter is recommended. [ABSTRACT FROM PUBLISHER]

In this article, stabilization methods of wind farms composed of fixed-speed wind turbines and variable-speed wind turbines are analyzed. A current-controlled voltage source converter scheme for a variable-speed wind turbine driving doubly fed induction generator is proposed. The proposed scheme is then compared with two other doubly fed induction generator control schemes to show the effectiveness of the proposed controller. Different types of symmetrical and unsymmetrical faults are analyzed considering wind farm recent grid codes. Though all of the schemes are able to stabilize the wind farms during transient conditions, the proposed current-controlled voltage source converter scheme offers the advantage of less intricacy of controller design, better performance during grid fault, and omission of DC-link protection scheme, which can reduce the overall cost of the system. Moreover, extensive simulation analyses by using PSCAD/EMTDC (Manitoba HVDC Research Center, Manitoba, Canada) are performed to determine the optimum amount of the fixed-speed wind turbine system that can be stabilized by variable-speed wind turbine system. [ABSTRACT FROM PUBLISHER]

This paper presents a system using an energy capacitor system (ECS) to smoothen the output power fluctuation of a variable-speed wind farm. The variable-speed wind turbine driving a permanent-magnet synchronous generator is considered to be connected to the ac network through a fully controlled frequency converter. The detailed modeling and control strategy of the frequency converter as well as variable-speed operation of a wind turbine generator system are demonstrated. Afterward, a suitable and economical topology of ECS composed of a current-controlled voltage-source inverter, dc-dc buck/boost converter, and an electric double layer capacitor (EDLC) bank is presented, including their control strategies. Exponential moving average is used to generate the real input power reference of ECS. Another novel feature of this paper is the incorporation of a fuzzy-logic-controlled reference signal adjuster in the control of the dc-dc buck/boost converter, in which the stored energy of the EDLC bank is utilized in an efficient way. Due to this controller, the energy storage capacity of the EDLC bank can be reduced in size, thus resulting in reduction of the overall cost of the ECS unit as well as decrease in irrepressible operations during high and low energy levels of the EDLC bank. Finally, extensive simulation results are presented that validate the effectiveness of the proposed system to smoothen the output power fluctuation of the variable-speed wind farm. [ABSTRACT FROM AUTHOR]

This paper presents a new method to enhance the transient stability of multimachine power system including wind farms, when a severe network disturbance occurs in the power system. For this purpose, the energy capacitor system (ECS) composed of power electronic devices and electric double layer capacitor (EDLC) is proposed. The control scheme of ECS is based on a sinusoidal PWM voltage source converter (VSC) and fuzzy logic controlled dc-dc buck/boost converter using insulated gate bipolar transistors (IGBT). Two wind farms are considered to be connected to the power system. Two-mass drive train model of wind turbine generator system (WTGS) is used in the analyses as the drive train modeling has great influence on the dynamic characteristics of WTGS. Real wind speed data are used in the analyses to obtain realistic responses. Different types of symmetrical and unsymmetrical faults are considered as the network disturbance. Simulation results clearly show that the proposed ECS can enhance the transient stability of wind generators in multimachine power system as well as their low voltage ride through (LVRT) capability. [ABSTRACT FROM AUTHOR]