Because of the commitment between the large-scale photovoltaic power plants and the main grid to cope with different LOW VOLTAGE conditions in the grid, LOW VOLTAGE RIDE THROUGH (LVRT) CAPABILITY of such plants is necessary. Handling this situation is more challenging when the main grid is under unbalanced conditions.  In this paper, a new LVRT approach is proposed to reduce oscillations in this situation. To this end, the simultaneous positive, negative, and zero sequences control (PNZSC) method is proposed to provide a suitable reference current for eliminating oscillations of the active power, and similarly to reduce VOLTAGE oscillations of the DC side. The zero sequence control is achieved THROUGH proper inverter switching.Also, this method limits the inverter output current to the maximum rated value. A Dual Second Order Generalized Integrator - Frequency Locked Loop (DSOGI-FLL) is used for better synchronizing of the inverter to the grid, in asymmetric faults. Besides, an interleaved DC-DC converter and a Neutral Point Clamped (NPC) Inverter are used to reduce Total Harmonic Distortion (THD) and losses. The performance of the proposed approach is confirmed using simulation of different possible scenarios in MATLAB/Simulink environment.

Under unbalanced grid condition, in a Doubly-Fed Induction Generator (DFIG), VOLTAGE, current, and flux of the stator become asymmetric. Therefore, active-reactive power and torque will be oscillating. In DFIG controlling Rotor Side Converter (RSC) aims to eliminate power and torque oscillations. However, simultaneous elimination of the power and torque oscillations is not possible. Also, Grid Side Converter (GSC) aims to regulate DC-Link VOLTAGE. In this paper, in order to regulate DC-Link VOLTAGE, an Extended State Observer (ESO) based on a Generalized Proportional-Integral (GPI) controller, is employed. In this controlling method, DC-Link VOLTAGE is controlled without measuring the GSC current, and due to using the GPI controller, the improved dynamic response is resistant against VOLTAGE changes, and the settling time is reduced. To improve the transient stability and LOW VOLTAGE Fault RIDE THROUGH (LVRT) CAPABILITY of DFIG, Statistic Fault Current Limiter (S-FCL) and Magnetic Energy Storage Fault Current Limiter (MES-FCL) are proposed in this paper. The proposed FCL does not only limit the fault current but also fasten VOLTAGE recovery. The simulations are implemented by MATLAB software in the synchronous positive and negative sequence reference (d-q).

Doubly-fed induction generators (DFIGs) are crucial in wind turbines due to their advanced control features and efficient power conversion, but they're vulnerable to grid issues like VOLTAGE dips and short circuits. This study explores enhancing DFIGs' LOW VOLTAGE RIDE-THROUGH (LVRT) capabilities using a superconducting fault current limiter (SFCL) system. The SFCL's superconducting coil plays a key role by limiting fault current to stabilize power output, reducing excessive currents during faults, and mitigating VOLTAGE fluctuations, protecting the rotor-side converter and gearbox. The research focuses on optimizing the coil's inductance to improve SFCL performance, showing THROUGH MATLAB/Simulink simulations that adjusting inductance can lessen rotor current oscillations during short circuits. The results indicate significant enhancements in LVRT capabilities, reducing electrical and mechanical stress on generators and converters, preventing severe VOLTAGE drops, and maintaining torque levels. Incorporating an SFCL into DFIG systems greatly increases stability, reliability, and fault tolerance, supporting more wind energy integration.

In the event of VOLTAGE sag at the nearing of a wind park, severe currents may pass THROUGH the power electronic devices of the Doubly Fed Induction Generators (DFIGs). Hence, according to the LOW VOLTAGE RIDE THROUGH (LVRT) requirements, the wind parks are alLOWed to disconnect from the network after a certain time. Therefore, to avoid such actions, the LVRT requirement of wind parks is considered to be enhanced in this paper. This aim is achieved by offering a novel objective function in order to find the optimum impedance of Superconducting Fault Current Limiter (SFCL). To evaluate the effectiveness of the proposed method, it is verified in MATLAB/SIMULINK and its great performance to amend the LVRT of such wind parks is corroborated by the simulation results.

In the conventional structure of the wind turbines along with the doubly-fed induction generator (DFIG), the stator is directly connected to the power grid. Therefore, VOLTAGE changes in the grid result in severe transient conditions in the stator and rotor. In cases where the changes are severe, the generator will be disconnected from the grid and consequently the grid stability will be attenuated. In this paper, a completely review of conventional methodes for DFIG control under fault conditions is done and then a series grid side converter (SGSC) with sliding mode control method is proposed to enhance the fault RIDE THROUGH CAPABILITY and direct power control of machine. By applying this controlling strategy, the over current in the rotor and stator windings will totally be attenuated without using additional equipments like as crowbar resistance; Moreover, the DC link VOLTAGE oscillations will be attenuated to a great extent and the generator will continue operating without being disconnected from the grid. In addition, the proposed method is able to improve the direct power control of DFIG in harmonically grid VOLTAGE condition. To validate the performance of this method, the simulation results are presented under the symmetrical and asymmetrical faults and harmonically grid VOLTAGE conditions and compared with the other conventional methods.

This paper studies the interest of the integration of battery energy storage with STATIC Synchronous Compensator (STATCOM) for improving the LOW VOLTAGE RIDE THROUGH CAPABILITY (LVRT) of a fixed speed wind turbine connected to the grid. For this reason and by applying a grid fault, a comparison is made between integrating the SSSC, the STATCOM and the STATCOM with battery energy storage.The system with the aforementioned flexible alternating current transmission system (FACTS) systems is simulated using MATLAB/SIMSCAPE and the results show that the STATCOM with a battery is most efficient in terms of improving the LVRT of a fixed speed wind turbine.

Using of DFIG-Based wind turbine in distribution network is increasing day by day. Despite different advantages of DFIG such as ease controllability، LOW cost and ability to work in different wind speeds، they are very sensitive to the grid VOLTAGE drop and when a fault occurs، the rotor current is increased and this may leads to damage the DFIG power electronics converters. Also، VOLTAGE swell caused by large loads switching off، large capacitor banks energizing and unbalanced faults may damage the DFIG converters. Dou to necessity of compliance of grid codes، a novel approach is proposed in this paper to improve the LOW VOLTAGE RIDE THROUGH (LVRT) and high VOLTAGE RIDE THROUGH (HVRT) CAPABILITY of the DFIG in microgrid by using superconducting magnetic energy storage (SMES) and superconductive fault current limiter (SFCL)، simultaneously. The simulation is carried out by using PSCAD/EMTDC software and the simulation resultsillustrate the effectiveness of proposed approach to improve fault RIDE THROUGH CAPABILITY of DFIG in microgrid during VOLTAGE swell and VOLTAGE sag.

In this paper, a sliding mode controller with an adjustable reactive power reference value is proposed. To improve the performance of the controller in a steady-state, an Integral Sliding Mode Control is designed and used. In addition, to improve the LOW-VOLTAGE RIDE-THROUGH CAPABILITY in the fault condition, a reactive power controller with an adjustable reference value is proposed. The performance of this control system, during the power track, is compared with two other control systems that have a fixed reference for reactive power and are based on SMC and PI controllers in 9 different fault modes. These 9 different modes include one-phase, two-phase, and three-phase short circuit faults in the sub-synchronous, synchronous, and super-synchronous mode of operation for DFIG. The proposed method has been implemented in Simulink/MATLAB software. The simulation results confirm the CAPABILITY and effectiveness of the proposed control system in comparison with two other aforementioned control systems.

Power grids are complex, interconnected and nonlinear systems, and this will be more severe when they are subjected to high wind resources penetration. STATIC synchronous compensators (STATCOM) are used to improve VOLTAGE regulation and to meet grid codes in power systems, including doubly fed induction generators (DFIG) based wind farms. Despite the nonlinear nature of STATCOM, the conventional control method in them is based on linear PI controllers with constant gains, which they cannot guarantee the optimal performance for STATCOM. In this paper, a comparison has been performed between two nonlinear control methods based on differential flatness theory for STATIC synchronous compensators in the WSCC system includes DFIG based wind farms and simulation results demonstrate the effective performance of both methods in comparison with the conventional linear control method.