Low Voltage Ride Through Enhancement Capabilities of Doubly Fed Induction Generator Based Wind Energy Conversion System

Abstract

Wind energy is considerably gaining much importance among the renew- able sources. The increase in wind power requires the system operators of different countries to frame the grid codes to ensure the safe wind in- tegration. The LVRT becomes mandatory for the grid integration of WT generator. Moreover, WT generators are flexible, and offer both fixed speed and variable speed. The DFIG is more preferred in WT generators mainly due to its low losses and less cost over the PMSG during grid in- tegration. DFIG is one of the variable speed WT generators, where the rotor speed is ±30% around the synchronous speed. Further, RSC and GSC are connected in back-to-back (B2B) form to the DFIG system. The main drawback of the DFIG system is sensitivity to faults that are occur- ring in it. As a result, it leads to huge generation losses and system would be disconnected. Based on these drawbacks, the literature has suggested the LVRT technique for the DFIG system. Basically, the LVRT keeps the DFIG to remain connected to grid even under the grid faults. Most challenging part in the LVRT enhancement is designing the con- troller for rotor side converter, grid side converter, DC-link capacitor and FACTS devices, which are considered as a part of this system. The de- signing of controller is very essential in LVRT strategy. The controller recovers the reactive power demand at the machine and the grid side, and reduces the rotor currents which are 8 to 10 times of the rated current. Also, it maintains the constant DC link voltage, brings down the elec- tromagnetic torque oscillations, enhances the power delivering capacity, and suppresses the dominant harmonics. Hence, the controller techniques are more significant in optimizing the parameters during the LVRT en- hancement. The observation made from the literature is that, controller strategies are more effective and economical in the LVRT enhancement of wind turbine generators. It has been shown that the existing controllers described in the literature fail to improve the overall DFIG performance under grid disturbance. The LVRT enhancement lacks the effective controller design. As a result, the new controller i.e Modified Super Twisting (MST) based Second Order iiiSliding Mode (SOSM) considers the DFIG’s overall performance for the LVRT enhancement under grid fault. Moreover, the performance of the proposed controller is compared with existing controllers in the literature with the help of MATLAB/SIMULINK. The hardware-in-loop (HIL) vali- dates the simulation results, which performed on the OPAL-RT setup. In addition, the proposed controller is also tested on an equivalent model of the Wind Farm (WF). Under grid disturbance, the DC link voltage does not perform well with the controller that was previously designed. So, a new controller called the modified Second Order Sliding Mode (MSOSM) for the DFIG-WT system under fault conditions is designed to only consider the DC-link parameter. Moreover, the performance of the proposed controller is compared with ex- isting controllers in the literature with the help of MATLAB/SIMULINK. The hardware-in-loop (HIL) validates the simulation results, which per- formed on the OPAL-RT setup. In addition, the proposed controller is also tested on an equivalent model of the Wind Farm (WF). The DFIG’s overall performance is hampered by the Total Harmonic Dis- tortion (THD) during grid fault. Thus, a new controller i.e Feed Forward Neuro-SOSM (FFN-SOSM) is designed to improve the THD in order to enhance the LVRT through the DFIG’s performance. Moreover, the per- formance of the proposed controller is compared with existing controllers in the literature with the help of MATLAB/SIMULINK. The hardware- in-loop (HIL) validates the simulation results, which performed on the OPAL-RT setup. In addition, the proposed controller is also tested on an equivalent model of the Wind Farm (WF). The grid fault has an impact on the system’s phase angle. It causes dis- tortion in the DFIG output. As a result, a new method (MSOSM with PR controller) is used to improve the PLL by taking the phase angle into account in order to improve the LVRT capability. Moreover, the perfor- mance of the proposed controller is compared with existing controllers in the literature with the help of MATLAB/SIMULINK. In addition, the proposed controller is also tested on an equivalent model of the Wind Farm (WF).