Transient Behavior Analysis of Grid-connected DFIG Based Wind Turbine During Power System Faults

Introduction: Nowadays, renewable energy sources, including wind energy, are gaining more attention due to some environmental concerns such as a decline in non-renewable energy sources, a rise greenhouse gases, and global warming, which are the consequences of utilising fossil fuels. Since the infiltration of wind power into the power system has been widely developed, the importance of transient behaviour analysis of wind turbine generators could be considered as a major issue in protecting relaying. Among various types of generators in the wind power industry, Doubly Fed Induction Generator (DFIG) is the most well-known technology. One of the advantages of this technology is the capability of keeping the generator output frequency constant at different wind speeds.  The DFIG is a wound rotor induction machine in which stator windings are connected directly to the grid, whereas rotor windings are attached to the network through a variable frequency converter (i.e. a back-to-back converter with the connection of DC-link capacitor). One of the significant concerns in providing power electricity is the transient instability of the power system at the instant of fault occurrence in the network. To this end, examining the transient behaviour of grid-connected generators during severe disturbances might be a feasible way for adopting effective solutions for protecting the power equipment. In the last decades, growing the wind farms integration into the grid confirms an essential investigation on the dynamic behaviour of DFIGs during transient conditions. Because it can gain a full understanding of the proper solutions for protecting wind farms against severe faults and subsequently improve the stability of the power network.
 
Materials and methods: DFIGs are intrinsically sensitive to the severe perturbations of the grid. These perturbations can be created by such factors as load shedding, short-circuit, etc. Hence, it is quite clear that DFIG based wind turbines (WTs) need specific protection circuits to protect them against grid faults. For this purpose, different protection schemes are suggested in literature, in which the crowbar circuit is the most commonly used one. During network disturbances, both the power converter and the rotor winding are protected by the crowbar circuit against excessive over-currents and over-voltages. Whenever the grid's voltage drops, the crowbar circuit is activated and separates the power converter from the rotor slip-rings. Afterwards, rotor windings are connected to the crowbar resistors, and, eventually, the rotor current is bypassed. Therefore, there will be no control on delivered reactive and active powers and the DFIG operational mode temporarily changes to squirrel-cage induction generator (SCIG). This can affect the transient characteristics of the generator such as critical speed and Critical Clearing Time (CCT), which are essential in specifying the stability margin of grid-connected induction generators. The first step for understanding the transient behaviour of a DFIG is to provide an appropriate mathematical model of it. In this regard, this paper uses a detailed model of grid-connected DFIG suitable for fault analysis. This model includes a 5th order mathematical model of the wound rotor induction machine along with a two-mass model of the mechanical drive train. In the next step, several transient fault responses of grid-connected DFIG based wind turbine are discussed using MATLAB/Simulink software.
 
Discussion and Result: In this study, transient responses of some mattering variables include stator voltage, electrical torque, rotor angular velocity, rotor current, DC-link voltage, and so on are comprehensively investigated. Obtained simulation results indicate that at the instance of fault occurrence in the power system, the stator voltage drops, and both the DC-link voltage of the converter and the rotor current increase dramatically. Additionally, under these transient conditions, the electrical torque experiences a sharp decrement, while the mechanical torque remains constant. Subsequently, the equilibrium between electrical and mechanical energy is dissipated, which leads to electromechanical fluctuations and acceleration of the rotor. These fluctuations can raise the possibility of instability. These conditions can damage the generator and power converter, and, consequently, jeopardize both the safe operation of the DFIG and even the stability of the whole system. It is worth mentioning that further investigations are also conducted to evaluate the vital role of various parameters of a DFIG such as the technology of the converter's control system, the capacity of the DC-link capacitor, the inertia constant of the turbine-generator system, the value of the crowbar circuit resistors, and the pre-fault speed of the generator on the stability margin and riding through the critical conditions. According to Simulation results, any increment in the capacitance of the DC-link capacitor will decline the DC-link voltage ripple under fault conditions. In addition, further increment in the inertia constant diminishes the acceleration of the rotor. This means that the CCT and stability margin can be improved by a greater value of the inertia constant. Another parameter that is effective in improving the CCT is the resistance of the crowbar circuit. As it was stated previously, the activation of the crowbar circuit enhances the equivalent resistance of the rotor. Accordingly, the intersection between electrical and mechanical torques in the torque versus rotor's angular velocity curve will moved away from the origin of this plate, and, thus, the critical speed and CCT will be improved. Furthermore, this study reveals that there exists a direct relationship between the DFIG's operating conditions and its stability margin. In other words, under sub-synchronous operating conditions, the duration of CCT is reduced, resulting in a low chance of generator stability.
 
Conclusion: This paper presented a comprehensive investigation on transient behaviour of grid-connected DFIGs under grid disturbances. A simulation study has been carried out using MATLAB/ Simulink software and the impact of various parameters such as the technology of the converter's control system, the capacity of the DC-link capacitor, the inertia constant of the turbine-generator system, the amount of the crowbar circuit resistors, and the pre-fault speed of the generator have been evaluated on the transient stability of the generator and its FRT capability. Obtained results have shown that in order to properly protect the generator, to ensure durability in the safe operation area, and to increase the FRT capability of the DFIG-based wind farms, it seems necessary to have a precise attention to aforementioned issues.