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PLL Approach to Mitigate Symmetrical Faults in Weak AC Grid of DFIG Based Wind Turbine for Voltage Stability Analysis

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PLL Approach to Mitigate Symmetrical Faults in Weak AC Grid of DFIG Based Wind Turbine for Voltage Stability Analysis International Journal of Electrical Electronics & Computer Science Engineering Volume 5, Issue 2 (April, 2018) | E-ISSN : 2348-2273 | P-ISSN : 2454-1222 Available Online at www.ijeecse.com PLL Approach to Mitigate Symmetrical Faults in Weak AC Grid of DFIG Based Wind Turbine for Voltage Stability Analysis Cholleti Sriram1, O. Rakesh2, M. Harish3, A. Sridhar4 1Assistant Professor, 2-4UG Scholars, EEE Department, Guru Nanak Institute of Technology, Hyderabad, India [email protected], [email protected] Abstract: Study of Instability issues of the grid-connected of wind turbines (WTs) based on doubly fed induction doubly fed induction generator (DFIG) based wind turbines generators (DFIG) that intends to improve its low- (WTs) during low-voltage ride-through (LVRT) have got voltage ride through(LVRT) capability. The main little attention yet. In this paper, the small-signal behavior of objective of this work is to design an algorithm that DFIG WTs attached to weak AC grid with high impedances would enable the system to control the initial over during the period of LVRT is investigated, with special attention paid to the rotor-side converter (RSC). Firstly, currents that appear in the generator during voltage based on the studied LVRT strategy, the influence of the sags, which can damage the RSC, without tripping it. high-impedance grid is summarized as the interaction As a difference with classical solutions, based on the between phase-looked loop (PLL) and rotor current installation of crowbar circuits, this operation mode controller (RCC). As modal analysis result indicates that the permits to keep the inverter connected to the generator, underdamped poles are dominated by PLL, complex torque something that would permit the injection of power to coefficient method (CTCM), which is conventionally applied the grid during the fault, as the new grid codes demand. in power system to study the interaction between mechanical A theoretical study of the dynamical behavior of the and electrical subsystems of synchronous generator (SG), is rotor voltage is also developed, in order to show that the generalized to analyze how the PLL-RCC interaction influence the phase motion of PLL. Then the concerned voltage at the rotor terminals required for the control small-signal stability of the PLL-synchronized DFIG system strategy implementation remains under controllable can be discerned by the developed complex phase limits. In order to validate the proposed control system coefficients. Impacts of PLL’s and RCC’s parameters are simulation, results have been collected using highlighted, as well as the system’s operating conditions MATLAB/SIMULINK and experimental tests have during LVRT. Finally, the analytical result is validated by been carried out in a scaled prototype [3]. experiment. For doubly fed induction generator (DFIG)-based wind Keywords: Doubly Fed Induction Generator, Phase Locked energy conversion systems (WECSs), large Loop, LVRT, Rotor Current Controlled. electromotive force will be induced in the rotor circuit I. INTRODUCTION during grid faults. Without proper protection scheme, the rotor side of DFIG will suffer from overcurrents, This paper aims to study how the small-signal behavior which may even destroy the rotor-side converter (RSC). of DFIG WTs is affected by the control parameters as To mitigate this problem, a new flux-linkage-tracking- well as operating point during severe grid faults, with based low-voltage ride-through (LVRT) control strategy the interaction between PLL and RCC highlighted. This is proposed to suppress the short-circuit rotor current. paper mainly deals with the instability issues of the grid Under the proposed control strategy, the rotor flux connected DFIG based wind turbine during low voltage linkage is controlled to track a reduced fraction of the ride through have taken special attention. During the changing stator flux linkage by switching the control fault conditions, there is more chance of losing the algorithm of RSC during grid faults. To validate the synchronization between the wind turbine and Grid. proposed control strategy, a case study of a typical 1.5- Due to this, there occurs voltage variations i.e.,sudden MW DFIG-based WECS is carried out by simulation increase or decrease of voltage levels so that the system using the full-order model in stability can be disturbed. In this, back to back SIMULINK/SimPowerSystems. In the case study, a converters are used for DGIG where RSC and GSC are comparison with a typical LVRT method based on RSC controlled by using the RCC and PLL so that the entire control is given, and the effect of the control parameter system can be stabilized. For the controlling of these on the control performance is also investigated. Finally, pulses, PI controllers are used and also helps in the validity of the proposed method is further verified improving transient stability. by means of laboratory experiments with a scaled-size The main aim of this paper is to maintain the Voltage DFIG system [4]. stability before the fault condition and also maintaining The outline of the paper is as follows: Section II the same voltage levels after the fault conditions also by discusses about description of paper. Section III compensating the faults using PI controllers. The output discusses about MATLAB simulation theory. Section pulses from PI controllers are given to GSC and RSC IV discusses about fault analysis in power system converter and active and reactive power delivered by Section V discusses DFIG based wind turbine the machine can be controlled.This paper presents a connected to grid Section VI discusses on Model new control strategy for the rotor-side converter (RSC) 128 International Journal of Electrical Electronics & Computer Science Engineering Volume 5, Issue 2 (April, 2018) | E-ISSN : 2348-2273 | P-ISSN : 2454-1222 Available Online at www.ijeecse.com working and its operation ,simulation results Section useful, for instance, for generators used in wind VII gives outlines of future scope and concludes the turbines. paper. II. DFIG BASED WIND TURBINE Extensive research on LVRT of DFIG WTs has been focused on the significant stress issues (including overcurrent in rotor winding and over-voltage in DC bus), which is induced by the coupling between stator and rotor and the limited control voltage of rotor-side converter (RSC). However, possible stability issues during faults are not well described yet. DFIG is Fig. 1. DFIG Block Diagram directly coupled to grid and the back-to- back converter usually works with heavy dependence on information Doubly fed electrical generators are similar to AC about terminal voltage. As control loops usually play a electrical generators, but have additional features which crucial role in dynamic behavior, PLL-based vector allow them to run at speeds slightly above or below ,control, which is widely implemented in on-site LVRT their natural synchronous speed. This is useful for large strategy for DFIG WTs [5] and is the scenario variable speed wind turbines, because wind speed can considered in this paper, concerns us. The easily change suddenly. disturbed terminal voltage in high-impedance grid brings undesirable dynamics to control loops and C. High Impedance Grids: probably cause small-signal oscillatory instability to trip In general, the grid-connected inverter is tending toward DFIG WTs during faults. In addition, the oscillation instability when the grid impedance is higher. A weak out of control may trigger transient instability issues grid system[5] with high grid impedance is usually easy further at the time of grid fault clearance since transient to cause stability problems, including large harmonic stability is dependent upon initial states. Thus, secure distortion in the inverter current and the grid voltage, LVRT technologies necessitate attention to stability and the false tripping of the inverter. issues during faults when WTs are connected to high impedance grid. D. Small-Signal Stability: A. Wind power Generation: Small-signal stability analysis is about power system stability when subject to small disturbances. If power In wind power generation systems, wind drives turbines system oscillations caused by small disturbances can be that generate electricity. This process has the great suppressed, such that the deviations of system state advantage of being free from carbon dioxide (CO2) variables remain small for a long time, the power emissions. There are several types of wind turbines. system is stable. Horizontal-axis three-bladed turbines, which are the main stream for large-scale power generation, can E. Low-Voltage Ride-Through (LVRT): convert about 40% of wind energy into electricity. Low voltage ride through. In electrical power On the other hand, the principal drawback of wind engineering [fault ride through] (FRT), sometimes power generation is its volatility, that is, electricity under-voltage ride through (UVRT), is the capability of generation fluctuates and output is inconsistent owing to electric generators to stay connected in short periods of changes in wind speed and direction. Output can be lower electric network voltage (cf. voltage dip). leveled to a great extent by locating many wind turbines F. Phase-Locked Loop (PLL): in a large area[4]. If insufficient electricity is generated by wind power, other sources such as thermal or A phase-locked loop (PLL) is an electronic circuit with hydroelectric power generation can make up for the a voltage or voltage-driven oscillator that constantly shortfall. adjusts to match the frequency of an input signal. PLLs are used to generate, stabilize, modulate, demodulate, B. Doubly Fed Induction Generator (DFIG): filter or recover a signal from a "noisy" Doubly-fed electric machines are electric motors or communications channel where data has been electric generators where both the field magnet interrupted. windings and armature windings are separately III. MATLAB SIMULATION SOFTWARE connected to equipment outside the machine. By feeding adjustable frequency AC power to the field With simpowersystems, we build a model of a system windings, the magnetic field can be made to rotate, just as we would assemble a physical system.
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