Research on Characteristics of Equivalent Sequence Impedance of Doubly Fed Induction Generator Wind Turbine

Abstract

Wind generation is widely utilized today as a promising form. As the major type of wind turbines, doubly fed induction generator (DFIG) wind turbines have been widely used. Now great attention is paid to the relay protection of power systems integrated with wind power of large capacity. Based on fault analysis, the equivalent sequence impedance is a key factor of protection strategy. In this paper, the model of DFIG wind turbines is built in PSCAD/EMTDC software. Then the characteristics of sequence impedance are calculated and analyzed at length. And the relay protection method is discussed as the conclusion. Wind generation is widely utilized today due to its renewable characteristics and environmental friendliness. Along with the enlargement of capacity of wind power, its integration affects both secure operation of power grid and correct action of relay protection evidently. Now great attention is paid to the secure protection of wind power increasingly. As the major type of wind turbines, doubly fed induction generator (DFIG) wind turbines have been widely used. The analysis of fault characteristics of DFIG wind turbines is the premise of the protection technology. Furthermore, the sequence impedance is a key factor affecting the relay protection strategy. The wind farm belongs to typical weak power-feed system. Due to the control strategy, the positive and negative sequence impedances of wind farm are not equal, which directly affects the protection elements. And the zero sequence impedance also plays an important role in ground fault protection. The fault current felt by protections of wind farm is almost the total zero-sequence component of the fault current. So it is crucial to analyze the sequence impedance. Several promising methods for FRT based on crowbar protection circuit have been provided, and some FRT control strategies considering rotor excitation control have been developed in (1-5). Some experts focus on the transient characteristics of wind turbines and its impacts on transient stability of power system (6-7). However, rare papers consider the sequence impedance characteristics of DFIG, which is a crucial element to the protection strategy of wind turbine. Without the analysis of sequence impedance, the influence factors of fault currents are unclear. In this paper, the electromagnetic transient model of DFIG wind turbine system is modelling in PSCAD, including the wind turbine, the DFIG and its control system. And the simulation of four types of fault are made and compared. The characteristics of sequence impedance, including the positive, negative and zero sequence impedances are calculated based on symmetrical component method. And the relay protection strategy is discussed as the conclusion.