Research On Control Strategy Of DFIG Based On Small Signal Stability Analysis

With the increased wind power installed capacity and higher penetration of wind power in the power system, challenges arises for the safety and stable operation of the power system and the wind power equipment. As the most widely used generator in the wind power system, the stability of doubly fed induction generator(DFIG) should be paid enough attention. This paper focuses on the stability analysis of DFIG, including modeling the equivalent input admittance of DFIG, small signal stability analysis for different grid conditions and the auxiliary controller design based on virtual inductance.Developing the input admittance model of DFIG is the key point of the research. The modeling is carried out in synchronous rotating frame in order to obtain the steady operation point. This paper firstly derives inherent input admittance matrix of the DFIG power stage based on the mathematical model of DFIG in synchronous rotating frame. The DFIG converter control system is also considered to develop the equivalent DFIG input admittance model introduced by control system. The linearized small signal model of PLL, inner current loop and outer power loop are deduced. Combined with the inherent admittance model, the complete DFIG input admittance model is derived, including the internal electromagnetic transients and the control loops.Based on the derived equivalent DFIG admittance model and the grid impedance model, small signal stability analysis is carried out according to the generalized Nyquist stability criterion(GNC). In synchronous rotating frame, the grid side of the DFIG wind power system is expressed in the form of voltage source with impedance matrix, while the DFIG side is expressed in the form of current source with admittance matrix. The small signal stability of the wind power system can be predicted with the characteristic loci of the return ratio matrix. Impact of the control system parameter variations and system operation point on the system stability is analyzed under weak grid and series-compensated grid condition.The auxiliary controller is proposed based on virtual inductance in view of the stability problems existed under weak grid condition. The proposed controller is easy for implementation and requires no extra current or voltage sensors. It can improve the small signal stability of the DFIG system without changing the PI parameters of the original control system.The validity of the theoretical analysis and the effectiveness proposed controller is verified on the DFIG experiment platform in the lab.