Small Signal Stability Analysis For DFIG Integration System

Doubly-fed induction generator(DFIG)has been dominantly used for wind power generation in China because of its excellent controllability and pertinent investment.With the increasing of wind power penetration in power system,the dynamic of DFIG has shown a strong influence on the power system small signal stability considering the stochastics of wind power.Phase-locked loop(PLL)which is the key block for DFIG vector control has a direct affect to the dynamics of DFIG and further more to the integrated power system.With the promotion of the concept of "Grid friendly",the virtual inertia has been introduced to DFIG to adapt its response to power system frequency change.Both the performance of PLL and virtual inertia should be fully studied to guarantee the small signal stability of the integrated system.Followings are the main contributions of the present thesis:Considering different PLL strategies for DFIG,characteristic roots analysis is employed for the studies of low frequency oscillation mode with stator voltage oriented control and stator flux oriented control.The relationship between control parameters and oscillation mode has been established.Numerical simulation verifies the theoretical study.Based on the small signal model of DFIG integrated power system,the small signal stability is analyzed considering the virtual inertia ratio and PI parameters.It has been shown that the smaller PI parameters for PLL,the smaller effect of inertia to the inter-section oscillation that the system with higher dumping ratio which is opposite with no virtual inertia included.The reasonability has been verified by numerical simulation.The high order for the detailed DFIG integrated power system can obscure the key performance of the dynamic system.Order reduction technique has been proposed to reduce the order of the small signal model.The time constants are analyzed according to different state variable transients.A 3-order concise model has been built with keeping the dynamics of PLL and virtual inertia control.Numerical simulation illustrates the similar model performance but a great reduction of calculation burden.