Methods of DTA to estimate the impact of integration of DFIG on single-machine infinite-bus power system

The linearized model of single-machine infinite-bus power system integrated with DFIG is derived. According to the linearized model, the figure of Phillips-Heffron model can be drawn. For single machine infinite bus system with DFIG integrated, the damping torque of electromechanical circuit of synchronous generator(SG) comes from two parts: one part is synchronous generator's excitation winding and automatic voltage controller(AVR), the other part is DFIG. The first part represents the synchronous generator's excitation's effect on the rotor motion equation; the other part represents the effect of dynamic interaction between DFIG and synchronous generator, which is directly reflected in the exchange of dynamic power between DFIG and synchronous generator intransient process. The damping torque to the electromechanical oscillation loop comes from excitation of synchronous generator and DFIG individually can be derived using the four different methods of damping torque analysis (DTA). Four types of DTA methods include direct, indirect and two simplified methods. The direct method calculates the damping torque provides by synchronous machine excitation and DFIG respectively according to the Phillips-Heffron model figure of single-machine infinite bus system with DFIG connected. The indirect method calculates the damping torque from DFIG using the principle of superimposition of linear systems. that is, total damping torque minus damping torque provides by synchronous machine excitation. In two simplified methods, only direct damping torque from wind farm to the synchronous machine electromechanical circuit is considered, Since the time delay in the signal path forming indirect damping torque weakens the signal strength, indirect damping torque is ignored. Finally, an example is conducted for validation. Results of the first two methods obtained are identical; results of two simplified methods are close to results of integrity algorithms, which verified the reasonableness of simplification. The conclusion of this paper has a certain practical significance to the long-distance transmission of bundled wind and fire power.