A Study On Control Strategy In Power System Stability Of The Unified Power Flow Controller

This thesis treats Unified Power Flow Controller as the investigative target. A throughout study of UPFC's control system is proposed in order to satisfy the basic criterion of power system stability. The relative model and control projects are presented and are examined by simulation. The dynamic model of UPFC was constructed which considers the process of UPFC's internal modulation and converters, the process of charge and discharge of DC capacitance. UPFC's linear optimal controller with full-state feedback is designed. Many advantages of the controller are proven by simulation, such as modulating power flow of transmission line and voltage of node, maintaining the voltage stability of capacitance's DC side, stabilizing system's power-angle character and good dynamic characteristic. The analytic expression of the nonlinear control law of control variable for the UPFC is obtained by using the approach of designing the nonlinear optimal controller based on the differential geometry theory. As a result, the control strategy for UPFC including linear and nonlinear laws is obtained in order to adapt the nonlinear character of power system. Simulation results verify its efficiency in improving dynamic quality and transient stability of power systems. By synthesizing the control functions of series and shunt-side of UPFC, the synthetical UPFC controller was proposed, which is combined by main control and secondary control. The strategy can make full use of UPFC in system control and stability by optimizing the weights of each control function. By simulation, the integrated system dynamic performance is improved in both first and subsequent swing.By applying DWT and MALLAT in wavelet analysis, the oscillation signal of active power on transmission line is decomposed and reconstructed; The strategy of UPFC's secondary control based on wavelet is proposed by using the reconstructed active power as input signal. Simulation results verify the efficiency of damping subsequent swing quickly under serious power oscillation circumstance that is hard to damp by ordinary UPFC controller. The system dynamic stability is apparently enhanced.