| Power system subsynchronous oscillation has been endangering the safe and stable operation of power system. In recent years, the FACTS devices are used more and more widely in power system. On the one hand, the inappropriate control or running status will cause subsynchronous oscillation. On the other hand, the strategies of using FACTS devices to damp subsynchronous oscillation also emerge in endlessly. Among all the FACTS devices based on converter, the unified power flow controller (UPFC) is the most powerful one. It not only can control the local bus voltage and transmission line trend, but also can control circuit impedance and voltage angle, so that keeps the power system in an optimal working state. In this paper, the subsynchronous oscillation characteristics of UPFC system is deeply studied, and the effects of UPFC on subsynchronous oscillation are analyzed, then the UPFC controller to restrain subsynchronous oscillation is designed.First of all, this paper introduces the basic theory of the analysis of the power system subsynchronous oscillation, and also summarizes various rejection strategies as well as the application of SSO, and then the research status of UPFC is introduced in detail. An equivalent model of UPFC is established for the analysis of subsynchronous oscillation, and the accuracy of the model is verified by the test signal method. Based on IEEE second standard model, the electrical system complex torque coefficient expressions are deduced from the linear dynamic mathematical model of whole system including UPFC by complex torque approach, which pointed out that UPFC can improve the electrical damping of series compensation capacitor system. Then the paper explains the mechanism of the effect of UPFC on SSO through the calculation of the subsynchronous frequency impedance of the whole system, and points out that UPFC can affect subsynchronous characteristics by changing subsynchronous impedance of line. Meanwhile, an equivalent impedance model of UPFC is established to qualitatively analyze the effect of different operating conditions on the electrical damping, which points out that the active power control of UPFC is of the greatest impact on the system damping characteristics compared to reactive power control and voltage control.Secondly, the precise electromagnetic transient simulation model of the IEEE standard model and UPFC are built in the MATLAB/Simulink simulation platform. The test signal method is used to analyze the effects on subsynchronous characteristics of placement of UPFC, power flow control, voltage control and system compensation degree, which points out that the UPFC shunt side voltage control has almost no influence on subsynchronous oscillation; The series side flow control has a certain influence on subsynchronous oscillation. And the influence of active control is more than the reactive power control. Above results verify the conclusions from the qualitatively analyzing of equivalent impedance model of UPFC. At the same time, the Nanjing network system including UPFC is built to calculate the electrical damping curve through simulation. And then the paper analyzes the subsynchronous characteristics and gets the oscillation curves of the turbine generator shaft. The results show that UPFC will not cause subsynchronous oscillation to the two power plants, but will improve the electrical damping of Xidu power plant.Finally, the mechanism of additional damping control rejecting subsynchronous oscillation is analyzed by complex torque coefficient method. And then the paper designs the additional damping controller of UPFC shunt side and series side respectively. The effects of the two damping controllers are verified through a numerical example. Then, the paper puts forward a strategy of coordinated control of two damping controller to suppress subsynchronous oscillation. And the genetic algorithm is used to optimize the controller parameters. The example analysis proves that the coordination of UPFC on damping controller can enhance the ability to suppress SSO. Then the oscillation of the turbine generator shaft system under various suppression strategies is obtained by the time domain simulation method, which verifies the effectiveness of the designed control strategies. |
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