Research On Coordinated Control Strategy Of UPFC For Transient Stability Enhancement

With the development for power grid stepped in a new stage of large-scale power grid,ultra-high voltage and long distance transmission,the grid has a higher demand for safety,stability,efficiency and flexible operation control.As the most typical integrated third-generation FACTS(Flexible AC transmission system)device,UPFC(Unified power flow controller)has flexible and rapid control capability for power grid,so that the research on UPFC power flow control and stability control have attracted more and more attention.With the successful commissioning of two UPFC pilot projects in Nanjing and Suzhou in China,UPFC has played a great role in the power flow control.Considering novel topology structure of practical project,it is of great significance to research on the effects of UPFC on transient stability and to formulate a practical coordinated control strategy.The thesis has established a novel UPFC model considering actual topology,and the influence of UPFC on the transient stability of actual large-scale power grid is analyzed considering multiple control modes of UPFC,then a UPFC transient stability coordinated control strategy considering system static security is proposed.The specific research contents are as follows:(1)According to the topology structure of actual project,a five-terminal UPFC model is proposed,which series part can control the power flow of double-circuit line and parallel node can be flexibly connected.The UPFC steady state model is established based on the power injection method and the mathematical expressions of the power injection model are derived.The UPFC dynamic model is established considering the process of UPFC DC capacitance.Based on PSASP/UD,the five-terminal UPFC steady state model and dynamic model are established.The normal operation state and the series line N-1 fault in Nanjing Project are simulated and the accuracy and flexibility of the model are verified.(2)The principle of UPFC four control modes for power system transient stability study is discussed,and the UPFC dynamic model with optional contol modes is established.Taking(Critical clearing time)CCT and generator rotor relative angle as criteria of transient stability level,the influence of UPFC on transient stability of actual large-scale power grid has been studied.Further,the influence of different control objectives corresponding to its control mode on the transient stability level of power grid is studied,and the rule of influence with different UPFC control modes is summarized.The result shows that UPFC constant power control mode and impedance compensation mode are more effective in improving the transient stability level of actual power grid.(3)A comprehensive evaluation index to measure the static security and transient stability level of power system is proposed,and a UPFC transient stability coordinated control strategy considering the static security of the system is proposed based on the comprehensive index.First,according to the demand of power flow under N-1 failure,the range of control targets under four control modes are determinged by inversion formulae;then,the target values are optimized under different control modes;finally,the control target value and control mode are optimized both and coordinated control of system static and transient stability is realized.The simulation results in Jiangsu power grid shows that the UPFC coordinated control strategy can simultaneously improve the static stability level and transient stability level.