Researsh On Control Of UPFC For Enhancement Of Voltage Stability

The problem of voltage stability of power system has been the focus of electric power researchers all around the world, and the problem of how to improve the voltage stability of the power system has been the research object of electric power researchers. After the emergence of flexible AC transmission technology, the staff of all over the world have carried out a lot of research, UPFC as the most representative of the FACTS equipment has also got more and more people’s attention. UPFC has fast and flexible reactive power control capability, and can support the voltage of access point dynamically. It plays a very important role in preventing voltage instability, so it is important to study how to use UPFC to improve the voltage stability of power system. This paper mainly studies the control strategy of UPFC to improve the voltage stability of the system. The specific contents are as follows:(1) The structure, working principle, control characteristics and working mode of UPFC are analyzed in this paper. A three node power injection model of UPFC is proposed in this paper for the special structure of the UPFC parallel converter transformer in China’s first UPFC project of Nanjing UPFC project, which is connected to the low voltage bus of the YanZIji Station. The mathematical expression of the UPFC three node power injection model is derived by using the power injection method. Using the user defined (UD) function of power system analysis software package (PSASP), the power flow model and the dynamic model are set up for the system level analysis. The correctness and effectiveness are verified by simulation.(2) Four commonly used control mode of UPFC:constant power control, angle regulator control, impedance compensation control, voltage regulator control were studied. Through a lot of simulation computing in PSASP, the relationship between parameters of the controller and control target in every kind of control mode and voltage stability of power system is discussed. The impact of the four control mode on voltage stability is concluded. At last, the ability of the four control modes of UPFC to enhance voltage stability are compared. The results show that the impedance compensation control is most favorable for improving the system voltage stability.(3) For the small disturbance, an improved constant power control of UPFC is proposed. For the large disturbance, based on the improved constant power control, a voltage stability control strategy based on control mode switching is proposed, which can improve the voltage stability of power system when UPFC regulating power flow. The effectiveness and correctness of voltage stability control strategy of UPFC was validated respectively in three machine nine node system and Nanjing Xi Huan network system. Compared to the constant power control of UPFC, results show that using the voltage stability control strategy of UPFC is more favorable on the system voltage stability, and can achieve the similar ability of constant power control in active power flow adjustment.