Research On Voltage Stability Of AC/DC Power System

High voltage direct current(HVDC) presents the characteristics of high transimission capacity, small power loss, power controllability and improving system stability, which is widely used in power system of recent years. With interconnecting DC transimission lines, the previous system will be significantly affected with complicated dynamic behavior. During certain faults, the DC transimission lines may induce commutation failure of converters. The restoration of commutation failure needs more consumption of reactive power, which worsen the system reactive power capacity and leads to the voltage stability of receiving-end power system.In this paper, researches on the transient voltage stability of AC/DC power grid have been carried on. The criteria of transient voltage stability are determined based on the voltage stability principle and practical operating mode of Shandong power grid. Based on analyzing DC system voltage equations, research method on voltage stability is proposed with considering short circuit ratio. The characteristics of load modeling are given in detail, and the corresponding transient voltage stability is discussed.During monopole operation of Yindong DC transimission lines, transient voltage stability of Shandong power grid under both ac and dc faults is analyzed and the weak voltage area are determined. Based on the give criteria, the transient voltage stability of power system is maintained under500kV transimission line faults,220kV line faults and single generator faults in Tsingdao power grid. Second, the converter faults and block faults of Yindong DC lines are simulated. Without the reactive power compensation of Yindong DC lines, bus voltages close to the inverter station declines with maintaining the system stability. With the block fault and cutting the reactive power compensation equipment, power oscillation happens in the interconnecting lines of Shandong power grid and bus voltage fluctuation is obvious in500kV stations of Tsingdao area.The principle and modeling of STATCOM is analyzed. The influence of STATCOM on voltage stability is presented under certain faults. The results illustrate that with installing the STATCOM equipment in35kV bus of Lishan station, the low-voltage issues is improved under the fault with cutting the#1and#3generation units of Tsingdao power plant. When the corresponding bus voltage declines, the more reactive power is giver by the STATCOM, which is good to the transient voltage stability.