Long-term Voltage Stability Analysis And Coordinated Control For AC/DC Power Systems

With the increase of load demands and the expanding of the power system scale,the operating state of the AC system gradually approaches its stable operating boundary.Due to the advantages of long-distance transmission of bulk power and connection to asynchronous power grids,the HVDC technology is widely used in the development of AC/DC hybrid power systems.Large blackouts caused by long-term voltage instability have occurred in recent years.Long-term voltage stability problem driven by slow dynamic characteristics such as load recovery during the system failure has attracted more and more attention recently.HVDC transmission systems have a great impact on the stability of the whole system while transmitting a large amount of power to the receiving power system.The interaction between the AC and DC systems under the fault may result in potential instability problems of the AC/DC power system.The voltage instability is mainly caused by the lack of reactive power.In the process of commutation,the DC converter absorbs a large amount of reactive power.When the long-term voltage instability occurs,the reactive power compensation from the capacitor of the converter station will decrease with the drop of the bus voltage.The AC system may need to provide reactive power from other areas to the converter station,which may degrade the system voltage.Therefore,it is of great significance to study the voltage stability evaluation method and coordinated control strategy of AC/DC systems under fault and load disturbance.For the AC/DC system with LCC-HVDC.the main research work of this paper is as follows:(1)Based on the theory of maximum transmission power,an online evaluation index for voltage stability of AC/DC systems is established.The formulation of the index combines the synchronous measurement data provided by the wide-area measurement system,which makes the indicator suitable for the on-line voltage stability assessment of weak buses.According to the critical condition of the voltage stability index under the maximum transmission power operating point,the system power margin is quickly predicted.The prediction value of power margin can provide real-time control basis for voltage control means such as DC modulation,machine and load shedding.The simulation of different scale systems verifies the effectiveness of the voltage stability index proposed in this paper.(2)Based on the model predictive control(MPC)theory,a coordinated voltage control method is proposed for mid-long term voltage stability of AC/DC power systems considering the DC modulation.The influence of DC system control on the voltage stability of AC/DC systems is analyzed.Based on the trajectory sensitivity,a receding horizon optimization model of MPC is established for long-term voltage stability coordination control of AC/DC system.To improve the efficiency of MPC and reduce the load shedding,the number and the location of multiple AC and DC control means are adaptively determined by using the voltage predictive trajectory.After bus voltages have recovered with the control,the system voltage stability margin is corrected according to the index proposed in this paper..(3)The voltage stability and control strategy of the AC/DC system with hierarchical connection mode is studied.The voltage stability index of the AC/DC system with traditional connection mode proposed in this paper is extended to the AC/DC system with hierarchical connection mode.The analytical sensitivity expression of the bus voltage to the power of high-level and low-level inverter is deduced.A coordinated voltage control method is proposed for mid-long term voltage stability of AC/DC power systems with hierarchical connection mode.The simulation results of Shandong power grid show that the proposed method can effectively coordinate multiple AC and DC control means.