System Voltage And Frequency Stability Analysis And Control With UHVDC Feed In

To satisfied the transmission needs of large-scale energy resources and change the energy allocation from partial balance to large-scale optimization in China,UHV grid has begun to construct extensively.Large capacity UHVDC project is connected to AC power system,which makes the system security and stability characteristics,grid structure and system operation control increasingly complex.As a result,the security and stability risk caused by DC fault is gradually increase.In this context,this thesis studies the issues of characteristics of voltage and frequency response of AC system after DC block fault,reactive power coordinated optimization control strategy to improve the voltage stability of AC system and the criterion for distributed active resources participating in frequency modulation after DC block.The main work includes the following three parts:The voltage response characteristics of AC receiving system after DC block fault is analyzed,the effect of different control measures on improving system voltage stability is studied.Then the frequency response characteristics of AC system after DC block fault and the possible risk the fault may cause are analyzed,and the frequency response characteristics of nodes on different voltage levels and different spatial distribution under several load types and load proportions are observed.Simulation results shows that DC block fault can lead to voltage instability for weak AC system,DC block fault will also cause frequency fluctuation and drop dramatically.The frequency response characteristics of nodes with different spatial distribution and voltage level are slightly different under different load types.Aiming at the voltage stability problems of the AC system,a sensitivity based voltage coordinated optimized control strategy is proposed,which considers the voltage control effect of synchronous condensers,capacitors,and reactors.The implementation of the control scheme has two types: voltage preventive control before the fault and voltage emergency control after the fault.The simulation results show that the strategy can effectively increase the voltage stability of the receiving-end system.While improving the system dynamic reactive power support capability,it also improves the overall AC grid voltage level after the fault.Aiming at the frequency stability problems of the AC system,a trigger criterion for high-speed distributed resources response to DC block faults at the terminal of distribution network is proposed.The Hausdorff algorithm is used to obtain the voltage characteristics after DC block.Combined with the frequency variation characteristics,the DC block can be quickly identified.The simulation results show that the criterion can help to trigger distributed resources to deal with serious faults before the primary and secondary frequency control finished,which offer a powerful power support for the AC system after DC block fault.