| Aming at serious load fluctuation and frequent short-circuit faults in the receiving system,the problem of voltage stability is becoming increasingly prominent,thus the safe and stable operation of power grid is facing great challenges.The variable speed pumped storage unit located in receiving power grid has large capacity,fast speed regulation and can operation in four quadrants.It can be used as a dynamic reactive power supply to flexibly participate in the voltage and phase regulation.In this paper,the basic theory of reactive power characteristics of doubly-fed variable-speed pumped storage units is studied,and the theoretical analysis and simulation verification for improving the voltage stability of the receiving system with grid-connected units are carried out.Firstly,on the basis of operation principle and control scheme of DFASPSU,this paper established the mathematical model of pump turbine,the electromechanical transient mathematical model of DFIG and the control model of converter and governor.Then the dynamic change process between pump turbine and unit is analyzed.Based on MATLAB/Simulink,the operation characteristics of the unit and the grid as the speed,grid voltage and frequency change are analyzed and studied.The electromechanical transient characteristics and the characteristics of frequency modulation,speed regulation and voltage regulation of the system are verified.This paper proposed a novel method to analyze the stator side reactive power characteristics of DFASPSU,and the influence of reactive power compensation of DFASPSU on transient voltage stability of receiving grid is discussed respectively when load fluctuation and three-phase short-circuit fault occur.According to the mathematical model of DFIG,the phasor relation of each electrical quantity under different operating conditions is analyzed,and the V-curve of stator current varying with rotor voltage is obtained.The characteristics of reactive power under different modes are analyzed based on V-curve,and the effect of slip on reactive power output is discussed.Finally,based on MATLAB/Simulink platform,the receiving system is built.The transient voltage changes of the receiving system are simulated by setting up three-phase short-circuit fault and abrupt load change respectively.The transient voltage changes of the receiving system before and after the DFASPSU connected to the power grid are compared.The simulation results show that the V-curve method is effective in analyzing the reactive power characteristics of DFASPSU,and the reactive power compensation of variable speed pumped storage units plays an active role in improving the transient voltage stability of the receiving power grid. |
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