Modelling And Control Of Doubly Fed Variable-Speed Pumped Storage Units For Grid Frequency Regulation

Pumped storage power station is an important means to solve the problem of peak load regulation,ensure the safety of power grid operation,and promote the economic operation of various types of power sources.The doubly fed variable-speed pumped storage(DFVSPS)system adopts doubly fed induction generator(DFIG)to replace the synchronous machine used in the traditional pumped storage.The stator of the DFIG is connected to the power grid,and the three-phase excitation windings are symmetrically distributed on the rotor,and the excitation current is supplied by the converter.The active power and reactive power of the unit can be quickly adjusted by adjusting the amplitude,frequency and phase of the rotor side voltage or current through the converter.What's more,the efficiency of the unit can be improved,and the stability of the system can be enhanced.Compared with conventional hydropower stations,pumped storage power stations have the characteristics of various operating modes and frequent start-up and shutdown.In the power system,pumped storage power station not only operates as a power source,but also as a load.At the same time,it also needs to switch from one working mode to another.The switching process of pumped storage power station is of great significance to the study of whether the pumped storage power station can operate safely,reliably and efficiently when it is connected to the grid.Firstly,this paper introduces the structure and principle of the DFVSPS unit.The mathematical model of the unit is established,including model of DFIG,model of pump-turbine,model of converter and its control,The two different control strategies for the unit model are proposed,which are fast power control strategy and fast speed control strategy.The model of DFVSPS unit of 300MW is established in MATLAB/Simulink,and the response characteristics in generating mode and pumping mode are studied respectively.Secondly,the process of mode switching is analyzed,which from start-up stage,load increasing stage,steady operation stage and shutdown stage of generating mode to that of pumping mode.The process of mode switching is simulated by using MATLAB/Simulink.According to the characteristics of the switching process of the unit,the output power model for mode switching process of the unit is established.Finally,the effect of doubly-fed variable speed pumped storage units on compensation of power fluctuation and frequency control in power grid are studied.The compensation characteristics of the unit at steady operation conditions are analyzed by using Bode diagram method.The spectrum characteristics of unit's output are obtained by using Fourier transform method,and the compensation of power fluctuation in power grid during the mode switching process of unit is analyzed.And the effect of units on frequency control of power grid is studied through simulation examples.