| The variable-speed pumped-storage unit with full-size converter has a fast power response characteristic.It can quickly smooth the power fluctuation of new energy,improve the capacity of new energy absorption,and ensure the quality of power supply.Therefore,that makes it full of importance in research value.However,the pumpedstorage unit with full-size converter,the generator decoupled from the grid,does not have the ability to suppress and improve grid disturbances.That affects the stability of the system.Variable-speed pumped-storage unit with full-size converter using Virtual Synchronous Generator(VSG)technology has the same external operating characteristics as synchronous generator,such as inertia,damping characteristics,active power frequency regulation,and reactive power voltage regulation.And these characteristics will contribute to the friendly grid connection.This article focuses on the research on the VSG technology used in variable-speed pumped-storage unit with full-size converter,and it includes the following aspects:(1)Research on coordinated controlling strategies between the source and grid side around the mathematical model of variable-speed pumped-storage unit with a full-size converter.Firstly,the mathematical model of the pump-turbine of the pumped-storage unit is given,and the speed regulation system of the pump-turbine is established.Secondly,the mathematical model of the synchronous motor is established,the principle of flux linkage observation of the synchronous motor is analyzed,and the voltage and current double closed-loop controlling strategy of the generator-side converter based on the air gap flux orientation is given.Then the VSG controlling strategy of the grid-side converter is studied,and the controlling principle,modeling process and operating characteristics of the VSG are analyzed in detail.The controlling strategies of the above three parts have been simulated to verify their correctness.Finally,on the basis of the variable-speed pumped-storage unit model,source-grid coordinated controlling strategies are proposed.Using these strategies make the variable-speed pumped-storage unit have the characteristics of rapid power response and automatic response to the grid frequency,thereby supporting the grid inertia and frequency,and verifying it through simulation.(2)With the research on the controlling of pumped-storage unit under unbalanced grid,this paper proposes a VSG controlling strategy based on adaptive virtual admittance.First of all,this paper analyzes the problems of frequency-doubled oscillation of active and reactive power,and unbalanced current under unbalanced grid of the pumped storage unit.Secondly,this paper analyzes the mathematical model of the VSG under unbalanced grid,and then sets the negative sequence current controlling commands of the adaptive virtual admittance with the goal of suppressing active oscillation and reactive oscillation,and balancing grid-connected current.Then,a method for VSG controlling under the positive sequence coordinate system and the negative sequence current controlling method using adaptive virtual admittance under the negative sequence coordinate system is proposed.Finally,the method is verified by simulation,which ensures the reliability of the VSG’s operation and switches among the three controlling targets flexibly at the same time.(3)This paper studies the low voltage ride-through(LVRT)of pumped-storage unit,and proposes a VSG-based LVRT controlling strategy for variable-speed pumped-storage unit with a full-size converter.Firstly,it analyzes the LVRT characteristics of the pumpedstorage unit,and points out that it cannot compensate the system for reactive power problems during the low voltage ride through period.Secondly,the principle of the LVRT of the VSG of the grid-side converter is explained,and the LVRT method of VSG controlling strategy switching is proposed.Then the machine-side converter adopts a constant torque controlling method during the low voltage ride-through period.The unbalanced energy generated on the machine-grid side is unloaded by the way of using DC side Chopper protection circuit.Finally,the method is verified by simulation,which improves the low voltage ride-through capability of the variable-speed pumped-storage unit. |
