Research On New Grid-connected Start-up Method And Reactive Power Regulation Capacity Of Doubly-fed Pumped Storage Unit

With the penetration rate of new energy power generation and DC transmission increasing year by year,the introduction of pumped storage can effectively promote the economic operation of all kinds of new energy connected to the grid.The doubly-fed pumped storage unit uses a low capacity converter for AC excitation at the rotor side,which realizes the variable speed performance of the motor under pumping conditions,and can improve the efficiency of the unit.In pumping condition,due to the low capacity of the rotor side of the converter will limit the starting torque,according to the traditional stator short-circuit starting mode is slow,according to the stator side of the grid-connected rotor side frequency conversion starting mode will make the stator current increase,so this paper seeks a new starting mode to integrate the speed increase and the current is too large,it is of practical value.In addition,the introduction of new energy and direct current transmission also puts forward higher requirements for reactive power regulation of units in pumped storage power stations.The traditional synchronous motor limits its capability of progressive operation due to stator end heating and static stability problems,while the reactive power adjustable range of doubly-fed motor is wider,which can not only make the reactive power regulation ability of the grid more efficient and flexible,but also benefit the voltage stability of the grid.Therefore,it is of great significance to study the reactive power regulation capacity and active power operating boundary value of DFPSU under different operating conditions and slip rates.Based on the basic principle of doubly-fed induction motor and vector control theory,a mathematical model of stator flux orientation is established in this paper.In order to solve the grid-connected startup problem of DFPSU,a three-stage startup control strategy is proposed based on the short-circuit startup of the stator windings of the doubly-fed motor.The strategy can reduce the stator current of the initial startup and control the rotor voltage of the weak magnetic suppression in the later stage to avoid overmodulation of the rotor inverter.Secondly,the stator side is directly or indirectly connected to the large power grid,and the rotor side is flexibly connected to the power grid through the AC-DC-AC converter and the corresponding control method.Based on the research of single unit with crowbar auxiliary circuit,a new method of stator side step-down grid-connected rotor side start-up is finally determined.This method not only reduces the constant rotor current amplitude at the beginning of startup,but also reduces the initial rotor side DC voltage amplitude and the steady-state value after startup,which is more conducive to the selection of rotor side inverter.Finally,a model of DFPSU is built in Matlab/Simulink platform,and the feasibility of the new variable frequency startup control strategy is successfully verified by simulation.In order to explore the reactive power capacity and the overall power limit operating boundary of DFPSU in late phase and advance phase operation,the Γequivalent circuit for power analysis is used to recombine the fixed rotor power and loss power of the unit,and the main factors limiting the power limit of the unit are obtained.According to the theoretical derivation and the actual parameters of DFPSU unit,the P-Q circle diagram with the limit of stator current and rotor current as the boundary is first drawn.Combined with the phasor diagram of the motor under different working conditions,the relationship between the slip s and rotor voltage limit circle diagram is analyzed.Finally,the empirical simulation finds that when the slip rate is large,The rotor voltage limit will be the key factor to limit the range of reactive power regulation.