Research On The Optimization Of The Start-up And Grid Connection And Excitation Control Strategy Of A New Type Of Dimming Camera

With the development of high voltage direct current(HVDC)technology,China has built the highest voltage level and the largest AC-DC hybrid power grid in the world.Thanks to the merit of longer transmission distance,larger capacity and lower power loss,HVDC transmission system has become a dominant way of electrical transmission in China.At the same time,the voltage instability caused by the lack of system reactive power with HVDC transmission has become increasingly prominent.As a large-scale synchronous reactive power compensation device,the synchronous condenser is widely installed in various converter stations due to its overload capacity and wide compensation characteristics,providing reactive and voltage support for the system.This thesis focuses on the 300 MVar new type synchronous condenser from three perspective,namely startup,grid-connection and excitation control of the new type synchronous condenser and proposes corresponding optimization strategies.(1)In view of the compatibility between the reliability and efficiency of the startup process,a new synchronous condenser start-up control strategy optimization method for segmented control of commutation advanced angle is proposed based on the variable frequency start-up method of synchronous condenser.To avoid the thyristor redundant actions,and the concept of seamless switching angle is proposed.The results show that the proposed start-up control strategy optimization method can improve the startup efficiency of the new condenser.Furthermore,the reliability of startup process is improved with the thyristor commutation failure avoided.(2)The influence of the exciting current on the speed of freewheel speed is summarized based on the new type synchronous condenser freewheel grid-connection mode,combined with various motor losses in the process of no-load freewheeling process.According to the variation of frequency difference and phase angle difference,the key parameter is extracted as the initial phase angle difference,and the influence of the initial phase angle difference on the success of grid connection is obtained.Finally,a new grid-connection strategy with the combination of excitation speed adjustment and idle speed point setting is proposed for improving the success rate of grid connection.Through statistical experiments,it is verified that the new grid-connection strategy can significantly improve the success rate of the new type of synchronous condenser freewheel grid-connection mode.(3)A third-order mathematical model suitable for the new type synchronous condenser is established according to its operating characteristics,based on the excitation control method of the synchronous generator.A new nonlinear excitation control strategy is established combing PI control with the differential geometry theory which uses power angle as the output function.According to the characteristic of stable operating power angle of "0",the steady-state PI control is introduced,and a new hybrid synchronous condenser excitation control method combining a small-disturbance PI control with a large-distortion differential geometry control is proposed.The simulation results indicate that with the new hybrid synchronous condenser excitation control method,the transient impact is smaller,and the transient time is shorter than the PI control under the system short-circuit fault.