| With the rapid development of UHVAC/UHVDC power grids,China has built a hybrid AC/DC power system with the largest transmission capacity,the highest transmission voltage level,and multiple regional power grids in the world.The problem of system stability and overvoltage will be caused by serious faults.How to accurately simulate the characteristics of AC/DC power grid is also a challenge to simulation technology.In order to improve the security and stability of AC/DC hybrid power grids,this paper proposes a new technology to improve system power angle stability,analyzes the overvoltage level of UHVDC transmission systems and proposes suppression measures,and a method for establishing a large-scale AC/DC digital-analog hybrid simulation platform is proposed.The main work is as follows:(1)A novel phase sequence exchange(PSE)principle and technology based on modular multilevel converters(MMC)is proposed.The designed topology can operate in both back-to-back DC mode and AC connection mode.A phase sequence exchange circuit based on MMC is designed.The circuit consists of 12 bridge arms cascaded by submodules.In the case of normal AC power transmission,A-a,B-b,and C-c are connected in three phases.When the generator accelerates out of step and the power angle increases,the PSE circuit operates in back to back VSC-HVDC transmission mode based on MMC.After the phase sequence exchange is completed,A-c,B-a,and C-b are connected in three phases to achieve phase sequence transformation of 120°,thereby reducing the power angle of the generator by 120°.The flexible and adjustable deceleration area is realized and the power angle stability and margin of power system are greatly improved.(2)An optimal control method for MMC-based PSE is proposed.The parameters of the PSE are optimized and the optimal PSE angle is compared.The power grid can enter a stable state directly after the PSE.The DC power,voltage and switching time in the process of PSE are analyzed theoretically,and the optimization method of parameters is proposed.The PSE system can be transformed when the deceleration area is equal to the acceleration area,the generator speed is synchronous,and the phase is 120° ahead.The generator can directly enter the stable state after the PSE,which reduces the power oscillation process when the generator repeatedly enters the acceleration and deceleration region during the recovery period,and further improves the power angle stability of the system.In addition,the optimal control angle of the PSE is studied.The 360°commutation sequence will not destroy the original phase sequence connection mode of the system after restoring the AC connection,which can avoid other problems caused by the PSE,but the recovery stability time is longer.For the system with high requirement of recovery time,120° and 240° PSE are the priority methods of commutation sequence.Simulation verifies the effectiveness of the optimal control method.(3)Based on the wave propagation theory,the mechanism and influencing factors of overvoltage in ±1100kV UHVDC transmission lines and 1000kV converter stations are analyzed.It is clear that the comprehensive impedance at the end of the converter station is the key factor affecting overvoltage.A scheme of installing lightning arresters on DC lines to suppress overvoltage levels is proposed.The research results have been used in the design and construction of actual DC projects.Based on the wave propagation theory and electromagnetic transient calculation method,the mechanism of DC overvoltage is analyzed,and it is pointed out that the comprehensive impedance of converters and DC filters at the ends of converter stations is a key factor affecting line overvoltage.The distribution characteristics and overvoltage levels along the DC line of the Zhundong-East China ±1100kV UHVDC transmission project are given.The installation scheme of lightning arrester for suppressing line overvoltage level is proposed.In addition,the power frequency transiant overvoltage and switching overvoltage levels of the 1000kV UHVAC bus in the UHVDC converter station are studied.The operating conditions and overvoltage levels of the overvoltage under different connection schemes are given,and measures to reduce the switching overvoltage of the converter bus are proposed.(4)Using optical fiber digital protocol communication technology,a digital-analog hybrid simulation method for HVDC transmission is proposed,and an electromagnetic transient real-time simulation platform for large-scale AC/DC hybrid power grid with multiple HVDC control and protection devices is established.Based on the fully interconnected super computer,the optimal decoupling method of power grid model and secondary signal,and the optimal sub task mapping method are proposed.The steady-state operation method of electromagnetic transient model of large power grid based on coordinated control of generator parameters is proposed.A digital-analog hybrid real-time simulation model of 6000 three-phase node-level AC power grid and 8 HVDC physical control and protection model is established.The comparison between the simulation test and the actual waveform shows that the simulation technology meets the accuracy requirements of the AC/DC hybrid power grid.Based on this platform,the effectiveness of MMC-based PSE in AC/DC hybrid power grid is studied.The digital-analog hybrid simulation platform can provide a research method for the stability control technology of AC/DC hybrid power grid,and has been used in the planning and operation calculation of State Grid Corporation of China. |
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