Research On Harmonics And Its Suppression Measures Of Series Multi-terminal HVDC Transmission Systems

With the rapid development of China's economy,energy demand has further increased.However,due to the serious reverse distribution of energy centers and load centers,large-capacity,long-distance transmission is required only through HVDC transmission.Multi-terminal DC transmission can solve the problem of multi-supply point feeding and receiving multi-point feeding.It has the advantages of high flexibility,high reliability and strong economy.Both the Line Commutation Converter(LCC)and the Voltage Source Converter(VSC)are developing multi-terminal DC transmission.LCC has large capacity and low cost,and some multi-end projects can upgrade and upgrade existing double-end LCC projects.Therefore,the research and design of multi-terminal LCC direct current transmission system characteristics is an important part of the development of direct current transmission.Among them,harmonic analysis and suppression measures are the basic content of multi-terminal DC transmission engineering design and operation.Therefore,this paper is of great significance for the research of harmonics and suppression measures of LCC-based Series Multi-terminal Direct Current(S-MTDC)transmission systems.This paper first introduces the S-MTDC transmission system,including its structural topology,working principle,operation mode and control method.Operating modes include wiring,DC voltage levels,and constraints.The control method includes the control mode of each converter station system and the mutual cooperation between the stations;then the characteristic subharmonic and non-characteristic subharmonic generation mechanism of S-MTDC transmission system is analyzed,and the 12-pulse exchange is analyzed.The harmonic model of the flow device is studied.The mathematical model of the non-characteristic harmonics of the 12-pulse converter is established by using the modulation theory.and the transformation characteristics of the AC side and the DC side of the 12-pulse converter are obtained.Considering the system asymmetry and distortion,the DC system is equivalent to the improved 3 pulsation model,and the advantaues of the improved 3 pulsation are verified by examples to accurately reflect the asymmetry factors,such as the commutation inductance deviation,the commutation angle deviation and the commutation angle in the AC system.The effects of background harmonics,compared to the conventional 3-pulse harmonic model,can more accurately calculate the DC-side harmonics of the converter.Finally,the DC filter system of S-MTDC transmission system is studied.The configuration method of the neutral point impact capacitor and DC filter in the S-MTDC transmission system is discussed by PSCAD/EMTDC simulation results.It can be concluded that in the S-MTDC transmission system,a neutral point impact capacitor is required,and each converter station needs to be equipped with a DC filter,and it is not possible to suppress the harmonic current through a certain station.Reduce the harmonic current of the entire system to an acceptable level.The method effectively suppresses the harmonic current in the S-MTDC transmission system,thereby making the system operation more stable.