Research On Harmonic Characteristics And Suppression Measures On DC Side Of Hybrid HVDC Transmission System

The line commutated converter high voltage current(LCC-HVDC)technology has matured day by day,and the flexible DC transmission based on the Modular Multilevel Converter(MMC-HVDC)has developed rapidly.In the future power grid,the hybrid DC transmission composed by LCC and MMC will be used widely,which can combine the advantages of both,not only meet the needs of large-capacity and long-distance transmission,but also control the power flexiblely.So that it can supply power for AC systems with weak receiving end or multi-drop points.However,hybrid DC transmission will also bring new resonance characteristics to the future power grid and generate a series of harmonic problems.Therefore,it is necessary to carry out research on the harmonic characteristics in the DC side of hybrid DC transmission.This paper first introduces the main topology of LCC-MMC hybrid DC transmission,its basic working principle and control operation characteristics from the rectifier and inverter side are analyzed separately.Next,a harmonic model on the DC side of the hybrid DC transmission was built.An improved three-pulse model was applied on the rectifier side.The zero-sequence harmonic voltage source model was built on the inverter side.The DC transmission line was equivalent using a distributed parameter model.This harmonic model has good adaptability compared with others,which can reflect the harmonic distortion and three-phase asymmetry of the AC system accurately,and calculate the harmonics current of the DC side in all situations.Then the generation principle and basic characteristics of the harmonics on the DC side and the transmission law of AC and DC harmonics are analyzed.For the AC system with weak electrical strength,the simulation verifies the harmonic transmission characteristics in the case of harmonic distortion and three-phase asymmetry.Finally,different suppression measures and control strategies are proposed for the hybrid DC transmission.The rectifier adopts a DC filter system,and the inverter adopts a control strategy for zero-sequence harmonic voltage suppression.The simulation under the PSCAD/EMTDC software shows that the harmonics can be effectively suppressed under the influence of suppression measures.