Study On The Transmission Characteristics Of 50Hz Harmonic Current In HVDC System

The steady operation of HVDC(High Voltage Direct Current)is the requirement for the continuous transmission of electric energy.While,there have been many accidents of misoperation of DC 50 Hz protection caused by the closing of the transformer,it seriously affects the DC security.At present,harmonic analyses of dc at home and abroad mainly focus on the converter harmonic problems,rarely on the transmission mechanism of 50 Hz harmonic current in DC.Therefore,the transmission characteristics of 50 Hz harmonic current in converter and DC line are studied in detail in this paper,taking JinZhong DC as the research object.At the same time,the harmonic characteristics of the two typical DC(ChuSui DC and NiuCong DC)are studied too.And the three typical projects are compared and analyzed,finally combined with the characteristics of each project,the impedance boundary of AC system before the converter transformer closing are given.Based on the modulation theory,the harmonic analysis model of converter is established,and the changes of the harmonic component after converter both from the AC side to the DC side and from the DC side to the AC side are derived.Then the source of the 50 Hz harmonic current on the DC side and the change of the its amplitude after converter are analyzed and verified.Taking JinZhong DC as the research object,based on the PSCAD/EMTDC platform,the effects of the amplitude of inrush current,the DC operation mode,AC system impedance,the value of smoothing reactor,DC line length and the DC transmission power on the transmission characteristics of the 50 Hz harmonic current in the DC system are studied.Transmission characteristics of 50 Hz harmonic current in two typical DC(the ± 800 kV single-circuit ChuSui DC and the ± 500 kV double-circuit NiuCong DC)in China southern power grid are studied.And the three typical projects are compared and analyzed,the influence of different factors on harmonic transmission is evaluated.The special features of NiuCong DC and ChuSui DC are analyzed,combined with their respective characteristics,in the case of the largest inrush current,the impedance boundary of AC system before the converter transformer closing of the three typical DC are given,it can provide a reference for practical engineering of converter transformer closing.The research on the 50 Hz harmonic transmission characteristics under different DC conditions in this paper can provide reference for the field operation personnel.The impedance boundary of AC system before the converter transformer closing of the three typical DC given in this paper can be used as reference for the minimum number of generators that should be switched on in the AC system.