Analysis for DC transmission line harmonics originated from AC transformer inrush current and improved method for DC harmonic protection

Abstract

Large-scale electricity has fed into Central and Southern China through DC transmission lines. Any one of the DC transmission lines suddenly outage will result in a large proportion of power-shortage. Recently, there are many DC harmonic protection misoperations caused by AC disturbance, resulting in DC transmission line exit operation. Therefore, it is necessary to analyse the effect of AC disturbance on DC harmonics and study methods for DC harmonic protections. In this study, based on the theory of switching function, the harmonics interaction in AC–DC systems is studied. The cause of DC line harmonic protection misoperation during unusual AC disturbances, such as inrush current and short circuit, is revealed. The DC line 50 Hz harmonic protection misoperates frequently due to inrush current on site. Taking it for example, an improved method using the second harmonic voltage in AC system bus is proposed. In this method, the 50 Hz harmonics generated by inrush current can be accurately identified by judging the correlation between the AC harmonic voltage and the DC line harmonic current. By automatically adjusting the threshold value of protection, the misoperation can be prevented effectively without affecting the original protection performance. The simulation results verify the correctness of the method.