Study on the influence of 1000kV AC lines on the parallel UHVDC transmission lines and its mitigation

With unceasingly increasing of electric power requirement and the shortage of corridor resource for transmission lines, in some regions the situation that the UHVDC lines are parallelly erected with UHVAC lines or the corridor is common-used. According to the main circuit parameters of UHVDC project from Xiangjiaba to Nanhui and the data of 1000 kV AC transmission line being planned, the model for simulation of UHVAC/UHVDC transmission lines is built by electro-magnetic transient in DC system (EMTDC) program. The power frequency voltages and currents induced in UHVDC lines under different parallel erection lengths and different adjacent distances are calculated. The relation between DC bias current at valve side of converter transformer and power frequency induced current in UHVDC lines is researched; and the corresponding calculation results are given. Several suggestions on the parallel length between UHVAC and UHVDC lines, their adjacent distance and the measure to suppress the DC bias current at valve side of converter transformer are put forward.