Study On The Overvoltages In Transmission Lines Of HVDC Systems

In order to improve the reliability of the High Voltage Direct Current Transmission (HVDC) system and select basic lightning impulse insulation level and switch impulse insulation level of equipment in converters station properly, the overvoltages of HVDC system must be studied in detail. In this thesis, the Gezhouba-Nanqiao HVDC system is simulated and two types of overvoltage in the transmission line are calculated by electromagnetic transient simulation software PSCAD/EMTDC, the outlines of this dissertation are as following:Based on the practical parameters of Gezhouba-Nanqiao HVDC system, the dissertation establishes the models of tower, transmission line, arrester, flashover characteristic of insulator, as well as other equipment inside converter station theoretically by PSCAD/EMTDC. Simulations are carried out to study the back flashover induced by lightning and ground fault on transmission line. The calculated results are closed to practical situation, thus make the models available for practical HVDC system.When study the overvoltage caused by back flashover, the HVDC system is working in monopolar mode with metallic return. The result indicates the tendency of wave-tail flashover with relative low amplitude of lightning current. The overvoltages caused by back flashover are damped with coupled and surge capacitors installed. With the protection of arresters, the overvoltages on most of equipments are lower than their BIL. But the overvoltage on the bottom of main capacitor of the DC filter is higher than its BIL in some occasion, which can be solved by enhancing the insulation level of the stanchion.When study the overvoltage caused by ground fault, the HVDC system is working in bipolar mode. The result indicates that the overvoltage on the health pole is largely dependent on the fault location. With the protection of line arresters, flashover will not occur on the health pole. When grounding fault is close to the terminal of converter station, the insulation of DC filter will be threatened by overvoltage. That can be solved by enhancing the insulation level of the stanchion.The modeling of the transmission lines, transmission towers, arresters, insulators and other devices in converter station built in this thesis can be used to study other types of overvoltage in HVDC system.