Overvoltage and Restriction of 1000kV Long-Distance Transmission Lines in Weak System

Power frequency and switching overvoltages may be high for long-distance transmission lines in the weak power systems, such as Southwest ultra high voltage (UHV) AC power grid at initial stage. On the other hand, discharge voltage of air gap decreases significantly with decrease of air pressure, and the cost of line insulation at high altitude occupy a large proportion in the UHV transmission project. Therefore, it is necessary to further restrict overvoltages in order to avoid large increase of insulation cost at high altitudes. Simulation models were established and overvoltage calculations were carried by using Electromagnetic Transient Program (EMTP). It is found that overvoltages are particularly serious when the weak system side is a switch station (or the unique main transformer is out of service). Under this condition, power frequency overvoltages could be limited by installing high voltage (HV) shunt reactors on the bus. Moreover, selection of the minimum air gap for 1000kV transmission lines under switching overvoltages is based on the statistical switching overvoltage level of 1.7p.u. according to the existing standards. However, in high altitude areas, the minimum air gap could be reduced (e.g. based on the statistical switching overvoltage level of 1.6p.u.) on the basis of deeply restricting switching overvoltage, thus effectively reducing the sizes of line towers.