Study On The Lightning Shielding Failure For HVDC Transmission Lines

Statistics show that the rate of lightning strike flashover is about forty percent to seventy percent of total flashover in our power grid. In Guangdong province, the number of lightning shielding failure is ninety percent in the total number of lighting strike flashover on 500 kV HVDC overhead lines. It relates with the current design method, such as regulations and electrical geometric model(EGM). Lately, the study of lighting shielding failure has made a great progress with the use of leader progression model(LPM) and EGM. The study result shows that this models work well only in the transmission line of low voltage level. With the increasing of the voltage level of the transmission line, this model cannot describe the real circumstance of UHVDC transmission lines, which made problems to explain the polarity effect in the process of shielding failure. So this model cannot meet the use in the engineering field.First, the article builds the calculation model of UHV DC transmission lines, tower, insulators and so on. It simulates the three-form over voltage of lightning strikes on the UHV DC transmission lines and compares with 500kV DC transmission lines. It indicates the possibility of lower current-flow of lightning strike on the transmission lines invasion converter station, and the existence threat of the equipment insulation.Then, according to the electromagnetism field theory, a lightning simulation model and a striking distance factor computation model considering the working voltage and the height of the tower are created. The simulation results show that given the same lightning current and tower height and along with the increase of working voltage, the striking distance factor will obviously decrease, and the striking distance will certainly be different between that of leading lightning to the lead and that of leading lightning to the shield. The calculation of a lightning shielding failure example indicates that the rate of lightning shielding failure will increase as the working voltage increases, and could, therefore, lead to misjudge in the case without considering the working voltage.Along with the increase in working voltage, the lightning shielding failure model is composed by one side of shield wire,one side of transmission line and lightning downward leader will be unable to describe the real circumstance of UHVDC transmission lines, which made problems to explain the polarity effect in the process of shielding failure. So this model cannot meet the use in the engineering field. Then, the article bases on Charge Simulation Method, and takes the lightning leader, two shield wires and bundle conductors as a unified whole to research the lightning shielding failure. Also the author considers the working voltage and conductor bundles, calculating the surface electric field of the conductors and shield wires when the lightning leader develops downwards and calculating the electric field with the different distance between the lightning leader and overhead lines. When the conductors induce the upward leader which is decided by Peek equation, the height H that named as the first lightning height away the earth of the lightning downward leader will be determined. Then the author explains the lightning shielding failure problems in UHVDC rationally. It has more probability for lightning shielding failure if the working voltage is considered. Finally, the author puts forward a new viewpoint of UHVDC lightning shielding failure: in UHVDC overhead lines, only anode conductor and cathode side shield wire will initiate positive leader to compete with each other to intercept lightning leader, the shielding function of anode side shield wire will be weakened and plays no role in the protection of the anode conductor.Finally the author has analyzed the insufficiency of the overall model, and the lightning shielding three-dimensional model is proposed. The article carries it on the discussion, the research, the computation and the analysis. And then it obtains with the above two-dimensional overall model consistent conclusion.