Research Of Lightning Induced Voltage Charactiristics On Metal Sheath Of Electrified Railway 27.5kV Cable

When the lightning current intrudes upon the cable core, it can occur induced overvoltage on the metal sheath due to the magnetic field lines coupling the metal sheath. And the induced overvoltage would damage the main insulation and outer sheath. Railways in our country, the distribution is too wide, lines are too long, and the geographical environment is more complex, so, the overhead lines are vulnerable to lightning strike. Due to the different climate and soil conditions along the railway, characteristics of the induced overvoltage which generated by lightning wave on the metal sheath may be different. According to statistic, a lot of cable faults occur because of the outer sheath on metal protecting layer being damaged. So, research on the lightning induced over-voltage characteristics of electrified railway 27.5 kV cable metal sheath has important significance to engineering.This paper uses unit length of electrical parameters to build cable distributed parameter models, and utilizes the theory of wave to verify the exactness of the models. Through in-depth studying on the generating mechanism of induced voltage on the single core XLPE cable sheath, this paper offers a calculation method of lightning induced over-voltage when only one end of the metal sheath grounding. And it analyzes and calculates the lightning induced over-voltage on two kinds of situations (when one end of the metal sheath grounds, the other end installs protector or not) combining with calculation example. For the longer high-voltage single-core cable, it is necessary to use a metal sheath grounding method of cross interconnection on the engineering. This paper also offers the calculation method of lightning induced over-voltage when there is no protector on the metal sheath under this grounding method.Based on the distributed parameter models, the paper utilizes Simulink software to simulate and calculate the maximum value of lightning induced voltage on the metal sheath under different working conditions when the lightning current intrudes upon the cable core along the overhead lines. And it discusses the influences on lightning induced voltage characteristics (for example, arrester grounding resistance, soil resistivity, cable length, lightning current waveform and impedance of the end). The simulation results indicate:the maximum value of sheath induced voltage increases with the increase of arrester grounding resistance linearly and proportionally; when the insulator flashover is caused by the damage of the arrester, the maximum value of sheath induced voltage may exceed the lightning impulse withstanding voltage of outer sheath if the soil resistivity> 800Ω·m, leading to the breakdown of outer sheath; the maximum value of sheath induced voltage may be the smallest when the length of cable is 500m; the maximum value of sheath induced voltage increases with the increase of lightning wave gradient(amplitude of the lightning wave), but its growth rate declines; the sheath induced voltage may be very high and exceed the lightning impulse withstanding voltage of outer sheath when the end of cable has a short circuit or a larger capacitive impedance; the sheath induced voltage may be small when the end of cable has an open circuit or a larger inductive impedance.