Analysis Of Induced Voltage Of Metal Sheath Of Buried Cable Under Lightning

With the acceleration of urbanization,it has become an inevitable trend that high-voltage cables replace overhead lines.However,the capacitance of the cable line is much larger than that of the overhead line,and the harm of the induced voltage generated in the cable sheath to the insulation is also prominent and complex.Lightning invades the cable core through overhead lines directly connected to the cable,or violent changes in the spatial electromagnetic field caused by direct lightning strikes on the ground near the cable may produce a higher amplitude of induced voltage on the cable metal sheath.Therefore,it has certain theoretical significance and practical value to study the overvoltage problem of metal sheath of cable line under lightning to improve line insulation,reduce line fault frequency and ensure safe and reliable operation of power system.Combined with a 132 kV cable ring network power supply project,a calculation model including power supply,cable and load components is built in the electromagnetic transient simulation software ATP-EMTP.The influences of different arrangement modes of three-phase cables and grounding modes of metal sheath on the induced voltage and circulating current of sheath are analyzed,and on this basis,the induced voltage of sheath under system closing operation and grounding fault is calculated.The results show that the induced voltage of the sheath in the triangle arrangement mode is the smallest,and the circulating current of the sheath in the cross interconnection grounding mode is the smallest.During the closing operation and short-circuit fault of the system,the induced voltage of the protective layer is mostly several hundred volts,which will not endanger the cable insulation.In view of the situation of lightn ing invasion of buried cables,the lightning model and parameters are determined,and the lightning induced voltage of cable metal sheath and the variation law under different lightning current amplitude are calculated.The calculation results show that the induced voltage on the metal sheath can reach up to hundreds of kV when lightning invades,which has exceeded the impulse withstand voltage level of line insulation.In view of this situation,according to the requirements of the guidelines,the corresponding protective layer protector is selected,and the simulation calculation model of protective layer protector is built in ATP.The lightning induced overvoltage of cable metal protective layer after the installation of protective layer protector is calculated and analyzed.The calculation results show that the protective layer protector can effectively suppress the lightning induced voltage of the protective layer within the cable insulation level.Aiming at the coupling voltage generated by lightning return stroke electromagnetic field on the metal sheath of buried cable,the lightning return stroke electromagnetic field below the ground is calculated by using the Finite-Difference Time-Domain(FDTD)algorithm and the lightning return stroke model.Aiming at the problem of convolution integral occupying memory and time consuming in the time domain field-line coupling model,the vector matching method is used to simplify the time domain form of the field-line coupling model,and the coupling voltage of the cable metal sheath under the action of the lightning return electromagnetic field is calculated by combining the obtained lightning return electromagnetic field.It is found that the lightning return stroke electromagnetic field and the induced voltage of the protective layer increase with the increase of the lightning current amplitude,and decrease with the increase of the distance from the lightning channel.However,the induced voltage generated by the coupling effect of lightning return stroke electromagnetic field on the metal sheath of buried cable will not endanger the cable insulation.