Factors affecting lightning impulse voltage of metal sheath

Author： GOZ Electric Time：2024-08-02 09:24:43 Read：13

    This study divides the influencing factors of metal sheath lightning impulse voltage into two aspects: cable parameters and lightning current parameters. Cable parameters include cable length, cross-interconnection segment uniformity and laying method, and lightning current parameters include lightning current amplitude, wavefront time, half-peak time and shielding intrusion phase. It is known that the metal sheath lightning impulse voltage at the ungrounded point is often greater than that at the grounded point, so the following mainly studies the effect of the above factors on the metal sheath lightning impulse voltage amplitude at the ungrounded point.

1.1 Cable parameters

1.1.1 Cable length

    Keeping the power frequency operation and other parameters of the cable unchanged, simulate the change of the amplitude of the metal sheath lightning impulse voltage when the cable length changes from 400m to 1500m in each section.

    From the results, it can be seen that when the cable length is reduced, the amplitude of the end of the head grounding can be reduced by 40%, while the amplitude of the cross-interconnection point 1 increases by 1% and the amplitude of the cross-interconnection point 2 decreases by 20%, but it is still far higher than the lightning impulse withstand voltage strength of the outer sheath. It can be seen that the reduction of cable length has a more obvious effect on the metal sheath impulse voltage amplitude far away from the intrusion point. This is because the longer the cable distance, the greater the inductance, which increases the amplitude superimposed on the cable end. In addition, under the principle of capacitive coupling, it is also shown that the cable length is positively correlated with the metal sheath lightning impulse voltage amplitude.

1.1.2 Uniformity of the cross-interconnection point

    Taking a cross-interconnection cable with a length of 1.5 km as an example, when other conditions remain unchanged, in order to eliminate the influence of the distance from the grounding point [2', the distance from one of the interconnection points to the grounding point is kept unchanged, and the uniformity of the other interconnection point on one side is changed to simulate the amplitude of the metal sheath lightning impulse voltage.

    From the results, it can be seen that when the distance from one of the cross-interconnection points to the grounding point is 500 m unchanged, changing the segment uniformity on the other side has different degrees of influence on the metal sheath lightning impulse voltage amplitude. Improving the uniformity of the cross-interconnection point on one side can only reduce the metal sheath lightning impulse voltage amplitude of interconnection point 1 by a maximum of 0.3%; improving the uniformity of the left segment of interconnection point 2 can reduce it by a maximum of 17.9%. In contrast, improving uniformity has a more obvious effect on reducing the metal sheath voltage at the cross-connection point 2, but the effect is not significant.

1.1.3 Laying method

    The arrangement and laying spacing of the cable will affect the electrical parameters of the cable 301, thereby affecting the electromagnetic coupling of the metal sheath impulse voltage 13". Keeping other parameters unchanged, the effects of horizontal arrangement, vertical arrangement, herringbone arrangement and laying spacing on the amplitude of the metal sheath lightning impulse voltage are discussed respectively. From the results, it can be seen that the amplitude under horizontal and vertical arrangements is basically unchanged because the parameters of the cables arranged horizontally and vertically are basically the same [2. The amplitude of the herringbone arrangement is 5% smaller than that of the other two arrangements. When the laying spacing is reduced from 1 m to 0.2 m, the amplitude of the metal sheath lightning impulse voltage at the ungrounded end is reduced by about 20%, but the amplitude at the interconnection point is still far higher than the rated lightning impulse voltage of the outer sheath.

1.2 Influence of lightning current parameters

    When the arrester is in action, the greater the steepness of the lightning current intruding the arrester, the higher the discharge voltage of the arrester. With the different time of lightning intrusion, the phase superimposed on the power frequency waveform is different, which will affect the amplitude of the lightning impulse voltage of the metal sheath. The following will study the amplitude change of the lightning impulse voltage of the metal sheath by changing the relevant parameters of the lightning current.

1.2.1 Amplitude and time of lightning current

    Keep other parameters unchanged, only change the amplitude of the lightning current intruding the cable, and only change the wavefront time and half-peak time of the lightning current.

    From the results, it can be seen that the amplitude of the lightning impulse voltage of the metal sheath is positively correlated with the amplitude of the lightning current. When the wavefront time remains unchanged, the larger the amplitude of the lightning current, the greater the steepness, and the amplitude of the lightning impulse voltage of the metal sheath increases accordingly.

    It can be seen from the results that when the amplitude of the lightning current remains unchanged, the amplitude of the lightning impulse voltage of the metal sheath is negatively correlated with the wavefront time of the lightning current and positively correlated with the half-peak time. This is because the shorter the lightning current wavefront time, the steeper the lightning intrusion wave, the higher the residual pressure on the wire core after the arrester is activated, and the amplitude of the lightning impulse voltage of the metal sheath becomes larger under the action of electromagnetic coupling. The longer the half-peak time, the longer the duration of the lightning current, which will affect the refraction and reflection process of the lightning intrusion wave, thereby affecting the amplitude of the lightning impulse voltage of the metal sheath.

1.2.2 Lightning current shielding intrusion phase

    Keep other parameters unchanged and change the intrusion power frequency voltage phase by changing the lightning current shielding time.

    The results show that when lightning invades the power frequency voltage trough (phase difference is 270°), the amplitude of the lightning impulse voltage of the metal sheath at the end of the head grounding is 10.56kV, which is 58% lower than the amplitude of 25.18kV at the invasion peak (phase difference is 90°). At the interconnection point of the cross-connected grounding, when the lightning invades the trough (phase difference is 270°), the amplitudes of interconnection points 1 and 2 are 37.24kV and 49.74kV respectively, which are about 80% lower than the amplitudes of 191.53kV and 234.62kV at the invasion peak (phase difference is 90°). It can be obtained that the same lightning current has a significant effect on reducing the amplitude of the lightning impulse voltage of the metal sheath when invading the power frequency voltage trough.