![]() ![]() At the assumed transmission and distribution system voltage levels (115 kV and 50 kV), power system voltage bias is not as important, but it does reduce I crit slightly, increasing the backflashover rate by a factor of about 20%, independent of footing impedance. The effects of impulse corona on I crit will be demonstrated for the transmission circuit, with and without the distribution underbuild to show the improvement in coupling. The computed value of I crit, and the probability of exceeding it, will then be compared to a composite result based on a realistic log-normal distribution of soil resistivity that changes from tower to tower. 1 that convert protected phases to extra groundwires and thus provide some benefits even to the unprotected transmission phases above.Īn initial calculation will compare results using a fixed value of footing resistance at each tower, incorporating a volt-time curve, surge impedance coupling and tower inductance. The effects of line surge arresters on the distribution circuit of Fig.The effects of line voltage, as one phase will always have an instantaneous voltage that adds to the stress.The effects of corona, which absorbs energy from the lightning and reduces stress on the insulators.The variability of soil resistivity and footing impedance from structure to structure.The additional voltage rise associated with fast-rising current flow in the inductance of the thin monopole structure.The surge impedance coupling from those OHGW, faulted and arrester-protected conductors that carry a fraction of the lightning surge current away in each direction from tower top.The volt-time curve for impulse flashover: often a calculation is done for time to flashover in the range of 1 to 3 microseconds, and the evaluation time depends on span length.There are a number of adjustments, each described in Standards for: This is the ‘critical current’ I crit that causes the voltage on one of the line insulators to slightly exceed its lightning impulse strength, resulting in a line-to-ground fault that follows and sustains the flashover arc.Ī rough estimate of I critcan be obtained by dividing the insulator critical flashover voltage by the footing impedance, measured at the base of the stricken tower. INMR TUTORIAL SERIES1.Ī series of calculations will be made to establish an important parameter in the ‘backflashover rate’ that establishes the lightning outage rate of most lines. The strength of the coupling is expressed as a set of ‘coupling factors’, C n, with individual values for each of the n conductors – twelve in Fig. Each flashover will introduce a new path for lightning that changes the coupling. The insulation strength of the distribution circuits is so low that, for most lightning surges, some or all six bottom phases will flash over to ground. The voltage stress on each insulator is the difference between the local tower voltage rise and this ‘coupled’ voltage. Depending on the distance from the single OHGW, each phase will take up a voltage wave that is a faithful but reduced amplitude copy of what appears on the tower top. This line configuration will be used to visualize the concept of electromagnetic surge impedance coupling. Assume the 115 kV transmission circuits have 1.0 m insulators and the 50 kV distribution circuits have the minimum recommended dry arc distance of 0.5 m. The lightning performance of a structure with four circuits, arranged as per Fig. Each steel pole is grounded but soil resistivity varies considerably from tower to tower so each pole will have a different value of footing resistance. Lightning protection for this line is provided by a single overhead groundwire (OHGW), mounted at the top of the pole. 1: Beton-Dribenenmast multi-circuit transmission line with twin HV, twin MV and single OHGW. 1 shows a visually appealing arrangement of “delta” transmission circuits and “horizontal” distribution circuits, with 1-2-3 conductors balanced on each side. With appropriate selection, arresters on the lower circuit can also mitigate cross-system contact faults.įig. William Chisholm explains that line surge arresters offer an opportunity to retain the benefits of improved electromagnetic coupling of lightning to the HV circuit above while mitigating induced and backflashover overvoltages on the MV circuit below. In this edited 2019 contribution to INMR, Dr. Often, the lightning performance of the HV circuit improves, while the trip-out rate of the MV circuit is disappointing. Distribution system engineers sometimes take advantage of existing transmission right-of-way to route their medium voltage (MV) circuits when standards show there is adequate vertical clearance to high voltage (HV) transmission circuits above. ![]()
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