Backflashover Performance Evaluation of the Partially Grounded Scheme of Overhead Lines With Fully Composite Pylons

Abstract

A design of a fully composite pylon has been proposed for new-generation 400 kV transmission towers to save line corridors and to reduce visual impact. Correspondingly, a method of external down-leads is proposed to bring grounding potential to the shield wires, together with a plan that not all pylons are grounded called 'partially grounded transmission lines' (PGTLs). This paper investigates backflashover performance of a partial grounding scheme of overhead lines (OHLs) supported by composite pylons. The transient analysis was carried out in PSCAD based on Monte Carlo method. For OHLs with every pylon grounded, reducing footing resistance and soil resistivity can improve backflashover performance effectively, but for PGTLs, these two methods do not have obvious effect and increasing insulation distance has limited effect. When lightning strikes at PGTLs, overvoltage is mainly dependent on the distance to the nearest grounded pylon and a longer distance will cause overvoltage with larger amplitude and longer wave front duration. Therefore, backflashover rate also increases along with the distance to the nearest grounded pylon until reaching a value limited by the inceptive condition of flashover. A coefficient recommended by CIGRE TB 63 to estimate backflashover rate is discussed and modified when using in PGTLs.

Transient Lightning Impulse Performance Analysis of a Fully Composite Pylon with an External-grounding Down-lead

“…The effect of decreasing footing resistance and soil resistivity will become even more insignificant in the PGTL schemes of longer length. The differences of the maximum overvoltage on the PGTLs of 1000 m, 1500 m and 2000 m and a strike at the middle of the span are 89.69 kV, 1.85 kV and 0 kV [120]. Moreover, at an precision level of 0.0001, the tiny differences of overvoltage have little influence on BFR results.…”

“…A lightning flash with the amplitude of 60 kA (3.83/77.5 µs) terminates in the middle. The maximum overvoltage when the grounded pylons are of lower footing resistance and soil resistivity is -3722.64 kV, which is only 4.5 % lower than when the grounded pylons are of higher footing resistance and soil resistivity, which is -3889.47 kV [120]. The span designed for composite pylons is 250 m, so the 500 m PGTL within one non-grounded pylon is the shortest scheme.…”

“…The fully composite pylon is featured by its insulated pylon shaft. The term "partially grounded transmission lines" (PGTLs) refers to a plan where not all pylons are grounded out of economic and aesthetic considerations [120]. A PGTL is created as an OHL segment that is supported by a number of non-grounded pylons.…”

“…The ratio relating to the probability distribution of lightning current for different spans in conventional OHLs lies in a small range between 0.58 and 0.67. 0.6 is therefore chosen as the compromised value [120].…”

“…For a PGTL, the distance between the lightning strike and the closest grounding device may typically be greater than most transmission line spans today, resulting in an overvoltage that has a higher amplitude and a longer duration. From another aspect, to sustain such a high overvoltage, the insulation strength at the cross-arm on a non-grounded pylon within a PTGL is often weaker than the air gap at mid-span [120]. Therefore, the ratio of the occurring probability of I c1 and I c2 increases along with the increasing length of a PGTL.…”