Lightning has been reported as the main cause of unscheduled outages on overhead sub-transmission and transmission lines (e.g. U.S.: 57%; Brazil: 50-70%; Japan: 70-80%; Denmark: 57%; Colombia: 47-69%). Reducing outages due to lightning has a major impact on overall reliability of both distribution and transmission lines. The main aim of installing transmission line arresters (TLAs) is to reduce tripping/outages on shielded or unshielded lines. Tripping generally arises from insulator flashovers – commonly known as back-flashovers, since the tower is no longer at earth potential but due to the lightning surge at higher voltage than the conductor:
• Unshielded Lines: Lightning strikes to the structures or the phase conductors will, in almost all cases, produce flashovers along the insulator strings.
• Shielded Lines: Strikes to the structure or earthwire have the possibility of backflashover occurring across the insulator strings depending, amongst other parameters, on the level of strike current and the transient grounding system behaviour.
Transmission line lightning performance also depends on correct choice of arrester type and positioning on the structure and along the line. It is possible to significantly reduce lightning caused outages by having TLAs on every phase on every structure but this will rarely be economical and the overall failure rate of the TLAs may then reduce transmission line performance. It is therefore necessary for a power supply utility to specify an acceptable performance level and work from there. This INMR article from 2015, contributed by Dr. Brian Wareing, an Overhead Lines & Lightning Protection Consultant based in the U.K., provided an example of choice and positioning of TLAs on a double circuit 400 kV line.
An evaluation was required to increase the reliability of the 400 kV Beauly-Denny line that crosses the North Scottish Highlands from near Inverness to just west of Edinburgh. The major problems affecting reliability are severe weather conditions (snow, ice and winds up to 65 m/s) and lightning triggered outages. Investigations into line design were introduced to reduce the impact of the effect of the severe weather expected, especially in the Corrieyairack area where the line rises to around 800 m. The risk of the line being struck by lightning with consequential back-flashover and tripping out of circuits was then investigated. The back-flashover situation commonly caused by lightning strikes is usually mitigated by a low value of tower earthing. However, in many parts of the line the ground is granite with soil resistivities >20,000Ωm. Obtaining low value tower earths would therefore be expensive and, in places, virtually impossible. The alternative to reducing back-flashovers is the transmission line arrester (TLA) and a full analysis of how this could used to produce a reliable line within financial constraints. This analysis was then used to determine the necessity or otherwise of providing possible costly earthing solutions. So the TLA method was compared with the cost of introducing earthing mitigation measures to produce the most reliable and economic solution. The initial target was to prevent 95% of strikes causing a back-flashover.
Back-flashover is the situation when the difference between the voltage on the cross-arm (created by strike current to the earthwire travelling to earth down the tower) and that on the phase conductor (both with respect to the same earth) exceeds the BIL of the phase insulator arc gap. This can put a sharp fronted overvoltage waveform onto the phase conductors – a particular problem on substation approach. A backflashover is generally agreed to be the situation where the tower is at high voltage compared with the phase wire instead of approximately at earth level, since it would be in the case of flashover from a direct strike to a phase conductor. The back-flashover-generated surge has a very sharp wavefront as the arc causes the phase wire to jump in <1μs from an induced voltage level (from the surge along the earthwire) to virtually the full lightning surge voltage as present on the tower cross-arm (the arc itself will drop a few hundred volts only).
The probability of a flashover is dependent on the voltage across the insulator i.e. between the tower crossarm voltage and the voltage on the phase conductor (induced voltage plus normal 50 Hz voltage). To calculate this it is necessary to calculate the surge impedance of all the phase and earthwires, which requires knowledge of local ground resistivity, and determine how much surge voltage is induced by the earthwire onto the phase conductors. This requires a calculation of the coupling factor between the earthwire and the phase conductors and requires knowledge of the ground resistivity. This is also dependent on the tower type (distance of conductors above ground) and the tower footing resistance. The crossarm voltage is also dependent on the strike current, earthwire impedance and the surge impedance of the tower and ground. In dealing with the latter it is always necessary to look at the system when subject to a MHz phenomenon and not a 50 Hz scenario. Hence the process is dealing with surge impedance and not resistance.
Once the surge voltage at the crossarm is known and compared with the phase wire voltage, the probability of flashover depends on the position on the 50 Hz cycle and the ±15% of the mean impulse (50%) breakdown level of the arc gap. The probability of flashover of the insulator will also depend on the waveform of the lightning surge as it appears at the crossarm level. This is currently the subject of work by SSE at Heriot Watt University in Edinburgh. The frequency of the flashover event can then be determined from the local lightning activity. The Heriot Watt PhD project takes a fresh approach at adding to the understanding of the effect of lightning strikes on an overhead transmission line in terms of its electrical behaviour such as charge and voltage propagation around towers and lines. It is reviewing and developing models to simulate this behaviour. This is to enable the answering of questions such as: where should transmission line arresters (TLAs) be placed, on the top, middle, bottom conductor or all and how efficient they are at reducing back-flashovers.