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Past Experience at Canadian Utility with Surge Arresters on Transmission Lines

July 22, 2017 • Arresters, ARTICLE ARCHIVE, Utility Practice & Experience
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Additional Work

NB Power has investigated use of a specialized lightning detection system as a tool to use along with present fault locating relays and other historical information. An effort was made to identify those areas most prone to lightning-related outages and which were suitable locations for application of line arresters to improve system performance. For example, a lightning event in August 2006 was studied using information from this detection system and is illustrated in Fig. 18. Fault locating relays identified the 138 kV lightning fault location as being near structure #30. But, information gathered from the lightning detection system identified a total of 6 different lightning strikes within 1.5 seconds of the recorded fault time.

Fig. 18: Example lightning detection system results for a transmission line fault. arresters Past Experience at Canadian Utility with Surge Arresters on Transmission Lines Screen Shot 2017 07 21 at 14

Fig. 18: Example lightning detection system results for a transmission line fault.
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Summary

When lightning protection is required for any particular transmission line, the various pros and cons of shield wire versus line surge arresters have to be considered. The length of the insulator string, the size of the structure grounding electrode and the cost of the arrester or shield wire should all be part of this decision-making process. In general, good lightning performance can be achieved using shield wires particularly on HV and EHV lines. Conversely, shield wires are usually not appropriate at distribution voltages that feature shorter insulators and higher values of ground resistance because the grounding electrode is smaller. To achieve good lightning protection, line arresters are appropriate to apply at line voltages such as 69 and 138 kV and are seeing increased use even at higher voltages. One significant difference in considering shield wires versus line arresters is the need for low ground resistance at the tower. For lightning strikes within the arrester’s energy rating, line arresters can provide zero lightning outages when installed across all insulator strings at every tower. If the towers are steel, no additional grounding is required for reasons of lightning performance. Conversely, with low ground resistance, shield wires can provide good lightning performance, approaching zero outages.

There is a fundamental difference between these two approaches for lightning protection of transmission lines. With shield wires, the goal is to get the lightning surge to pass into the ground before an insulator back-flashover. If such a backflash occurs, system power-follow current will flow through the ionized air. Protective relays will detect this and signal the breaker to trip resulting in a transmission line outage. In the case of line arresters, the goal is to prevent the lightning strike from passing through the air by giving it another path. There is no opportunity for power-follow current to flow because the arrester switches off after the lightning strike has passed through it. With no power-follow current, the protective relays and breaker at the end of the line do not operate and there is no outage. Effectively, lightning which hits the transmission line will pass through to the tower steel and ground. The ability of more than one arrester to share in conducting the lightning strike to ground can be an important consideration when determining arrester energy carrying capability. This is an important topic for future research.

Conclusions

Line arresters installed on the NB Power transmission system clearly demonstrated their ability to eliminate lightning outages. For example, a three-day lightning storm struck the Saint John area and caused numerous transmission line outages. However, there were no line outages due to lightning on those towers equipped with line arresters, even though these were located in the very center of the storm area. All outages were triggered at structures without any arresters. Probably, the various problems experienced with arresters and described in this article can be eliminated by using arrester/insulator assemblies similar to those shown in Figs. 13 & 17. More research needs to be undertaken to investigate the energy sharing capability of line arresters under lightning conditions.

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