The goal of a lightning proof transmission line has been attainable since the early 1990s when the first polymeric-housed transmission line arresters became available. Indeed, based on IEEE 1410 and IEEE 1243, installation of arresters on every phase of every tower of a shielded line will ensure no lightning induced insulator flashovers – essentially a line that is ‘lightning proof’. However, for a number of reasons, most engineers involved in overhead line design have still not adopted this guaranteed strategy to improve lightning performance. Perhaps the most cited reasons for this are that the arresters themselves are seen as a potential factor in reduced reliability and could also require a lot of maintenance.
The dominant design of transmission line arrester used over the past 20 years – the non-gapped type (NGLA) – has demonstrated that such concerns are generally not valid. The NGLA has demonstrated that it is highly reliable and failures are comparatively few. Nevertheless, it seems clear that another approach towards the goal of lightning proofing a line is necessary in order to eliminate all possible concerns by line designers.
This past article by arrester expert and INMR columnist, Jonathan Woodworth, discussed the basics of the externally gapped line arrester (EGLA) in order to increase knowledge and confidence in applying them for line protection.
The EGLA concept dates back to the earliest stages of surge protection on power lines, although in subsequent years it became an internal gap that performed the same function. The reason for this long history is that air gaps have an excellent capability to withstand voltage and predictable breakdown at a very specific voltage level. It is therefore no accident that arrester designs over the years continue to make use of it and that there are currently millions of arresters in service that use the air gap to hold off line voltage until a surge appears. In the presence of a surge, they sparkover and protect equipment on the line.
When highly non-linear metal oxide varistors (MOV) are connected in series with the age-old air gap, the combination takes on unique features not available in earlier designs of gapped arresters. It is these unique and repeatable characteristics that make the externally gapped MOV arrester the ultimate in distribution and transmission line protection. The EGLA schematic in Fig. 1, for example, has been applied in Japan, Europe and Mexico as an IEC certified device for many years.
The IEC type EGLA differs in that it is designed to only protect lines from lightning. This is achieved simply by increasing the gap spacing to a level that will not sparkover in the presence of switching surges. For systems above 230 kV, the IEC type EGLA does not need larger varistors to handle the energy associated with switching surges. It is characterized by a larger gap section and smaller tolerance in gap dimension than the EGLAs that are discussed below. This type of EGLA is also an arrester recognized by the IEC community and has its own specific test standard.
It is important to emphasize that the EGLA is being referred to in this article is a line arrester designed to be applied to eliminate insulator flashovers and not to protect sensitive oil-insulated equipment such as transformers. Because it is being used only to eliminate insulator flashover, the front-of-wave sparkover of this type of arrester is not relevant and not an application issue. Fig. 2 contrasts the front-of-wave characteristics of EGLAs and NGLAs. It also demonstrates that while EGLAs work well to eliminate insulator flashover, they cannot be expected to protect sensitive transformer insulation.
Mode of Operation of EGLAs
The well-coordinated gap and varistor of an EGLA work in conjunction to turn the arrester on and off. In spite of the high sparkover characteristic of the gap, it is more than adequate to protect the self-restoring insulation of line insulators – whether on a distribution system or even a 500 kV transmission system. When a lightning surge from a direct strike travels along the line and reaches a standard insulator, it will flashover the insulator and cause a power system fault. Such a fault can be prevented by using an EGLA to protect the insulator. In such a case, lightning will be directed through the arrester, which turns on immediately because the voltage is above the sparkover of the gap and turn on point of its varistors. Immediately following the lightning charge transfer, the varistors terminate all current flow and there is no power frequency follow current or fault. The robust withstand characteristics of line insulators, combined with the self-restoring capability of an EGLA, make it a perfect candidate for this type of protection.
Gap Function & Design Criteria
The gap of all externally gapped line arresters is the critical component in determining its turn on voltage. In this regard, it is important that this gap be large enough to withstand typical voltage swings during temporary overvoltage events yet small enough that it will always sparkover before the protected insulator flashes over. Fig. 3 is a graphic that outlines the various considerations determining gap settings for a unit that sparks over for both switching and lightning surges. The minimum acceptable gap setting of an EGLA is determined by three factors:
1. system voltage;
2. expected temporary overvoltage levels (TOV); and
3. safety factor.
The maximum acceptable gap is set by both the CFO (U50) of the protected system plus relevant safety factors.