Given the possible application of line arresters to mitigate the effects of lightning, it strikes me as inexplicable that arresters are not being used more often to reduce momentary lightninginduced outages on transmission lines. I have asked experts to explain why this is the case but most could offer no explanation. Some speculated it was due to concern about arrester failure.
I have recently been involved in an economic study funded by the State of New York R&S Association and conducted in partnership with R&D firm, Ceralink, as well as professors at Rensselaer Polytechnic Institute and Cornell University. Our work focused on using arresters in place of shield wires on transmission lines, with the initial goal being to demonstrate that inductive losses due to overhead ground wires could be eliminated by replacing these with arresters. Other possible advantages included reducing overall energy needs. That goal seemed worthy in itself but our team soon realized that the accompanying improvement in reliability offered even greater potential for gain.
In the initial study, it was found that the financial benefits from eliminating the OHGW came in two parts: lowering construction costs and preventing induced losses over the lifetime of the line. When considering initial construction costs, the variables included tower type, system voltage, number of circuits, phase configuration, desired protection level and type of arresters used. The table below summarizes findings:
The second to last column in the Table is the net present value of the savings that would be realized if shield wires were eliminated, assuming a 10% return on investment, 30-year life of the line, power cost of 50 cents per MWHr and 5% annual inflation.
The study demonstrated that the savings from reducing losses is not as significant as the reduction in upfront material and construction costs. When combined, savings of using arresters in place of OHGWs ranged from 5% to 8% of the initial total investment – certainly worth thinking about on its own merit.
* The per mile savings by not installing a shield wire includes the cost of the wire, the cost of installation as well as reduced tower and foundation costs. The cost of using arresters includes arresters on each phase of each circuit, using Class 3 line arresters on the exposed conductors and Class 1 arresters on the conductors shielded by the phase above. The spacing selected was 8 towers per mile (1.6 km).
But what the calculation does not reveal is that reliability of a line protected by arresters is better than for one that relies on the OHGW. For lines protected by shield wire and having very low tower ground resistance and good insulator BIL, the probability of backflash outage can be as low as 5% of all challenges faced by the line. But if footing resistance is poor and/ or the insulator BIL low, backflash of insulators can account for as much as 20 to 30% of such challenges. With arresters installed on all phases of all towers, however, the probability of outage due to lightning is zero, i.e. arresters applied to all phases on every tower render the line ‘lightning proof’. Moreover, ground resistance is no longer a factor in the reliability equation.
Knowing that line arresters eliminate lightning related outages, it might seem that quantifying the benefit of improved reliability would be easy. But this proved not to be true. Assigning a cost value to a transmission line outage is not as easy as for a substation or distribution system since, in the latter cases, the utility incurs a specific cost if equipment fails due to lack of protection. In the case of transmission lines, arresters are preventing insulator flashover, not transformer failure. An insulator flashover may be a system failure, but the insulator is self-healing and most of the time remains undamaged. This fundamental difference means that power system operators do not see any direct cost from a momentary outage that occurs when an insulator flashes over.
In fact, the impact of such an outage is borne more by end users than line operators and, since there is no associated direct cost, there is less incentive among operators to eliminate it. Moreover, most end users don’t even realize that the lightning outage could have been avoided. This combination of little financial incentive by power system operators and lack of awareness among end users therefore seems to be preventing greater application of transmission line arresters.
The economic benefits of increased reliability cannot easily be quantified without doing an analysis of all the customers served and we have therefore not been able to quantify them. Still, the figures are most probably significant.
So, one might well ask power system operators: how do you justify your transmission line usage?