This past INMR article contributed by John Williamson, former Manager of Transmission Engineering at NB Power, described the experience at a Canadian utility regarding application of surge arresters on 69 kV and 138 kV transmission lines over a 6-year period. During this time, about 2000 gapless surge arresters were installed with about 1500 of these put into service on double circuit steel towers in an urban area.
NB Power is a public utility located on the eastern seacoast of Canada and serving the province of New Brunswick. Its transmission system includes 69 kV, 138 kV, 230 kV and 345 kV lines. Lightning protection on these lines typically employs shield wires, which have been installed on all transmission lines at 230 kV and 345 kV. Shield wires, however, have been only selectively applied on the utility’s 69 kV and 138 kV lines. While New Brunswick has a relatively low-flash density, on the order of only 0.5 flashes per 100 km per year, lightning outages still posed a significant problem in areas with soil having high electrical resistivity. Much of this soil is rocky and therefore it is difficult to obtain good lightning performance due to the high resistance of structure grounding systems.
Experience with Shield Wires at 345 kV
NB Power once experienced a 345 kV fault under lightning conditions that led to an outage at the utility’s nuclear power station. Subsequent field investigation found that all three strings of 18 insulators flashed over on one structure (#331). A similar lightning fault had been recorded at this same structure two years before that. Since flashover of insulator strings on multiple phases following a lightning strike is evidence of poor grounding at the tower, an engineering study was commissioned to investigate grounding problems along the line and propose mitigation techniques to eliminate this problem. The investigation included:
• gathering soil electrical resistivity data,
• design and construction of different grounding electrodes,
• computer modeling of soil and electrodes,
• field measurement to verify designs using a new impulse injection technique,
• selection of a preferred grounding electrode design.
A helicopter survey using electromagnetic scanning was used to gather soil electrical resistivity data in order to create a two-layer soil model. The data gathered covered a distance of 29 km along the twin 345 kV line right-of-way.
Test installations of 6 different electrode designs were constructed and field measurements were carried out to determine electrode ground resistance. The test laboratory providing this work proposed a new impulse injection technique to measure ground resistance on a multi-grounded system. Measurements were then carried out to verify the electrode design and the computer model. Reviewing the soil resistivity data with the electrode design, it was determined that 25% of the tower electrodes did not require additional grounding to achieve the targeted 22 ohms ground resistance. Field installation of grounding electrodes was subsequently carried out in selected areas of lightning activity and high soil resistivity, resulting in improved performance of these 345 kV lines.
Transmission Lines Near Saint John
Lightning performance improvements were first considered 15 years ago on 69 kV and 138 kV lines supplying the area around the city of Saint John. Many of these lines are supported on double circuit, lattice steel towers installed over very rocky terrain. These towers were built during the 1950s with shield wires included as part of their design for lightning protection.
Over the years, salt and industrial contamination resulted in failures of the shield wires due to rust and corrosion. Difficulty in taking lines out of service prevented replacing these and shield wires began to disappear on these lines in many areas. With no shield wires and with numerous towers sitting on rock (i.e. having poor grounding), the number of outages due to lightning became problematic. In particular, customers were concerned as industrial loads and processes were being affected. Initially, to improve lightning performance, it was thought that the shield wires would have to be reinstalled along with improved grounding systems. However, the required line outages to accomplish this were difficult to obtain. Acceptable lightning protection with shield wires depends on obtaining low tower grounding resistance. With towers sitting on rock, this was a huge challenge as well as an environmental issue in this mainly urban location. So, although successful at 345 kV, it would be difficult to achieve improved lightning performance on transmission lines near Saint John using only shield wires and improved grounding systems. Other solutions had to be considered.
Application of Line Arresters
Line arrester application offered a possible alternative to traditional shield wires. Partial application of arresters at a tower requires a low grounding resistance so as to prevent backflash on the unprotected insulator string. The cost of upgrading the tower grounding system given the rocky soil as well as the disruption to property owners in this urban area was of concern. It was therefore decided to apply arresters to all insulator strings on the selected towers. Historical information of lightning events from fault locating relays was then used to identify those locations most prone to lightning strikes. Arresters were installed on all phases of the double-circuit towers in areas where such lightning-induced outages had been experienced in the past. Fig. 2 illustrates a typical double circuit 138 kV tower with arresters installed on all phases. With this solution, no improvements needed to be made to existing tower grounding systems.
Arrester components were supplied as a kit (see Fig. 3) that included leads, chain, disconnector and a hot line clamp. Installation required only drilling a single hole in the steel cross-arm to hang the assembly and then clamping the arrester lead onto the conductor. This work was then carried out under de-energized conditions.
The purpose of the disconnector was to automatically disconnect the arrester from the transmission system under conditions where the arrester failed internally and thereby isolate the faulty arrester from the system (see Fig. 4). Under normal lightning strikes, within the arrester’s rating, this disconnector would not operate.
Table 1 provides a summary of lightning outages on transmission lines in the Saint John area. The summary specifically looks only at lightning outages at structure locations where arresters were installed. (Note that installation of these 1500 arresters was not completed until 2004.)
Between 1996 and 2002, a total of 28 lightning-related outages occurred at the identified structure locations prior to arrester installation. From 2003 to 2006, however, only 4 outages were experienced under lightning conditions and all of these resulted from failure of an arrester. Two of these failures were attributed to a manufacturing defect while the remaining two were the result of a lightning strike beyond the arresters’ energy handling capability. Historically, NB Power would experience at least two outages each year where the conductor would fall to the ground after insulator flashover under lightning conditions. No events such as this have occurred since arresters were installed. Equally important, the lightning performance of these lines has improved to the point that industrial customers have recognized and acknowledged this to NB Power.