Over the initial years after installation, there were four line outages due to failed arresters. Two of these resulted when a 69 kV arrester failed under conditions where the high magnitude of the lightning strike was beyond the arrester’s energy carrying capability. The other two outages from failed arresters were at 138 kV and were attributed to a manufacturing defect. Investigation concluded that both arresters had been supplied with an improper moisture seal on the end cap of the weathershed, allowing moisture ingress and causing an internal fault.
None of the other problems described above resulted in transmission line outages. These problems were either found during inspection or were self-isolating, without causing an outage. However, the hardware problems have been problematic as well as time-consuming and bring into question concerns about the longevity of the installed design. For this reason, the condition of line arresters at NB Power has been being closely monitored through field inspection and thermovision scanning of those units in service.
Other Line Arrester Applications
Other installations of transmission line arresters on the NB Power transmission system include:
• 138 kV single steel pole design with post insulator and arrester assembly (three arresters at every second pole)
• 138 kV steel H-Frame (suspended arresters at every insulator string)
• 69 kV wood pole H-Frame (arresters installed on poles below the conductors)
• 69 kV and 138 kV arresters installed at terminal stations across transmission line dead-end insulator strings
138 kV Single Steel Pole
The 138 kV installation shown in Fig. 12 was constructed with arresters installed at every second pole. Poles were at 80 meter spacing and every second pole was chosen with the goal of minimizing cost. Each pole includes a grounding conductor around the pole one meter out from the structure. It has since been learned that a single arrester on the top phase of every pole would be a more efficient design, assuming grounding resistance is low. The insulator/arrester assembly which has been found to be more appropriate for this application is shown in Fig. 13.
138 kV Steel H-Frame
In 2003, NB Power constructed a 25 kilometre long 138 kV line using a steel H-Frame design shown in Fig. 14. Arresters were installed on all phases at every structure. No lightning outages have been experienced up to now on this section of line.
69 kV Wood Pole H-Frame
In 2002, NB Power installed arresters on an existing section of a 69 kV wood pole H-Frame transmission line (see Fig. 15). Since it was difficult to install these arresters along with grounding conductors with the line energized, a single transmission line outage was necessary to clamp the arrester lead to each phase conductor.
Arresters Installed at Terminal Stations
In 2002, NB Power experienced failure of a 138 kV SF6 dead tank circuit breaker during lightning conditions. Following this event, a study was commissioned to investigate the application of line arresters on towers near the station. This study concluded that arresters installed at the transmission line dead-end tower at terminal stations provides a margin of lightning protection to a station SF6 breaker located within 100 meters of the tower. NB Power has therefore undertaken installation of line arresters at transmission line dead-end towers for stations with such breakers (see Fig. 16).