On the South Island of New Zealand lies a wide range of medium-voltage electrical distribution topologies. Mountainous terrain, thousands of lakes, numerous glaciers and extinct volcanoes dot the landscape, making it an outdoorsman’s dream. But that same beauty also creates challenges, particularly for rural electrification.

Despite being the larger of the two islands, only about 25% of New Zealand’s 5.1 million people reside there. The lush vegetation and open space make the island ideal for animal husbandry and agricultural production. These two industries are the leading economic drivers for the entire South Island, which has more sheep than people living on it.
To provide electricity effectively and efficiently to its more than 70,000 customers, PowerNet Limited operates one of the country’s largest single-wire-earth-return (SWER) networks. These lines were originally designed in New Zealand nearly 100 years ago as a cost-effective way to bring electricity to rural communities. Because of its simple construction and low cost compared to two- or three-phase systems, the design is still preferred across parts of New Zealand and Australia for rural power distribution.

As New Zealand’s fourth-largest investor-owned utility based on the regulatory value of the network it manages, PowerNet needed to conscientiously provide excellent customer satisfaction but also maintain costs. Because PowerNet’s customer base was spread throughout a sparsely populated area, the region was vulnerable to frequent outages that were causing escalating operations and maintenance (O&M) costs. That was also a major problem for energy-intensive industries such as farming, ranching, and general manufacturing that rely on consistent power to function as a business. An outage impacting a large lateral line could knock out operations for an entire farming community.

PowerNet, as were many other utilities, was using hydraulic reclosers for protection and fault-testing along its SWER lines. However, because its fleet of hydraulic reclosers was aging, the vast majority were disabled to avoid potential catastrophic failures. Because reclosing had been turned off, this led to more frequent outages and resulted in higher O&M costs to operate the lines. And, critical to the community, more customers were without power when faults occurred. Moreover, because these problem areas were impacting approximately 15 farms spread over several miles, it created lengthy delays in restoring power because a single truck roll could take several hours.

PowerNet needed a solution that could help it transition away from hydraulic reclosers and meet customer expectations.

Hydraulic recloser replacement
Hydraulic reclosers have many downsides compared to modern vacuum-based SWER-line reclosers. For one, because hydraulic reclosers rely on oil and relatively complex mechanical parts for operation, when they become older there is a constant need for frequent maintenance and lengthy repairs. When significant repairs were needed, PowerNet found it difficult and expensive to procure parts. It had to procure spare parts from backup inventory and refurbished units, which added to the overall operation costs because backup inventory of reclosers was also needed to cover emergency replacements when units failed in the field. Storing the units also adds additional costs.
That’s only one drawback in using hydraulic reclosers. Other performance issues are caused when oil purity and temperature are not maintained properly, which is an ongoing task throughout the lifecycle of the device. To maintain oil quality, service is required every 3 to 5 years for every single hydraulic recloser. Failing to properly service an aging fleet of hydraulic reclosers could create gaps in a lateral protection strategy because of compromised reliability and lagging coordination between devices.

Another concern that emerges involves setting protection parameters. Because hydraulic reclosers’ protection capabilities are mechanically bound, changing TCC curves is difficult and curve options are limited. Mechanical limitations for hydraulic reclosers mean there are only three or four fixed TCC curves, and utilities have limited options to address this drawback. On one hand, they can continuously send reclosers in for re-configuration, which can be a hassle, especially with larger fleets. Or they could build up a large inventory of backup hydraulic reclosers to account for multiple TCC curve profiles, adding additional storage fees to a utility’s O&M costs.

PowerNet was contending with most of these issues. As the overall fleet of hydraulic reclosers aged, the devices rarely altogether worked properly. The growing O&M costs, limitations around TCC curves, a need for backup inventory to service existing devices and a constant maintenance schedule caused PowerNet to reexamine its protection strategy.

Discovering a new solution
After struggling with servicing and replacing hydraulic reclosers, PowerNet explored alternatives that required less maintenance, could also reclose up to four times, reduce inventory requirements and ultimately decrease the number of outages — all while improving upon the limited basic protection-operation characteristics found in hydraulic devices.
The utility found the solution in a new generation of vacuum-based automatic lateral reclosers. These devices provided PowerNet adaptability and configuration options, but they also provided several benefits over other SWER-line protection options. Unlike hydraulic reclosers, which are fixed mechanical devices, automatic reclosers are user-configured and contain a microprocessor-based relay that gives utilities programmable precise, highly repeatable TCC curves and improved coordination capabilities – including fuse-saving features.

The reclosers’ integrated intelligent controls record and log operational and fault data, allowing PowerNet to better analyze events and optimize its system. Using the automatic reclosers also means PowerNet could reduce inventory costs, trim annual maintenance expenses and allow crews to avoid multi-hour road trips for outages.

For line crews, cutout-mounted vacuum reclosers are designed for easy adoption with existing infrastructure because they are installed and operated similar to a fuse and do not require separate power or control modules. Like legacy fuses, cutout-mounted reclosers visibly drop out of the cutout mount when a permanent fault occurs, making it easier for line crews to recognize the location of the fault. And because the automatic reclosers offer an unlimited amount of TCC curves and are configurable from 4 to 200 amps, PowerNet was able to achieve the flexibility it lacked from having hydraulic reclosers. It also led to rapid deployment because the automatic lateral reclosers could easily slot into the existing coordination scheme by using the same TCC curves used previously for both fuses and hydraulic reclosers.

This flexibility and accuracy provide utilities with options in how to approach a deployment. These vacuum-based automatic lateral reclosers are designed to be easily deployed in any quantity. PowerNet could target a troublesome area without committing to a full deployment. Recognizing how the automatic reclosers could improve reliability in its most remote locations and better serve its customers, PowerNet moved forward with an initial trial of six units.

Results
PowerNet’s lead project manager, Jacques Vergottini, said the new generation of automatic reclosers was exactly the solution the utility needed. “[Automatic] reclosers are projected to save us 10 truck rolls per week, resulting in a significant reduction in O&M costs versus conventional hydraulic reclosers. More importantly, it means improved reliability for our customers located in the most remote areas of our system.”
PowerNet was thrilled with the initial success of the project and decided to increase the number of devices deployed on its power grid. The expanded trial, composed of roughly two-dozen units, has already saved the utility more than 121 truck rolls. By successfully keeping temporary faults from becoming permanent, the automatic lateral reclosers have saved an average of eight truck rolls per unit per year, providing PowerNet a swift return on investment.

The clear benefits have led PowerNet to expand the pilot to a deployment of 65 reclosers across all its SWER lines in the next few years. When the devices are fully deployed, PowerNet expects they will save approximately 520 truck rolls annually, or more than 1,000 crew hours per year on avoided fault-management tasks. That translates to roughly 10 avoided truck rolls per week for PowerNet and a significant reduction in the utility’s O&M costs versus conventional hydraulic reclosers.