The electricity network of Powerco, a distribution utility that supplies some 320,000 customers in the lower and central regions of New Zealand’s North Island, mainly consists of overhead lines totaling 30,000 km (18,641 miles). Much of the network covers remote hilly terrain where customer density is typically less than one customer per line kilometer, and the energy demand of individual customers is low.

The 11-kV lines have a service life of around 40 years. As the overhead lines age, the hard wood poles decay and split, and the crossarms break. The conductors, a combination of aluminum conductor steel reinforced (ACSR) and No. 8 steel wire, weaken as they corrode. Pasture converted to forestry presents further challenges for the overhead line reliability.

These factors affect supply reliability and create public safety hazards. Distance, trees and storms together with low populations and economics mean the overhead lines cannot supply remote customers with the same reliability of supply as customers in urban and commercial centers.

In New Zealand, distribution utilities have an obligation to continue to provide lines services to all existing pre-1993 consumers. To meet this obligation, the estimated cost of rebuilding the time-expired remote rural overhead lines throughout the country is estimated to be NZ$300 million.

Therefore, Powerco has developed a stand-alone power system (SAPS) that offers an economic alternative to rebuilding the aging overhead lines in remote areas. The utility’s SAPS allows the overhead lines to be decommissioned while electricity supply to customers is maintained at a lower cost than connection to the grid.

Powerco’s work with customers has confirmed they want a reliable supply of electrical energy not necessarily afforded by overhead lines. This being the case, the utility decided to prove the erection of overhead lines was not the sole, or always the best, way to supply energy needs and could result in significant savings.

Stand-Alone Power System

Powerco first commissioned the SAPS in 2008 as a pilot trial, supplying a wool shed and the shearer’s quarters. The results were used to help lobby the New Zealand government to allow for energy supplies that met customer needs without the need for a connection to an overhead line network. This pilot trial involved an expenditure of $70,000, which alleviated the need to spend around $200,000 for rebuilding the existing overhead line with a like-for-like configuration, the lowest-cost overhead line rebuilding option.

The supply regulations were updated in October 2010, permitting distribution utilities to meet their supply obligations from alternative sources to the overhead line network. Consequently, Powerco began work on its first combined stand-alone unit, which was installed at a remote sheep station in March 2011.

While many stand-alone electricity options were already on the market, they had several drawbacks for customers such as risk of interference, safety and inconvenience. Therefore, Powerco focused on creating a supply-on-demand product that would deliver electrical energy. The manufacturer awarded the SAPS supply contract has developed its own module, but Powerco added some additional features before the units were installed.

Prior to the trial, the supply of energy to the wool shed and small shearer’s dwelling was delivered by a 4-km (2.5-mile)-long single-phase spur from the main 11-kV feeder to an 11-kV/230-V transformer near the dwelling. The line, routed through rugged terrain, was due for a major refurbishment at a cost of around $200,000.

The SAPS, installed in a single day, included photovoltaic (PV) cells, a battery bank for energy storage, a diesel generator and a load management system. All the components, including site foundation pads and PV framing, were assembled at the Powerco depot. The modular unit was tested by supplying electricity for security lighting at the depot for six months to gain confidence in its performance and correct any design issues.

The operation was monitored for one year, with data analyzed to confirm whether the system met the requirements in terms of supply continuity, power quality, maintenance regimes and customer acceptability. The results were promising, with the PV panels in conjunction with batteries proving to be cost-effective and reliable for supplying energy at non-peak times.

The customer’s overall energy costs were lower than when connected to the distribution network, and the reliability of supply improved as SAPSs are not vulnerable to the supply interruptions that occur on long rural networks due to adverse weather conditions, tree contact and equipment failure.

A fully operational stand-alone power system module