Hydro Storage Improved During September

Dramatically improved inflows ended the possibility of a springtime energy crisis during September 2008. Generous rainfall improved hydro storage in both islands.

At the beginning of September 2008, New Zealand’s hydro-electric storage lakes stood at 1403 GWh (GigaWatt hours), 62% of average for the time of year. By month’s end, this had increased to 2172 GWh, 96% of average.

National storage steadily improved during the month, and rose above last year’s levels on the 17th of September. The trend continued, with levels remaining above those recorded during September 2007 until the end of the month.

Generous rainfall in some South Island catchments removed the imbalance between North and South islands which had persisted throughout August. Inflows were above average on 25 days.

In recent months, north-to-south transfers via the Cook Strait cables have exceeded northbound transfers on most days. This pattern continued during September with southbound transfers dominating on 19 days. However, an improvement in South Island lake levels allowed the more usual pattern for energy transfer between the two islands to be restored on September 22nd.

Demand figures were similar to September last year, with daily demand (adjusted to match weekday for weekday) being less than the same day last September on 18 days. The average daily load was lower than last year, being 106.8 GWh this year compared with 107.9 GWh in September 2007.

Peak load of 121.2 GWh was drawn on September 27th. The peak load day during September 2007 occurred on the 6th when 121.7 GWh were drawn.

The Minzone calculation, which determines when thermal plant must be kept running to maintain hydro storage and ensure continuity of supply, showed an improved situation from early September 2008. The complex calculation suffered a “fracture” in May when storage levels in both islands began a rapid divergence, with northern lakes benefitting from rain while a dry spell saw South Island lakes plummet to alarming levels at a time when the capacity of the Cook Strait cables was compromised. This resulted in a South Island Minzone being calculated in addition to the New Zealand Minzone, with the South Island figure keeping thermal plant running through winter even though total lake levels were above the national minimum from mid-July.

During the first week of September, improving hydro storage in the South Island resulted in calculations breaking through the South Island Minzone, and the improvement continued. This allowed a relaxing of market restraints, with the Whirinaki plant coming off-line for the first time in months. With national storage now well above the Minzone calculation, the running of Whirinaki will be determined by spikes in market prices rather than the need for security of supply.

In a bizarre illustration of market mentality some commentators suggested during September that the Whirinaki plant should be dismantled and relocated to either Auckland or the South Island. This myopic attention to the latest hydro shortages overlooks the value of the plant’s current site.

The plant in its current configuration uses one million litres of fuel per day when running at full capacity, with on-site storage allowing up to four days’ continuous operation. Nearby bulk storage tanks at the port of Napier can hold up to 19.5 million litres of fuel.

The plant is located in Hawke’s Bay, conveniently close to a distribution network that includes the Waikaremoana hydro-storage lake. This gives Whirinaki a role as an alternative supply for the eastern North Island should a distribution network problem arise, or the three stations (Kaitawa, Piripaua and Tuai) taking water from Lake Waikaremoana be unable to operate.

Experience shows that New Zealand’s boisterous tectonic environment provides a number of challenges for operating electricity networks. Nevertheless, the sound design of a national grid in the 1920s was a key element in the quick restoration of electricity to parts of Hawke’s Bay and Gisborne following the Hawke’s Bay earthquake of 1931. Power supply to most of Gisborne was restored within hours of the quake, and Napier was supplied via a jury-rigged solution the following day. Steam plant was rapidly brought online in the southern North Island to make up for the loss of feeds from the Tuai power station at Lake Kaitawa (below Waikaremoana) which had been commissioned in 1929.

As recently as 1996, the eruption of Mt. Ruapehu caused two days of disruption to North Island power supplies when ash deposits caused arcing-over on pylons running through the Volcanic Plateau. The resulting power surges caused damage in Wellington and loss of supply in northern parts of the island.

In the event of a major earthquake disrupting the Cook Strait cables, the collapse of Mt. Taranaki destroying gas plant in Taranaki or a volcanic eruption disrupting transmission through the central North Island (to list just three of many threats), Whirinaki is uniquely located to augment supply locally or to the north or south. Its siting in Hawke’s Bay fulfills a role beyond mere market price or counteracting a Minzone threat. It is an essential backup for local supply in the event of a serious disruption at the 138 MW Waikaremoana power scheme and it has provided a valuable contribution to managing our recent power threat.

The Whirinaki plant is a reminder that ensuring continuity of electricity supply requires us to pay attention to local needs, national demand, market prices, access to fuel supplies, and distribution networks.

[Compiled from data supplied by M-co and the Electricity Commission.]

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