Hydro Storage Improves

New Zealand’s hydro lake levels improved during July 2008, with good inflows boosting North Island lakes while South Island storage languished.

At the beginning of July 2008, New Zealand’s hydro-electric storage lake levels stood at 1517 GWh (GigaWatt hours), 57% of average for the time of year. By the end of the month, storage had improved to 1670 GWh, 69% of average.

A succession of low pressure weather systems brought heavy rain to both main islands, with some areas experiencing floods. However, while lake catchments in the North Island were boosted by the rain, the major South Island lakes received little benefit. Lake Tekapo continued to decline during July and, while the level of Lake Te Anau improved during the early part of the month, its level was drawn down as the month progressed. The pattern was the same for Lake Manapouri with the lake remaining close to its minimum level for normal operation.

Nationally, inflows were above average for the time of year on 19 days.

Southern lake levels were preserved by tight management. North-to-south transfers via the Cook Strait cables exceeded the more normal northbound transfers on all 31 days. On 22 days, no energy at all was transferred northward.

Matching weekday for weekday and weekend for weekend with July last year, demand was lower than the equivalent day last year on 18 days, reflecting the influence of a public savings campaign. However, as North Island storage improved, rumours began circulating that the savings campaign would be abandoned and demand began rising on the 21st and remained above last year’s levels from July 24th.

Average daily demand for July 2008 was 114.6 GWh, lower than the average of 116.7 GWh recorded during July of 2007.

Peak load of 124.3 GWh was recorded on the 8th of July 2008, lower than the previous July’s peak of 126.5 GWh which was drawn on the 9th of July 2007.

During July energy managers faced a continued headache in calculating the Minzone, the minimum hydro storage level at which all thermal plant must be online and operating with the Whirinaki plant generating at full capacity.

The Minzone calculation became fractured during June as North Island and South Island storage levels diverged, with northern storage going against the trend and improving while southern storage languished. Whilst the Operational Minzone remained the same the South Island Minzone, which was in a weaker state, dictated what was labelled “New Zealand Minzone.”

With stronger inflows in the North Island during July, the Minzone situation continued, even though national storage broke through the Operational Minzone level during the week of July 24th (the Electricity Commission’s fortnightly reporting date). Since then, South Island storage has rapidly declined, holding the electricity network just within the New Zealand Minzone.

The good news is that the Emergency Zone calculation which factors in the need to hold water in reserve to meet future demand has drawn away as winter has progressed and the months of replenishment approach. Had lake levels entered the Emergency Zone, as they threatened to do during the early part of June, compulsory savings would have been imposed, and blackouts could have been likely.

National storage levels usually continue to decline until mid-September while wintertime demand slowly eases and the lakes await the boost provided by the spring snowmelt.

In essence, the fragile state of the South Island hydro lakes and the split Minzone calculation continued the suspension of the wholesale electricity market during July. Government-owned generation and thermal plant continued running to maintain continuity of supply and preserve hydro levels, instead of market price determining which generators were online and supplying energy.

[Compiled from data supplied by M-co.]

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