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Dry Year Planning 2008

Transpower established a dry year planning group through the CEO Forum. This group will look at initiatives for maximising the hydro storage available for the generation of electricity going into winter of 2008.

Power System Limits in South Transfer

In previous dry years with power transfer from north to south in 2001, 2003 and 2005, the two potential power system limits on transfer into the South Island are due to the:

• Power transfer on the North Island Grid from Bunnythorpe (near Palmerston North) to the HVDC terminal station at Haywards in Wellington
• Provision of sufficient instantaneous reserves in the South Island to cover the sudden loss of the HVDC link when sending power to the South Island

The recent stand-down of HVDC Pole 1 and reconfiguration of the undersea cables to the South Island has not reduced the capability of the HVDC link to send power to the South Island. The south transfer limit remains at 626 MW. In previous years there has also been a limit on power export from generators in the Taranaki from Stratford to Bunnythorpe. With the decommissioning of New Plymouth power station this limit is unlikely to occur in 2008.

Bunnythorpe - Haywards Transfer
Transfer over this route is the critical constraint for south transfer on the HVDC link. The transfer on the HVDC link is limited to the amount transferred on the Bunnythorpe to Haywards circuits less the Wellington area load (which varies in winter from 250-600 MW over the day).

Details of the limits for different power system conditions is set out in the table below.

Bunnythorpe to Haywards Limits

Transpower determines the status of the Mangamaire autochange-over based on grid flows and constraints in consultation with the affected line companies. Limits assume all transmission assets including capacitors, condensers and filters at Haywards are available for service.

HVDC Flows
The HVDC link has a stability limit for south transfer. That is, the limit is less that the physical limit of the equipment. After a review in 2003 the limit was raised from 520 MW to 626 MW. In 2005 the 626 MW limit was approached on a few occasions with HVDC South transfer up to 590 MW overnight.

Southland Transfer

Power transfer into the Southland region from the north reduces the amount of generation in the Southland region needed to meet Southland demand. Power transfer into the Southland region is limited by several constraints. A constraint is required to avoid the Livingstone-Waitaki circuit exceeding its thermal capacity following the loss of one of the Clyde-Cromwell-Twizel circuits. This constraint can be alleviated (allowing greater transfer of power into the Southland region) by grid re-configuration albeit at the cost of reducing security in some parts of the grid and the introducing of some generation constraints in the Waitaki area.

Some background and implications for potential grid re-configurations are contained in our Maximising Southland Transfer Capability presentation. Other constraints include limitations on power transfer between Twizel and Roxburgh to avoid those circuits exceeding their thermal capability and limitations on transfer between Roxburgh and Invercargill.

South Island Instantaneous Reserves

With Pole 1 only being used for north transfer in a grid emergency, the total HVDC south transfer is now on Pole 2.  As a result the South Island reserve risk is set by south transfer on the HVDC when this exceeds that of the largest generating unit in the South Island (121 MW at Manapöuri).

The charts below show the available and cleared fast and sustained South Island instantaneous reserves for the last 7 days.

Related Information

Previous Daily and Weekly Updates