Hydro Spill Terms:

kWh/MWh/GWh Unit:
The kilowatt-hour (symbolized kWh) is a unit of energy equivalent to one kilowatt (1 kW) of power expended for one hour (1h) of time. The consumption of electrical energy by homes and small businesses is usually measured in kilowatt-hours. Larger businesses and institutions sometimes use the megawatt-hour (MWh), where 1 MWh = 1,000 kWh. The energy outputs of large power plants over long periods of time, or the energy consumption of states or nations, can also be expressed in gigawatt hours (GWh), where 1 GWh = 1,000 MWh = 10^6 kWh.

Hydro Release:
The release of water that could potentially have been used for the purposes of energy production, but was released for other reasons.

Hydro Releases are carried out for a number of reasons. The following list describes the standard reasons for a release:

Plant:
Hydro-release was due to a plant malfunction including plant owned by a third party. For instance, if enough water to enable generation at full load was released via the upstream power stations or control gates and a TrustPower generator developed a fault (for example, an electrical or mechanical fault) and stops operating, the station would be unable to use all of the released water for generation. The lake at the station may be unable to store the excess water that has resulted from the generator fault, therefore, the water must be spilled.

Obstruction:
Hydro-release was due to physical obstructions preventing normal operation of the generating plant. For example, each of TrustPower’s generators has water intake screens to prevent large debris such as logs from damaging the machinery. Debris from water ways (logs, water weed etc.) can severely restrict the water passing through the screens to the generator, and must therefore be removed. The generators must be turned off and if the lake is unable to store the excess water then it will be spilt.

High Flow:
Hydro-release was due to high inflow events (flooding or very high rainfall). This type of release applies when the flows exceed the ability of the generation scheme to operate at that level, and when the operator has no discretion over avoiding the release.

Regulation:
Hydro-release was due to regulatory obligations. It includes statutes, resource consents, use permits, bylaws etc. For example, the resource consent associated with the Matahina power station requires a minimum amount of water to be flowing down the river at all times – despite the state of the plant.

Legal:
Hydro-release was due to legal obligations. In most cases this type of spill refers to a release due to contractual obligations, and only occurs when TrustPower has no discretion in avoiding a release.

Recreational:
Hydro-release was for recreational use. That is where recreational, social or cultural interests have negotiated hydraulic profiles, and hydro-release has occurred as a result. Typical examples include, but are not limited to, jet boat racing and kayaking etc …

Support:
Hydro-release was for economic reasons. This either means that energy demands made the releases economic, or for economic reasons the quantity could not be delivered to market, and releases occurred as a result.

Commercial:
Hydro-Spill was due to spot price not meeting the hydro generator’s threshold for that plant’s short run price for operating.

Transmission:
Hydro-spill was due to transmission or distribution constraints.

Other:
Hydro-release was for reasons other than those described previously.