The need for reserves, or back up generation, is not unique to wind generation. Electricity supply must be continually matched to demand. Reserves are required to meet fluctuations in demand and to cover all forms of generation, as demand varies constantly and no one power station or form of generation is totally reliable.
The system operator sets aside reserves to cover a range of events, such as a thermal plant going offline with no warning, daily fluctuations in demand, and variability in wind generation. It is highly unlikely that shifts in wind patterns will cause either:
- instantaneous power changes as large as those currently managed when a thermal generator goes offline without warning because of a fault
- changes in power supply over an hour or two that are as great as currenly managed every morning when demand increases several hundred megawatts.
Wind energy is naturally variable, however this does not necessarily mean new thermal stations are required to provide electricity generation on calm days. Wind speeds can be predicted days and hours ahead, and the output of a wind farm can be forecast in advance. Generation from other, existing sources can be planned to accommodate expected fluctuations in wind generation.
New Zealand's existing storage-based hydro generation is particularly good for balancing wind generation as its energy can be stored (in the form of water in a hydro dam), and electricity output can be altered quickly.
The role of reserves in the electricity system
In order to maintain electricity supply, a second-by-second balance between generation and demand must be achieved. An excess of generation causes the system frequency to rise, and an excess of demand causes the system frequency to fall. The electricity system is designed and operated in such a way as to cope with large and small fluctuations in supply and demand.
To balance supply with demand, the system operator (Transpower) sets aside reserves to provide the capability to respond to the variations expected over different timescales. The system operator pools reserves for the whole electricity system, rather than backing up each power plant with a second plant.
The range of factors considered when setting reserve levels include changes in demand, fluctuations in wind generation and the risk of a large generator going offline unexpectedly because of a fault.
Frequency-keeping reserves are used to respond to instantaneous imbalances. The system operator buys sufficient reserves such that system frequency remains near to 50Hz for the continuous demand and generation fluctuations, and within set limits following any sudden tripping of the largest generating units, or any sudden disconnection or reduction of demand.
The system operator also requires reserves to meet daily fluctuations in demand. Reserves are particularly valuable at times when large power stations are connecting (or disconnecting) from the system or when demand is changing rapidly. For example, on a typical morning, electricity demand can increase by several hundred megawatts over two or three hours.