The need for reserves, or back up generation, is not unique to wind generation. Short term variations in wind generation are just one of many components of supply and demand that are considered when the Grid’s System Operator (Transpower) sets reserve generation levels.
Operating an electricity system is all about managing risks, and what matters most is the overall risk. Over short timeframes, an electricity system is managed to minimise interruptions to supply caused by constantly varying demand, equipment failure, the weather, operational error and other factors.
Over longer timeframes, a system is managed to ensure there is enough generation capacity to meet peak demands, there is enough fuel (including fossil fuels, water inflows into hydro lakes and wind) to generate sufficient electricity at all times, and the transmission system can convey the electricity from where it is generated to consumers.
In order to maintain supply, a second-by-second balance between generation and demand must be achieved. An excess of generation causes the system frequency to rise, 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 both large and small fluctuations in supply and demand.
Instead of backing up each power station with a second power station in case the first suddenly fails, the System Operator pools reserves for the whole system, including for wind farms. This allows it to respond to a variety of potential events.
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, every morning electricity demand increases by several hundred megawatts over two or three hours.
NZ’s wind farms do not currently provide frequency keeping and generation reserves. However, recent developments in wind turbine technology mean that new wind farms will be able to offer these services if the system operator and wind farm operators agree how to implement the technology.