Only limited forecasting possible for wind power infeed - demand for wind - based reserve capacity increases with new wind power construction

Large quantities of electrical energy cannot be directly stored. This means that every second, exactly the amount of energy must be fed into the grid that is taken out at the same time. If the amount fed in differs from the amount tapped, this can cause faults or even failure of the supply - as confirmed in 2003 by the wide-scale power failures in the USA, Italy, Sweden and Denmark. The transmission system operators must therefore at all times ensure a balance in their control areas between generation and tapping (power-frequency control).
Generation in traditional power stations can be easily controlled in line with demand. As a result, in the past it was mainly only the time pattern of tapping from the grid that was relevant to power-frequency control. Thanks to constant consumption behavior, this tapping can be forecast with a high degree of accuracy.
However, the increased use of wind power in Germany has resulted in uncontrollable fluctuations now also occurring on the generation side due to the stochastic character of wind power infeed, thereby increasing the demands placed on control and bringing about rising grid costs. So that stable grid operation is possible despite the high volatility of wind power infeed, transmission system operators depend on the most accurate possible forecasts of the expected wind power infeed.

To forecast wind power, E.ON Netz uses a complex forecasting system developed by ISET and based on the forecasting data of the German Meteorological Service.

The quality of wind power forecasting is to a great extent limited by the quality of the wind forecasting. Like all weather forecasting, this is only partly reliable.
In 2003, the average negative forecasting error for the E.ON control area was -370 MW, and the average positive forecasting error was 477 MW.

Power-frequency control

The purpose of power-frequency control is to create an equilibrium between generation and consumption in a defined grid territory (control area) at a stable grid frequency of 50 Hz. This is achieved by briefly activating additional generating capacity or by deactivating surplus generating capacity for seconds and minutes. However, during individual hours the deviations reached much higher levels of up to +/-2,900 MW. This was equivalent to just under half the installed wind power capacity. The transmission system operator must balance out differences between the wind power forecast and the wind power actually fed in by using the controlling power range and reserve capacity. Of crucial importance to the wind-related demand for reserve capacity is the expected maximum forecast deviation and not, for example, the mean forecast error. This is because even if the actual infeed deviates from the forecast level only on a few days in the year, the transmission system operator must also be prepared for this eventuality and have sufficient capacity available so that a reliable supply is still guaranteed.

The massive increase in the construction of new wind power plants in recent years has greatly increased the need for wind-related reserve capacity in Germany. In 2003, costs amounting to around 100 million € for this were incurred in the case of E.ON Netz alone. Operational experience over the past few years has shown that reserve capacities in the order of magnitude of up to 60 % of the installed wind power capacity must be kept for wind balancing in years when wind levels are normal. The need for reserve capacity and the resulting costs will therefore continue to rise in future parallel to the further expansion of wind power.
In 2003, wind levels and therefore also the absolute fluctuation range of the wind power infeed were at above-average high levels. This meant that in retrospect, only reserve capacity amounting to around 50 % of the installed wind power capacity actually had to be used.