An assessment of the variability of wind energy

Wind energy introduces a dynamic component to energy supply. The generation industry has adapted to significant variations in demand, but intermittent non-dispatchable sources such as wind energy pose new and different challenges to those seeking to maintain a consistent supply of adequate power at an effectively constant frequency. In order to understand the implications, a study of the British grid was undertaken. Gridwatch provides data on the average output (in MW) of all forms of generation feeding the grid every 5 minutes. Four years of such data, from May 2011 to March 2015, were analysed. The results show that the variation in wind supply is normally not a matter for great concern. The standard deviation of wind supply over 5 minutes is only about 20MW when the installed wind capacity is 4GW, rising to 40MW when the installed capacity is 8GW. However, there are occasions when the wind drops rapidly to very low levels, and others when it recovers equally rapidly from very low levels. These are comparatively rare events, occurring on average about ten times a year, and they form an increasing large part of the annual large changes which the grid must meet as the installed wind capacity increases. The average power output from this large grid was close to 20% of the installed generating capacity, and the maximum output was about 55% of the installed capacity. These results are discussed in terms of the need to increase the spinning reserves as the proportion of wind capacity to conventional capacity increases.