Sources of intermittency on the UK National Grid
The largest source of intermittency on the UK National Grid is the power stations; in fact, the single largest source is Sizewell B nuclear power station. Whenever Sizewell B is operating the entire 1.3GW output is liable to stop at any time without warning. Its capacity is 2.16% of the national grid maximum demand, making it the single largest power source and therefore the largest source of intermittence. Despite this issue, NGT readily copes with it. An industry wide rate of unplanned scrams of 0.6 per 7000 hours critical, means that such a shut down without warning is expected to happen about once every year and a half. However, no matter how low the rate of unplanned scrams, this is largely irrelevant - what matters is the fact that it can and does happen, and measures have to be in place to deal with it.
In a recent case, both Sizewell and Longannet power stations, both stopped unexpectedly within minutes of each other, in fact causing widespread power failures, as sub stations were tripped of using prearranged under frequency relays.
Paradoxically, although wind power is inherently intermittent and variable it is in fact much more reliable than conventional plant. Consider a 660 MW plant, which could be replaced by perhaps 900 x 3MW wind turbines to give the same annual output of energy. On a day when wind strength is enough to give a total output of 600 MW, then these simply cannot all fail simultaneously, since a drop in wind would not affect all stations equally at the same time.
Furthermore, the most reliable form of wind forecasting is to simply look at the total output of the wind turbine themselves – in all probability, what they are producing at one point in time, is likely to be produced one hour later, or only a small change from that. If this prediction window is decreased – 20 minutes, 10 minutes 5 minutes, the difference in total national wind power output becomes less and less, and even at 5 minutes, that is ample time to raise or lower spinning reserve accordingly. There is thus ample time to cope with these changes by calling up or standing down more or less plant. If the 5 minute estimates are wrong then the Frequency Service and Reserve Service diesels will clearly have the resilience to cope with it.
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