Ancillary services are activated when the power grid is out of balance - that is, when supply and demand do not match. This typically occurs when the demand for electricity exceeds what is available, or vice versa.
Here are some examples of when this can happen:
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When weather forecasts are inaccurate.
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In the event of outages in electricity-generating units.
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In case of failures in transmission connections.
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During unexpected surges in electricity consumption.