Upper panels: Power-grid frequency time series for Singapore (SGP), Indonesia (IND), Malaysia (MYS), Ireland (IRL), Australia (AUS), Iceland (ISL), and Mallorca Island of Spain (PM_ESP). Lower panels: Estimated drift coefficient of the power-grid frequency time series. The black dashed lines show a piecewise-linear drift function with a deadband, corresponding to an idealised primary control law [2].