Frequency regulation in AC power supply system

A system for frequency regulation in an AC power supply system (10) is disclosed wherein the system comprises an uninterruptible power supply (12) electrically connected to the AC power supply system (10), and the uninterruptible power supply comprises a phase-locked loop (17) and/or a frequency-locked loop for monitoring a frequency of an AC input voltage received from the AC power supply system (10), wherein the phase-locked loop (17) and/or the frequency-locked loop are/is designed to operate at an operating frequency higher than a nominal grid frequency, which operating frequency is selected for detecting a deviation of the monitored frequency from the nominal grid frequency within a predetermined time span after a frequency drop of the monitored frequency occurs, wherein a control signal (19) is generated on the basis of the detected deviation of the monitored frequency from the nominal grid frequency, and wherein the generated control signal is used to regulate the power flow between the uninterruptible