The first stage of FrequencyThe number of cycles occurring in each second in an Alternat... Click for more control in a Power SystemA highly complex cyber-physical and transactional System tha... Click for more. It is the response of generating systems and loads to arrest and correct locally detected changes in Frequency by providing a proportionate change in their active power output or consumption.
PFR is automatic; it is not driven by a centralised system of control and begins immediately after a frequency change beyond a specified level is detected.