The development of a high accuracy mains frequency measurement for plant control purposes, and the derivation of a steady state system instability warning parameter

The equipment described in this paper has been developed over a period of more than 15 years. Originally intended as a device for indicating grid instability it has acquired additional features to provide automatic dispatch of plant on low frequency conditions, and event recording facilities. The system has evolved through three generations, the system described being the latest, uses an industrial computer and real-time operating platform. The original system concentrated on measuring the variation in frequency to provide a measure of system instability. The later systems have also derived the rate of change of frequency. Many proprietary instruments are available which monitor quality of the electricity supply. Most monitor the local voltage waveform to provide information about effects on the supply system in a particular locality, however, if information about the state of the system as a whole is required, a measurement of frequency is more appropriate since this is a characteristic of the whole system. The system described acquires high accuracy data for further processing to derive output signals for frequency, rate of change of frequency and frequency variation (wobble), and digital outputs based on a frequency/rate of change of frequency algorithm