P940 FREQUENCY MEASUREMENT

P94x Frequency Measurement (v10 Software)

The relay samples the analogue channels at 24 samples per cycle. Every 2 samples a 1 cycle DFT calculation is performed using the latest 24 samples. This gives a basic frequency measurement 12 times per cycle. In other words, we are performing a rolling 1-cycle DFT 12 times per cycle.

(According to these measurements, the time period is calculated and used to maintain the frequency tracking and ensure that exactly 24 samples are being taken in each cycle).

The basic frequency measurements are stored in an n-cycle buffer (user selectable as 1-12 cycles, 0-48 cycles on v11) that is updated 4 times per cycle. The entire contents of this buffer are averaged every cycle to give a rolling n-cycle average frequency value. These averaged values of frequency are used in all frequency based protection algorithms.

P94x Rate of Change of Frequency Measurement (v10 Software)

The relay samples the analogue channels at 24 samples per cycle. Every 2 samples a 1 cycle DFT calculation is performed using the latest 24 samples. This gives a basic frequency measurement 12 times per cycle. In other words, we are performing a rolling 1-cycle DFT 12 times per cycle.

(According to these measurements, the time period is calculated and used to maintain the frequency tracking and ensure that exactly 24 samples are being taken in each cycle).

The basic frequency measurements are then used to calculate raw ROCOF values by comparing the current frequency value with one calculated 72 samples previously. In other words, we are performing a rolling 3-cycle raw ROCOF calculation 24 times per cycle. This information is passed onto an n-cycle buffer (user selectable as 1-12 cycles, 0-48 cycles on v11) that is updated 4 times per cycle. The entire contents of this buffer are averaged every cycle to give a rolling n-cycle average ROCOF value. These averaged values of rate of change of frequency are used in all rate of change of frequency based protection algorithms.