P940 FREQUENCY MEASUREMENT

P94x Frequency Measurement

The relay samples the analogue channels at 24 samples per cycle. Every 6 samples a 1 cycle DFT calculation is performed using the latest 24 samples. This gives a basic frequency measurement 4 times per cycle. In other words, we are performing a rolling 1-cycle DFT 4 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. Currently there is a limit applied to the logic to ensure that the maximum rate of change of frequency measured by the relay doesnt exceed 20Hz per second. This means that difference in frequency between successive values is limited to 0.1Hz).

The basic frequency measurements are stored in a 3-cycle buffer that is updated every time a new measurement is made i.e. every cycle. The entire contents of this buffer are averaged every cycle to give a rolling 3-cycle average frequency value. These 3-cycle averaged values of frequency are used in all frequency protection algorithms.

P94x df/dt Protection

The df/dt protection uses both the basic frequency measurements and the 3cycle average values as shown in the previous section. Every 4th basic frequency sample is taken and inverted to give a time period, t1, for the previous cycle (remember 4 measurements per cycle). This is repeated for another 2 cycles, and the 3 values of time period are added together to give a 3-cycle time measurement, dt. When this time value is available, the difference between the current average frequency measurement and the average frequency measurement from 3 cycles before is calculated, df. This value is then divided by the 3-cycle time period to give the rate of change of frequency, df/dt.

The current average value of frequency is then stored for the next calculation, which is performed 3 cycles later when the time period has been calculated again. The ROCOF protection uses a fixed 3-cycle calculation.