Frequency Measurement - GE PACSystems RX3i User Manual

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Frequency Measurement

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Healthy three-phase power systems produce three almost identical sinusoidal voltage
waveforms that swing from positive to negative voltage at a frequency of either 50 or 60
times per second (50–60Hz).
The PSM determines the frequency of the power line by looking for the zero crossings,
which are places where the voltage crosses zero volts going from positive to negative or
from negative to positive. Only phase A voltages are used for the zero-crossing detection,
so phase A must be always present on one or the other grid.
Frequency Measurement Notes

PSM preferentially chooses Grid 1 VA zero crossing as the reference for signal
sampling.

In the absence of Grid 1 VA, PSM uses Grid 2 VA zero crossing.

In the absence of either VA signal, grid frequencies will be reported as 0Hz, and the
remaining inputs will be sampled at a rate of 128 samples per 25ms. New data will
be updated at 25ms intervals. This may create sporadic looking individual readings,
but a 10 cycle average will be fairly steady and accurate.

When the difference in frequency between Grid 1 and Grid 2 is large, the Grid 2
RMS readings will vary from cycle to cycle, however a 10 cycle average will be fairly
steady and accurate.
PACSystems* RX3i Power Sync and Measurement System – February 2013
Time
Zero-Crossings
GFK-2749