Methods for frequency determination
What you will find in this section:
Frequency determination
The frequency is determined exclusively on the basis of the voltage inputs.
In multiphase systems, all voltage inputs are evaluated. The voltage input with the first valid frequency measurement is selected as the measuring channel. Only when there is no longer a valid frequency at the selected measuring channel the other voltage inputs are re-evaluated and, if necessary, selected as the measuring channel.
The voltage inputs are first DC-decoupled and all harmonic components are attenuated using a low-pass filter. The basic signal thus extracted is sampled and analyzed.
The frequency measurement is based on the evaluation of the period duration. For this purpose, the sum of the signal periods is determined over a sampling window of 200 ms using the positive zero crossings of the signal, as well as the sum of the duration of all signal periods.
These values are smoothed using an equally weighted moving mean value over 3 seconds. This time is necessary to achieve the specified accuracy (Specifications according to IEC 61557-12).
The frequency results from the quotient of the number of periods divided by the duration of all signal periods.