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.
Firmware <= 1.6.0
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.
The frequency results from the quotient of the number of periods divided by the duration of all signal periods.
Firmware > 1.7.0
From firmware version 1.7.0 , the frequency measurement of EMpro is based on a Phase-Locked-Loop (PLL) procedure. Based on this procedure, two different frequency measurement registers are provided. From version 1.7.1 on these are available at:
- Averaged values for maximum accuracy (registers 32780 / 49164)
- Unaveraged values for determining rapid frequency changes (registers 32856 / 49240)
The frequency shown in the WBM, on the display of the device and used for the other (not Modbus) interfaces is based on the averaged values. The unaveraged frequency value is especially needed to fullfill the requirements for frequency determination in energy supply networks. Respective requirements for speed and accuracy of frequency determination in Germany are for instance defined by VDE application guidelines (VDE-AR-N 4110 / VDE-AR-N 4120). The unaveraged value is updated after each signal period (e.g. every 20 ms for 50 Hz), the averaged value is updated after ~200 ms (10 signal periods 50 Hz / 12 signal periods 60 Hz) - like the other instantaneous (TRMS) values provided by EMpro.