Harmonics

The harmonics are calculated using Fourier transformation over a time window synchronized to the grid frequency. The signals are recorded over several grid periods. The number of grid half-waves taken into account can be set via the parameter HarmonicsWindowCycles.

In order to calculate harmonics using Fourier analysis, the sampling rate must meet the Shannon-Nyquist criterion. A multiplier of 2.5 to 10 is usually applied to the fundamental frequency in order to take into account conversion inaccuracies, measurement errors and transients. A higher sampling rate also improves the resolution and reduces alias effects and leakages in the spectral analysis. The EL3783 record the measured values at 20 kHz. Based on this sampling rate, it was decided to provide harmonics up to the 63rd order in order to ensure a uniform interface for all modules. The 63rd harmonic corresponds to 63 times the fundamental frequency: at 50 Hz this corresponds to 3150 Hz, at 60 Hz to 3780 Hz.

The harmonic analysis is used to evaluate grid perturbations, ensure compliance with standards and identify non-linear loads.

The measured values are provided via the STcPowerMeasureHarmonics structure.

The TF3650 | TwinCAT 3 Power Monitoring, whose license is included in TF8350, is available for the calculation of higher-order harmonics.