6-22 L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUALMETERING CHAPTER 6: ACTUAL VALUES6If the 87L function is enabled, then dedicated 87L frequency tracking is engaged. In this case, the relay uses the METERING TRACKING FREQUENCY TRACKING FREQUENCY value for all computations, overriding the SOURCE FREQUENCY value.6.4.3.10 Current harmonics and THD meteringACTUAL VALUES METERING SOURCE SRC 1 CURRENT HARMONICSThis menu displays metered current harmonics values. The "SRC 1" text is replaced by the name was programmed for theassociated source (see SETTINGS SYSTEM SETUP SIGNAL SOURCES). Current harmonics are measured for each sourcefor the total harmonic distortion (THD) and 2nd to 25th harmonics per phase.6.4.3.11 Voltage harmonics and THD meteringACTUAL VALUES METERING SOURCE SRC 1 VOLTAGE HARMONICSThis menu displays metered voltage harmonics values. The “SRC 1” text is replaced by the programmed name for theassociated source (see the SETTINGS SYSTEM SETUP SIGNAL SOURCES menu).To extract the 2nd to 25th voltage harmonics, each harmonic is computed on a per-phase basis, where:N = 64 is the number of samples per cycleω0 = 2πf is the angular frequency based on the system frequency (50 or 60 Hz)k = 1, 2,..., N – 1 is the index over one cycle for the Fast Fourier Transform (FFT)m is the last sample number for the sliding windowh = 1, 2,..., 25 is the harmonic numberThe short-time Fourier transform is applied to the unfiltered signal:Eq. 6-3 CURRENT HARMONICS SRC 1 SRC 1 THD Ia: 0.0Ib: 0.0 Ic: 0.0% SRC 1 2ND Ia: 0.0Ib: 0.0 Ic: 0.0% SRC 1 3RD Ia: 0.0Ib: 0.0 Ic: 0.0% SRC 1 25TH Ia: 0.0Ib: 0.0 Ic: 0.0% VOLTAGE HARMONICS SRC 1 SRC 1 THD Va: 0.0Vb: 0.0 Vc: 0.0% SRC 1 2ND Va: 0.0Vb: 0.0 Vc: 0.0% SRC 1 3RD Va: 0.0Vb: 0.0 Vc: 0.0% SRC 1 25TH Va: 0.0Vb: 0.0 Vc: 0.0%
