Distortion measurementThe distortion measurement module measures harmonics up to the 11th harmonic.The total harmonic distortion THD for voltage is calculated from the measuredharmonic components with the formulaTHD UUkkN= =∑ 221GUID-83A22E8C-5F4D-4332-A832-4E48B35550EF V1 EN (Equation 98)Uk kth harmonic componentU1 the voltage fundamental component amplitudeDemand calculationThe demand value for THD is calculated separately for each phase. If any of thecalculated demand THD values is above the set alarm limit THD alarm limit, theALARM output is activated.The demand calculation window is set with the Demand interval setting. It hasseven window lengths from "1 minute" to "180 minutes". The window type can beset with the Demand window setting. The available options are "Sliding" and "Non-sliding".The activation of the BLOCK input blocks the ALARM output.10.2.5 ApplicationVMHAI provides a method for monitoring the power quality by means of thevoltage waveform distortion. VMHAI provides a short-term three-second averageand long-term demand for THD.10.2.6 SignalsTable 647:VMHAI Input signalsName Type Default DescriptionU_A_AB SIGNAL 0 Phase-to-earth voltage A or phase-to-phasevoltage ABU_B_BC SIGNAL 0 Phase-to-earth voltage B or phase-to-phasevoltage BCU_C_CA SIGNAL 0 Phase-to-earth voltage C or phase-to-phasevoltage CABLOCK BOOLEAN 0=False Block signal for all binary outputsSection 10 1YHT530004D05 DPower quality measurement functions770 615 seriesTechnical Manual
