Harmonic detection and filtering - Schneider Electric

En brefThe primary indicator is the THD, asingle value that reflects the level ofdistortion in voltage and currentwaves.The harmonic spectrum provides a"fingerprint" of the distorted signal.2.6 Usefulness of the various indicatorsb The voltage THD indicates the distortion of the voltage wave.The measured THDu can provide information on phenomena observed in theinstallation. A THDu value of less than 5 % is considered normal and there is virtuallyno risk of equipment malfunctions.A THDu value between 5 % and 8 % indicates significant harmonic distortion. Someequipment malfunctions may occur.A THDu value higher than 8 % indicates high harmonic distortion. Equipmentmalfunctions are probable. In-depth analysis is required and an attenuation systemmust be installed.b The current THD indicates the distortion of the current wave.To identify the load causing the disturbance, the current THD must be measured onthe incomer and the outgoers of the different circuits.The measured THDi can provide information on phenomena observed in theinstallation. A THDi value of less than 10 % is considered normal and there is virtuallyno risk of equipment malfunctions.A THDi value between 10 % and 50 % indicates significant harmonic distortion.Temperature rise may occur, which means cables and sources must be oversized.A THDi value higher than 50 % indicates high harmonic distortion. Equipmentmalfunctions are probable. In-depth analysis is required and an attenuation systemmust be installed.b The power factor PF indicates the extent to which the source of the installationmust be oversized.b The crest factor is used to determine the capacity of a generator (UPS orgenerator) to provide high instantaneous currents. For example, computers drawhighly distorted current with crest factors that may reach 3 or even 5.b The spectrum (signal broken down into frequency) provides a different view ofelectrical signals and may be used to assess distortion.15