Harmonic detection and filtering - Schneider Electric

Measuring the valuesof the indicators3.1 Measurement devices3.1.1 Selection of a measurement deviceOnly digital analysers, based on recent technology, provide sufficiently accuratemeasurements for the indicators presented above.Other measurement devices were used in the past.b oscilloscopes for observation purposesA general indication of the distortion of a signal may be obtained by viewing thecurrent or the voltage on an oscilloscope.When the wave form is not sinusoidal, the signal is distorted by harmonics. Thevoltage and current peaks can be displayed.Note that using an oscilloscope, it is not possible to precisely quantify the harmoniccomponents.b analogue spectral analysersImplementing old technology, these devices are made up of a passband filtercombined with an rms voltmeter.These devices, now outdated, produce mediocre results and do not provide anyinformation on displacement.3.1.2 Functions of digital analysersThe microprocessors used in digital analysers:b calculate the values of the harmonic indicators (power factor, crest factor,distortion power, THD)b offer a number of additional functions (correction, statistical detection,management of measurements, display, communication, etc.)b when they are multi-channel devices, provide simultaneously and nearly in realtime the spectral breakdown of voltage and current.3.1.3 Operating principle of digital analysers anddata-processing techniquesAnalogue signals are converted into a series of digital values.On the basis of the digital values, an algorithm implementing the Fast FourierTransform (FFT) calculates the amplitude and the phases of the harmonics overa large number of observation time windows.Most digital analysers measure harmonics up to the 20 th or 25 th order for calculationof the THD.Processing of the various values calculated using the FFT algorithm (smoothing,classification, statistics) can be carried out by the measurement device or by externalsoftware.16