Harmonic detection and filtering - Schneider Electric

En bref2.5 Total harmonic distorsion (THD)2.5.1 Definition of total harmonic distortionTHD stands for Total HarmonicDistortion.The level of harmonic distortion isoften used to define the degree ofharmonic content in an alternatingsignal.For a signal y, the total harmonic distortion (THD) is defined by the equation:THD =∞∑ y2hh=2y1This definition complies with that of standard IEC 61000-2-2.Note that the resulting value may exceed one.According to the standard, h can generally be limited to 50. This equation producesa single value indicating the distortion of a voltage or a current flowing at a given pointin a distribution system.Harmonic distortion is generally expressed as a percentage.2.5.2 Current and voltage THDWhen dealing with current harmonics, the equation becomes:THDI=∞∑I2hh=2I1The above equation is equivalent to the one below, which is more direct and easierto use when the total rms value is known:THDIIrms= ⎛ ⎝ ⎜ ⎞⎟ − 1I ⎠12When dealing with voltage harmonics, the equation becomes:THDu=∞∑ U2hh=2U12.5.3 particular case (thd)In certain countries with different work habits, a different equation is used to determineharmonic distortion. In this equation, the value of the fundamental voltage U1 or thefundamental current I1 is replaced by the rms values U rms and I rms respectively.To distinguish between the two equations, we will call the second the total harmonicfactor (thd).Example of a voltage thd:thdu=¥åU2hh = 2UeffThe thd, whether for voltage or current, is always less than 100 %. It makes analoguemeasurements of signals easier but is used less and less because the result is veryclose to the THD defined above when a signal is not significantly distorted. What ismore, it not well suited to highly distorted signals because it cannot exceed the valueof 100 %, contrary to the THD defined at the beginning of this section.13