Harmonic detection and filtering - Schneider Electric

standard UTE C15-112 indicates a derating factor for transformers calculated as afunction of the harmonic currents:k =T= hhI 11⎛1+0,1. ⎜⎝I40∑h=21 ,6.T2⎞h ⎟⎠typical values:v "square-wave" current (spectrum inversely proportional to h (1) ): k = 0.86v current drawn by a frequency converter (THD ≈ 50 %): k = 0.80.(1) in fact, the current wave form is approximately that of a square wave form. This is the casefor all current rectifiers (three-phase rectifiers, induction furnaces, etc.).b "K factor":Standard ANSI C57.110 defines a derating method based on the "K factor", with theequation below.∞∑2 2Ih.hh=1K =∞=2I∑h=1hThe K factor produces more severe derating and is widely used in North America.In the example presented below, the resulting "K factor" is 13.Order h I h (%)5 307 2011 1413 1117 819 723 525 4h∞∑h=1⎛ I⎜⎝ Ihrms2⎞⎟ . h⎠2The increase in cost for a transformer sized using the "K factor" varies from 30 % to60 % depending on the rating, in a range from 15 to 500 kVA.4.3.4 Asynchronous machinesb standard IEC 60892 defines a weighted harmonic voltage factor (HVF) for whichthe equation and the maximum permissible value are presented below:13UhHVF = ∑∆ ≤2hh=2002 ,b example:consider a supply voltage with the following levels of individual harmonic distortion(u h) where U 1 is the fundamental voltage:-u 3 : 2 % of U 1-u 5 : 3 % of U 1-u 7 : 1 % of U 1(i.e. a voltage THD of 3.7 % and a HVF of 0.018).In this example, the harmonic voltage factor is very close to the maximum value atwhich the machine must be derated.Practically speaking, an asynchronous machine must not be supplied with powerhaving a THDu greater than 10 %.23