Harmonic detection and filtering - Schneider Electric

5.4 Maximum permissible harmonic valuesOn the basis of data drawn from a number of international studies, it was possible toestimate the typical harmonic values encountered in distribution systems.Formulated on the basis of work carried out by the CIGRE organisation, the tablebelow reflects the opinion of a large number of utilities concerning harmonic limitsthat should not be exceeded.Odd harmonics, non-multiples of 3 Odd harmonics, multiples of 3 Even harmonicsOrder h LV MV VHV Order h LV MV VHV Order h LV MV VHV5 6 6 2 3 5 2.5 1.5 2 2 1.5 1.57 5 5 2 9 1.5 1.5 1 4 1 1 111 3.5 3.5 1.5 15 0.3 0.3 0.3 6 0.5 0.5 0.513 3 3 1.5 21 0.2 0.2 0.2 8 0.5 0.2 0.217 2 2 1 > 21 0.2 0.2 0.2 10 0.5 0.2 0.219 1.5 1.5 1 12 0.2 0.2 0.223 1.5 1 0.7 > 12 0.2 0.2 0.225 1.5 1 0.7> 25 0.2 + 25h 0.2 + 25h 0.1 + 25h5.5 Installation standardsThe impact of the THD value on the safety of equipement has led the installationstandards to impose thresholds in determining their capactity to be detrimental.Therefore the French installation standard NF C 15-100 imposes the choice ofneutral conductors cross section according to the thresholds of 15 %, 33 % and 45 %of the level of rank 3 harmonics in the phase current (§ 524.2):S N = 1/2 S PhS N = S PhS N > SPhTHDI H3 y 15 % 15 % < THDI H3 y 33 % THDI H3 > 33 %PermittedProhibitedProhibitedNeutral protectioncompulsoryPermittedNeutral protectionoptionalPermittedCurrent in the phasesdetermines the crosssection of allconductorsPermittedThe current in theneutral determines thecross section of all theconductorsPermittedThe current in theneutral onlydetermines the crosssection of the neutralIn this case, Schneider Electric proposes special trip units (OSN trip units) whichallow using a cross section of phase conductors smaller than the neutral conductorcross section, in the case of large THDI H3.Harmonic three in the phasecurrent (%)Reduction factorChoice based on the phasecurrent0 - 15 1.0 -15 - 33 0.86 -33 - 45 - 0.86> 45 - 1.0Choice based on the neutralcurrent29