Danfoss VLT 380-500 V Design Manual

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Danfoss VLT 380-500 V Design Manual Manual pdf 103 page image

Current Harmonic currentIRMS I1 I5 I7 I11-49Input current 1.0 0.9 0.5 0.2 <0.1Table 9.8 Harmonic Currents Compared to the RMS InputCurrentThe voltage distortion on the mains supply voltagedepends on the size of the harmonic currents multipliedby the mains impedance for the frequency in question. Thetotal voltage distortion (THDi) is calculated based on theindividual voltage harmonics using this formula:THDi = U25 + U27 + ... + U2nU9.8 Effect of Harmonics in a PowerDistribution SystemIn Illustration 9.13, a transformer is connected on theprimary side to a point of common coupling PCC1, on themedium voltage supply. The transformer has an impedanceZxfr and feeds several loads. The point of common couplingwhere all loads are connected is PCC2. Each load isconnected through cables that have an impedance Z1, Z2,Z3.PCC Point of common couplingMV Medium voltageLV Low voltageZxfr Transformer impedanceZ# Modeling resistance and inductance in the wiringIllustration 9.13 Small Distribution SystemHarmonic currents drawn by non-linear loads causedistortion of the voltage because of the voltage drop onthe impedances of the distribution system. Higherimpedances result in higher levels of voltage distortion.Current distortion relates to apparatus performance and itrelates to the individual load. Voltage distortion relates tosystem performance. It is not possible to determine thevoltage distortion in the PCC knowing only the harmonicperformance of the load. To predict the distortion in thePCC, the configuration of the distribution system andrelevant impedances must be known.A commonly used term for describing the impedance of agrid is the short circuit ratio Rsce. Rsce is defined as the ratiobetween the short circuit apparent power of the supply atthe PCC (Ssc) and the rated apparent power of the load(Sequ).Rsce = SscSequwhere Ssc = U2Zsupplyand Sequ = U × IequNegative effects of harmonics• Harmonic currents contribute to system losses (incabling and transformer).• Harmonic voltage distortion causes disturbanceto other loads and increases losses in other loads.EMC and Harmonics Design GuideMG37N102 Danfoss A/S © 6/2016 All rights reserved. 1019 9