The VLT Low Harmonic Drive is a VLT High Power frequency converter with an integrated active filter. An active filter is a device that actively monitors

harmonic distortion levels and injects compensative harmonic current onto the line to cancel out the harmonics.

Illustration 3.1: Basic layout for the Low Harmonic Drive

3.1.2 IEEE519 Compliance

Low harmonic drives are designed to draw an ideal sinusoidal current waveform from the supply grid with a power factor of 1. Where traditional non

linear load draws pulse shaped currents the low harmonic drive compensates that via the parallel filter path lowering the stress on the supply grid. The

Low harmonic drive meet the toughest harmonic standards and has a THiD of less then 5% at full load for <3% pre-distortion on a balanced three-phased

grid. The unit is designed to meet IEEE519 recommendation for Isc/Il >20 for both uneven and even individual harmonic levels. The filter portion of the

low harmonic drives has a progressive switching frequency which leads to a wide frequency spreads giving lower individual harmonic levels above the

50th.

Illustration 3.2: Typical harmonic frequency spectrum and THD value at the mains terminals of the drive

n = harmonic order

⃟.....IEEE519 (Isc/IL>20) limits for individual harmonics

I [%]

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

[n]

MG.20.T1.02 - VLT

is a registered Danfoss trademark

3 Introduction to the Low Harmonic Drive

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