3 Introduction to the Low Harmonic Drive
3.1.1 Working Principle
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
VLT AQUA Low Harmonic Drive Operating In-
structions
3 Introduction to the Low Harmonic Drive
MG.20.T1.02 - VLT
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