What is the best way to mitigate harmonics? PQM-106

There are number of types of equipment that will reduce harmonic distortion (tuned harmonic filters, line reactors, active harmonic filters, multi-pulse rectifiers, 5% THD harmonic filters, phase shifting transformers, etc).  Each has its merits in the proper circumstances. Each has its advantages as well as disadvantages.  Each has a typical performance and cost.  There is no single method that is best for all cases.  

This best solution will depend on the unique aspects of the power system, facility operations, type of loads and the target distortion levels.  In a facility with many loads, often the best technical and economical solution involves a combination of methods. An examination of the single line diagram supplemented by simulation of the power electronic loads can result in a determination of the best economical and technical solution to a harmonics concern.

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What harmonic current distortion should I expect from a VFD? PQM-105

An 18-pulse drive to typically will achieve about 4% to 6% THD-i when operating at full load (assumes balanced and non distorted voltage source). Distortion will increase as load is reduced and when voltages are not balanced.

 

A 12-pulse drive typically achieves 12 – 15% THD-i when operating at full load, (assuming balanced and non distorted voltage source). Distortion will increase as load is reduced and when voltages are not balanced. Filters are available for 12-pulse drives that will reduce current distortion to 5% or less.

A 6-pulse drive without a DC or AC inductor may be as high as 60-100% THD-i.  However a 6-pulse drive with 5% line reactor (or equivalent DC choke) achieves about 35% THD-i. at full load (45% at 60% load) and with 3% line reactor (or equivalent dc choke) achieves about 45% THD-i at full load (about 60% THDi at 60% load).  Filters are available for 6-pulse drives that will reduce current distortion to 5%, 8%, 10%, 12%.  

What is a Detuned Power Factor (PF) Capacitor? PQM-104

Since power factor capacitors have capacitance reactance (ohms) that decrease as frequency increases, they will naturally offer a low impedance path to harmonics. If the power system voltage is distorted, then current will be driven into the capacitor, at each of the frequencies that are present in the harmonic voltage spectrum. Additionally, the capacitance (Farads) will combine with upstream circuit inductance (Henries) to form a parallel tuned circuit and a series tuned circuit, each with their respective natural resonant frequency. If harmonic voltage is present at one of these frequencies, then harmonic resonance can cause amplification of harmonic voltage (parallel resonance) or harmonic current (series resonance).  A detuning reactor is used to prevent harmonic resonance, by purposefully shifting the capacitor circuit resonant frequency beneath the lowest harmonic that is present. It also restricts the flow of harmonic current into the capacitor by increasing the capacitor circuit impedance relative to harmonic frequencies.  Power factor capacitor systems that use detuning reactors are sometimes referred to as anti-resonant capacitors, capacitors with harmonic blocking reactors, or simply detuned capacitors.