Harmonic current considerations
...This paper intentionally assumes that a facility does not have significant harmonic currents present. Harmonics, their impact on true power factor, and the economics of harmonic mitigation will be the topic of another paper (true power factor includes the effects of a harmonic power factor component). However, some caution must be taken when applying capacitors in a circuit and this topic deserves some consideration when applying capacitors to improve power factor.
Although capacitors themselves do not generate harmonics, problems can be created when capacitors for power factor correction improvement are applied to circuits with nonlinear loads that interject harmonic currents. Those capacitors may lower the resonant frequency of that circuit enough to create a resonant condition. Resonance is a special condition in which the inductive reactance is equal to the capacitive reactance. As resonance is approached, the magnitude of harmonic current in the system and capacitor becomes much larger than the harmonic current generated by the nonlinear load. The current may be high enough to blow capacitor fuses, create other “nuisance” problems or develop into a catastrophic event. A solution to this problem is to detune the circuit by changing the point where the capacitors are connected to the circuit, changing the amount of applied capacitance or by installing filter reactors to a capacitor bank which obviously increases its cost.
Sources of harmonic currents include variable speed drives, frequency changers for induction furnaces, soft starters for motors, light dimmers, electronic ballasts for arc‐discharge lamps, as well as switch mode power supplies used in instrumentation, PC’s, televisions and uninterruptible power supplies. If a facility has more than 15% non‐linear load, a harmonic study should be performed before applying capacitors. The presence of harmonics can affect the proper operation of machinery, equipment, and processes, which can have an economic impact. The results of problematic harmonics need to be considered when evaluating operating costs.
"The Economics of Improving Power Factor"
By Ed Kwiatkowski, BSEE, MS President, Staco Energy Products Company
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