Passive Filters for Minimum Life Cycle Cost

Harmonic currents can originate from systems that do not contain energy and in which the sign of the current matches that of the voltage throughout the cycle (e.g. a phase-angle controller for an incandescent lamp). The term ‘wattless current’ is sometimes applied to harmonic currents that do not have substantial voltage harmonics of the same orders to multiply them. The result is a product of current and voltage that is zero. Those harmonic currents have a lot in common with reactive currents:

  • They are both undesirable in that they require part of the capacity of generators, cables, and transformers, while contributing nothing to the generation and transport of electrical energy.
  • They both cause additional losses—because the voltage drop is phase-related to the current so the product is real and non-zero.
  • Harmonics originate mostly from the power-consuming load and flow back to the energy source, against the normal energy flow. (An exception is a renewable energy source connected to the grid by a power electronic converter where the harmonics flow away from the source.) Fundamental reactive power does not have a defined direction—intake of inductive reactive power is synonymous with output of capacitive reactive power and vice versa.

It should therefore be possible to combat both reactive power and harmonics by similar means.

This is indeed the case. This publication explains how it can be accomplished effectively and how dedicated filters can be created for individual frequencies.

In addition to the general principles and guidelines, this paper tackles the importance of using high-quality components for accurate filter tuning. It discusses whether to compensate centrally or dispersed, and how to choose the inductor for a given L/C ratio. Finally, it warns that special caution must be taken to (i) prevent the filtering out of sound frequencies used by utilities, and (ii) that despite filtering, harmonics should still be taken into account for rating cables and equipment.

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