Problems Mains Harmonics Cause

Single-phase power converters are specifically problematic since they generate significant levels of triplen harmonics, such as the 3rd, 9th, 15th, etc. As they do not undergo phase cancellation, they add up linearly in the neutral conductor to create a particular nuisance. Apart from this, they are also present in zero-phase transformer flux, and heat up cables and transformers. Although three-phase converters also generate harmonic emissions, the triplen currents produced by them are of much lower levels.

Other non-linear loads also contribute to harmonic currents in the mains supply. Such loads include motors and transformers, welding equipment and arc furnace rectifiers. Another source is the fluorescent lamp with magnetic ballast. However, rectifiers produce much higher frequencies as compared to that from fluorescent lamps.

The harmonic currents an equipment draws from the AC mains supply do not alter the power the equipment consumes when measured in Watts. However, the harmonic currents increase the VA rating of the equipment. Since Power Factor is the ratio of the Watts to the VA the equipment consumes, the equipment that produces significant emissions of harmonics also has a lower power factor.

A resistive load, such as an incandescent lamp, has a PF of 1.0 since it consumes the same amount of power in Watts, as it does in VA. Therefore, an incandescent lamp cannot emit any harmonic content. On the other hand, electronic equipment with rectifiers at the input and with no harmonic reduction techniques have power factors of around 0.6, implying they generate harmonic currents. Fluorescent lamps with magnetic ballast, running at 50/60 Hz, usually have PF of the order of 0.3, so they generate significant amounts of triplen harmonics.

The power factor of the load is significantly different from the power factor traditional electrical generation and distribution engineers use—the latter is the cosine of the angle between the sine-wave supply voltage and its load current. While the traditional PF assumes all loads are linear using sine wave voltages, engineers adjust this PF by adding capacitance or inductance to the power line, depending on whether the load is resistive, inductive, or capacitive.

However, the traditional method of PF correction for linear loads fails when trying to correct the PF of a rectifier-input electronic power converter. Mains power distribution networks are now driving significant numbers of electronic loads as these operate at higher efficiencies, and electronic loads are now replacing most linear loads.

The standard IEC 61000-4-7 [6] offers a survey of harmonics present in power supply systems. Typically, there are four major kinds of problems that harmonic currents cause when they are flowing in mains power supply networks:

  • Problems that harmonic currents themselves cause
  • Voltage distortion from harmonic currents
  • Problems that voltage distortions cause
  • Interference to telecommunication networks

In large installations with several single-phase electronic loads, such as in modern offices, the total neutral currents may reach as high as 1.7 times the highest phase current. This is the effect of harmonic currents, mainly the triplens, as these flow without being cancelled, in the neutral conductor. As many older buildings have half-sized or even smaller diameter neutrals, there can be a risk of fire hazard.