Tag Archives: Harmonics

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.

What are Harmonics and What do they do?

In the 19th century, Jean-Baptiste Joseph Fourier presented his theorem, which is known as the Fourier’s theorem. According to this theorem, a periodic function of period T can be represented as the summation of a sinusoid with the identical period T, additional sinusoids with frequency same as integral multiples of the fundamental, and a possible continuous component, provided the function has a non-zero average value in the period. The first two are known as harmonics, while the third is known as the DC component.

Of the three, the first waveform with a frequency matching the period of the original waveform is called the fundamental harmonic, while the second may have more than one component. Those with frequency equal to ‘n’ times of the fundamental are called harmonic components of order ‘n’. A conclusion drawn from the above discussion about Fourier’s theorem is a perfectly sinusoidal waveform can have only the fundamental component, and no other harmonics.

This also means an electrical system with sinusoidal current and voltage waveforms has no harmonics. However, protective devices and malfunctioning equipment in an electrical system can lead to distribution of electrical power with distortions of the voltage and current waveforms, creating harmonics. In other words, harmonics represent the components of a distorted waveform, and their presence allows analysis of any repetitive non-sinusoidal waveform from a study of the different sinusoidal waveform components.

Most non-linear loads generate harmonics. When a sinusoidal voltage encounters a load of this type, it produces a current with a non-sinusoidal waveform. It is possible to deconstruct these non-sinusoidal waveforms into harmonics. Provided the impedances present in the network are low, the distortions of the voltage resulting from the distorted current are also low, and the pollution level in the system from harmonics is below the acceptable level. Therefore, even in the presence of distorted currents, the voltage can remain sinusoidal to some extent.

Typically, the operation of many electronic devices leads to cutting the sinusoidal waveform to change its rms value or to obtain a direct current from the alternate value. In such cases, the current on the line transforms to a non-sinusoidal waveform. Several such equipment produce harmonics—welding equipment, variable speed drives, continuity groups, static converters, fluorescent lamps, personal computers, and so on.

In most cases, waveform distortion results from the bridge rectifiers present within the above equipment. Although these semiconductor devices allow the current to flow for a major duration of the whole period, they stop conducting for the balance part. This creates discontinuous waveforms with the consequent addition of numerous harmonics.

Apart from electronic equipment, transformers can also be the cause of harmonic generation and pollution. Even when a perfectly sinusoidal voltage waveform is applied to a transformer and it generates a sinusoidal magnetizing flux, the magnetic saturation of its iron core may prevent the magnetizing current from remaining sinusoidal.

The distorted magnetizing current waveform from the transformer now contains several harmonics, with the third one being of the greatest amplitude. Fortunately, compared to the rated current of the transformer, its magnetizing current is only a small fraction. As the load on the transformer increases, this percentage becomes increasingly negligible.