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Harmonics – dismissing the AC-drives myth

Product News Thursday, March 15, 2018: Danfoss (Pty) Ltd

The subject of harmonics currents and voltages is one that has received increased attention over the last few years. But, did you know that the biggest sources of harmonic current can be found in the large number of consumer items installed such as fluorescent lighting, LEDs, TVs and many other common pieces of equipment?

Drives are often wrongly accused of being the culprits. In many cases, disconnecting the drives has very little influence on the existing harmonic levels, but turning off all lighting can improve the situation significantly. All commercial and consumer equipment has to comply with reasonable harmonic requirements, but the huge number of common devices has a major impact on the harmonic levels.

The level of harmonic distortion in the supply network is on the increase, and this higher level of distortion can cause malfunctions in equipment connected to the supply. As the utilities supplying power must guarantee a certain voltage quality to their customers, the trend is obviously worrisome to them.

Harmonics are defined as signals (voltage or current) that are not at the fundamental frequency, but at multiples of it. In addition, interharmonics exist between the normal ones. The harmonics are mainly caused by non-linear loads – loads that draw current that is not linearly dependent on the voltage. Typical examples are diode rectifiers used in the power supplies for TVs, PCs and other electronic devices and fluorescent lighting; in both cases the supply voltage is a sine wave, but the load current is not sinusoidal, it contains harmonics. The harmonics in the current interact with the supply network impedance to create a voltage distortion in the supply. The allowed distortion in the supply is defined in different standards, notably EN 50160 and the IEC/EN 61000-2 series.

EN 50160 defines the voltage quality that is required at the terminals of a load. The IEC standards define the compatibility levels that the distortion in the supply must meet – actually with a good margin, as the allowed levels are derived from the compatibility levels and a margin must exist between what is allowed and what is used as a basis for planning. The standards are valid for public networks, where anybody can connect a load. In private networks, where the supply transformer is owned by the user, no formal limits apply, as it is within the user’s power to rectify any overly large distortion levels. In practice, owners of private supplies also apply the same EN/IEC standards.

As previously mentioned, the main sources of harmonic currents are lighting, both fluorescent and modern LED lamps, and power supplies in consumer equipment such as TVs, radios, refrigerators, freezers, PCs, you name it. Using a standardised network impedance and the allowed distortion levels, limits for the harmonic currents individual pieces of equipment may draw have been defined in IEC/EN 61000-3-2 and IEC/EN 61000-3-12.

Danfoss AC drives comply with the requirements of these standards. We also have other solutions that allow you to connect large loads to your supply without fear of creating an unacceptable distortion level. Under certain conditions, our solutions can actually ‘eat’ existing harmonics and thus improve the general situation.

For further developments on low harmonic solutions, keep an eye on this space…

Author: Michael Björkman, Technical Director, Marketing, Danfoss Drives.

Photo source: ‘Gleaming Lights of the Souls’ installation, Yayoi Kusama