Harmonics in drives and innovative solutions

Paul Boughton
Any repeating or periodic waveform can be broken down into a fundamental sine function plus a number of lesser sine waveforms of higher frequency called harmonics. A step wave for instance can be broken down into the fundamental frequency f plus a harmonic of 1/5 the amplitude at five times the frequency plus another of 1/7th the amplitude at seven times the frequency and so on to infinity.

There will be no even harmonics or any divisible by three (triplens). The breakdown of such a wave is called Fourier Analysis after the French mathematician and physicist, Joseph Fourier. The step wave analysis is f + 1/5f5 + 1/7f7 + 1/11f11 + 1/13f13, etc to infinity. Where the wave is not symmetrical around the axis it is because a dc-component is present. Second and third order harmonics are not present.

Any electrical device drawing non-linear current from the mains supply causes voltage harmonic distortion on the mains to a greater or lesser degree. This is because the high frequency components of the waveform interact with the supply impedance and create voltage distortion of varying magnitude and frequency. The effect is less on low impedance supplies, such as where a high voltage supply is provided and transformed down to low-voltage locally. However, it is exacerbated on high impedance systems, for example where the supply is by a long low-voltage network.

Why does this matter? Any deviation from the pure-sine wave supply for which electrical equipment is designed will function less well if there is a harmonic component in the supply. This is particularly true of transformers and cables where overheating and power-loss occurs, creating inefficiency and necessitating oversizing to compensate for the distortion. Skin effects are a minor side effect and not critical. More likely is premature ageing, synchronising failures and in extreme cases, erratic operation. Thus a cleaner supply is very much to be desired rather than a dirty one for a number of reasons.

In industry, control equipment utilising thyristor or diode input bridges is a recognised generator of harmonic currents, resulting in voltage distortion of the mains. This is commonly referred to as 'reflected' harmonics. Variable speed dc and ac drives particularly fall into this category.

Electronic dc and ac drives have been with us now for many years and their harmonic impact on the supply system was recognised and supply authorities produced guidelines as to the power capacity of such devices that might be connected to a 'point of common coupling' in an industrial environment. If the factory mains supply was a high impedance low-voltage (380/400/415V) supply, then the connected harmonic load would be more limited than if the supply was a low impedance high voltage supply, simply because the low-impedance supply can cope with more current harmonics. Simply, Ohms low U=RxI voltage distortion = grid impedance x total load current harmonics.

As industry has increasingly adopted ac drives for all manner of applications, so harmonic distortion of the mains supply has become an increasingly urgent concern. New regulations such as EN61000-3-2 and the forthcoming EN61000-3-12 regulations mandate a reduction in generated harmonics which is normally met with standard VLT drives (with DC-coils). Complying with regulations such as IEEE519, G5/4 and EN61000-2-4 does however sometimes impose limits beyond that which are possible by the use of standard drives alone. Danfoss is able to offer a number of techniques for diminishing the harmonic impact of drives upon the mains supply.

There are several solutions for larger three-phase drives. Standard drives universally have a 6-pulse three phase input, leading to the generation principally of 5th, 7th, 11th, 13th, 17th, 19th harmonics of diminishing value. Note that in a 6-pulse system the harmonics are all of the order of h=6p+/-1, where p is a positive integer. If a 12 pulse drive were to be created, then this would eliminate the larger harmonics up to the 11th and 13th and eliminate the 23rd and 25th. Creating such 12 pulse systems entails creating non-standard drive inputs with 12 switching elements and feeding these with a 6 phase supply, derived from a special input transformer with phase displaced windings. Danfoss offers 12p drive solutions, particularly for larger drives >250kW.

A more common solution for smaller drives is to employ a harmonic filter at the drive input. The Danfoss VLT AHF 005 and AHF 010 units are advanced passive harmonic filters - not to be compared with traditional harmonic trap filters. The AHF 010 provides suppression down to 10 per cent harmonic current. The AHF005 will provide suppression down to 5 per cent harmonic currents (THiD).

VLT active filters employ a much more pro-active approach to harmonic diminution. Proven VLT power electronics re-establish optimal sinusoidal power and unity power factor by generating counter-phased harmonic and reactive currents. The modular build-up offers the same characteristics as Danfoss' High Power VLT family, including high energy efficiency, user-friendly operation, back channel cooling and high enclosure grades. Danfoss Active Filters can compensate individual VLT drives as a compact integrated solution or be installed as a compact stand-alone solution at a common point of coupling compensating several loads simultaneously.

Finally, Danfoss offers a low harmonic drive combining an active harmonic filter and a standard drive in an 'all-in-one-box' solution. This solution is similar to the active front end - easy to specify and reduces harmonics to meet the toughest harmonic specifications.

Enter √ at www.engineerlive.com/ede

Gregers Geilager is Product Manager, Active Filters & Low Harmonic Drives, Danfoss Drives, Gråsten, Denmark. www.danfoss.com/drives

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