Halting harmful harmonics

April 10, 2018 // By John Mitchell
The rise of non-linear loads in industrial environments over the last two decades has resulted in the growing problem of harmonic currents and utility-level voltage distortion. Facing a lack of awareness, the industry has struggled to implement effective mitigation techniques. 

When The Beach Boys tell us they're "pickin' up good vibrations" it's fair to say that they probably hadn't heard of the importance of power quality. Ironically given that musical reference, the very issue that the industrial community has increasingly struggled with has always been an essential and positive technique in pop music. The acoustics created by a guitar amplifier often rely on the distortion of the fundamental frequency, by adding multiple sound waves, or overtones to create a warm, fuzzy sound.

It's a different story in industry, where voltage distortion, caused by current harmonics can wreak havoc on the plant, its equipment and the mains power supply. Damage can be serious and varied with the most common symptoms including voltage notching, motor vibration, arcing on bearings, nuisance tripping, electromagnetic interference (EMI/RFI) and overheating. The thermal stress on components can cause them to wear out quicker and inefficiency through heat loss results in increased energy costs in the long term.

So why is this now becoming such a cause for concern? The last few decades have seen a rise in the use of non-linear loads such as transistor based variable speed drives (VSDs) and line commutated DC drive systems. The processes of high frequency switching and pulse width modulation (PWM), introduce unwanted multiples of the fundamental 50Hz frequency in the form of harmonics.

 

Industry challenges

Various approaches have been used to combat harmonics over the years. This has led to many suppliers using setups which are not meant for harmonic mitigation, in configurations that are often unnecessarily complex, outdated, take up lots of space or are inefficient, ultimately raising costs.

There is the added issue of meeting international harmonic control requirements such as IEEE-519 which limits "the maximum frequency voltage harmonic to 3% of the fundamental and the voltage total harmonic distortion (THD) to 5% for systems with a major parallel resonance at one of the injected frequencies." Some form of filtering is subsequently recommended.

We're lucky in the UK to have a stiff grid, but this is not true everywhere. Developing countries often suffer from weak grids, with an unreliable supply and inadequate infrastructure. The power ratings on products are often based on calculations performed in ideal conditions. Buyers would be wise to note that these products may perform adversely in weak grids and may not perform to IEEE 519 standards in these conditions.

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