Get rid of harmonics before they cost you money

This short technical note is about minimising harmonics; yet there will be those who don't as yet realise the extent to which harmonics cost money.

For example, harmonics can defeat the opportunity for the installation of energy saving power factor correction equipment, burn up energy, cause equipment failure in transformers and motors and unexpected tripping of protective devices.

Chances are that some readers will be unaware that their maintenance and repair bills are in fact due largely to circulating harmonic currents in their plant.

Without taking precautions, much industrial and commercial plant generate up to 50% of actual useful-energy load current in the form of harmonics.

Harmonics are minimised by using filters or reactors and I discuss their applications here. As a general rule, all industrial power electronics cause harmonic currents to circulate in the internal busbar and cable network of a factory.

Examples include variable speed drives, uninterruptible power supplies, induction furnaces, electric welding lines, etc.

Variable speed drives are of particular interest because (a) they are ubiquitous and (b) as they can also generate high frequency components in their output lines that drive motor - and drive them to destruction.

High frequency components heading towards the RF spectrum are not to be confused with harmonics which are multiples of line frequency and generally under 100 kHz, but both are ameliorated by filters or reactors.

I will stick to the variable speed drive as a typical industrial example but UPS systems and many other industrial electronics incorporate the same structural components consisting of AC input, converter, DC link, and inverter output, or at least the first three of these.

At the input end of a drive input, filters or reactor can be selected. Another possibility is a 5th harmonic trap consisting of a series inductor-capacitor combination in each of the phases.

The 5th harmonic is not, however, the only component that should be minimised, although because of its opposite phase sequence it has particular importance as it can interfere with the smooth running of line connected motors.

Harmonics suppression can reduce the load capacity of the drive, with power loss increasing, generally, as the total harmonic current (I) distortion (THID - which is the percentage of harmonic currents related to the power delivering 50 Hz fundamental current) is decreased.

The principal reason for this is because of the saturation characteristics of reactor cores, requiring a reduction in line current.

Selection of core materials and the combination of reactors, resistors, and capacitors into a filter design, such as is the case with the MTE Matrix filter (a T-element low-pass LCR design), can preserve suppression efficiency without necessarily reducing power output of the drive through core saturation.

As an example, a 5% reactor will require a decrease of 60% or more in drive output if a THID of 10% is desired.

A Matrix filter with an equivalent percentage impedance (ie, the percentage by which it reduces the prospective short circuit current of the power circuit feeding the drive) of 5% does not reduce the drive output.

Although variable speed drives are often fitted with output filters whose function is to smooth the voltage waveform to a more or less acceptable sinusoid, harmonic filters for the drive input are seen as an 'as required installation' item for the user.

Most smaller drives are of the so-called six-pulse variety with an inherent THID of 110% at light loading.

Note: the term 'pulse' refers to the number of individual phase rectifiers in the input stage converting the AC input to a DC voltage which in turn powers the inverter - or frequency control - for the connected motor or motors.

Higher pulse numbers reduce the harmonic content. These are achieved in polyphase networks derived from three-phase supplies through wye-delta secondary transformers and generally through phase-shifting transformers. The larger the number of phase rectifiers, the smoother the DC link voltage and the smaller the THID.

Even so, 12 pulse or 18 pulse converter input drives are not common because of the additional expense of construction.

In any event, at light loading (Figures 1 and 2) all unfiltered poly phase converters still have a high THID, the 12-pulse being at about 50%.

It can be seen from the THID curves that the harmonic contribution is not only sharply reduced when Matrix filters are installed on a drive input but also that the THID stays constant over virtually 100% of the load range.

This is an important feature since many machines employing variable speed drives are highly dynamic, eg, CNC machines, robotics, and these give rise to highly variable harmonics generation.

The subject of harmonics in power circuits has not been exhausted but this note should at least alert readers to the opportunity to 'clean up' their 'electrical act', thus reaping benefits of increased plant reliability, power savings and so on.