Matrix filters promise harmonic solutions

With mains-borne harmonic corruption increasing year on year, filter technology needs to develop significantly from its current level.

With mains-borne harmonic corruption increasing year on year, filter technology needs to develop significantly from its current level.

Matrix harmonic filters promise a significant step forward in the battle to ensure power quality, as Terry Hutson, Senior Project Engineer at Silverteam reports.

Twenty years ago power quality was uniformly high across virtually the whole of the British national grid.

But the steady rise in the use of equipment such as variable speed drives, UPSs, electronic welders, pumps and fans, induction heaters, battery changers, contactors, circuit breakers and servo drives, all of which cause harmonic corruption means this statement is no longer true.

Other major corruption contributors include passenger lifts, office automation equipment such as computers and printers, magnetic imaging and other medical equipment.

And with all these technologies becoming more and more common, the problem could reach epidemic proportions if not addressed.

The same problem is becoming apparent around the world.

Some countries have taken the view that it is up to the individual power users to protect themselves from dirty mains; others, including Britain, have adopted the stance that users of harmonics generating equipment need to "secure their premises" so that no corruption is fed back into the mains by offending equipment.

Whether you are, as in the UK, ensuring that you don't leak harmonics or, as in say America, protecting yourself from a potentially dirty supply, you use the same technology -filters.

These are mainly fitted to individual machines, but a single big one could be used to cover an entire production line, factory or office.

Filters are made up of capacitor and reactor circuits and have to be selected to deal with the particular frequencies present.

Most common filters are supplied with a "percentage performance rating", recognising that they do not stop all harmonics.

Achieving sufficient performance has traditionally required the installation of specialist filters, the expense of which make them prohibitive to all but the most special mission critical cases.

But the state of the art is now advancing with the development of matrix filter technology, which can meet or exceed the harmonic mitigation performance of other common filtration methods.

Unlike alternative solutions, matrix filters come with a performance guarantee, typically substantially exceeding the performance of a 12- or 18-pulse rectifier from a six-pulse configuration.

That is not to say that there is 100% harmonic containment, this is impossible to achieve, but (unlike most filters) the specified performance of say 5% THID (total harmonic current distortion) or 8% THID is achievable over the whole working range from no load to full load - and also through periods of rapid load change.Additionally, matrix filters will not cause power system resonance nor attract harmonics from other nonlinear loads.

No system analysis is required to select and apply matrix filters.

Currently the most common type of filter is the bandpass, which is built up by an array of capacitors, each in series with a reactor.

Each array acts as a filter, tuned to a frequency just below the harmonic frequency that is to be filtered.

By contrast a matrix filter is a multistage lowpass filter which achieves a broadband attenuation of all harmonic frequencies, while maximising the attenuation of the fifth harmonic.

Three phase AC supply is connected to the input section which contains a three phase AC reactor and proprietary circuitry which inhibits oscillation of the filter with the AC power system.

The centre leg consists of a series reactor and capacitor bank.

Because of the capacitor bank the matrix filter operates with leading power factor at all loads, but unlike band-pass filters the matrix filter does not produce significant voltage rise at the common coupling with the power system.

Matrix filters reduce the stresses on electrical equipment by reducing the true RMS current, peak current and harmonic frequency distortion that are generated by nonlinear loads.

They also absorb transient overvoltages to prevent trips and rectifier damage.

This increased system reliability means higher productivity.

There are also significant side benefits, such as a reduction in the amount of heat generated by rectifying field equipment, thus extending their life expectancy.

Matrix filters improve the input current waveforms of nonlinear loads to nearly sinusoidal.

This results in lower peak current and true RMS current demands while achieving cleaner power supplied to the equipment.

By nearly eliminating the wasted energy associated with harmonics, matrix filters reduce the true RMS real power (kVA) demanded from a power source.

Mains-borne harmonic corruption is becoming more and more common.

Factories and production facilities, are automating by using ever more potentially corruptive equipment; the population of computers, photocopiers and other office automation equipment is growing exponentially, hospitals and healthcare are being transformed by new technologies.

So filtering to maintain power quality is becoming not only essential but also a legal requirement.

For mission critical applications such as passenger lifts, hospitals, airports and where wireless communications technologies are in use matrix filters have already established themselves amongst the most effective solutions, and now users with less critical applications are beginning to discover their benefits too.

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