The importance of power quality

Apr 2, 2013 | Electrical & electronics

With the need to reduce energy consumption becoming essential, the importance of power quality has been marginalised. According to Steve Hughes of REO UK, however, improving power quality can result in a reduction in the cost of electricity

A power quality issue is generally defined as a voltage, current or frequency deviation in the applied mains that has a detrimental effect on the performance or longevity of connected equipment. Usually, if the voltage or current is not a pure sine wave then there is a problem.

In the UK the useful power supplied to a system is at a frequency of 50Hz and at unity power factor – anything that deviates from this will result in higher costs. Therefore, if additional harmonic currents are being drawn, users will pay for them in cash; and if a site has a low power factor, it is likely they will be paying for that too.

More often than not it is the energy user’s own equipment and that of their neighbours that is the root cause of quality issues, not the power utility companies. Almost anything connected to the mains network can cause difficulty, but components and systems that switch high power at low frequencies – such as power factor correction (PFC) capacitor banks, thyristor-controlled equipment like heater banks, and the DC input sections of variable speed drives (VSDs) – create the most issues.

These can cause sags, swells, transients and harmonic issues which affect equipment performance and longevity. Despite the known benefit of increased power quality, however, it is likely that users are not bothered by this as long as their equipment and site is running satisfactorily.


Smoothing out harmonics

Most VSD manufacturers will recommend that users deploy some element of harmonic mitigation, and that will typically involve an iron core mains choke. An input choke will effectively smooth out these harmonics to make the current waveform more sinusoidal and less harmonic-laden. This is demonstrated in a Fourier analysis of harmonic content with and without mains chokes.

REO makes single-phase and three-phase line chokes with one, two and four per cent inductance, allowing OEMs to make engineering decisions to mitigate harmonics based on the application and environment. Ideally they would measure the harmonics and select a choke accordingly, but often they would select a two percent choke with the understanding that this offers the best performance/price trade-off.

Input chokes and mains RFI filters are typically discrete components. REO can, however, easily integrate them as a single choke-filter combination. These can be water-cooled and even added to an output choke or brake resistor, so the customer does not need three separate parts. Of additional benefit, these offer IP64 ingress ratings, useful if the equipment is to be used in a harsh environment or where there are space constraints.

Addressing issues

Recently, REO was asked to look at a site that was experiencing some reliability issues. The site was using a large number of high current DC power supplies, comprising thyristor bridges in a typical B6 configuration.

The phase current was approximately 1500A, but this comprised of a number of higher order harmonics. REO determined that this was the first issue that should be addressed and subsequently designed, specified, manufactured and fitted appropriate harmonic filtering. The result was two-fold: an increase in power quality with an improvement in process reliability, but also a 500A reduction in metered current. For a site that operates continuously the payback on the filtering equipment is many times over.

This example is perhaps one of the clearest that could be produced to illustrate the cost savings that go hand in hand with power quality engineering. A 500A reduction in metered current on a 1500A application represents a cost reduction of one third. Imagine combining this kind of saving with more conventional energy reduction methods.

So instead of thinking of the cost of power quality, think instead of saving money and improving power quality into the bargain.


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