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Power supply harmonics: Pay attention or pay the price
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March 2008
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Bob Liddle, managing director at Block UK, provides useful advice on how industrial companies can reduce the effects of harmonic distortion from their power supply lines, thereby eliminating equipment failures and cutting production downtime, repair and replacement costs
Power supply networks are the life support of modern industry, but their quality
and reliability are rarely questioned. Manufacturers are basically interested only in whether current is flowing or not. The majority of companies never question how good or bad the quality of the power source they are using actually is. However, on closer examination, the destructive effects of distortion sources, such as harmonics, can be seen. But with the help of suitable harmonic filters, the effects of harmonic distortion can be practically eliminated.
Although the power supply grids in Europe are amongst the most reliable in the world, this does not guarantee the quality of the power they deliver. Unfortunately, machinery and equipment can fail with no apparent reason, or can show signs of failure with expensive repairs. In these cases, an analysis of the mains supply is critical.
Harmonic distortion on modern power grids can harm equipment in different ways. For example, vibration and noise levels are annoying symptoms, but their effect on the operation of machines is minimal. Another is the harmonic distortion present in communication lines and in electronic circuits, which can cause malfunction and fault conditions.
Overheating of transformers, cables, motor windings and in capacitor banks can also be caused by harmonic distortion. These faults can reduce the life expectancy of machines and equipment, causing unnecessary expense and production downtime for a company.
Rectifiers, frequency inverters, UPS devices and electronic power supplies are generally found as loads in all modern power grids. The current that they draw is not sinusoidal but pulsating. One cause of this is the pulse current drawn for charging smoothing capacitors. Inevitably, this results in a reverse current flowing back into the supply grid. There are numerous national and international standards limiting these harmonic levels to certain values. For example, DIN EN 61000-4-2; DIN EN 61000-3-12 and IEEE 519-1992.
Unfortunately, in industrial everyday practice, these standards are not always being adopted. For reliable industrial power distribution, having in the main three phase loads, the fifth harmonic at 250Hz and the seventh harmonic at 350Hz are of primary concern.
Three phase systems such as variable speed drive units and large UPS systems utilise three phase rectifiers (6-pulse bridge). The order of the harmonics depends on the number of pulses the rectifier has. For a B6 bridge circuit, the resulting harmonics has the order of V=K*P±1 (where K = integer number i.e. 1,2,3,4 etc. and P = Pulse Number i.e. 6 Pulse bridge). This means that harmonics with the number five, seven, eleven, thirteen, etc. are present. Single phase loads such as computers and monitors in administration buildings generally only produce the third harmonic, which if left undetected could cause fires.
In every electrical system, losses are caused by current flowing and magnetising currents. These are taken into account when calculating the gauge of cables and the size of transformers and drives equipment. All of these losses have to be accepted, because they cannot be completely eliminated, even with modern technology. But harmonics cause other losses, including copper and iron losses in transformers and copper wires. Adding all these losses together means that for a typical industrial company, around 5% of its total electricity bill is comprised of losses.
Until now, very little attention has been paid to these cost factors, mainly due to lack of knowledge or lack of adequate means of monitoring. Modern supply analysing equipment makes these problems more transparent. With these aids, it is easier to track down the source of distortion, which is the first step in solving the problem. Block UK assists its customers, analysing the line supply voltage, and helping to decide which harmonic filter is best suited for the application.
Up to now, industry has used reactors or active filters to protect the line supply from harmonic distortion. With both systems, a reduction in the level of distortion can be achieved. However, both methods have distinct disadvantages. The effectiveness of a reactor is not always adequate, so reactors only have a limited implementation. Active filters, however, eliminate harmonic distortion almost completely; the drawback being that the technical expenditure is comparatively high.
Block has developed a third method of dealing with these harmonic distortions - the ‘harmonic filter module’ (HFM). During development of the filter module, priority was given to the fifth and seventh harmonics, which cause most problems in industrial applications.
With the harmonic filter the proportion of THD (total harmonic distortion) for any frequency inverter and intermediate circuit using B6 input bridges is reduced significantly, typically by 84% up to 95%.
Depending on the application, a filter can be used either directly in front of the source of the harmonic distortion or as a complete system filter in the distribution cabinet. Typically, filters are designed for the individual equipment and its harmonic distortion level, which means the design is relatively straightforward.
Normally, a filter is designed for the power rating of the equipment that is causing the harmonic distortion. In order to optimise the design of the required filter, Block offers an on-site analysis of the power quality. This information of THD current levels enables a more effective design of the required filter module.
Block’s family of harmonic filter modules range from 7kW up to 800kW. Larger power ratings can be achieved by connecting modules in parallel. Due to the very high production quality and the use of only high quality components, the system loss caused by the filter module is very low (HFM efficiency is higher than 98%). A comparison between industrial applications with and without the harmonic filter module, show the advantages of the system with a filter.
The primary advantage of these filters is that they need no servicing and are relatively easy to install. Practical experience has shown that far fewer fault conditions or equipment failures caused by harmonic are present when harmonic filter modules are installed. The thermal stress on a transformer and motor windings, installation wiring and premature ageing of electronic circuits, is reliably prevented.
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Author
Chris Rowlands
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