There is ever increasing pressure for plant operators to conserve energy by fitting variable speed drives, but the equipment can cause corruption of the mains supply – against which there is equal official scorn. Until recently, the solution has been seen as fitting a small filter to each drive, but Guy Kennett of Mitsubishi Electric Automation Systems is now tending to favour a single active harmonic filter for the entire plant.
If you want to watch a plant engineer squirm, ask about harmonic corruption. Most engineers have only vague ideas on the subject: they know its important and that it can cause all sorts of problems, but are too busy to get to the bottom of the matter.
The fact that no preferred nomenclature has evolved indicates that the subject has not been widely discussed at a plantfloor user level.
It is generally thought that the following all mean essentially the same thing: harmonic disturbance; mains-borne corruption; dirty power; radio frequency interference, rfi, feedback, noise.
But this is not the case, there are differences which can be very significant. For instance, harmonics are usually present in the background, most problematic in multi-drive or multi-servo installations and often affect power equipment, whereas EMC can be caused by a single piece of equipment (often as small as a valve actuator or mobile phone) and tend to affect just one or two devices rather than the whole system.
Interference is easy to recognise when you hear it on the telephone line or see it on the TV screen, and the industrial version is not so very different.
Technically there are three major causes, high speed electronic switching, power supplies inadvertently spilling a tiny part of their power to nearby equipment, and changing electric and magnetic fields causing unplanned coupling between circuits.
One way to visualise it is to image the power supply as being like waves at sea; if a section of a wave hits something solid that part is reflected back into the path of the next oncoming wave and disturbs the otherwise smooth flow of waves.
Mains-borne harmonics of a frequency close to that of the power supply have the potential to cause problems of corruption, noise, heat and vibration both within the mains to other equipment.
Plant and equipment that includes rectifier circuits, such as variable speed drives, arc welding equipment, and transformers are the worst offenders. Often harmonics from a single source, while irritating, are not a major problem. But when you have multiple sources, the harmonics become additive and the cumulative effect can be very significant.
Because the corruption has the potential to travel considerable distance and disturb not only adjacent plant but also other factories and power users.
But harmonics are a fundamental part of the physics of electricity. You cannot eradicate them completely so, like the spam emails and evening telesales calls, you have to develop strategies for dealing with them.
Many pieces of equipment generate harmonics, namely those that convert between AC and DC. In an industrial situation, perhaps the greatest culprit is the variable speed drive, but others include generators, universal power supplies, power factor correction equipment, battery chargers, arc furnaces and some temperature controllers.
In offices, single phase equipment with corrupting potential includes computers, fax machines, photocopiers, UPSs, TVs, VCRs, etc. Lighting dimmers and electronic ballasts for high efficiency lighting in the home and ultra-violet disinfection systems in hygienic environments are equally problematical.
Twenty years ago, harmonics were not much of a problem, simply because there was not much of the above type equipment around. But manufacturing industry, along with just about every other sector of human endeavour, has spent the intervening years working very hard to automate previously manual processes and achieve productivity gains.
More recently a new front has opened up, as we try to reduce energy consumption to cut both costs and the production of greenhouse gases. These trends have lead to a steady and continuous increase in the number of variable speed drives and other harmonics generating equipment installed in factories and plants everywhere.
A decade ago for instance, about 100,000 drives a year were installed in the UK; now the figure is several multiples of that and expected to perhaps double again before this decade is out.
Legislation In the mid-1990s legislation and official guidelines were introduced to contain the problems of harmonics. (Interestingly, the British and European laws required users of harmonic generating equipment to control their disruption of the mains, whereas in some other parts of the world the view was that people likely to be affected by the problem should protect themselves.)
So plant operators had a flurry of fitting filters to drives and hoped that would be an end to the problem. But now a new technology in the form of ‘active’ filters has spun out of the US military’s stealth programme and has opened up new architectures for dealing with harmonics on a plant-wide basis.
The effects of harmonics are many. They reduce the efficiency of power generation, transmission, and utilisation; age the insulation of components; cause malfunctioning and failure of electronic equipment; overheating and failure of electric motors and power factor correction capacitors. They also create problems with resonance due to the interaction of capacitors with harmonics and huge inaccuracies in metering equipment. Fuses can be burned out and circuit-breakers tripped; computers, TVs, radios and telephone systems disrupted.
There are several methods for controlling harmonics, including phase shifting, phase staggering, passive filters and active filters.
In phase shifting a phase transformer is used to effectively split a supply into two with a 30º phase shift between them. This has the effect of some of the harmonics cancelling each other out.
Phase staggering is the phase shifting of individual loads to achieve a cancelling effect. However it does require loads of similar ratings to be matched against each other.
Currently the most popular way of dealing with harmonics is passive filters. These are made up of capacitor and reactor resonant circuits ‘tuned’ to present a low impedance path to the frequency of specific harmonics. Typically they are connected to individual loads (e.g. drives) or small groups of similar loads, so can be time consuming and expensive to install. They even create ‘anti-resonance’ problems!
AIM (active injection mode) or active harmonic filters are far more technically advanced and offer an effective solution for reducing the total harmonic current distortion to below 5%, in line with IEEE 519. They work by continuously monitoring the harmonic current demanded by the load and generating an adaptive waveform which matches exactly the shape of the non-linear portion of the load current.
Active harmonic filters represent the first new thinking in the field of harmonics control for half a decade or more, and their potential seems to be enormous, so let’s look at them in more detail: Active injection compensates for all harmonics from the 2nd to the 51st. It has a very fast response time (typically 100 micro-seconds), so is especially suitable for dynamic loads. Further, it is not affected by changes in system impedance so is inherently non-resonating.
The active filter can mitigate individual or multiple non-linear loads, i.e. it can be used with a single piece of equipment, with a number of items (i.e. fitted at motor control centre level), or installed centrally to control all harmonics within a plant or factory.
Active filters are ‘current limited’ and will function regardless of the magnitude of the harmonic current. They also provide ‘electronic power factor correction’ via reactive power compensation – typically 37kVA per 415V, 100A AIM. Reliability is high, with an MTBF well above 10 years.
So, where are active filters likely to find their niche? They represent such a major advance in the control of harmonic corruption that they are leading to a fundamental reassessment of current practice. They are unlikely to find their way into home installations, but that looks like being the only limitation!
One of the key points is that they are very simple to install, requiring only three phases and two current transformers to be connected. As such they are highly attractive for retrofitting to existing installations and equally good for new installations.
Their ability to deal with multiple loads reduces the number of individual filters that require fitting to a very low number. And finally their highly dynamic response will prove particularly useful in situations where the load can change suddenly, e.g. where large drives are likely to be brought on or off line during the course of a shift, or where arc equipment is in use.