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The drive to maintain harmony

With the national grid under ever more strain, harmonics are likely to remain an issue for drives users for many years to come. Dr Martin Payn of Parker SSD Drives offers an overview, and suggests ways to prevent issues becoming problems.

Mains-borne harmonic disruption, also known as radio frequency interference or noise, is a two-way street: you can receive and suffer from it, but you can also generate and transmit it to others (including other parts of your own plant or installation). In fact it is very difficult for certain types of electrical equipment not to generate it, so if you use such items it is incumbent upon you to contain the problem. Inverter drives are particularly prone to generating noise because of their constant power switching. 

A drive works by rectifying the sinusoidal mains voltage wave form into a DC link, resulting in a series of current spikes from the supply. The spikes, although necessary, are essentially harmonics, which if allowed to escape from the drive to the mains, can cause problems to other equipment. It is often thought that the problem is restricted to big drives, but this is not the case; a drive of any size can produce harmonics. Naturally there are regulations in place and less formal customs and practises that address the situation. Essentially, Europe and America takes the view that everybody should do their bit to minimise the problem; Asia works more to the principle of leaving individuals to deal with the quality of their local power supply themselves. Interestingly marine and offshore systems generally have poor mains quality, due to their limited on-board generating capacity, so use international maritime standards as their yardstick.

Harmonics can disrupt any equipment, but are particularly noticeable with lighting and sensitive electronic equipment. The latter\'s increasing popularity in industrial systems means harmonics are becoming less and less tolerable. The presence of harmonics means you have to use bigger cables and transformers than you would with a clean supply, which naturally cost considerably more. This is the responsibility of the end-user company on one side of the Point of Common Coupling, and the responsibility of the supply utility on the other. 

The utilities, of course, frown on this extra expense and thus have an interest in driving the Standards along. There are several potential solutions to harmonic problems, each with advantages and disadvantages and each finding favour in different industries. One basic principle for lowering harmonics is to add supply inductance, by way of chokes, either before or after the rectifier bridge that convert the AC wave to DC link. This is a fairly common solution and can be built into small and medium sized drives by their manufacturers. But this has the effect of reducing the DC link voltage.  

Alternatively you can reduce the capacitance across the DC link which in turn reduces the incoming harmonics. But this in turn reduces braking efficiency and tolerance to faults. It can compromise the drives braking abilities to the point where additional brakes become necessary. However, it is a popular solution because it is relatively cheap and adequately effective in many situations.

Active Front End
Another solution, used in nearly every marine installation and with many large capacity drives is called an Active Front End (AFE). This is in effect two identical drives running back to back. This has the effect of allowing the current to follow the supply side waveform and effectively makes the drive input current follow the supply side voltage wave form, effectively making it invisible to the mains. Advantages include the ability for power to flow in either direction without causing distortion and an inherent ability to compensate for mains borne distortion. The disadvantages are of course cost and size.

A similar solution is to use a standalone filter. These are tuned to specific harmonics and are passive rather than reactive like an AFE. However they don\'t work as well as an AFE and they cost nearly as much. The final solution is to develop transformer circuitry that increases the number of pulses in the supply side wave form from three to six, 12, 18, or 24. This is a bespoke solution that has to be developed for each individual application, so is always going to be an expensive option. 

In reading the above, it is possible to get the feeling the harmonics is an almost insurmountably large problem. However, that is not the case. Often problems simply never manifest themselves, or they are small enough and rare enough to be ignored. However, we can expect that the issues will become steadily more common over the coming years. This is partly due to the increasing strain being placed on the available generating capacity, partly due to the unstoppable proliferation of sensitive electronic equipment, and partly to the aging of the cable and transformer supply infrastructure which simply cannot be modernised fast enough. We must expect legislation and standards to tighten steadily over time.

Legislation already exists around the world and in the UK to prevent corruptions of the mains by users of equipment such as drives. This is embodied in the G5/4 Engineering Recommendations and measures power quality at the \'point of common coupling\' - ie the mains incoming point for a given plant, factory, large office, hospital, shopping centre or other site. It does not cover power quality within the site.

However for many sites - indeed most sites - internal power quality is important so that the performance of individual machines can be guaranteed. It is worth considering the end consquence of a poorly operating life-support machine, or the effect on the mains of an x-ray machine. In a commercial environment, computers and other delicate office equipment will not tolerate too much disruption to a steady power supply. Retailers will not put up with moving displays or lighting looking anything other than perfect. Furthermore, in every environment there are human safety issues if machinery cannot be relied upon to operate to incredibly high levels of consistent performance.

Most drive manufacturers, such as Parker SSD Drives, offer a range of solutions to harmonic issues. They will also have considerable in-house expertise so can offer advice and guidance to most situations. It should also be noted that the technology is continuing to develop, so a solution that was acceptable say five years ago may now be seen as approaching the end of its life and in need of upgrading.
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