Harmonics threaten your electrical installation
By: Subodh Bhatia, Managing Director, Westek Electronics Pty Ltd
The introduction of the carbon tax, and the frequently reported charge of ‘gold plating’ of poles and wires for the delivery of electrical power may have focussed the attention of building owners and operators on electricity bills they might face in the future. There’s certain inevitability about the cost of electricity for consumers. However as we point out here there are measures that can be adopted for existing and planned electrical installations that may not only save electrical energy but also future-proof your building and thus ward off the effects of increased energy charges. An important part of the solution is the installation of active harmonic filters.
Technically the issue addressed is that of harmonics mitigation. Harmonics, without getting into electrical engineering to any extent, are a modern phenomenon. They have come about because of electrical loads such as electronic ballasts for fluorescent lighting, compact fluorescent lighting (CFL), LED lighting, computers, electronic cash registers, scanners and modern heating, ventilation and air conditioning (HVAC). Harmonics basically cause significantly additional current to flow to that needed to provide energy to these types of loads.
Energy is measured in kilowatt-hours as is evident from electricity charges. You are not as yet charged, in all likelihood, for the additional current that has to flow as a result of harmonics but that is not the end of the story as the introduction of electronic electricity meters to replace the more traditional ‘spinning disk’ types makes the additional charging for excessive demand caused by harmonics possible. Harmonics imperil electrical facilities because they can stress the electrical wiring, distribution boards, switchboards and substation transformers to breaking point. Instances of switchboard fires as a result of excessive harmonic current have occurred. Yet based on kilowatts there was no excess power usage.
Electricity supply companies have not been vocal about harmonics. However they are increasingly insisting that harmonics causing voltage distortion at the customer’s connection point (point of common coupling—pcc) does not exceed the maximum values imposed by Australian Standards AS/NZS 61000.3.6.and this specifies a total distortion level for voltage of 8% and in addition specifies much lower levels for individual harmonics. The limitation on voltage distortion is likely to affect new installations.
We’ll explain harmonics without digging too much into electrical engineering. Generally power companies want to supply AC voltage to consumers as a smooth sine wave, oscillating at a frequency of 50 cycles per second. Electrical loads like electric bar heaters will draw an equally smooth sine wave current. However electronic loads, virtually without exception, will not—instead very peaky current flows. This is called a ‘distorted current’. Such a current is composed of harmonics and these are multiples of the 50 cycles per second AC current (i.e.: 150, 250, 350 cycles per second, etc.).
In addition to voltage distortion mentioned above, harmonics cause additional heating of wiring, switchboards, and transformers. These heating effects are serious—not only are they proportional to the square of the harmonic currents (for example if the 5th harmonic—a frequency of 250 cycles per second—might typically be 30% of the 50 cycle current, that will add a further 70% heat load. But there’s worse—the heating effect is also proportional to the square of the frequency. And this can very easily double or even quadruple the heating the installation has to withstand.
A good solution for harmonic mitigation—active filters
Harmonic filters—as their name implies—filter out the harmonic current contribution, preventing that current from flowing through the electrical installation. Or they do that to whatever extent possible as dictated by technology constraints—and they are particularly effective if placed close to the harmonic current generating loads. Active filters as opposed to passive filters are able to ‘roll with the punches’ adapting to rapid changes in the harmonic load as is typical of many installations. They basically function by measuring the harmonic components, and then ‘sucking up’ just that part of the load current.
It is important to note that not all active filters perform the same correction task. A critical parameter in electrical installations is that of flicker. This is caused by the switching in and out of loads, start up of heavy current drawing loads, and active filters need to have a broadband correction methodology so that response within two-thousands of a cycle period is possible. The filters should also permit tailoring of the selective filtering of designated harmonics such as the fifth, and other negative sequence harmonics, which can cause mal-operation of direct-on-line motors. Of particular importance is the elimination of so-called ‘zero sequence’ harmonics as these like the 3rd can severely stress building wiring.
Spending money on more hardware needs to be balanced against the costs involved in breakdowns, catastrophic failure (costs of interruption to business—danger to personnel) and of course, the cost of electricity. The latter is in part dependent on the tariff structure of your electrical energy supplier. In terms of future-proofing your electrical installation against capital expenditure when additional load has to be supplied (more tenancies, physical extensions, etc) investment now in active harmonic filters may well be a smart choice.