How harmonics threatens your electrical installation
How electrical energy can be saved and a facility future-proofed by paying attention to harmonics is explained by SUBODH BHATIA from Westek Electronics.
There is a certain inevitability about the cost of electricity; however, there are measures that can be adopted for existing and planned electrical installations that may not only save electrical energy, but also future-proof a building, thus warding 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, is a modern phenomenon. It has come about because of electrical loads such as electronic ballasts for fluorescent lighting, compact fluorescent lighting, LED lighting, computers, electronic cash registers, scanners and modern HVAC systems. Harmonics basically causes a significant amount of additional current to flow over and above that needed to provide energy to these types of loads.
Energy is measured in kilowatt hours as is evident from electricity charges. We 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 charging for excessive demand caused by harmonics possible.
Harmonics endangers electrical facilities because it 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 Standard AS/NZS 61000.3.6. The standard specifies a total distortion level for voltage of 8 percent and specifies much lower levels for individual harmonics. This limitation on voltage distortion is likely to affect new installations.
I’ll explain harmonics without getting 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. Electronic loads, however, virtually without exception, will not – instead they draw 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 (for instance: 150, 250, 350 cycles per second and so on).
In addition to this voltage distortion, harmonics also causes additional heating of wiring, switchboards and transformers. This heating effect is serious. It is proportional to the square of the harmonic currents, for example, if the fifth harmonic – a frequency of 250 cycles per second – might typically be 30 percent of the 50 cycle current, that will add a further 70 percent heat load.
This is not the worst of it, however. The heating effect is also proportional to the square of the frequency and this can very easily double or even quadruple the heat the installation has to withstand.
HARMONIC MITIGATION USING ACTIVE FILTERS
Harmonic filters, as their name implies, filter out the harmonic current contribution, preventing that current from flowing through the electrical installation. They do this to whatever extent possible as dictated by technology constraints and 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, and start-up of heavy current drawing loads. 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 that 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 third, can severely stress building wiring.
Spending money on more hardware needs to be balanced against the costs involved in breakdowns, catastrophic failure (interruption to business and danger to personnel) and, of course, the cost of electricity. The latter is in part dependent on the tariff structure of electrical energy suppliers. In terms of future-proofing an electrical installation against capital expenditure when additional load has to be supplied due to issues such as more tenancies and physical extensions, investment in active harmonic filters may be a smart choice.
Subodh Bhatia is managing director of Westek Electronics.