Harmonics and the new, energy-efficient world
Future-proofing electrical installations
By: Subodh Bhatia, Managing Director, Westek Electronics Pty Ltd
Commercial buildings in Australia consume some 50 Terawatt-hours annually in electrical energy. The major loads are lighting, mechanical services, and increasingly IT systems. In addition to useful power required by these loads (watts, kilowatts), the electrical supply system has to provide reactive power (volt-amps, kilovolt-amps).
Reactive power for many years was delivered “under the radar”—it was not the consumer’s problem. That situation is no longer, the principal reason being that reactive power requirements have shot up because of the large number of electronic loads. These loads cause the flow of large additional harmonic currents, and additional harmonic reactive power. Examples include chillers, fans, etc driven by variable speed drives, CFL lighting and electronic ballasts for fluorescent lighting, power supplies for LED lighting.
Harmonic reactive power can easily add anywhere from 20% to even 100% to demand. But in addition harmonic currents frequently cause significant problems, in particular overheating and breakdowns in switchboards and sometimes substation transformers. Many commercial installations are underrated for harmonics. Traditionally neutral conductors have been beefed up to allow for unbalanced phases but even then they do not cope with third harmonics (150 Hz) and higher ‘zero sequence harmonics. As to switchboards, fires have occurred in half-loaded boards (in terms of kilowatts) but with very high harmonic reactive power loadings.
Combating harmonics effectively requires the use of active harmonic filters
These filters provide a bypass of the harmonic demand imposed by loads, thus preventing the flow of harmonic current in the installation. The term ‘active’ means that the filters are able to ‘compensate’ harmonics under widely varying load conditions. The placing of these in an installation is very important and is beyond the scope of this short article. However software is available to map the placement of active filters, and to predict the mitigation effect.
The power demand diagrams below explain graphically the operation of active harmonic filter harmonics mitigation. The diagrams show two situations. The figure on the left is the power triangle of a load without harmonics, for example an induction motor. The horizontal axis is the useful power axis and represents kilowatts drawn by the load. The vertical axis shows the reactive power drawn by the load in kilovolt-amps. The hypotenuse of the right-angle triangle shows the total demand in kilovolt-amps. Note that the three components of the triangle are all 50 Hz parameters. The diagram on the right is drawn as a three-dimensional figure with the reactive component due to harmonics at right angles to the plane in which the power triangle of a non-harmonic load is. This arises because the harmonic demand is at multiple frequencies of the 50 Hz fundamental. The example is typical of a variable frequency fan motor drive. As is evident the total demand has increased.
Differences in Active Filters
It is important to note that not all active filters perform the same correction task. Shown here is the way Reinhausen MR filters can operate. As already mentioned active filters basically trap the harmonic current due to the load and prevent from circulating in the building’s electrical installation. The Reinhausen MR filters allow tailored compensation so that in many instances it is not only possible to totally compensate the harmonic component but to use excess compensation capacity to also wholly or part compensate the power line 50 Hz reactive component. The filters also permit tailoring 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.
Revenue metering in kilowatt-hours as the sole means of assessing liability for electrical energy use is becoming a thing of the past. The reason is that electrical supply systems are built on volt-amp, kilovolt-amp, etc demand basis. Demand peaks strain the system to breaking point and although national electricity consumption is not growing as fast at present, maximum demand is! It is a factor, which is adding increasingly to electricity bills as supply companies, and authorities charge in effect a fixed cost component for households. For commercial consumers maximum demand in kilovolt-amps (kVA) in addition to energy costs in kilowatt-hours will become the norm. Unless harmonics current flow in electrical installations is mitigated, there will be significant additional maximum demand and costs. Installing active harmonic filtering is a way of future-proofing electrical installations, making the installation capable of expansion without immediately having to consider beefing up transformers, switchboards, busbars and cabling. As important is the maximum harmonic voltage pollution restriction power companies are placing on installations.