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06/07/2010
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Every consumer has an allowance for the total harmonic currents that they can pass back to the supply, which are defined in the Energy Networks Association Engineering recommendation G5/4-1.
There are basically two methods of installing a drive and achieving low network harmonics, either use a standard diode rectifier, and apply mitigation and filtering techniques, or avoid harmonics at the outset.
 Every consumer has an allowance for the total harmonic currents that they can pass back to the supply, which are defined in the Energy Networks Association Engineering recommendation G5/4-1.
There are basically two methods of installing a drive and achieving low network harmonics, either use a standard diode rectifier, and apply mitigation and filtering techniques, or avoid harmonics at the outset.
An active front end drive (AFE) by its construction produces extremely low levels of harmonics. It is well known that the harmonics passed by a modern inverter to the motor under control are very low, as evidenced by the minimal de-rating needed, therefore an AFE comprises an inverter, but used as a rectifier, with a low pass filter.
Most frequency converters use diode rectifiers which create high levels of harmonics. These have to be cleaned up after they have been created, while an AFE will go into battle on your behalf by actively modulating the voltage and frequency to reduce harmonic overtones. When looking at filtering, there are active and passive types. Active filters compensate harmonics by generating the same harmonic components in the opposite phase.
Harmonics occur when non-linear loads are connected to the power supply, and these may include electronic soft starters for motors, variable speed drives, computers, TV, radio and other electronic devices, electronic lighting, welding supplies and uninterruptible power supplies.
A plain diode rectifier will generate relatively high levels of harmonics, while reputable manufacturers may mitigate the levels of harmonic by fitting passive filtering. If this is inadequate, then one of the active approaches will be worth considering.
When choosing between AFEs and an active filter, there are a number of factors to bear in mind. Firstly, the active filter will have substantial losses that need to be taken into account. There must also be an allowance for the standing distortion of the network. This may well be out of phase with the new harmonic load and the use of a filter can result in a poorer power quality.
It will also be necessary to consider the need for regeneration or raised DC levels. In addition, the choice between AFEs and an external active filter is dependent on the number of drives installed and their sizes.
In general, a single drive up to 37kW meeting EN 61000-3-12 will not need any additional filtering on a 400 V network. A large number of independent small drives are best filtered by an active filter, while a few larger drives will in most cases be best suited for AFE solutions.
In all work with harmonic mitigation, it is important to remember that it is not only the drives on a system that need to be considered. The network also has a huge influence and a skilled pre-installation survey and evaluation can save enormous costs.
www.abb.co.uk/energy
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ABB Ltd
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