• • • EDITOR’S CHOICE • • •
This means that the VSD’s level of harmonic content needs to be considered from the planning stage of the project to evaluate its effect and its consequences for the facility. Limits for harmonic currents are specified by several national and international standards – this is commonly known in the UK as G5/5.
Many transmission and distribution system operators also have requirements for high power equipment connected directly to medium or high-power grids. Certain industries have even set factory-specific regulations.
Solving the challenge Mitigation strategies will only get you so far. As we connect more non-linear loads such as EV chargers and solar panels to the grid, harmonics are becoming an increasingly widespread problem. A well-known way to mitigate harmonics is to add additional harmonic filters. However, these add to the cost of the project and introduce another potential point of failure and an extra maintenance burden.
Another mitigation strategy is to use a 12-pulse
rectifier, formed by connecting two 6-pulse rectifiers in parallel to feed a common DC-bus. The benefit with this arrangement is that some of the harmonics are in opposite phase and are thus cancelled out. The major drawbacks here are the need for special transformers and the added costs compared to a 6-pulse rectifier.
Instead, some modern drives are now able to prevent harmonics from being generated in the first place. Active Front End (AFE) drives, such as ABB’s ACS880 Ultra Low Harmonic drives, have an active supply unit (also known as an IGBT supply unit) and an integrated line filter –
these can help to reduce its harmonics level down to 3%.
No other commonly used switching device has in-built mitigation, and because drives are
responsible for large amounts of harmonics, removing them can make a huge difference to the overall harmonic content on a network. A further advantage is that there is no need to replace all the drives - only one AFE drive is needed for the critical application that needs protection. Although AFE drives have a higher upfront cost compared to conventional units, they save money in other ways. They include allowing the use of a smaller transformer, and smaller cables, while avoiding the need for additional harmonic filters. As well as being cheaper than a conventional drive and dramatically reducing the risk of disruption from harmonic content, an AFE solution will also improve the overall system efficiency.
With a THDi of 3%, facilities can use AFE drives to avoid massive oversizing of power network components like generators, transformers, switchgear, and cables. By using less material, projects featuring AFE are also more sustainable.
As we all seek to use energy and resources more efficiently, the power savings achievable with VSDs combined with the material savings possible from ultra-low harmonic drives make increasing sense.
12 ELECTRICAL ENGINEERING • NOVEMBER 2023
electricalengineeringmagazine.co.uk
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