Expert view: Harmonic mitigation in the water industry

Harmonic currents are a headache for any industry, and water is no different. However, as ABB's Martin Richard explains, there is a solution.

Left unchecked, harmonics generated by electrical equipment can cause overheating, nuisance breaker tripping and a reduction in a system’s true power factor. To compound this, systems are often undersized because harmonics have not been adequately considered, meaning harmonics account for a proportion of the available current capacity on that supply. There is a solution though.

Ultra low harmonic (ULH) drives feature an active front end employing insulated gate bipolar resistor (IGBTs) which convert incoming AC power to DC. With ULH drives, the active front end monitors the waveform of the input current, shaping it to be sinusoidal, before employing LCL filters to combat low frequency (<5kHz) disturbances caused by the constant IGBT switching. Frequencies up to 500kHz are managed by EMC filters, which is of comparable standard to six-pulse variable speed drives (VSDs). In conjunction, this reduces total harmonic distortion (THDi) to under four percent.

In practice, this allows a pump system’s operator to correctly specify an application without needing to over-dimension. Imagine, for example, a plant hiring or fitting a generator to ensure consistent pumping operation during a mains power loss. For plants with softstarters or six-pulse VSDs, a generator will need to be over-dimensioned by 200-300 percent to allow for the high initial current demands but pairing the generator with ULH drives removes the need for over-dimensioning, thus lowering the upfront cost of the equipment significantly.

ULH drives also mean a system can be designed differently, with the emphasis placed on eliminating the harmonics at source. ULH drives negate the need for separate filter banks being installed in any application, for example. This means one less part of the chain to potentially go wrong; essentially, if the drive is running, there is no problem with the filtering.

Efficiency is the big winner with ULH drives, with the potential to reduce carbon emissions and energy bills considerably. They also offer significant benefits to the inherent resilience of the systems in which they’re installed. Applications using ULH drives offer full redundancy compared with applications using a regular filter system, where failure can lead to operational issues, resulting in a site being unable to operate at full capacity.

There are claims that low harmonic drives produce significant higher order harmonics (i.e. above the 50th harmonic). However, with the filters integrated within the drive, this issue is mitigated such that levels are comparable with a standard six-pulse VSD. Indeed, tests have shown that modern regenerative drives conform with all applicable standards, and applications using this technology actually offer superior overall harmonic mitigation and improved resilience compared to other solutions. Harmonics are often blamed for interference at a site when there are usually other factors at play, like poor earthing practices at the installation stage. It’s also important too that harmonics are not confused with RFI and other disturbances like common mode voltages.

Concerns over efficiency regarding low harmonic drives, when compared to standard diode-rectified drives, can be allayed by viewing the bigger picture rather than single components. The technology behind low harmonic drives aids in reducing power losses from the transformer and to the motor, offering potentially higher overall system efficiency and helping reduce the carbon footprint of an application while delivering a simpler and easier installation and engineering process.

As with any system, however, it’s vital to understand the bigger picture and not the single pieces alone. Drives are, after all, a single component. Yet for water industry applications where maintenance, running costs and space are an issue, low harmonic drives offer a practical, reliable alternative to traditional technologies and means harmonics won’t disrupt the overall harmony of your application.

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