Know your flow / Managing harmonics

Managing harmonics, to minimize disturbances and energy losses

When electrical equipment disturbances like flickering lights, erratic instrument readings or unstable motors appear without any seemingly good explanation it may well be due to harmonics. By managing harmonics in the electrical network running your pump system, you can reduce energy costs and prolong the system’s lifetime. Detecting the problem may require expert help, which we can provide either directly or within our Authorized Value Provider Network.

Harmonics in electrical networks are current or voltage waveforms which have a different frequency than optimally functioning networks have. In the water industry where networks can be older and distances between the power source and the site quite long, harmonics is a common problem. Unfortunately, the overall net effect of harmonics, if left unmanaged, is increased energy costs.

There are ways to manage the “pollution” in the electrical network caused by harmonics. One is by using drives labeled as ultra-low harmonic (ULH) designs. When these are controlling motors and pumps, the disturbances are kept at a low level. Therefore, weak networks using ULH drives experience minimal additional harmonics, while also maintaining unity power factor. High power-quality levels in weak networks means increased usable capacity of supply, thus saving energy and costs.

Variable speed drives for ultra-low harmonic results
ABB’s offering of ultra-low harmonic drives means harmonics produced by e.g. large motors in pumping stations can be minimized. Depending on the pumping solutions in use, properly selected drives can provide the correct level of harmonic mitigation, while reducing cost to an acceptable level. Additionally, since these drives are compact they use less space, even when installed in cabinets.

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Various possible strategies to manage harmonics

Essentially, there are three options for handling harmonics – reduction, cancellation, or diversion. Reduction is achieved by added inductance or an active rectifier; cancellation by a 12 or 24-pulse drive (or a PQF harmonic filter); while diversion involves a standard 6-pulse rectifier with a passive harmonic filter that is specially designed to divert certain harmonic frequencies into the filter.

The most basic technique for harmonics management is to use AC line reactors, usually known as chokes, fitted inside or outside a drive (if it does not have a low-harmonics solution built-in). This is called reduction, because with a correctly sized AC/DC choke in an ordinary 6-pulse variable speed drive, harmonics levels can be substantially reduced to a level where they cause less problems. Another approach to reduction is to use an active rectifier. This has an inverter working “backwards” onto the mains supply, treating the mains as it would treat an electric motor and controlling the current flow more effectively, resulting in essentially no harmonic contribution.

Cancellation is an older approach

Cancellation has traditionally been achieved using a 12 or 24-pulse drive. Total harmonic load is divided between several isolated sources sharing a common primary supply (via a 3-winding input transformer). A 12-pulse drive uses two sources, with each source phase-shifted by 30 degrees relative to the other. A 24-pulse drive uses four sources shifted 15 degrees relative to the other. A more modern approach is to use either a low harmonic variable speed drive or a closed loop active filter, although cabling and engineering costs are often more expensive and complex with the active filter method.

Active filter tackles harmonics, leaving clean current

A low harmonic drive uses an insulated-gate bipolar transistor (IGBT) input bridge and cancels the harmonics in the drive’s system in the first place. This closed loop active filter injects an ‘anti’ harmonic current spectrum that sums to zero (or client specific values) with the load harmonic currents. What remains is a ‘clean’ current on the mains supply. 12 and 24-pulse drives tend only to be used these days if the required transformers are already on-site or a new installation requires a dedicated transfer per drive for powers over 500kW.

We are experts in managing harmonics - Let’s talk

Although the subject of harmonics can be confusing, our network of experts is available wherever you are to work with you. They perform harmonic assessments to determine the best strategies for lowering costs, as well as insuring long life cycles for the pumping system and the automation products controlling it. With these assessments, combined with our ultra-low harmonic drives, we can actively manage and minimize harmonic issues to ensure the highest overall stability and productivity in your water operations.

Passive filter can be less flexible and effective

A passive filter, which is less dynamic and requires more engineering resources, works by diverting harmonics away from the affected equipment. This is a fairly inflexible solution with few application areas and several drawbacks. For instance, it increases voltage and heat loss, and reduces the power factor. At low fundamental load (25Hz pump load) it requires capacitor disconnect to protect the capacitors against overvoltage, which means no harmonic mitigation occurs at low speed. So, in general, passive filter solutions are less efficient as a system than ULH and active filter solutions, especially at low load.

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