Water & Wastewater Treatment Magazine
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www.wwtonline.co.uk | WWT | SEPTEMBER 2016 | 29 STUART FOSTER WATER INDUSTRY MANAGER DRIVES & CONTROLS ABB The hidden pollutant Harmonic distortion can be diffi cult to diagnose, but, once found, it is relatively easy to remedy In the know Getting to grips with... harmonic distortion Southern Water has cut the risk of harmonics occurring at its Peacehaven wastewater treatment plant in Sussex by installing a combination of low harmonic and 6-pulse drives H armonic distortion is a common problem in the water industry, and one that is becoming more prevalent. While harmonic distortion itself does not cause problems, its symptoms do. These range from overheating motors to recurring problems with lighting and equipment. With the advent of Ofwat's Asset Management Period 6 (AMP6), water companies are being encouraged to re-focus attention away from the traditional capital and operating expenditure (capex/opex) spending models towards a total expenditure, or TOTEX, approach, one that emphasises controlling operational and maintenance costs. Because of this, keeping harmonics and their damaging e… ects in check should be a priority for water companies. What is harmonic distortion? Harmonic distortion is a type of electrical interference caused by non-linear loads on the network. A harmonic current or voltage is one with a frequency that is several times that of the basic supply frequency of 50 Hz. As an example, a 250 Hz current on a 50 Hz network is Š ve times the basic frequency and so would be classed as the Š ‹ h harmonic. Harmonic currents are caused by loads such as capacitors and power electronic circuits. In these loads, the current through them does not vary in simple proportion to the voltage across them – they are therefore said to be non-linear. All electronic power supplies from computer power supplies, ' uorescent lights and welding supplies, as well as both AC and DC drives, will give rise to harmonic currents. These are then passed to the supply network. What problems do harmonics cause? The energy in harmonic currents cannot be used by devices on the network and so it is simply turned into heat. This can cause all sorts of problems. Cables may overheat, damaging their insulation. Motors can overheat too or become noisy, while oscillation in the rotor can lead to mechanical damage. Lights can ' icker while meters can produce false readings. What makes harmonics particularly challenging is that while all of these symptoms can be caused by harmonic distortion, they can be caused by a number of other problems as well. This means by the time harmonic distortion is conŠ rmed, signiŠ cant amounts of time and money may have already been spent on treating other suspected causes. What industries are aff ected by harmonics? Harmonic distortion can occur anywhere but certain industries are more susceptible to it. Water facilities can be particularly a… ected, as many sites are in rural locations with weak networks that are especially vulnerable to harmonics. This means that for the water industry, harmonics can be a major factor in increasing maintenance bills. Why are we hearing about this now? While harmonic distortion is nothing new, it is becoming more prevalent within the water industry. This is in part due to the growing use of AC drives, which are used to improve productivity and reduce energy consumption. The arrival of AMP6 means water