Water & Wastewater Treatment Magazine
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www.wwtonline.co.uk | WWT | MAY 2014 | 33 • Three ways to approach harmonics There are basically three options when working with harmonics: reduction, cancellation or diversion. Reduction is achieved by added inductance or an ac- tive rectifier; cancellation by a 12- or 24-pulse drive or an active front-end drive; and diversion by a passive filter. Reduction The most basic technique is to use AC line reactors, usually known as chokes, fitted inside or outside the drive. With a correctly sized AC/DC choke in an ordinary six-pulse drive, harmonics levels can be substantially reduced. The objective is to reduce harmonics to a level where they no longer cause a problem. Another approach to reduction is to use an active rectifier. This has an inverter working "backwards" onto the mains supply, treating the mains supply as it would treat an electric motor and mitigat- ing the incoming wave. Cancellation Cancellation has tradition- ally been achieved by using a 12- or 24-pulse drive. Total harmonic load is divided between several isolated sources sharing a common primary supply. A 12-pulse drive uses two sources, with each source phase-shi ed by 30 degrees relative to the other. A 24-pulse drive uses four sources shi ed 15 degrees relative to the other. This technique can be used to eliminate many harmonic frequencies. A more modern approach is to use an active front- end drive. This cancels out harmonics by creating an equivalent wave in the opposite direction. 12- and 24-pulse drives tend only to be used these days if the required transformers are already on the site. Diversion The passive filter works by diverting harmonics away from the affected equipment, but it is a fairly inflexible solution with few application areas and it has several drawbacks. For instance, it increases voltage and heat loss and reduces power factor. At low fundamental load (25Hz for pump load) it requires capacitor disconnect to pro- tect its capacitors against over-voltage, which means no protection is offered at low speed. When reconnect- ing, there is a time delay for capacitor discharge, which means this method is unsuitable for cyclical control. If used with a gen- erator supply, the sudden disconnection can cause unacceptable harmonics levels on the network. The filter is also likely to import background harmonics from the network which increases total harmonics. Some harmonic frequen- cies are difficult to address with a passive filter and are likely to remain. All these issues combined increase the heat loss in the active filter. Bearing in mind its limitations – and we still haven't mentioned all of them – the passive filter requires a number of fairly complicated calculations to be used successfully. Still, if the conditions are met, it can provide a cost-effective alternative. The problem arises when vendors that do not have a full range of harmonics mitigating equipment try to sell pas- sive filters for applications where they are not suitable. All these measures can be applied on both low voltage and medium volt- age networks. However, mitigation of harmonics on a medium voltage network can be quite costly. It is o en more cost-effective to work on the low voltage side and let the corrected wave go through the trans- former to the MV side. sponsored by Smart liquid lime www.neutralac.co.uk