Water & Wastewater Treatment

32 | MAY 2014 | WWT | www.wwtonline.co.uk In the know Getting to grips with Harmonics Troubleshooting harmonics in electrics When electrical equipment runs erratically without explanation, chances are it is down to harmonics T he incidence of harmonics in the water industry has risen in recent years due to the increased use of variable-speed drives (VSD) installed for reasons of energy efficiency. It is a particular problem on weak networks o•en found in rural locations. Harmonics are waveforms on the electricity network that have a different frequency than the network frequency, such as a 250Hz waveform on a 50Hz network. This is energy that the devices on the network cannot use and instead may start to behave erratically, causing motors to run hot or to slow down. Harmonic waveforms are created by the VSD as it first converts the incoming AC 50 Hz waveform into DC and then back to pulses that control the AC motor. This conversion causes parts of the spectrum to multiply, resulting in waveforms in multiples of the network frequency. Symptoms of harmonics can be vague and solutions are o•en sought in other areas. But once a harmonics issue has been identified, it can be effectively remedied. However, harmonics expertise can be difficult to find as most electrical engineers have had little experience in the field. When suspicions arise there may be a harmonics issue on the site, it is usually best to contact the VSD manufacturer. All harmonics problems have a cure, but the solution varies with Frank GriFFiTh Drives ConsulTAnT engineer, ABB 1. symptoms of harmonics are o en vague and solutions sought in other areas 2. Harmonics can cause motors to run hot or slow down the conditions on the site and it is important to apply the measures that will work where you are. There is no universal remedy that will fix all harmonics issues. Some level of harmonics on the network is acceptable and to eliminate it completely would not be practicable. The issue is to find out what level is acceptable at your particular site. A VSD configuration can be acceptable at some sites while the same setup will cause substantial problems elsewhere. This difference is down to the network. A weak network has a low fault level and will be more susceptible to harmonics. Fault levels differ widely depending on the type of earthing system used, the supply type and the proximity to the supply. Unfortunately, there is no way of knowing, without prior installation, what the fault level will be. It is possible to carry out a harmonic survey at the site, but if awareness of the issue is low in the first place, this is unlikely to happen. It is also important to bear in mind that all VSDs are not the same. Some will cause more than double the amount of harmonic distortion compared to others. The amount of harmonic distortion is mitigated by chokes in the drive. More chokes will generate more heat, which means all components have to be larger and more robust, which in turn will be reflected in the price. So the chances are that cheap drives will cause more harmonics than higher priced ones. For further reading please go to wwtonline.co.uk 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

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