Improving power quality by reducing harmonics

Sey-Veng Lu, product manager for ABB’s high-power drives, looks at the problems caused by harmonic distortion. He examines how plant operators can win their battle against an invisible enemy which has the potential to cause equipment failures that negatively impact on business.

Harmonic distortion or network noise is a form of pollution in the electricity supply that can make equipment behave erratically. Many industries are affected by network noise, especially where there is a large installed base of AC drives and other nonlinear loads. Harmonics can be difficult to identify and users often attribute the symptoms to other causes, frequently applying mitigating action that fails to solve the problem. Harmonics can be effectively dealt with if properly identified.

Harmonic distortion can cause problems if the sum of harmonic currents increases above certain limits.  For instance, a mine site tends to operate many loads that are both susceptible to harmonics and which generate harmonics. A harmonic frequency is a multiple of the network frequency, for instance, a 250 Hz waveform on a 50 Hz network is the 5th harmonic. A harmonic waveform represents energy that cannot be used by the connected equipment and this can make the equipment behave erratically - motors can overheat or become noisy, cable insulation can get damaged, circuit breakers can trip and meters can give false readings.

Harmonics are produced by non-linear loads, such as computer power supplies and electronic lighting, as well as both AC and DC drives. For example, a mining company tends to have large numbers of AC drives across many sites. Many of these sites are in remote locations with weak supplies and these are particularly prone to harmonics. Also the back-up generators used at many sites can be an additional source of, or be susceptible to, harmonics.

Identifying the cause

Harmonics caused by AC drives can be managed and suppressed to a level where they no longer cause a problem, or can be eliminated altogether. However, problems caused by harmonics are frequently not recognised as such. As the user is not aware that the problems are caused by harmonics, inappropriate action is often taken, for instance, additional cooling and higher-rated transformers, cables and capacitors. Although such action may limit the problem it does not address the fundamental cause.

Reducing harmonics

Most AC drives use diode rectifiers which create high levels of harmonics. These have to be mitigated after they have been created. Filtering can be achieved with either a passive or an active filter. A simple line reactor can reduce the harmonic levels; a passive filter will remove some or part of the spectrum; while a more effective way to get rid of harmonics is the active filter, which can target specific harmonic frequencies.

An alternative approach is to use a low harmonic drive. This has an active rectifier and a built-in LCL (inductor-capacitor-inductor) filter. The active rectifier is controlled to eliminate low-order harmonics, while the LCL filter suppresses the switching frequency components caused by the rectifiers’ semiconductors.

The type of filtering needed depends on the network and on the drive installation. On a 400 V network with just a single drive of no more than 37 kW, meeting the product standard EN 61000-3-12, no additional filtering is needed. A large number of independent small drives are best filtered by an active filter, while a few larger drives will, in most cases, be best suited for solutions involving low harmonic (active front end) drives. ABB has low harmonic drives for 415 V to 6.6 kV and ideal for new and upgrade projects.  

Evaluating the cost

Many solutions exist to mitigate harmonic distortion.  However, it is not just the drives that need to be taken into account. The network also has a great influence and there is no substitute for a skilled pre-installation evaluation, which can save significant costs. ABB can also provide expertise from initial energy audits, power network surveys and harmonic analysis, to delivering tailored solutions across all areas of power quality.

There is no need to miss out on the benefits of variable-speed drives just for fear of harmonics. Variable-speed drives can vastly improve the efficiency and performance of motors, pumps and fans in many applications and offer plant operators enormous energy savings.