Power line harmonics cause trouble for smart buildings. They result in unreliable and inefficient operation of electrical equipment as well as a poor power factor that brings down the energy efficiency of the whole building. Arnold Taddeo, global head of product management for Drive Products at ABB Motion, explains the origin of harmonics and how ultra-low harmonic (ULH) drives can eliminate them at source.
Power line harmonics cause trouble for smart buildings. They result in unreliable and inefficient operation of electrical equipment as well as a poor power factor that brings down the energy efficiency of the whole building. Arnold Taddeo, global head of product management for Drive Products at ABB Motion, explains the origin of harmonics and how ultra-low harmonic (ULH) drives can eliminate them at source.
Variable speed drives (VSDs) are vital for the energy efficient operation of the motors, pumps and fans installed in smart buildings. They save energy by controlling the speed of electrical motors to match the exact needs of the application. This is particularly important as many motors usually run at well below their peak load. However, VSDs can introduce power line harmonics. Should their relative presence in the network become too high, harmonics can be a troublesome power quality issue that impacts both reliability and energy efficiency.
Harmonics can cause damage to sensitive electronic equipment, interference in communication equipment, and false readings on measurement devices. They can also trip circuit breakers, blow fuses and damage capacitors as well as resulting in overheating of transformers, cables, motors, generators and capacitors, wasting energy and shortening their life.
Non-linear loads cause harmonics
The alternating current (AC) power supply serving a building will, ideally, take a pure sinusoidal wave form – see figure 1. In reality, the use of non-linear loads creates harmonics that cause the sine wave to deviate as shown in figure 2.
In addition to VSDs, harmonics can result from equipment such as EC (electronically commutated) motors, LED or fluorescent lighting, mobile phone chargers, computers, uninterruptible power supplies (UPS) and Wi-Fi routers. They are caused by the constant switching of power electronics elements in their design.
The impact of harmonics is measured as the total harmonic distortion (THD). This is the ratio of the RMS (root mean square) harmonic content to the RMS value of the fundamental frequency. Where no voltage or current harmonics exist the THD is 0%. As the level of harmonics increases, the THD value increases. THDi is the total harmonic distortion on current.
The higher the THDi, the higher the losses in the power network – for example, a 40% THDi results in 16% greater losses than a network with no harmonics. That means increased energy costs. And it also requires the electrical system to be sized to carry the excess current.
A single 4 kilowatt (kW) drive, even with 100% THDi, will not always cause problems for the whole network. But it is important to consider the cumulative effect of harmonic distortion from multiple drives.
Tackling harmonics
One approach to tackling the overheating caused by harmonic losses is to oversize the critical electrical equipment, such as transformers, generators and cables. This is usually expensive, and often ineffective. Instead, it is better to use equipment that does not cause harmonics in the first place. That is why building developers are adopting a new generation of ULH drives.
ULH drives have harmonics mitigation built in. This includes an active front end (AFE) and integrated low harmonic line filter. There is no need for external filters, multi-pulse arrangements or special transformers. The simple installation offers significant savings in space, time and money.
Compared to a conventional drive, the harmonic content is reduced by up to 95%, this results in a typical THDi of 3%. In contrast, with an external passive filter the typical total harmonic distortion is between 5 to 10%. Furthermore, as the risk of overheating is reduced, there is no need to oversize equipment.
The importance of power factor
Harmonics also affect the power factor (PF), which describes how effectively the building is using its power. True power factor must consider two elements. They are the displacement power factor (also known as cosφ) - related to reactive power, and distortion power factor – related to harmonics. In the very best case, a network will have a PF of unity.
In some cases, utilities impose penalty charges on buildings with a poor power factor. Using a standard VSD with a motor will improve its displacement power factor but add to the distortion due to the drive rectifier. Because ULH drives mitigate harmonics they have a positive effect on the distortion power factor. They also compensate for reactive power to improve the displacement power factor.
To illustrate the difference, a standard drive might have a true PF of around 0.78, causing it to draw an increased line current of 128% of the nominal. In contrast, a ULH drive with a true PF of 1 will draw only the nominal current (100%).
Essentially, ULH drives not only optimize the energy efficiency of the building’s electrical equipment, they help make the overall power network more efficient. The result is lower electricity bills.
Saving building capital costs
We have already seen that deploying ULH drives reduces the need to oversize key electrical assets. Instead, they can be optimized to match the actual load more closely. This “right-sizing” effect ripples through the capital costs of the building. For example, compared with standard VSDs, cables might be reduced in size by about 10%. Furthermore, the size of distribution transformers might be reduced by 20%, generators by 50% and switchgear and circuit breakers by 10–30%.
A proactive approach to harmonics pays dividends
When designers and operators of smart buildings take appropriate action to address harmonics in their electrical networks they soon experience a positive return on investment. This comes in terms of improved reliability, longer equipment life, lower energy bills and reduced capital costs.
A particularly elegant and cost-effective solution is to deploy ultra-low harmonic (ULH) drives such as ABB’s ACS880 range that mitigate harmonics at source.
For further information: https://new.abb.com/drives/har...