Using variable speed drives (VSDs) to control the large number of electric motors, pumps and fans used in the water industry results in energy efficiency and performance gains. However, VSDs and many other types of electrical loads, such as certain motors, lighting, computers, uninterruptible power supplies (UPS) and Wi-Fi routers, can cause harmonic distortion, an undesirable event in the electrical network.
Harmonics can damage sensitive electronic equipment, interfere with communication equipment, and produce false readings on measurement devices. They can also trip circuit breakers, blow fuses and damage capacitors as well as overheat transformers, cables, motors, generators and capacitors, wasting energy and shortening system life.
Even though the benefits of VSDs outweigh the negative effects of harmonics, it is vital to understand the problems harmonics can cause and the solutions available to eliminate them.
Explaining harmonics
Alternating current (AC) should present as a sinusoidal wave, running at a base frequency of 50 or 60 Hertz (Hz). In reality, however, the use of non-linear electrical loads creates harmonics – multiples of the base frequency - that cause significant deviations in both current and voltage from this pure sine wave. The resulting “electric pollution” means that some energy is lost within the network.
To measure the impact of harmonics we use a THDi figure - the total harmonic distortion on current. The higher the THDi, the more energy is lost in the network. As an example, a 40% THDi results in 16% more losses than a network with no harmonics. As well as increased energy bills, the electrical system needs to be sized appropriately to be able to carry the excess current. A drive with no harmonic protection, for example, might add 80% additional load to a transformer.
While a single drive is unlikely to cause any issues, multiple drives will have an effect as the excess current produces more heat. Overheating equipment is more unreliable and wastes energy, and can shorten its life expectancy.
These issues can have a negative financial impact. System inefficiency adds to daily running costs, while premature equipment failure carries unnecessary repair and replacement costs. Therefore, it is crucial to ensure that selected equipment does not have an adverse effect on connected devices or the power system.
Addressing harmonics
Commonly, specifiers would oversize power components or cables to deal with overheating. However, if the incoming transformer is a higher rating, for example, the local utility will charge higher fees. Oversizing backup generators is another common method of mitigating the challenges induced by harmonics. However, this requires more space, increases the costs of a project and negatively affects project sustainability.
Many power companies in water supply and wastewater treatment require operators to prevent increases in the harmonic distortions their equipment may cause.
A sensible alternative to oversizing equipment is to install devices that simply reduce or eliminate harmonics.
Variable speed drives (VSDs) control the speed and torque of a motor driving a water application such as a pump or fan. Conventionally, external filters or using multi-pulse transformers would have been installed to address problems created by harmonics when using these drives. However, a more modern and efficient method is to use an ultra-low harmonic (ULH) drive. These have built-in harmonic mitigation, including an active supply unit and an integrated low harmonic line filter.
While traditional 6-pulse VSDs decrease THDi to around 40% (Fig:1), the best ULH drives reduce harmonic content in a network to lower than 3%. Water facilities can now avoid the massive and unnecessary oversizing of power network components and be more sustainable due to decreased material usage.
With “right-sized” components, the overall investment costs of the entire system are transformed. For example, compared with standard VSDs, incoming power cables might be reduced in size by about 10%. Furthermore, the size of distribution transformers can be reduced by 20%, generators by 50% and switchgear and circuit breakers from 10 to 30%.
Because of the minimal harmonic content, ULH drives (Fig:2) also increase facility reliability as they eliminate process interruptions from network overloads caused by increased line current. The drives also eliminate malfunctions in connected devices caused by the distorted current. So, ULH drives reduce the footprint of an installation and the total cost of ownership (TCO). ABB ACQ580 ultra-low harmonics drives for water and wastewater is the ideal solution for this purpose.
The power factor
Harmonics also affect the power factor (PF), which describes how effectively the system uses its power. In an ideal world, a network will have a PF of 1. In some cases, utilities can even impose penalty charges on customers with a bad power factor.
Because ULH drives reduce harmonics they have a positive effect on the power factor. 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 will draw only the nominal current (100%).
Essentially, ULH drives can optimize the energy efficiency of the electrical system. The result is lower electricity bills.
Eliminating electrical disturbances
AlburyCity Council in New South Wales, Australia, operates two water supply systems to manage and treat raw water, providing clear, potable water.
A water boosting pump station responsible for pumping water to storage tanks on a nearby hill features in one section of the system. When the station’s pumps were started or stopped, it would cause an intrusive electrical disturbance causing the lights in nearby buildings to dim or flicker.
The problem was that the pump’s installed soft starter motor controller was causing power quality disturbance. Starting and stopping the motor introduced harmonics into the power network.
A water and wastewater industry ULH variable speed drive with a premium efficiency motor was found to be the best solution. THDi remained below 3% and the drive also enabled the system to advance from a fixed speed application to variable speed.
By lowering motor speed by 10%, the Council now benefits from better performance overall while reducing energy consumption. Not only did the ULH drive solution solve the original problem, it has optimized the demands of the process with less wear and tear, lower pump cavitation and hydraulic stress through the pipes.
ABB’s drive and motor package, installed in AlburyCity Council’s pump room 
Installed ABB ACQ580 ULH variable speed drive on site at the water pumping station operated by AlburyCity Council 
Second ABB ACQ580 ULH drive installed in the pump room on site, used to optimise existing application as a customised approach by Remtron to give AlburyCity Council the best outcome. The Control System also features ABB automation equipment including an ABB AC500 PLC and ABB CP610 HMI touchscreen visual display unit for accurate, digitalised monitoring.
ULH drives in action
Hamburg Wasser’s local power company requires them to minimize harmonic distortions from its equipment. Hamburg Wasser decided to test an ULH drive to explore ways to minimize harmonics.
A wall-mounted water and wastewater industry ULH drive was installed, controlling a feed pump delivering 450 m3 per hour of water from a water tank in the old section of the water plant to another tank in the new section. Another feed pump, controlled by an existing standard drive, is doing the same task on the same tanks and these two pumps alternate operating on a weekly basis. This made this installation ideal for comparative testing.
When running close to the nominal operating point, the ULH drive had a THDi of 2.73%, compared to the standard drive which had a much higher THDi of 45%. This means that the harmonics content with the ULH drive is 94% lower than with the standard drive. The results have proven the benefits of a ULH drive.
The future is ULH
ULH variable speed drives are an efficient and cost-effective solution that eliminate harmonics at source. They ensure the highest overall stability and productivity in water operations to deliver secure water supplies and a fast return on investment.
