Imperfect harmony: ultra-low harmonic drives

Ultra-low harmonic drives ensure disturbance-free electrical power, as well as better efficiency and reliability, says ABB

Electrical equipment runs most efficiently and reliably when it is supplied by clean, disturbance-free electrical power. Often, however, power networks – for multiple reasons – are subjected to continuous or transient electromagnetic disturbances. 

The most frequently seen continuous disturbances in almost any network are harmonics, which can have a significant effect not only on power-network efficiency, but also reliability.

Harmonics can cause damage to sensitive electronics and interference in communication equipment, and give false readings. They can trip circuit breakers, blow fuses and cause capacitor failures. 

Harmonics are electromagnetic pollution in the power network resulting in current and voltage waveform distortion that makes it different from the pure sinusoidal waveform generated by the electrical supplier. Harmonics are generated by non-linear loads such as LED lighting, uninterruptible power supplies, computers, or variable speed drives (VSDs) as a result of the constant switching of power electronics elements in their design.

VSDs help to save a tremendous amount of energy in processes with varying loads, such as data centres, and they do this by adjusting application motor speed to the exact process need. 

At the same time, it is important to remember that, just as with other non-linear loads, drives generate harmonics in the power line that, as already mentioned, can have an adverse effect on the network and the connected equipment when they exceed certain limits. Hence, the VSD harmonic content level needs to be considered at the project stage to evaluate its effect and its consequences for the facility.

The presence of harmonic content is measured as a percentage value, known as the total harmonic distortion (THD) – which is the relationship between all the current or voltage harmonics and the fundamental current or voltage. Where no voltage or current harmonics are present, THD is 0.1%.

The higher the harmonic content, the higher the line current, which means higher losses in the network, including its components, such as transformers, switches, circuit breakers and cables. Increased line current also means that power network equipment overheats, which causes premature failure – and because a current with harmonics in it is a distorted current, there is a risk of connected equipment malfunctioning.

Harmonics may not cause immediate issues; malfunctions may take time, or may only happen when certain combinations of equipment come ‘on line’. Often, site issues are not even attributed to harmonics by the users. But it’s important to understand that, in mission-critical facilities, such as data centres, harmonics can lead to serious consequences, causing financial and reputational damage. 

When selecting equipment, therefore, it’s crucial to make sure it does not have an adverse effect on the power system or connected devices, which, in turn, ensures higher process reliability and efficiency. 

Instead of trying to tackle harmonics by often ineffective actions, such as adding cooling or over-dimensioning equipment, one solution is to employ equipment that does not cause harmonics in the first place.

ABB’s ultra-low harmonic (ULH) drives are based on active front-end technology with DC bus capacitors, and they produce a harmonic content that is less than 3%. In systems that require minimal harmonic content, this can lead to improved overall efficiency and contribute to process reliability. With a passive filter, the typical total harmonic distortion is between 5% and 10%.

The compact drive features built-in harmonics mitigation. This includes an active supply unit and a low harmonic line filter. As there is no need for external filters, multi-pulse arrangements or special transformers, the simple installation offers significant space, time and cost savings.

As there is less risk of overheating with lower harmonic currents, there is no need to over-dimension equipment, such as transformers and cables.

The possibility to stabilise the output voltage of a ULH drive is an advantage compared with passive filters. At nominal loads with a passive filter, there is typically a 5% voltage drop. On partial loads, the voltage over the passive filter can rise by up to 10%, creating the possibility of an over-voltage fault occurring.

ULH drives achieve a unity power factor, indicating that electrical energy is being used efficiently.

They offer the possibility for network power factor correction to compensate for the low power factors of equipment connected to the same network. They can also help avoid penalty charges set by electrical utilities for poor power factors.

Lower harmonics and full motor voltage at all times mean reduced system losses and better overall system efficiency.