23 Jul 2024


Defence against the dark arts

If your facility suffers from flickering lights and overheating cables, the cause could be ghosts – but is more likely to be harmonics. ABB’s George Dodwell explores why harmonics are such a problem, and how drives can be used to solve them.

Halloween is almost upon us, and with that in mind I thought I’d use this column to tackle one of the hidden gremlins that could be lurking within your power networks. Harmonics are distortions of current and voltage waveforms caused by introducing non-linear loads to your systems. These can include switched-mode power supplies, LED lighting systems and computers, as well as solar inverters and EV chargers.

Add too many of these devices to your system, and they will affect power quality and cause equipment to behave erratically. Sometimes this will manifest itself in flickering lights or overheating cables – suitably spooky. It can also lead to failure of protection devices, and damage to sensitive electronics.

Harmonics are heavily regulated. The current applicable standard, G5/5, sets out the assessment criteria and harmonic limits of harmonic generating loads. If a site is not G5/5-compliant and is generating excessive amounts of harmonic content, then network operators are allowed to issue fines, or to turn off the supply altogether. Any harmonics should therefore be taken very seriously.

An electricity supply is rarely a pure sine wave, and there will almost always be distortion, at least to some extent. However, non-linear loads draw current in a periodic non-sinusoidal manner, which can exacerbate the problem significantly. This causes inefficiencies and system losses, while also potentially interfering with other equipment as the harmonics flow through the power network.

Drives are switching devices, and one of the major culprits for harmonic distortion, because they typically draw large amounts of current. Filter remedies are available, but are often bulky and expensive. An alternative approach is to mitigate drive harmonics using the drive itself.

G5/5 specifies active rectifier technologies to mitigate harmonics, and these can make a huge difference to total harmonic distortion (THD). A conventional drive with no mitigation whatsoever can have a THD of around 30-40%. By contrast, active front end (AFE) drives use IGBTs instead of diodes in the rectifier circuit, resulting in much lower THD. For example, ABB’s latest generation Ultra-Low Harmonic (ULH) AFE drives can offer THD figures of less than 3%.

For an 11kV site, the harmonic calculations would ordinarily allow a maximum of 76kVA of six-pulse load. If instead you were to use an AFE drive, this would allow a maximum of 672kVA, while also significantly improving the resilience of your systems.

For more information on harmonics, or ABB’s ULH drive offering, contact George Dodwell at george.dodwell@gb.abb.com.