Harmonics in electrical power systems – what causes them, what do they do, and how can you deal with them? Markus Bakker, Field Application Engineer at Fluke Corporation, has the answers.
Power quality is a good indicator of the health of an industrial three-phase electrical system. It can cause many problems, not least the risk of damaged equipment and wasted energy if energy usage is not optimised. One of the main issues needing to be addressed to ensure maximum efficiency in any company’s electrical power systems is harmonics – distortions caused by variations in the waveform of voltage or current.
There are around 40 countries that use a fundamental power frequency (FPF) of 60 Hz, with the rest running on 50 Hz power, which is the case in Europe. Whichever system is in operation, harmonics are a nuisance and can lead to overheated neutral conductors, motor windings, and transformers.
What causes harmonics in electrical power systems?
Usually, the voltages and currents in a power system have a “perfect” sinusoidal waveform with a frequency of 50 Hz (60 Hz in the USA, the Caribbean, Saudi Arabia, and some other countries). This 50 Hz is the only harmonic component that composes the waveform. As soon as this waveform is distorted, there will be more harmonics contained in the waveform of the power.
These so-called harmonics have a frequency equal to the fundamental frequency multiplied by an odd integer number. So, for example, the third harmonic has a frequency of 150 Hz and so on, up until the 50th harmonic.
The most important cause of this waveform distortion are loads connected to the power system, like variable-frequency drives, PC power adaptors, and LED lighting. These loads distort the current, and depending on the quality of the network cabling, they may also distort the voltage waveform.
Variable frequency drives are devices that find application in the industry more and more and are used to control the speed of an electric motor that drives a pump, fan, or compressor, for example.
What is THD?
Total harmonic distortion (THD) is a way of expressing all the combined harmonic components that are present in an electric network as a percentage of the 50 Hz component. The greater this percentage, the greater the number of harmonics. So, this number shows how many harmonics are present at a glance. This THD can be calculated for voltage harmonics and current harmonics.
As a rule of thumb, voltage harmonic effects should never represent more than 8%. If this figure is exceeded, the percentage of each individual harmonic would need to be checked on a measuring instrument that shows the operator the harmonic spectrum, usually in the form of a graph.
There are two established ways of reducing harmonics in power systems, but they both have their downsides and can be costly. The first is the use of filters, and the second is to replace the transformer with one with a high K factor capable of handling the distortion.
Harmonic surveys
Depending on what is causing the harmonic distortion, filters can offer many benefits, but it is essential to carry out harmonic surveys of whatever equipment is connected to a system to locate the specific source(s) of distortion.
The first area to be checked for the highest degree of THD would be the biggest electronic drives, looking at the kind of equipment that is responsible for drawing the highest current. These would typically include high-power variable-frequency drives.
Best practice would see as much harmonic data as possible being collected over a number of days to get a picture of how THD levels change with the processes in that industry and to highlight where and when the peaks are. Armed with this information, a filter supplier would be able to recommend the best solution.
In reality, it is usually only a couple of pieces of equipment that may be causing any concerning distortion issues, but the numbers could be significantly higher in a larger system.
The second option, replacing a transformer, would involve a greater degree of difficulty, although harmonic surveys would still be required, this time to ascertain the K factor.
The K factor relates to the heating effects of harmonics and can be calculated using a suitable measuring instrument. Today’s power quality analysers often calculate the K-factor automatically. However, because K-factor rated transformers are significantly more expensive than standard transformers, and replacing a K-factor transformer can be costly and disruptive, it is critical to minimise downtime. Nevertheless, sometimes it can be the only solution.
The importance of measuring
What the scenario above underlines is the importance of measuring, because only by having a full picture of a system’s power quality health can a user get the most out of their equipment while ensuring energy usage remains at optimal levels.
A crucial part of any routine maintenance plan should be to carry out regular power quality surveys and take reasonably regular measurements. These actions would enable users to identify whatever changes are taking place (if any) in order to flag up anything that could be a problem and get it sorted in advance of this happening.
How often these surveys take place – monthly, quarterly, half-yearly, or annually – is usually the choice of the user, but it should be remembered that the greater the requirements the user has in terms of system reliability, the more often a survey should be carried out. Clearly, by obtaining accurate data via a power quality survey, users can gain complete control over their electrical systems, ensuring that they operate efficiently and that all electrical equipment in the system has the longest possible working life.
Keep those harmonics under control
It is important to take care when selecting where measurements will be taken and then to stick with the same location every time. This means identifying critical points on the network where equipment might be more sensitive and be expected to cause problems. The experience of those who operate the equipment can be invaluable in this regard because they often have the greatest insight into where problems might occur – they know better than anyone what is happening at their level.
Also important is the need to monitor trends and ensure that like is being compared with like in order to obtain data that is genuinely useful. At the same time, all historical data should be saved and stored, and any alterations or moves should be meticulously recorded, including updating all electrical diagrams.
Finally, take measurements and log your findings over several days, because taking a single snapshot over any given 24 hours will only show what is happening at that time. Equipment operation can remain constant over that period but then change during the following 24-hour period, so it’s wise to make any power quality survey last for a number of days.
In summary, distortions caused by variations in voltage or current can cause significant issues in an industrial three-phase electrical system, so checking on the behaviour of harmonics can help prevent many problems from occurring. Nobody wants to use more energy than is necessary or risk expensive and vital equipment being damaged, so start planning that power quality health survey now and keep those harmonics under control.
