Power quality is a good indicator of the health of a three-phase industrial electrical system. It can lead to many problems, including equipment damage and wasted energy if not optimized. Harmonics, the distortions caused by variations in the voltage or current waveform, are one of the most important issues that need to be addressed to ensure maximum efficiency in a company's power systems.
About 40 countries use a fundamental mains frequency of 60 Hz, the others use 50 Hz, as in Europe. Regardless of the system used, harmonics are a nuisance and can cause overheating of neutral conductors as well as motor windings and transformers.
What causes harmonics in electrical systems?
In general, the voltages and currents in an electrical system have a "perfect" sine waveform with a frequency of 50 Hz (60 Hz in the United States, the Caribbean, Saudi Arabia and a few other countries). This 50 Hz frequency is the only harmonic component that makes up the waveform. As soon as this waveform is distorted, the power waveform contains more harmonics. These harmonics have a frequency equal to the fundamental frequency multiplied by an odd integer. So, for example, the third harmonic has a frequency of 150 Hz, and so on until the 50ème harmonic distortion. The most important causes of this waveform distortion are loads connected to the power system, such as variable frequency drives, PC power adapters, and LED lighting. These loads distort the current and, depending on the quality of the network wiring, can also distort the voltage waveform. Variable frequency drives are increasingly used in industry to control the speed of an electric motor driving a pump, fan or compressor, for example.
What is THD?
The rate of harmonic distortion (THD, Total Harmonic Distortion) is a value for expressing all the combined harmonic components present in a power system as a percentage of the 50 Hz component. The higher the percentage, the more harmonics are present. This value therefore indicates at first glance the number of harmonics present. The THD can be calculated for both voltage and current harmonics. According to basic principles, voltage harmonics should never exceed 8 %. If this figure is exceeded, the percentage of each harmonic should be checked on a measuring instrument that shows the operator the harmonic spectrum, usually in the form of a graph. There are two well-established methods for reducing harmonics in power systems, but both have drawbacks and can be costly. The first is to use filters; the second is to replace the transformer with a high K-factor transformer that can handle the distortion.
Depending on the cause of the harmonic distortion, filters can offer many benefits, but it is essential to perform harmonic surveys of all equipment connected to the system to locate the specific source(s) of distortion. The first area to inspect for the highest THD is the large electronic drives to identify the equipment responsible for the highest current draw. These are typically large variable frequency drives. The best practice is to collect as much harmonic data as possible over several days to get a good representation of how THD levels change with the processes in the area, and to highlight where and when the peaks occur. Armed with this information, a filter provider will be able to recommend the best solution. In practice, usually only a few pieces of equipment cause the observed distortion problems, but the numbers can be much higher in a larger system. The second option, replacing a transformer, can be more complex to accomplish, although harmonic readings are still required, this time to determine the K-factor. The K-factor refers to the heating generated by the harmonics and can be calculated with an appropriate measuring instrument. Current power quality analyzers often calculate the K-factor automatically. However, since specified K-factor transformers are significantly more expensive than conventional transformers, and replacing a K-factor transformer can be costly and disruptive, it is essential to minimize downtime. Nevertheless, sometimes it is the only solution.
Importance of the measures
The above scenario highlights the importance of measurements, as only by having an overview of a system's power quality can the user get the most out of their equipment, while ensuring that energy is used optimally.
An essential step in any routine maintenance program is to perform periodic power quality surveys and measurements. These actions allow users to identify ongoing developments (if any) so that any potential risks can be flagged and addressed before problems arise. The frequency of these inspections (monthly, quarterly, semi-annually or annually) is usually left to the user's choice, but it should be kept in mind that the higher the user's requirements for system reliability, the more frequent these inspections should be. Clearly, by obtaining accurate data through a power quality survey, users have full control over their electrical systems, ensuring that they are operating efficiently and that all electrical equipment in the system will have the longest possible life.
It is important to carefully choose the points where measurements are made and to always use these same points. To do this, it is important to identify critical points in the network where equipment may be more sensitive and likely to cause problems. The experience of equipment users can be valuable in this regard, as they are often in the best position to know where problems occur and to understand better than anyone else what is really going on.
It is also important to detect and track trends by comparing historical data with each other to obtain truly useful information. At the same time, all historical data should be backed up and stored, and any changes or moves to electrical equipment should be meticulously recorded, including the updating of all electrical schematics. Finally, take your measurements and record the readings over several days, as measurements taken over a single 24-hour day will only reflect the situation during a given period. Indeed, the operation of equipment may remain constant during a whole day, but change during the next 24 hours. It is therefore wise to conduct the power quality study over several days. In summary, distortions caused by voltage or current variations can cause significant problems in a three-phase industrial power system, so it's important to monitor harmonic behavior to prevent problemsNo one wants to consume more power than necessary, nor do they want to risk damaging expensive and vital equipment. So start planning your power quality study now and keep your harmonics under control.
by Markus Bakker, Fluke Corporation