Harmonics: Understanding a Goblin in Your Electrical System
Many of the potential readers of this blog may have heard the term “harmonics” frequently enough to know it relates to VFDs, but that is possibly where their understanding may end. If you are experiencing mysterious issues (motors experiencing unusually high wear and tear, medical imaging scans with unusually low resolution, a very low power factor listed on electric bills, among other things) then a better understanding of harmonics may help you guide a conversation that ultimately flushes out the culprit.
Harmonics have been present in electrical systems since the first alternating current (AC) device was powered on. However, they had negligible impact on systems of the time. Electrical loads can be broken into 2 categories, Linear and Non-linear. Linear loads (heaters, incandescent lights) have both current and voltage moving together throughout their electrical sine waveform. Non-linear loads (computers, LED lights, Variable Frequency Drives (VFD’s), printers, battery chargers, etc.) have power supplies that create electrical sine waveforms where current and voltage do not move together throughout. Current will pulse on/off while voltage follows a steady rise/fall typical of a sine waveform.
Non-linear loads contribute harmonics to the overall electrical system because the current pulses create currents that return into the electrical distribution system. These contributions are amplified when loads are unbalanced between phases; ideally, loads should be balanced within 2-3% between phases. Individual contributions are generally negligible, but are cumulative within an entire electrical system. Harmonic distortion does not have to stay at the point of generation and will circulate throughout the entire electrical system. When measuring power within a system, a smooth sine wave illustrates power without harmonics. A system experiencing harmonics will have a sine wave that may look like it was drawn by hand during an earthquake resulting in poor power quality and the degradation of equipment being run on the system.
Harmonics can be in the form of either voltage or current distortion since the frequency is affected. Harmonics are generally referred to by their order (1st, 2nd, 3rd, etc.); this order refers to the integer multiple of the fundamental frequency (60 Hz in the United States) where the electrical contribution is present. Harmonics of the 2nd order are present in a system at 120 Hz (2 x 60 Hz), 3rd order harmonics are present in a system at 180 Hz (3 x 60 Hz) and so on. Odd order harmonics are generally of bigger concern than even order ones, due to the rotational torque they can cause in motors and most dominant harmonics are the lower orders from the 2nd to the 19th with negative impacts increasing as you approach the 2nd. Harmonics can affect a motor’s rotation in the positive or negative direction (or not at all). The reason the odd order harmonics are of concern, is because they can cause a braking torque to occur in motors causing inefficiency (manifested in heat and diminished power factor) and/or damage, the effect is like pressing both the accelerator and brake in your car at the same time. The braking torque caused by harmonics can potentially approach the torque rating of motors attached to the system. Why should this concern you? A motor rotating clockwise at 25 horse power will be inefficient and will experience higher than normal wear on parts if harmonics attempt to turn the same motor counter clockwise at 15 horse power.
Variable Frequency Drives contribute harmonic orders relative to their pulse rate (+ or – 1) but eliminate or result distortion on the octaves below. For example, a 6-pulse drive will primarily contribute 5th and 7th harmonics but limit the lower order harmonics. Based on this, you may have already guessed that an 18-pulse drive will primarily contribute 17th and 19th harmonics but limit the harmonic orders below. This is why we’ve seen the VFD industry develop cost effective solutions to 18-pulse drive manufacturing and the consulting world lean towards their selection when harmonics are of concern.
Most people do not notice electrical issues until a failure occurs, often in a dramatic fashion, as capacitor bank and non-dry type transformer failures can involve fires. An example of a frequency issues that you may be familiar with is the Tacoma Narrows Bridge. In this example, the resonance frequency of the steel structure was matched by wind passing through the canyon and as the bridge undulated in the wind like a sine wave it eventually broke apart in dramatic fashion. While the risk is minor, harmonic frequency contributions can match the resonance frequency of a piece of equipment and shake it apart with electricity instead of wind. A much less dramatic manifestation of this same example is the hum sometimes heard coming from a piece of electrical equipment.
Capacitors tend to draw harmonics to themselves, making capacitor banks and Uninterruptible Power Supplies (UPS) potential targets. Transformers can trap them within and experience higher temperatures than designed, which can shorten the life of transformers. For example, a sustained increase of 10o C above design temperatures can lessen the life of electrical components by over a third. Capacitor and transformer failures tend to be fairly extensive, not just themselves but also equipment located in near proximity. Other examples include but are not limited to: excess heating of cables, lighting fixture ballasts burning out, motor inefficiency, static on phone lines, data corruption, a humming noise, motor damage, transformer failure, nuisance circuit breaker trips, incorrect meter readings, Automatic Transfer Switch (ATS) transfer inhibition, and generator operation disruption. All of these conditions reduce the life expectancy and operating reliability of the equipment and system.
Where to Look
Finding harmonics in a system is a little like playing hide-and-seek since they can migrate away from the source of generation and be apparent anywhere that is attractive to them. Luckily for the facility, harmonics are bad at the game since electrical issues resulting from harmonics are always near the point of manifestation. Placing true RMS meters on the electrical system near suspected manifestation points, potential generation points, and upstream of those locations for a period of time (1-2 weeks) is the easiest way to flush their locations out and determine if there is a goblin wreaking havoc in the system.
If harmonics are in an electrical system, there are ways to better withstand and even remove them. For example, K-rated transformers are purposefully built with more metal core material to withstand the negative effects. Use of K-rated transformers to mitigate harmonics must also include HVAC considerations since K-rated transformers generate additional heat as a byproduct. Delta-wye transformer configurations can trap harmonics on the delta side preventing them from reaching parts of the electrical system. Harmonic Mitigating Transformers (HMTs) are specifically built to minimize voltage distortion and power losses generated by non-linear loads and cancel the effects of harmonics, however, HMTs are most effective when multiple identical loads are being fed from the transformers. Oversizing neutral conductors at transformers and panelboards can also help with conductor heating caused by harmonics. Line reactors and harmonic filters are common pieces of equipment that can be installed to help negate harmonic contributions from an associated drive. Lastly, on large variable speed motor applications (50HP) 18 pulse drives should be considered as noted above to limit the lower order harmonics.
Most of the time harmonics need not be considered, however, it is worth a conversation for facilities with significant non-linear loads (computers, rotating machines, VFDs, PV Inverters, medical imaging equipment) that are experiencing odd electrical issues (static on phone lines, data corruption on servers, humming of electrical equipment, equipment burning out). Similarly, large renovations that include the addition of significant non-linear loads should consider what harmonics may be introduced to the electrical system. While there is nothing harmonious about harmonics in your system, with a bit of knowledge and planning you can keep your systems running efficiently.