- Jul 12, 2023 10:57 pm GMT
History and Context
The effect of harmonics on the electrical power system is not a new issue. This phenomenon has been of particular interest to electrical engineers since around the turn of the century when self-taught mathematician and electrical engineer, Oliver Heaviside, published works that became influential in understanding harmonics on electrical systems. And in 1916 renowned electrical engineer, Charles Steinmetz, published concerns about the effects of harmonics and the possibility of resonance on the electrical system.
Since then, harmonics levels on the distribution grid have continued to be a problematic issue for utilities and their customers. Elevated levels can cause power quality issues and disturbances, often resulting in eventual equipment problems, outages, and customer dissatisfaction. What’s more, because harmonics measurement and identification of their source can be challenging, many utilities find harmonics problems difficult to address.
For utilities and electrical engineers alike, it’s apparent that harmonics is becoming a more significant issue as load becomes more digitized and non-linear and new non-linear loads like EV charging are added.
To explore this topic more fully we posed the following questions to a team with years of utility industry, electrical engineering, and power system harmonics experience, the Sentient Energy Analytics team:
What causes harmonics on the distribution system?
Harmonics are caused by the non-linear or non-continuous draw of current from loads such as switch mode power supplies. In the simplest terms, a switch mode power supply is anything that takes the incoming AC from the utility and changes it to DC. This means that harmonics are produced by a wide variety of devices that we use daily. Examples of harmonic producing loads that are readily found in homes include LED lights, computer power supplies, phone chargers, HVAC units, and TVs.
As might be expected, harmonic loads in industrial environments have the potential to be even more problematic for utilities and their customers. Sources of these loads include variable frequency drives (VFDs), lighting, welding, arc furnaces, and servers. Each broad type of load tends to have its own characteristic harmonic signature. A single-phase power supply that changes AC to DC for use tends to produce a lot of third harmonic while a six-pulse variable speed drive will dominantly produce the fifth and seventh harmonic along with other multiples of those harmonics to a much smaller degree.
An increasing need for energy efficiency and process control requirements has driven the growth of many of these harmonic-producing loads. The switch to energy-efficient LED lighting for homes and businesses in recent years is one example of a prominent driver of harmonic loading increases. Where traditional filament-based lighting is resistive in nature, more energy efficient lighting such as LED is non-linear and has a switch mode power supply in it. This has increased the amount of third harmonic flowing on the electrical system. Similarly, many industrial and commercial processes have also pushed forward with energy efficiency measures and now require sophisticated control capability for many aspects of production and operation, leading to a dramatic increase of VFDs.
And as the push for electric vehicles (EV) adoption and more wide-spread charging infrastructure dramatically accelerates, so too will the occurrence of harmonics on the electrical system as EV chargers have non-linear front ends.
What other factors contribute to elevated harmonics levels?
Although the root source of harmonics on an electrical system is from customer loads, utilities can inadvertently make changes that amplify the situation. For instance — in recent years, the need for improved energy efficiency has driven utilities to optimize voltage profiles, from the circuit breaker all the way through to the end of the line. Deployment of capacitor banks is a common way for utilities to accomplish voltage optimization.
Capacitor banks are beneficial electrical devices that help provide reactive power to the system and reduce the line losses seen upstream. But in doing so, they also change the impedance characteristics of the electrical environment and can create amplification or even catastrophic resonance of harmonics if misapplied.
What system impacts and safety issues can be caused by harmonics?
The three main concerns about harmonics on the electrical system are resonance, heating, and electromagnetic compatibility with equipment.
Resonance can lead to catastrophic failure of equipment or systems. It is akin to putting a microphone in front of a speaker and creating a feedback loop where the signal continues to amplify and rise until something fails. This occurs on the electrical system when the capacitance and inductance are such that they align with the harmonic frequency to create a feedback loop, which is why installation of capacitor banks should always be investigated for harmonic concerns.
Heating of equipment due to harmonics is a well-known system impact and described within literature. IEEE C57.110 IEEE Recommended Practice for Establishing Liquid Immersed and Dry-Type Power and Distribution Transformer Capability when Supplying Nonsinusoidal Load Currents even provides guidance on derating of transformers serving non-linear load.
In addition to heating of transformers, the phenomenon of skin effect should also be accounted for. Skin effect is the term used to describe how harmonic frequencies penetrate conductors. In short, the higher the frequency of the current the less the cross-sectional area of the conductor is used as it is only able to flow across the surface thus increasing system impedance at that frequency and heating up the conductor.
The increased impedance becomes a critical concern when dealing with third harmonic return flow in the neutral conductor. Third harmonics do not cancel out but instead sum together in the neutral wire. This coupled with the increased impedance due to skin effect can cause an increase in 3rd harmonic stray voltage along return paths back to utility sources.
The least understood effect of harmonics is misoperation of equipment due to electromagnetic compatibility issues. Numerous cases have been documented where harmonics have caused process control issues with industrial customers, flickering led lights, misoperation of utility control equipment such as SCADA systems and regulator controls, and audible noise interference. This type of harmonics issue is often the most difficult to solve as it requires a lot of data gathering of both the electrical system and customer locations, as well as research into phenomenon and interaction.
Moving forward, will harmonics become an increasing problem?
Yes, for a couple reasons. First, non-linear loads including those associated with EV charging, will continue to increase the occurrence of harmonics on the electrical system. The push for utilities to achieve further energy efficiency via voltage optimization and capacitor banks will also increase.
Second, harmonics are complex. Many revenue meters, protection relays, and data acquisition systems used by utilities are not setup or enabled to capture harmonic information, so measuring harmonics requires the use of specialty devices. These specialty devices often require involved and costly substation deployment. And a deep understanding of harmonics is not typically widespread within a utility. Often, only specialized utility groups that focus on power quality are knowledgeable resources on complex harmonic issues.
The negative power system effects of harmonics like fluctuating power quality, maintenance issues caused by heating, and equipment failure have impacted utilities and their customers for many decades. But growth in non-linear power system loads driven by the electrification of everything and a need for energy efficiency has given new importance to this old problem.
To understand and address the harmonics on their systems utilities must be able to measure and identify elevated levels, and zero in on the source. This is often not easy. An intelligent line sensing and advanced analytics solution like Sentient Energy’s MM3ai System enables utilities to measure harmonics and uncover likely sources of elevated levels, providing the information needed to address harmonics issues, improve power quality, and increase customer satisfaction.
For information on Sentient Energy’s solution for harmonics measurement along overhead feeders visit: www.sentientenergy.com/solutions/harmonics
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