Jan 15, 2026
Managing Substation Harmonics to Preserve Power Quality
Summary: Among the many things that can destabilize power quality are substation harmonics. A wide variety of nonlinear loads produce substation harmonics. Fortunately, engineers have devised clever strategies for keeping harmonics under control.
The word 'harmonics' might conjure up images of a world-class symphony orchestra producing an incomparable sound while playing a renowned classical piece. In such a setting, harmonics are a good thing. But when it comes to substation design, it is just the opposite. Substation harmonics can hinder power quality and reliability. Therefore, they must be addressed.
Harmonics are 'extra frequencies' that ride on top of normally expected frequencies. In a symphony orchestra setting, the extra frequencies can add a level of depth and tonal color that truly make a piece of music memorable. But they are unwanted frequencies in a substation environment. They can hinder meters and relays, cause equipment to overheat, and otherwise cause a substation to experience performance issues.
Normal and Distorted Power
Ideally, we want the voltage and current in the grid to remain smooth and steady. We want a single frequency of 60 Hz. When wave frequency is no longer smooth, you have harmonics. Power transmission becomes lumpy or jagged instead of clean and consistent.
A normal 60 Hz wave is considered the fundamental frequency in the U.S. Substation harmonics introduce copycat waves at higher frequencies, including:
120 Hz (the second harmonic)
180 Hz (the third harmonic)
240 Hz (the fourth harmonic)
300 Hz (the fifth harmonic)
Perhaps you've noticed that each harmonic is a multiple of sixty. What you essentially have is a duplicate wave of current that is just flowing at a faster rate. That is why the multiples are always at 60 Hz.
What Causes Substation Harmonics
To better understand what causes substation harmonics, let us go back to the music example. A standard acoustic guitar has six strings. Both the first and sixth string are tuned to the 'E' note. However, the first string is an octave higher than the sixth. Therefore, it vibrates at a higher frequency even though it is playing the same note. In order to play that note, you would pluck the string.
In a substation scenario, there are multiple things that can 'pluck the string'. They include things like computers, variable speed drives, UPS systems, etc. Rather than drawing current steadily, these devices pull current in short pulses. As they do, they break the smooth and steady 60 Hz waves that run through the substation.
Loads of this sort are known as nonlinear. But these nonlinear loads do not come from within the substation itself. They come from the customers whose homes and businesses are connected to the substation. The more customers generating non-linear loads, the greater the potential for substation harmonics to degrade power quality.
Inside the Substation
What happens inside the substation as nonlinear loads increase? The loads can create several practical problems:
Overheating – Harmonic currents produce increased losses in transformers, cables, and other equipment. This can lead to overheating.
Distorted Voltage – Harmonic currents create distortion to the extent that the voltage waveforms at both substation and customer load are no longer clean.
Compromised Performance – Harmonic currents can cause meters to malfunction, leading to performance issues.
In order to control the potential damage substation harmonics can cause, engineers rely on being able to accurately measure how severe waveform distortion is. That measurement is represented in total harmonic distortion (THD). A higher THD suggests a lower quality of power.
Why It Matters to Utilities
Utilities are concerned about substation harmonics because they directly impact customers. In fact, excessive harmonics at a single substation impact numerous customers simultaneously. To keep harmonics in check, utilities establish limits at acceptable harmonic levels. Some rely on IEEE 519 as the standard.
By taking steps to mitigate harmonics, utilities keep customers happy and protect sensitive equipment against damage. Limiting THD also keeps the waveform clean and stable. The big question is how they pull it off.
How Harmonics Are Managed
Managing harmonics is part of the substation design process. Engineers have access to a variety of strategies designed to keep harmonics under control. They can insist on better equipment, and they can implement harmonic filtering. They can build short, low-impedance connections into the system from the ground up.
Here are some practical examples:
1. Capacitor Banks
A substation acts as a checkpoint, if you will. It pulls power from an external source and then delivers it to customers on the other side. With this in mind, one very effective strategy for controlling harmonics is to deploy capacitor banks that help with power factor and voltage support.
The banks supply reactive power locally, mitigating the need to pull excess power from the source just to keep the power factor as close to 1 as possible. This reduces both current and losses in lines and transformers.
2. Reactors
Engineers deploy reactors – coil inductors that add electrical stiffness to the system. These reactors limit short-circuit currents. They smooth out sudden changes in the wave form. And by combining them with capacitor banks, they can shift natural resonance away from the harmonic frequencies engineers are trying to avoid.
3. Filtering
Just like sound waves can be filtered, so can electrical waves. Filtering in a substation environment is about cleaning up distorted waveforms in order to produce a smoother and cleaner sine wave. Engineers rely on a combination of:
Passive Filters – Passive filters are combinations of resistors, capacitors, and reactors tuned to a specific harmonic frequency. Harmonics are absorbed rather than flowing through the grid.
Active Filters – Active filters are powered devices that can sense distortion and respond by injecting an equal and opposite current. This cancels out the harmonics, restoring balance.
Both passive and active filters work together to reduce TDH. This results in a cleaner and more stable power supply while also preventing unnecessary wear and tear on equipment.
Substation design is a lot more complicated than it might appear. Engineers need to account for so many variables, including substation harmonics. By keeping harmonics under control, engineers ensure that stable, high-quality power continues to flow to customer homes and businesses. Customers are happy, and substation equipment is protected against premature wear and failure.
FAQs
Why are harmonics a problem for substations?
Harmonics are essentially extra copies of the normal AC waveform. They distort voltage and current, thereby reducing the quality of a substation's output and putting its equipment at risk.
What is reactive power and how do capacitors supply it?
Reactive power is power that oscillates between source and load. Capacitors store this power until it is needed. It can be injected into a system to cancel out harmonics.
Why are reactors often installed along with capacitor banks?
Reactors are installed in series with commensurate capacitor banks to de-tune them. Together, the components shift a system's natural resonance away from common harmonic frequencies.
Do harmonic filters target specific frequencies?
Yes. By doing so, engineers provide an easy path for absorbing or canceling the extra waveforms. This smooths out the remaining voltage.
Do substation harmonics amplify power?
Substation harmonics do not amplify power. Rather, they distort the power waveform by mimicking the standard wave but at a higher frequency.
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