Harmonics: What Electrical Contractors Need to Know — Part 1

Unplanned downtime is a significant expense. In fact, the average large industrial plant loses about 27 hours of production monthly to unscheduled outages. According to a report from Siemens, “The True Cost of Downtime 2024,” for the world’s 500 largest companies, these disruptions result in more than $1.4 trillion in losses each year.

With maintenance teams under pressure to reduce downtime, understanding how harmonics can impact plant operations will help keep costs under control. If you have dealt with unexplained overheating, reduced motor life, tripped breakers, or equipment malfunctions, you may not have realized that harmonics was the culprit. In this first article in the series, we will introduce harmonics and review why they’re a concern in electrical systems. Then we’ll dive into some specific examples from plant operations.

What are harmonics?

In an AC power system, the ideal voltage and current waveforms are pure 60 Hz or 50 Hz sine waves. This is the fundamental frequency: 60Hz is used mostly in the Americas, while 50 Hz is used in Europe and other parts of the world. Harmonics are “pollution” in an electrical system, distorting the normal sine wave by adding higher-frequency components on top of the fundamental waveform (see graphic below). For a 60 Hz system:

• Fundamental frequency (f₁): 60 Hz
• 2nd Harmonic (f₂): 120 Hz
• 3rd Harmonic (f₃): 180 Hz
• … and so on.

Utilities and alternative energy power sources produce mostly pure sine waves at the fundamental frequency.

So where do harmonics come from? They are caused by the things that we plug into the power system. Harmonic distortion is usually produced by loads that draw current in non-linear ways, such as pulses (think computer switching power supplies) rather than smoothly. 

Common harmonic-producing loads include:

• Variable-frequency drives (VFDs)
• LED lighting
• Computers and office electronics
• UPS systems
• Battery chargers
• Welders and arc furnaces
• Inverters and converters

Why are harmonics a concern?

While some harmonic content is expected in systems with modern electronic loads, harmonics can create a variety of negative consequences. A few of these are defined below.

Overheating

Transformers and motors can experience excessive heating and reduced life, particularly from 3rd, 5th, and 7th harmonics. Neutral conductors can overheat due to triplen harmonics (3rd, 9th, etc.) that add arithmetically in 3-phase systems. In addition, harmonics increase eddy currents and core losses, reducing equipment life.

Protective device misoperation

Circuit breakers, relays, and fuses may trip due to distorted current waveforms. Harmonics can also affect the timing and sensitivity of protection schemes.

Reduced power factor

Harmonic current does not perform useful work, but still increases the RMS current. This can reduce the power factor and may affect utility charges or require costly correction.

Equipment malfunction

Drives and inverters may trip or fault due to distorted waveforms. Control systems may experience communication interference if harmonic noise enters signal lines.

Resonance

In certain conditions, harmonics can resonate with capacitive or inductive elements in the system. This can significantly amplify voltage distortion or current spikes, sometimes leading to insulation failure, damaged capacitor banks, and other failures.

Multiples of three

Triplen harmonics are harmonics that are multiples of three. The most common concern is the 3rd harmonic and other odd triplens, such as the 9th and 15th (see graphic below).

In a balanced 3-phase system, the phase voltage and currents are 120 degrees apart and tend to cancel in the neutral. Triplen harmonic currents behave differently. They are in phase with each other and can add in the neutral. That means a neutral conductor may carry more current than expected, even when the phase currents look reasonable.

Watch for triplen harmonics in systems with:

• Shared neutrals
• Heavy single-phase nonlinear loading
• Office areas with many electronic loads
• LED lighting conversions
• Data-heavy facilities
• UPS-fed panels

Harmonics examples from plant operations

An overheated neutral

A commercial panel serves office electronics, LED lighting, and other single-phase nonlinear loads. Phase current does not appear excessive, but the neutral is running hot. A likely next step is to measure neutral current and look for triplen harmonics.

The capacitor bank failure

A facility adds or changes large loads. Soon after, fuses open or capacitors fail. One possible cause is harmonic resonance between the capacitor bank and the system impedance. PQ and harmonic measurements should be taken before selecting a correction method.

A drive trips without an obvious voltage problem

A variable frequency drive trips during normal production. Standard RMS voltage readings look acceptable. Harmonic distortion, voltage notching, or higher-frequency noise may be involved. A power quality recording can help separate supply issues from equipment or process issues.

EV chargers go offline randomly.

EV chargers or banks of chargers randomly go offline for unexplained reasons. The AC power supplying the EV charger(s) could have high harmonic content, which can result in random shutdowns. 

Key takeaways

To help electrical professionals better understand why harmonics matter, here are several key critical considerations to remember.

• Harmonics are like "power pollution." Unlike a clean 60 Hz sine wave, harmonics are higher-frequency distortions caused by nonlinear loads (like VFDs, LEDs, and UPS systems) that pull current in pulses rather than a smooth flow.
• With the average large plant losing 27 hours of production monthly to unscheduled outages, identifying harmonics is as much a financial necessity as a technical one. They often hide behind symptoms like unexplained overheating or nuisance trips.
• Triplen harmonics (multiples of three, such as the 3rd or 9th harmonic) add up in the neutral conductor rather than canceling out. This can lead to dangerously hot neutrals even when phase currents appear balanced.
• Harmonics do more than distort waves; they increase eddy currents in transformers, reduce equipment lifespan through heat, cause resonance that can destroy capacitor banks, and trigger power factor penalties.

In the next article, we’ll cover harmonics standards and measurement as well as ways to mitigate their impact on electrical systems.