What is Total Harmonic Distortion (THD) and Why Should I Care?

Total Harmonic Distortion (THD)

One of the most misused and least understood terms in power quality is Total Harmonic Distortion or THD. So, what is it, and why should I care? Let’s break it down.

Harmonics in Music

Music is one of the best ways to understand harmonics. Various disciplines employ the term harmonics. For instance, music, physics, acoustics, electronic power transmission, and radio technology are just a few of the many. Generally, harmonics apply to repeating signals, such as sinusoidal waves. 

harmonic is a wave whose frequency is a positive integer (whole-number) multiple of the frequency of the original wave, known as the fundamental frequency. The original wave is additionally known as the 1st harmonic. 

The following harmonics are known as higher harmonics. Below, the image shows a fundamental wave and the harmonics associated with it. Musicians who play stringed instruments (guitar, bass, harp, etc.) will recognize this immediately.

Fundamental and Harmonic Waveforms

On stringed instruments, you play harmonics by touching an exact point on the string while sounding the string (plucking, bowing, etc.). As a result, this allows the harmonic to sound at a pitch higher than the string’s fundamental frequency.

Electrical Harmonics

In North American electrical systems, the fundamental frequency is 60 Hz. The 60 Hz tone is almost precisely halfway between A♯ (58.24 Hz) and B (61.68 Hz). In other words, a B flat.

Electrical harmonics (also known as Distortion Factor) result from the electrical characteristics of the circuit. For example, non-linear loads like Variable Frequency Drives (VFDs) and Uninterruptible Power Supplies (UPS) increase the 60 Hz waveform distortion and increase harmonics. Conversely, linear loads like three-phase motors and three-phase electric heaters reduce the 60 Hz waveform distortion and decrease harmonics.

Electrical Harmonic Waveform

You can mathematically calculate the Distortion Factor or Total Harmonic Distortion through Fourier Series. In electrical systems, many instruments and meters have this calculation function built-in. Visually, the more the voltage or current waveform looks distorted, the more harmonics are present.

Total Harmonic Distortion (THD) Limits for Current and Voltage

Fortunately, the Institute of Electrical and Electronics Engineers (IEEE) published a standard widely accepted by the industry. This standard is an adequate guideline for acceptable limits of Total Harmonic Distortion (for both voltage and current waveforms) in electrical systems.

The standard IEEE 519-2014 is known as the Recommended Practice and Requirements for Harmonic Control in Electric Power Systems.

Voltage THD Limits
Current THD Limits

Harmonic Mitigation

Harmonics are a long-term problem for electrical system equipment due to:

  • Excessive current flow causing heating
  • Lower efficiency due to a distorted voltage and current waveform

Harmonics rarely cause Power Quality events. However, they are often a symptom of a power quality problem.

For instance, to protect a critical load from a voltage sag or outage while a generator starts, a UPS is often installed. However, if the UPS isn’t sized to match the load, the output voltage and current waveform are no longer sinusoidal. Instead, it will become like a sawtooth wave. This remains a persistent problem when installing UPS systems.

There are several solutions to reducing harmonics in electrical power systems:

  • Zig Zag Transformers
  • Tuned Harmonic Filters
  • Active Linear Voltage Supplies

Interested in determining the Total Harmonic Distortion (THD) at your business?  

Contact APT. We specialize in the measurement and analysis of Total Harmonics Distortion (THD) and designing the best solution to save you money and maximize your reliability.

Andy Taylor PE, APT Chief Executive Officer