An engineer’s guide to harmonics

Harmonics degrade power quality, reduce the operational lifespan of equipment, and can compromise system stability. This FAQ provides a technical overview of harmonics, their primary sources, and the industry standards established for their control.

How are harmonics defined in an electrical system?

An ideal ac power supply delivers a pure sinusoidal waveform at a fundamental frequency, such as 50 Hz or 60 Hz. In practice, the increasing use of modern electronic devices creates distortion in this waveform. This distortion is caused by higher-frequency waveforms known as harmonics.

A distorted electrical waveform is a composite signal formed by the summation of multiple sine waves at different frequencies. The primary, intended waveform is the fundamental frequency. Harmonics are sinusoidal waveforms with frequencies that are integer multiples of the fundamental frequency.

Figure 1 illustrates this concept by deconstructing a distorted current waveform, I(t), into its fundamental component and its subsequent odd harmonics.

Figure 1. A distorted current waveform, I(t), is the sum of the 1st fundamental frequency and its odd harmonics (3rd, 5th, and 7th). (Image: IEEE)

For a 50 Hz system, the relationship is as follows:

1st Harmonic (Fundamental) = 50 Hz
3rd Harmonic: 3 x 50 Hz = 150 Hz
5th Harmonic: 5 x 50 Hz = 250 Hz
7th Harmonic: 7 x 50 Hz = 350 Hz

In three-phase power systems, odd harmonics (3rd, 5th, 7th, etc.) are the most prevalent and disruptive. Their cumulative effect transforms a clean sine wave into a distorted, less efficient waveform that can cause operational issues.

What are the primary sources of harmonics?

The primary cause of harmonics is the operation of non-linear loads. Unlike linear loads (e.g., incandescent bulbs, resistive heaters) that draw a sinusoidal current in response to a sinusoidal voltage, non-linear loads draw current in short, abrupt pulses. This process injects harmonic currents back into the power system.

The number of non-linear loads in industrial and commercial facilities has grown significantly. Figure 2 maps the most common sources of harmonics in electrical networks.

Figure 2. Major sources of harmonic distortion in power systems. (Image: Wiley)

As the diagram shows, power electronic converters are a principal source. This category includes:

Variable speed drives are used for motor control in industrial and HVAC systems.
Photovoltaic inverters that convert dc from solar panels to ac for grid use.
Switched-mode power supplies, which are integral to computers, chargers, and most electronics.

Other significant sources include electric car charging stations, fluorescent and LED lighting, and industrial equipment like arc furnaces.

What are the impacts of harmonics and associated standards?

The presence of harmonics has tangible, negative consequences for electrical systems. These include:

Overheating: harmonic currents increase I²R losses, leading to excessive heat in wiring, transformers, and motors, which reduces efficiency and shortens equipment life.
Equipment malfunction: voltage distortion can disrupt the operation of sensitive electronics, leading to data errors, control system failures, or the false tripping of circuit breakers.
Energy losses: harmonics represent energy that does not perform useful work, contributing only to system losses and increasing operational costs.

Due to these adverse effects, international bodies such as the Institute of Electrical and Electronics Engineers (IEEE) and the International Electrotechnical Commission (IEC) have established standards to limit harmonic distortion. Figure 3 displays the most prominent of these standards.

Figure 3. An overview of key international harmonic standards. (Image: IEEE)

These standards define acceptable limits for harmonic injection to ensure power quality and system stability. They cover several key areas:

Consumer limits (e.g., IEEE 519): Specifies the maximum distortion a facility is permitted to inject into the utility grid.
Equipment constraints (e.g., IEC 61000-3-2): Set limits on harmonic currents produced by specific types of equipment.
Measurement protocols (e.g., IEC 1000-4-7): Standardize the methods for measuring harmonics.

A primary objective of these standards is to maintain the total harmonic distortion below a specified threshold, typically 5% for general applications. This is a key metric quantifying the overall waveform distortion.

Summary

Harmonics are an inherent byproduct of power electronic loads. Their impact on system efficiency, reliability, and cost is significant. By understanding the principles of harmonics, their sources, and the governing standards, engineers can design, analyze, and maintain more robust and efficient electrical systems.