What is Harmonics?

One of the most often mentioned and misunderstood power quality issues we see with commercial, industrial, and power-generating electrical equipment is harmonics. Since it’s impossible to explain the theory of harmonics, the causes of harmonic issues, how to identify and meter harmonics, and offer solutions to remedy harmonic issues in one short article, this will serve as Part 1 of a multi-part series to discuss how to identify, monitor, diagnose, and deal with harmonics in large electrical systems.

Harmonics have increasingly become a concern for facilities and customers, given the ever-increasing number of non-linear and power electronic switching equipment being placed on electrical systems. The types of equipment that can create problems include:

Uninterruptible power supplies (UPSs)
Variable-frequency drives (VFDs)
Computers, PLCs, etc.

Identifying and correcting harmonics and other power quality problems can lower energy costs and extend the useful life of electrical equipment. This occurs because harmonics can create uneven heating in electrical equipment, conductors, motors, etc. Simply put, harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency, which in the United States is 60 Hz. When waveforms deviate from a sinewave shape, they contain harmonics. Harmonics can best be described as the shape or characteristics of a voltage or current waveform relative to its fundamental (or base) frequency (Fig. 1).

Fig. 1. Harmonics is demonstrated by the shape of a voltage or current waveform relative to its fundamental frequency. — **Fig. 1**. Harmonics is demonstrated by the shape of a voltage or current waveform relative to its fundamental frequency.

As we will discuss in future parts of this series of articles, sophisticated power quality meters can measure the many harmonic frequencies that may be present, and that data can be used to design filters, such as LC (inductive/capacitive) or RLC (resistive/inductive/capacitive). The most common filter we see in commercial industrial facilities is a delta-wye step-down transformer that limits the pass-through of the larger 3^rd and 5^th harmonics back up to the higher voltage power system, reducing the effect of damaging harmonics on adjacent feeders, circuits, transformers and customers. As can be seen in Fig. 2, filtering can restore a very close to fundamental 60Hz sinewave, reducing the heating and other negative effects of harmonics.

Fig. 2. Filtering can reduce the heating and other negative effects of harmonics. — **Fig. 2**. Filtering can reduce the heating and other negative effects of harmonics.

The density of non-linear loads (e.g., lighting, computers, VFDs, etc.) and distributed generation (e.g., photovoltaic inverter, etc.) is not going away. These types of loads are ever-increasing, and we will continue to need to learn to identify, measure, and mitigate harmonic currents/voltage in our power systems to maintain safe reliable operation.

Next month, we’ll focus on how to identify harmonics. Check out last month's column, "When Is the Power Quality Issue Not the Issue?" for more power quality content.

David Colombo, P.E. is a professional engineer located in Massachusetts, and the owner and principal of Power Engineers, LLC, a design, engineering, and consulting firm. He has more than 30 years in the electrical engineering and construction industry and is involved with the design of utility substation & distribution infrastructure, large-scale renewable energy projects, and commercial / industrial power systems. He specializes in the areas of medium-voltage design, power distribution, protection, metering, power quality, power studies, and arc flash analysis. He also provides owner’s engineering support for developers, EPC contractors, and project owners. Prior to starting Power Engineers, LLC and being an engineering consultant, he was a utility distribution supervising engineer. He holds a bachelor of science degree in electrical engineering from Worcester Polytechnic Institute (WPI) and a master of engineering degree in electric power engineering from Rensselaer Polytechnic Institute (RPI). He is a registered professional engineer in 13 states.