Imagine being in a room listening to a relaxing song. Then a group of people in the next room turn on a football game and begin eating, talking and reacting to the ball game. The noise in the other room becomes loud enough to distort the ability to hear the song so it is no longer possible to enjoy the relaxing music. This is an example of harmonic distortion, and it is analogous to what harmonic distortion can do to power quality on the electric grid.
What Are Harmonics?
Historically, harmonics have been a key component of power-quality issues utilities face in the reliability of their distribution and transmission systems. These issues were primarily the result of industrial and large-commercial loads. However, recently the issue of harmonics has grown to non-traditional loads, which is having a greater effect on power distribution systems. This change is making system reliability a greater challenge.
In an alternating-current (AC) transmission and distribution system, the voltage and current are represented by a sine wave based on the fundamental frequency of the system. Harmonics are sine waves that are multiples of the fundamental frequency and added to the fundamental AC sine wave. The greater the magnitude of each harmonic multiple, the greater the distortion of the fundamental AC sine wave on the system. In the music example, the song is the system frequency and the party is the harmonic distortion.
Industrial and Commercial Loads
Traditionally, utilities work with commercial and industrial (C&I) customers concerning the mitigation of harmonics to levels meeting utility standards, which are usually based on IEEE Standard 519 or IEC Standard 61000-4-7, or on both standards.
In the case of C&I customers, utilities ideally work with the customer engineers during the facility's design to meet the standards, including harmonics. C&I customers traditionally have equipment within their facilities that is nonlinear in nature. By working at design, the utility and the customer can install filters and equipment to mitigate harmonics to acceptable levels.
When a facility has a power-quality issue affecting its operation or the utility's system, the utility will use power-quality equipment to determine the cause of the issue. The power-quality equipment can determine whether the power-quality issue is a result of harmonics from a particular piece of equipment or a result of several factors within the facility.
In some cases, the load requirements of C&I customers are large enough that a dedicated substation is required to meet their needs. A plant might contain motors, compressors, refrigeration units or computer equipment with nonlinear components that contribute to harmonics emissions. The utility may then install power-quality devices within the substation control panels or include them within the specifications of the relays used to control substation equipment.
Another commercial load contributing to harmonics may be outside a plant in the form of dairies, irrigation or oil pump jacks. Dairies contain motors, pumps and refrigeration within their facilities that can contribute to harmonics if not properly filtered. Farmers use a wide range of irrigation techniques from flooding to low-energy precision application systems to grow crops. In most cases, the application used for irrigation requires a motor and a pump, which can contribute to system harmonics.
Examples of Power-Quality Issues
One example of harmonics from commercial equipment affecting other loads connected on the same part of the system is a utility receiving a call about power-quality issues affecting some equipment at a customer's residence. The utility sends a technician, who uses a multimeter with an oscilloscope to obtain initial readings and consults with the customer to get more information about what the customer observed. The technician then installs a power-quality monitor at the customer service entrance. After a period of time, usually determined by the type of power-quality issue, the technician returns to download the information from the monitor. The technician consults again with the customer to determine if the conditions of the power-quality issues have changed and to advise that the utility will be in contact after the data evaluation.
An evaluation of the data may show voltage or current distortion exceeding the standards used by the utility. During the visit with the customer, it is usually determined the customer's equipment was not the major contributor to the observed distortion. Further examination may reveal nearby equipment, such as an oil pump jack with no filtering equipment, that may be a harmonic source. Readings are then obtained from the nearby equipment. If the equipment data shows harmonics exceed the standards used by the utility, then the equipment owner is notified the equipment does not comply with the rules and regulations of service set forth by the utility. The equipment owner must then mitigate the harmonics and come into compliance with the rules of service usually by installing filter equipment. The utility then monitors the residence again to determine whether the issues have been corrected.
However, in cases where the customer's equipment is found to exceed the utility harmonic standards, it is the customer who must take action to be within the regulations of service. The utility provides the customer with information about what can be done to be compliant and, therefore, mitigate the harmonic distortion.
Rise of Harmonics in Residential Services
A residential service shown to have harmonic distortion exceeding the utility's standards is becoming a more frequent finding. Sometimes, the levels of harmonics found are the result of a combination of local and remote harmonic distortion. While residential services traditionally have not been a major contributor of high harmonic content, that trend has changed with the advent of digital and energy-efficient devices.
In residential services, digital devices include televisions, computers, sound systems and other devices containing digital components that are nonlinear in nature. One component of focus is the compact florescent light (CFL), which emits the lighting level of a traditional incandescent lightbulb but uses less energy. However, some CFLs produce harmonics beyond the levels set forth in the IEEE and IEC standards. Recently, as CFL technology has evolved, the harmonic contribution CFLs make has decreased; however, some still exceed the levels set forth in the standards.
With the advent of digital devices, the harmonic contribution of a residence to the utility's power system has increased due to the increased harmonic currents on the system's neutral, caused by device power supplies. The harmonic current contributed to the neutral does not vary enough in its phase angles to provide for cancellation and results in an addition of the harmonic components and overloading of the neutral.
With a larger percentage of residential services displaying harmonic content that exceeds industry standards, the aggregate of the residential services on a circuit could cause harmonic issues for utilities. While a statement cannot be asserted that residential services are the major contributor to power-quality issues for a utility, residential loads must be given more consideration concerning power quality than in the past.
Key Issues
With demands being made on a utility's power system, from market demand, capacity issues and system stress, utilities must run their systems at an optimum reliability level. For a utility to operate a reliable power system, power quality must become a key issue. The mitigation of harmonics within the system improves the efficiency of the system.
The evolution of the power system in the form of the smart grid will challenge the future approach of utilities and their engineers; therefore, utilities must improve every aspect of their power system including power-quality issues to ensure reliable and quality service to their customers.
Mike Swearingen ([email protected]) earned a bachelor's degree from the department of computer science and mathematics at Eastern New Mexico University in 1990. He is the manager of engineering/NERC compliance at Tri-County Electric Cooperative in Oklahoma. He is a member of IEEE and the Power & Energy Society, the Computational Intelligence Society and the Standards Association. He also is a member of the Centre for Energy Advancement though Technological Innovation's Power-Quality Interest Group, is a technical advisor for the National Electric Energy Testing Research and Applications Center, and has served as vice chair of the National Rural Electric Cooperative Association's Power-Quality Subcommittee.
Standards in Development
The IEEE Standard 519 is a valuable tool for application in commercial and industrial services, but it is not the best application for residential services. The concern over the years has been the misapplication of IEEE Standard 519.
Two IEEE working groups have been working on a revised version of IEEE Standard 519 in the form of a standard and an application guide. IEEE Standard 519-1992 contained calculations, tables to define harmonic levels and examples of application. The new IEEE Standard 519, which is currently being reviewed by balloters at the IEEE Standards Association, contains the tables and calculations to determine harmonic compatibility levels. The application standard IEEE Standard 519.1, which also is currently being reviewed by balloters at the IEEE Standards Association, contains application examples of the standard for applying harmonic limits on industrial facilities, commercial customers, residential customers and utility system considerations.
The two working groups currently are developing IEEE Standard 1836 and IEEE Standard 1837, both of which deal with harmonic current emissions at different current levels. IEEE Standard 1836 provides harmonic compatibility limits for currents less than or equal to 16 A per phase while IEEE Standard 1837 provides harmonic compatibility limits for currents between 16 A per phase and 75 A per phase.
While IEEE Standard 519 and IEEE Standard 519.1 provide one resource for harmonic compatibility levels, they do not provide a complete source of tools for evaluating harmonic levels of individual equipment. With IEEE Standard 1836 and IEEE Standard 1837 providing for an additional reference on harmonic compatibility levels for equipment, the power-quality engineer will have a full range of tools to use in combination with IEEE Standard 519, IEEE Standard 519.1 and IEC Standard 61000-4-7.
Residences such as these may be affected by harmonics generated from adjacent homes, remote commercial installations or from equipment within the residence.
Companies mentioned:
IEC | www.iec.ch
IEEE | www.ieee.org
Tri-County Electric Cooperative tri-countyelectric.coop
