Control harmonic distortion to reduce energy consumption and extend asset life
Mitigate electrical harmonics: It improves system reliability, uptime and energy efficiency.
By Timothy Skell, Eaton Corp.
Increasingly, facilities are using drives to achieve energy savings, precision control and intelligent automation. Yet it’s critical to understand how harmonics can affect your assets. Working with a knowledgeable drive manufacturer supplying a range of solutions to meet unique application requirements, businesses are better able to find the most effective solution to mitigate harmonics in their environments. Solutions that mitigate harmonics also reduce electrical consumption and extend equipment life.
The basics: What are harmonics?
Figure 1. Producing harmonics is easy. A perfect wave with a frequency of 60 Hz (upper left) is close to what the utility supplies. Second wave (upper right) is the 5th harmonic of the fundamental frequency — 300 Hz — which is typical of the frequency a fluorescent lamp adds to the line. When the two waves combine (lower), the result is a symmetrical, harmonic-rich, 60-Hz supply with the same frequency as the fundamental but devoid of its original sinusoidal character.
Harmonics are distorted electrical waveforms (Figure 1) that introduce inefficiencies into an electrical system. Harmonics produce wasteful heat and can’t be converted into useful energy.
The effects of harmonics on a system are comparable to the consequences of putting bad fuel in a car – it burns inefficiently and can net more trips to the service station. The engine works harder, runs hotter and loses efficiency. It causes more pollution and failed emissions testing, resulting in fines. Although it might cost less initially, bad fuel is likely to cost more in the long run.
Nonlinear loads that convert AC line voltage to DC inject harmonics into an electrical system. A range of equipment can do this; nonlinear electronic switching devices include:
- Adjustable frequency drives (AFDs)
- UPS systems
- Computer power supplies
- Energy-efficient lighting
- Punch presses
- DC power supplies
Linear loads, such as motors that run across the line, don’t add harmonics. Many fluid handling applications use a throttling valve to reduce the flow of material, but without reducing the load to the motor significantly. Although it yields some level of control, a throttling valve consumes considerably more energy than using an adjustable frequency drive. All in all, large linear loads draw more energy than variable frequency drives, because they can’t control motor speed.
Hard-to-see harmonic-induced problems are easily overlooked. It’s critical that plants with heavy motor, AFD and lighting loads understand the problems harmonics can trigger. Too much harmonic distortion can result in slow and steady damage to sensitive electronic equipment and motors that run across the line. In the long term, harmonics can cause:
- Increased electrical consumption
- Added wear and tear on motors and other equipment
- Greater maintenance costs
- Upstream and downstream power-quality problems
- Utility penalties for causing problems on the power grid
- Overheating in transformer
Adhering to applicable standards
IEEE 519-1992 is the standard that addresses harmonic distortion. Written from the point of view of electrical utilities, it’s a system recommendation and establishes the level at which electrical energy is determined to be “clean.” IEEE 519-1992 provides guidelines for commercial and industrial users using medium- and low-voltage equipment.
“The first approach to reducing harmonics from a drive is to add impedance, either in the form of an AC line reactor or DC bus choke.”
- Timothy Skell, Eaton Corp.
To comply with the IEEE standards, total harmonic distortion is calculated under worst-case conditions. Harmonic calculations are based on the kVA capacity and short-circuit-to-load ratio of the local medium- to low-voltage step-down transformer feeding the system. The calculations are made with the point of common coupling being the juncture where the utility feeds multiple customers.
IEEE 519 looks at both current and voltage distortion. The intent of the standard is to ensure that one plant isn’t producing harmonics that could affect another plant on the power grid. In other words, your harmonics shouldn’t interfere with your neighbor’s system.
Clean power, drives to meet specific requirements
There’s a spectrum of solutions to help minimize harmonic distortion. A harmonic analysis can provide guidance about the best way to mitigate any harmonic contributions to an electrical system.
Reactors or chokes: The first approach to reducing harmonics from a drive is to add impedance, either in the form of an AC line reactor or DC bus choke. Typically, this is a low-cost and technically simple solution, but it’s likely to reduce only higher-order harmonics and has little effect on the 5th and 7th harmonics. Additionally, there’s an associated voltage drop, which means there are limits to the amount of impedance that can be added.
Both AC and DC reactors deliver similar harmonic mitigation, but AC reactors provide additional protection to the drive rectifier bridge. Reactors are typically rated between 1.5% and 5% — the industry standard is 3%. Because of diminishing returns and voltage drop, higher percentage reactors seldom are used. Typically, 3% reactors yield about 35-38% current distortion, with 5% reactors dropping this to 33% to 35%.