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Power networks contain an ever-increasing presence of harmonics from power conversion and common electrical appliances such as computers and LED lighting.
Harmonics lead to any number of results and can cause both technical and financial issues. Rather than suffer the potential effects of harmonics on power systems, solutions such as harmonic filters are often applied.
Choosing the right variable frequency drive (VFD) for harmonic mitigation can be daunting with many different options available. Use the information on this page to help select the VFD that best fits your situation. If you need additional help determining the best harmonics mitigation for you, contact us to have a harmonics expert walk you through your options.
By adding a line reactor or an isolation transformer to attenuate harmonics, you get a low-cost, technically simple solution. However, this solution tends to offer mitigation in only higher-order harmonics and has little effect on the 5th and 7th harmonics. And, because of the associated voltage drop, there are limits to the amount of reactance that may be added.
Typical reactance is either AC or DC, with both delivering similar harmonic mitigation, but AC reactors provide additional protection to the drive rectifier bridge. Reactors are typically rated between 1.5% and 5%, with 3% being the industry standard due to the diminishing returns and voltage drop issues that higher levels can deliver. Reactors rated 3% typically deliver approximately 35–38% current distortion, with 5%-rated reactors or equivalent DC link chokes dropping this only nominally.
A 12-pulse converter incorporates two separate VFDs input semiconductor bridges, are fed from 30-degree phase-shifted power sources with identical impedance. The 12-pulse arrangement allows the harmonics from the first converter to cancel the harmonics of the second, especially at the 5th and 7th harmonics.
The 12-pulse converter is lower in cost than the 18-pulse unit and produces a substantial reduction (up to approximately 85%) in voltage and current harmonics compared to 6-pulse VFDs. The 12-pulse converter also provides the VFD and its semiconductors with increased protection from line transients. Additionally, this converter may not reduce distribution harmonic levels to below IEEE-519 1992 guidelines. Twelve-pulse converters have been largely superseded by 18-pulse converters due to the significant harmonic attenuation benefits provided by the newer 18-pulse technology for a marginal price difference.
The Eaton 18-pulse clean power converter will consistently meet IEEE-519 standards by reducing current distortion to 5% or less. The 18-pulse drive offers 50% better harmonic ratings (10% for 12-pulse vs. 5% for 18-pulse) at only a slightly higher cost.
The efficiencies of this new drive have helped to take harmonic mitigation to a new level for manufacturing facilities across the globe. It meets IEEE standards in every case, attenuates all harmonics up to the 35th, stops harmonics at the source, is insensitive to future system changes, and increases the life of the drive through incredibly stable DC bus voltage (18 small inputs instead of six large ones). The 18-pulse converter is the most cost-effective solution at 50 hp or higher.
This method of harmonic mitigation blocks harmonics from being transferred to the electrical distribution system through the use of an inductive and capacitive (L-C) filter. A primary inductor with relatively high impedance blocks higher order harmonics, and a shunt-connected tuned reactor is connected with a capacitor to mitigate the 5th and 7th harmonics.
These filters have a relatively low cost compared to 12- or 18-pulse converters, though there are a few concerns with their use. They can be challenging to size as they can act as a magnet for existing harmonics that are on the system. When the drive is off, the capacitor can cause power factor and voltage rise problems, and in the event of capacitor failure, standard units offer no indication of this failure. These filters can be sensitive to future system changes, and careful application is required by the design engineer.
The active front end drive is a bi-directional power converter for the front end of a common DC bus drive lineup. Although it does not affect other harmonics and cannot be retrofitted into existing drives, it does reduce total harmonics at any load to 2–3% THD. This power converter is immune to voltage imbalance, is available in 10–2000 hp, and can be used on multiple drives with a single front end. It provides voltage sag ride through capabilities, a unity power factor and a regenerative power flow.
The active front end drive is a newer technology designed for regenerative loads, such as test stands and centrifuges. It requires an inductive-capacitive inductive filter to filter the high frequency IGBT switching from the line. Because of the added technology introduced by the additional IGBTs, the cost of an active front end drive can be prohibitive in many applications. Also, the capacitive filter and the IGBTs are not as robust as the simple, but reliable, magnetics and diode technology of the 18-pulse drives.
Active harmonic correction units are high-performance inverters that measure system harmonics through current transformers and inject harmonics of equal amplitude and opposite phase into the system. These systems are very expensive and are rarely applied for low-diversity, high-horsepower applications. They can be well-suited to systems where low horsepower loads are numerous and diverse, as a single active filter can be applied to mitigate the harmonic distortion effects of many drives.
Active harmonic correction filters also can make an excellent retrofit for existing systems because they are a shunt-connected device. When lightly loaded, most devices can correct power factor using the capacitance built into them. Drawbacks to this technology include high cost per amp and lower energy efficiency.
Harmonics in power systems result in increased costs and downtime. Get help from Eaton.
Technology |
How it works |
Typical THD |
Advantages |
Disadvantages |
Inductive Reactors |
Mitigates higher order harmonics by providing high input impedance that limits high frequency currents | 33-38% | Low cost, simple application | Low effectiveness Voltage drop concerns |
12-Pulse Converters |
Two parallel 6-pulse converters fed by parallel isolation transformer paths, phase shifted 30°, which mitigates 5th and 7th order harmonics. Input impedance mitigates higher order harmonics as well | 12-18% | Simple to apply Extremely robust |
Not as effective as other methods Relative high cost compared to passive filters |
Passive Filters |
Provides high input impedance for higher order harmonics and a shunt-tuned reactor and capacitor to mitigate 5th and 7th harmonics | 6-8% | Simple retrofit Low cost for performance Insensitive to voltage imbalance |
Challenges with generator design Power factor issues Less robust than 18-pulse |
Active Harmonic Correction Filters |
Actively measures and injects equal and opposite harmonics into the system, canceling most harmonic currents below the 49th harmonic | 5-10% | Simple retrofit Good for high diversity systems |
High cost per ampere Relatively low efficiency System compatibility issues |
Active Front End Drives |
IGBT-based front end pulls relatively linear power from the line and uses an L-C-L filter to mitigate the IGBT switching noise | 3-5% | Slightly higher efficiency Immune to voltage imbalance Regeneration capability |
High cost, not as robust as 18 pulse |
18-Pulse Converters |
Three parallel 6-pulse converters fed by a single phase shifting of 20° to cancel all harmonics below the 17th. Input impedance mitigates higher order harmonics as well | 3-6% | Simple to apply Extremely robust |
Not available as a retrofit High cost on small hp |
Eaton’s Paul Olis discusses harmonic risks and solutions, such as clean power drives, to reduce energy costs and protect systems on FlowControlNetwork.com.
For any motor over 50 hp, the 18-pulse converter offers maximum efficiency. At smaller loads, an inductor or passive filter is usually sufficient to provide the needed harmonic mitigation.
Some high diversity low-power application can cost-effectively benefit from an active harmonic-correction filter due to the small individual load requirements.
Harmonics FAQsNo. According to industry market research studies, the compound annual growth rate (CAGR) for 18-pulse technology stands at 10.1%, which is greater than the 8.3% share that active front end technology holds.
Applications that require multiple VFDs typically produce a higher level of harmonic distortion due to the intrinsic nature of VFD technology. Low harmonic drives are used in these applications to limit the amount of harmonic distortion and improve the power quality.
The Institute of Electrical and Electronics Engineers developed guidelines called IEEE 519 to establish design objectives for low harmonic applications.
Total low harmonic VFDs sales by horsepower.
| Std | Low harmonic | |
| 1-100HP | 91% | 9% |
| 125-250HP | 53% | 47% |
| 300-1000HP | 46% | 54% |
