In recent years, variable frequency drives (VFDs) have become more crucial in modern applications, including water infrastructure. VFDs provide precise motor control, energy savings and better system performance, and as VFDs become more common, concerns around power quality—especially harmonics—have grown.
This article explores harmonics, how they affect VFD solutions, common mitigation methods and the role of standards like Institute of Electrical and Electronics Engineers (IEEE) 519 in pumping and water-related VFD applications.
Understanding Harmonics
Most electrical equipment is designed to operate on a clean sinusoidal supply; however, many modern devices—including VFDs—draw power in a disruptive manner. Instead of drawing current in a smooth and continuous fashion, they draw current in rapid pulses, disrupting the waveform and causing harmonics that impact other devices using the same supply.
Harmonics are higher frequency currents and voltages that ride on top of the fundamental frequency, distorting its shape from the traditional sinusoidal waveform. Distorted waveforms can lead to cascading problems and accelerating equipment wear, reducing system reliability and negatively impacting energy efficiency. These effects often appear in the form of:
- Overheating motors, transformers and conductors
- Additional noise in signal or feedback circuits
- Nuisance tripping of breakers and protective devices
- Reduced system efficiency
- Premature failure of equipment sensitive to power quality
Harmonics & VFDs in Water Applications
VFDs control motor speed by first converting alternating current (AC) power waveforms into direct current (DC) power and back to a variable frequency AC waveform through various power electronic components. Most harmonic distortion is generated during the AC to DC conversion stage. As the application load increases, so does the significance in harmonic distortion.
In wastewater treatment facilities and municipal water systems where pumps, blowers and process equipment operate continuously, the long-term effects of harmonic distortion become especially consequential. In remote agricultural installations, where power is frequently delivered over long feeder lines and may already be subject to voltage irregularities, power quality concerns are amplified. The addition of nonlinear loads such as VFDs can create further risks to system performance unless harmonics are properly managed.
These effects can lead to unplanned shutdowns, accelerated wear and costly maintenance. Most notably, harmonic distortion can propagate back through the utility connection, potentially affecting neighboring facilities and violating power quality agreements. Without proper mitigation, systems risk falling out of compliance and compromising critical infrastructure.
IEEE 519 Harmonic Limits
The IEEE has developed Standard IEEE 519 to define acceptable limits of harmonic distortion in electrical systems. This standard looks at total harmonic levels at the point of common coupling (PCC)—where the facility connects to the utility grid. IEEE 519 establishes a target toward which installations can be guided. Adherence is based on the authority having jurisdiction (AHJ).
In general, IEEE 519 sets limits for:
- Total harmonic distortion (THD) for voltage – usually 5% or less (dependent upon line voltage)
- Individual and total harmonic distortion for current – usually 8% or less for most systems (dependent upon short circuit current and total load measured at the PCC)
It is important to consider that individual VFD installations are typically downstream of the PCC. Managing harmonics local to the VFD help to ensure compliance with the levels outlined above. These limits help utilities maintain power quality across the grid and avoid issues like transformer overheating or voltage instability. For facility owners and engineers, adhering to IEEE 519 ensures that systems operate reliably and avoid penalties or utility-imposed corrective requirements where required. Working toward THD improvements in any system can increase the longevity and performance of the equipment and utility. Other guidance may also be in place to regulate harmonic levels differently from the IEEE 519 Standard.
Harmonic Mitigation Techniques
Various technologies and strategies are available to ensure long-term equipment performance, reduce harmonics and, where necessary, meet local standards. Each solution has ideal applications based on system size, criticality and budget.
1. Input reactors (line reactors)
An input reactor is an inductor installed on the supply side of a VFD. It limits the rate of current change and smooths the input waveform, reducing the magnitude of the harmonics produced by the drive.
Typical results: Can limit THD current distortion to 25%-35%, with minimal voltage waveform improvement
These are best for smaller pump systems or installations where a basic level of harmonic mitigation is acceptable.
2. Multipulse drives (e.g., 12-pulse or 18-pulse systems)
These systems use phase-shifting transformers and multiple rectifier bridges to cancel out specific harmonics. A 12-pulse drive, for example, cancels the 5th and 7th harmonics, which are typically the most dominant.
Typical results: Can achieve 8%-12% THD
These are best for large pumping systems requiring large motors (250 horsepower [hp] and above) in municipal water or industrial treatment plants.
3. Passive harmonic filters
Passive filters use a combination of inductors and capacitors to “tune out” specific harmonic frequencies. When properly sized, passive filters can bring current distortion well within IEEE 519 limits.
Typical results: Can reduce THD to 5% or less
These are best for systems with multiple VFD-motor pairings, or where compliance with harmonic limits is mandatory.
4. Active front end (AFE) drives
AFE drives use advanced electronics to actively shape the incoming current, drawing nearly sinusoidal waveforms and producing very low harmonic distortion. Although AFE drives offer the highest performance, they may be excessive for systems where loads are consistent and not regenerative.
Typical results: Can achieve <3% THD
These are best for high-performance applications or facilities with very strict utility requirements or regenerative applications.
See the table below for a side-by-side comparison of these harmonic mitigation techniques and their results:
| Solution | Typical THD (Current) | Typical Applications | Incremental Solution Cost | Notes |
| Baseline VFD | 35%-50% | General-purpose VFD use without harmonic concerns | None | No harmonic mitigation |
| Input reactors | 25%-35% | Any VFD installation; providing basic protection and harmonic mitigation | $ | Minimal harmonic mitigation at low cost |
| Multipulse drives | 8%-12% | Large motors ≥ 250 hp; municipal water systems; medium-voltage applications | $$$ | Large footprint, high cost due to multipulse transformer |
| Passive harmonic filters | ≤5% | Facilities with multiple VFD-motor pairings or where IEEE 519 compliance is required | $$ | Balanced cost and capable of meeting IEEE 519 requirements |
| Active front end (AFE) | <3% | High-performance systems with strict compliance or regenerative needs | $$$$ | High cost due to additional electronics balanced against highest performing drive system |
In agricultural irrigation systems, harmonic mitigation is often needed to protect equipment powered by long feeder runs or undersized transformers. In municipal systems, pump stations may be located near industrial or residential loads, meaning current and voltage distortion must be carefully managed to not adversely impact neighboring equipment. Water treatment plants, which often run large VFDs continuously, are subject to the long-term impacts of unmanaged harmonics.
A practical understanding of harmonics in VFD applications helps engineers and operators ensure that water-related systems—from farmland irrigation to urban treatment plants—operate efficiently and in compliance with the required standards. Input reactors, passive filters, multipulse designs and AFE drives offer a variety of solutions to meet these needs.
IEEE 519 offers a strong foundation for appropriate harmonic levels. With the appropriate harmonic mitigation method in place, water systems can achieve the full benefit of VFD technology, resulting in sustainable improvements in efficiency, performance gains and precision control.
