Harmonic Filters for Reducing VFD Distortion

Many facilities and applications, like conveyors and automated packaging lines, rely on VFDs to save energy and improve motor control. However, these VFDs sometimes create harmonic distortion, which leads to issues like overheating and increased costs. To eliminate those problems and prevent them from hurting the system in the future, harmonic filters can be installed. This article overviews VFDs, how the VFD input stage creates harmonic distortion, the negative effects of harmonics, and how harmonic filters reduce these harmonics in industrial applications.

 

VFD Basics

Variable frequency drives (VFDs) used in industrial applications provide many benefits, including energy savings, better process control, extended component lifetime, and increased machine safety when utilizing functional safety features.

Variable frequency drives consist of three sub-systems: the diode bridge converter, DC bus, and IGBT output inverter.

The diode bridge input rectifier typically consists of a full-wave 6-pulse rectifier, which rectifies the incoming AC voltage into a DC voltage. The DC bus system utilizes DC capacitors to help smooth the rectified AC voltage and provide voltage storage for the system.

Finally, the IGBT output system utilizes the DC bus voltage and creates the variable frequency and voltage output to the motor, using a pulse width modulated (PWM) control.

Any VFD which uses a 6-pulse rectifier on the input stage may be one source of current and voltage harmonics on the mainline. (Other sources of mainline harmonics include: Arc furnaces, any power supply using a static power converter, or inverters for distributed generation.) This is due to the non-linear load produced by the 6-pulse rectifier.

A load connected to an AC input voltage is a linear load if the current draw is in the same form as the voltage (resistive, inductor, or capacitive load, for example). A non-linear load draws a current that is not in the same form as the AC voltage waveform – non-sinusoidal.

In the VFD system, once the DC bus capacitors are charged, the input current to the capacitors will flow only when the incoming AC voltage is greater than the DC bus voltage. This is near the top of the arc of the sine wave voltage waveform.

As the voltage of the sine wave drops below the DC bus level, the current will stop flowing as shown in the diagram below. This nonlinear capacitor current results in a current pulse on the main incoming voltage line.

What Causes Harmonic Distortion in VFD?

VFDs create harmonic distortion. Because VFDs cause harmonic distortion, a harmonic filter for VFDs reduces harmonics.

In other words, any VFD using a bridge rectifier inherently draws harmonic current from the utility. The harmonic current draw causes voltage distortion on the mains that can create power quality issues for other electrical loads.

Electrical loads like resistors and line-fed induction motors draw a sinusoidal current from the supply. Most commercial VFDs use a 6-pulse bridge rectifier to convert from AC to DC power. 6-Pulse inverters draw a non-sinusoidal input current which then creates distortion on the supply voltage.

The voltage distortion is undesirable because it can negatively affect the operation of other electrical loads. Critical applications using VFDs like wastewater treatment plants, elevators, and airports might need to mitigate harmonic distortion.

Negative Effects of VFD-Caused Harmonics

Harmonics on the mainline can have the following effects:

• Cause interference with communication circuits
• Transformer overheating
• Nuisance circuit breaker trip
• Neutral overloading
• Capacitor bank failure

How much harmonic content on the mainline is too much?

To assist with this question, the Institute of Electrical and Electronics Engineers (IEEE) developed a standard for harmonic levels. The current standard for acceptable harmonic content is IEEE519-2014.

The main components of this standard define the acceptable levels of current and voltage harmonics based on the short circuit current rating (SCCR) of the incoming power distribution system. The amount of distortion a system can tolerate depends on the input impedance level of the voltage distribution system.

The standard considers the distribution system impedance by determining acceptable harmonic levels based on the system SCCR. Lower impedance or stiffer distribution networks can tolerate a higher level of harmonics. Where higher impedance or softer distribution networks can tolerate a lower level of harmonic content.

Understanding IEEE 519 Harmonic Limits

The IEEE519-2014 standard also defines the point of common coupling (PCC) where the harmonic levels are to be measured. This provides a standard measurement location for the harmonic content of the facility to be measured.

Since most industrial environments consist of linear and non-linear loads, the overall effect at the PCC may be negligible even if an individual system produces some harmonic content. The IEEE519-2014 standard provides some clarification on the PCC over earlier versions of the standard:

“…the PCC is usually taken as the point in the power system closest to the user where the system owner or operator could offer service to another user. Frequently for service to industrial users (i.e., manufacturing plants) via a dedicated service transformer, the PCC is at the HV side of the transformer. For commercial users (office parks, shopping malls, etc.) supplied through a common service transformer, the PCC is commonly at the LV side of the service transformer.”

What Happens When You Exceed IEEE 519?

Facilities often face penalties or connection limits from utilities if they exceed IEEE 519 limits. Meeting these standards not only protects equipment but also ensures future scalability as more drives are added.

The IEEE519-2014 standard provides guidelines for the allowable harmonic content on a voltage distribution system. Standards are provided for both voltage and current harmonic levels.

KEB Z1 Harmonic Filters

While multiple solutions exist for addressing harmonics in electrical systems, harmonic filters provide an optimal balance between performance and cost-effectiveness.

KEB’s Z1 Harmonic Filters are engineered to meet IEEE 519 harmonic distortion standards, ensuring reliable power quality in industrial applications. Available in graduated sizes up to 400 HP, these filters accommodate a wide range of motor drive applications.

To deliver optimal performance across diverse electrical infrastructures worldwide, KEB offers filters specifically designed for different electrical mains configurations:

• Voltage Options: 230V and 480V AC mains
• Frequency Optimization: Models available for both 50Hz and 60Hz power systems

The KEB Harmonic Filters effectively limit and control distortion generated by Variable Frequency Drives. By reducing harmonic current injection from the drive, these filters significantly decrease voltage distortion on the electrical mains, protecting sensitive equipment and improving overall power system quality.

Explore our white paper for an in-depth understanding of harmonic filter implementation.

Reducing VFD Distortion for Industrial Applications

KEB Harmonic Filters deliver exceptional performance in the most challenging industrial VFD applications. Built to withstand real-world operating conditions, these filters feature a robust 150% current overload capacity for 60 seconds, ensuring reliable operation during startup surges and temporary load spikes.

The Z1 series achieves remarkable 99% efficiency at full load through advanced engineering principles. This exceptional performance stems from precision-manufactured, high-grade steel laminations combined with optimized winding configurations that minimize energy losses while maximizing harmonic suppression.

KEB’s Z1 filters incorporate high-performance 3-phase capacitors specifically selected for extended operational lifetime and enhanced reliability. These premium components ensure consistent performance over years of demanding industrial service.

The design philosophy prioritizes installation versatility. The compact form factor and flexible mounting system accommodate diverse enclosure requirements, while the unique split-component design allows the capacitor bank to be positioned up to 2 meters from the main core assembly. This flexibility enables machine builders to optimize component placement within space-constrained enclosures.

For larger applications requiring turnkey solutions, pre-wired NEMA1 enclosures are available as a factory option, streamlining installation and reducing field labor requirements.

KEB Harmonic Filters maintain the highest standards through comprehensive certification programs. UL listing and CE marking demonstrate compliance with rigorous safety and performance standards, making these filters the ideal choice for mission-critical industrial applications where reliability cannot be compromised.

Choosing the Right Harmonic Filter for Your System

• Are VFD-driven loads exceeding 10 HP?
• Have you measured THD over 8% at your PCC?
• Are nuisance trips or transformer heating common?

If you answered yes to any of the above, it’s time to consider a harmonic filter assessment and contact a KEB America engineer to discuss your application.