What Is a Harmonic Mitigating Transformer?
Modern electrical systems contain more electronic equipment than ever before. Variable frequency drives, UPS systems, computers, LED lighting, rectifiers, and other power-electronic equipment can create non-linear electrical loads.
These loads can introduce harmonic currents into an electrical system.
When harmonics become significant, they can contribute to additional heating, losses, interference, and other power-quality concerns.
A Harmonic Mitigating Transformer (HMT) is a specialized transformer designed to help reduce the impact of certain harmonic currents within an electrical distribution system.
For facilities with substantial non-linear loads, selecting an appropriately designed transformer can be an important part of power-system planning.
What Are Electrical Harmonics?
In a simple AC power system, the electrical waveform ideally follows a smooth sinusoidal pattern.
Non-linear loads, however, do not always draw current in the same smooth waveform.
Instead, they can draw current in pulses, producing frequency components that are multiples of the fundamental power-system frequency.
These additional frequency components are called harmonics.
For example, in a 60 Hz system:
Fundamental = 60 Hz
3rd harmonic = 180 Hz
5th harmonic = 300 Hz
7th harmonic = 420 Hz
The presence and magnitude of harmonics depend on the equipment and electrical system.
What Causes Harmonics?
Many modern electrical devices contain power-electronic components that can produce harmonic currents.
Common examples include:
Variable Frequency Drives (VFDs)
VFDs are widely used to control the speed of motors in industrial facilities.
Uninterruptible Power Supplies (UPS)
UPS systems can contain rectifier circuits that contribute harmonic currents.
Computers and Electronic Equipment
Power supplies used by computers and electronic equipment can produce non-linear current waveforms.
LED Lighting
Large installations using electronic LED drivers can contribute to harmonic distortion.
Rectifiers and Power Converters
Industrial equipment using rectification or power conversion can generate harmonic currents.
Battery Chargers
Large charging systems may also introduce harmonics into the electrical network.
Why Are Harmonics a Concern?
Not every harmonic level represents a problem. The impact depends on the magnitude of the harmonics, the electrical system, and the equipment connected to it.
However, significant harmonic currents can contribute to several issues.
1. Transformer Heating
Harmonic currents can increase losses in transformer windings and other components.
This additional heating can become important when a transformer supplies substantial non-linear loads.
2. Increased Electrical Losses
Harmonic currents can contribute to additional losses in conductors, transformers, and other electrical equipment.
3. Neutral Conductor Loading
In three-phase, four-wire systems, certain triplen harmonics—particularly the third harmonic and its multiples—can accumulate in the neutral conductor.
This is an important consideration for facilities with significant single-phase non-linear loads.
4. Power Quality Problems
Significant harmonic distortion can affect the quality of electrical power supplied to equipment.
Depending on the system, this may contribute to malfunction, overheating, nuisance tripping, or other operational problems.
5. Reduced Equipment Performance
Electrical equipment may operate less efficiently when exposed to significant waveform distortion or other power-quality problems.
How Does a Harmonic Mitigating Transformer Work?
A Harmonic Mitigating Transformer uses transformer design techniques to help reduce the transfer of selected harmonic currents through the electrical system.
One important technique involves the transformer's phase-shifting characteristics.
By using appropriate winding configurations and phase relationships, certain harmonic components from different loads can partially cancel each other when they combine on the upstream side of the transformer.
The exact harmonic-mitigation performance depends on the transformer design, winding configuration, phase shift, load characteristics, and system configuration.
This means that a Harmonic Mitigating Transformer should be selected based on the actual electrical application rather than simply choosing a transformer based on kVA.
Harmonic Mitigating Transformer vs. Standard Transformer
A standard transformer primarily performs voltage transformation and electrical isolation as required by its design.
A Harmonic Mitigating Transformer is designed with additional consideration for systems containing significant non-linear loads.
FeatureStandard TransformerHarmonic Mitigating TransformerVoltage transformation✓✓Electrical isolationDepending on designDepending on designNon-linear loadsCan supply themSpecifically designed with harmonic considerationsHarmonic mitigationLimited by standard designDesigned to mitigate selected harmonicsPhase shiftingDepends on configurationOften incorporated for harmonic cancellationTypical applicationGeneral loadsFacilities with significant non-linear loads
The appropriate choice depends on the electrical system and its load characteristics.
Where Are Harmonic Mitigating Transformers Used?
Harmonic mitigating transformers are particularly relevant in facilities with substantial non-linear electrical loads.
Manufacturing Facilities
Industrial plants may operate numerous VFDs, motor drives, control systems, and electronic equipment.
Data Centers
Data centers can contain large quantities of UPS systems, servers, power supplies, and other electronic equipment.
Commercial Buildings
Modern commercial buildings may have extensive computer systems, LED lighting, HVAC controls, and variable-speed equipment.
Hospitals and Healthcare Facilities
Healthcare facilities often contain sensitive electronic equipment and substantial non-linear loads.
Industrial Automation Systems
Automated production lines can include drives, PLC systems, robotics, power supplies, and other electronic equipment.
Telecommunications Facilities
Telecommunications equipment can involve significant electronic power-conversion loads.
What Is the Difference Between a K-Rated and Harmonic Mitigating Transformer?
K-Rated transformers and Harmonic Mitigating Transformers are related to non-linear-load applications, but they address the problem differently.
A K-Rated transformer is designed to withstand the additional heating effects associated with non-linear loads. Its design accounts for the additional losses produced by harmonic currents.
A Harmonic Mitigating Transformer, on the other hand, incorporates design characteristics intended to reduce or cancel selected harmonic currents.
In simple terms:
K-Rated Transformer:
Designed to handle the heating effects of harmonic currents.
Harmonic Mitigating Transformer:
Designed to mitigate selected harmonic currents.
The correct solution depends on the facility's electrical characteristics and power-quality requirements.
Does a Harmonic Mitigating Transformer Eliminate All Harmonics?
No.
A Harmonic Mitigating Transformer should not be considered a device that simply eliminates every harmonic in an electrical system.
Its effectiveness depends on factors such as:
Transformer winding configuration
Phase-shift arrangement
Harmonic order
Load distribution
Load balance
System impedance
Number of transformers
Type of non-linear equipment
Actual harmonic spectrum
For more complex installations, a complete power-quality assessment may be appropriate before selecting the transformer.
How Do You Know If Your Facility Needs One?
A Harmonic Mitigating Transformer may be worth evaluating when a facility has a significant concentration of non-linear loads.
Examples include facilities with:
Multiple VFDs
Large UPS systems
Extensive computer equipment
Large LED lighting installations
Industrial rectifiers
Battery charging systems
Automated manufacturing equipment
Sensitive electronic equipment
If the facility is experiencing unexplained electrical issues, excessive transformer heating, neutral-current concerns, nuisance trips, or other power-quality problems, harmonic measurements can help determine whether harmonics are contributing to the issue.
What Information Is Needed to Specify a Harmonic Mitigating Transformer?
Proper transformer selection requires more than just the required kVA.
Useful project information includes:
Primary voltage
Secondary voltage
kVA requirement
Phase
Frequency
Load type
Percentage of non-linear loads
VFD/UPS/rectifier information
Existing harmonic measurements, if available
Required enclosure
Installation environment
System grounding and connection requirements
Providing this information allows the transformer design to be evaluated against the actual application.
Can a Dry-Type Transformer Be Used for Harmonic Mitigation?
Yes.
Dry-type Harmonic Mitigating Transformers can be designed for applications where non-linear loads are a significant part of the electrical system.
Dry-type construction can be suitable for many indoor commercial and industrial installations where liquid-filled transformers are not required.
The transformer can be specified according to the project's requirements, including:
kVA
Primary voltage
Secondary voltage
Phase
Frequency
Winding configuration
Phase shift
Enclosure
Taps
Accessories
Installation requirements
The final configuration should be determined from the project's electrical requirements.
Benefits of a Harmonic Mitigating Transformer
When properly applied, a Harmonic Mitigating Transformer can provide several benefits.
Better Management of Harmonic Currents
The transformer can be designed to reduce selected harmonic components entering the upstream electrical system.
Support for Non-Linear Loads
It provides a transformer solution specifically considered for facilities with substantial electronic loads.
Reduced Harmonic-Related Heating
Mitigating harmonic currents can help reduce some of the additional heating associated with harmonic currents.
Improved Power-System Planning
Using the appropriate transformer as part of a broader power-quality strategy can help engineers manage non-linear loads more effectively.
Customized Design
Transformer specifications can be developed according to the project's voltage, kVA, winding configuration, enclosure, and other requirements.
Why Proper Engineering Matters
A Harmonic Mitigating Transformer is only one part of a facility's overall power-quality strategy.
Before specifying one, engineers should consider the entire electrical system.
This may include:
Load analysis
Harmonic measurements
Transformer sizing
Cable sizing
Neutral conductor sizing
Grounding
Generator compatibility
VFD configuration
UPS systems
Harmonic filters
Power-factor correction equipment
For larger or more complex facilities, a power-quality study can help identify the actual source and magnitude of harmonic problems.
Frequently Asked Questions
What is a Harmonic Mitigating Transformer?
A Harmonic Mitigating Transformer is a specialized transformer designed to reduce the impact of selected harmonic currents generated by non-linear electrical loads.
What causes harmonics in electrical systems?
Harmonics are commonly associated with non-linear loads such as VFDs, UPS systems, computers, LED drivers, rectifiers, and other power-electronic equipment.
Is a Harmonic Mitigating Transformer the same as a K-Rated transformer?
No. A K-Rated transformer is designed to withstand the additional heating effects associated with harmonic currents, while a Harmonic Mitigating Transformer uses specific design characteristics to mitigate selected harmonic currents.
Can a Harmonic Mitigating Transformer eliminate harmonics?
No. It is designed to mitigate selected harmonics, and its effectiveness depends on the transformer configuration and the electrical system.
Does every facility with VFDs need a Harmonic Mitigating Transformer?
Not necessarily. The need depends on the number and type of VFDs, the overall load profile, system design, and actual harmonic levels.
Can VERIL provide a Harmonic Mitigating Transformer?
VERIL Transformer Sales and Services provides dry-type transformer solutions that can be specified according to project requirements. For harmonic-sensitive applications, the transformer should be evaluated based on the specific electrical system and load characteristics.
Need a Harmonic Mitigating Transformer?
If your facility operates significant non-linear loads, transformer selection should take harmonic performance into consideration—not just voltage and kVA.
VERIL Transformer Sales and Services provides dry-type transformer solutions for industrial and commercial applications, with specifications developed according to project requirements.
Contact VERIL to discuss your kVA, primary and secondary voltage, load type, enclosure, and harmonic requirements.
Request a quotation or technical consultation for your project.
