Harmonics in Power Systems: Why Normal Operation Does Not Mean the System Is Healthy

08 Sep 2026

Quick Answer

The biggest challenge with harmonics is that they usually do not cause immediate equipment failure. A power system may continue operating normally, while distorted current gradually increases losses, temperature rise, and stress on the entire power distribution system.

For systems with rectifiers, UPS systems, battery chargers, variable frequency drives, and switching power supplies, checking only voltage, power consumption, or whether equipment is running properly is not enough. Harmonic distortion in current waveforms may create long-term impacts on cables, transformers, capacitors, and protection devices.

Key Takeaways

Harmonic problems often develop without immediate equipment shutdown.
Nonlinear loads can distort current waveforms and increase RMS current even when active power does not increase significantly.
Harmonics can affect multiple parts of the power system, including cables, transformers, capacitors, and protection equipment.
THD is an important reference, but harmonic evaluation should also consider individual harmonic components and actual operating conditions.
Harmonic control requires analysis of both equipment characteristics and the overall power system.

Why Should We Pay Attention to Harmonics When Equipment Still Runs Normally?

In many applications, a power system is considered healthy when:

Voltage remains normal.
Equipment can start and operate properly.
No alarms appear.
No power interruption occurs.

However, this judgment may not fully reflect the actual condition of the electrical system.

For systems with a large number of nonlinear loads, the current waveform may no longer remain sinusoidal. Equipment such as rectifiers, UPS systems, chargers, variable frequency drives, and switching power supplies may draw current in a non-linear way.

Instead of following a smooth sine wave, the current may become concentrated at certain points during the voltage cycle, causing waveform distortion.

These distorted currents can be separated into:

Fundamental current
Harmonic currents with different frequencies

Harmonics may not immediately trigger a fault, but they can continuously increase electrical stress throughout the power distribution system.

Over time, they may contribute to:

Additional losses
Higher temperature rise
Increased interference
Reduced system margin

This is why a system can appear normal while harmonic-related problems are gradually developing.

Why Does Current Increase When Active Power Does Not Increase?

Harmonic Current Increases RMS Current and System Stress

Under ideal conditions, voltage and current waveforms are close to sinusoidal. In this situation, system losses are easier to predict and equipment operates under expected conditions.

However, when a large number of nonlinear loads are connected, harmonic components appear in the current waveform.

The result is:

RMS current may increase.
The fundamental component that contributes to useful power does not increase at the same level.

Therefore, the system may experience higher electrical stress even when active power has not increased significantly.

Possible impacts include:

Component	Possible Impact
Cable and busbar	Higher temperature rise and increased conductor losses
Transformer	Increased copper loss and eddy current loss
Switching devices	Higher current stress
Power system	Reduced operating margin

This is why only monitoring kW values may overlook power quality problems.

The system may consume similar power, while electrical components experience additional current stress.

Where Do Harmonics Affect the Power System?

Harmonic problems can affect different parts of the power distribution system.

Location	Possible Phenomenon	Potential Impact
Load side	Current spikes and waveform distortion	Higher equipment heating and increased EMI pressure
Cable and busbar	Increased RMS current and higher temperature	Increased conductor losses under the same power level
Transformer	Increased copper loss and eddy current loss	Higher temperature rise and possible capacity derating
Capacitor	Current amplification and abnormal heating	Higher risk when resonance conditions occur
Protection and measurement	Reading differences, incorrect operation	Reduced measurement accuracy and possible operation issues

The influence of harmonics is not limited to the equipment that generates them. Harmonic currents can travel through the distribution network and affect other system components.

Why Are Third Harmonics Important in Three-Phase Four-Wire Systems?

Why Do Triplen Harmonics Accumulate in Neutral Conductors?

In an ideal balanced three-phase system, the fundamental currents of three phases can cancel each other in the neutral conductor.

However, third-order harmonics, including:

3rd harmonic
9th harmonic
15th harmonic

have different characteristics.

These triplen harmonics have the same phase relationship, meaning they do not cancel like fundamental currents. Instead, they can accumulate in the neutral conductor.

This issue is especially important in applications with many single-phase switching power supplies, such as:

Office buildings
Data centers
Communication rooms

Even when three-phase loads appear balanced, the neutral conductor current may still become high.

If cable selection, system design, and temperature evaluation are still based on traditional linear load assumptions, potential risks may be overlooked.

Why Is THD Not Enough to Judge Harmonic Problems?

Current Harmonics and Voltage Harmonics Should Be Distinguished

THD is commonly used to evaluate harmonic distortion. However, looking only at THD values cannot fully determine the impact of harmonics.

First, current harmonics and voltage harmonics should be considered separately.

Nonlinear loads usually create current distortion first. The impedance of the power network can then convert this distortion into voltage distortion.

Therefore:

Current harmonic distortion
Voltage harmonic distortion

represent different conditions and should not be treated as the same issue.

Both THD Percentage and Actual Current Matter

The THD percentage alone cannot determine the actual risk level.

For example:

Under light load conditions, THD may appear high, but the actual harmonic current may be small.
Under full load conditions, THD may be lower, but the actual current stress may be more significant.

Therefore, both harmonic ratio and actual current level should be considered.

Individual Harmonic Orders Need Attention

Looking only at total harmonic distortion is not enough.

Different harmonic orders may have different impacts, including:

5th harmonic
7th harmonic
11th harmonic

Certain frequencies may interact with capacitive and inductive components and create resonance conditions.

Therefore, individual harmonic components should also be analyzed.

Harmonic Mitigation Requires System Analysis

Equipment-Level Improvement

When harmonic problems come from individual equipment, possible improvement methods may include:

Input rectifier design
PFC design
Switching strategy
Equipment parameters

The solution should start by identifying the source of harmonic generation.

System-Level Solutions

When multiple devices operate together, harmonic problems may become a system-level issue.

The analysis should consider:

Grid short-circuit capacity
Transformer impedance
Load distribution
Capacitor compensation configuration

Solving harmonic problems requires understanding the relationship between equipment and the overall electrical system.

Passive Filter vs Active Filter

Different filtering solutions have different characteristics.

Solution	Characteristics
Passive filter	Cost-effective and suitable for relatively fixed harmonic frequencies
Active filter	Provides dynamic compensation for multiple harmonic orders

For new projects, considering power quality during the design stage can help avoid problems after installation.

Four Steps for On-Site Harmonic Diagnosis

Step 1: Check Waveforms

Use a power quality analyzer or oscilloscope to check:

Current waveform distortion
Current spikes
Waveform clipping
Changes under different load conditions

Testing under different operating conditions helps provide a more complete evaluation.

Step 2: Analyze Harmonic Spectrum

Check:

THDi
THDu
Individual harmonic components

This helps identify the main harmonic frequencies affecting the system.

Step 3: Check Temperature Rise

Inspect:

Neutral conductors
Transformers
Capacitor cabinets
Cables
Connection terminals

This helps locate areas where abnormal heating occurs even when current does not exceed rated values.

Step 4: Review System Conditions

A complete evaluation should include:

Load types
Compensation capacitors
Filtering equipment
Distribution impedance

Avoid making conclusions based only on one device or one measurement point.

FAQ About Harmonics in Power Systems

What are harmonics in a power system?

Harmonics are additional frequency components caused by waveform distortion. They usually appear when nonlinear loads draw current that is different from a normal sinusoidal waveform.

Why can harmonics be a problem if equipment still works normally?

Because harmonic problems usually develop gradually. They can increase losses, temperature rise, and stress on electrical components without immediately causing equipment failure.