Harmonics in power systems are unwanted frequency components that distort voltage and current waveforms. These harmonics cause numerous problems such as overheating, increased losses, resonance, nuisance tripping, and malfunction of sensitive equipment.
In this article, we will explore the adverse effects of harmonics on electrical equipment and why managing them is critical for reliable power system operation.
Effects of Harmonics on Power System
1. Heating and Losses in Electrical Equipment
Overheating of Transformers and Rotating Machines
Motors and generators can be adversely affected by the presence of harmonic voltage and current due to increased heating caused by iron and copper loss. In addition to this, a harmonic current can increase audible noise emissions and reduce machine efficiency.
All these effects combined together to increase energy consumption and reduce machine life considerably. Iron loss of a machine for the fifth harmonics can be calculated with the following formula.
Iron Loss KW5 = KW1 X (V5/ V1)2 X (f5 / f1)2
Iron losses increase as harmonic frequency increases.
Calculation of Iron loss
Example:
An induction motor of 275 HP, 415 V, 3 Ph, 50 Hz is fed by a 1000 KVA, 11/0.433 KV transformer. No-load iron loss at 50 Hz is 3 kW.
Harmonic voltage components:
- V₅ = 7%
- V₇ = 6%
- V₁₁ = 4.2%
Let’s calculate iron loss due to each harmonic using the formula:
KWₙ = KW₁ × (Vₙ / V₁)² × (fₙ / f₁)²
1. Iron Loss Due to 5th Harmonic (KW₅):
- KW₅ = 3 × (0.07)² × (5)²
- KW₅ = 0.3675 kW
2. Iron Loss Due to 7th Harmonic (KW₇):
- KW₇ = 3 × (0.06)² × (7)²
- KW₇ = 0.52 kW
3. Iron Loss Due to 11th Harmonic (KW₁₁):
- KW₁₁ = 3 × (0.042)² × (11)²
- KW₁₁ = 0.649 kW
Total Harmonic Iron Loss = 0.3675 + 0.52 + 0.649 = 1.5367 kW
Total Iron Loss (Fundamental + Harmonics) = 3 + 1.5367 = 4.5367 kW
% Increase Due to Harmonics =
(4.5367–3)/3×100(4.5367 – 3) / 3 × 100%(4.5367–3)/3×100 = 51.23% increase
In induction motors, additional losses also arise due to harmonic-generated fields. Each harmonic has a sequence (positive, negative, or zero) which determines the direction of the resulting magnetic field.
- Harmonics of 3rd order and multiples (3, 6, 9…) produce a stationary field that increases magnetic losses.
- Negative sequence harmonics produce a counter-rotating field that reduces torque.
- Positive sequence harmonics contribute to the main torque.
The interaction between positive and negative sequences causes vibration and reduces the service life of the motor.
Overheating of Neutral Conductor
Under balanced load conditions without harmonics, phase currents cancel each other in the neutral, resulting in zero neutral current. However, in a 4-wire system with nonlinear single-phase loads, the triplen harmonics (3rd, 6th, 9th, etc.) add in the neutral instead of canceling out.
This leads to dangerously high neutral current. Since neutral conductors typically lack protective devices like circuit breakers, overheating becomes a serious issue.
A case study found 150 A of neutral current while phase current was only 100 A. Hence, proper neutral conductor sizing is critical in harmonic-rich systems.
Read: Neutral Conductor Size Selection For Non Linear Loads
I2R Loss & Skin Efffect
At higher harmonic frequencies, conductor resistance increases due to the skin effect. As a result, all current-carrying conductors suffer greater I²R losses.
Furthermore, harmonics increase the RMS current, further raising copper losses in cables and equipment.
Related: Effect of Harmonics on Power Factor
Overloading of Transformers
Transformers are designed to operate at 50 Hz. Their iron losses consist of eddy current losses (which increase with the square of frequency) and hysteresis losses (which increase linearly with frequency). As harmonic frequencies increase, so do the losses, leading to overheating and decreased transformer life.
Read detailed Article on: Effects of Harmonics on Transformers
Losses in Distribution Equipment
Besides transformers, harmonics also increase losses in cables, busbars, and protective devices like fuses — all of which must now carry distorted current waveforms.
2. Equipment Malfunction and Reliability Issues
Malfunctioning of the Electronic Control and Computers
Electronic controls and computers are sensitive to waveform distortion. Harmonics cause distorted waveforms and voltage fluctuations, especially in the neutral conductor.
A rise in neutral-to-ground voltage beyond 3V can affect electronic reliability and cause malfunctioning.
Measurement Error in the Metering
Harmonics reduce the accuracy of energy meters. While watt-hour meters can register power direction at harmonic frequencies, their magnitude errors increase with frequency.
Demand meters and VAR meters perform even worse under harmonic conditions. True RMS meters should be used for accurate readings in harmonic-prone systems.
Zero-Crossing Noise
To reduce transients and EMI, many electronic devices detect voltage zero-crossing points. Harmonics distort these points, causing rapid voltage changes at the zero crossing — leading to erratic equipment behavior.
3. Impact on Protection Systems
Nuisance Tripping of Breakers
Protective relays that respond to neutral current may malfunction due to harmonics. Additionally, relays that operate based on peak voltage/current or zero-crossing are affected by distorted waveforms.
Resonance caused by harmonics can also lead to excessive current, resulting in nuisance tripping of breakers and blown fuses — especially in industries reliant on high-quality, uninterrupted power.
Overloading of Power Factor Correction (PFC) Capacitors
Capacitor impedance decreases with frequency, making them absorb more harmonic current. This causes overheating and shortens capacitor life.
Moreover, resonance between the capacitor and the transformer/line inductance can occur at harmonic frequencies, amplifying current levels and further stressing the capacitors.
Read detailed Article on: Effect of Harmonics on Power Factor
Resonance
Resonance may occur between the inductance of transformer windings and the capacitance of the feeder, amplifying harmonic levels. There are two types:
Series Resonance
When inductive and capacitive components form a series circuit, resonance occurs at the frequency where their reactances cancel. The impedance becomes minimal, and harmonic currents rise sharply.
At resonance, this low impedance path causes large harmonic current flow, especially if the capacitor on the LV side aligns with the transformer’s primary.
Parallel Resonance
The LV transformer side and power factor correction capacitor may form a parallel resonant circuit. At the resonant frequency, the circuit impedance becomes very high, resulting in increased voltage distortion and harmonic drop.
4. Communication and Signal Interference
Electrostatic Interference with Communication Circuits
Higher-order harmonics can couple into nearby communication circuits, causing interference and signal degradation, especially in analog or voice-grade systems.
Harmonics & Effects Summary
The table below summarizes the common harmonic orders and their effects on various electrical components:
| Harmonic Order | Effect | Affected Equipment |
|---|---|---|
| 3rd (Triplen) | Add in neutral → overheating | Neutral conductor, switchboards |
| 5th | Negative sequence → reverse torque | Motors, generators |
| 7th | Torque pulsation, overheating | Induction motors |
| 9th (Triplen) | Accumulates in neutral → increased neutral current | 4-wire systems, cables |
| 11th, 13th | Iron loss, core heating | Transformers, reactors |
| High-order (>15th) | Measurement inaccuracy, EMI, signal interference | Energy meters, communication lines |
| All odd orders | Distorted waveforms, increased losses | Entire distribution system |
Why Are Odd Harmonics More Harmful Than Even Harmonics?
Odd harmonics — particularly the 3rd, 5th, and 7th — are more damaging than even harmonics in power systems. Here’s why:
- Triplen harmonics (3rd, 9th, etc.) accumulate in the neutral and cause overheating.
- 5th and 7th harmonics cause reverse torque and torque pulsations in motors, reducing efficiency.
- Odd harmonics align with resonance points in many systems, amplifying their impact.
- They distort current and voltage waveforms more significantly due to their higher magnitude in typical nonlinear loads.
Even harmonics, on the other hand, tend to cancel out in symmetrical systems and are usually of lower magnitude.
How to Reduce Harmonics in Electrical Systems
Minimizing harmonic distortion is essential for safe and reliable power system operation. Here are effective strategies:
- Use Passive Filters – Tuned filters to trap specific harmonic orders like 5th or 7th
- Use Active Filters – For dynamic compensation of multiple harmonics
- Install K-Rated Transformers – Designed to withstand harmonic-rich environments
- Avoid Resonance Conditions – Coordinate PFC capacitors and system inductance
- Oversize Neutral Conductors – In 4-wire systems with nonlinear loads
- Apply Multi-Pulse Converters – Such as 12 or 18-pulse rectifiers for harmonic mitigation
- Conduct Power Quality Audits – Regular testing using harmonic analyzers
Key Takeaways
- Harmonics increase losses and overheating in transformers and motors.
- 3rd, 5th, and 7th harmonics are most dangerous.
- Harmonics can distort waveforms and interfere with metering and communication.
- Use filters, K-rated transformers, and avoid resonance to manage harmonics.
Conclusion
Harmonics in power systems may seem like invisible threats, but their impact is far-reaching—ranging from excessive heating, equipment malfunction, increased system losses, and shortened equipment lifespan to power quality degradation. As modern electrical systems rely heavily on nonlinear devices, managing harmonics is no longer optional but essential.
By understanding the sources, effects, and mitigation strategies of harmonics, industries and utilities can ensure reliable power delivery, protect expensive equipment, reduce energy wastage, and comply with power quality standards such as IEEE 519.
Proactive harmonic analysis and corrective measures like proper load balancing, harmonic filters, and system design optimization are key to a healthier, efficient power system.
Frequently Asked Questions( FAQs)
Harmonics are typically caused by nonlinear loads—devices that draw current in abrupt pulses rather than smooth sine waves. Common sources include variable frequency drives (VFDs), UPS systems, computers, LED lighting, and switch-mode power supplies. These devices distort the waveform and inject harmonic currents into the system.
Harmonics increase both eddy current and hysteresis losses in transformer cores, leading to excessive heat generation. Higher-order harmonics elevate the frequency, which in turn multiplies these losses, potentially reducing transformer lifespan and increasing operational costs. K-rated transformers are often used to handle such conditions.
Odd harmonics—especially the 3rd, 5th, and 7th—tend to have higher magnitudes and cause more significant waveform distortion. The 3rd harmonic and its multiples, known as triplen harmonics, accumulate in the neutral conductor, causing overheating. The 5th and 7th can induce negative torque in motors, reducing efficiency and increasing wear.
Harmonics lead to a wide range of issues such as overheating of electrical equipment, increased I²R losses, nuisance tripping of circuit breakers, reduced power factor, capacitor overloading, interference with communication lines, and malfunction of sensitive electronics. They also result in measurement errors in metering systems and reduce system efficiency.
To reduce harmonics, engineers can use passive filters (tuned to specific harmonic frequencies), active filters (for dynamic harmonic cancellation), install K-rated transformers, oversize neutral conductors, and avoid resonance conditions. Using multi-pulse rectifiers (like 12-pulse or 18-pulse converters) and conducting regular power quality audits also help maintain harmonic levels within acceptable limits.
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