Total Harmonic Distortion (THD) in Low Voltage Networks
Introduction
Modern low-voltage distribution networks experience increasing total harmonic distortion (THD) due to widespread nonlinear residential loads, EV chargers and PV inverters. These distortions impact voltage quality, equipment lifetime and grid stability. This article provides a comprehensive overview of harmonic behavior from the 2nd to the 40th order, sources of distortion, regulatory limits (EN 50160, IEC 61000-3-2) and practical mitigation strategies. Engineers, DSOs and technical planners will find detailed insights based on real measurements and simulation results.
Key Takeaways
- Harmonic levels are rising in residential LV grids due to electrification (EVs) and distributed generation (PV).
- Odd harmonics dominate; triplen harmonics accumulate in the neutral conductor.
- Compliance with EN 50160 (voltage limits) and IEC 61000-3-2 (equipment emissions) is crucial.
- Harmonics impact efficiency, reliability, safety and equipment lifetime.
- Filters, PFC, modern inverter tech and monitoring significantly reduce THD.
What Are Harmonics in Low-Voltage Networks?
Harmonics are sinusoidal components whose frequencies are integer multiples of the fundamental 50 Hz. In European LV grids, relevant harmonic orders range from the 2nd (100 Hz) to the 40th (2000 Hz).
Odd harmonics - 3rd, 5th, 7th, 9th, 11th, 13th - dominate because they are produced by typical rectifier and switching topologies.rmonics increase RMS current → higher I²R losses → heating in cables, switchgear and transformers.
Sources of Harmonics in Modern Residential and Commercial Systems
Electric Vehicle (EV) Chargers
Modern EV chargers use switch-mode rectification and generate odd harmonics and supraharmonics. Evening charging peaks cause significant THD rises.
Photovoltaic (PV) Inverters
PV inverters inject harmonic currents and interact with grid impedance, which can amplify certain orders such as the 15th and 21st.
LED Lighting & Switching Power Supplies
LED drivers and SMPS draw current in pulses, generating broad harmonic spectra.
Heat Pumps & Variable-Speed Drives
PWM-controlled compressors and drives introduce mid- to high-order harmonics.
Effects of Harmonics on Power Quality and Equipment
Increased Losses & Heating
Harmonics increase RMS current → higher I²R losses → heating in cables, switchgear and transformers.
Neutral Conductor Overload
Triplen harmonics (3rd, 9th, 15th) add up in the neutral instead of cancelling - a major risk in three-phase four-wire systems.
Reduced Equipment Lifetime
Higher thermal stress accelerates insulation degradation in transformers and motors.
Voltage Effects Caused by Harmonic Distortion
Unfavorable grid impedance can amplify specific harmonics, especially the 15th and 21st.
Regulatory Framework (EN 50160, IEC 61000-3-2, D-A-CH-CZ)
EN 50160 - Voltage Quality
- Defines maximum voltage distortion levels
- Total harmonic distortion (THD) ≤ 8% (95% of the time)
IEC 61000-3-2 - Equipment Emission Limits
Applies to all devices ≤16 A; limits harmonic currents for each order up to the 40th.
Harmonic Levels from 2nd to 40th Order
| Harmonic order | Frequency | Relevance | Note |
|---|---|---|---|
| 2nd | 100 Hz | low | indicator of asymmetry / unbalance |
| 3rd | 150 Hz | very high | triplen (neutral conductor) |
| 5th | 250 Hz | very high | dominant |
| 7th | 350 Hz | high | rectifiers / SMPS |
| 9th | 450 Hz | medium | triplen |
| 11th–13th | 550–650 Hz | medium | PV/EV systems |
| 15th | 750 Hz | often critical | resonances |
| 17th–21st | 850–1050 Hz | increasing | EV/PV share |
| 23rd | 1150 Hz | medium–low | relevant depending on grid impedance |
| 25th–31st | 1250–1550 Hz | low | typically small magnitude |
| 33rd–39th | 1650–1950 Hz | low | typically small magnitude / device-dependent |
| 40th | 2000 Hz | low | upper limit (50 Hz) in many approaches |
Practical Approach - Measurement, Interpretation & Mitigation
Measurement
Use IEC 61000-4-30 Class A analyzers for accurate harmonic measurement.
Interpretation
Compare each harmonic order with EN 50160 limits and IEC 61000-3-2 emission limits.
Mitigation Strategies
- Passive filters (5th & 7th tuned)
- Active power filters (APF)
- Detuned capacitor banks
- Power factor correction
- Modern PV/EV inverter firmware
Results & KPI Effects
- Dominant harmonics: 3rd, 5th, 7th, 9th, 11th, 13th
- In EV/PV areas, mid-order harmonics (15th, 21st) rise significantly
- THD correlates with evening charging peaks & daytime PV export
- High THD reduces efficiency, increases aging and risk of malfunction
- THD must remain below <8% (EN 50160 baseline)
FAQ
What is THD?
THD is the sum of all harmonics and describes the deviation from an ideal sinusoidal waveform.
Why are odd harmonics dominant?
They arise from standard power converter topologies (six-pulse designs).
Which harmonics overload the neutral?
Triplen harmonics: 3rd, 9th, 15th.
How can harmonics be reduced?
Mitigation can be achieved using active filters, passive filters, PFC, detuned capacitor banks, and modern inverters.
Do EV chargers increase THD?
Yes - strongly during evening charging.
Summary
Harmonic distortion is a defining challenge of modern LV grids due to electrification and distributed energy. Odd harmonics and triplen harmonics dominate and can exceed thermal, regulatory and functional limits. Understanding harmonic behavior, continuous monitoring and targeted mitigation measures are essential for reliable network operation.
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Author
Rafael Wagner - Embedded Electronics Consultant and Business Owner at W4 MARISANA TECH.
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