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Glossary · Energy industry & energy transition

Harmonics (harmonic distortion)

Harmonics harmonics power quality electricity grid alternating current power electronics

Harmonics, in German usually called Oberschwingungen or Harmonische, are voltage or current components in an alternating current system whose frequency is an integer multiple of the fundamental frequency. In European power grids the mains frequency is 50 hertz. The harmonic of order h = 2 is therefore at 100 hertz, order 3 at 150 hertz, order 5 at 250 hertz, and so on. Caution is needed with the way of counting: technically, harmonics are usually counted by order h, i.e. by the multiple of the fundamental frequency. In German usage, however, 100 hertz is sometimes also called the "first Oberschwingung" (first overtone). Only stating the order or the frequency is therefore unambiguous. An ideal sinusoidal alternating current contains only the fundamental. Real grids additionally contain smaller frequency components, because devices and installations do not always draw or feed in current sinusoidally.

One can picture it like this: any periodic but distorted waveform can be represented mathematically (Fourier decomposition) as the sum of a 50-hertz sine wave and several faster sine waves. The more the curve deviates from a sine shape, the larger these additional components are.

Characteristics: order, individual level and THD

Harmonics are described by their order, their individual level and by distortion indicators. The most important indicator is Total Harmonic Distortion (THD), in German Gesamtoberschwingungsgehalt. To calculate it, the RMS values of the individual harmonics are added quadratically, and the square root of the sum is taken. The result is related to the fundamental. In voltage quality, orders 2 to 40 are usually included. THD is therefore not a simple sum of the individual components. It is stated separately for voltage (THDu) and current (THDi).

Current harmonics and voltage harmonics must be clearly distinguished. Current harmonics arise mainly from non-linear loads or converters. Voltage harmonics only arise when these distorted currents cause voltage drops across the frequency-dependent grid impedance. A device therefore emits or absorbs harmonic currents. Whether this results in significant voltage distortion depends on the point of connection, the short-circuit power there and the grid impedance. In a weak grid with high impedance, the same harmonic current produces considerably more voltage distortion than at a strong connection point.

Why harmonics arise

Harmonics typically arise where electrical energy is converted using power electronics. This includes switched-mode power supplies in computers and household appliances, chargers, frequency converters for motors, rectifiers, inverters of photovoltaic systems, battery storage, charging infrastructure for electric vehicles and many industrial installations. Magnetically saturated iron cores and arc furnaces also distort the current. A simple power supply with a rectifier and smoothing capacitor, for example, draws a current pulse only shortly before the voltage peak. This results in a strongly non-sinusoidal waveform with high components of the 3rd, 5th and 7th order. A three-phase six-pulse rectifier mainly produces the so-called characteristic orders 5, 7, 11, 13 and so on.

This does not mean that all power electronics inevitably cause poor power quality. Modern converters with active power factor correction or pulse-width-modulated control can produce very clean current waveforms in the range of the classical harmonics if they are suitably designed, controlled and filtered. To do so, they often shift disturbance components to their high switching frequencies. Conversely, simple devices in large numbers can produce considerable distortion. For the grid, what counts is therefore the specific design, the mode of operation, the connection point and the sum of many devices in the same grid area.

Harmonics must be distinguished from other phenomena of voltage quality. A deviation of the mains frequency concerns the fundamental frequency itself. Voltage dips, flicker or rapid voltage changes describe changes in voltage magnitude over time. Reactive power in the classical sense concerns the phase shift between fundamental current and voltage. Harmonics, by contrast, concern the waveform. There is, however, an overlap: distorted currents increase the apparent power without transmitting active power. This is sometimes called distortion reactive power. A device's power factor can therefore be considerably below its cos φ. Classical compensation with capacitors does not eliminate this share.

Technical effects on the grid and in installations

Harmonic currents increase the RMS value of the current and thus the losses and heating in lines, transformers and machines. Because current displacement (skin effect) and eddy currents increase with frequency, higher orders weigh disproportionately. In a transformer, eddy-current and stray-flux losses in particular increase. Transformers for heavily distorted loads are therefore sometimes dimensioned larger or built specially. In motors, harmonics cause additional losses, torque pulsations and noise. Capacitors of reactive power compensation form a resonant circuit with the grid inductance. If its resonance frequency is close to an existing harmonic, that harmonic is strongly amplified, even if the actual source is small. Remedies are detuned (reactor-protected) compensation systems or specifically tuned filters.

Particular attention is due to the orders divisible by three, such as the 3rd, 9th or 15th. In a symmetrically loaded four-wire system in the low-voltage grid, they are in phase in all three line conductors (zero-sequence system). In the neutral conductor they therefore do not cancel but add up. The neutral current can then even become larger than the current in the line conductors, although one would rather expect a small current with a symmetrical load. Buildings with many single-phase electronic loads, such as offices or data centres, are particularly affected.

Protection systems, measuring devices and signal transmission can also be affected. Protective devices can react differently to distorted currents than to pure sinusoidal currents, which in extreme cases leads to nuisance tripping. Energy meters and instrument transformers must correctly capture non-sinusoidal quantities. Ripple control signals, powerline communication and other signals on the grid can be disturbed if frequency ranges coincide. Harmonics are therefore not purely an efficiency issue, but also concern operational safety, the service life of installations and measurement accuracy.

Distinction from interharmonics and supraharmonics

Not every additional frequency component is a harmonic in the narrower sense. Interharmonics do not lie at integer multiples of the fundamental frequency. They arise, for example, from certain converters, arc furnaces or variable-speed drives. Supraharmonics usually refers to distortion in the range of 2 to 150 kilohertz, i.e. above the classical harmonics, which in 50-hertz grids extend up to about 2 kilohertz. Above 150 kilohertz begins the range of conducted radio disturbances. Supraharmonics arise mainly from the high switching frequencies of converters, active rectifiers and chargers. They propagate differently and behave differently over time than classical harmonics. Limits and measurement methods for them are still under development.

This distinction is practically important because standards, measurement methods and filter concepts differ depending on the frequency range. A filter for the 5th or 7th order does not solve problems in the kilohertz range. A low THD says nothing about disturbances above the 40th order. Anyone assessing power quality must therefore state which frequency range and which measured quantity is meant.

Standards, responsibility and cost-effectiveness

Harmonics are a typical interface topic that is dealt with under the umbrella term grid disturbances (power system perturbations). In Europe, EN 50160 describes the characteristics of the supply voltage in public grids (current edition: 2022). For low and medium voltage it specifies a voltage THD of 8 percent. The assessment is statistical: 10-minute mean values are formed over one week, and 95 percent of them are to comply with this value. In addition, the standard contains values for individual orders. EN 50160 is a description of voltage characteristics and not an immunity standard for devices. Even a grid that complies with the values can disturb sensitive devices.

For devices, the emission standards of the IEC 61000 series apply (in Europe as EN IEC 61000). IEC 61000-3-2 limits the harmonic currents of devices with a rated current of up to and including 16 A per conductor that are connected to public low-voltage grids, for orders 2 to 40. The European version EN IEC 61000-3-2:2019 was supplemented in 2024 by amendment A2. For larger devices there are separate parts, such as IEC 61000-3-12. IEC 61000-2-2 sets compatibility levels for public low-voltage grids. It has meanwhile also been extended to the range of 2 to 150 kilohertz. For larger generation and consumption installations, the rules for grid connection of the respective grid operator additionally apply. They usually assess the emission of an installation in relation to the short-circuit power at the connection point.

These rules distribute technical responsibility: a single device should not place excessive burden on the grid, and the grid should offer a quality in which standard-compliant devices function reliably. Economically, harmonics often show up only indirectly. They cause additional losses, shorter service life, nuisance tripping, filters, larger neutral conductors, more robust transformers and measurement campaigns. The costs do not always arise at the source. One installation may cause harmonic currents while another connected customer in the same grid area experiences resonance or heating. Because cause and impact can diverge, the topic is regulatorily demanding.

Common misunderstandings

  • "Renewables cause harmonics": Photovoltaic and battery storage systems do work with inverters, but their effect depends on device design, control, filtering and grid connection. Modern inverters generally comply with the emission limits. At the same time, the growing number of power-electronic devices is changing the electrical environment. Rotating machines, which used to act as damping, are becoming fewer, while switched converters are increasing. This creates a new need for measurement, standardisation and grid planning.

  • "Dirty power": The expression mixes different phenomena. A high 5th harmonic, a voltage dip, flicker and high-frequency disturbances have different causes and countermeasures. Without precise naming, a filter is easily installed even though the grid actually needs to be reinforced. Or a device comes under suspicion although a resonance in the local grid is the main cause.

  • "THD says it all": A single percentage value says little if it is unclear whether voltage or current is meant and which measurement duration, load condition and frequency range apply. A high current THD at low load can mean an absolutely small harmonic current. A grid with low distortion in normal operation can be prone to resonance in other switching states. For the assessment, relative indicators and absolute currents or voltages are needed together.

Harmonics show that power quality does not consist only of voltage magnitude and frequency. The waveform influences losses, equipment, protection systems, device function and connection rules. In a power system with many converters, chargers and electronically controlled installations, this level becomes more important. Harmonics are neither a single cause nor a blanket fault. They are a measurable property of voltage and current whose effect results from the device, grid impedance, operating state and regulatory framework.

Sources

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Edited by eCMS on 12.11.2026 05:05

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