# Interharmonics
**Interharmonics** are components of voltage or current in a power system whose frequencies are not integer multiples of the fundamental supply frequency (50 Hz or 60 Hz). The term was officially defined by IEC 61000-2-1 as frequencies observed between the harmonics of the power-frequency voltage and current that are not an integer multiple of the fundamental, appearing either as discrete frequencies or as a wide-band spectrum[1](https://www.sciencedirect.com/science/article/pii/S0378779603000701). A component with a frequency below the fundamental is called a subharmonic; the term has no official definition and is simply a special case of interharmonic[2](https://sagroups.ieee.org/harmonicswg/wp-content/uploads/sites/445/2022/01/interharmonics-paper.pdf).
| Key fact | Detail |
|---|---|
| Definition | Any frequency that is not an integer multiple of the fundamental frequency; the ratio of interharmonic frequency to fundamental (the interharmonic order, m) is not an integer[3](https://www.standards.iteh.ai/catalog/standards/iec/bb59b84d-efc6-4e43-a0bf-f2fc21f318ac/iec-61000-2-2-2002) |
| Standard measurement | IEC 61000-4-7 uses a 10-cycle window on 50 Hz systems or a 12-cycle window on 60 Hz systems (about 200 ms), giving a 5 Hz spectrum resolution[4](https://sagroups.ieee.org/harmonicswg/wp-content/uploads/sites/445/2022/01/ih519b.pdf) |
| Reported quantities | Interharmonic groups and subgroups, plus total interharmonic distortion normalized to the fundamental, the overall r.m.s. value, or a declared value such as the average maximum demand load current[4](https://sagroups.ieee.org/harmonicswg/wp-content/uploads/sites/445/2022/01/ih519b.pdf) |
| Main sources | Variable frequency drives, cycloconverters, electric arc furnaces, HVDC converters, induction motors, and photovoltaic and wind generation[5](https://www.mdpi.com/1996-1073/16/14/5538) • [6](https://www.mdpi.com/1996-1073/14/5/1218) |
| Dominant effect near the fundamental | Light flicker; a 1% interharmonic voltage component near the fundamental can produce flicker severity above 4 Pst[4](https://sagroups.ieee.org/harmonicswg/wp-content/uploads/sites/445/2022/01/ih519b.pdf) |
| Compatibility levels | IEC 61000-2-2 gives indicative interharmonic voltage levels in low-voltage networks tied to the flicker effect, for example 0.23% of nominal voltage for 58–62 Hz components on a 50 Hz system[3](https://www.standards.iteh.ai/catalog/standards/iec/bb59b84d-efc6-4e43-a0bf-f2fc21f318ac/iec-61000-2-2-2002) |
| Measurement difficulty | Non-stationary waveforms and spectral leakage make accurate determination of interharmonic frequency and magnitude difficult[1](https://www.sciencedirect.com/science/article/pii/S0378779603000701) |
## Definition and terminology
The IEC 61000-2-2 standard defines an interharmonic frequency as any frequency which is not an integer multiple of the fundamental frequency, and an interharmonic component as a component having such a frequency, normally expressed as an r.m.s. value[3](https://www.standards.iteh.ai/catalog/standards/iec/bb59b84d-efc6-4e43-a0bf-f2fc21f318ac/iec-61000-2-2-2002). By extension from harmonic order, the interharmonic order m is the ratio of the interharmonic frequency to the fundamental frequency, and when m is less than 1 the term subharmonic frequency may be used[3](https://www.standards.iteh.ai/catalog/standards/iec/bb59b84d-efc6-4e43-a0bf-f2fc21f318ac/iec-61000-2-2-2002). The IEEE Interharmonic Task Force adopted the IEC definition and recommended including interharmonics in the next revision of [IEEE 519](https://www.edgepedia.org/ieee-519)[1](https://www.sciencedirect.com/science/article/pii/S0378779603000701).
A defining property follows from periodicity: if every cycle of a distorted waveform is identical, only integer harmonics of the fundamental can be present, so interharmonics indicate waveform behavior that does not repeat at the fundamental period[7](https://powermonitors.com/whitepapers/defining-interharmonics/).
## Sources
Interharmonics arise wherever equipment operates asynchronously with the power frequency. Documented sources include double-conversion systems connecting two AC systems of different frequencies through a DC link, such as IGBT-based voltage source inverters used in STATCOMs and HVDC applications, as well as [cycloconverters](https://www.edgepedia.org/cycloconverter), electric arc furnaces, and power communication lines[5](https://www.mdpi.com/1996-1073/16/14/5538). Variable frequency drives and AC choppers driving induction motors also contribute, through the interaction of motor inertia with converter switching frequencies, DC-link filters, and soft starters[5](https://www.mdpi.com/1996-1073/16/14/5538).
**Loads and machines.** Induction motors can generate an irregular magnetizing current because of the slots in the stator and rotor, possibly combined with iron saturation; at normal speed this produces interharmonics at frequencies between 10 and 40 times the power frequency, and during starting the components sweep through the whole frequency range[3](https://www.standards.iteh.ai/catalog/standards/iec/bb59b84d-efc6-4e43-a0bf-f2fc21f318ac/iec-61000-2-2-2002). [Arc welding](https://www.edgepedia.org/arc-welding) machines generate a continuous wide-band spectrum associated with an intermittent process[3](https://www.standards.iteh.ai/catalog/standards/iec/bb59b84d-efc6-4e43-a0bf-f2fc21f318ac/iec-61000-2-2-2002). Power supplies to traction systems can produce interharmonics at fixed frequencies, for example 16.7 Hz[3](https://www.standards.iteh.ai/catalog/standards/iec/bb59b84d-efc6-4e43-a0bf-f2fc21f318ac/iec-61000-2-2-2002).
**Renewable generation.** Photovoltaic plants and wind power stations are sources of substantial interharmonics[6](https://www.mdpi.com/1996-1073/14/5/1218), and the IEC recommends considering interharmonics as one of the assessment criteria in grid code compliance assessment for grid connection of wind and PV power plants[8](https://ietresearch.onlinelibrary.wiley.com/doi/10.1049/iet-rpg.2018.5697). In grid-connected PV systems, the disturbance of the maximum power point tracking algorithm is a main source of interharmonic components in the output current[9](https://www.mdpi.com/1996-1073/18/13/3426).
## Effects
**Flicker.** The most studied effect is light flicker. An interharmonic voltage component combines with the fundamental to produce a beat frequency, and components near the fundamental cause visible changes in light intensity in incandescent, LED, and fluorescent lamps[5](https://www.mdpi.com/1996-1073/16/14/5538). When the flicker algorithm is applied to a 1% interharmonic voltage component near the fundamental frequency, flicker severity levels above 4 Pst can result[4](https://sagroups.ieee.org/harmonicswg/wp-content/uploads/sites/445/2022/01/ih519b.pdf). The most visible flicker occurs at around ±8 Hz of a power system harmonic[4](https://sagroups.ieee.org/harmonicswg/wp-content/uploads/sites/445/2022/01/ih519b.pdf).
**Motors and mechanical systems.** Voltage interharmonics cause fluctuations of rotational speed in induction motors, and the impact depends strongly on the moment of load inertia[6](https://www.mdpi.com/1996-1073/14/5/1218). Investigations on seven cage induction motors rated from 3 kW to 5.6 MW showed that voltage interharmonics can cause high vibration and torque pulsations that can coincide with the natural frequency of the first elastic mode, so torsional resonance may destroy the powertrain[6](https://www.mdpi.com/1996-1073/14/5/1218). For motors running at constant speed, the resulting current interharmonics stayed below roughly 5% of rated current, but with a negligible load inertia both current interharmonics and subharmonics reached about 9.5% of rated current for one 200 kW motor[6](https://www.mdpi.com/1996-1073/14/5/1218).
**Other effects.** Interharmonics cause distortions and energy losses leading to equipment heating, mechanical system oscillations, acoustic disturbances, and interference with power communication lines[5](https://www.mdpi.com/1996-1073/16/14/5538). Unwanted currents flowing in supply networks generate additional energy losses[3](https://www.standards.iteh.ai/catalog/standards/iec/bb59b84d-efc6-4e43-a0bf-f2fc21f318ac/iec-61000-2-2-2002). In PV systems, reported impacts include harmonic instabilities and sub-synchronous oscillations during parallel operation of multiple inverters, instability of phase-locked-loop-based control and measurement instruments, amplification of emission around resonant frequencies, unintended tripping of protection circuits, and peak-voltage fluctuations[8](https://ietresearch.onlinelibrary.wiley.com/doi/10.1049/iet-rpg.2018.5697).
## Measurement under IEC 61000-4-7
The governing standard is IEC 61000-4-7, first published in 1991 and revised in 2002 with an amendment in 2008; the consolidated edition applies to instrumentation measuring spectral components up to 9 kHz superimposed on the 50 Hz or 60 Hz fundamental, and distinguishes harmonics, interharmonics, and other components above the harmonic frequency range[10](https://webstore.iec.ch/en/publication/4228). The key to the method is a 10-cycle sample window for 50 Hz systems or a 12-cycle window for 60 Hz systems, on which the [Fourier transform](https://www.edgepedia.org/fourier-transform) is performed, producing a spectrum with 5 Hz resolution for both system types[4](https://sagroups.ieee.org/harmonicswg/wp-content/uploads/sites/445/2022/01/ih519b.pdf).
Because individual 5 Hz bins are sensitive to leakage, the standard defines groupings. To obtain a harmonic group, one takes the root-mean-square of a harmonic together with the neighboring spectral lines around it; a harmonic subgroup keeps only the central harmonic and its two closest components; and an interharmonic group combines the rms values of every interharmonic lying between two successive harmonic frequencies[11](https://onlinelibrary.wiley.com/doi/10.1155/2014/730362). From these definitions, measurements can be reported for each interharmonic group, together with a figure for total interharmonic distortion normalized to the fundamental, the overall r.m.s. value, or a declared value such as the average maximum demand load current, and these quantities form the basis for defining limits[4](https://sagroups.ieee.org/harmonicswg/wp-content/uploads/sites/445/2022/01/ih519b.pdf).
The grouping approach has two stated limitations: spectrum leakage still exists, and the method cannot identify individual component frequencies and amplitudes, so it is not suitable for system diagnostic purposes[11](https://onlinelibrary.wiley.com/doi/10.1155/2014/730362). Grouping 5 Hz bins can also yield misleading information when an interharmonic frequency lies very close to a harmonic frequency, a case in which interharmonics can cause more severe waveform modulation[12](https://doi.org/10.1109/tpwrd.2012.2183394).
**Near the fundamental.** The 61000-4-7 methodology does not provide the information needed to identify interharmonic distortion near the fundamental at the 0.2% level, because of the 5 Hz bins and spectral leakage in bins adjacent to the fundamental[4](https://sagroups.ieee.org/harmonicswg/wp-content/uploads/sites/445/2022/01/ih519b.pdf). For this situation the IEC flicker meter of IEC 61000-4-15 is recommended, with results compared against flicker compatibility levels, limits, and guidelines[4](https://sagroups.ieee.org/harmonicswg/wp-content/uploads/sites/445/2022/01/ih519b.pdf).
**Measurement duration.** Some interharmonic sources vary in magnitude and frequency over time, so some loads require measurements over a one-week period while others need only a few hours; variations in network impedance also affect the results[4](https://sagroups.ieee.org/harmonicswg/wp-content/uploads/sites/445/2022/01/ih519b.pdf).
## Limits and standards landscape
IEC 61000-2-2 specifies compatibility levels for public low-voltage networks and gives indicative interharmonic voltage levels corresponding to the compatibility level with respect to the flicker effect; for example, on a 50 Hz system the indicative level is 0.23% of nominal voltage for interharmonic frequencies between 58 and 62 Hz, and 0.68% for the 10–30 Hz range on a 120 V system[3](https://www.standards.iteh.ai/catalog/standards/iec/bb59b84d-efc6-4e43-a0bf-f2fc21f318ac/iec-61000-2-2-2002). To tackle flicker in incandescent and fluorescent lighting, the IEEE Interharmonic Task Force advised that the limits in IEC 61000-2-2 be adopted for the 20–90 Hz band, and that above 90 Hz and up to 3 kHz each interharmonic voltage component be kept below 1%, 2%, or 3%, according to the voltage level.[4](https://sagroups.ieee.org/harmonicswg/wp-content/uploads/sites/445/2022/01/ih519b.pdf)
Coverage remains uneven. IEEE 519 does not provide a general technical description of interharmonics, methods of measurement, or guidelines for limits[2](https://sagroups.ieee.org/harmonicswg/wp-content/uploads/sites/445/2022/01/interharmonics-paper.pdf), and power quality standards generally do not limit interharmonics; the IEEE 519 voltage interharmonic limits are presented for informative purposes and are based on lamp flicker, not on effects on motors, generators, transformers, or communication systems[6](https://www.mdpi.com/1996-1073/14/5/1218). A study of induction motors under voltage fluctuations found that the flicker-severity requirements and the subharmonic and interharmonic limits proposed in IEEE 519-2022 do not protect induction motors against excessive vibration[13](https://ieeexplore.ieee.org/document/10360212).
## Measurement difficulties and alternative methods
Because an interharmonic is a non-integral order component, adequate frequency resolution requires a long analysis window, but power system signals are seldom stationary, and this non-stationarity can corrupt spectrum analysis results[1](https://www.sciencedirect.com/science/article/pii/S0378779603000701). When interharmonic components appear in a spectrum, it can be debatable whether they really exist and, if so, what their actual frequencies and magnitudes are[1](https://www.sciencedirect.com/science/article/pii/S0378779603000701).
A range of techniques addresses these problems. Dual-stage methods first estimate and remove the fundamental and harmonics, then evaluate interharmonics on the residual signal; a difference-filtering variant has a wider passband than the IEC 61000-4-7 method, which helps detect interharmonics close to the fundamental or to harmonics[8](https://ietresearch.onlinelibrary.wiley.com/doi/10.1049/iet-rpg.2018.5697). Kalman filtering tracks dynamic changes of interharmonic signals in noise, [adaptive notch filters](https://www.edgepedia.org/adaptive-notch-filter) adjust to variable frequencies in real time, and parametric methods such as ESPRIT offer high frequency resolution at high computational cost[9](https://www.mdpi.com/1996-1073/18/13/3426). Energy-retrieval approaches collect the power dispersed by spectral leakage to recover the true amplitude and frequency of interharmonics adjacent to harmonics[14](https://ietresearch.onlinelibrary.wiley.com/doi/full/10.1049/iet-smt.2015.0115).
## References
1. [Chun Li, Wilsun Xu, Thavatchai Tayjasanant (2003). Interharmonics: basic concepts and techniques for their detection and measurement. Electric Power Systems Research.](https://www.sciencedirect.com/science/article/pii/S0378779603000701)
2. [IEEE Interharmonic Task Force and Cigré/CIRED Working Group. Interharmonics in Power Systems.](https://sagroups.ieee.org/harmonicswg/wp-content/uploads/sites/445/2022/01/interharmonics-paper.pdf)
3. [IEC 61000-2-2:2002+AMD1:2017, definitions and Annex B (compatibility levels for low-frequency conducted disturbances).](https://www.standards.iteh.ai/catalog/standards/iec/bb59b84d-efc6-4e43-a0bf-f2fc21f318ac/iec-61000-2-2-2002)
4. [IEEE Interharmonic Task Force Working Document IH0101: Interharmonics (measurement).](https://sagroups.ieee.org/harmonicswg/wp-content/uploads/sites/445/2022/01/ih519b.pdf)
5. [Gudiño-Ochoa et al. (2023). Analysis of Interharmonics Generation in Induction Motors Driven by Variable Frequency Drives and AC Choppers. Energies.](https://www.mdpi.com/1996-1073/16/14/5538)
6. [Gnaciński et al. (2021). Effects of Voltage Interharmonics on Cage Induction Motors. Energies.](https://www.mdpi.com/1996-1073/14/5/1218)
7. [Power Monitors, Inc. Defining Interharmonics.](https://powermonitors.com/whitepapers/defining-interharmonics/)
8. [Ravindran et al. (2019). Interharmonics in PV systems: a review of analysis and estimation methods. IET Renewable Power Generation.](https://ietresearch.onlinelibrary.wiley.com/doi/10.1049/iet-rpg.2018.5697)
9. [Mao et al. (2025). Monitoring and Suppression Strategies of the Interharmonic in the Grid-Connected Photovoltaic System: A Review. Energies.](https://www.mdpi.com/1996-1073/18/13/3426)
10. [IEC 61000-4-7:2002+AMD1:2008 CSV, General guide on harmonics and interharmonics measurements and instrumentation.](https://webstore.iec.ch/en/publication/4228)
11. [Lin (2014). Sources, Effects, and Modelling of Interharmonics. Mathematical Problems in Engineering.](https://onlinelibrary.wiley.com/doi/10.1155/2014/730362)
12. [A Method to Improve the Interharmonic Grouping Scheme Adopted by IEC Standard 61000-4-7. IEEE Transactions on Power Delivery.](https://doi.org/10.1109/tpwrd.2012.2183394)
13. [Induction Motors Under Voltage Fluctuations and Power Quality Standards. IEEE Transactions on Energy Conversion (2024).](https://ieeexplore.ieee.org/document/10360212)
14. [Lin (2016). Separation of adjacent interharmonics using maximum energy retrieving algorithm. IET Science, Measurement & Technology.](https://ietresearch.onlinelibrary.wiley.com/doi/full/10.1049/iet-smt.2015.0115)
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*Topic: Encyclopedia › Technology and the built world › Energy technology › Grids and transmission › Grid equipment and concepts › Power quality*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license