An interharmonic is a frequency component that is not an integer multiple of the system frequency, such as 75 Hz in a 50 or 60 Hz system. IEEE 519-2022 sets no interharmonic limits: its Annex A is informative, the values it gives are non-prescriptive examples, and current limits are developed case by case.
Short answer
An interharmonic is a frequency component that is not an integer multiple of the system frequency. IEEE 519-2022 sets no mandatory limits for them. Its Annex A is informative: it describes the working group’s reasoning, gives example curves and leaves the values to the judgment of each case. For current, clauses 5.3 to 5.5 state that limits are developed case by case.
What an interharmonic is
Clause 3.1 defines interharmonics as frequency components of a periodic quantity that are not an integer multiple of the frequency at which the system operates. The standard’s example: 75 Hz in a 50 or 60 Hz system.
An integer harmonic, such as 120 Hz or 180 Hz in a 60 Hz system, falls exactly on a multiple of the system frequency. An interharmonic falls between two of those multiples. In a 60 Hz system, 75 Hz lies between the fundamental (60 Hz) and the 2nd harmonic (120 Hz).
One consequence of the definition: the THD and TDD of IEEE 519 consider harmonics up to the 50th order and expressly exclude interharmonics (clause 3.1). A record with a compliant THD can contain interharmonics that do not show up in that number.
What Annex A.1 records about the 2014 edition and what changes
Annex A.1 recalls that the 2014 edition of IEEE 519 suggested, in its Annex A, voltage interharmonic limits related to a short-term flicker severity (Pst) of 1.0 for 60 Hz systems. They were evaluated with the measurement technique of IEEE 1453 and IEC 61000-4-15, and were based on avoiding flicker effects in incandescent lamps.
Those recommendations did not consider the effect of interharmonics on other equipment and systems, such as the mechanical systems of generators, motors, transformers, signaling and communication systems, and filters. The 2014 edition already suggested developing current interharmonic limits case by case, with knowledge of the supply system, the connected loads and the provisions for future users.
The 2022 edition keeps the case-by-case treatment for current and leaves Annex A as guidance, with an explanation of the reasoning behind a possible limit curve.
What Annex A contains
Annex A explains the work of the IEEE working group on interharmonic distortion. The standard warns that the numerical values of the curves and the frequency range should not be read as prescriptive: suitable values must be based on specific local, national or site needs.
| Clause | Content |
|---|---|
| A.1 Premise | Definition, history of the 2014 edition and its rationale |
| A.2 Basis of the limits | General shape of a possible limit curve for low voltage in non-generation facilities |
| A.2.1 Countries that do not use ripple control | Example curves with the interharmonic subgroups of IEC 61000-4-7 and the IEC flickermeter |
| A.2.2 Countries that use ripple control | Compatibility with ripple control signals |
| A.3 Working group summary | Proposed levels if sensitive equipment is present |
Centered subgroups. To evaluate interharmonics, Annex A starts from the IEC 61000-4-7 definition: spectral components between two consecutive harmonics are grouped into “subgroups”. With the window of about 200 ms (12 cycles at 60 Hz) each component is 5 Hz apart. Around the fundamental, the Annex relies on the IEC flickermeter because of the difficulty of measuring very low voltages with IEC 61000-4-7 instruments.
Example values. Figure A.1 shows an example curve for 60 Hz systems with a limit of 0.3 % of the subgroups up to 1 kHz and 0.5 % up to 2.5 kHz. Figure A.2 shows another example with a different subgroup definition: 0.5 % around even harmonics, 0.3 % at a suitable distance from odd ones, and 0.5 % between 1 kHz and 2.5 kHz. The text presents them as examples of suitable values, not as limits.
Ripple control
Annex A.2.2 explains that in some countries, including Germany, France and Sweden, ripple control signaling is used: a pulse-modulated carrier that uses harmonic and interharmonic frequencies from 168 Hz to 1,350 Hz, depending on the region. For that reason, compatibility of voltage interharmonics with that signal requires limits specific to each country.
IEC 61000-2-2 recommends a reference level of 0.2 % of the nominal voltage at the defined frequency, according to local practice. That level is justified because the ripple control receiver can respond from 0.3 % of the nominal voltage; an unwanted interharmonic above that value can cause interference if it coincides with the operating frequency of the receivers.
Proposed levels for sensitive equipment
Summary A.3 gathers the working group’s analysis. The sensitivity of lighting (LED) to individual interharmonic components showed that very strict limits must apply near the fundamental (for example 0.2 % of the nominal voltage at the PCC), and that at more than twice the fundamental, up to 0.3 % is justified over a suitable range around the odd harmonics. For non-lighting equipment, even stricter limits (for example 0.1 %) in the subharmonic range, that is, below the system frequency.
If sensitive equipment is present and a risk analysis shows it is necessary, interharmonic voltages should be limited to:
| Range | Level |
|---|---|
| Subharmonic | 0.1 % |
| Around the fundamental (about ±10 Hz away) | 0.2 % |
| Around odd harmonics (about ±10 Hz away) | 0.3 % |
| Around even harmonics | More than 0.3 % |
The standard warns that these limits look very restrictive, so they should be applied only if sensitive equipment is present and a risk analysis showed they are needed.
What it means for a project
- There is no automatic compliance limit. For interharmonics there is no table in IEEE 519-2022 against which to declare compliance.
- It is defined case by case. With the supply system, the loads and the provisions for other users, as clauses 5.3 to 5.5 indicate.
- It is worth measuring them when there are symptoms of flicker or interference with signaling, and not only THD.
- The effect on frequency protection is covered in Harmonics and Interharmonics in Frequency Protection.
Table 1 and the current tables cover integer harmonics up to the 50th order (IEEE 519-2022 Voltage Harmonic Limits at the PCC and Harmonic Current Limits and TDD by Isc/IL). How the spectrum is measured, with the 12-cycle window and the 5 Hz bins, is in Measuring Harmonics for IEEE 519: the 12-Cycle Window.
We help with your case
At Tiza Group we carry out power quality studies, in Colombia, or elsewhere on request: we measure the interharmonic content of your facility when there is flicker, interference with sensitive equipment or loads with their own modulation, and we define the acceptance criteria for the case with you.
Related guides
- Measuring Harmonics for IEEE 519: the 12-Cycle Window
- IEEE 519-2022 Voltage Harmonic Limits at the PCC
- Harmonic Current Limits and TDD by Isc/IL
- Harmonics and Interharmonics in Frequency Protection
- Power Quality Monitoring: IEEE 1159
References
- IEEE Std 519-2022, IEEE Standard for Harmonic Control in Electric Power Systems: clause 3.1 (interharmonics, THD and TDD); clauses 5.3 to 5.5 (current interharmonics case by case); informative Annex A (A.1 premise, A.2 basis, A.2.1 and A.2.2, A.3 summary).
- IEC 61000-4-7, measurement of harmonics and interharmonics (subgroups); IEC 61000-4-15, flickermeter; IEEE Std 1453; IEC 61000-2-2.
Frequently asked questions
Does IEEE 519-2022 have a mandatory limit for interharmonics?
No. The limit tables apply to integer harmonics. For current interharmonics, clauses 5.3 to 5.5 state that the values and statistical indices are developed case by case from Annex A, which is informative.
What changed compared with the 2014 edition?
According to Annex A.1 of the 2022 edition, the 2014 edition suggested, in its Annex A, voltage interharmonic limits tied to a short-term flicker severity (Pst) of 1.0 in 60 Hz systems, intended to avoid flicker in incandescent lamps. The 2022 edition does not set those limits: it offers example curves and leaves the values to the judgment of each case.
What are the 0.1 %, 0.2 % and 0.3 % values in Annex A for?
They are the levels that, according to the working group summary, should apply if sensitive equipment is installed and a risk analysis shows it is needed. They are guidance: Annex A is informative and the standard clarifies that the numerical values of its curves are not prescriptive.