Supraharmonic Emissions from Photovoltaic Inverters — An Emerging Power Quality Challenge
| Phenomenon | Supraharmonic (SH) emissions in the 2–150 kHz range from grid-connected PV inverters |
| Source | PWM switching in modern high-frequency PV inverters using SiC and GaN semiconductor switches |
| Emission types observed | Narrowband (at switching frequency and multiples) · Broadband · Time-varying |
| Key paradox | New wide-bandgap semiconductors reduce classical harmonics (<2 kHz) but increase supraharmonics (>2 kHz) |
| Regulatory status | No specific emission limits currently exist for the 2–150 kHz range — standards gap |
| Measurement standard | IEC 61000-4-7 and IEC 61000-4-30 — both inadequate for SH characterisation; under revision |
| Intermodulation risk | PV inverter + EV charger switching frequencies interact to create new frequency components not present in either device alone |
| Known effects | Cable heating · LED lamp interference · Capacitor aging · PLC communication failure · Control circuit malfunction |
01 Context — The New Frontier of Power Quality
Power quality engineers have spent decades characterising and mitigating harmonics in the range up to 2 kHz — the fifth, seventh, eleventh, thirteenth harmonic orders that are the signature of six-pulse rectifiers, arc furnaces, and saturated transformers. The measurement methods are well established, the standards are comprehensive, and the mitigation technology is mature. Above 2 kHz, however, the landscape changes fundamentally.
Supraharmonics — electrical disturbances in the 2 kHz to 150 kHz range — are not a new phenomenon, but they are a rapidly growing one. The proliferation of grid-connected power electronic devices: photovoltaic inverters, EV chargers, battery storage systems, LED drivers, and variable-speed drives using modern wide-bandgap semiconductor switches, is filling the supraharmonic frequency range with emissions that the existing power quality measurement framework was not designed to capture and that no current regulatory standard limits.[1]
This case study presents the findings of research by Pinto, Grasel, and Baptista (2024) at the University of Trás-os-Montes (Portugal) and Technikum Vienna (Austria), analysing supraharmonic emissions from multiple PV inverters in an electrical network under different penetration scenarios. The study provides one of the clearest published accounts of the emission characteristics, propagation mechanisms, and interference potential of PV-generated supraharmonics on low- and medium-voltage networks.
Previous-generation power electronics used diodes and thyristors — passive switching devices limited to line-frequency commutation. They produced substantial harmonic distortion in the 0–2 kHz range. Modern inverters use Silicon Carbide (SiC) and Gallium Nitride (GaN) switches operating at switching frequencies of 20–100 kHz or higher. These devices dramatically reduce low-frequency harmonic distortion — but the high switching frequencies shift the emission spectrum upward into the supraharmonic range, where measurement is more difficult and regulatory limits do not yet exist.[1]
02 What Are Supraharmonics?
Supraharmonics are frequency components present in the power system voltage or current waveform in the range from 2 kHz to 150 kHz. They are distinct from both classical harmonics (integer multiples of the 50/60 Hz fundamental, typically addressed up to the 40th harmonic — 2 kHz at 50 Hz) and from radio-frequency electromagnetic interference above 150 kHz, which is addressed by CISPR standards.[1]
The supraharmonic range sits between two well-regulated domains — and falls through the gap between them. Neither the power quality standards framework (IEC 61000 series, IEEE 519) nor the electromagnetic compatibility framework (CISPR) adequately covers this range with specific emission limits for grid-connected power electronics.[1]
Emission types in the supraharmonic range
The study identified three distinct emission types from PV inverters, each with different characteristics and propagation behaviour:[1]
- Narrowband emissions — concentrated at the inverter’s switching frequency and its integer multiples. For a PV inverter switching at 20 kHz, narrowband emissions appear at 20 kHz, 40 kHz, 60 kHz, etc. These are deterministic and directly related to the PWM modulation frequency
- Broadband emissions — spread across a wide frequency range, typically caused by switching transients and the finite rise and fall times of the semiconductor switches. The faster the switching (as with SiC and GaN devices), the broader the high-frequency content of the transient
- Time-varying emissions — changing with solar irradiance, load, and the inverter’s operating point. At low power levels or during cloud transients, the MPPT (maximum power point tracking) algorithm changes the switching pattern, altering the emission spectrum dynamically
03 Sources and the Intermodulation Problem
PWM switching — the primary generation mechanism
The supraharmonic emissions from a PV inverter originate from the Pulse Width Modulation (PWM) switching process that converts the PV panel’s DC output to the grid-frequency AC output. Every switching event — turning the semiconductor switch on or off — creates a current transient whose frequency content extends far above the fundamental switching frequency. The faster the switching transition (characterised by dI/dt and dV/dt), the higher the frequency content and the broader the emission spectrum.[1]
When measuring supraharmonic emissions at the PCC, the instrument always measures the sum of primary emissions (from the device under test) and secondary emissions (supraharmonic currents from other devices on the network flowing through the measurement point). This distinction is critical for assigning responsibility correctly — and it is one reason why supraharmonic source attribution is significantly more complex than classical harmonic source identification. The impedance network between devices determines how much of each device’s primary emissions appears at every other measurement point.[1]
Intermodulation — when two devices interact
One of the most important findings in current supraharmonic research is the intermodulation phenomenon. When two power electronic devices with different switching frequencies are connected to the same network — for example, a PV inverter switching at 20 kHz and an EV charger switching at 32 kHz — their supraharmonic emissions interact through the network impedance to produce new frequency components at sum and difference frequencies (52 kHz, 12 kHz, 72 kHz, etc.) that were not emitted by either device individually.[1]
This phenomenon — well known in telecommunications as intermodulation distortion — is now being observed in power distribution networks as the density of high-switching-frequency devices increases. It means that the supraharmonic environment at any point in the network is not simply the superposition of individual device emissions — it is a complex mix of primary emissions, secondary emissions, and intermodulation products whose composition changes with the connected device population.
The intermodulation problem means that supraharmonic emissions from a distribution feeder with multiple PV inverters and EV chargers cannot be predicted by summing individual device emission measurements. The network impedance, the spatial distribution of devices, and the relationship between their switching frequencies all matter. This requires a fundamentally different approach to supraharmonic assessment than the harmonic summation methods used for classical harmonics.
04 Effects on Equipment and Networks
Supraharmonic emissions cause a range of effects on power system components and connected equipment, some of which are analogous to classical harmonic effects and some of which are specific to the higher frequency range:[1]
- Cable heating — skin effect: At high frequencies, current concentrates on the conductor surface (skin effect), reducing the effective cross-section and increasing the effective resistance. A cable carrying significant supraharmonic current runs hotter than its power-frequency loading alone would predict. Thermal calculations based on power-frequency current rating are non-conservative in the presence of significant supraharmonic content
- Capacitor aging: Capacitors present low impedance at high frequencies, drawing supraharmonic currents in proportion to the frequency. The dielectric losses at supraharmonic frequencies can significantly exceed the losses at power frequency, accelerating insulation degradation and reducing service life. Aluminium electrolytic capacitors in lighting equipment are particularly vulnerable
- LED lamp interference: LED drivers are sensitive to high-frequency interference on the supply voltage. Supraharmonic distortion can cause perceptible variation in LED light output — a flicker mechanism different from the 8–10 Hz voltage fluctuation flicker addressed by IEC 61000-4-15, and not captured by the standard flickermeter
- Power Line Communication (PLC) interference: Smart metering systems, SCADA communications, and demand response signals often use power line carrier frequencies in the supraharmonic range (typically 9–150 kHz). Supraharmonic emissions from PV inverters and EV chargers can overwhelm these signals, causing communication failures in smart grid infrastructure
- Control circuit malfunction: High-frequency emissions can couple into control and protection circuits through electromagnetic induction or conducted paths, causing spurious relay operation, measurement errors, or communication faults
- Audible noise: Supraharmonic frequencies in the range 20 Hz–20 kHz are within the human auditory range and can cause audible noise from transformers, cables, and other magnetic components
Smart metering and demand response systems — which are foundational to modern grid management and load control — depend on power line carrier communications in exactly the frequency range where supraharmonic emissions are most concentrated. A distribution feeder that is being equipped with PV inverters and EV chargers to reduce carbon emissions may simultaneously be degrading the communication infrastructure that manages those devices. This is not a hypothetical concern — PLC communication failures in areas with high PV penetration are already being reported by network operators.
05 Measurement — The Standards Gap
The measurement of supraharmonics requires sampling rates above 300 kHz (by the Nyquist criterion, to capture signal content up to 150 kHz) — significantly higher than what classical harmonic measurement instruments, which typically sample at 12–16 kHz, are designed to provide. This means that most existing power quality monitors — even Class A instruments compliant with IEC 61000-4-30 — do not capture the supraharmonic range.[1]
Current measurement standards and their limitations
- IEC 61000-4-7: Specifies harmonic and interharmonic measurement using 200 Hz frequency bands up to 2 kHz. Does not address the supraharmonic range
- IEC 61000-4-30: Specifies PQ measurement methods including a non-continuous grouping method using 2 kHz frequency bands for frequencies above 2 kHz. This provides only 8% signal coverage — 92% of the supraharmonic signal is not captured. The 2 kHz band grouping also loses frequency resolution that is essential for identifying individual device switching frequencies. This standard is currently under revision by IEC SC 77A WG9 specifically to address these deficiencies[1]
- CISPR 16: Electromagnetic interference measurement standard used above 9 kHz. Designed for conducted and radiated EMI from equipment, not for power system PQ monitoring. Uses quasi-peak and average detectors rather than the RMS measurements appropriate for PQ assessment
A PQ survey conducted with a Class A instrument fully compliant with IEC 61000-4-30 will report voltage and current parameters from DC to 2 kHz with high accuracy. Above 2 kHz, the same instrument provides fragmentary, low-resolution data that misses the majority of supraharmonic signal energy. The survey report will be technically correct — and will completely fail to characterise the supraharmonic environment. This is not a deficiency of the instrument or the measurement practice — it is a gap in the standard itself, which the IEC is actively working to fill.
06 Key Findings from the Study
The study by Pinto, Grasel, and Baptista analysed real supraharmonic signals from PV systems under several network scenarios, examining the propagation of emissions and the correlations between different PV inverter models and penetration levels. The key findings were:[1]
- Each PV inverter model has a distinct emission signature — the switching frequency and its harmonics appear as characteristic narrowband peaks in the supraharmonic spectrum, allowing individual inverter models to be identified from their emission pattern. A constant narrowband emission at the switching frequency (for example, 20 kHz) is the most reliable identifier
- Broadband emissions vary with operating conditions — at partial load (low solar irradiance), the MPPT algorithm changes the switching pattern, and the broadband emission profile changes accordingly. This time-varying character makes characterisation under a single operating condition misleading
- Intermodulation products are measurable — when multiple PV inverters with different switching frequencies are present on the same network, intermodulation products at sum and difference frequencies are detectable, confirming that the supraharmonic environment is not simply the sum of individual emissions
- Propagation depends on network impedance — supraharmonic emissions propagate through the network according to the impedance distribution. Capacitive loads (including power factor correction capacitors) present low impedance at supraharmonic frequencies and draw significant supraharmonic currents, potentially amplifying local emission levels
- No current regulatory framework adequately addresses the findings — the study concludes that specific regulations for the 2–150 kHz range are urgently needed, covering both emission limits and measurement methodology
The narrowband emission at the PV inverter’s switching frequency is the most reliable field identification marker. If a power quality analyser with sufficient bandwidth (300 kHz+ sampling rate) is available, scanning for narrowband peaks in the 10–100 kHz range will reveal the switching frequencies of connected inverters and chargers. The intermodulation products — at sum and difference frequencies — appear as additional narrowband peaks that shift when any device’s switching frequency changes, which distinguishes them from primary emissions.
07 Power Quality Perspective
Supraharmonics represent the next frontier of power quality engineering — a phenomenon that is growing in significance at exactly the moment when the tools to measure and limit it are still being developed. The parallel with classical harmonics in the 1980s and early 1990s is striking: a new class of non-linear loads (then, VFDs and UPS systems; now, PV inverters and EV chargers) is introducing disturbances that the existing measurement and regulatory framework was not designed to handle, and the engineering community is racing to characterise the problem before it becomes unmanageable.
From a utility distribution perspective, the most immediately consequential effect is the threat to power line carrier communications. Smart metering, demand response, and grid control systems that depend on PLC frequencies in the 9–150 kHz range are directly vulnerable to the same frequency range where supraharmonic emissions are concentrated. As PV penetration and EV charger density increase on LV distribution feeders, the signal-to-noise ratio for PLC communications will degrade — potentially undermining the smart grid infrastructure that is intended to manage the energy transition.
Power quality engineers who built their practice on harmonic measurements up to the 40th order need to be aware that the PQ problem space is now extending above 2 kHz — and that the instruments, standards, and mitigation tools for this range are still maturing. A PQ assessment that does not address supraharmonics is not wrong — it is simply incomplete for any site with significant PV generation or EV charging. The question is not whether supraharmonics matter, but when the measurement tools and regulatory framework will catch up with the physical reality that is already present on the network. Based on the pace of standards development at IEC SC 77A WG9, that convergence is likely within the next 3–5 years. Engineers who build familiarity with the supraharmonic range now will be well positioned when it becomes a mandatory part of every PQ survey.
References
- Pinto J, Grasel B, Baptista J. “Analysis of Supraharmonics Emission in Power Grids: A Case Study of Photovoltaic Inverters.” Electronics, vol. 13, no. 24, p. 4880, 2024. DOI: 10.3390/electronics13244880. Open access under CC BY 4.0.
- IEC 61000-4-7:2009+AMD1:2021. Electromagnetic compatibility (EMC) — Part 4-7: Testing and measurement techniques — General guide on harmonics and interharmonics measurements and instrumentation. IEC, Geneva.
- IEC 61000-4-30:2015+AMD1:2021. Electromagnetic compatibility (EMC) — Part 4-30: Testing and measurement techniques — Power quality measurement methods. IEC, Geneva.
- IEC 61000-2-2:2002. Electromagnetic compatibility (EMC) — Part 2-2: Environment — Compatibility levels for low-frequency conducted disturbances and signalling in public low-voltage power supply systems. IEC, Geneva.
- Rönnberg SK, Bollen MHJ. “Power Quality Issues in the Electric Power System of the Future.” The Electricity Journal, vol. 29, no. 10, pp. 49–61, 2016.
Pinto J, Grasel B, Baptista J. “Analysis of Supraharmonics Emission in Power Grids: A Case Study of Photovoltaic Inverters.” Electronics, 13(24), 4880, 2024.
DOI: 10.3390/electronics13244880 · Read the original article at MDPI →
Published open access under CC BY 4.0. This case study is presented in summary and commentary form. The PQ Perspective section (Section 7) is original IPQDF editorial commentary by Denis Ruest, M.Sc. (Applied), P.Eng. (ret.). IPQDF does not claim authorship of the original research.