Solar Inverter RFI: Diagnose the Installation, Not the Topology

PV power electronics switch large voltage and current rapidly. Their interference becomes a radio problem when unwanted energy leaves the converter on cables or fields and reaches the receiving antenna above the local noise floor.

Where the Spectrum Comes From

An inverter or optimizer synthesises its output with switched semiconductor stages. The nominal switching frequency may be in the kilohertz or higher, while fast voltage and current edges contain spectral energy far above it. Gate timing, modulation, resonances, filter layout and cable impedance shape the resulting combs, carriers and broadband components.

It is therefore too simple to blame only the switching frequency. A 20 kHz converter can create components across HF, while a higher-frequency design can still be quiet if edge control, filtering and layout are effective.

Both Differential and Common Mode Matter

Differential-mode current flows out on one conductor and returns on its paired conductor. Keeping PV positive and negative close together reduces loop area and external magnetic field, but differential voltage can still couple or be converted into common mode by imbalance.

Common-mode current flows in the same longitudinal direction on the grouped conductors and returns through frame capacitance, protective earth, building wiring, soil and surrounding structures. Because the return path is spatially separated, even modest net current on long cables can radiate effectively.

Either mode can dominate at a given frequency. “The real problem is always common mode” is as unsafe a shortcut as “all noise is differential.” Measure both.

Transformerless Is a Topology, Not a Verdict

A transformerless inverter has no galvanic isolation between its PV and grid power circuits. Parasitic capacitance from modules and wiring to frames and earth then participates in a frequency-dependent common-mode network. The switching topology must control common-mode voltage and leakage current while its EMC filter controls emissions.

A transformer can interrupt the DC path, but interwinding capacitance and deliberate safety capacitors still couple RF. Isolation therefore changes the common-mode circuit; it does not guarantee a quieter installation. A poorly filtered isolated converter can be noisier than a well-designed transformerless one.

Modern transformerless topologies can clamp or shape common-mode voltage and achieve good EMC. Their higher efficiency and lower mass are not evidence of corner-cutting. Conversely, a low price alone does not prove poor filtering. Judge a model and installation by documentation and measurement.

Micro-Inverters and Optimizers Change the Geometry

Module-level electronics create more switching nodes distributed across a roof. That can increase the number of possible sources and make diagnosis harder, but it does not make every installation noisy. Shorter DC runs, different modulation, filters and cable geometry can also change coupling favourably.

“Fewer converters is always quieter” is not a technical specification. Consider the exact products, firmware, harness, frame bonding, cable loops and distance to the receiving antenna.

What Current PV EMC Standards Cover

The current IEC 62920:2017+A1:2021 specifies EMC requirements and test methods for DC-to-AC power conversion equipment used in PV systems, including grid-interactive and stand-alone equipment. Its value is that PV-specific port arrangements and operating conditions are considered rather than treating the inverter as a generic box.

Safety is separate. IEC 62109-2 addresses particular safety requirements for PV inverters. Meeting an emissions test does not replace electrical safety, and meeting a safety standard does not prove radio quietness.

A product standard is not the same as a ham's spectrum screenshot. Standardised limits, detectors, bandwidths, impedances, cable arrangements, operating modes and distances make results repeatable. A field measurement is essential for diagnosis but is not automatically a compliance test.

What CE Marking Means in the EU

Under the EMC Directive 2014/30/EU, equipment must meet essential protection requirements: it should not generate electromagnetic disturbance at a level that prevents radio, telecommunications or other equipment from operating as intended, and it must have adequate immunity for its environment.

CE marking is not merely decorative and is not normally a third-party “quiet radio” certificate. The European Commission explains that the manufacturer performs the applicable conformity assessment, creates technical documentation, issues the EU Declaration of Conformity and affixes the mark.

For a purchase or investigation, useful documentation includes:

• the exact product and hardware/firmware variant;
• the EU Declaration of Conformity;
• the legislation and standards editions cited;
• the installation conditions needed to maintain conformity; and
• manufacturer or installer support for an interference case.

The full technical file and test reports are generally held for market-surveillance authorities; their absence from a public website does not itself prove non-compliance. A CE mark also cannot guarantee that a sensitive nearby HF station will hear no increase in noise.

Do Not Call It an “Illegal Transmitter”

An inverter is apparatus that can produce unintended electromagnetic disturbance; it is not thereby a radio transmitter. A troublesome field observation does not by itself establish which product or installation requirement was breached, who is responsible, or whether an authority will find non-compliance.

Use neutral language: suspected interference associated with the PV installation. Preserve evidence, involve the owner and installer, and let the competent authority make legal findings.

A Safe Diagnostic Workflow

1. Establish a baseline. Record the station noise with the same antenna, preamp, attenuator, bandwidth and receiver settings.
2. Correlate with operation. Compare dawn, full sun, cloud transitions, evening and known inverter states. Correlation is evidence, not final proof.
3. Capture a waterfall. Record absolute frequency, time, detector/bandwidth and amplitude reference. Note recurring comb spacing and drift.
4. Use a portable receiver carefully. A small loop or near-field probe can localise cables and boxes, but close-field amplitude does not predict far-field compliance.
5. Measure net cable current where safe. A suitable RF current probe around defined conductor groups can identify common-mode paths without electrical contact.
6. Request controlled switching. Only the owner, installer or other qualified person should operate PV isolators and approved shutdown procedures. Compare states without unplugging live DC connectors.
7. Separate paths. With qualified assistance, compare inverter, AC cable, DC strings, frame/bonding conductors and module electronics rather than assuming one box is the radiator.
8. Repeat. Weather, array voltage, load, MPPT state and firmware modes can alter the signature.

Mitigation Must Match the Mode

Ferrite is not a generic cure. Material, geometry, turns, RF current, voltage, temperature and DC safety approval matter. Passing both DC conductors through the same aperture can create common-mode impedance while their equal-and-opposite operating currents largely cancel magnetically, but the assembly must still be designed for the PV voltage, insulation and fault environment.

Do not place improvised impedance in a protective-earth conductor. Do not add capacitors from PV rails to earth; they can change leakage current, insulation monitoring, residual-current detection, MPPT behaviour and touch current.

Installation Geometry Is Part of EMC

• Route positive and negative conductors together to minimise differential loop area.
• Avoid unnecessary cable loops and long separated runs.
• Keep PV and inverter wiring away from receiving antennas and feedlines where practical.
• Follow the manufacturer's frame-bonding and protective-earth scheme; more random bonds are not automatically better at RF.
• Keep filtered and unfiltered conductors physically separated so the filter is not bypassed by coupling.
• Use specified connectors, glands and cable types so safety and EMC assumptions remain valid.

How to Report Interference in Belgium

Start constructively with the installation owner, installer and manufacturer. Provide dates, operating correlation, frequencies, recordings and the exact equipment identification. This often enables a firmware update, repair or approved filter without a dispute.

If significant interference remains unresolved, Belgium's BIPT Spectrum Monitoring service accepts interference reports. Its engineers—not a neighbour's receiver screenshot—can investigate the radio-frequency case with the appropriate authority and methods.

The Practical Verdict

The topology label is a clue, not a verdict. Galvanic isolation changes the common-mode circuit; it does not guarantee silence. Transformerless equipment can be compliant and quiet; isolated equipment can be defective, poorly filtered or badly installed.

Choose a documented product, preserve compact cable geometry, plan antenna separation, and agree before purchase who will support an EMC problem. Then measure the real installation. The useful question is not “does it have a transformer?” but “which unwanted current flows where, under what operating state, and what evidence supports the fix?”

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