Solar PV and Storage Harmonics: Why Inverters Need an AHF

Summary. An active harmonic filter for solar PV systems is not a luxury add-on for a plant that already meets its grid code: it is often the device that makes compliance achievable once inverter count rises and background distortion from the utility is high. PV inverters generate switching sidebands, interact with existing capacitor banks, and change the network impedance as they connect and disconnect. This guide explains where inverter harmonics actually come from, why background distortion from the grid matters as much as the plant's own emission, how a fast-response filter damps resonance rather than amplifying it, and how to size compensation for a plant that also contains battery storage. It closes with a coordination checklist for EPC contractors working to grid-code limits at the point of connection.

Where Do Harmonics in a PV Plant Actually Come From?

A grid-tied inverter switches at several kilohertz and modulates a fundamental output. The switching process produces sidebands around multiples of the switching frequency, while the current control loop produces low-order distortion, particularly the 5th and 7th orders, when the inverter operates at light load or under unbalanced grid conditions. Modern inverters with high switching frequency and LCL filters deliver low current THD at rated output, but the figure rises sharply below 20 percent of rated power — which is precisely where a plant operates for much of the morning and evening.

The second source is the grid itself. Background distortion from upstream loads appears at the point of connection, and the plant's capacitor banks, cable capacitance and transformer impedance form a resonant network that can amplify those background components. A plant that measures its own inverters as compliant may still fail a connection assessment because the measured distortion at the metering point is dominated by resonance rather than by generation.

The third source is interaction. Multiple inverters operating in parallel share a common AC collection system, and their individual control loops can interact at low frequency, producing oscillation that appears as sub-harmonic distortion or as power oscillation. Adding a filter changes the impedance seen by every inverter, which is why coordination between devices is a design activity, not a procurement one.

Storage adds a fourth dimension. A battery system charges and discharges, so the current direction reverses on a schedule, and the PCS behaves as a load during charging. Harmonic current during charging can exceed the export condition, and the filter must be sized for the worse of the two states rather than the conventional export-only case.

Measurement method decides the verdict. Harmonic limits are assessed differently depending on the standard referenced: some require very short aggregation windows, others ten minutes or two hours. A plant can pass one method and fail another with the same hardware. Agree the method with the network operator before the test, use a Class A analyser, and record the aggregation interval in the report. Getting this wrong is the most common reason a compliant plant is asked to repeat a commissioning test, and repetition at a remote site is expensive for everyone involved.

Why Filtering Changes the Outcome for Renewable Plants

1. Resonance is damped rather than excited. A capacitor bank on a feeder with high cable capacitance can present a parallel resonance close to the 5th or 7th harmonic. A fast-response filter injects opposing current at the distorted frequency and effectively damps the resonance, whereas a passive filter tuned near the same frequency may simply move the problem to another order.

2. Grid-code compliance is measured at the point of connection. Harmonic emission limits apply where the plant meets the network, not at the inverter terminals. Compensating at the collection substation addresses the measurement that matters, and per-order logging provides the evidence a network operator will request.

3. Light-load distortion is handled. Inverter THD rises at partial output, and a fast-response AHF for photovoltaic systems with sub-cycle detection keeps the point of connection clean during the hours when a conventional design would be non-compliant.

4. Reactive capability reduces equipment count. A filter with reactive output can supply the reactive current required by the grid code during the transition periods around sunrise and sunset, when reactive demand changes quickly. That can remove a separate compensation device from the design.

5. Cable and transformer losses fall. Harmonic current circulating between inverters and the transformer contributes to I²R losses continuously. Reducing it improves the plant's measured performance ratio, which matters for both revenue and warranty discussions with the EPC contractor.

6. Protection coordination improves. Distorted current affects relay pickup and fuse behaviour. Reducing distortion makes protection settings predictable, particularly on long collection feeders where the fault current is modest compared with load current.

7. Future expansion is accommodated. Plants are expanded in blocks. A modular frame, such as the 150 kvar modular compensation cabinet family or a parallelable AHF cabinet, allows the compensation to grow with the array rather than being replaced.

8. Inverter availability improves. Some inverters derate or disconnect when voltage distortion exceeds internal limits, particularly on weak feeders. Cleaner voltage at the inverter terminals reduces those events, which shows up as higher annual yield rather than as an engineering metric. On plants with aggressive availability guarantees, that difference can affect contract performance.

How to Size and Coordinate Compensation in a PV or Storage Plant

Step 1 — Measure background distortion before the plant is energised. A pre-connection survey establishes what the network already contains. Without it, the plant's contribution cannot be separated from the background, and the filter may be sized to solve a problem it did not create.

Step 2 — Survey the plant at three output levels. Record the spectrum at low, medium and rated output, plus the charging state if storage is present. Light-load spectra are usually the design case for renewable sites, not rated output.

Step 3 — Check the resonance map. Calculate parallel resonance between the existing capacitor banks, cable capacitance and transformer impedance, and confirm that no resonant point coincides with a dominant harmonic order during normal operation. If it does, compensation must be placed and tuned to damp that point.

Step 4 — Size on the worse operating state. Take the larger harmonic current of charge and discharge, add headroom for ambient and future blocks, and confirm the filter's response time is faster than the inverter's own current loop so that the two do not interact.

Step 5 — Coordinate with the plant controller. The filter's reactive settings must not fight the inverter's voltage or power factor control. Define which device regulates voltage, which regulates current, and document the priority in the plant controller logic. Where a 75A cabinet serves a load board inside the plant, its CT placement should exclude the inverters so that it compensates loads only.

Step 6 — Verify with a witnessed test. Many network operators require a compliance test at the point of connection with the plant at several output levels. Prepare the test procedure in advance, including the filter settings, and record the results in the same format the operator uses.

For plants that also include a small industrial load — a workshop, an O&M building, a storage HVAC system — compensation at the load board is generally more effective than a larger device at the incomer, because it stops harmonic current from travelling through the collection system at all. A 30A module at that board is often sufficient, and the main filter then only needs to handle generation-side distortion. Review the full AHF range to compare frames and mounting options before fixing the electrical layout.

What the network operator will ask for. Expect requests for the resonance study, the harmonic model, the filter datasheet with declared ratings, evidence that the filter does not interfere with ripple control or power line carrier systems, and a witnessed commissioning report. Preparing that package during design rather than after energisation turns a potential delay into a routine review, and it demonstrates the kind of engineering discipline that shortens future grid connection approvals for the same developer.

Seasonal and daily variation. Harmonic performance changes with irradiance, temperature and grid conditions. A plant verified on a clear spring day may behave differently in midsummer haze or during a winter storm when output is low and background distortion is high. Include a low-irradiance test in the commissioning programme, because that is the condition under which light-load inverter distortion and resonance effects combine.

Losses in the collection system. Harmonic current flows from inverters through transformers and cables to the point of connection, and every conductor dissipates I²R losses along the way. On a large plant with long collection feeders these losses are not trivial; they reduce measured output and increase conductor temperature, which in turn derates cable capacity. Compensation placed close to the source reduces the current that travels, and the gain shows up in the plant's performance ratio as well as in the harmonic compliance report.

Plan for the second phase. Most PV sites are built in blocks, and the harmonic assessment for phase one rarely reflects phase two accurately. Specify frames with spare capacity or clear paralleling provision, keep the resonance study as a living document, and re-run the assessment when the next block is energised. A filter that is 30 percent oversized at phase one completion is usually the right size two years later.

FAQ

Do modern PV inverters still need harmonic filtering?

At rated output, often not. At low output, frequently yes: THD rises sharply below 20 percent of rating, and background grid distortion plus resonance can push the plant above its connection limits. Survey at light load before concluding that no filter is needed.

Why does background distortion matter to a new plant?

Because compliance is measured at the point of connection, not at the inverter. Existing upstream distortion plus the plant's own emission determines the measured value, and a plant can fail an assessment even when its inverters individually test as compliant.

Can an AHF cause resonance with inverter filters?

Not if it is correctly coordinated. The filter changes the network impedance, so the resonance map must be recalculated after installation. Commissioning should include a sweep or a step test to confirm no new resonant point has appeared near a dominant order.

Do I need a separate device for reactive power?

Sometimes. A filter with reactive capability can cover modest requirements and transitional periods, while plants with larger reactive obligations or voltage-support duties may need a dedicated SVG. The decision depends on the grid code and the plant rating.

How does battery storage change the calculation?

Charging reverses current direction and can produce higher distortion than export. Size on the worse state, and confirm the filter remains stable during transitions between charge and discharge, when the current steps rapidly.

Where should the filter be installed in a large plant?

At the point that matches the compliance measurement — usually the collection substation or the point of connection — plus local units at internal load boards. Central-only compensation leaves internal load harmonics circulating through the collection system.

Conclusion

Renewable plants fail harmonic assessments for reasons that have little to do with inverter quality: light-load operation, background distortion and resonance with existing capacitance. Measure the pre-connection baseline, survey at three output levels, check the resonance map, size on the worse of charge and discharge, and coordinate control with the plant controller. A fast-response filter used in this way turns compliance from an argument into a measurement.

Commissioning a PV or storage plant with harmonic limits? Send us the single-line diagram, inverter datasheets and grid-code limits, and we will model the resonance, size the compensation and provide a witnessed-test procedure. Contact our renewable energy team.