Harmonics are one of the most frequently cited causes of grid connection rejections for commercial and utility-scale solar projects. Utilities, distribution network operators (DNOs), and transmission system operators (TSOs) all set limits on the harmonic current injection that grid-connected inverters may contribute. Failing to meet these limits can delay grid approval, require expensive filtering equipment, or — in the worst case — result in a connection refusal.
This guide explains what harmonics are in the context of solar inverters, which standards apply, and how modern inverters from Sungrow and Huawei achieve compliance — along with cases where additional external filtering is required.
A pure AC power supply is a sine wave at the fundamental frequency — 50 Hz in Europe, Asia, Africa, and Australia; 60 Hz in North America. Harmonics are voltage or current components at integer multiples of the fundamental frequency: 2nd harmonic = 100 Hz, 3rd = 150 Hz, 5th = 250 Hz, and so on.
Solar inverters use pulse-width modulation (PWM) switching to synthesise the AC output from the DC input. Imperfect synthesis — particularly at high switching frequencies with limited LC filter bandwidth — produces harmonic distortion. The Total Harmonic Distortion (THD-I) of the current waveform is defined as:
THD-I (%) = √(I₂² + I₃² + I₅² + … + Iₙ²) / I₁ × 100
where I₁ is the RMS fundamental current and I₂, I₃… are the RMS harmonic components. Grid codes specify limits both on individual harmonic orders and on total THD-I.
The IGBT (Insulated Gate Bipolar Transistor) switches inside a solar inverter commutate at high frequency — typically 8–16 kHz for string inverters and 2–4 kHz for large central inverters. Each switching event creates a step change in voltage that, when decomposed, contains harmonic components. The LC output filter smooths these high-frequency components but cannot eliminate them entirely.
Additional harmonic sources in a solar plant include:
| Standard | Scope | Individual Harmonic Limit (odd) | THD-I Limit |
|---|---|---|---|
| IEEE 519-2022 | USA, point of common coupling (PCC) | 3rd–11th: 4.0%; 11th–17th: 2.0%; 17th–23rd: 1.5% | 5.0% |
| IEC 61000-3-12 | Equipment ≥ 16 A/phase, LV grid | 5th: 10.7%; 7th: 7.2%; 11th: 3.1% | 8.0% |
| IEC 61000-3-2 | Equipment < 16 A/phase, LV grid | 3rd: 2.3 A; 5th: 1.14 A; 7th: 0.77 A (absolute A, Class A) | N/A (per-order limits) |
| EN 50549 / VDE-AR-N 4110 | Europe, MV grid connection | Per national grid code; typically follows IEC 61000-4-7 | Typically ≤ 3% |
| AS/NZS 4777.2 | Australia/NZ, grid-tied inverters | 5th: 4.0%; 7th: 4.0%; 11th–23rd: 2.0% | 5.0% |
| GB/T 14549 | China, utility grid | 5th: 5.0% (HV: 3.0%) | Based on system voltage level |
Note: IEEE 519 applies limits at the Point of Common Coupling (PCC) — the metering point shared between the utility and the customer — not at the inverter output terminals. This is critical: a single inverter with 4% THD may produce less than 1% THD at the PCC due to grid impedance attenuation. The standard allows harmonic emission proportional to the short-circuit ratio (Isc/IL) at the PCC.
Tier-1 inverter manufacturers publish THD-I performance data in their specifications. Typical values for modern string inverters at rated power:
These figures typically satisfy IEEE 519 and IEC 61000-3-12 requirements at the inverter terminals. At the PCC, performance will generally be even better due to grid stiffness and transformer impedance.
External filtering becomes necessary in three main scenarios:
When the short-circuit ratio at the PCC is low (Isc/IL < 10), the grid impedance is high and harmonic voltages become significant even with low harmonic current injection. Utility interconnection studies using load flow and harmonic penetration analysis (per IEEE 519-2022 Annex A) are required to confirm compliance. In these cases, passive LC harmonic filters or active power filters (APF) at the PCC may be required.
A 100 MW solar farm may have 300–400 string inverters all feeding into a common MV bus. Even if each inverter meets its own THD spec, aggregated harmonic injection can exceed PCC limits. Harmonic currents from different inverters partially cancel (due to random phase diversity) but not entirely — allowance factors in IEC 61000-3-6 and IEEE 519 account for this but studies are still needed for large plants.
Industrial sites with variable frequency drives, arc furnaces, or large UPS systems already have elevated background harmonic distortion. Adding solar inverter harmonic injection to an already-polluted grid can push total distortion above limits, requiring a combined harmonic mitigation study.
| Inverter Model | Rated Power | THD-I (Full Load) | THD-I (20% Load) | DC Injection |
|---|---|---|---|---|
| Sungrow SG350HX | 350 kW | <3% | <5% | <0.5% |
| Huawei SUN2000-330KTL-M3 | 330 kW | <3% | <4% | <0.5% |
| SMA Sunny Tripower 150 | 150 kW | <3% | <4% | <0.5% |
| GoodWe GW250K-HT | 250 kW | <2.5% | <4% | <0.5% |
| SolarEdge SE100K | 100 kW | <3% | <5% | <0.5% |
The step-up transformer between inverter LV output and MV distribution plays an important role in harmonic management. Key considerations:
Econo Solar's procurement team works with customers to specify transformers with appropriate K-factor ratings and winding configurations for grid code compliance. For a full BOM including inverters, transformers, and harmonic mitigation equipment, request a quote here.
Modern solar inverters are among the cleanest grid-connected devices available. With THD-I below 3% at full load, they compare favourably to variable frequency drives (VFDs) at 5–30% THD, switch-mode power supplies (10–150% THD), and fluorescent lighting (15–30% THD). The main risk is aggregated injection from many inverters at a common PCC in a weak grid, which requires harmonic penetration studies for large plants exceeding 1 MVA at a single connection point.
THD-I (current THD) measures harmonic distortion of the current waveform injected by a device. THD-V (voltage THD) measures the resulting distortion of the grid voltage waveform at the PCC. IEEE 519 sets limits on both: THD-V limits are 5% (general systems) or 3% (special systems), while THD-I limits depend on the Isc/IL ratio. In practice, meeting THD-I limits at the inverter level generally ensures THD-V limits at the PCC are also met for most grid connections.
Yes — most inverters exhibit higher THD-I at low power levels (below 20–30% of rated output). This is because the dead-time distortion becomes proportionally larger relative to the fundamental current, and the LCL filter operates in a less optimal region. Sungrow and Huawei use adaptive dead-time compensation and active damping algorithms that significantly improve partial-load THD compared to older inverter generations. Check the manufacturer's THD-I vs. load curve (not just the rated-power figure) when evaluating inverters for sites with frequent low-irradiance operation.
Econo Solar procures Sungrow, Huawei, GoodWe and SMA inverters direct from the factory with full grid code compliance documentation. Get a quote in 24 hours.
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