Every grid-tied inverter converts DC to AC by switching at high frequency. That process is not perfectly clean, and it injects harmonic currents into your electrical system.
In most installations this is a non-issue. Compliance limits are strict and modern equipment meets them comfortably. The cases where it becomes a problem share recognisable characteristics.
Inverters sold for grid connection must meet harmonic emission limits – IEEE 519 in the US, IEC 61000-3-12 and related standards in Europe, referenced through national grid codes. Typical requirements hold total harmonic distortion of current below about 5% at rated output.
A certified inverter operating normally into a reasonably stiff supply will meet this. The problems arise from interaction effects that individual equipment certification does not capture.
Weak supplies. Harmonic voltage distortion results from harmonic current flowing through source impedance. On a strong supply, impedance is low and distortion is minimal. On a weak rural feeder or behind a long cable run and a small transformer, the same current produces much more voltage distortion.
Resonance with power factor correction. The important one. Capacitor banks and system inductance form a resonant circuit. If the resonant frequency lands near a harmonic the inverters produce – commonly the 5th or 7th – the result is amplification rather than attenuation.
This can produce distortion far worse than either the inverters or the capacitors would cause alone, and it can damage the capacitors themselves. Detuned reactors on the capacitor bank shift the resonant frequency away from problem harmonics and are the standard remedy.
Many small inverters in parallel. Individual compliance does not guarantee aggregate compliance. Emissions can add, and in some cases interact.
Existing distorting loads. Variable speed drives, rectifiers, arc furnaces and induction heating already inject harmonics. Solar adds to an existing background rather than starting from clean.
Intermittent problems that correlate with sunny conditions are the strong hint. If issues appear at midday and vanish in the evening, and the plant runs the same load throughout, the solar is involved.
For a straightforward installation on a strong supply with no capacitor bank, a certified inverter and normal design practice is sufficient. Do not over-engineer this.
Commission a power quality study when any of the following apply:
The study models the system impedance, identifies resonance points and predicts distortion. It is not expensive relative to a project, and it is far cheaper than diagnosing intermittent faults after commissioning.
A power quality recording at the main switchboard for a week before installation gives you a baseline. Repeat after commissioning.
Without a baseline, any subsequent power quality complaint becomes an argument about whether the solar caused it. With one, the question is answerable in an afternoon. This is a cheap insurance policy that almost nobody buys.