Harmonics

What are harmonics

Harmonics are sinusoidal components of voltage or current at integer multiples of the fundamental frequency. In a 50 Hz Indian grid, the 3rd harmonic is at 150 Hz, the 5th at 250 Hz, the 7th at 350 Hz, and so on. Any periodic non-sinusoidal waveform can be decomposed into a sum of harmonics through Fourier analysis.

A perfect sine wave has only the fundamental. A distorted waveform has the fundamental plus harmonic content. Power systems are designed for sine waves. Harmonics cause heating, vibration, equipment ageing and measurement errors. Standards like IEEE 519 and IEC 61000 limit harmonic content to keep power quality manageable.

Solar inverters, like all switching converters, generate some harmonic content as part of their normal operation. Output filters, multilevel topologies and high switching frequencies keep modern inverter harmonic output well within applicable limits.

Harmonics explained simply

Imagine pure music: a single sine-wave tone. Now imagine you add a second tone three times the frequency, a third tone five times the frequency, all mixed in. The waveform no longer looks like a clean sine wave. It looks ragged.

The grid is supposed to deliver a clean 50 Hz sine wave. Anything that distorts it, switching electronics in inverters, computers, drives, fluorescent lamps, adds ragged shapes that are mathematically expressible as a stack of higher-frequency sine waves. Those are harmonics.

A small amount of harmonic ripple is harmless. Too much causes real problems: transformers run hot, motors vibrate, capacitors blow up, breakers nuisance-trip, and meters read wrong. The measure engineers quote to summarise all this ripple in one number is Total Harmonic Distortion (THD).

Why harmonics matter

Equipment lifetime. Heat from harmonic currents shortens transformer, motor and capacitor life. Even 5 percent THD can halve capacitor life.

Neutral overload. Triplen harmonics (3rd, 9th, 15th) add in the neutral of three-phase systems, sometimes carrying more current than each phase.

Resonance. Harmonics excite LC resonances on the network, causing voltage spikes.

Metering. Older meters mis-record under high distortion. Even modern meters specify accuracy only up to defined THD levels.

Anti-islanding interference. PLL inside inverters can lock onto harmonic components in highly distorted grids, causing trips. Facilities that also manage power factor penalties alongside harmonics often specify inverters with reactive power support; see Reactive Power Support from Solar Inverters, India Grid Compliance for the CEA Grid Code requirements.

DISCOM compliance. Indian large C&I and utility connections include harmonic limits referencing IEEE 519, and hospital or data-centre loads typically demand tighter limits still; see THD in Solar Inverters, Why It Matters for Sensitive Loads for the sensitive-load thresholds.

How harmonics arise

1. A non-linear load draws current in pulses rather than smoothly.
2. The pulsed current contains the fundamental plus harmonics.
3. The current flows through source impedance, dropping harmonic voltage on the line.
4. The harmonic voltage appears at other loads on the same feeder.
5. Sensitive loads experience distorted voltage and may malfunction.

A switching converter inside a solar inverter operates at 16 to 50 kHz, well above audio frequencies. Without a filter, switching ripple would inject harmonics at the switching frequency and its sidebands. An LCL filter attenuates this. Lower-order harmonics from PWM control imperfections are kept within limits through modulation design.

Technical deep dive

Common harmonic orders

IEEE 519 current limits

Voltage limits

IEEE 519 voltage limits for systems below 69 kV: 3 percent individual, 5 percent total. CEA Grid Code uses equivalent limits with state-specific adjustments.

Inverter output filters

Real-world applications

Solar inverter compliance. All BIS-certified Indian inverters meet THD below 3 percent at rated output.

Industrial reactive and harmonic compensation. Solar inverters with active filter mode compensate factory harmonic loads.

Data centre power quality. Multiple non-linear loads need clean voltage. Solar plus filter approach maintains it.

EV charging stations. DC fast chargers are non-linear loads. Co-located solar with active filtering manages site harmonics.

Hospital power quality. Sensitive imaging equipment requires low harmonic distortion. Solar inverter design must demonstrate compliance.

Advantages of low-harmonic inverters

Meets grid code with margin.

Reduces equipment heating and ageing.

Supports neighbouring sensitive loads.

Avoids resonance with capacitor banks.

Compatible with future smart grid functions.

Limitations and trade-offs

Lower THD requires larger filter inductors and higher switching frequencies, increasing cost and losses.

Active filtering capability uses inverter kVA capacity, reducing solar export headroom.

Filter design depends on grid impedance, which varies. Off-spec sites can resonate.

External harmonic loads can saturate the inverter active filter.

Common problems

High THD at light load. Inverters often have higher THD at low output. Quoted THD figures are at rated load.

Voltage resonance with site capacitor banks. Capacitor failure within months.

Neutral current overload from triplen harmonics in offices with many single-phase IT loads.

Meter error at high THD. Class 0.5 meters can drift to 1 percent error above 10 percent THD.

PLL false trips during severe grid distortion events.

Filter capacitor failure in coastal humidity. Replace at 7 to 10 years.

Best practices

Specify THD compliance in inverter purchase contracts.

Measure THD at commissioning with power quality analyser, at multiple load levels.

Avoid placing fixed capacitor banks near solar inverter output without resonance analysis.

Use active filtering inverters at sites with severe external harmonic loads.

Audit harmonic levels annually during preventive maintenance.

Coordinate with DISCOM if grid voltage THD is high upstream.

Comparison: harmonic sources and mitigation

Indian market context

CEA Grid Code 2019 references IEEE 519 and IEC 61000 for harmonic limits applicable to distributed generators. Inverter manufacturers typically declare THD against these references on the datasheet.

Major Indian inverter brands like Sungrow, Sineng, Solis, Goodwe, Growatt and Tata Power Solar list THD below 3 percent in datasheets.

State DISCOM commissioning protocols often include a harmonic measurement at point of common coupling.

C&I customers using VFDs alongside solar increasingly specify active filtering inverters to manage site harmonics holistically.

Standards and certifications

Common mistakes

Quoting inverter THD at rated load and ignoring light-load THD.

Installing capacitor banks without resonance check.

Ignoring neutral current measurements in three-phase systems.

Skipping harmonic measurement at commissioning.

Using budget inverters without filter specification.

Treating harmonics as a maintenance-only concern.

Forgetting that grid-side voltage harmonics affect every load, including the solar inverter.

Key takeaways

Harmonics are unwanted higher-frequency components on AC power, generated by switching converters and non-linear loads. Modern solar inverters control output harmonics well below the harmonic limits referenced in the CEA connectivity regulations (IEEE 519), using LCL filters and high switching frequencies. Site design, capacitor bank placement and external harmonic loads decide whether actual site harmonic levels stay within compliance.

Further reading

For how Harmonics plays out in real projects, these guides go deeper:

• THD in Solar Inverters, Why It Matters
• Solar Inverter Specifications Decoded: 15 Parameters
• How Does a Solar Power System Work in India?
• String vs Microinverter, Indian Roof Reality

Sources

• IEEE Std 519-2014, IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems.
• IEC 61000-3-2, IEC 61000-3-6, IEC 61000-4-7, IEC 61000-4-30, harmonic emission and measurement standards.
• CEA Technical Standards for Connectivity to the Grid (Amendment) Regulations 2019.
• Arrillaga J., Watson N. R., Power System Harmonics, John Wiley and Sons.
• NREL Technical Report TP-5D00-71341, Inverter Performance for High Penetration PV.
• IEA PVPS Task 14 publications on PV grid integration and power quality.
• Sandia National Laboratories Report SAND2014-0840, PV Inverter Modelling for Harmonic Analysis.
• Akagi H., Watanabe E. H., Aredes M., Instantaneous Power Theory and Applications to Power Conditioning, Wiley-IEEE Press.