Harmonics (Power Quality)

Harmonics in electrical power systems refer to voltage or current waveforms that deviate from a pure sinusoidal shape because of distortion created by nonlinear electrical loads. In solar PV systems, harmonics are primarily introduced by inverters, variable-speed motors, LED lighting, EV chargers, and certain electronic equipment.

Excessive harmonics can cause overheating, reduced inverter efficiency, nuisance tripping, transformer stress, and poor power quality. For grid-tied PV systems, maintaining low harmonic distortion is essential for interconnection approval and compliance with standards such as IEEE 519.

In modern solar design workflows, harmonics analysis is important for inverter selection, grid compliance, and electrical modeling — often performed alongside layout and electrical design using tools like Solar Designing.

Key Takeaways

Harmonics represent unwanted frequency distortion that reduces power quality.
Solar inverters, nonlinear loads, and switching devices commonly generate harmonics.
Excessive harmonics cause overheating, inefficiency, equipment stress, and grid compliance issues.
THD is the primary metric used to measure harmonic distortion.
Maintaining low harmonic levels is essential for safe, efficient, and grid-approved solar PV operation.

What Are Harmonics in Power Quality?

Harmonics are unwanted frequencies in the electrical system that are integer multiples of the fundamental AC frequency (50 or 60 Hz). Instead of a clean sine wave, these higher-frequency waveforms distort the power system and reduce efficiency.

In solar PV systems, harmonics typically originate from:

Grid-tied inverters
Switching components inside power electronics
Nonlinear customer loads
Improper grounding or wiring
Poor inverter filtering

Harmonic distortion is measured using Total Harmonic Distortion (THD), which quantifies how far the waveform deviates from ideal.

Related concepts include Solar Inverter, Voltage, and Stringing & Electrical Design.

How Harmonics Work

1. Nonlinear loads distort the waveform

Devices that draw current in pulses or irregular shapes generate harmonics.

2. Harmonics propagate through the AC system

They travel through conductors, transformers, and distribution panels.

3. Inverters manage harmonics via filters

Modern inverters use switching controls, LCL filters, and firmware to minimize harmonic emissions.

4. Grid codes limit allowable harmonic distortion

Utilities often require THD < 3–5% at the point of interconnection.

5. Excessive harmonics cause performance and safety issues

This includes overheating cables, tripping protection devices, or decreasing inverter lifespan.

In electrical site evaluations, harmonics may be measured during load analysis using specialized power analyzers. See Load Analysis.

Types / Variants of Harmonics

1. Voltage Harmonics

Distorted voltage waveforms caused by nonlinear loads or system impedance.

2. Current Harmonics

More common in solar; created by inverter switching and customer loads.

3. Odd Harmonics

3rd, 5th, 7th, 9th…

Usually the most problematic in distribution systems.

4. Even Harmonics

2nd, 4th, 6th…

Less common but harmful when present.

5. Interharmonics

Frequencies between harmonic multiples; can arise from variable-speed equipment.

6. Total Harmonic Distortion (THD)

The most common metric for evaluating harmonic performance in PV systems.

How Harmonics Are Measured

Harmonics are quantified using:

Total Harmonic Distortion (THD)

The ratio of all harmonic frequencies to the fundamental frequency.

Harmonic Order (n)

Example:

3rd harmonic = 3 × fundamental frequency
5th harmonic = 5 × fundamental frequency

Harmonic Spectrum

Shows which harmonic orders are present and their magnitude.

Point of Common Coupling (PCC) Measurements

Utilities evaluate THD at the grid interconnection point.

Typical limits:

Voltage THD < 5%
Current THD < 8%

Typical Values / Ranges

Well-designed PV systems maintain THD under utility limits to avoid interconnection rejection.

Practical Guidance for Solar Designers & Installers

1. Select inverters with low THD ratings

Modern grid-tied inverters include filtering to minimize distortion.

2. Avoid overloading circuits

High current distortion leads to heat and tripping.

3. Use proper conductor sizing

Harmonics increase RMS current, requiring careful electrical design.

4. Balance phases in three-phase systems

Uneven loads amplify harmonic issues.

5. Keep DC/AC ratios within recommended limits

Extreme oversizing can increase switching noise.

6. Evaluate harmonics during commissioning

Power analyzers help diagnose THD issues onsite.

7. Follow utility harmonic limits

Interconnection approval can be delayed or denied if THD exceeds grid standards.

8. Incorporate harmonics into electrical layout planning

SurgePV assists with cleaner grid-tied design workflows and BOS routing:

Solar Designing

Real-World Examples

1. Rooftop PV System Causing Breaker Tripping

A residential solar inverter with poor filtering produces high current harmonics, overheating the panelboard conductors until a breaker starts nuisance-tripping.

2. Commercial Building with LED Lighting & Solar

A shopping center experiences voltage distortion due to combined LED drivers + inverter harmonics. A harmonic filter is installed, reducing THD to acceptable levels.

3. Utility PV Farm Interconnection Delay

A 10 MW solar project fails harmonic testing at the PCC. Engineers adjust inverter firmware and add line reactors to reduce THD so the utility approves the interconnection.