Harmonics is distortion to the voltage or current waveforms whose frequencies are integer multiples of the system’s fundamental frequency. As an example, the fundamental frequency is 60Hz in North America and 50Hz in Europe.

• Fundamental Frequency (f₁): 60 Hz, 50Hz
• 2nd Harmonic (f₂): 120 Hz, 100Hz
• 3rd Harmonic (f₃): 180 Hz, 150Hz
• …and so on.

Instead of a clean, single-frequency sine wave, harmonics create waveform distortion by overlaying higher-frequencies on top of the 60/50 Hz fundamental.

They’re caused by nonlinear loads: devices where current doesn’t follow the voltage in a linear fashion throughout the AC cycle. This is common in:

• Electronic loads
• Variable Frequency Drives (VFDs)
  
• LED lighting
  
• Computers and office electronics
  
• UPS systems
  
• Battery chargers
  
• Welders and arc furnaces
  

These loads can distort the current waveform which in turn affects the voltage across the impedance of the circuit. Some offer built-in harmonic filtering or mitigation techniques to limit harmonics.

Harmonics are more than just waveform distortion. They carry real physical consequences:

1. Overheating

• Transformers and motors experience excess heating, particularly from 3rd, 5th, and 7th harmonics.
• Harmonics increase eddy currents and core losses, reducing equipment life.
• Neutral conductors can overheat due to triplen harmonics (3rd, 9th, etc.) that add arithmetically in three-phase systems.

2. Mis-operation of Protective Devices

• Circuit breakers, relays, and fuses may trip due to distorted current waveforms.
• Harmonics can affect the timing and sensitivity of protection schemes.

3. Reduced Power Factor

• Harmonic current doesn’t do useful work but still contributes to the apparent power, reducing the true power factor.
• This can increase utility demand charges or require costly correction.

4. Equipment Malfunction

• Drives and inverters may trip or fault due to distorted waveforms.
• Control systems may experience communication interference if harmonic noise enters signal lines.

5. Resonance

• In certain conditions, harmonics can resonate with capacitive or inductive elements in the system.
• This can amplify voltage distortion or current spikes significantly, sometimes leading to insulation failure or capacitor bank damage.

Most PQ meters report:

• Total Harmonic Distortion (THD) for voltage and current – Expressed as a percentage of the fundamental. THD is that one number that represents all harmonic content. Voltage THD is often used for harmonic limits.
• Individual Harmonic Orders – You can view harmonic spectrums to isolate which frequencies are most dominant.
• Crest Factor & K-Factor – Useful for evaluating transformer sizing in harmonic-rich environments.

Passive filters – Tuned to absorb specific harmonic frequencies.
Active harmonic filters – Monitor and inject counteracting waveforms in real-time.
Phase shifting transformers – Can cancel out specific harmonics in balanced loads.
Line reactors or chokes – Limit the rate of current change, reducing harmonic generation at the source.

IEC 61000-4-7 – Defines harmonic measurement techniques.

IEEE 519 – Recent versions adopt the measurement techniques of IEC 61000-4-7, but add 3 second and 10 minute harmonic measurements called Very Short Time and Short Time harmonics. Defines acceptable harmonic current and voltage distortion limits at the point of common coupling (PCC).

IEC 61000-2-4 – This and either IEC standards define compatibility levels of harmonics in low-voltage and other systems.