Harmonics in electrical networks

Accordions — These are current or voltage components whose frequency is an integer multiple of the fundamental (fundamental) frequency of the power grid (usually 50 Hz or 60 Hz). For example, a 2nd order harmonic is 100 Hz, a 3rd order harmonic is 150 Hz, etc.

Ideally, a power system has sinusoidal current and voltage waveforms, but nonlinear loads cause distortions that generate harmonics. The degree of this distortion is measured by the total harmonic distortion (THD — Total Harmonic Distortion), usually expressed as a percentage.

Why do harmonics occur?

  • Nonlinear loads — main source: power converters, inverters, UPS, power supplies, LED lighting, IT equipment, industrial drives. All of them consume current in short pulses, not smoothly.
  • Symmetrical distortions usually generate odd harmonics (3, 5, 7…), and even (2, 4, 6…) are almost absent. The presence of even — signal of a defect.
  • Harmonic multiples of three (3rd, 9th, 15th) in three-phase systems are not compensated and create excess current in the neutral, which can lead to overheating of the neutral, losses, and even fire hazards.

Historical background

Historically, power supply was based on linear loads — resistors, coils, traditional incandescent lamps. They supported sinusoidal current and voltage waveforms.

Starting in the mid-20th century, nonlinear electronic devices (transistors, diodes, PWM converters, LED lighting, etc.) became widespread, which increased the level of harmonic distortion in networks.

What problems do harmonics cause?

  • Equipment Overheating: Transformers, cables, and neutrals can overheat due to eddy and additional losses.
  • Failures and Damage: Capacitors, contactors, fuses, and sensitive electronic devices can fail or operate incorrectly.
  • Resonances: In networks with compensation capacitors, unexpected resonances are possible, which create overcurrents and overvoltages.
  • Electromagnetic Interference (EMI): Electromagnetic interference can interfere with other devices, reducing power quality.

How to deal with harmonics?

  • Passive filters: circuits with inductors and capacitors tuned to specific harmonics; effective and relatively cheap.
  • Active filters: more complex, but capable of adaptively compensating for a wide range of harmonics by injecting countercurrents.
  • Special transformers: e.g. «zigzag», which reduces the impact of multiples of three harmonics in the neutral.
  • System analysis: Regular monitoring of the harmonic spectrum (e.g. up to the 50th harmonic) allows you to identify sources of distortion and adjust the system.
  • Additional measures — phase balancing, load adjustment, optimization of reactive power compensation schemes.

Conclusion

Question Answer
What are harmonics? Frequencies that are multiples of the fundamental that distort the sinusoidal shape.
Why do they occur? Due to nonlinear loads — electronics, drives, LED lighting.
What can happen? Overheating, failures, resonances, reduced equipment life.
How to fight? Filters (passive/active), transformers, system analysis, balancing.