Voltage and Current Distortion Quiz

Harmonics in Power Systems

• Power systems operate at frequencies of 50 or 60Hz, but certain loads produce currents and voltages with frequencies that are integer multiples of the fundamental frequency, known as power system harmonics.

Sources of Harmonics

• The most common sources of harmonics are power electronic loads, such as:
  • Adjustable speed drives (ASDs)
  • Switching power supplies
• Other sources include:
  • Magnetic core equipment (e.g. transformers, electric motors, generators)
  • Conventional equipment (e.g. arc furnaces, arc welders, high-pressure discharge lamps)
  • Electronic and power electronic equipment

Characteristics of Harmonics

• Harmonics are steady-state, periodic phenomena that produce continuous distortion of voltage and current waveforms.
• Harmonics can cause:
  • Malfunction or loss of efficiency in computers, communication equipment, and other power systems
  • False tripping of ground fault circuit interrupters (GFCIs)
  • Nuisance tripping of circuit breakers
  • Instrument and relay transformer accuracy issues

Quantities Describing Voltage and Current Distortion

Individual Harmonic Distortion (IHD)

• The ratio of the RMS value of an individual harmonic to the RMS value of the fundamental component.

Total Harmonic Distortion (THD)

• The ratio of the RMS value of the sum of all harmonic components to the RMS value of the fundamental component.
• Often limited to 50 or 25 harmonics, depending on the risk of resonance.

Peak Factor

• The ratio of the peak value to the RMS value of a periodic waveform.
• A value other than 1.41 indicates waveform distortion.

Crest Factor

• The ratio of the RMS value to the average value of a waveform.
• Crest factor indicates waveform distortion.