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Harmonics in power system

  • A non sinusoidal waveform can be considered to be composed of several sinusoidal waveforms of fundamental frequency and other harmonic frequencies
  • The frequency of nth harmonic is nth times fundamental frequency
  • Even harmonics, such as 2nd, 4th, 6th, etc.
  • Odd harmonics , such as 3rd , 5th, 7th, etc.
  • The r.m.s. value of nth harmonic is 1/nth of r.m.s. value of fundamental

Causes of harmonics generation

  • Caused by the use of high flux densities in the core of the transformers
  • If the core is of insufficient cross section, the core gets saturated
  • Secondary waveform may be non sinusoidal because of magnetic core
  • Hence the primary requirement of transformer is to provide a sinusoidal secondary voltage waveform which is achieved by
(a)    providing sufficient cross section of core and
(b)    Achieving operating point in the linear region of B/H curve.

Disadvantages of harmonics

  • Harmonic currents cause additional  I2R loss , core loss, magnetic interference with protective relays and interference in communication circuits
  • Harmonic voltage causes increased dielectric stresses, interference with communication circuits and resonance  between inductance of transformer winding and the capacitance of the transmission line.
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