Harmonics and Effects of Harmonics

Introduction

       While dealing with alternating voltages and currents ,it had been assumed that they have sinusoidal waveform or shape.Such a waveform is an ideal one and much sought after by the manufactures and designers of alternators, but it is nearly impossible to realize such a waveform in practice.All the alternating waveforms deviate to a greater or lesser degree, from this ideal sinusoidal shape,Such waveforms are referred to as non-sinusoidal or distorted or complex waveforms.
        The fundamental and its higher multiples form a harmonic series.Fundamental wave itself is called the first harmonic.The second harmonic has frequency twice that of fundamental,The third harmonic has frequency thrice that of the fundamental and so on.
fig.
Complex Waveforms
         Complex waveforms are produced due to the superposition of sinusoidal waves of different frequencies.On analysis, it is found that a complex wave essentially consists of 
  • A fundamental wave it has the lowest frequency say f.
  • A number of other sinusoidal waves whose frequencies are an integral multiple of the fundamental or basic frequency like 2f,3f and 4f etc.
fig.

       Even Harmonics .   Waves having frequencies of 2f,4f and 6f etc are called even harmonics.

       Odd Harmonics .    Waves having frequencies 3f,5f and 7f etc are called odd harmonics.

    In alternators which are symmetrically designed we should expect odd harmonics only.
  
Effects of Harmonics
       
       The distortion in the waveform in the load current of any non linear device ( Electrical element which do not have linear relationship between current and voltages) causes similar changes in the voltage waveform relative to the harmonic impedance of the source network.This voltage distortion affects both the current and voltage for all other loads connected to that system.
        The common effects of such harmonic distortion are as follows.
Motors and Generators
       Generators and motors are adversely affected by harmonics in the networks to which they are connected.
Typical effects are
  • Increase heating due to iron and copper losses at the harmonic frequencies.
  • Higher audible noise emission as compared with sinusoidal excitation.
  • Harmonic current in the rotor.
Transformers

        The harmonics generated by non linear load will impose non-sinusoidal current on the power transformers that supply such loads, resulting in a substantial increase in losses and temperature rise.
Capacitors
  
        Effects of harmonics on capacitors and capacitor banks are as follow.
  • Resonance imposes considerably higher voltages ans currents in capacitors.
  • The capacitor bank acts as a sink for higher harmonic currents which increases the heating and dielectric stresses.
  • These losses shortened the capacitor life.
Power Cables
       
       Effects of harmonics on power cables are as follow
  • Cable involved in system may be subjected to voltage stress and corona.
  • Increased heating due to higher rms current ,skin effect and proximity effect.

       

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