Current Harmonics and Voltage Harmonics

Total harmonic distortion

• The harmonic distortion is expressed by total harmonic distortion            ( THD ). 
• It is defined as the ratio of the RMS voltage of all harmonics and RMS voltage of fundamental frequency. 
• The THD of the system voltage is given by

       THD = √ ( V₂² + V₃² + V₅² +…….. +  V_n² ) / V₁

       Where

       V_n = nth Harmonic voltage in RMS

       V₁ = Fundamental frequency voltage

Current harmonic ( THD )

• It is defined as the ratio of the root mean square of the current harmonic and fundamental current.

       THD = √ ( I₂² + I₃² + I₅² +…….. +  I_n² ) / I₁

Origin of Current Harmonic

The origin of the current harmonic is due to non linear load such as

• UPS
• Television
• Variable Speed drive
• Welding
• Inverter
• Semiconductor circuit

Therefore the current waveform for such type of load is fundamental frequency and of its multiple frequency.

Voltage harmonic

• The voltage harmonic does not produce due to non linear load. 
• The current harmonic flows through system impedance and this will result in voltage harmonic drop across impedance. 
• This will distort also supply voltage waveform, in this way voltage harmonics are generated.

Sources of Voltage Harmonic

• Long cable
• High impedance equipment
• High impedance transformer

Importance of Impedance

• The harmonic voltage distortion in the grid is very low because the grid impedance is very low whereas the source impedance of the DG set is very high therefore the voltage harmonic distortion is very high in spite of small current harmonic.

Effect of Current Harmonic

• The current harmonic increases the power losses due to RMS current flows through circuit. 
• This current affects entire distribution system.
• They may increase hysteresis loss and eddy current loss in the transformer and motor resulting heating also increase. 
• This will cause overloading in the neutral conductor, false tripping of circuit breaker, and interference with communication lines.

Effect of Voltage Harmonics

• The voltage harmonics affects irrespective type of load and affects entire system.
• It affects sensitive equipment as they introduce voltage distortions. 

Voltage harmonics distortion problems

• Electronics loads : Malfunction
• Motors : Mechanical Fatigue
• Transformers : Increased noise, Possible insulation failure

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