21 March 2018

Linear and Non - Linear load : Source of harmonics

Linear loads

  • It is a load in which impedance remains constant. The current input is sinusoidal if the applied voltage is sinusoidal.

Non linear loads

  • It is a load in which impedance varies for a given time period. The current input is non sinusoidal if the applied voltage is sinusoidal.

Sources of harmonics

( a ) Six phase bridge rectifier

  • The current pulse is rectangular in shape which produces harmonic in the order of
         n = 6k ± 1 ( Where k = 0, 1, 2, …. )
        Amplitude of current pulse
        In = I / n
       Where n = harmonic
                  I = Fundamental current
                 In = nth harmonic current
  • The calculation of harmonic current according IEC is calculated as below
               In = I / ( n – 5 / n ) 1.2    5 < n < 11
  • The harmonic current as a percentage of fundamental current

Fundamental current
5th harmonic current
7th harmonic current
11th harmonic current
I
I5
I7
I9
100%
18.9%
11%
5.9%

  • The inductance of the DC circuit affects the harmonic current. Lower value of inductance means high ripple content.

( b ) Alternator

  • The alternator does not provide pure sinusoidal voltage in spite of linear loads. It supplies pre – existing harmonic voltages

( c ) Switch Mode Power Supply ( SMPS )

  • The harmonic current induced by the SMPS depends upon type of load.

( d ) Lighting load ( Fluorescent lamp, Discharge lamp )

  • The harmonic current supplied by the electronic ballast and fluorescent lamp is given as below. 
  • The electronic ballast supply lower harmonic content as compared to switch   mode power supply.

Fundamental current
3rd harmonic current
5th harmonic current
7th harmonic current
I
I3
I5
I7
100%
35%
27%
10%

( e ) Uninterruptible power supply

  • The harmonic current supply by UPS is shown as shown below

Fundamental current
5th harmonic current
7th harmonic current
11th harmonic current
I
I5
I7
I9
100%
33.0%
2.5%
6.1%

( f ) Arc furnace

  • The arc furnace is used for production of steel supplied through either AC or DC. 
  • The arc is unstable, asymmetric and non linear. 
  • It will produce odd and even harmonics. 
  • The level of harmonic depends upon type of furnace, its power rating, process time etc. 
  • When arc furnace is supplied through DC , arc produced being more stable than the AC.

 ( g ) Magnetic saturation in the transformer

  • When the applied voltage to transformer primary is 10% higher than the rated voltage, magnetic circuit will saturate and current waveform distorted resulting generation of odd harmonics.

( h ) Voltage sources

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