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

        I_n = I / n

       Where n = harmonic

                  I = Fundamental current

                 I_n = nth harmonic current

• The calculation of harmonic current according IEC is calculated as below

               I_n = 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

• The generation of harmonic due to voltage sources such as alternator, UPS even though the load is linear. 

You may also like :

Compare Power Transformer - Distribution Transformer

Compare Synchronous Motor - Induction Motor

Torque - Slip Characteristic of Induction Motor

Crawling and Cogging in the Induction Motor

Auto Transformer and Saving in Copper