## 21/03/2018

### Linear and Non - Linear load : Source of harmonics

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

• 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.

#### ( 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

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