Monday, 12 September 2016

Suppression of Harmonics in Transformer


In this post we will focus on various methods adopted to suppress harmonics in a Transformer. Harmonics in a Transformer can be suppressed by flowing methods:

Lower Flux Density:

Harmonics in Transformer can be suppressed by having lower flux density in core but it has one disadvantage that, this method of reducing flux density in core will lead to higher cross sectional area of the core and more number of turn for maintain voltage ratio.

Because V = 1.414πfNØ

So if B is reduced then for having a constant value of V, we need to increase cross sectional area A to maintain flux Ø (as Ø = BA) and number of turns N. Thus this method of reducing harmonics is uneconomical and is not used.

Type of Connection:

Triplen harmonics are those which are having frequency multiple of 3. Triplen harmonic current and voltage in the line are suppressed by using Star or Delta connection of the winding in the Transformer.

Primary or Secondary Winding in Delta:

As we know that harmonic voltages have more nuisance effect than harmonic current. Thus every effort is made to suppress the third harmonic voltages in the Transformer.

When any of the Primary or Secondary Winding is connected in Delta, triplen harmonic voltages are suppressed considerably. In view of this, one of the Transformers winding either Primary or Secondary must be connected in Delta to suppress triplen harmonics.

Use of Tertiary Winding:

As seen in previous paragraph, we need to connect either Primary or Secondary must be connected in Delta to suppress triplen harmonics but in case it is not possible to connect either of the Primary or Secondary in Delta then three phase Transformer must be designed to have an extra i.e. tertiary winding connected in Delta. This closed Delta formed by the tertiary winding provides a path for the third harmonic currents and therefore the EMF and flux becomes approximately sinusoidal in nature.

It shall be noted that tertiary winding has no any effect on the fundamental frequency component voltages because the phasor sum of three EMFs mutually displaced by an angle of 120° is zero in the closed Delta connected tertiary winding.

Star Delta Earthing Transformer:

The third harmonic voltages in the two winding Star-Star Transformer can also be suppressed by using Star-Delta Earthing Transformer as shown in figure below.

 

The closed Delta provides a path for the third harmonic currents and hence the EMF and flux remains sinusoidal.


For Star-Star Transformer, the third harmonic voltages can also be suppressed by using 4 wire supply or earthing Transformer Neutral point if Alternator / Generator neutral is grounded. The neutral wire in this case provide a path for the flow of third harmonic current and hence the flux & EMF remain sinusoidal.

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