Neutral Shift in the Transformer

Describe the effect on transformer when unbalanced load is connected to Star / Star connected transformer? What is neutral shift? Describe its causes?

• The star – star connected transformer works on satisfactory if the load is balanced. 
• When unbalanced load is applied, neutral shifting causes line to neutral voltage unequal. 

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Let us understand how it becomes? 

• Load is placed between phase y and neutral as shown in the figure A.
• The power to the load has to be supplied by the primary phase winding Y. 
• This primary winding Y cannot supplied required power because it is in series with primary winding R and B whose secondary winding is open circuited.
• The primary winding R and B acts as high impedance therefore primary winding Y provides only little current from the line. 
• Therefore the winding y cannot supply requires power to the load. 
• As the resistance between secondary winding y and neutral is very low, it acts as short circuit path resulting very little current flow through it.
• This is happened due to reduction of secondary voltage V_yn due to neutral shift. 
• The neutral pulled towards secondary winding y resulting reduction of V_yn but increase of V_rn and V_bn under short circuit condition. 
• The primary voltage V_YN reduces to zero whereas V_RN and V_BN increases nearly full primary line voltage. This is known as neutral shifting. 
• These difficulties can be overcome by connecting star point of the primary winding to the star point of the alternator.

How stabilizing neutral provides sinusoidal waveform ( or reduces distortion in the secondary phase voltage ) in the alternator?

• It is necessary to flow sinusoidal flux through the core in order to obtain pure sinusoidal waveform. 
• The sinusoidal flux requires a third harmonic component of the exciting current due to property of core material. 
• As the frequency of third harmonic is thrice the supply frequency, it flows towards / away from the neutral point in all the three transformers. 
• If the primary neutral of the transformer is isolated, triple frequency current cannot flow. 
• Therefore the flux in the core becomes non – sinusoidal and output voltage waveform are distorted. 
• The path for the triple frequency current is provided by connecting star point of the transformer to the star point of the alternator. 
• To avoid this difficulty, a third or tertiary winding is provided with each of the transformer. 
• The kVA rating of the tertiary winding is very small and it always connected in delta to provide path for third harmonic component.

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