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. 
  • neutral-shifting.png
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 Vyn due to neutral shift. 
  • The neutral pulled towards secondary winding y resulting reduction of Vyn but increase of Vrn and Vbn under short circuit condition. 
  • The primary voltage VYN reduces to zero whereas VRN and VBN 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.

You may also like :

No comments:

Post a Comment