What are the problems associated with the YY threephase tran

What are the problems associated with the Y-Y three-phase transformer connection? How they can be solved? A 100 VA, 110/10 V conventional transformer is to be used to supply power from a 110-V source to a 120-V load. Consider the transformer to be ideal. Sketch the transformer connection that will do the required job. What is the kilovoltampere rating of the transformer in this configuration? Find the maximum primary and secondary currents under this condition.

Solution

(a).

The problems associated with star star connection of three phase transformer are the following:

1. Problem:If the load on the secondary side of the transformer is unbalanced, the phase voltages of load side changes unless the load star point is earthed.

Solution:The difficulty of shifting neutral can be overcome by connecting the primary star point to the star point of the generator.

2.Problem:The primary of the transformer draws a magnetizing current which has third and fifth harmonics. If neutral of primary winding is not connected to neutral of generator, the third and fifth harmonics current cannot flow hence the flux in the core cannot be of sinusoidal wave and, therefore, the voltages will be distorted.

Solution:By connecting primary neutral to the generator neutral, the path for return of these third and fifth harmonics current is provided and, therefore, the trouble of distortion of voltages is overcome.

Due to these problems the star to star connections are rare and used only for small high voltage transformers.