THREE PHASE TRANSFORMERS CONNECTIONS Questions 1 For the Y

THREE PHASE TRANSFORMERS CONNECTIONS Questions

1) For the Y- ? and ? -Y connections determine the phase shift between the primary and secondary line-to-line voltages. What are the necessary conditions for parallel operation of two three-phase transformers?

2) What are the problems associated with the Y-Y three-phase transformer connection? Discuss the harmonics in the Y-Y connection and your observation. With isolated neutrals do the phase voltages contain third harmonics?

3) For the Y-? connection, why was the loop voltage nearly zero with a grounded neutral and not zero with an isolated neutral?

4) In a ? - ? connection, can one of the transformers be removed with the remaining ones operating satisfactorily? Why? What is the name of this connection?

CA cn ab 30 AB an bn A-connected HV side Y-connected LV side Figure 2 Phase shift of line voltages for the -Y connection shown in Fig. 1

Solution

1. Conditions for parallel operations of two three-phase transformers

a. Same voltage ratio of transformers

b. Same percentage impedence

c. Same polarity

d. Same phase sequence

2.

Thus harmonic phase voltages may be high in-shell type 3-phase transformers.