Can somebody please explain the differences/advantages between three phase three leg AND three phase five leg transformers.

When do we use what. I also read that the three phase 5 leg transformer's zero sequence impedance is almost the same as its posi

 

WB... simply put, the 5-leg design, and yes, there is 4-leg design, overcomes some electrical and mechanical constraints associated with 3-leg transformers. The term 'leg' (also called 'limb') refers to the number of vertical magnetic-structures, called, vertical 'yokes' employed.

The 3-leg design has three yokes. One paired set of high and low-voltage windings or coils (associated with a phase) are assembled on each yoke. The 4-leg and 5-leg designs also have 3 sets of coils, mounted on 3 yokes, but additional yokes are provided without coils.

Summarizing, the 3-leg design has 3-yokes, each with coils. The 4-leg design has 4-yokes, 3 with coils and 1 without. And, the 5-leg design, has 5-yokes, 3 with coils, and 2 without.

Following are the advantages/disadvantages when compared to a 3-leg, core-type, transformer:

1) They are appreciably lower in height, thus facilitating land-transport when travel through tunnels is mandated.

2) The additional yokes, directionally, yield a more symmetrical flux distribution.

3) Additional yokes allow for an increase in coolant.

4) Typically, Zero-Sequence impedance (Z0) is not the same as the transformer's Positive-sequence (Z1) impedance.

Regards, Phil Corso (epsicon [at] aol [dot] com)