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autotransformers single three phase Brioschi electric transformers
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guide to the connection groups

Connection STELLA - STELLA, Group Yy0 - Group Yy6
Relatively less used because of a tension instability of the nautral point due to:
a - single-phase loads connected between line and neustral
b - differences in the magnetization currents
c - currents of third harmonic

Connection TRIANGOLO - STELLA, Group Dy11 - Group Dy5
It is one of the most used connections, used also for the distribution transformers because it offers all benefits due to the secondary neutral and none of the disadvantages of the connection star-star. Furthermore the triangle at the primary guarantees a correct magnetization of the nucleus and a secondary tension through sinusoidal.

Connection TRIANGLE - TRIANGLE, Group DdO - Group Dd6
A less used connection the two previous ones because it has the disadvantage of not having the neutral at the secondary (it does not enable to use a stellar tension).
As regards to the working, the connection triangle-triangle offers the possibility of a free circulation of the currents of third harmonic of excitation without them showing outside the terminals.