Phase U is taken for example to be analyzed. When the
upper arm of phase U is opened, in the other word, the current
flows through T1 or D1 , the voltage between the point U and
the point N1 is uUN1 = Ud / 2 . When the nether arm of phase U
is opened, the voltage between the point U and the point N1 is
uUN1 = −Ud / 2 . The voltages of the three phases are