The average load voltage error is depicted on Fig. 6 for the
first phase (u1). The waveform of uk can be seen as the sum
of two contributions: the sign of the k-th leg current –sign(ik)
and average of signs of all output currents 1/3Σsign(ik), considering
modified sign functions. This two contributions are
depicted in Fig. 7. The first one corresponds to unity square
wave signal with period T, but modified with taking into account
the zero values due to zero crossings, and the second
one corresponds to a square wave signal, also modified, with
amplitude 1/n and period T/n.
The Fourier development of the load voltage error can
be obtained as a combination based on these two signals for
the three-phase inverters, leading to: