*2.1. Principle of USPWM*

Figure 5 shows the principle of USPWM. The di fferential-mode voltage *V*AB has three levels: <sup>+</sup>*V*PN, 0, and −*V*PN. There are four modes in a total line-frequency period. The inverter is working in positive conduction (PC) mode and negative conduction (NC) mode when *V*AB equals to <sup>+</sup>*V*PN and −*V*PN. There are two modes if *V*AB = 0: one is the positive freewheeling (PF) mode when the grid current is positive, and the other is the negative freewheeling (NF) mode when the grid current is negative.

**Figure 5.** The principle of USPWM.
