**6. Simulation Results**

Two types NPC topologies with a Direct Connection NPC cell or an Indirect Connection NPC cell are provided based on the topologies provided in Part I of this paper. To further verify the theoretical analysis in the coming sections, simulations based on two proposed topologies are made, and the simulation results are given. One is the H6 topology (R5) and the proposed H6 topology with a Direct Connection NPC cell (R5S2-2) under USPWM. The other is the proposed H8 topology (R13) and H8 topology with an Indirect Connection NPC cell (R11S1) under DFUSPWM. 0shows the simulation parameters.

#### *6.1. H6 Topology Without*/*With Direct Connection NPC Cell under USPWM*

Figure 27 shows the H6 topology without/with a Direct Connection NPC cell. The H6 topology R5 is illustrated in Figure 27a. Switches TP2 and TN3 are used to allow the freewheeling current to flow. The freewheeling branch is cut o ff and the Direct Connection NPC cell is injected into the topology R5 to form R5S2-2, as shown in Figure 27b.

**Figure 27.** Topology H6 without /with NPC cell under USPWM. (**a**) R5; (**b**) R5S2-2 (new).

Figure 28 shows the waveforms of CM voltage and leakage current. In *t*0–*t*1, the topology H6 without the NPC cell works, and the CM voltage (*V*AN + *V*BN)/2 does not remain constant. The CM voltage includes high frequency resonant voltage in freewheeling mode, as terminals A and B are floating (*V*AN = *V*BN). The leakage current is 100 mA. In *t*1–*t*2, the topology H6 with the NPC cell works and the CM voltage (*V*AN + *V*BN)/2 always remains constant. The leakage current is reduced from 100 mA to 2 mA.

**Figure 28.** Waveforms of grid current, terminal voltage, CM voltage, and leakage current.

Figure 29 shows the waveforms of the grid current, terminal voltage, CM voltage, and leakage current with the parameters listed in Table 7. The THD of the grid current is about 1.12%. As we can see, the CM voltage is maintained at 200 V with small fluctuations over the whole power line period; thus, the RMS value of the leakage current is only 3 mA. Figure 30 shows the waveforms when the load steps at *t* = 0.06 s. The THD is still 1.12%. The CM voltage and leakage current have the same values as those in Figure 29.

**Figure 29.** Waveforms of grid current, terminal voltage, CM voltage, and leakage current.

**Table 7.** Simulation Parameters.


**Figure 30.** Waveforms of grid current, terminal voltage, CM voltage, and leakage current in the case.

#### *6.2. H8 Topology Without*/*With Indirect Connection NPC Cell under DFUSPWM*

Figure 31 shows the topology H8 without/with "Indirect Connection" NPC cell under DFUSPWM.

**Figure 31.** Topology H8 without /with NPC cell under DFUSPWM. (**a**) R13 (new); (**b**) R11S1 (new).

Figure 32 shows the waveforms of CM voltage and leakage current. It may be observed that the topology H8 without the NPC cell works, and the CM voltage is not constant between *t*0 and *t*1. The CM voltage contains high frequency resonant voltage in freewheeling mode due to the floating terminals A and B. The leakage current is 100 mA. In *t*1–*t*2, the topology H8 with NPC cell works, and the CM voltage always remains constant. The leakage current is reduced from 100 mA to 3 mA.

**Figure 32.** Waveforms of grid current, terminal voltage, CM voltage and leakage current.

Figure 33 shows the waveforms of the leakage current, voltage *V*AB, and PWM signal. The same PWM signals are provided to couple switches TP1 and TP3, TP2 and TP4, TN1 and TN3, as well as TN2 and TN4. To achieve good clamping performance, complementary PWM signals are given to couple switches TP1 and TN2, as well as TP2 and TN1. The frequency of voltage *V*AB is double the switching frequency.

**Figure 33.** Waveforms of leakage current, voltage *V*AB, and PWM signal.
