5.2.2. Modified PWM Method

The simulation results for the modified Min–Max method and third harmonic injection methods as applied in the three-level NPC inverter. Figure 10a–d obtained waveform of CMC using modified Min–Max method, waveform CMV using modified Min–Max method, waveform of CMC using modified third harmonic injection methods, and waveform of CMV using modified third harmonic injection methods, respectively. Figure 10e,f show the FFT analysis of CMV and CMC with filter, respectively. It is observed that the THD of the CMV and CMC are 12.95% and 5.91%, respectively. Before modifying the PWM scheme, the common mode voltage and current of the inverter are experimentally investigated and shown in Figure 11a. Without modification, the RMS values of the common mode current and common mode voltage are 1.95 A and 4.73 V, respectively.

**Figure 10.** Simulation results: (**a**) CMC using the modified Min–Max method; (**b**) CMV using the modified Min–Max method; (**c**) CMC using the modified third-harmonic injection method; (**d**) CMV using the modified third-harmonic injection method; (**e**) with filter, common mode voltage FFT analysis; (**f**) with filter, common mode current FFT analysis.

**Figure 11.** Experimental results: common mode current and common mode voltage, (**a**) before modifying the PWM strategy, and (**b**) after modifying the PWM strategy.

After modifying the PWM scheme, the common mode voltage and current of the inverter are experimentally investigated and shown in Figure 11b. After modification, the RMS values of the common mode current and common mode voltage are 452.6 mA and 5.16 V, respectively. In Table 4, the observed results are tabulated and it is clear that, after modifying the PWM method, the common mode current is reduced significantly with a small increment in the common mode voltage.


**Table 4.** Experimental results.

In Table 5, a comparison of PWM strategies for reduction in CMC, CMV, and voltage THD of the three-level inverter is tabulated based on the obtained results. The simulated results show that when the modified PWM method is implemented there is a considerable reduction in CMC. The voltage THD of the system is also observed to be reduced considerably. This satisfies the objective of optimizing the PWM technique to reduce the CMV and current in grid-tied inverters. From the above comparison, it is clear that the modified third harmonic injection approach shows a significant amount of reduction in the CMV and CMC. EMI mitigation techniques are investigated with the aim to reduce the CM voltage and current in PV grid-tied power inverters. The common mode undesirable e ffects for grid-tied inverter systems has been discussed and compared for di fferent PWM schemes. Two small passive filters are connected between the rectifier input and grid neutral point, and in between the grid and output port of the inverter and tested for a three-phase two-level inverter using a passive cancellation method.

**Table 5.** Comparison of PWM strategies for reduction in the CMC, CMV, and voltage THD of the three-level inverter.

