# Novel Common-Mode Current Suppression Method in Transformerless PV Grid-Connected System
## 4 Simulation Analysis  
In Matlab/Simulink, a grid-connected PV system of two stages with 5 kW H6 bridge is built. The comparison and analysis of CM current with three conditions of no internal CM loop, single CM loop, and double CM loop are performed using the simulating results. The parameters of the simulation platform are shown in Table 1.  
Figure 9 shows the waveform and spectrum of the CM current when there is no CM internal loop. From the results in Figure 9b, it can be concluded that the main component of the CM current is consistent with the analysis of Section 1. The CM current component is 25.41 mA, which meets the requirement of grid connection. The total distortion is 202.68% (compared with the power frequency component, the same below). In the high frequency component, the modulation frequency section of 10 kHz and 20 kHz is the main component, as well as the resonance frequency component caused by the circuit resonance. Figure 10 shows the waveform and spectrum for the CM current using the single loop method in CM, where the switching frequency component is suppressed and the total harmonic distortion is decreased to 72.79%. However, the harmonic component of 50 order accounts for about 50% of the fundamental component in the CM current. The reason is that \(L_{\text{cm2}}\) and \(C_{\text{PV}}\) produce local resonance. In the resonant frequency, the impedance of the external parasitic branch decreases dramatically compared with in the non-resonant frequency. Hence, the higher frequency components in the CM current are higher.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline \(C_{\text{PV1}},C_{\text{PV2}}/\text{\mu F}\) & 0.25 \\ \(L_{1},L_{2}/\text{mH}\) & 1 \\ \(C_{\text{x1}}/\text{\mu F}\) & 47 \\ \(L_{\text{cm1}},L_{\text{cm2}}/\text{mH}\) & 1.3 \\ \(C_{\text{Y1}},C_{\text{Y2}}/\text{nF}\) & 10 \\ Pre-boost operating frequency/kHz & 10 \\ Post-level H6 bridge operating frequency/kHz & 20 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The relevant parameters of simulation platform.  
Figure 10: The common-mode current waveform and spectrum with single inner loop: (**a**) current waveform; and (**b**) current spectrum.  
Figure 9: The common-mode waveforms and spectrum without the common-mode inner loop: (**a**) current waveform; and (**b**) current spectrum.  
Figure 11 shows the waveform and spectrum of CM current using the dual CM loop method. The total distortion rate of the harmonics is reduced to 23.04%. The main part of the CM current is line frequency component, and the resonant frequency component is suppressed. The component of 10 kHz is less than 10% of fundamental frequency, and the components of other frequencies are no more than 10% as well. The components of switching frequency, resonant frequency and low frequency in the CM current are well suppressed. On the one hand, compared with the single CM inner loop method, the damping of the external parasitic branch is introduced by the damping resistors \(R_{5}\), \(R_{6}\), \(R_{7}\), and \(R_{8}\) in the double CM internal loop increase. On the other hand, the external CM current is shunted by the internal loop paralleled with the external parasitic branch and the component of high resonant frequency in the external CM current is effectively suppressed.