## 4 Simulation Results and Analysis  
In this section, the simulation results of conventional and the proposed GCI current control methods are presented. SRFPI current control is implemented as a conventional GCI current control method for performance comparison because SRFPI control is widely used for GCI current control applications and is a similar control system structure to the proposed GCI current control system. CFFC and DRC are also applied to the SRFPI current control method for fair performance comparison. Simulation is implemented using PLECS software and the GCI system parameters used during simulation are summarized in Table 2.  
Simulation results at a normal steady state operating condition with and without the CFFC and DRC are shown in Figure 7a,b, respectively. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & **Value** & **Parameter** & **Value** \\ \hline Grid voltage (V\({}_{\text{g}}\)) & 50 [V\({}_{\text{AC}}\)] & Inverter side inductor (L\({}_{1}\)) & 1.5 [mH] \\ \hline Frequency & 50 [Hz] & Filter capacitor (C\({}_{\text{f}}\)) & 2.75 [μF] \\ \hline DC link voltage & 180 [V\({}_{\text{DC}}\)] & Damping resistor (R\({}_{\text{d}}\)) & 4.9 [\(\Omega\)] \\ \hline Switching frequency & 10 [kHz] & Grid side inductor (L\({}_{2}\)) & 1 [mH] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: GCI system and controller parameters. Figure 7: Simulation results of GCI current control system for the proposed stationary reference frame proportional-integral (PI) current control and synchronous reference PI (SRFPI) current control (**a**) without and (**b**) with the CFFC and DRC. As seen in Figure 7a, steady state error and phase lag occur when the stationary reference frame PI current control is used for current control of a GCI system. However, the command tracking performance of the proposed GCI current control system, the stationary reference frame PI current control with the CFFC and DRC, is improved as shown in Figure 7b. The effect of the CFFC and DRC is verified through the simulation in Figure 7 and the simulation results are summarized in Table 3 below. When the CFFC and DRC are not applied to the stationary reference frame GCI current control system, the steady state error and phase delay are shown, and the errors are larger than the results from the SRFPI grid current control system. By applying the CFFC and DRC into the stationary reference frame GCI current control system, the command tracking performance can be improved as much as the performance of the SRFPI grid current control system. Though CFFC and DRC are applied, approximately 3 degrees of phase delay appears when the proposed control method is used as shown in Table 3. At a power factor point of view, 3 degrees corresponds 0.998 power factor (cos 3\({}^{\circ}\)), which is 0.2% power factor error or difference and do not significantly affect power factor. Although 0.2% error in power factor appears using the proposed GCI current control method, relatively simple GCI current control system can be developed by avoiding reference frame transformation, cross-coupling issues and use of PLL which are necessary for implementation of the SRFPI grid current control system. Figure 8 shows the simulation results when a line to ground fault occurs in a grid system. When grid fault occurs, the overcurrent occurs. Figure 9 shows the simulation results at the moment of grid fault in greater detail. Figure 9a shows the current at the time of fault without CFFC and DRC, while Figure 9b shows the result with the control method applied. Table 4 compares the transient response for each current controller. Without CFFC and DRC, 150% overcurrent occurs in the proposed method and SRFPI current controllers. With the application of CFFC and DRC, overcurrent decreased by about 10%. Because of the increased likelihood of remaining under the commercial inverter trip condition, it is more likely that linkage with the grid would have been maintained.