# Topology and Control Strategy of PV MVDC Grid-Connected Converter with LVRT Capability
## 4 Simulation and Experimental Results  
A 5 kV/50 kW PV dc-dc module based on SiC MOSFETs and SiC Diodes was developed with proposed dc-dc topology, as shown in Figure 18. Four modules are input parallel and output series connected to form \(\pm\)10 kV/200 kW PV MVDC converter, as shown in Figure 19. The main parameters are shown in Table 3.  
Figure 20 shows the waveforms of the dc-dc module in boost mode and buck mode. In Figure 20a-c, the duty cycles are 0.74, 0.65 and 0.53, respectively. The output voltage is 5 kV. The circuit works in boost mode. Figure 20d shows the waveform of the module in buck mode. The duty cycle of 0.45, the input voltage of the module is 500 V, the output voltage is 2.5 kV. Figure 19: \(\pm\)10 kV/200 kW SiC PV MVDC converter. Figure 18: 5 kV/50 kW SiC dc-dc module. Figure 20: Experimental waveforms of dc module in two modes. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Parameters** & **Value** \\ \hline  & Rated output power \(Pco\) & 200 kW \\  & Rated output voltage \(Vco\) & \(\pm 10\) kV \\ Converter & Input voltage \(Vin\) & 450–850 V \\  & Number of modules & 4 \\  & Output inductor \(Lo\) & 10 mH \\  & Switching frequency \(fs\) & 50 kHz \\ \hline  & Rated power \(P_{0}\) & 50 kW \\  & Rated output voltage \(Vo\) & 5000 V \\ DC module & Output capacitor \(Co\) & 1 uF \\  & Input inductor \(Lin\) & 75 uH \\  & Transformer ratio \(N\) & 5.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Parameters of converter and dc module. The efficiency curves of the dc-dc module are shown in Figure 21. Efficiency curves under different input and output voltages are shown. In boost mode, as the input voltage increases, the efficiency increases. Because the current is larger in buck mode than boost mode, the efficiency is lower in buck mode. The maximum efficiency is 98.9%. Figure 22 is the voltage gain range of the dc-dc module and converter. The output voltage curve in Figure 22a conforms to the derivation result in Section 2.3. The voltage gain of the dc-dc module is between 6 and 11. It is between 24 and 44 for the converter in normal to adapt to the changing PV voltage. In the process of LVRT, the converter operates in buck mode and the voltage gain is less than 8 or even close to 0. The blue shade is the voltage gain range of the converter, as shown in Figure 22b. A wide range of voltage gain is realized in the converter. Table 4 shows the comparison between DAB, isolated boost and proposed topology. As the turn ratio and input voltage are the same, DAB has the lowest output voltage. Isolated boost and proposed topology have the same boosting ability. However, the output voltage of isolated boost cannot be lower than 2.3 kV in Table 4. The proposed topology has wider output voltage. Figures 23 and 24 are the simulation results of inserting active damping by the proposed method. By only applying the PI regulator, the amplitude of the oscillation is more than 20% of the average output current. By inserting active damping, the ripple of the current is reduced to 0.8%. The waveform is stable and oscillation is effectively eliminated. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **DAB** & **Isolated Boost** & **Proposed Topology** \\ \hline Output voltage range & 0–2.3 kV & 2.3–5 kV & 0–5 kV \\ Input voltage & 400 V & 400 V & 400 V \\ Turn ratio & 5.8 & 5.8 & 5.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of the output voltage range of different topologies. Figure 21: Measured efficiency of the dc–dc module. Figure 22: Voltage gain of the dc–dc module and converter. Figure 23: Simulation result of inserting active damping (\(I_{b}=5\)\(A\)). Figure 25a,b are the experimental results with the PI regulator and proposed active damping control, respectively. Figure 26 is the experimental result of inserting active damping by the proposed method. It can be seen from the experimental results that the proposed method effectively suppresses oscillation. Table 5 is the comparison between PI and the proposed method.