# Modular Combined DC-DC Autotransformer for Offshore Wind Power Integration with DC Collection
## 5 Simulation Study
### Simulation of Wind Speed Variation  
To validate the performance of the MCAT during wind speed variation, the transient-state process is designed as follows: at \(t=1.0\) s, wind speed reduces from 12 m/s to 8 m/s, and then maintains 8 m/s for 2.5 s, finally, rises to 10 m/s. Figure 5 shows the dynamic process of the test system.  
Figure 6a illustrates that the transmission power of each MMC varies according to the wind speed quickly and stably. Figure 6b,c shows that the AC output current of each MMC and DC currents on both sides of the MCAT responds to the change of wind speed fast. Figure 6d,e shows the DC voltage of each MMC and DC voltages on both sides of MCAT, which are well balanced and stable. Figure 6f shows that the fluctuation of \(u_{\text{CH,1}}\) and \(u_{\text{CH,2}}\) is within 1.2 pu, proving the calculation of DC-isolation capacitors and compensation  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Item** & **Value** & **Item** & **Value** \\ \hline Rated MMC capacity/MVA & 200 & \begin{tabular}{c} Rated submodule \\ voltage/kV \\ \end{tabular} & 2.0 \\ Rated voltage/kV & 300 & \begin{tabular}{c} Number of \\ submodules per arm \\ \end{tabular} & 150 \\ Transformer rated power/MVA & 240 & \begin{tabular}{c} Submodule \\ capacitance/\(\mu\)F \\ \end{tabular} & 666 \\ Transformer radio & 220/157.5 & \
\begin{tabular}{c} Arm inductor \(L_{0}/\mathrm{H}\) \\ Smoothing reactor/H \\ \end{tabular} & 0.076 \\ HVDC cable resistance/\(\Omega\) & 4.5 &
\begin{tabular}{c} HVDC cable \\ inductance/mH \\ \end{tabular} & 20 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Parameters of the onshore converter and HVDC cable.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Item** & **Value** & **Item** & **Value** \\ \hline Rated capacity/MVA & 25 & d-axis inductance/pu & 0.41 \\ Rated voltage/kV & 2.96 & q-axis inductance/pu & 0.77 \\ Frequency/Hz & 10 & Field flux/pu & 0.83 \\ Inertia constant/s & 2.5 & DAB converter rated capacity/MVA & 28 \\ Stator resistance/pu & 0.1 & DAB converter rated input voltage/kV & 2.96 \\  & & DAB converter rated output voltage/kV & 100 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Parameters of the aggregated WT.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Item** & **MMC\({}_{\text{L,1}}\)** & **MMC\({}_{\text{H,1}}\)/MMC\({}_{\text{H,2}}\)** \\ \hline Rated MMC capacity/MVA & 70 & 40 \\ Rated voltage \(U_{\text{dc}}\)/kV & 100 & 100 \\ AC side frequency/Hz & 100 & 100 \\ Rated submodule voltage/kV & 2.0 & 2.0 \\ Number of submodules per arm & 50 & 50 \\ Submodule capacitance/\(\mu\)F & 666 & 666 \\ Arm inductor \(L_{0}/\mathrm{H}\) & 0.076 & 0.076 \\ \(C_{\text{H,1}}\), \(C_{\text{H,2}}\)/\(\mu\)F & & 41.3 \\ \(L_{\text{H,1}}\), \(L_{\text{H,2}}\)/mH & & 84.3 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Parameter in the MCAT.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Item** & **Value** & **Item** & **Value** \\ \hline Rated MMC capacity/MVA & 200 & \begin{tabular}{c} Rated submodule \\ voltage/kV \\ \end{tabular} & 2.0 \\ Rated voltage/kV & 300 & \begin{tabular}{c} Number of \\ submodules per arm \\ \end{tabular} & 150 \\ Transformer rated power/MVA & 240 & \begin{tabular}{c} Submodule \\ capacitance/\(\mu\)F \\ \end{tabular} & 666 \\ Transformer radio & 220/157.5 & \
\begin{tabular}{c} Arm inductor \(L_{0}/\mathrm{H}\) \\ Smoothing reactor/H \\ \end{tabular} & 0.076 \\ HVDC cable resistance/\(\Omega\) & 4.5 &
\begin{tabular}{c} HVDC cable \\ inductance/mH \\ \end{tabular} & 20 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Parameters of the onshore converter and HVDC cable.  
inductors are correct in Section 3. Figure 6g demonstrates that the AC bus voltage are balanced well. Conclusively, the simulation results validate the feasibility of the proposed MCAT and the effectiveness of the control strategy.