# Modular Combined DC-DC Autotransformer for Offshore Wind Power Integration with DC Collection
## 6 Conclusions  
This paper presents the MCAT without the transformers. In the MCAT, the expensive converter transformers are substituted with the DC-isolation capacitor and compensation inductor in series to reduce the footprint of the offshore platforms. The selection methods of main circuit parameters for the MCAT are discussed in detail. Then, the control strategies of the MCAT are investigated under the steady-state operation and the AC fault recovery occurring at the AC bus. A detailed complicated OWF simulation model is constructed and the feasibility and effectiveness of the proposed MCAT and corresponding control strategies are validated. The conclusions are as follows:  
Figure 7: Simulation results of fault and recovery process: (**a**) Three-phase short circuit currents; (**b**) A phase AC output current of MMCs; (**c**) DC currents of high-voltage and low-voltage DC grids respectively; (**d**) DC voltages of MMCs; (**e**) DC voltages of high-voltage and low-voltage DC grids respectively; (**f**) active power transmitted by MMCs; (**g**) voltages of \(C_{\text{H,1}}\) and \(C_{\text{H,2}}\); (**h**) A phase voltage of the AC bus. 1. The proposed MCAT utilizes the DC-isolation capacitor to isolate the DC offset voltage of the converter transformers. The application of the compensation inductors facilitates improving the operation efficiency of the MCAT. The time-domain simulation results show that the calculation method of the main circuit parameters is correct, and the calculated values are highly consistent with the simulated values. 2. Ignoring the dynamic characteristics of MMC submodules, the output characteristics of the MCAT and the AC bus voltage could be regulated by MMCs adopting DC voltage control, reactive power control, and passive control strategies, respectively. The time-domain simulation proves that the voltage across the MCAT can remain stable under the condition of wind speed changes, and the output power can quickly respond to the wind speed changes. This indicates that the MCAT has favorable voltage control performance. 3. The fault recovery strategy proposed in this paper can help the MCAT to quickly enter steady state. The test system is built in PSCAD/EMTDC, and the time-domain simulation shows that the MCAT could resume operation in a short time after the AC fault, which is conducive to maintaining the stable operation of the OWF. Conceptualization, Z.Z. and Z.X. ; methodology, Z.X. ; software, Z.Z. ; validation, Y.S. ; formalism analysis, Y.S. ; investigation, Y.S. ; resources, Z.X. ; data curation, Z.X. ; writing--original draft preparation, Y.S. ; writing--review and editing, Z.Z. ; visualization, Z.Z. ; supervision, Z.Z. and Z.X. ; project administration, Z.X. ; funding acquisition, Z.Z. and Z.X. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. Data available on request due to privacy. The authors declare no conflict of interest. [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] [PERSON] Comparison of the modular multilevel DC converter and the dual-active bridge converter for power conversion in HVDC and MVDC grids. _IEEE Trans. Power Electron._**2015**, _30_, 124-137. [CrossRef]
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