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Wind power is a type of renewable energy source which has vast potential for further development as it is featured as pollution-free and plentiful. However, wind power turbines also have negative consequences for the environment (in particular visual impact). Therefore, wind power bases should be built away from centres of population, and in particular offshore where the wind speeds are higher and more consistent. As is known, most of the electric energy is consumed in cities, so one needs to find a way to transfer the energy long distances efficiently. HVDC transmission lines/cables are often appropriate when wind farms are built more than about 80km from the shore. This thesis is concentrated on the research of offshore wind farm based Multi-Terminal HVDC systems where the Multi-Terminal HVDC system can extract and deliver power from and to more than one connection point. In the thesis, the study of a full system PLECS+ Simulink simulation model with single or dual branch, including wind model, wind turbine, PMSG, PWM rectifier, SAB DC-DC converter, IPOS DC-DC converter, HVDC cables, simplified onshore system, are presented, focusing on the investigation of the output ripple of multiple DC-DC converters on DC cables with different voltage and power ratings. THD and cable AC losses on the DC cables are also monitored and studied in the thesis. In the hardware part, two different down-scaled DC-DC converter models are built to verify the simulation results.