Prototype Design And Control Strategy For Isolated Modular Multilevel DC-DC Converter

High voltage direct current(HVDC) is beneficial for long-distance transmission and large-scale renewable energy generation. Although there are large numbers of HVDC lines operated around the world, most of them are worked as the ”point-to-point” connection.One of the reason is that it is short of a DC transformer or a DC-DC converter to connect different HVDC transmissions. From the perspective of application, DC transformer is very important for HVDC to develop DC grids, and it is also the one of most probable solution for new and legacy HVDC links, which have different operating voltage as the different cable or overhead line technology. Therefore, DC transformer will be significant for future DC grids.Nowadays, there are many DC transformer topologies having been proposed, one of which is based on LC resonance, while others are based on modular technique. Among them, LC resonant topologies are not competitive for HVDC transmission since highvoltage components are used and unavoidable high dv/dt or di/dt.On the contrary, modular technique has been utilised in HVDC for more than 10 years and has high reliability.Isolated modular multilevel DC-DC converter(IMMDC) is one of the most promising DC transformer topologies based on modular technique, which has galvanic isolation and can stop the dc fault current effectively.In this paper, the IMMDC will be focused and analysed from three different parts.Firstly, the mathematics model and state equation will be set up, and a 600V/10 kW IMMDC experimental prototype will be built based on its mathematics model. Next, a novel precharge strategy is proposed to solve the large inrush current when the IMMDC is operated without any precharge control, where two different implemented scenarios have be analysed and the effectiveness has been verified by experiments. Finally, an optimal reactive power control has been proposed to suppress the influence of reactive power, which has been ignored by previous research and actually has a significant impact on active power transferring capability and components. All of these have been proved by simulations and experiments.