| Energy problem is a worldwide problem, has got more and more people's attention. New energy technology is an important means to solve energy problems, especially wind power. Offshore wind farms have variety of features such as high wind speed, wind stability, less interference and large power generation, wind farms extend from land to sea, is the trend of future development of wind power. Offshore wind farm connecting to grid by HVDC way can overcome the limitations of transmission capacity and transmission distance, so based on HVDC offshore wind power generation system has become a research hot spot.DC offshore wind farm connecting grid system began to enter the stage of multi-terminal DC network, in multi-terminal HVDC offshore wind farm connecting grid system, there are 2 to 3 DC conversions, then DC/DC converter must be required. Constructing power converters that are suitable for medium to high voltage range applications is a great challenge for power electronics. Modular design, low switching frequency, high ef?ciency, and excellent output voltage waveforms are the advantages offered by Modular Multilevel Converter(MMC). The MMC has recently been accepted as one of the most competitive converter alternatives for VSC-HVDC applications.This thesis is study on the DC/DC converter with MMC. The topology is the DC/DC half-bridge MMC topology. Analysis of its basic principle, and building its mathematic model and calculating the power flow of the system. Then based on the conventional modulation method of MMC, a modified NLM modulation method is proposed for the topology due to the converter can work in the high frequency and very good waveform is not necessary. Then analyzing the switching device losses of the converter with the modified NLM method, and an calculation example is given; finally build a prototype, getting waveforms through, the experiments results validate the effectiveness of the modified NLM method and the accuracy of power flow calculation. |
PDF Full Text Request