Medium Frequency Soft Switching DC/DC Converter Based HVDC Transmission System for Offshore Wind Farms

As the most developed form of renewable energy, wind power continues to be a focus of academia and industry. Large-scale offshore wind farms are preferred for environment and economic consideration. High voltage DC (HVDC) transmission system has many advantages over high voltage AC (HVAC) for the growing distance from offshore wind farms to the shore (>100 km). As the essential part of HVDC system, DC/DC converter draws much research attention. Soft switching DC/DC converter with voltage-fed/current-fed topology is found most suitable for this application, for it can significantly reduce the switching loss at large operation range, allowing for higher operation frequency and smaller magnetic components.;A HVDC transmission system based on isolated medium frequency zero voltage zero current switching (ZVZCS) DC/DC converter is proposed for offshore wind farms in this thesis. In the proposed system topology, 40 wind generators are series connected and then attached to +/-500 kV HVDC lines. Turbines equipped with permanent magnet synchronous machine of 6 kV output are considered in this thesis. For the converter, all primary side switches are turned off with zero voltage and all secondary switches are turned on with zero current. Soft switching is achieved over the entire load range, via resonance between transformer leakage inductance and capacitance across primary switches.;Design example of a 2.5 MW, 5.6/25 kV converter is then presented. Steady state analysis and small signal model are developed. MatlabRTM routines are created to assist the calculation of lengths of operation modes and the choice of resonant parameters. With ZVZCS, the operation frequency is chosen as 2 kHz, which is much higher than traditional DC/DC converter of this power level, allowing for smaller transformer and inductor. Since transformer leakage inductance is utilized in the resonant circuit, its design procedure is elaborated as well. Finite element simulation aided iterative design ensures transformer has leakage inductance close enough to desire value. Controller design is completed and the closed loop system is simulated in PLECSRTM. Converters series connected at the output are also simulated to show scalability.