Research On DC/DC Based Dc Power Converter For Offshore Wind Farm

Offshore wind power generation has been paid more and more attention and research by experts and scholars both at home and abroad for its better continuity and stronger wind energy capture capability.There are two ways to collect the electric energy in the offshore wind farms,which are DC collection and AC collection.Compared to AC collection mode,DC collection mode has the following advantages:when the offshore wind power generating units are not fully synchronized fields,DC collection mode can be used,AC collection requires each generating unit complete synchronization;in DC collection mode,due to maximum power tracking characteristics of individual units and the use of the DC/DC converter units with high frequency transformer,volume and weight of the converter system has been reduced,the efficiency of the converter system has been improved;compared with AC collection mode,DC collection mode uses one less transmission line etc..Therefore,the DC collection mode will become an important development and research direction of the wind farm.In this paper,a DC/DC based DC power converter of offshore wind farm is proposed.The converter is mainly used in the power conversion system of marine permanent magnet synchronous motor(PMSG)directly connected to VSC-HVDC.The converter consists of a three-phase three level PWM rectifier,a single-phase DC/AC high frequency inverter,a single-phase diode rectifier bridge and a high frequency transformer.The single-phase DC/AC high frequency inverter,single-phase diode bridge rectifier,high frequency transformer can be encapsulated into an insulated type DC/DC sub module,a plurality of sub modules of DC/DC parallel input and series output can make the output voltage rise.The input parallel and output series structure has the advantages of high modularity,easy to add,large total power,small total output,and the output voltage can be directly connected to the VSC-HVDC.This paper analyzes the topology structure and working mode of parallel input and series output DC/DC converter and DC/DC sub module,and proposed a coordinated control strategy of DC bus voltage outer loop of the three-phase three level PWM rectifier and output voltage inner loop of the DC/DC sub module.The waveform difference of each sub module is analyzed when the output capacitance of each DC/DC sub module and the transformer leakage inductance are unbalanced,the influence of the output capacitor and transformer leakage inductance on the power system is studied,and the corresponding solution is discussed.The relationship between the wind turbine generators of each independent VSC-HVDC transmission system is analyzed theoretically,the equivalent circuit model is established.and through the analysis of the circuit relationship,we can get the conclusion that each unit is independent of the grid connected DC/DC converter.A simulation system of PMSG with output power of 2MW is built by using MATLAB simulation platform,and the validity and correctness of the theory is verified.