Topology And Control Technology Of Large-capacity DC Collector Converter For Offshore Wind Farms

As the proportion of wind power in new energy generation continues to increase,it is becoming increasingly important to study offshore wind power generation with more concentrated and abundant wind energy resources.Compared with traditional AC wind farms,full DC wind farms using "DC collection-DC transmission" have the advantages of small equipment size,low system loss and having no reactive voltage problems,and have gradually become a research hotspot for offshore wind power generation,but high-voltage and large-capacity DC gathering and sending converters,as the key equipment in full DC wind farms,hinder their large-scale development.Therefore,this paper proposes an isolated DC converter with the characteristics of double-layer modularity,and designs a modulation strategy with voltage boost function and a set of fault handling schemes,which realizes the technical requirements of large capacity,high boost transformation ratio and fault isolation of high-voltage sending converter.In order to meet the requirements of fault isolation and high voltage and large capacity,this paper first analyzes the technical requirements of large scale wind farm on converters,and proposes to combine Modular Multilevel Converter(MMC)with input series and output series DC/DC converters to form a composite modular topology.Secondly,in order to reduce the step-up pressure and insulation manufacturing problems of isolation transformer,a s/m modulation strategy based on double phase shift is proposed,which makes MMC itself has the boost characteristics,and a destocked capacitor voltage equalization algorithm is designed according to the modulation characteristics.Compared with the traditional equalizationmethod,the proposed one can effectively reduce the switching frequency within the allowable fluctuation range of the capacitor voltage,thereby reducing the switching loss.At the same time a constant voltage control strategy is put forward to maintain the stability of the input voltage.Then,in order to suppress the sudden increase in current and isolate the fault under DC fault conditions,a set of fault handling schemes and one kind of novel sub-modules are designed,in which the short-circuit current is firstly reduced by active current limiting control at the beginning of the fault,and soon afterwards,the fault current is blocked by using the topology characteristics of the novel submodule after a latching signal is send to MMC.Finally,the high-voltage large-capacity DC converter is built by MATLAB/Simulink software,which verifies the voltage boost capability of the modulation strategy and the effectiveness of the voltage equalization algorithm to reduce the switching frequency,and verifies the rationality of the fault handling scheme by setting a bipolar short-circuit fault on the high-voltage DC line.The double-layer modular structure proposed for the high-voltage sending converter of full DC offshore wind power can effectively reduce the floor space of the converter,and its modular characteristics increase the flexibility of the application of the converter with different transmission capacity and voltage levels,and the proposed modulation algorithm,control scheme and capacitor voltage equalization control strategy realize the normal operation control of the converter,and the treatment scheme under the bipolar short circuit fault also effectively reduces the current stress of the switching device and realizes fault current limiting and protection.The DC converter topology and control protection scheme proposed in this paper provide theoretical and technical reference for promoting the development of all-DC offshore wind farms.