Resrarch On Voltage Control Of Multi Terminal Flexible Medium Voltage Dc Distribution Network

Flexible DC distribution network has become a research hot spot at home and abroad because of its high power quality,flexible power supply mode and convenient access to distributed generation.For the multi terminal DC distribution network,its topology is complex and its operation mode is diverse,so the reasonable control strategy is the key to ensure the safe and stable operation of the DC distribution network.In this paper,the voltage control strategy of multi terminal flexible DC distribution network is studied.The main contents are as follows:In the paper,first of all,analyzed the research status of voltage level and topology of DC distribution network,determined that the research object is±10kV medium voltage ring type DC distribution network.Choose voltage source converter(VSC)as AC/DC converter.Adopt the DC solid state transformer(DCSST)based on Dual Active Bridge(DAB)as the DC converter connecting the medium DC voltage bus and the low DC voltage grid and select photovoltaic and energy storage as the distributed power source in this paper.The mathematical models of voltage source converter,DC solid state transformer and distributed power are established.And based on the mathematical models,the controller of each part is respectively designed.Secondly,in view of the shortcomings of traditional DC distribution network voltage control,the voltage margin control and droop control are combined.to coordinate the control strategy among the multi converter stations.The converter station is divided into master station and slave station,and the DCSST connected with the energy storage is used as the backup station.In normal operation,the master station controls the system voltage with constant DC voltage control as the power balance node.The slave station adopts the droop control scheme with voltage margin.When the master station has the regulating capacity,it operates according to the constant power.When the master station loses the regulating capacity,it operates according to the droop control scheme.Backup station work as the emergency voltage regulating equipment when the whole DC distribution network is about to reach the upper limit of power.When the master station and the slave station reach the upper limit of voltage regulation,the backup station changes control mode from constant low DC voltage control to constant medium DC voltage control.According to this control strategy,the operation mode of distribution network is analyzed,then has the simulation under the PSCAD/EMTDC.The normal operation of the system,converter station fault and line disconnection are simulated respectively.The results show that the control method is effective.Finally,In view of the voltage deviation of DC distribution network caused by fault,a kind of voltage control strategy based on depth first search is proposed.The control mode of the converter station in the distribution network can be switched quickly by identifying the topology structure of the distribution network after failure.That is,on the basis of the original control strategy,a second voltage adjustment based on the real-time topology is realized.In the last,the voltage control strategy based on depth first search is simulated,and the simulation results show that the method can effectively reduce the abnormal voltage offset caused by large disturbance and improve the voltage quality.