Coordinated Control Of The DC Power Distribution Network With Distributed Generations

With the increase of the distributed power supply in the distribution network,the stability and reliability problems become more complicated.DC distribution network,which has the advantages of flexibility and efficiency,and without reactive power and power angle stability as well as other issues,is a more ideal way of distributed power supply network.Because there is no reactive power and frequency problem in the DC distribution network,its power-voltage control is the core technology of stable and optimized operation.For DC distribution network with a large number of distributed generation,the control method which combine the primary voltage and secondary voltage regulation can be used to adjust the system power,so that the stabilized operation and optimized operation of the complex power grid can be decoupled in different time scales.In addition,the distributed energy based on power electronic converter has little contribution to the inertia of DC distribution network,which will become a new problem after large scale access of distributed energy resources to distribution network.With the diversification of energy structure and the leanness of user needs,the requirements on power quality and operating characteristics of DC distribution network have also increased.This paper mainly studies the coordinated control method of DC power distribution network with distributed power supply to improve its transient operation stability as well as system efficiency and power supply quality.The main research results are as follows:1.The mathematical model of each unit in the wind power DC power distribution network is established,which include the dynamic model of permanent magnet wind turbine,battery energy storage system,grid-connected converter and loads.In addition,the traditional control strategy of each unit and the operation characteristics are studied.In this way,foundation is laid for the establishment of predictive control model for each unit and the proposal of inertia control strategy.2.Voltage regulation system which incorporates primary and secondary voltage control of DC power distribution network is proposed.Based on primary voltage regulation method,the response characteristic of each unit is further studied and the self-adaptive inertia control is proposed to solve the problem of low inertia of DC power distribution network.For wind power unit,the output power characteristics can be improved through the absorption or release of rotational kinetic energy of wind turbine.The control parameters of energy storage and grid-connected units can also be changed to virtual out inertia response corresponding to the change of DC voltage,which enhances the anti-disturbance capability and the transient stability of DC power distribution network.3.A model predictive control method of local converters is proposed to coodinate with inertia control which requires fast converter regulation.In this method,predictive control models of the generator side converter,the grid side converter and the energy storage converter are established based on the analysis of the mathematical model of each converter.In order to eliminate the single step time delay of the digital execution process,the two step prediction error function is introduced which forms a multi-objective optimization problem.A novel sorting method is used to select the target control vector,which avoids the setting process of the weight coefficients in traditional multi-objective optimization problem.By combining the model predictive method of converters and inertia control,the control response speed as well as the transient stability of the DC distribution network are enhanced.4.The distributed control system of wind power distribution network based on model predictive controller is designed.In order to meet the requirements of economic operation of the system and high quality electric energy provision,the model predictive controller is adopted as the main control unit in the distributed control system to generate the optimized reference signal of local controller.The local controllers conduct the fine-tuning of the operating state of the system based on the optimized reference value according to system operating characteristics.At the same time,redundant reference signal is utilized to provide power allocation scheme based on DC voltage during communication failure to avoid stability collapse.In this way,not only efficient operation and high power quality of the system can be guaranteed,but also system stability is ensued during communication failure.