Study On Control Strategy And Optimal Operation For DC Micro-grids

With the social development and scientific and technological progress,clean and efficient distributed generation technology has been widely used to solve the problem of environmental pollution and energy shortage.However,the grid-connected operation of a large number of distributed power sources will have a non-negligible impact on the safe and reliable operation of large power grids.In order to overcome this drawback,the concept of micro-grid came into being.DC micro-grids have become an important development direction for micro-grids because they do not need to consider factors such as reactive power,phase,and frequency.There are two modes of operation of the DC micro-grid: islanding and grid-connected operation.In order to ensure stable operation,smooth switching,and economical operation in both modes,it is necessary to further study the stable operation control method and optimization of the DC micro-grid.This paper starts from the modeling of distributed power supply and studies the stability control and operation optimization of DC micro-grid with hybrid energy storage devices.Changes in the external environment will affect the distributed generation output,causing the DC bus voltage to fluctuate.Based on the degree of deviation of the instantaneous bus voltage from the rated voltage and the state of charge(SOC)of energy storage device,a DC micro-grid coordination control strategy based on distributed generation(DG)and hybrid energy storage device(HESS)is proposed;based on the SOC of energy storage device and principles of selling electricity to the grid during the peak hours and buying electricity from the grid at the time of valley,an optimization model of grid-connected operation mode was constructed and examples were verified.The main work is as follows:Firstly,the concept,structure and key technologies of DC micro-grid are introduced.The component models of the DC micro-grid units,such as photovoltaic power supply,direct-drive fan,energy storage component,bidirectional AC/DC converter,bidirectional DC/DC converter,etc.are established.By studying the energy storage characteristics of lithium batteries and supercapacitors,combined with the charge/discharge characteristics of bidirectional DC/DC converters,a hybrid energy storage device model based on the rapid multi-frequency charge and discharge of supercapacitors and supplemented by the charge and discharge of lithium batteries was established.This paper summarizes the current control technologies commonly used in DC micro-grids and compares and analyzes the principle and characteristics of master-slave control,peer-to-peer control and hierarchical control.Secondly,according to the various operating states of DC micro-grid,based on the variation of DC bus voltage and SOC of energy storage device,a coordinated control strategy of DC micro-grid based on DG and HESS is proposed.According to the SOC of energy storage device,it is determined that the DC micro-grid is operating in an island mode or a grid-connected mode,and the distributed power supply is controlled to operate in a maximum power point tracking output and a constant voltage constant power mode.Using coordinated control of DG and HESS to guarantee the operational stability and reliability of DC micro-grids.The corresponding DC micro-grid simulation model was established using the power system simulation program DIgSILENT.It was verified that under the circumstance that the environmental change affects the output power of the distributed generation,the coordinated control strategy proposed in this paper can effectively control the stability of the DC bus voltage.Finally,Based on the premise of satisfying the load demand,based on the principle of selling electricity to the grid as much as possible during peak hours and purchasing power from the grid as much as possible during electricity troughs,and the SOC of energy storage device,an economical optimization scheme for grid-connected operation of DC micro-grids is proposed.According to the optimization scheme,a corresponding mathematical model is established,and the scheme is verified with examples.