Research on the operation control strategy of optical storage and DC microgrid

Compared to traditional alternating current (AC) power grids, direct current (DC) microgrids have outstanding technical and economic advantages and bear great development potential and application value. In this paper, the operation control strategy of optical storage DC microgrid is studied. Firstly, the structural composition and related characteristics of the DC microgrid are systematically analyzed. Secondly, the coordinated control strategy for the DC microgrid during off-grid operation, grid connection operation, and load optimization is studied, and the mathematical model of the DC microgrid is established. Finally, the simulation analysis of the model of the DC microgrid is conducted, and the results show that the coordinated control strategy can effectively stabilize the output power, promote the power balance of the power grid, and provide fundamental guidance and reference for the efficient conversion and control of power grid, which has important practical significance.


Introduction
The comprehensive utilization of distributed output power supply, the DC microgrid control is characterized by a simple structure, high efficiency, and high stability, which has become a hot topic of many scholars [1].In order to realize the optimization of control, each process plays different roles [2].Literature [3] studies the low-voltage crossing control strategy of DC microgrid.In order to solve the problems of randomness and indirection of renewable energy, Hu put the literature to solve the problems.Hu et al. [4] put forward three working modes for the independent operation of the optical storage and DC microgrid.Yin et al. [5] proposed the optimization design of the microgrid coordinated control strategy in the traditional mode, which is prone to a large power quality fluctuation range.Eghtedarpour and Farjah [6] established a DC microgrid capacity configuration model for an independent DC microgrid system composed of photovoltaic, ultracapacitors, batteries, and fuel cells.The above literature of light storage DC microgrid operation structure and coordinated control strategy for comprehensive optimization and perfect, for efficient and reliable operation of DC microgrid solid foundation, at the same time, also provides the subsequent study for reference experience, to promote the DC microgrid coordination continuous optimization control strategy has a significant role.

( )
where I is the current flowing through the load, Iph is the photogenic current directly proportional to the sunlight intensity, I0 is the reverse saturation current, Ish is the leakage current of the solar photovoltaic cells, Rs is the series resistance of the photovoltaic cell, U is the load end voltage, q is the electronic load (1.610-19C), K is the Boltzmann constant (1.3810-23 J/K), T is the absolute temperature (t + 273K), A is the ideal factor of PN junction, and Rsh is the parallel resistance of the photovoltaic cells.
In order to maximize the use of light energy and further improve the power generation efficiency of the DC microgrid system, it is important to always control the operation of the microgrid system near the maximum power point.This control technology is called maximum power tracking.Figure 1 shows a schematic model of photovoltaics.( ) where Ns take part in; Np represents the number of batteries connected in series and in parallel.It should be noted that the compensation factor of the feature at 25℃ as reference is: To sum up, the digital model of the battery is:

Pv model
The parameters of photovoltaic are as follows: we set the initial external light intensity to 1000 W/m2,and setting that at 0.2 s, the light intensity will be reduced from 1000 W/m2 to 500 W/m2.
These parameters are the packaging data of the photovoltaic cell module.In order to simulate the light intensity change in the study area, the sunshine intensity was selected as 1000W / m according to the average daily light intensity change data within one month2In addition, 0.2 seconds per unit time was selected as the change point of illumination intensity, and at 0.2 seconds, the illumination decreased to half the initial given amount to simulate the change of real-time illumination.

Simulation and analysis
When the load of the DC microgrid changes, test the operation characteristics of each unit on the DC side.In the following simulation, each micro-power supply adopts a coordinated control strategy.At time t=0.5 s, the load increased from 7.0 kW to 8.5 kW, and at time t=1.0 s, the load decreases to 5.8 kW.The simulation results are shown in Figure 3-4.As shown in the figure, Figure 5 and Figure 6 show the output power waveform and the DC bus voltage of each module when the DC microgrid battery does not adopt the coordinated control strategy.In the t [0,0.5]interval, each unit operates in the constant power mode and the DC voltage is 0.7 kV; in the t [0.5,1.0] and [1.0,1.5], the load increases is 0.625 kV, and the load decreases is 0.775 kV; beyond the specified voltage range (± 5%), the voltage output quality will be affected to a large extent.As shown in the Figure5-6, it can be seen from the figure, after the coordinated control strategy is added to the DC microgrid, with the change of DC load, the coordinated control strategy adjusts the power output of each unit of the DC subnetwork in real time according to the voltage, maintains the rated DC pressure of the system at about 0.7kV, and maintains the power balance of the system.

Conclusion
This paper analyzes the structure and related characteristics of the DC microgrid and systematically analyzes the main functional modules.Secondly, the coordinated control strategy of optical storage and DC microgrid is studied.Under the grid-connected operation mode, the DC bus voltage is mainly stabilized through the power grid converter.Make the power grid has the supporting role of the microgrid.In the off-grid operation mode, the energy storage system should adopt sag control mode, and the DC microgrid voltage control and energy storage system should adopt voltage sag control mode to realize the functions of suppressing photovoltaic output, load, and supporting fast charging of charging pile.It provides a significant reference for the subsequent control optimization strategy research.

Figure 1 .
Figure 1.Photovoltaic simulation model 3.2.Battery mathematical model This paper selects the storage battery as the main component of the energy storage system.Figure 2 is shown in the battery model diagram.
Figure 2 is shown in the battery model diagram.

Figure 2 3
Figure 2 Equivalent model of the battery

Figure. 3
Figure. 3 The active power of each module on the DC side without coordinated control

Figure 5 Figure 6 A
Figure 5 The active power of each module on the DC side with coordinated control