Researches on Operation Optimization of Charging Device Participating in Demand Response of Power Grid

The application of electric vehicles has been an important mean to solve crisis of oil supplies and environment protection pressure. The number of charging devices has increased rapidly due to the orientation of policy and market in recent years. As a result, the load peak-to-valley difference will be aggravated and the electricity cost of residents will be increased. The purpose of the paper is researching the path to achieve the demand response with orderly charging of electric vehicles when a large number of electric vehicles are connected to the grid. Firstly, technological development trends of electric vehicles are analyzed. Secondly, a scheme that can uniformly manage and regulate load resources is designed by considering the charging needs of users in a comprehensive way. The core devices such as energy controller and energy router are applied in the scheme. Thirdly, a series of experiments are conducted in a residential district with 660 households and 24 charging piles in Beijing. Finally, the results indicate that the load peak-trough difference of the residential district has reduced by 58% if all of users choose to participate in demand response based on the control strategy of orderly charging.


Introduction
The number of electric vehicles has increased rapidly in recent years.At the same time, how to establish an effective operating mechanism of charging pile participating in response of power grid has gradually become the focus of the industry.Due to the rapid power regulation capabilities, charging pile has become the flexible resources which can provide various ancillary services for power grid such as peak regulation and frequency regulation.Therefore, orderly charging of electric vehicles is one of the current research hotspots, which is of great significance for charging load to participate in the operation of the power grid.
Much work has been done to achieve the demand response of power grid, which is based on orderly charging of electric vehicles.Xun Dou [1] established cluster-based control strategies of electric vehicles for integrated energy system with AC-DC hybrid distribution network, which could make the most of the interactive capabilities of electric vehicles and effectively reduces the operating cost of the system.Yanhong Li [2] proposed a kind of charging load management strategy based on the energy generated by photovoltaic system hourly, which could provide reference for solar energy storage and charging in integrated energy systems.Jie Yang [3] analyzed the benefits and costs of V2G service providers participating in various applications based on the current electricity price policy and power battery cost through calculation examples.Xun Ding [4] established a multiobjective optimization model of intelligent charging, in which the electricity cost, temperature, time and comfort level were taken as objective functions.Tianning Li [5] proposed a charging strategy that adjusts the charging time by using the time-of-use price of electricity，which uses the load variance of the residential quarter and the charging cost of electric vehicles as evaluation indexes to optimize the charging time of electric vehicles.Lu Chen [6] proposed an orderly charging control strategy for EV in three-phase unbalanced distribution network by analyzing the reactive power supporting capacity of EV charging inverter, which can improve the voltage level of the distribution network，reduce the network loss and three-phase unbalanced rate.Xiaotong Chen [7] analyzed the optimization objectives and control strategies of the current orderly charging control, introduced the coordinated optimization of electric vehicles and new energy, and summarized the current problems.
In this paper, a kind of optimization control strategy is proposed to achieve intelligent and orderly charging of electric vehicles.Through a series of experiments, we can draw the following conclusions: the load peak-trough difference of the residential district has reduced by 58% due to the orderly charging of electric vehicles.

Analysis of Charging Mode
The large-scale of electric vehicles for charging to the power grid will lead to an increase of peaktrough difference which can improve the instability of the power system.This situation will pose a serious threat to the safe and economic operation of the electrical equipments.According to the prediction of the Society of Automotive Engineers, the total number of electric vehicles in China will exceed 80 million by 2030.There are many charging methods for household electric vehicles, such as fast charging in public area, charging in residential area.For users of household electric vehicles, charging in residential areas is an economical and convenient way of supplying electricity.But the charging load severely will overlap with the peak period of normal electricity consumption, forming a peak load that lasts for about 3 hours.
For example, if all existing fuel powered vehicles in Beijing are replaced by electric vehicles, the grid load in residential areas will increase by 12 million kilowatts.If large-scale renovation of the power grid is carried out to address this challenge, the investment amount will be at least 64 billion Yuan.
In response to the above issues, an overall solution based on orderly charging of electric vehicles is established in this paper.

Optimization Strategy
The users of household electric vehicles can present their charging orders by the APPS of mobile phone, enabling interaction between electric vehicles and the power grid.According to the load situation, the power company will reasonably arrange the charging timing and power of each electric vehicle, which can realize the energy optimization based on charging during low load periods.On the basis of this, a design method is proposed in this paper, which can realize the efficient matching of electric vehicle charging and photovoltaic power generation [8].
The design method follows the concept of industrial internet, which can flexibly connect to new energy consuming devices on the customer side, such as electric vehicles.The entire system will collect real-time operating status of the power grid, analyze load trends, and carry out coordinated control of charging loads.The design method applies Bluetooth and 5G technology to build a dual channel communication network, which is backup and instant response to each other.At the same time, the design method fully utilizes the advantages of Bluetooth near-field communication to achieve rapid human-machine interaction on site.It can provide users with personalized and customized services by applying mobile software.A widely applicable and self correcting ordered charging control strategy is applied to load optimization, which is based on big data and artificial intelligence technology.

Control System and Core Device 1) Smart Energy Control System
Smart energy control system based on industrial internet is responsible for balancing the needs of the power grid and electric vehicles.The control system has functions such as collection management, operation monitoring, strategy generation, data statistics and collaborative control.The overall architecture of smart energy control system is constructed based on internet & ubiquitous internet of things [9].The system has been deployed interconnecting with the intelligent internet of vehicles platform and other systems, which can realize data sharing and resource sharing. 2

) Energy Controller
The energy controller is the core control unit on the user side.It is responsible for real-time collection of power grid operation status, analysis of load trends, and coordination and control of various resources.At the same time, energy controller can also respond to the scheduling instructions of the control system, promoting clean energy consumption.Energy controller is a new type of intelligent control terminal, which is mainly installed next to transformer and acts as a gateway of transformer. 3

) Energy Router
The energy router can interact with the charging pile in real-time.Data perception, collection, and control of the charging process can be achieved based on energy routers.Energy router is mainly installed in the customer side near the charging Device.
4) Intelligent and Orderly Charging Device A miniaturized, standardized, and intelligent orderly charging device has been designed in the paper, which can effectively reduce the construction and installation costs.Intelligent and orderly charging devices receive instructions from energy routers.
The design prototypes of the three core devices mentioned above are shown in figure 1.

Overall Architecture
A scheme is designed based on the system, equipment, and communication channel mentioned above, which can uniformly manage and regulate load resources such as charging piles, distributed photovoltaics, smart appliances, and household energy storage in residential areas.The scheme can realize functions such as wide access of multiple devices, monitoring and control, orderly charging of electric vehicles, and management of energy storage device charging and discharging.The scheme has technical advantages such as low investment, easy implementation, and easy promotion.The overall architecture of the scheme is shown in figure 2.

System Flow
The interaction paths from electric vehicle users to energy routers can be established the application of APP.Users can decide whether to participate in demand response based on their own wishes.If users choose to participate, the system will generate control strategies based on operational information, which can realize flexible control of charging load.If users do not choose to participate, the devices will immediately start charging.The control flow of the smart energy control system is shown in figure 3.

Experimental Result
We do a series of experiments as a case for calculation at a residential district with 660 households and 24 charging piles Beijing.A large number of charging piles are running during the peak electricity consumption period when none of users choose to participate in demand response.The load peaktrough difference of the residential district is 600 kW in this case.
The number of charging piles in operation can significantly reduced during the peak electricity consumption period when all of users choose to participate in demand response based on the control strategy of this paper.As a result, the load peak-trough difference of the residential district has been reduced from 600 to 250kW.The data comparison is shown in figure 4.

Conclusion
A scheme of orderly charging strategy is designed based on industrial internet under carbon peaking and carbon neutrality goals in this paper.A series of experiments are conducted in a residential district with 660 households and 24 charging piles in Beijing.According to the experimental results, it can be found that the load peak-trough difference of the residential district has reduced by 58% due to the control strategy in this paper.Therefore, orderly charging of electric vehicles will be conducive to improve the utilization rate of power equipments.
In the future, we will strive to conduct in-depth research in the fields of distributed photovoltaics, smart appliances, and household energy storage to achieve collaborative management of multiple energy sources based on smart energy control system.On the basis of the platform, the applications of electric vehicles and energy storage orderly charging and discharging will be established, which can be a path to verify the practicability and effectiveness of the scheme.

Figure 1 .
Figure 1.The design prototypes of three core devices.

Figure 2 .
Figure 2. The overall architecture of the scheme.

Figure 3 .
Figure 3.The control flow of the system.

Figure 4 .
Figure 4. Comparison of the data curves.