Study on the interaction of vehicle-to-grid and its impact on power quality of electric grid

With the continuous increase in the number of electric vehicles (EVs) in the market, both EVs and charging stations are rapidly developing. This paper provides an overview of the negative impacts of EV charging on the distribution grid at the current stage. It summarizes specific solutions proposed by scholars both domestically and internationally to address the effects on voltage, harmonics, and power quality. These solutions mainly involve the addition of filters to the distribution grid, the use of 12-pulse rectifiers, and series resonance. Furthermore, the paper analyzes how Vehicle-to-Grid (V2G) technology can balance the grid and regulate power quality issues caused by EV charging, ensuring stable system operation. V2G technology also enables EVs to function as mobile energy storage devices and allows for rational energy distribution using bidirectional charging stations. The integration of charging stations with the grid, forming a connected vehicle network, is considered the future direction of EV development.


Introduction
The number of EVs in China has been rapidly increasing, leading to significant market development for electric vehicles.As the number of EV users grows, various challenges arise during the charging process.The penetration level of EV fast charging affects the voltage of the distribution grid [1], and as more EVs connect to the grid, harmonic pollution becomes more severe [2], negatively impacting EV charging.Additionally, uncontrolled charging of EVs for prolonged periods increases the load on the distribution grid and can result in a decline in power quality [3].To mitigate the negative impacts of EV charging on the distribution grid, scholars at home and abroad have studied loss optimization management strategies that can suppress excessive peak power [4] and improve power quality in the distribution grid.
Currently, the application of V2G and V2X technologies, which utilize EVs as energy storage systems, is gaining traction.V2G technology provides reliable energy storage without compromising battery life and offers significant advantages for industrial frequency regulation services.Using V2G technology, EVs can serve as energy storage systems to reduce the gap between peak and off-peak electricity demand on the grid [5].Some researchers propose using direct current control strategies to provide bidirectional chargers for V2G EV batteries, enabling fast charging, fast discharging, slow charging, and slow discharging.
Simultaneously, optimizing intelligent charging stations and enabling vehicle-grid interaction are crucial development directions in the field.To enhance user convenience, the optimal number and location of charging stations can be determined [6].Intelligent charging stations can automatically adjust the charging power based on the number of vehicles, maximizing the utilization of station resources.
One key technology involves using load prediction to calculate the required number of charging stations, correcting the estimated demand and designing charging station locations and capacity.Vehicle-grid interaction is also a current research focus, where scholars propose an EV charging route strategy based on prospect theory to achieve coordination and optimization among charging stations, networks, and road infrastructure.Moreover, by acquiring the positional information of each user and aggregating it in the network [7], EVs can interact effectively with the internet, enabling the design of optimal charging routes for users and addressing urban traffic congestion issues.

Negative impacts of electric vehicle charging on power distribution grid
Currently, the electric vehicle (EV) industry is characterized by rapid technological advancements and fast-paced development.As the number of electric vehicles and charging infrastructure continues to grow, the impact on the power distribution grid is also increasing.Factors such as the size and proximity of charging stations to the grid, as well as the integration of distributed generation, can result in voltage fluctuations, harmonic distortion, power quality issues, and increased load capacity demands.Consequently, the requirements for EV charging stations and power distribution grid in terms of power quality have become increasingly stringent.

Impact on voltage fluctuations of power distribution grid
With the widespread adoption of electric vehicles, the integration of EVs into the power distribution grid has become a pressing concern.The simultaneous connection of a large number of EVs to the grid can have a significant impact on voltage fluctuations.Several studies have been conducted to investigate the effects of EV charging on voltage fluctuations in the power distribution grid.Reference [8] analyzed the impact of EVs on grid voltage.The research indicated that vehicle charging involves the parallel operation of multiple nonlinear loads.The use of unsafe chargers can result in load-induced fluctuations in direct current (DC) voltage and power factor reduction.The voltage of uncontrolled rectifier chargers fluctuates greatly during the charging process, but when pulse width modulation (PWM) rectifiers are used, the voltage variation in the distribution grid is minimal.Reference [9] established a simulation model for EV integration into the power distribution grid based on charging characteristics.The study revealed that under uncontrolled conditions, EV charging causes larger voltage fluctuations compared to traditional charging methods.Additionally, the proximity of each distribution node to the EV charging points affects the efficiency of the grid, with closer distances leading to larger voltage fluctuations.Based on various studies, it is evident that simultaneous charging of a large number of EVs can have a significant impact on voltage fluctuations in the power distribution grid.Therefore, current research focuses on optimizing equipment and incorporating filtering techniques to effectively mitigate voltage fluctuations.

Impact on harmonics of power distribution grid
In addition to voltage fluctuations, EV charging also has a notable impact on harmonics in the power distribution grid.Harmonics not only affect voltage variations but also compromise the compatibility of the grid.The presence of harmonics constitutes a form of pollution in the power distribution grid, degrading the operating environment of electrical equipment, generating excessive harmonics, and potentially causing line short circuits and even power outages.Reference [10] analyzed the harmonic characteristics of the power distribution grid resulting from EV charging through a simulation model.The study revealed that higher power charging generates fewer harmonics compared to lower power charging.Reference [11] demonstrated that as the number of EVs being charged increases, the harmonic current from charging batteries decreases.When multiple charging stations are connected simultaneously, the harmonics tend to cancel each other out.
Since harmonics cannot be naturally eliminated, their mitigation has become a primary focus.
Filtering techniques have proven to be practical methods for reducing harmonics.After passing through an active power filter circuit, the input current still exhibits low harmonics, primarily third harmonics, as shown in Figure 1 below.Series resonance is highly effective in eliminating third harmonics [12].By employing harmonic detection and control, it is possible to implement control-delayed current feedback in network management, reducing harmonic generation [13].Furthermore, enabling Vehicle-to-Grid (V2G) mode can reduce the maximum charging stages in the network, thus accelerating charging speed and improving the overall performance and efficiency of the grid [14].Effective control of grid harmonics holds great significance for power systems, and innovative approaches should be explored to develop more convenient and efficient methods for harmonic elimination.

Impact on other power quality aspects of power distribution grid
During the EV charging process, high-performance transformer switches continuously toggle, generating a large amount of high-frequency currents and noise harmonics, which severely affect power quality and the electromagnetic environment of the grid.Different charging stations and varying numbers of EVs being charged have different impacts on the power quality of the grid.The influence of loads on power quality can be analyzed from the perspective of electromagnetic interference (EMI) radiation noise.EMI conducted noise generated during EV charging can be transmitted through power lines, affecting the normal operation of other electrical devices connected to the power supply.DC charging introduces harmonic distortion, making conducted noise and EMI radiation noise significant.In the DC charging mode, distortion does not increase with the increase in charge capacity.However, in the AC charging mode, harmonic current distortion significantly increases with the increase in the number of EVs being charged [15].Regarding AC and DC charging stations, as the number of EV charging cycles increases, distortion rates for AC batteries increase, while DC charging remains relatively stable, with higher distortion rates compared to AC batteries.When EVs are connected to charging batteries, temporary voltage drops occur in the power distribution grid, accompanied by overvoltage transients during the charging process [16].
Charging batteries can cause harmonic distortion in the distribution bus, thereby affecting the grid harmonics and voltage deviations at charging stations.The more rectifier pulses, the lower the harmonic distortion.Research has shown that the harmonic distortion generated by a PWM rectifier charging stack remains low, with distortion rates maintained at around 2.45% for a 12-pulse rectifier charging stack, indicating good performance.Considering the transient characteristics of power distribution grid quality, the installation of these two control devices, including fast charging stations, within the distribution grid can effectively improve power quality [17].

Impact on power distribution grid's load capacity
The rapid development of electric vehicles (EVs) presents significant challenges to the planning and construction of urban power distribution grids due to the substantial increase in charging load.The types of charging in the distribution grid have also changed, with the introduction of new loads such as electric heating and EVs connecting to the grid, leading to increased demand.If the structure of the distribution grid is incompatible with the penetration rate, the connection of these loads poses a significant risk to the safe operation of the distribution grid.Currently, scientists both domestically and internationally have developed various methods to assess the load capacity of power distribution grids.One approach is to evaluate the distributed generation with new loads and a high percentage, establishing a comprehensive scoring index system for the distribution grid in terms of safety, reliability, performance quality, business management, and flexibility.As the penetration rate of distributed generation gradually increases, the safety and reliability of power sources in the distribution grid slightly improve, while power quality, business management, and flexibility experience a slight decline [18].
Residential areas serve as gathering places for a large number of EV charging activities, making it necessary to develop a procedure for assessing the maximum charging capacity that can be deployed in residential areas.If transformer overload occurs in the system, it will continue to be overloaded as the coverage of EVs expands.At the same time, providing permanent coverage for EVs is crucial, as higher charging capacity results in a higher proportion of overload in the distribution grid [19].Statistical estimation of the charging capacity of EVs in residential distribution grids can be conducted by first conducting a macro analysis of the charging capacity of EVs in the area.The maximum charging load in residential areas can be roughly calculated based on basic data such as the penetration rate of EVs, the number of households in residential areas, electricity loads, and transformer capacity.If the future penetration rate of EVs slightly increases, some residential distribution transformers may be overloaded [20].Additionally, the higher the proportion of private cars to slow chargers, the lower the proportion of private cars to low-speed chargers, and the smaller the proportion of normal loads.If the access points of charging stations are located near the end of the distribution grid, there is a higher likelihood of severe violations in the distribution grid [21].
The power supply mode for EVs can be selected based on the parking environment of EVs to assess the charging capacity of the distribution grid receiving EVs.Factors that limit the access of EVs to the distribution grid include voltage offset, harmonics, branches, and transformer capacity.As the number of EVs increases, the total frequency of harmonic distortion reaches its peak earlier, limiting the continuous use of EVs [22].In order to assess the congestion of the distribution grid in a targeted manner, another scientist has developed a control strategy that enables the control of closed-loop EV loads, online optimization, and real-time control.After collecting transformer operating data and charging point data, the system determines whether the transformer is overloaded, allowing for continuous monitoring of the transformer in subsequent control circuits.This strategy prevents transformer overload, optimizes the use of charging stations, effectively realizes the construction of charging stations during peak charging demand with insufficient power supply, and resolves the issue of transformer overload after the construction of charging stations [23].
To address the issue of limited load capacity in the distribution grid, measures such as timely adjustment of grid load or grid upgrading are needed to ensure the safe operation of the distribution grid.Methods such as network transformation, power capacity expansion, and double-fed line power supply can be employed to enhance the load capacity of charging stations in the grid.

Application of electric vehicles in power grid
With the accelerating pace of new energy vehicles entering the automotive market, the idea of using electric vehicles as energy storage has emerged.Currently, some emerging technologies such as V2G (vehicle-to-grid) and V2X (vehicle-to-everything) enable the effective control of the charging and discharging processes of electric vehicles.Research on energy storage in electric vehicles can better serve users and improve economic efficiency.

V2G technology
V2G technology refers to the effective control of the charging and discharging processes of electric vehicles through the interaction between electric vehicles and the power grid.Currently, research has been conducted both domestically and internationally on the interaction between electric vehicles and the power grid as energy storage devices, but there is limited research on the mobile energy storage characteristics of electric vehicles.
Research on the application of batteries in electric vehicles indicates that orderly management of the charging and discharging processes of electric vehicles can effectively improve the efficiency of peak power allocation in the power grid.V2G technology can provide reliable energy storage without affecting the battery's lifespan, thereby offering significant advantages for industrial frequency regulation services.With V2G technology, electric vehicles can participate in grid frequency response faster and at a lower cost [24].It is well known that electric vehicles also have intermittent and random consumption patterns, and when a large number of electric vehicles are connected to the grid, it can cause significant fluctuations and shocks.However, V2G technology overcomes the limitations of "bidirectional communication and unidirectional transmission" between traditional power sources and the network, thereby improving users' economic benefits and reducing network losses.The overall impact of V2G on grid configuration includes deep involvement of electric vehicles in the grid, user response to demand, and technological and economic research on optimization operation.In terms of energy storage, renewable energy can be consumed locally, and used batteries can be gradually utilized.In environmental terms, this helps conserve energy, reduce emissions, and lower environmental costs [25].
The integration of mobile energy storage systems into the power grid can effectively cope with power fluctuations on the generation side caused by high penetration of renewable energy.It enables mobile energy storage to perform better in grid peak regulation, main grid frequency control, reactive power in microgrids, and energy storage.V2G technology's control strategy realizes power factor control, making it suitable for mobile energy storage in distribution networks [26].In order to optimize the charging of electric vehicles, a universal road selection and charging-discharging algorithm has been developed.By calculating routes and joint optimization, an extended temporary V2G network is established to simulate the behavior and operating time of electric vehicles, thereby limiting the limited resources of charging stations.The results show that the combination of route planning and processing solutions can reduce the costs of all electric vehicles in the vehicle network [27].

Research on electric vehicles as energy storage
Currently, optimizing the configuration of electric vehicle energy storage systems has become one of the research hotspots.However, the difficulty lies in accurately assessing the response behavior of V2G electric vehicles due to the uncertainty of electric vehicle behavior.
Comprehensive quantitative evaluation of electric vehicle eligibility has been conducted nationwide, and scholars have studied the capacity distribution of energy storage systems.Four typical response limits of electric vehicles have been established to calculate the response capability of electric vehicle clusters.In order to ensure power supply security, electric vehicles and energy storage systems can be used to support power supply in situations of power shortage in the power grid [28].With the continuous connection of new charging equipment such as electric vehicles, the end of charging is gradually changing, and charging characteristics are also changing.Therefore, flexible adaptation measures such as wind power, photovoltaic power, energy storage, electric vehicles, and flexible interconnected power sources can be used to achieve the maximum absorption of wind and photovoltaic power in regional networks.Energy management strategies of the system can be considered, and the specific topology structure of urban power systems and the optimal configuration of energy storage systems can be selected to fully utilize energy storage resources [29].
To fully utilize the role of electric vehicle demand response in energy storage configuration, a combination of wind energy and energy storage in the form of an electric vehicle charging-discharging response microgrid can be considered.Proper control of electric vehicles can help reduce the cost of allocating mixed energy storage in microgrids, promote the absorption of clean energy in microgrids, and reduce power exchange with the main grid during peak periods [30].In the scenario of electric vehicle charging, fast charging stations for electric vehicles in the distribution network can be optimally configured.Considering the charging requirements of electric vehicles and the impact of power fluctuations at fast charging stations, an optimal configuration model is proposed.This general configuration method can be applied to different distributed generation scenarios and different charging requirements for electric vehicles [31].Another approach is the use of bidirectional chargers for V2G electric vehicle batteries with a direct current control strategy, where bidirectional chargers allow for fast charging, fast discharging, slow charging, and slow discharging.The results show that this strategy is effective and efficient [32].The topology is shown in Figure 2.

Interactivity technology of charging stations
For smart charging stations, a patent provides a method for building charging stations in smart cities.This method is based on an Internet of Things (IoT) system management platform and is used for the construction of charging stations in smart cities.The method involves determining at least one candidate facility based on characteristics, assigning traffic routes to at least one person within the region, allocating existing charging stations within the expansion area, and determining at least one destination within the main station.Charging stations can also automatically adjust the charging power based on the number of vehicles, maximizing the utilization of station resources [33].
The development of charging infrastructure is the foundation for the popularization of electric vehicles, and a well-planned charging infrastructure will directly impact the development of electric vehicles.Therefore, a load model can be created for electric vehicles, and the number of charging piles required can be calculated based on the load curve.From the perspective of energy demand, a method has been studied to calculate the number of load piles based on the prediction of electric vehicle loads, and the maximum number of different types of electric vehicles arriving simultaneously is input to correct the required number of load piles [34].The focus is on fast charging on the road, understanding the route selection behavior of battery electric vehicle users, and how much remaining battery capacity they usually have when they are willing to make a detour for fast charging.The model is applied to regions with different densities of fast charging stations, investigating the relationship between charging patterns and the development level of charging infrastructure.Another important aspect is that this is an initial study on the behavior of selecting fast charging stations.Future research based on the tourism industry can provide a more comprehensive understanding of how users decide when and where to fast charge [35].

Economic benefits of charging station interactivity
Due to the significant investment in charging infrastructure and its operation, it brings new opportunities and challenges to the construction of distribution networks.The impact of the vehicle-station-grid interactive mode on the overall efficiency of distribution networks is analyzed from three aspects: economic efficiency, safety and stability of the power system, and environmental considerations.In residential buildings, the vehicle-station-grid interactive mode allows for reasonable load distribution during the period from 6 pm to 12 am and has the function of load leveling.In commercial places, the vehicle-station-grid interactive mode can evenly distribute the load throughout the operating hours.In the charging network of densely populated cities, the orderly increase in electric vehicle charging capacity reduces electricity costs.The operation of the vehicle-station network reduces the load factor and improves the power quality of the distribution network [36].

Key technologies of the internet of vehicles
Currently, it is challenging to integrate large-scale charging stations into the power grid.Considering the incentive effect of electricity prices and the interests and requirements of the distribution network, energy storage stations, and charging stations, an optimization model for the distribution network, including energy storage stations and charging stations, has been designed to effectively reduce active power losses and suppress power fluctuations in the network [37].Another approach is to create an optimal management model for the distribution network by integrating daily renewable energy production and distribution network load predictions.This model connects the electric vehicle load converter to the microgrid system and employs a consensus algorithm for coordinated control.It treats the load converter as an agent, thereby reducing the complexity of iterative calculations and effectively addressing the output current problem of each charging battery converter.When the load converter operates under a fixed information structure, it can also ensure the stable operation of the electric vehicle microgrid [38].

Applications of the internet of vehicles in daily life
As a typical application of the Internet of Things, the Internet of Vehicles has become an important tool for urban traffic congestion management.The integration of traditional automobiles with the mobile Internet is a crucial step in addressing urban traffic congestion and achieving intelligent transportation [39].
Today, "Internet of Vehicles + New Energy Vehicles" has become a new production and operation model for automobile manufacturers.The application of the Internet of Vehicles in new energy vehicles includes integrated electronic control, remote monitoring systems, and car rental services.By utilizing big data analysis techniques to analyze load, energy consumption, driving data, and more, owners can receive a wider range of experiences and services.The Internet of Vehicles can also analyze smart charging station data in real-time, rapidly transmit accurate load data to new energy vehicle users, and improve user experience.However, there are still challenges to be addressed, such as the relatively backward operation and maintenance level of charging stations, lagging charging pile status evaluation technology, high charging pile failure rates, and safety risks associated with some charging piles [40].The topology is shown in Figure 3. Internet of Vehicles technologies have promising prospects and can also be utilized to address urban traffic congestion issues.An electric vehicle charging routing strategy has been proposed to achieve coordination and optimization among charging stations, networks, and road networks.The road traffic time can be understood as dynamic data, including the number of people waiting at charging stations, arrival time, and waiting time, to calculate daily electricity consumption.By examining the operation status of road networks, charging stations, and distribution networks, real-time charging routing strategies for users can be implemented, effectively improving the efficiency of user charging and reducing the negative impact of the increasing number of electric vehicles on power and road networks [41].

Discussion and prospects
In order to address the negative impact of electric vehicle (EV) integration on the power distribution network during charging, PWM rectifiers are currently used to reduce voltage fluctuations.However, in order to analyze the charging process in more depth, it is possible to establish a simulation model to effectively reduce errors.Delayed current control can suppress and regulate a portion of the harmonic generation at the source, while series harmonic compensation can effectively eliminate third-order harmonics.PWM rectifiers not only reduce voltage fluctuations but also maintain a low level of harmonic distortion, thereby improving power quality effectively.The assessment of the distribution network's capacity is closely related to the actual site, and reasonable allocation and deployment of power can achieve better carrying capacity results.
Leveraging the role of EVs as energy storage, the main strategies are V1G, V2G, V2X.V2G is suitable for energy storage distribution, providing reliable energy storage, reducing grid losses, and improving economic benefits.In terms of system stability, it plays a coordinating role.The prerequisite for implementing V2G is that EVs are interconnected with charging stations and the grid when not in operation.As a part of energy storage, V2G can serve as a power quality control device to regulate power quality issues caused by EV charging, thus fundamentally solving power quality problems in the grid.
The optimization of charging stations and their integration with networks will be the future direction of development, where smart interactions between vehicles and networks will drive the green benefits of renewable energy development.Smart charging stations, based on the Internet of Things (IoT) and connected to mobile phones, can provide users with the most suitable charging routes.At the same time, based on the number of charging vehicles, charging stations can autonomously adjust charging efficiency to maximize resource utilization.
Currently, the key technologies of the vehicle-network interaction system include analysis of EV travel and load, battery warning, management of energy flow between charging stations, and the establishment of communication information platforms.

Conclusion
Electric vehicles will gradually become mainstream in the automotive market.Based on the research conducted in this paper, the following conclusions can be drawn: By employing appropriate methods and technologies, the shortcomings of EVs in terms of charging and energy storage can be optimized.The use of advanced techniques such as PWM rectifiers and filters can effectively reduce harmonics and voltage fluctuations during the charging process.V2G technology can effectively manage the charging and discharging processes of EVs, turning them into flexible mobile energy storage systems and enhancing grid stability.To improve the user experience of EV owners and address traffic congestion issues, an integrated vehicle-charging network system is an effective solution.By integrating the location of each EV user and the status of charging stations on the IoT platform, algorithms can be employed to plan the optimal routes for each user, making full use of existing resources.

Figure 1 .
Figure 1.Harmonic current at the connection of different number of charging points.

Figure 3 .
Figure 3. Structure of the vehicle-charging network.Internet of Vehicles technologies have promising prospects and can also be utilized to address urban traffic congestion issues.An electric vehicle charging routing strategy has been proposed to achieve coordination and optimization among charging stations, networks, and road networks.The road traffic time can be understood as dynamic data, including the number of people waiting at charging stations, arrival time, and waiting time, to calculate daily electricity consumption.By examining the operation status of road networks, charging stations, and distribution networks, real-time charging routing strategies for users can be implemented, effectively improving the efficiency of user charging and reducing the negative impact of the increasing number of electric vehicles on power and road networks[41].