The impact of massive EV charging on distribution lines

The large-scale charging of electric vehicles at the same time is bound to have a large impact on the distribution network and then reduce the stability of the grid, especially during peak hours when it is easy to overload the lines. In this paper, we combine battery capacity and charging power data of electric vehicles and user usage habits to model the charging load changes of electric vehicles at different scales. The impact of charging by EV owners on network load was also analyzed through case studies. Our results have a certain practical significance for the planning of power grid lines.


Introduction
As a new energy vehicle, electric vehicles have the advantages of being environmentally friendly, energy efficient and a good driving experience, which, combined with strong state support, are gaining more and more popularity.According to the statistics, from 2020 to 2022, annual sales of electric vehicles in the Chinese market increased from 1.367 million to 6.87 million, a fourfold increase in just two years.As of the end of June 2023, the total number of pure electric vehicles in China had reached 12.6 million, a figure that is still growing rapidly.In the State grid and the Southern Power Grid, while the layout of charging piles "full coverage of counties and counties", it is also constantly accelerating the construction of charging facilities in rural areas, making the charging of electric vehicles more and more convenient.
However, while EVs give us convenience, they also have a certain impact on the grid, especially when large-scale EV charging occurs at the same time, the grid load goes straight up and a charging peak is formed.If the current carrying capacity of the distribution line is small at this time, it is prone to overload.At the end of EV charging, a trough forms and the charging power is extremely imbalanced.Therefore, it is of great practical importance to investigate large-scale EV charging [1].Reference [2] analyzed the impact of electric vehicle charging on distribution network load based on energy adequacy in distribution network reliability.This research method establishes a probability reliability model for the charging distribution network of electric vehicles.Based on the analysis of the impact of electric vehicle charging on the load of the distribution network, reference [3] proposes to use the energy storage characteristics of electric vehicles to supply power to isolated areas of the distribution line.

Parameter model
Establishing an EV load model is closely related to the performance parameters of an EV [4].In this paper, we analyze the charging load of this electric vehicle by referring to the performance parameters of the previously better-selling BYD Song PLUS in the market: • The BYD Song PLUS has a pure electric range of 520 km; • The BYD Song PLUS battery has a capacity of 71.8kWh; • The BYD Song PLUS charges with a power of 18kW; • Electric vehicles are charged to the maximum battery capacity on each charge; • Assume that the EV battery state is charged at 20% of the battery capacity.

Models of mobility demand for EV
As a means of transportation, the duration and frequency of charging of electric vehicles is closely related to factors such as their own battery capacity, daily mileage, and commuting time.According to the survey, 15 percent of the private cars on the market are not used during the working day and are not charged, and the remaining EV is assumed to have a three-day charge and to start charging after working [5].
According to relevant investigations, residents' travel meets the following formula where   is equal to 18 and   is equal to 1 of (1).Electric vehicle owners' daily mileage meets the following formula In Equation ( 2),   is equal to 3.2 [6] and   is equal to 0.88.

Probabilistic model of charging power
Thanks to the rapid development of electric vehicle technologies, EV charging methods are also diverse.Among the current mainstream charging methods, the battery charging process is mainly constant current-constant voltage, that is, when electric vehicles are simply connected to charge, the current remains unchanged and the voltage rises, but only for a short period, and then it will then charge in a constant voltage-constant current manner.At this point, the charging power is also maintained at a stable value.From this, we can assume that the electric vehicle is always charged at constant power [7].Based on this assumption, we can calculate the length of time it takes for each EV to reach battery saturation from the start of charging, and the calculation formula is as follows where   is the length of charging time (ℎ) ;   is daily mileage () ;  100 is the power consumption of 100 km ( • ℎ/);   is the charging power ().
The duration of each electric vehicle's battery capacity reaching saturation can also be calculated using the following formula where   represents the state of the battery when charging an electric vehicle,   represents the battery capacity of an electric vehicle ( • ℎ).
The battery load state   of an electric vehicle depends on the range of the electric vehicle and the endurance of the electric vehicle during the day, which is calculated as follows where  1 stands for the range capability ();   stands for the daily mileage ().Combining (4) and (5), we can obtain that Thus,

The impact of EV charging on the load
Large-scale electric vehicles, once connected to the distribution network, will inevitably have a significant impact on the grid, especially during peak hours when electricity consumption is high during the off-peak hours.Large-scale charging can easily reach maximum load throughout the day.Moreover, the current on the line will increase as the number of electric vehicles increases.At this point, the current carrying capacity and the load-carrying capacity of the line should be taken into account [8].If not predicted and controlled, it can easily lead to line overload and even power equipment failure.At the same time, prolonged overload operation can also affect the service life of power equipment component [9].If we divide a 24-hour day into 1440 points, each separated by a time difference of 1 minute, and the electric vehicles start charging randomly at these 1440 points according to their travel characteristics, we can simulate the load curve of the battery when charging each electric vehicle.Figure 1 shows the load curve of the residential area simulated with 200 electric vehicles as an example.From the load curves shown in the above figure, we can see that most EV charging start times are concentrated during the peak electricity consumption period of the night.If at this moment, a large number of electric vehicles are connected to the power grid for charging, the load of the power grid will reach the highest load value of the whole day.At this point, we need to take into account the load capacity of the line.

Example analysis
This section uses the IEEE-RBTS Bus6 [10] as the test system, as shown in Figure 2. In this test system, we assume that load points 3 and 9 are industrial loads, load points 5 and 11 are government loads, and load points 7 and 14 are commercial loads, while the other load points are residential loads.See Table 1 for details of the various load cases.The following figures show typical daily load curves for residential load, industrial load, government load, and commercial load simulated based on different load type electricity consumption patterns.Figure 3 shows the daily load curve of residents; Figure 4 is the daily load curve in industry; Figure 5 shows the daily load curve in business; Figure 6 shows the daily load curve of government.In order to obtain the impact of different quantities of electric vehicle charging on power lines, we will consider the following five scenarios: Case1: 0 electric vehicles; Case2: 50 electric vehicles; Case3: 75 electric vehicles; Case4: 100 electric vehicles; Case5: 125 electric vehicles.Note that load points 3 and 9 are industrial loads, load points 5 and 11 are government loads, load points 7 and 14 are commercial loads, and these types of load points do not participate in EV charging.Electric vehicles are only charged at residential load points.The results of the power flow calculations are shown in Table 2. From the calculations, we can conclude that when 75 EVs are charged at each residential load point, the system's lines 1 and 15 are already close to the capacity of the line.Lines 1 and 5 are already experiencing line overload when 100 EVs are charged at each residential load point.By overlaying various residential load points, we can conclude that the distribution network system can accommodate up to 675 electric vehicles for charging simultaneously.

Conclusions
In this paper, we analyze the impact of EV charging on line carrying capacity at different scales using an example.It can be seen that when a certain number of EVs is exceeded, the distribution network lines

Table 1 .
Load situation at each load point.

Table 2 .
Current values under different conditions (A).