Design of AC charging pile system for electric vehicles

0 Introduction
As the global energy crisis continues to deepen, oil resources are becoming increasingly depleted, and the harm of air pollution and global temperature rise is increasing, governments and automobile companies around the world generally recognize that energy conservation and emission reduction are the future direction of automobile technology development, and the development of electric vehicles will be the best way to solve these two problems. China attaches great importance to the development of electric vehicles, and the country has successively issued a series of standards to support and regulate the development of electric vehicles. However, there is still a long way to go to achieve the large-scale popularization of electric vehicles in China, and there are still many problems to be solved. The recently released draft of the "Energy-Saving and New Energy Vehicle Industry Plan" pointed out that pure electric vehicles will be the main strategic orientation. Relevant experts pointed out that there are three bottlenecks in the development of pure electric vehicles: one is the lack of standards, the second is the imperfection of supporting policies, and the third is the orderly advancement of infrastructure planning and construction. The electric vehicle AC charging pile studied in this paper is of great significance to the promotion of the popularization of electric vehicles as part of the charging infrastructure.

1 Introduction to AC charging piles for electric vehicles
AC charging piles, also known as AC power supply devices, are fixed on the ground or wall, installed in public buildings (office buildings, shopping malls, public parking lots, etc.) and residential parking lots or charging stations, and use conduction to provide electric vehicles with on-board chargers with a human-machine interactive operation interface and AC charging interface, and have corresponding measurement and control protection functions. The AC charging pile uses a large-screen LCD color touch screen as the human-machine interactive interface. It can choose four charging modes: fixed power, fixed time, fixed amount, and automatic (until full). It has functions such as operation status monitoring, fault status monitoring, charging time-sharing metering, historical data recording and storage. The AC working voltage of the charging pile is (220±15%)V, and the rated output current (AC) is 32A (seven-core socket). It takes about 6 to 8 hours for an ordinary pure electric car to be fully charged with an AC charging pile. The charging pile is more suitable for slow charging. The AC charging pile generally consists of four parts: pile body, electrical module, metering module, and account management module. According to different installation methods, the pile body can be divided into two types: floor-standing type and wall-mounted type. Floor-standing charging piles are suitable for ground installation in various parking lots and roadside parking spaces; wall-mounted charging piles are suitable for wall-mounted installation in crowded spaces and fixed buildings with walls around them, such as underground parking lots or garages.

2 Working principle of AC charging pile system
According to the relevant requirements of GB/T 20234.2-2011 "Connection device for conductive charging of electric vehicles; AC charging interface", a control pilot circuit is used as a judgment device for the connection status and rated current parameters of the charging connection device. Its typical control pilot circuit is shown in Figure 1.

After the power supply device plug is connected to the socket, the power supply control device determines whether the power supply plug and the power supply socket are fully connected by the voltage value of the detection point 4 shown in Figure 1. At the same time, the electric vehicle vehicle control device determines whether the vehicle plug and the vehicle socket are fully connected by measuring the resistance value between the detection point 3 and PE. After completing the plug and socket connection status detection, the operator completes the charging start setting for the power supply device, and the switch S1 switches from the +12 V connection state to the PWM connection state, and the power supply control device sends a PWM signal. The power supply control device determines whether the charging connection device is fully connected by measuring the voltage value of the detection point 1. After the vehicle control end detects correctly, S2 is closed. The power supply control device determines whether the vehicle is ready by measuring the voltage value of the detection point 1 again. If it meets the requirements, the AC power supply circuit is turned on by closing K.

3 AC charging pile system solution
The system consists of basic parts such as LCD touch screen, printer, RS 485 interface power meter, leakage protection circuit breaker, AC contactor, card reader and LED light. The LCD touch screen can provide a friendly human-machine interface and a quick and simple operation method to meet the requirements of customers to charge electric vehicles in different ways. It can display the current charging status, charging power and charging costs. The friendly user interface allows customers to make corresponding choices. When the collected voltage exceeds the overvoltage protection setting or is lower than the undervoltage protection setting, the charging pile stops charging. The leakage protection circuit breaker can ensure that charging is stopped in the event of an emergency fault such as leakage during the charging process. When an unexpected situation occurs and charging needs to be stopped urgently, the emergency stop button can be used to interrupt charging. The electrical connection diagram of the system is shown in Figure 2.

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4 Control system unit circuit
4.1 Main controller selection
The main controller selects STMicroelectronics' STM32F107VCT6 microcontroller. The STM32F107VC interconnect series uses a high-performance ARM Cortex-M3 32-bit RISC core with an operating frequency of 72 MHz. The device contains 2 12-bit ADCs, 4 general-purpose 16-bit timers and 1 PWM timer, as well as standard and advanced communication interfaces: up to 2 I2Cs, 3 SPIs, 2 I2Ss, 5 USARTs, a USB and 2 CANs. The device also provides an Ethernet interface, which greatly facilitates circuit design.
4.2 Serial interface circuit
The system uses a total of four serial interfaces to communicate with the LCD touch screen, thermal printer, card reader and RS 485 interface electric energy meter. The LCD touch screen and thermal printer are RS 232 level, and communicate with the MCU after level conversion. The communication protocol between the LCD touch screen and the MCU adopts the Modbus RTU communication protocol, with the MCU as the host and the LCD touch screen as the slave. The thermal printer communicates according to the protocol provided by the printer module. The card reader is TTL level and can be directly connected to the MCU, and communicates using the protocol provided by the card reader module. The energy meter for charging measurement uses a multifunctional single-phase meter. The meter uses a 2.0-level energy meter with a current specification of 5 (40) A. The meter provides an RS 485 interface and communicates with the MCU through the DL/T 645-2007 communication protocol. The energy value of the energy meter is read as the energy measurement value of the charging pile, and the current and voltage values ​​of the meter are read to determine whether overcurrent and overvoltage occur during the charging process and deal with them. The circuit diagram of the energy meter interface is shown in Figure 3.

4.3 CAN bus interface circuit
According to the relevant instructions in the draft for comments on the Communication Protocol between On-board Charger and AC Charging Pile of Electric Vehicles, the draft for comments recommends that the communication system between the on-board charger and the AC charging pile adopt the CAN bus, so the CAN bus interface is designed. The data link layer provides reliable data transmission between physical connections. The data frame format between the on-board charger and the AC charging pile of this system complies with the provisions of the CAN bus version 2.0B, and uses the 29-bit identifier of the CAN extended frame. The corresponding definition of each bit allocation and the transmission protocol and other functions comply with the provisions of SAE J1939-21.
4.4 Charging voltage measurement circuit
Voltage measurement first requires the voltage and current to be converted into a small signal that can be measured by measuring the transformer. For example, for the measurement of a 220 V voltage signal, the transformer ratio used is 2 mA/5 mA. Using the circuit shown in Figure 4, it can be seen that the output of the transformer is exactly 5 mA at 220 V. Ignoring the influence of large resistance shunt, 27 Ω is equivalent to a sampling resistor. Since the sampled signal is an alternating current, the signal has positive and negative signals, and the input range of the A/D converter is 0-3.6 V, the sampled voltage cannot be directly input into the A/D converter. This problem can be solved by connecting a positive reference voltage to the positive input of the op amp and selecting a suitable amplification factor so that the output can be within the input range of the A/D converter. After quasi-synchronous sampling, the data is calculated using a rectangular self-convolution window to obtain its effective value.

4.5 Control and guidance circuit
The control and guidance circuit completes the tasks of confirming the connection between the charging pile and the electric vehicle before charging, identifying the power supply and the current carrying capacity of the charging connection device, and monitoring the charging process. The MCU determines the state by detecting different voltage values ​​at the point. The circuit schematic is shown in Figure 5.

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5 Design of the electrical part of the pile
The electrical part of the AC charging pile mainly completes the functions of charging control and protection of the charging process. It has protection functions such as leakage protection, short circuit protection, overcurrent, overvoltage, and undervoltage protection. In addition to short circuit and leakage protection, other protection functions are realized by controlling the contactor through the charging controller to achieve self-recovery; short circuit and leakage protection are realized by miniature circuit breakers with leakage protection. In addition, the system also has a lightning protection module, the nominal discharge current of the lightning protection module is not less than 20 kA, and the protection voltage level is less than or equal to 1.5 kV. When the power supply is single-phase, the wiring method of the lightning protection module adopts the P+N wiring method. The charging pile is equipped with an emergency stop button so that charging can be forcibly terminated in an emergency.

6 Software design
The charging pile completes interactive control through the touch screen. If the card is swiped during operation, an interrupt is triggered to read the card, and the type of card is determined for related operations. The charging mode provides a variety of options. It can be set to charge by time, power, and amount, or it can be set to charge directly until it is fully charged. The overall flow chart of the program is shown in Figure 6.

7 Conclusion
This paper analyzes the hardware design and software design of the AC charging pile control system, and describes the design of the electrical part of the charging pile. The system uses STM32F107VCT6 as the control core, and realizes a variety of perfect functions such as human-computer interaction, charging control, energy metering, IC card payment, ticket printing, operation status monitoring, charging protection, and charging information storage and upload. The system can meet the general slow charging requirements of electric vehicles. As part of the charging infrastructure, it is of great significance to promote the popularization of electric vehicles.