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The electrical system is an important part of an electric vehicle. According to different voltage levels and uses, electric vehicle electrical systems can be divided into two parts: low-voltage systems and high-voltage systems. The low-voltage system adopts DC 12V or 24V power supply. On the one hand, it provides power for conventional low-voltage electrical appliances such as lights and wipers, and on the other hand, it provides power for vehicle controllers, control circuits and auxiliary components of high-voltage electrical equipment. The high-voltage system is mainly composed of high-power, high-voltage electrical equipment such as power batteries (or fuel cells), power converters and drive motors. The energy conversion from power batteries (or fuel cells) to drive motors is completed according to the power requirements of the vehicle. with the transfer process.
1.Low-voltage electrical control logic
Taking a fuel cell electric vehicle as an example, while the low-voltage electrical system provides power for conventional electrical appliances and control systems, it should also realize the order and coordination of various electrical equipment according to the driver's operation. Figure 1 is the control logic ladder diagram of the low-voltage electrical system.
In Figure 1, the control objects of the low-voltage electrical system mainly include the hydrogen leakage detection unit J0, the conventional low-voltage electrical system TLE, the control unit of the high-voltage electrical equipment, and the relays J3 and J4 of the high-voltage electrical main circuit. When the driver operates, first close the hydrogen leakage detection switch K0, if the hydrogen leakage detection unit J0 detects that the hydrogen leakage concentration reaches the set value, the normally closed contact J-H2 is disconnected, the relay J0 is de-energized, and its contact J0- 1 Without closing, the vehicle electrical system cannot energize other devices. Only when no hydrogen leakage occurs, J0 is energized, and J0-1 is closed, can the electrical system be powered in sequence. The specific operation steps are as follows.
Figure 1 Control logic of low-voltage electrical system of fuel cell vehicle
①Close the switch K1, the relay J1 is energized, the contacts J1-1 and J1-2 are closed, and the 24V power supply supplies power to the conventional low-voltage electrical system TLE of the vehicle, which can realize the operation of the doors, lights, wipers and other components;
②Close the switch SW-1, and the 24V power supply supplies power to the vehicle controller (ECU) and the control circuit of the electrical equipment. After the ECU judges that each control circuit is working normally, it closes the relay contact J-ECU, so that the 24V power supply passes through the contact J- The ECU supplies power to the relay J2, and the corresponding contact J2-1 is closed;
③Close the switch SW-2, the 24V power supply supplies power to the high-voltage relays J3 and 4 through the contact J2-1, so that the fuel cell engine and the power battery are connected to the DC high-voltage bus respectively.
2.High voltage electrical system
Depending on the configuration of the electric vehicle powertrain, the high-voltage electrical system has different electrical components. Also taking a fuel cell vehicle as an example, the largest high-voltage electrical system is a dual power supply structure composed of a fuel cell stack and a power battery stack. The fuel cell stack is the main power source for vehicle operation, and the power battery stack is the auxiliary power source. The high-voltage electrical system of a fuel cell electric vehicle is shown in Figure 2. When the fuel cell stack is used as the main power source, the power battery stack provides energy for the start of the fuel cell stack through the start control unit during vehicle start-up. During the acceleration process of the vehicle, when the output power of the fuel cell group is insufficient, the power battery group is discharged to supplement the energy required for vehicle acceleration. When the vehicle decelerates and brakes, the braking energy is absorbed by the power battery pack. This hybrid structure reduces the requirements for the peak power and dynamic characteristics of the fuel cell during vehicle operation, which is beneficial to improve the reliability of the vehicle's electrical system. Because the fuel cell stack and the power battery stack have different output voltage ranges and external characteristics of the power supply, it is difficult to use them in parallel. Therefore, a boost DC/DC converter is connected in series at the output end of the fuel cell stack. The output voltage is boosted and regulated. The output voltage of the DC/DC converter matches the working voltage of the power battery pack, which is called the bus voltage of the high-voltage electrical system. The bus voltage provides electrical energy to high-power electrical equipment such as the motor in the drive mechanism, power steering mechanism, and air brake mechanism through various power converters, so as to realize the functions of driving, steering and braking of the vehicle.
Figure 2 High-voltage electrical system of a fuel cell electric vehicle