The high-voltage safety protection measures of full electric cars are shown in Figure 1, which are mainly reflected in four aspects: maintenance safety, collision safety, electrical safety and functional safety.
(1) Maintenance safety
Maintenance safety mainly includes two aspects, namely the maintenance safety of traditional fuel cars and the special maintenance safety of electric cars. The maintenance safety of full electric cars is mainly to prevent high-voltage electric shock. Therefore, maintenance personnel should ensure that there is no risk of electric shock for high-voltage type cars. For this reason, most full electric cars are designed with a maintenance switch on the system. Figure 2 shows the BYD e6 full electric car maintenance switch. When the maintenance switch is turned off, the power output of the power battery is interrupted, but it still needs to wait for more than 5m. Access to high-voltage components (during this period is used to discharge system capacitors).
(2) Collision safety
When a car collides, the safety system of the car must ensure the personal safety of the relevant personnel during and after the collision. For full electric cars, in addition to the relevant protection requirements of traditional cars, the following requirements should also be met.
① Avoid the risk of electric shock for occupants and pedestrians during the collision, and try to protect key components from damage while ensuring the safety of personnel.
②Ensure that maintenance and rescue personnel have no risk of electric shock after collision. For this reason, some cars are designed with a protection circuit as shown in Figure 3: the inertia switch is connected in series to the power supply circuit of the high-voltage contactor, and the inertia switch is disconnected when a collision occurs, thereby cutting off the high-voltage The power supply of the contactor, the high-voltage output of the power battery will be disconnected at this time, ensuring the high-voltage safety of passengers, pedestrians, maintenance and rescue personnel.
(3) Electrical safety
The electrical safety of full electric cars mainly includes the following aspects
① Prevent personnel from coming into contact with high-voltage electricity.
②Reasonable distribution of battery energy.
③ High voltage safety during charging.
④High pressure safety during driving.
⑤ Electrical safety in the event of a collision.
⑥ Electrical safety during maintenance, etc.
The following safety measures are mainly used in the electrical system.
①Lines of different colors represent different voltages, and the color of high-voltage lines is uniformly orange. Therefore, we must attach great importance to high-voltage lines. Figure 4 shows the high-voltage lines on the BYD e6 full electric car controller.
② Warning signs are set on the high-voltage components. There is a high-voltage warning mark on the shell of the high-voltage component of each full electric car, and after-sales service personnel or car owners can intuitively see that the component is high-voltage through the mark. The warning signs used are the patterns specified by international standards, as shown in Figure 5.
③Anti-contact protection of high-voltage parts. Multi-layer (triple) insulation prevents accidental direct or indirect contact with live parts.
④ Electrical isolation. The positive and negative poles of the high-voltage electricity are insulated from the car ground. This protection prevents electric shock in the event of a simple failure.
⑤ The connectors of high-voltage components are designed with safety, as shown in Figure 6, which can not only prevent personnel from directly contacting high-voltage, but also waterproof and dust-proof, reducing the risk of insulation problems in the high-voltage system.
⑥ High voltage contactor and short circuit protector. As shown in Figure 7, a high-voltage contactor is designed between the power battery and the external high-voltage circuit to ensure that when the driver has no intention of driving or charging, the high-voltage system of the car except the inside of the battery is free of high-voltage electricity. The contactor may close only when the driver puts the car key in “Start” or when the power battery is being charged.
When a dangerous situation such as a short circuit occurs in the high-voltage system, in order to protect the occupants and key components, a short-circuit protector needs to be designed. If the current flowing through the short-circuit protector is greater than a certain value, the protector will be blown.
⑦ Precharge circuit. As shown in Figure 8 , before the power battery outputs high-voltage electricity, the high-voltage system outside the battery is pre-charged through the pre-charging circuit. The precharge circuit is mainly composed of precharge resistors. Since compensation capacitors are designed between the high-voltage positive and negative electrodes of high-voltage components, if there is no pre-charging resistor, the compensation capacitor will be burned due to excessive instantaneous current when the high-voltage loop is turned on.
⑧ Insulation resistance monitoring. The insulation resistance monitoring system detects whether there is an insulation fault in the entire high-voltage system, and displays the fault with sound or light in the instrument. If the insulation resistance value is too small, the car controller sends the contactor disconnection command.
The insulation condition of electric cars is measured by the insulation resistance of DC positive and negative busbars to ground. The international standard for electric cars stipulates that the insulation resistance value is divided by the nominal voltage U of the DC system of the electric car, and the result should be greater than 100Ω/V to meet the safety requirements. The traction battery insulation resistance measurement method recommended in the standard is suitable for static testing, but does not meet the requirements of real-time monitoring.
By measuring the voltage between the DC busbar and the electric chassis of the electric car, the insulation resistance value of the system is calculated. Assuming that the DC system voltage of the electric car (that is, the total battery voltage) is U, the insulation resistance values between the negative and positive, busbars and the electrical chassis to be measured are Rp, RN, respectively, and the voltages between the positive and negative busbars and the electrical chassis are Up, UN, the insulation resistance measurement principle is shown in Figure 9.
In Figure 9, RC1 and Rc2 are standard resistors with known resistance values used for measurement. The working principle is as follows: when the electronic switch tubes VT1 and VT3 are all disconnected, the voltages between the positive and negative busbars and the electrical chassis are measured as Up0 and UN0 respectively, which can be obtained from the circuit law.
When the electronic switch tube VT1 is closed and VT3 is disconnected, a standard bias resistor Rc1 is added between the positive busbar and the electrical chassis, and the voltages between the positive and negative busbars and the electrical chassis are measured as Upp and UNP, respectively. The same can be obtained.
The software solves the insulation resistance between the positive and negative busbars and the electrical chassis through the above two formulas as RP and RN, respectively. Similarly, the insulation resistance can also be obtained in the following two cases: VT1, VT3 are all disconnected and VT1 is disconnected, VT3 is closed; VT1 is closed, VT3 is disconnected and VT1 is disconnected, and VT3 is closed. It can be seen from the above calculation formula that the specific resistance values of the insulation resistances Rp and RN are calculated from the four measured voltage values and the known standard resistance, and the accuracy of the final result is directly related to the accuracy of the voltage measurement and the standard resistance. In addition, before and after the switching action, the influence of the battery voltage with the acceleration and deceleration of the car on the results should also be analyzed. The insulation resistance of electric cars is usually a slowly changing parameter, and the measurement process is fast, so it can be considered that the actual insulation resistance value to be measured remains unchanged during the measurement process.
The insulation resistance monitoring module mainly completes the following functions: voltage measurement of positive and negative busbars to the electrical chassis, control of standard bias resistance, alarm parameter setting, sound and light alarm, liquid crystal display and communication.
Usually, the nominal voltage of an electric car is 90~500V, and the actual bias resistance varies depending on the voltage. During operation, the battery voltage has a certain fluctuation range, and the insulation resistance to be measured also has a certain range of change. Therefore, the general-purpose monitoring The voltage measurement circuit of the system must ensure the same precision measurement in the full range, and the measurement of the positive and negative busbar-to-ground voltage must be completed at the same time.
⑨ High voltage interlock. Set a pass ring for the entire high voltage system. If the signal transmitted by the conduction ring is interrupted, the voltage is cut off and the capacitance of the high voltage system is discharged.
⑩ Service off/high voltage on lock. After disconnecting the voltage using the diagnostic aids, the staff not only ensures that the entire high-voltage system is turned off (high-voltage interlock is open), but also prevents the high-voltage system from being re-engaged by “ignition switch on”. By means of the insertion (connection) of the press-on lock, an additional insurance against switching on is added to the high-voltage system. Figure 10 shows the outline drawing and installation position of the high-voltage switch-on lock for Mercedes-Benz electric cars.
⑪ Power supply reverse polarity protection. If the positive and negative poles of the power supply are accidentally connected, the system will automatically cut off the high voltage.
⑫ Open cover detection protection. A low-voltage switch is set on the cover of the high-voltage battery and components. When the low-voltage switch is opened (the cover is opened), the system cuts off the high voltage.
⑬ Active discharge and passive discharge. Actively and passively monitor whether there is a short circuit to the body, and automatically and quickly discharge the power of the battery pack to prevent the battery from heating and burning.
In addition, each high-voltage circuit of the electric car high-voltage system has a fuse for overcurrent protection. A certain number of fuse box contactors are added inside the power battery assembly for protection. Each sampling line of the power battery also has a separate fuse protection. Even if a short circuit occurs, it can ensure that high-voltage components such as battery packs and wiring harnesses will not be damaged or damaged by a short circuit. catch fire.
(4) Functional safety
①Torque safety management.
In order to prevent the undesired motion of the car, it is necessary to add a torque safety control strategy to the car controller. The specific torque safety strategy is as follows.
a. The car controller is responsible for calculating the torque demand of the car. If the difference between the calculated torque requirements is greater than a certain calibration value, the torque output is considered to be a safety risk. At this time, the car controller will limit the car speed to a safe level. within range
b. If the difference between the demand torque calculated by the car controller and the actual torque of the motor is greater than a certain calibration value, it is considered that there is a risk in the torque control of the motor, and the car controller will limit the torque output of the motor. , if the difference between the two is always too large, the power output of the power battery will be cut off.
②Charging safety measures.
When charging, it is necessary to prevent the car from moving and avoid conflicts between fast charging, slow charging, and driving modes. The following measures are taken for this purpose.
a. Charging is only allowed when the gear is placed in the P gear.
b. During the charging process, both the torque demand and the actual torque output should be zero.
c. When the charging gun is plugged in, it is not allowed to close the contactor controlling the high voltage output.
d. When the insulation resistance of the charging circuit is less than the resistance value required by the standard, the charging should be stopped and the high-voltage contactor should be disconnected.
③ Battery pack safety management
a. Correction of available battery capacity. The battery management system (BMS) determines the available capacity and reports it to the car controller based on factors such as the discharge capacity of the single battery at ambient temperature and the fact that the battery system is not fully charged due to poor cell consistency during the slow charging process. (VCU), the VCU calculates the driving range based on this value.
b. SOC estimation and correction strategy. The SOC is corrected according to the highest voltage of the single battery in the on-board charging mode and the driving mode.
c. The current control strategy of the discharge process. During the driving discharge process, the discharge current cannot exceed the maximum allowable discharge current value reported by the BMS to the VCU. The current control strategy in the discharge process is that the BMS adjusts the “maximum allowable discharge current” value in real time according to the current SOC and maximum temperature of the power battery.
d. Energy feedback process control strategy. The BMS reports the “maximum allowable charging current” to the car controller to show the ability of the power battery to accept the maximum feedback current in the current state.
e. On-board charging current control strategy. When on-board charging, the BMS requests the maximum charging current according to the current minimum temperature request. When the battery is charged to the highest voltage 36 (for lithium batteries, the same below), the BMS requests the charging current to drop to 5A. The highest voltage of the monomer reaches 3.7V, the charging is stopped, and the SOC is corrected to 100%.
f. Ground charge control strategy. During fast charging, the interactive information and workflow between the power battery system and the ground charging pile are strictly implemented in accordance with the “Communication Protocol between Off-Board Conductive Chargers and Battery Management Systems for Electric cars” (GBT279302015). Limited by the charging capacity of the power battery, in order to better realize the fast charging function, a heating function is designed during the fast charging process.
The end condition of fast charging is that the highest cell voltage Umax of the battery is greater than or equal to 3.7V; SOC correction is not performed during the fast charging process; when the minimum temperature of the battery is Tmax <0°C, the heating relay is closed and the heating function is turned on.
g. Insulation process control strategy. After the on-board charging is completed, it is judged whether it needs to be kept warm according to the temperature of the battery.
Enter the heat preservation condition: Among the battery temperature data monitored by all temperature monitoring points, the highest value is lower than 25°C and the lowest value is lower than 10°C. When entering the heat preservation process, the BMS requests the on-board charger for heating demand voltage and current, and closes the heating relay to heat the battery. During the heating process, when the lowest value of the battery temperature data monitored by all temperature monitoring points is higher than 8°C, the heating relay is disconnected, the heating is stopped, and the heat preservation stage is entered. After the heat preservation time reaches 6h, the heat preservation is stopped and the heat preservation process is exited.
h. Power battery fault handling strategy. The power battery system diagnoses and reports the faults, processing measures and recovery conditions in the driving mode/car charging mode/ground charging mode.