Analysis and application of active short-circuit protection strategy for electric vehicles

The safe working state is triggered based on the system architecture, software architecture and hardware architecture. Different faults need to be combined with information such as the time and location of the fault and functional safety to select the corresponding safe working state to be triggered at the software or hardware level.

5. Trailer protection application

When a vehicle breaks down or in some special circumstances, it needs to be temporarily towed with the driving wheels on the ground. When there is no disengagement device between the electric vehicle drive motor and the transmission shaft, the motor will generate back electromotive force. At this point, we need to consider the safe working state of the electric vehicle when it is temporarily towed:

• If the electric drive system enters OC mode, there will be back electromotive force, which will pose a safety threat to the motor controller switch tube, bus capacitor and other devices;

• If the electric drive system enters ASC mode, the back-electromotive force energy is released through the motor stator winding, effectively protecting the safety of electrical appliances. However, there is also a risk of overheating due to large braking torque during low-speed towing and long high-speed towing time.

As the saying goes: choose the lesser of two evils, so electric vehicles often put the electric drive system into ASC mode when they are temporarily towed. Friends who have purchased new energy electric vehicles will find that the electric vehicle manual is marked: it is not allowed to tow the vehicle with the drive wheels on the ground, as shown in Figure 8. If necessary, it is allowed to tow at a low speed (such as 5km/h) for a short distance and a short time, because the electric drive system enters the ASC working state when being towed. In addition, some people may propose that 0-torque control is used for temporary towing, but temporary towing is often required when the vehicle has a fault. As mentioned earlier: based on functional safety considerations, the bus voltage is disconnected after a fault occurs, resulting in the inability to achieve 0-torque control.

In special cases, such as when the battery is low and no backup power is available, and a temporary towing truck is needed, the electric vehicle can be towed with the driving wheels on the ground. The specific operation is as follows: the towed electric vehicle must be put into D gear and the accelerator pedal must be pressed to maintain a certain stroke. This operation simulates the operating condition of the towed vehicle's electric drive system entering the power generation mode, provided that the vehicle's power/control system can work normally and the towing speed can reach a higher speed, such as 45 kilometers per hour, and the towed vehicle is equivalent to charging.