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

As the power source of electric vehicles, when the electric drive system fails, it is a necessary task to analyze its hazards and risks and ensure that the electric drive system enters a safe working state: active short-circuit working state ASC, open-circuit working state OC or 0-Torque control state.

The following will introduce the safety protection strategy of high-voltage electric drive system from the following points:

• What is ASC?

• The trend of current changing with speed in ASC state;

• Variation trend of motor torque with speed in ASC state;

• What is ****OC;

• What is 0-torque control?

• Safety protection strategy: OC, ASC, 0-torque selection mechanism

1. Active short circuit working state: ASC

Active short circuit is also called ASC (active short circuit). Taking the electric drive system equipped with a three-phase IGBT power module as an example, the three tubes of the upper bridge arm of the IGBT are turned off and the three tubes of the lower bridge arm are turned on at the same time, as shown in Figure 1; or the three tubes of the upper bridge arm of the IGBT are turned on and the three tubes of the lower bridge arm are turned off at the same time, which is the safe working state of active short circuit protection.

Figure 1. IGBT operating state when the lower bridge arm is actively short-circuited

In the active short-circuit working state, the motor stator winding and the IGBT of the lower bridge arm form a closed loop, the back electromotive force energy generated by the motor is released through the stator winding, and the motor output end generates a corresponding braking torque. Taking a permanent magnet synchronous motor with a peak value of 150kw as an example, the changes of current and motor braking torque with speed in the ASC working state are simulated.

The flux equation of a permanent magnet synchronous motor is as follows:

When entering the ASC working state, the motor input d-axis and q-axis voltages are 0. When reaching a steady state, we get:

When the ASC is in operation, at different speeds, the steady-state Id current increases rapidly with the increase of speed. After reaching a certain speed, the current remains basically unchanged and is equal to the characteristic current, as shown in Figure 2, for reference only. The specific value is related to the design parameters of the motor: flux linkage, inductance, number of pole pairs, winding resistance, etc.

Figure 2. Id current simulation curve under ASC state

When the ASC is in working state, at different speeds, the steady-state Id current increases rapidly with the increase of speed in the low-speed zone, then decreases rapidly with the increase of speed, and tends to be stable in the high-speed zone, as shown in Figure 3.

Figure 3. Iq current simulation curve in ASC state

When the ASC is in operation, at different speeds, the motor output torque increases rapidly with the increase of speed in the low-speed zone, then decreases rapidly with the increase of speed, and tends to be stable in the high-speed zone, as shown in Figure 4.

Figure 4. Simulation curve of motor torque under ASC state

When actively short-circuiting ASC****, the characteristics of the electric drive system are:

• Significant braking torque is generated in the low-speed area;

• The continuous current generated by the back EMF may cause the motor to overheat;

• The risk of rotor magnet demagnetization caused by motor overheating;

• Motor overheating causes inverter overheating, resulting in inverter damage;

2. Open circuit working state: OC

The open circuit protection working state is also called OC (open circuit) or Freewheeling. Taking the electric drive system equipped with a three-phase IGBT power module as an example, by turning on all the tubes of the IGBT upper and lower arms, the inverter enters a passive rectification state, which is open circuit protection, as shown in Figure 5.

Figure 5. IGBT operating state when open circuit

When the motor runs in the high speed area, if it enters the open circuit protection working state, the back electromotive force generated by the motor is higher than the bus voltage, and is fed back to the high-voltage battery through the freewheeling diode to form a closed loop, as shown in Figure 5. At this time, a large braking torque is generated at the motor end. At the same time, this uncontrollable passive rectification causes the motor back electromotive force to have a large impact on the devices hung on the DC bus, such as bus capacitors, IGBTs, etc.

When the motor runs in the low speed area, if it enters the open circuit protection working state, the back electromotive force generated by the motor is lower than the bus voltage, and cannot be fed back to the high-voltage battery through the freewheeling diode, and a closed loop cannot be formed. At this time, the motor end runs at no load. At this time, the motor back electromotive force will not cause impact damage to the devices hanging on the DC bus.

When the OC is open circuit protected, the characteristics of the electric drive system are:

• The phase current in the high-speed region flows through the freewheeling diode;

• High back electromotive force in high-speed area brings impact damage to the devices on the busbar;

• Unexpectedly large braking torque is generated at the motor output end in the high-speed zone;

• In the low-speed zone, there is only friction torque such as bearings at the motor output end.

3. 0-torque control working state

As the name implies, 0-torque control means that the inverter enters the 0Nm control state, that is, the motor output torque is 0Nm. However, the prerequisite for the 0-torque control working state is that the high-voltage and low-voltage power supplies are normal and the electric drive system can perform 0Nm output.

4. Safety protection strategy: ASC, OC, 0-torque selection mechanism

The automotive functional safety standard ISO26262 strictly requires that when a fault occurs in the electric drive system and other vehicle systems (such as batteries, DCDC, chargers, and vehicle VCU), the motor controller receives the fault and responds promptly to enter a safe working state, so that the electric drive can operate in an appropriate working state to avoid personal injury and, at the same time, avoid further damage to the electric drive system as much as possible. When the electric drive system is operating in a safe working state, the following events must be avoided:

Avoid causing casualties due to inappropriate torque and speed output of the vehicle;

Avoid casualties caused by excessive back electromotive force or high battery pack voltage output;

Avoid excessive back electromotive force from damaging the components (such as IGBT, DC capacitor, etc.) hung on the busbar;

Avoid excessive temperature from causing damage to the inverter or demagnetization of the rotor magnet;

etc……..

By analyzing the safe working state of the electric drive system: the characteristics of ASC and OC, the safe working state can be entered roughly according to the following mechanism, as shown in Figure 6:

• When the bus voltage is large enough, the open circuit protection working state OC is considered;

• When the bus voltage is not large enough and there is a phenomenon that the back electromotive force exceeds the bus voltage, consider entering the open circuit protection working state OC in the low-speed area and the active short circuit protection working state ASC in the high-speed area. Therefore, the use of OC in the low-speed area avoids the use of ASC to generate a large braking force that causes a large impact on vehicle operation and affects driving comfort; the use of ASC in the high-speed area avoids the use of OC to generate a large back electromotive force that causes impact damage to the devices on the bus.

Figure 6. Safe working state selection mechanism 1

Considering that the active short-circuit working state can easily cause overheating of the motor or inverter, a combination of active short-circuit and 0-torque control can be designed in the high-speed area to adjust the safe working state, as shown in Figure 7. When 0-torque control is introduced, the functional safety standard ISO26262 needs to be considered. Then the control system of the entire safe working state selection mechanism will need to be designed to be more complex: when a fault occurs, it is necessary not only to monitor in real time whether the 0-torque control can be executed normally, but also to monitor that the path of the 0-torque control will not cause random torque output. The control of the fault-tolerant time interval (FTTI) is also a severe challenge.

Figure 7. Safe working state selection mechanism 2

When a fault in the vehicle system or electric drive system affects driving safety, the vehicle system will disconnect the high-voltage relay immediately and prohibit the battery from outputting high voltage based on the safety of the high-voltage function to avoid danger to personnel caused by the high-voltage power supply still outputting when a fault occurs. At this time, 0-Torque control cannot be executed.

Relatively speaking, the implementation of OC and ASC is simpler and faster. Regardless of the occurrence of any system failure or hardware failure, the inverter hardware level can monitor each other through devices or circuits to achieve rapid implementation and switching of OC and ASC. Therefore, in pursuit of simplicity and efficiency, most of the current electric drive systems have adopted the safe working state selection mechanism 1 as shown in Figure 6. Of course, a small number of suppliers use a combination of ASC and 0-torque control, as shown in Figure 7, safe working state selection mechanism 2.