Active Fast Discharge Method of Bus Capacitor for Electric Vehicle with Total Power Loss

When electric vehicles (EVs) encounter an emergency, the dc-bus voltage of EVs equipped with the high-voltage drive system needs to be reduced to safe voltage (60 V) as soon as possible. To drop the dc-bus capacitor voltage to a safe voltage, a winding-based active discharge method was proposed. However, traditional active discharge based on PI controller has poor robustness, long discharge time, and low safety, which cannot meet the discharge requirements of the United Nation Vehicle Regulation ECCE R94. This paper proposes an active discharge method for EVs with total power loss to reduce the discharge time and enhance the robustness. Firstly, the d-axis weakened current is calculated using the permanent magnet synchronous motor winding as the bleeding resistor. Then, the traditional active discharge method based on the PI controller is analyzed. Secondly, the dc-bus capacitor energy flow model is established. More importantly, the extended sliding mode observer (ESMO) is introduced to address long discharge time and poor robustness issues. The ESMO can observe the total loss, which consists of inverter loss, motor winding copper consumption, motor inductance energy storage, and other losses. The stability of ESMO is proved by the Lyapunov stability theory. Finally, observed total discharge loss can be feedforward compensated by the designed control law. Compared with the conventional discharge method, the influence of total power loss and motor speed on the dc-bus capacitor voltage is well suppressed, and the dc-bus capacitor voltage and motor speed are decoupled in the proposed strategy. Experimental results show that the DC-bus voltage of the proposed method drops to a safe voltage within 0.2 s, faster than traditional active discharge method-based PI controller (1.2 s) and disturbance observer (0.3 s). In contrast, the discharge time is shorter with the active discharge method-based disturbance observer because the convergence rate is faster than the disturbance observer. The experiment with parameter mismatch proves that the parameter error can be observed and compensated by the ESMO. Furthermore, electric vehicle drive systems' power density and reliability are further improved by eliminating the complicated drain circuit. The following conclusions can be obtained: (1) The traditional discharge method-based PI controller has the problem of changing the steady-state operating point with the motor speed, which could not meet the requirements of fast and safe discharge of the dc-bus capacitor in emergencies. (2) The discharge model with PMSM winding as the bleeding resistor is established, and the relationship between the dc-bus capacitor voltage and the total power loss of the system is deduced. Moreover, the observed total loss of the system can be feedforward compensated in the proposed discharge method, effectively suppressing the voltage pulsation when the dc-bus voltage reduces to the safe voltage. Hence, the discharge time is short, and the robustness and safety of the system are improved. (3) The simulation and experimental results show that the proposed control method for electric vehicles with total power loss estimation has strong robustness, high safety, strong practicability, and engineering application value. © 2024 China Machine Press. All rights reserved.