The hybrid-cascaded multilevel converter proposed in this paper can actualize the charging and discharging of the battery cells while the terminal voltage or SOC balance control can be realized at the same time. The proposed converter with modular structure can reach any number of cascaded levels and is suitable for the energy storage system control with low-voltage battery cells or battery modules. The fault module can be bypassed without affecting the running of the other ones, so the converter has a good fault-tolerant character which can significantly improve the system reliability.
The PWM method with low switching loss for both discharging and charging control is proposed considering the balance control at the same time. The output of the circuit is multilevel ac voltages where the number of levels is proportional to the number of battery cells. So the output ac voltage is nearly the ideal sinusoidal wave which can improve the control performance of the motor control in EVs. A dc bus current control method for battery charging with external dc or ac source is also studied where the constant-current control can be realized and the additional charger is not needed any more. Experiments are implemented and the proposed circuit and control method are verified.
TOPOLOGY OF THE HYBRID CASCADED MULTILEVEL CONVERTER
One of the popular voltage balance circuits by energy transfer is shown in Fig. 2.1. There is a half-bridge arm and an inductance between every two nearby battery cells. So the number of switching devices in the balance circuit is 2∗n–2 and the number of inductance is n–1 where n is the number of the battery cells. In this circuit, an additional inverter is needed for the motor drive and a charger is usually needed for the battery recharge. In fact, if the output of the inverter is connected with the three-phase ac source by some filter inductances, the battery recharge can also be realized by an additional control block which is similar with the PWM rectifier.
The recharging current and voltage can be adjusted by the closed-loop voltage or power control of the rectifier. The hybrid-cascaded multilevel converter proposed in this paper is shown in Fig. 2.2, which includes two parts, the cascaded half-bridges with battery cells shown on the left and the H bridge inverters shown on the right. The output of the cascaded half-bridges is the dc bus which is also connected to the dc input of the H-bridge. Each half-bridge can make the battery cell to be involved into the voltage producing or to be bypassed.
Therefore, by control of the cascaded half-bridges, the number of battery cells connected in the circuit will be changed, that leads to a variable voltage to be produced at the dc bus. The H-bridge is just used to alternate the direction of the dc voltage to produce ac waveforms. Hence, the switching frequency of devices in the H-bridge equals to the base frequency of the desired ac voltage. There are two kinds of power electronics devices in the proposed circuit.