High Voltage Side DC-Bus Capacitor Voltage Balancing Control of a 350 kW Multiport EV Charging System

DSIM results

DSIM is used to verify the control scheme for a 6.6 kV and 350 kW system in this paper which consists of 21 cell units (contain 252 switches).

“DSIM is used due to the large numbers of converter system simulation, which would not be possible to study using the conventional simulation software like PSIM or MATLAB/Simulink”.

The system performance with balanced and unbalanced load condition is demonstrated from the simulation with DSIM. Under unbalanced load condition, DSIM simulation results show that (1) When the charging power of Electric Vehicle 2 is reduced from the rated power of 90 kW to 70 kW (a decrease of 23%), all three-phase high-voltage capacitor voltages stabilize at 1050 V. The modulation signal of Unit 2 (Vam_up2) is lower than that of Units 1 and 3, demonstrating the controllability of the system. (2) When the charging power of Electric Vehicle 2 is further reduced from the rated 90 kW to 45 kW (a decrease of 50%), all high-voltage capacitor voltages stabilize at the reference value, thereby verifying the stability of the control scheme proposed by the paper.

Abstract:
Solid state transformer (SST) based power converter is now days very popular for high power EV charging, while keeping a reduced overall footprint size as compared to the conventional low frequency-based solutions. The output from all cell converter units are connected together to form a common DC-bus, which can be then connected to EVs with the help of additional DC/DC converters. Although, this topology is quite simple and convenient as an EV charging solution, the numbers of the output side DC/DC converters are proportional to the connected EVs. Hence, the overall system cost is high, and the overall system efficiency is also lower due to the multiple series connected converter units. To overcome this limitation a matrix switch-based charging solution is proposed in this research work. In this topology the output from the cell converter units are connected to a matrix switch in place of connecting them together, which helps to provide three separate fast charging ports (100 kW, 100 kW, 150 kW respectively). However, this strategy has a common problem of the DC bus voltage balancing, while serving inequal load power. Hence, to solve this issue a DC-Bus voltage balancing strategy is proposed which can keep the high voltage side DC-bus voltages balanced, while even feeding inequal charging power to the connected EVs. Detailed simulation studies and experimental verification is presented to verify the control scheme for a 6.6 kV and 350 kW system.