University of South Alabama,
Keywords: battery storage, control strategies, efficiency, electric vehicles, power electronics, ultracapacitor (UC).
Summary:This paper presented a novel control technique for hybrid SC bi-directional DC/DC converter, applicable for HEV/PHEV ESS applications. SCCs offer essential features of voltage step-down, voltage step-up, and bidirectional power flow, associated with two or more HEV energy storage devices. Furthermore, detailed efficiency modeling and analyses are conducted for the specific 4-quadrant topology presented in this paper, using the designed novel SCC control strategy. Tests conducted on the proposed topology depict the following major advantages: (a) lower source current ripple, (b) simpler dynamics, (c) control simplicity, and (d) continuous input current waveform in both modes of operation (boost as well as buck). Future work includes hardware-in-the-loop (HIL) implementation of the 4-Q SCC topology with the proposed novel control strategy, for EV/HEV dual energy storage systems (ESS), running on various driving load patterns. The typical bidirectional SCC consists of 6 switches and 2 capacitors, C1 and C2. Each switch consists of two MOSFETs for current flow in both directions. For an HEV application, the high voltage (HV) side typically consists of battery modules and the low voltage (LV) side could consist of ultra-capacitor (UC) modules. In this case, a Lithium-ion (Li-ion) battery is used, with total battery bank voltage of 215 V. The low voltage side is set at 120V and UC initial voltage is at 110V. This type of converter operates in four quadrants (forward and reverse mode). When the converter operates in motoring mode (P ≥ 0), two conditions are chosen: If the battery current gradient is between -2mA/sec and 2mA/sec, battery modules supply their energy to the load and motor side. Otherwise, UC modules supply their power to load and motor side. Secondly, during motoring mode, power of motor (Pm) is compared with 90% of load power (PL). If Pm < 0.9*PL, with attention given to battery current gradient. UC or battery modules supply their power to the motor. Otherwise, if the battery modules need energy and fully discharge, the UC modules transfer their energy to battery or HV side. If the UC modules need energy and fully discharge, the battery modules deliver their energy to UC or LV side. On the other hand, when the converter operates in generating mode (P < 0), only the first condition is considered. In generating mode, the motor tends to give up its power; thus it is not compared with load power.