No abstract

The rapid development of electric vehicle ultra-fast battery chargers is increasingly demanding higher efficiency and power density. In particular, a proper control of the grid-connected active front–end can ensure minimum passive component size (i.e., limiting design oversizing) and reduce the overall converter losses. Moreover, fast control dynamics and strong disturbance rejection capability are often required by the subsequent DC/DC stage, which may act as a fast-varying and/or unbalanced load. Therefore, this paper proposes the design, tuning and implementation of a complete digital multi-loop control strategy for a three-level unidirectional T-type rectifier, intended for EV ultra-fast battery charging. First, an overview of the operational basics of three-level rectifiers is presented and the state-space model of the considered system is derived. A detailed analysis of the mid-point current generation process is also provided, as this aspect is widely overlooked in the literature. In particular, the converter operation under unbalanced split DC-link loads is analyzed and the converter mid-point current limits are analytically identified. Four controllers (i.e., dq-currents, DC-link voltage and DC-link mid-point voltage balancing loops) are designed and their tuning is described step-by-step, taking into account the delays and the discretization introduced by the digital control implementation. Finally, the proposed multi-loop controller design procedure is validated on a 30 kW, 20 kHz T-type rectifier prototype. The control strategy is implemented on a single general purpose microcontroller unit and the performances of all control loops are successfully verified experimentally, simultaneously achieving low input current zero-crossing distortion, high step response and disturbance rejection dynamics, and stable steady-state operation under unbalanced split DC-link loading.

This paper presents a fast and effective approach to Li-ion battery performance modeling, particularly suited for automotive applications (i.e. HEV, PHEV, BEV). A second-order electrical equivalent circuit model made up by one voltage source, one series resistor and two series RC blocks (dual-polarization model), is here selected as the best trade-off solution for the task, addressing both acceptable levels of accuracy and complexity. While a lithium-iron-phosphate cylindrical battery cell is chosen for the purpose of the study, the presented procedure has broader validity and is mostly independent of Li-ion chemistry and/or cell format. The battery model is parametrized through a low timeconsuming current pulse test, performed during both charging and discharging, at different state of charge levels. The temperature and load-current effects on the battery performance are not considered for simplicity and lightness of the presented model. Validation is carried out by comparing measured and simulated results during the dynamic current pulse test, showing a high level of agreement between the two.