Abstract
State-of-the-art ultra-fast battery chargers for electric vehicles simultaneously require high efficiency and high power density, leading to a challenging power converter design. In particular, the grid-side filter, which ensures sinusoidal current absorption with low pulse-width modulation (PWM) harmonic content, can be a major contributor to the overall converter size and losses. Therefore, this paper proposes a complete analysis, design and optimization procedure of a three-phase LCL filter for a modular DC fast charger. First, an overview of the basic LCL filter modeling is provided and the most significant system transfer functions are identified. Then, the optimal ratio between grid-side and converter-side inductance is discussed, aiming for the maximum filtering performance. A novel design methodology, based on a graphical representation of the filter design space, is thus proposed. Specifically, several constraints on the LCL filtering elements are enforced, such that all feasible design parameter combinations are identified. Therefore, since in low-voltage high-power applications the inductive components typically dominate the overall filter volume, loss and cost, the viable LCL filter design that minimizes the total required inductance is selected. The proposed design procedure is applied to a 30 kW, 20 kHz 3-level unidirectional rectifier, employed in a modular DC fast charger. The performance of the selected optimal design, featuring equal grid-side and converter-side 175 μμH inductors and 15 μμF capacitors, is verified experimentally on an active front-end prototype, both in terms of harmonic attenuation capability and current control dynamics. A current total harmonic distortion (THD) of 1.2% is achieved at full load and all generated current harmonics comply with the applicable harmonic standard. Moreover, separate tests are performed with different values of grid inner impedance, verifying the converter control stability in various operating conditions and supporting the general validity of the proposed design methodology.
Highlights
Even though the performance of Li-ion batteries is constantly increasing, their weight and cost still pose a major barrier to a widespread vehicle electrification [1]
The major contribution of this paper is to present exclusive experimental results, aimed at validating the proposed design methodology with a purposely built LCL filter for a 30 kW 3-level unidirectional active front-end (AFE) unit for electric vehicles (EVs) ultra-fast charging
Each of the N AFE modules consists of a 3-level unidirectional T-type rectifier and an LCL filter, both rated at 30 kW nominal active power
Summary
Even though the performance of Li-ion batteries is constantly increasing, their weight and cost still pose a major barrier to a widespread vehicle electrification [1]. The design methodologies adopted in [30,32] determine the converter-side inductance from a maximum current ripple target, select the filter capacitance to achieve a fixed reactive power generation and determine the grid-side inductance from the harmonic attenuation requirement These procedures fail to identify the role of the ratio between the grid-side and the converter-side inductance values in the filter size and loss minimization, leading to sub-optimal filter designs. This procedure is applied to the input filter of a 30 kW, 20 kHz 3-level AFE for EV ultra-fast charging, identifying the feasible LCL filter design with lowest total inductance.
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