Nonisolated Three-Phase Current DC-Link Buck-Boost EV Charger With Virtual Output Midpoint Grounding and Ground Current Control
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2024-03
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Journal Article
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Abstract
Nonisolated three-phase AC/DC converter concepts facilitate more compact and more efficient realizations of future electric vehicle (EV) chargers. However, without the galvanic isolation and/or high common-mode (CM) impedance provided by an isolation transformer, nonisolated chargers must employ other means to suppress CM leakage currents to ground sufficiently and to prevent nuisance tripping of mandatory residual current devices (RCDs). Typically, the required electromagnetic interference (EMI) filters reduce high-frequency (HF) CM leakage currents to uncritical values. However, low-frequency (LF) CM voltages, e.g., generated by third-harmonic injection, may drive significant LF CM currents through the parasitic capacitances of the DC output (including the battery pack) to protective earth (PE). Therefore, considering a nonisolated three-phase buck-boost (bB) current DC-link PFC rectifier system that consists of a buck-type current-source rectifier (CSR) stage and a three-level boost-type DC/DC-stage, this article first proposes a virtual grounding control (VGC) of the DC output voltage midpoint. VGC employs the DC/DC-stage to compensate the LF (third-harmonic) CM voltage inherently generated by the CSR-stage and thus controls the LF CM voltage between the DC output midpoint and PE to zero. This enables further a direct connection of the DC output midpoint to PE, where an additionally proposed ground current control (GCC) ensures near-zero LF CM leakage current. The proposed concepts are verified with a 10-kW hardware demonstrator (power density of 6.4 kW/dm 3 or 107.5 W/in 3 and full-load peak efficiency of 98.5%) considering terra-terra (TT) and terra-neutral (TN) grounding systems. With a direct connection of the DC output midpoint to PE, GCC limits the LF CM leakage current to < 6 mA RMS, i.e., significantly below typical RCD trip levels, and, using the human-body impedance model according to UL 2202, achieves a test voltage of 110 mV that is clearly below the most stringent limit (250 mV) of the standard.
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published
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Volume
10 (1)
Pages / Article No.
1398 - 1413
Publisher
IEEE
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Subject
Ground current control (GCC); nonisolated electric vehicle (EV) charger; synergetic control; three-phase buck-boost (bB) current DC-link PFC rectifier; two-third pulsewidth modulation (PWM); virtual grounding control (VGC)
Organisational unit
03573 - Kolar, Johann W. (emeritus) / Kolar, Johann W. (emeritus)