Nonisolated Bidirectional Three-Phase Three-Level Wide-Output-Voltage-Range Voltage DC-Link EV Charger

Nonisolated three-phase ac/dc electric vehicle (EV) chargers show improved efficiency and power density compared with their counterparts with a galvanic isolation stage, but residual current devices (RCDs) are mandatory to ensure electrical safety. However, RCDs are prone to nuisance tripping caused by low-frequency (LF) common-mode (CM) leakage currents through the ground, which therefore must be suppressed. Therefore, first, modulation schemes that do not result in LF CM voltages (i.e., do not use third-harmonic voltage injection) that could drive LF CM currents through parasitic capacitors from the dc output to ground must be used. Second, closed-loop ground current control (GCC) ensures near-zero LF CM leakage currents even with a direct connection of the charger dc output midpoint to protective earth (PE). Considering a voltage DC-link power-factor-correction (PFC) rectifier system that consists of a boost-type three-level T-type (Vienna) ac/dc-stage and a dc/dc-stage with two stacked buck converters, this article proposes a new modulation scheme for buck-mode operation at low dc output voltages: the dc/dc-stage then shapes the DC-link voltage such that only one of the ac/dc-stage’s three bridge-legs operates with high-frequency (HF) switching [1/3-pulse-width modulation (PWM)] at any given time, and, different from previous methods, does not require third-harmonic injection to do so. Furthermore, a synergetic GCC is proposed, which operates the two converter stages in the loss-optimum mode for any output dc voltage (buck-mode and boost-mode) and regulates the LF CM ground current to near zero. The proposed concepts are verified using a 10-kW hardware demonstrator with a wide output voltage range (200–800V) and a direct connection of the dc output midpoint to PE, considering terra-terra (TT) and terra-neutral (TN) grid grounding systems, whereby the proposed GCC results in LF CM leakage currents below 7mA, i.e., far below typical RCD trip limits (30mA). The test voltages obtained with the human-body impedance model from UL 2202 are below 120mV, i.e., below 50% of even the most stringent limit of 250mV of the standard.