Single-Phase Grid-Forming Strategy with Power Decoupling Implementation for Electrolytic-Capacitor-Free EV Smart Battery Charger

Abstract

Smart Battery Chargers (SBCs) implementing grid-forming (GFM) control strategies are a promising solution to provide voltage and frequency support, increasing the grid reliability. Typically, GFM studies consider inverters with high DC-link capacitance. Therefore, there is a research gap in two-stage DC-AC converters with small DC-link capacitance implementing GFM strategies. This article proposes a novel approach to implement a GFM control strategy along with active power decoupling (APD) in an isolated, single-phase, electrolytic-capacitor-free two-stage DC-AC structure. The structure is composed of a voltage source inverter (VSI), DC-linked by film capacitors, with a dual-active-bridge series-resonant (DABSR) DC-DC converter. High DC-link ripple is allowed and managed by the APD. Hence, electrolytic capacitors are avoided, increasing the converter lifetime. In the proposed approach, the VSI implements the GFM strategy, operating in the four quadrants of the active and reactive power plane. However, the DABSR allows galvanic isolation, average DC-link voltage control, and suppression of the low-frequency ripple on the battery current, minimizing the impact in the battery lifetime. Design criteria are given for the DC-link voltage controller, active-reactive power controllers, inner inverter controllers, and APD technique. The control strategy is validated for vehicle-to-grid and stand-alone vehicle-to-home applications, using hardware-in-the-loop for a 2 kW test setup.

HESS Management for Virtual Inertia, Frequency, and Voltage Support Through Off-Board EV Bidirectional Chargers

“…For simplicity, no power losses are considered. Moreover, in the proposed control strategy, the controllers are classified according to timescale as fast, medium, and slow response [24]. A fast response is required for the inner AC current controller on the AC-DC stage, with a timescale of around a few milliseconds.…”

“…The equivalent circuit to control battery current 𝐼 𝑜 is shown in Fig. 9(a), where 𝐿 𝑜 , 𝐶 𝑜 are the inductance and capacitance for the DC filter and 𝑅 𝑜 is the inductor series resistance, whereas the averaged current source 〈𝑖 𝑜ℎ𝑓 〉 is given by [24]:…”

“…In ( 16) 𝜙 is the phase-shift angle, 𝑛 is the turns-ratio of HF transformer, 𝑉 𝐷𝐶 is the DC-Link voltage, 𝑍 is the tank impedance, and 𝐹 is the ratio between the switching frequency 𝜔 𝑠 and the resonance frequency 𝜔 𝑟 . The parameters 𝑍, 𝐹 and 𝜔 𝑟 are defined in (17), where 𝐿 𝑠 and 𝐶 𝑠 are the inductance and capacitance of the resonant circuit [24].…”

“…Hence, since electric vehicles (EVs) remain plugged into the grid most of the day [1], EV batteries along with their bidirectional chargers (EVBCs) present a high potential to supply both VI and long-term frequency support. However, according to the state-of-the-art review, very few studies have been carried out to integrate VI support in EVBCs [1]- [2], [18]- [24]. Moreover, the fast-dynamic response of active power required for VI implementation would decrease the battery lifetime [3].…”

The Scope of Application of Power Decoupling Technology and Its Evolution

“…Fig. 1 Two-stage DC-AC charger structure and control overview [3] The analyzed two-stage DC-AC structure and the overall control overview are depicted in Fig. 2.…”