An Overview on Medium Voltage Grid Integration of Ultra-Fast Charging Stations: Current Status and Future Trends

Abstract

The emphasis on clean and green technologies to curtail greenhouse gas emissions due to fossil fuel-based economies has originated the shift towards electric mobility. As on-road electric vehicles (EVs) have shown exponential growth over the last decade, so have the charging demands. The provision of charging facilities from the low-voltage network will not only increase the distribution system's complexity and dynamics but will also challenge its operational capabilities, and large-scale upgrades will be required to meet the inevitably increasing charging demands. An ultra-fast (UF) charging infrastructure that replicates the gasoline refueling network is urgently needed to facilitate a seamless transition to EVs and ensure smooth operation. This paper presents a review of state-of-the-art DC fast chargers, the charging infrastructure's current status, motivation, and challenges for medium-voltage (MV) UF charging stations (UFCS). Furthermore, we consider the possible UFCS architectures and suitable power electronics topologies for UF charging applications. To address the peak formation issues in the daily load profile and high operational expenses of UFCSs, integration of renewable energy sources and energy storage systems due to their technological and economic benefits is being considered. The benefits of line frequency transformer (LFT) replacement with a solid-state transformer (SST), SST models, SST-based UF chargers, and MV SST-based UFCS architectures, as well as related MV active front-end and back-end power electronics topologies, are presented. Finally, the application of microgrids' hierarchical control architecture is considered for chargers and system-level control and management of UFCSs.INDEX TERMS Electric vehicles (EVs), DC fast chargers, ultra-fast charging stations (UFCS), renewable energy sources (RESs), energy storage systems (ESSs), line frequency transformer (LFT), solid state transformer (SST).

New single‐stage bidirectional three‐phase ac‐dc solid‐state transformer

“…Similarly, the proposed SST's volumetric power density can be estimated based on the main components. First, assuming forced air cooling, the heat sink volume can be estimated with a cooling system performance index [43] of CSPI = 10 W/(K dm 3 ); with the total semiconductor losses calculated above, a maximum heat sink temperature of 70 • C, and an ambient temperature of 45 • C, we find about 50 dm 3 . The volume of the MFT is estimated at 285 dm 3 by conservatively assuming a power density of 3.5 kW/dm 3 based on built prototypes reported in the literature [44].…”

“…Conventionally, a low-frequency, that is, grid-frequency transformer (LFT) provides galvanic separation and steps down the MVac voltage to low-voltage ac (LVac) levels like 400 or 690 V. The subsequent power-factor-correction (PFC) rectifier is then not exposed to MV, but, on the other hand, the LFT's relatively large volume might limit the overall power density. To enable more compact system realizations and/or to provide extended functionality [1], MVac-LVdc solid-state transformers (SSTs) employing medium-frequency transformers (MFTs) instead of LFTs are considered for applications like EV charging [2][3][4][5][6][7][8][9][10][11], electrolyzers [12], and future datacenters [13][14][15].…”

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“…Recognizing this need, the International Electrotechnical Commission (IEC) has recommended a substantial increase in ESS capacity within the next 30 years, proposing a doubling or tripling of the current levels. [ 1 ] This strategic initiative aims to ensure a reliable and sustainable energy infrastructure capable of accommodating the growing global energy consumption while fully leveraging the potential of renewable energy sources. Pumped hydroelectric storage (PHS), a technology that harnesses the potential energy of water to store and release electricity, currently dominates the ESS market relying on its low capital cost.…”

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“…Both ESS and EV chargers can have similar voltage conversion interfacing with the MV-AC grid, e.g., 13.8 kV and 30 kV classes, to provide low voltage (LV)-DC on the EV charging pole (CP) or battery. In many cases, ESS is employed with EV chargers to help with peak shaving [4], with such integration already realized by EVgo [7], Volkswagen, and Tesla [8]. Several EV charging infrastructures have been proposed for off-board EV charging [9]- [11] but can be divided into three major categories shown in Fig.…”