Novel fast charging solution for electric vehicles

With surging popularity for Electric Vehicles (EVs), rapid charging is a challenge as it requires power delivery exceeding 1 MW (which can power about 1,000 homes). Conventional charging stations rely on bulky line frequency transformers (LFTs), which are expensive due to extensive use of copper and iron. These stations also have multiple conversion stages involving stepping up or down current, or converting AC to DC and vice versa. This can greatly increase cost and reduce efficiency.

To solve this problem, researchers at the Department of Electrical Engineering (EE), Indian Institute of Science (IISc), in collaboration with Delta Electronics India, have developed a novel cascaded H-bridge (CHB)-based multiport DC converter that directly connects to the medium-voltage AC (MVAC) electricity grid. This eliminates the need for large and expensive LFTs.

Published in IEEE Transactions on Industrial Electronics, the study shows that such converters can help address the growing power demands of fast charging EV stations, crucial for scaling up India’s EV infrastructure.

“Today's high-power chargers are complex, with multiple conversion stages between the grid, the energy storage, and the vehicle battery,” explains Kaushik Basu, Associate Professor in EE and corresponding author of the study. “Our invention simplifies this process and offers a 3-5% improvement in energy efficiency. At a megawatt scale, this leads to energy saving, significant reductions in cost, and a smaller material footprint, which makes the charging stations environmentally friendly.”

Such converters would be especially helpful for stations where multiple vehicles need to charge simultaneously, points out Harisyam PV, first author and PhD student in EE. “Going from a line frequency transformer-based solution to solid state transformers, we made the solution much smaller and more compact compared to existing solutions,” he adds.

Along with power conversion, the technology facilitates a compact bidirectional charging hub that provides key advantages. In addition to powering multiple vehicles, it also enables local storage of charge in batteries, and can easily be hooked up to renewable energy sources like solar panels.

Storing charge locally in batteries allows for charging when there are sudden high-power demands on the distribution grid, when multiple vehicles are charging simultaneously. The bidirectional converter can not only transfer power from the grid to the vehicles, but also from the station's storage back to the grid in critical conditions. This feature would allow the charging station to function with uninterrupted power supply. It can also potentially support essential facilities like hospitals during a power outage.

The team built and tested a 1.2 kW lab-scale prototype and found that it was able to supply DC power for charging EVs at an efficiency higher than 95%. They now plan on scaling up the system to handle higher loads.

Beyond EV charging, this technology also holds promise for applications in next-generation data centres, wind energy systems, and railway traction by enabling high-efficiency power conversion directly at medium voltages.

“This project is part of a national effort to implement fast EV charging technology across India within the next five years. With funding from the Department of Science and Technology (DST) and in partnership with Delta Electronics, we are establishing a 2,500 square feet lab at IISc,” adds Basu. “The facility will be used to demonstrate a megawatt-level converter capable of simultaneously charging a 200 kW bus and a 50 kW car directly from an 11 kV grid line.”