This article is part of our special IEEE Journal Watch series in partnership with IEEE Xplore.
rapid construction of Fast-charging stations for electric vehicles Testing the limits of today’s power grid. While individual chargers draw 350 to 500 kilowatts (or more), making charging times for EVs functionally equivalent to filling up times for a gasoline or diesel vehicle, full charging sites can reach megawatt-scale demand. It’s enough to stress Medium-voltage distribution network – The section of the grid that connects high-voltage transmission lines to low-voltage lines that serve end users in homes and businesses.
DC fast charging stations are clustered in urban centres, highways and fleet depots. Because the load is not spread evenly across the network, particular substations are overworked – even when the total grid capacity is rated to accommodate the load. Overcoming this problem as more charging stations with higher power demands come online requires power electronics that are not only compact and efficient, but also able to manage local storage and renewable inputs.
One of the most promising technologies to modernize the grid so that it can meet the demands of vehicle electrification and renewable generation is the solid-state transformer (SST). An SST performs the same basic function as a conventional transformer – stepping the voltage up or down. But it uses high-frequency conversion and digital control with semiconductor, silicon carbide or gallium nitride switches rather than passive magnetic coupling alone. The setup of an SST allows it to dynamically control power flow.
For decades, charging infrastructure has been relied upon Line-Frequency Transformer (LFT) – huge assemblies of iron and copper that carry medium-voltage AC to low-voltage AC before or after the external conversion from alternating current to direct current required for EV batteries. A typical LFT may contain a few hundred kilograms of copper windings and a few tons of iron. that’s all metal Expensive and becoming difficult to source, These systems are reliable but cumbersome and inefficient, especially when energy flows between local storage and vehicles. SSTs are much smaller and lighter than the LFTs they are designed to replace.
“Our solution achieves the same semiconductor device count as a single-port converter while providing multiple independently controlled DC outputs.” -Shashidhar Mathapati, Delta Electronics
But most multiport SSTs developed so far are too complex or expensive (between five and 10 times the upfront cost of LFT). That difference – plus SST’s reliance on auxiliary battery banks that add more expense and reduce reliability – explains why the clear benefits of solid-state have not yet materialized. Transfer from LFT to technology was encouraged.
Surajkant Majumdar, Saichand Kashicheyanula, Harisyam PV and Kaushik Basu place their SST prototype in a laboratory.Harisyam PV, Saichand Kashicheyanula, et al.
How to make solid-state transformers more efficient
one in Study Published on August 20 in IEEE Transactions on Power Electronicson researchers Indian Institute of Science And Delta Electronics IndiaIn Bengaluru, the two proposed a Cascade H-Bridge (CHB)-based multiport SST that circumvents those compromises. “Our solution achieves the same semiconductor device count as a single-port converter while providing multiple independently controlled DC outputs,” says Shashidhar MathapatiCTO of Delta Electronics. “That means no extra battery storage, no extra semiconductor devices, and no extra medium-voltage insulation.”
The team built a 1.2-kilowatt laboratory prototype to validate the design, which achieved 95.3 percent efficiency at rated load. They also modeled a full-scale 11 kilovolt, 400 kilowatt system divided into two 200 kilowatt ports.
At the heart of the system is a multi-winding transformer located on the low-voltage side of the converter. This configuration avoids the need for expensive, bulky medium-voltage insulation and allows power balancing between ports without an auxiliary battery. “Previous CHB-based multiport designs required multiple battery banks or capacitor networks to equalize the load,” the authors write in their paper. “We have shown that you can achieve similar results with simpler, lighter and more reliable transformer arrangements.”
A new modulation and control strategy maintains a unity power factor at the grid interface, meaning that any current coming from the grid is not wasted by oscillating back and forth between source and load without doing any work. The SST described by the authors also allows each DC port to operate independently. In practice, every vehicle connected to the charger will be able to receive the appropriate voltage and current, without affecting neighboring ports or disturbing the grid connection.
Using series-connected silicon-carbide switches, the system can handle medium-voltage inputs while maintaining high efficiency. An 11-kilovolt grid connection would require only 12 cascade modules per stage, about half as many as some modular multilevel converter designs. Fewer modules ultimately means lower costs, simpler controls and greater reliability.
Although still in the laboratory stage, the design could enable a new generation of compact, cost-effective fast-charging hubs. By removing the need for intermediate battery storage – which adds cost, complexity and maintenance – the proposed topology can extend the operational lifetime of EV charging stations.
According to researchers, this converter is not just for EV charging. Any application that requires medium-voltage to multiport low-voltage conversion – such as data centers, renewable integration, or industrial DC grids – can benefit.
For utilities and charging providers facing megawatt-scale demand, this streamlined solid-state transformer could help make the EV revolution more grid-friendly, and faster for drivers waiting to charge.
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