This article is part of our exclusive IEEE Journal Watch series in partnership with IEEE Xplore.
The rapid construction of fast charging stations for electric vehicles tests the limits of the current electricity network. With individual chargers consuming between 350 and 500 kilowatts (or more), making electric vehicle charging times now functionally equivalent to the fueling time of a gasoline or diesel vehicle, comprehensive charging sites can achieve megawatt-scale demand. It’s enough to force medium-voltage distribution networks: a segment of the network that connects high-voltage transmission lines to low-voltage lines that serve end users in homes and businesses.
DC fast charging stations tend to be clustered in urban centers, along highways and at fleet depots. Because the load is not distributed evenly across the network, some substations become overloaded, even when the overall network capacity is designed to accommodate the load. To overcome this problem, as more charging stations, with higher power demands, come online, there is a need for power electronics that are not only compact and efficient, but also capable of handling local storage and renewable inputs.
One of the most promising technologies for modernizing the grid so that it can meet the demands of vehicle electrification and renewable energy generation is the solid-state transformer (SST). An SST performs the same basic function as a conventional transformer: to increase or decrease voltage. But it does so using semiconductors, high-frequency conversion with silicon carbide or gallium nitride switches, and digital control, instead of just passive magnetic coupling. Configuring an SST allows it to control power flow dynamically.
For decades, charging infrastructure has relied on line frequency transformers (LFT): Massive iron and copper assemblies that step down medium-voltage alternating current to low-voltage alternating current before or after the external conversion from alternating current to direct current needed by electric vehicle batteries. A typical LFT can contain up to a few hundred kilograms of copper windings and a few tons of iron. All this metal is expensive and increasingly difficult to find. These systems are reliable but bulky 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 number of semiconductor devices as a single-port converter while providing multiple independently controlled DC outputs.” –Shashidhar Mathapati, Delta Electronics
But most multiport SSTs developed so far have been too complex or expensive (between five and ten times the initial cost of LFTs). This difference, along with SST’s reliance on auxiliary battery banks that increase expense and reduce reliability, explains why the obvious advantages of transistors have not yet been realized. prompted a move to LFT technology.
Surjakanta Mazumder, Saichand Kasicheyanula, Harisyam PV and Kaushik basu hold their sst prototype in a laboratory.Harisyam PV, SAichand KASICHYANLA, ETL.
How to Make Solid-State Transformers More Efficient
In a study published on August 20 in IEEE Transactions on Power ElectronicsResearchers from the Indian Institute of Science and Delta Electronics India, both in Bangalore, have proposed what is called a cascaded H-bridge (CHB)-based multiport SST that eliminates these tradeoffs. “Our solution achieves the same number of semiconductor devices as a single-port converter while providing multiple independently controlled DC outputs,” explains Shashidhar Mathapati, CTO of Delta Electronics. “That means no additional battery storage, no additional semiconductor devices, and no additional medium-voltage insulation.”
The team built a 1.2-kilowatt laboratory prototype to validate the design, achieving 95.3 percent efficiency at rated load. They also modeled a full-scale 11-kilovolt, 400-kilowatt system split 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 and bulky medium voltage isolation and allows power balancing between ports without auxiliary batteries. “Previous CHB-based multiport designs required multiple battery banks or capacitor networks to equalize the charge,” the authors write in their paper. “We have shown that you can achieve the same result with a simpler, lighter and more reliable transformer arrangement. »
A new modulation and control strategy maintains unity power factor at the grid interface, meaning no current from the grid is wasted oscillating between source and load without doing any work. The SST described by the authors also allows each DC port to operate independently. Concretely, each vehicle connected to the charger would be able to receive the appropriate voltage and current, without affecting neighboring ports or disrupting the network 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 cascaded modules per phase, about half that of some multilevel modular converter designs. Fewer modules ultimately means lower cost, simpler control and greater reliability.
Although still in the laboratory stage, the design could enable a new generation of compact and cost-effective fast charging hubs. By removing the need for intermediate battery storage, which adds cost, complexity and maintenance, the proposed topology could extend the operational life of electric vehicle charging stations.
According to the researchers, this converter is not only intended for charging electric vehicles. Any application requiring medium voltage to low voltage multiport conversion, such as data centers, renewable energy integration or industrial DC networks, could benefit.
For utilities and charging providers facing megawatt-scale demand, this streamlined solid-state transformer could help make the electric vehicle revolution more grid-friendly and faster for drivers waiting to charge.
From the articles on your site
Related articles on the web
