ERC Researchers Develop Prototype Medium-Voltage Fast Charger for Electric Vehicles
Researchers at the NSF-funded Future Renewable Electric Energy Delivery and Management (FREEDM) Systems Engineering Research Center (ERC), headquartered at North Carolina State University, have developed a prototype medium-voltage (MV) fast charger for electric vehicles (EV). That prototype has met all design goals and is now operational and undergoing tests.
The MV fast charger has many advantages over the state-of-the-art solution for EV charging; it substantially simplifies system installation and eliminates the costly and bulky low-frequency step-down transformer. Additionally, its direct-connection single-phase MV line allows for simpler system isolation where only a single-phase distribution line is available. The prototype fast charger will be a building-block for commercialization of a whole new product line serving a set of emerging applications, including DC data centers or other DC power distribution systems. The research team brought this concept from Technology Readiness Level (TRL) 3-4 to TRL 6-7, thereby proving concept viability in an “open-source” manner so the technology can be offered to FREEDM partners and industry members for further commercialization and mass production.
The MV fast charger is based on a technology similar to the solid state transformer (SST) in that it uses power electronics for isolation and voltage step-down from a distribution line, and it delivers a controlled output voltage to a load. Development of the MV charger advances the SST concept further and demonstrates viability in a real-world application.
The objective of this task was to design and build the MV charger prototype using wide-bandgap (WBG) devices. Motivation for introducing a WBG solution for this application includes the potential for higher efficiency, higher power density, and system-level cost savings. This charger's ability to feed power directly from a single-phase MV line will reduce the system's footprint (3x size reduction, important in densely populated areas). Further, the Electric Power Research Institute (EPRI) estimated that installation costs would be reduced by 50% or more, depending on the site. As a result, the WBG solution would have a distinct competitive advantage if it can be manufactured at parity or even at a premium over the conventional solution (see figure).
The proposed converter will utilize off-the-shelf SiC and potentially GaN devices to step down and rectify the single-phase MV input. The first-year goal has been to demonstrate a 25kVA, 2.4kVac/400Vdc system with very high efficiency (above 95%), and high power quality (power factor [PF] ≥ 0.98 and total harmonic distortion [THD] ≤ 2%).
