Detection of DC injection and measuring AC current with a single system for electric vehicle charging and discharging

Abstract

Modern power systems increasingly include distributed generation and storage. In addition to solar, these include newer technologies such as the Vehicle-to-Grid (V2G) technology, where energy, stored in electric vehicle (EV) batteries, is injected back to the AC grid when there is a need. Injected power is regulated by many standards and laws, with electrical requirements including anti-islanding, limitation of harmonics, low-voltage ride-through, fault current and DC injection. For solar and battery storage, including V2G, the power to the grid is provided by a power converter taking DC from the solar panels or battery, converting to AC, and injecting AC power onto the grid. This thesis addresses the problem of accurate measurement of AC power, while using the same device to detect DC injection. Any faulty or improperly designed power converter can inject DC into the AC grid, but the high power converters of electric vehicles represent a particular concern. DC injection may cause saturation of transformer cores, overheating of grid-connected equipment, and acceleration of cable corrosion. Therefore, it is important to ensure that its value stays within the allowable limit stated in the "Limitation of DC injection" in the IEEE 1547-2018 standard. Although the power converter is responsible for limiting DC on the AC lines, additional verification of DC injection by the Electric Vehicle Supply Equipment (EVSE) would be a cost-effective extra safety measure, allowing diverse EVs to be checked by one EVSE. Importantly, a V2G-capable EVSE must be able to carry out concurrent high-precision AC and moderate-precision DC current measurements to fulfill revenue metering requirements in the most economical way. ☐ In the first part of the work, we will review the DC injection problem and commonly used current sensing techniques to evaluate the most promising current-sensor candidates. In the second part of this work, we will present an Integrated Shunt Current Sensing System that we have designed, prototyped, and validated. The design accomplishes cost-effective AC current measurements within 1.0 accuracy class required for revenue metering. In addition, the prototype is able to simultaneously detect DC injection of ≥ 400 mA in AC current up to 80 A in accordance with the "Limitation of DC injection" section in IEEE 1547-2018 standard. Positive DC injection detection accuracy at the limit value of 400 mA is 8.25%. Negative DC injection detection accuracy is 16.5%. We will discuss system design challenges, test results, and the cost analysis. In the third and final part of the work, we will review alternative solutions and recommend an optimal solution for the concurrent measurement of the AC current and DC injection to be used for EV charging and discharging.