Converter-Based Power System Protection Against DC in Transmission and Distribution Networks

The dc in a power system may be caused by geomagnetic disturbances that are the result of solar storms and high-altitude nuclear detonations. Increased inverter-based generation is also contributing to small dc injection into the power systems. The resultant dc could have serious consequences for the power systems as it may drive power transformers into saturation, cause transformer internal heating, cause large draw of reactive power, and misoperation of protective relays. This letter proposes a novel power electronics-based dc mitigation approach that involves a transformerless series active filter integrated between the neutral point and ground of power transformers serving multiple purposes. The first objective is to surpass the effect of dc injection, the second goal is voltage regulation, and the third is to provide harmonic isolation or impedance balancing. The proposed device is currently being developed on a 7.2-kV/240-V single-phase transformer; however, the solution is also relevant for 24-500-kV networks. The system circuitry, operation, and control are implemented and verified for this letter in a controller hardware-in-the-loop (C-HIL) test setup using a Typhoon HIL-402 simulator. Results indicate that our approach is a promising alternative to traditional neutral capacitor-blocking strategies.