Innovative Differential Protection of Power Transformers Using Low Energy Current Sensors

Traditional differential protection systems are applied on large power transformers using current transformers (CTs). However, because of high secondary currents (often exceeding 100 kA RMS) differential protection systems for electric arc furnace (EAF) transformers have not been applied in the past due to the lack of commercially available CTs. This paper will present differential protection systems that have been in use for many years using Rogowski Coil (RC) current sensors. These protection systems use high precision printed circuit board (PCB) Rogowski Coil (RC) current sensors. The paper reviews the characteristics, designs, and application of these RC sensors for advanced protection, control, and metering systems with new multifunction relays. The paper compares performance characteristics of new solutions based on RC sensors with the conventional differential protection systems based on CTs, demonstrating that the new systems do not have the limitations of conventional technology. Operating experience from several site applications will be included.