Restricted Earth-Fault (REF) Protection Challenges due to Extensive Use of Cables: A Case Study

Capacitive zero-sequence currents do exist in any power system during an earth-fault regardless network grounding principle. However, existence of these currents is typically ignored in traditional protection practice. In many countries some type of high-impedance grounding is used which limits the earth-fault current to a certain level. Increased use of cables in medium voltage distribution grids and high voltage sub-transmission grids can also significantly raise the capacitive earth fault currents, which in combination with the used grounding principle in the power network can pose a problem for proper operation of some relays.