Applying digital line current differential relays over pilot wires

Digital current differential protection attracts the user for application simplicity, operation speed, and high sensitivity. Traditionally, current differential protection communication channels are either direct fiber optic cable or multiplexed channels using short copper links from a relay to a multiplexer. Commonly, pilot wire analog differential relays used to be applied for protection of short lines. Many utilities still have such copper, twisted pair, links with pilot wire relays in service. However, these relays do not meet nowadays requirements and are likely to be replaced. Utilities, however, are reluctant to invest into new communication links to replace existing pilot wires, if they can avoid it. This paper presents both the problems and practical solutions (modern HDSL communications modems appeared to be a good solution) for applying digital line current differential protection over copper wires.