Communicating line differential protection for urban distribution networks

Modern urban power distribution networks are gradually developing from a radial-type topology towards ring-type and even mesh-type topologies. There are many drivers behind this development, one significant being the need to increase the reliability and availability of networks. From the network protection point of view the ongoing development poses new challenges for the prevailing protection philosophies and the protection relaying of today. Varying fault current conditions, magnitudes and contributions from several fault current sources represent challenges that are difficult to resolve with conventional overcurrent protection relaying. One solution to this problem is to introduce unit type protection schemes utilizing the differential current protection principle. Communicating multifunction line differential relays offer basic in-zone unit protection, and local and remote back-up overcurrent protection for the downstream power system, and dedicated earth-fault protection for enhanced line to ground fault sensitivity. The IEC 61850 standards combined with Ethernet communication technology already caters for new possibilities using horizontal communication within a substation. The suggested approach can also be extended to inter-substation communication and used in conjunction with line differential protection. The protection communication channel between the two line ends can also be used for transferring additional information between the substations, such as information related to other protection schemes or primary equipment interlockings or for enhancing performance of the unit protection under certain switching conditions.