Substation primary plant and instrument transformers generally remain inservicefor many years and are only renewed when physically or mechanicallylife-expired. However, substation protection and control systems, also known assecondary systems, are changed more frequently, especially electronic orcomputer based systems. One way to overcome the lifecycle-coordinationproblem is to develop a new architecture for substation secondary systems bydeploying new technology such as standard interface modules, the bay processbus and IEC 61850 communication protocol.IEC 61850-9-2 "process bus" has proposed an Ethernet based communicationnetwork to replace traditional hardwiring and cabling, for communication ofanalogue and binary signals with Ethernet messages. The application of processbus in substations is able to reduce the life-time cost of the secondary systemsand improve flexibility and functionality. Sufficient confidence must bedemonstrated in the application of this new technology.This thesis describes the design and performance evaluation of a prototypeprotection scheme suitable for application on a corner of a mesh transmissionsubstation of the type used in the UK on the transmission network.