A design scheme of line current differential protection based on IEC61850

Current differential relay plays a very important role in transmission line protection because of its absolute selectivity for isolating the failure equipment. Nowadays, in digital substation, lots of new technologies (such as non-conventional instrument transformers, process bus, time synchronization, embedded Ethernet and information modeling following IEC 61850) bring huge challenge for relay design and application. This paper reviews traditional technical solutions to the transmission line current differential protection, addressing its basic principle, typical architecture, differential criterion and communication requirement. Then a new design scheme of line current differential protection is discussed in detail. Network time synchronization protocol - IEEE 1588 and process bus communications based on IEC 61850-9-2 are introduced into the new scheme. The communication between substations adopts directly optical fiber link or wide area Ethernet networks. Consequently, the protections in different substations receive synchronous sampled value of both sides though its own merging unit is just like communication in the same substation. Information exchange models (Logic Devices and Logic Nodes) in the transmission line protection are also established. There are many advantages behind this scheme, one significant being to increase the interoperability and availability of protective relay.