Introduction

The IEC 61850 standard is a milestone in substation automation, replacing a plethora of busses and links by a hierarchy of well-specified switched Ethernet networks, namely the station bus between the bays and the process bus within a bay. A great achievement of IEC 61850 is the description of all communication in the substation in the Substation Configuration Language (SCL), which allowed for the first time engineering of a substation consisting of protection, measurement and control devices (called IEDs, for "Intelligent Electronic Devices") of different manufacturers. However, the IEC committee TC57 Working

Group 10 that created IEC 61850 did not specify in detail the underlying hardware of these busses, believing that solutions standardized for industrial Ethernet would find their way into substation automation. This applied especially to two indispensable network features: time synchronization and network redundancy. Time synchronization was solved by the SNTP (Simple Network Time Protocol) and for stricter requirements by the IEEE standard 1588 [8] (which is not subject here), but redundancy was a major hurdle. Indeed, the lack of a commonly accepted redundancy solution was threatening the whole interoperability concept, since manufacturers started putting on the market proprietary redundancy solutions, preventing effectively to build a substation when the interfaces did not fit. Fortunately, the IEC committee SC65 WG15 "Highly Available Automation Networks" published just in time the IEC standard 62439 which specifies several redundancy methods, one of them applicable to substations of any size and topology, for the station bus as well as for the process bus.

The Parallel Redundancy Protocol (PRP) IEC62439-3 Clause 4, relies on the parallel operation of two local area networks, and provides completely seamless switchover in case of failure of links or switches, thus fulfilling all the hard real-time requirements of substation automation. PRP can also be applied to build a simple, seamless ring by treating each direction as a separate network, a method called HSR (High Availability Seamless Ring), currently circulated as IEC CDV 62439-3 Clause 5. This method has been now proposed as an integral part of IEC 61850.