Merging SONET and Ethernet Communications for Power System Applications

The type and amount of information exchanged between intelligent electronic devices (IEDs) within a power system are increasing. Initially, communication in protection systems was the exchange of a few bits of information to indicate the direction of a fault or to signal a remote relay to trip or block. The communications media consisted of hard wire, pilot wire, and other similar communications methods. In modern power communications systems, the range of supported applications includes line current differential schemes, synchrophasor data collection, supervisory control and data acquisition (SCADA), engineering access, voice, surveillance, pilot protection, event report collection, and many other types of tasks. Each application data type has different latency and reliability requirements. Modern power communications systems are dominated by time-division multiplexing (TDM) communication, such as synchronous optical network (SONET) and synchronous digital hierarchy (SDH), and packet-based communication, such as Ethernet. TDM communication provides deterministic data by dedicating bandwidth to each data service. Ethernet incorporates a bandwidth-sharing scheme that allows each service to use bandwidth when it is available. TDM communication is ideal for real-time protection and control applications due to deterministic characteristics. Packetbased communication is ideal for transporting event reports and performing similar services because these services generally do not have deterministic requirements and require the transport of large amounts of data. However, with the availability of preengineered Ethernet packet navigation techniques, such as virtual local-area network (VLAN) segregation and message priority, system designers are attempting to develop near realtime communications schemes with Ethernet systems, such as IEC 61850. Too often, TDMand packet-based communications are perceived as mutually exclusive and competing technologies. However, TDMand packet-based communications can be integrated to operate together in a way that leverages the benefits of each technology. This paper describes the benefits and shortcomings of TDMand packet-based communications, when each technology is appropriate for different types of services and/or data, and how combining both transport technologies provides a robust network system that meets the needs of both real-time protection engineers and corporate information technology (IT) professionals.