Performance considerations in digital substations

Today's substation automation, protection and control applications use IEC 61850 messaging typically only for control related and relatively static functions like blocking and interlocking schemes. With the introduction of process bus communication in digital substations copper wiring is being replaced by digital communication using IEC 61850. This shift towards a higher usage of Ethernet-based communication increases the overall performance requirements of IEC 61850 communication systems, as well as of protection and control devices. The process bus transmits sampled analogue values according IEC 61850-9-2 supplied by merging units as well as binary GOOSE messages for dynamic and time critical applications such as circuit breaker tripping. This poses new requirements on products and communication systems to ensure timely handling of real-time data for proper performance of the substation protection, automation and control functions. The paper discusses performance considerations in digital substations by looking into requirements of products on process level such as merging units, sensors and breaker IEDs (intelligent electronic devices), as well as on bay level protection and control devices. Also system aspects like communication network design are taken into account. Two examples for AIS (air insulated system) and GIS (gas insulated system) substations are used to verify the standardized performance figures defined in IEC 61850, IEC 60044 and IEC 61869 standards. As supporting information a short overview of IEC 61850 GOOSE performance testing explains how GOOSE performance for IEDs is assessed and certified.