In digital substation applications based on IEC 61850 technology, protection Intelligent Electronic Devices (IEDs) obtain analog quantities and binary signals via Ethernet communication. The digitized analog quantities are published as Sampled Values (SVs), and binary signals as GOOSE messages, with the IEDs that need these quantities/signals subscribing to receive them.
In conventional substation applications, IEDs obtain analog quantities via copper wires that connect the CT/PT secondaries to the IED’s analog inputs, and binary signals via copper wires that connect the signal source to the IED’s binary inputs.
All protection personnel are well accustomed to connecting CT and PT secondary analog quantities in the conventional way to their protection IEDs. With the continued evolution of digital technologies, and the coming to fruition of the IEC 61869-9 standard for instrument transformers with digital outputs that encompass the IEC 61850-9-2 process bus standard, the wired connection of the secondary analog quantities to the protection IEDs is no longer the only method by which these devices can receive these quantities. A question that now arises is whether the performance of the protection functions remain unchanged irrespective of the way in which the analog quantities are connected to the IED, or whether performance differences arise? This is one of the questions a project was set up to answer. The procedure adopted was to connect to the same IED the same analog quantities in the conventional way (i.e. connected to the IED via a traditional copper wire interface) as well as via process bus (i.e. connected to the IED via a fiber interface as sampled values), and then internally to connect each input type to identical protection functions (in the same IED), with identical settings, and to compare their performance.