Comparative Performance Analysis of Identical Protection Functions In The Same Ied Fed By Conventional and Process Bus Connected Analog Quantities

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.

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, i.e. ̵ in one IED, connect protection functions (21, SOTF, power swing blocking, 50/50N, negative sequence overcurrent, 59/59N, etc.) to receive conventional copper wire connected analog quantities (currents and voltages). ̵ in the same IED, connect a second set of identical protection functions, with exactly the same settings, to fiber connected analog values (currents and voltages as SVs). - for any disturbance causing operation of any protection function, record all function operate signals to the internal disturbance recorder (conventionally connected and process bus connected protection functions are in the same IED and connected to the same disturbance recorder, as opposed to comparing the functional performance in one IED with conventional connections vs. a second IED with process bus connection); incorporate logic to automatically determine whether both protection functions operated identically, or not, and if not (e.g. operation of identical protection functions was outside a very tight timing difference) flag this as a comparative performance discrepancy, and record this also on the disturbance recorder; for all protection functions, where applicable, compare operation per phase. - check also for discrepancies between the conventionally connected and process bus connected analog quantities by applying any difference between them to a low set function, for both current and voltage. - as well as the protection functions, measured quantities from the measurement functions are also checked to be withing a certain measured value of each other (e.g. per phase current, per phase phase-ground voltage, active power P, reactive power Q, etc.).

The site chosen for the installation is a double circuited 500kV rated transmission line that spans 224 miles and averages 12 faults a year.

Faults have occurred (three single-phase faults), as well as other transients, causing protection functions to operate and trigger captured recordings. The paper will include details of the application, overview of the IED engineering, as well as analysis of the recordings, and will answer the question whether the performance of the IED’s protection functions is unchanged, or not, by the way in which the analog signals are connected to the IED.