Case Study: Increasing Reliability, Dependability, and Security of Digital Signals Via Redundancy and Supervision
Paulo Franco, Geraldo Rocha, David Dolezilek · 2015
Protection and control functions within modern microprocessor-based intelligent electronic devices (IEDs) directly measure data representing the present state of the power system. Communications-assisted protection and control functions collect signal data from other IEDs via digital messaging. The contents of the digital messages are combined with the local measurements in the relay to perform accelerated and/or more selective protection and automation logic. NERC PRC-005 describes a protection system application to include protective relays, voltageand current-sensing inputs, dc control circuitry, station dc supply, and the communications media. Application redundancy is accomplished via multiple protection system components, such as IEDs, instrument transformers, dc circuits, and power sources. Communications media redundancy is accomplished via multiple communications connections, communications devices, and messaging protocols. Self-tests within the application and communications media perform supervision and immediate detection of failure. Application and media system reliability and dependability are best improved via the use of robust devices and designs. Both application and communications media redundancy are performed via primary and backup devices, hot standby devices, or dual primary devices. Ethernet media is made resilient with technology designed to satisfy reliability standards, such as the IEEE 1613 standard for the environmental and testing requirements for communications networking devices in electric power substations. Ethernet media is made more robust via optimized redundant connections and hot standby reconfiguration algorithms. Dual primary message technologies provide redundant signaling with no common-mode failure. Traditionally, protection and automation engineers search for the best and most secure logic designs. Security in applications using MIRRORED BITS® communications or IEC 61850 Generic Object-Oriented Substation Event (GOOSE) digital messaging is improved via the supervision of the message quality. Though designs intend to maximize good message quality, signaling message quality fails when monitoring detects that messages are not received as expected, are received out of sequence, or are corrupted. Status of system-wide digital messaging for protection and interlocking signaling, control, and monitoring is now easily displayed for quick review by operators similar to other system data. This provides 100 percent visibility for supervision and performance auditing. Failures are automatically time-stamped, logged, reported, and alarmed similar to other power system or control system malfunctions. This paper presents a comparison of various application and communications media redundancy methods and redundant and duplicate connections. Finally, case studies provide examples of improvements to applications, such as logic selectivity, circuit breaker failure (50/62BF), and automatic line transfer (ALT) based on signal message supervision. © 2015 by Schweitzer Engineering Laboratories, Inc. All rights reserved. 20150422 • TP6695