Assessment of precision timing and real-time data networks for digital substation automation

Title

Assessment of precision timing and real-time data networks for digital substation automation

Creator

David M. E. Ingram

Publisher

Queensland University of Technology

Date

2013-02

Format

application/pdf

Language

en

Type

Thesis

info:eu-repo/semantics/doctoralThesis

info:eu-repo/semantics/publishedVersion

Identifier

https://eprints.qut.edu.au/60892/

Coverage

Brisbane, Queensland, Australia

Abstract

The high voltage power grid, with its interconnected network of transmission lines and substations, is an important part of everyday life. Increasing demand and limitations on the building of new power lines have led to increased operating voltages, and consequently, larger substations. This in turn has increased the cost of copper cabling to connect circuit breakers, disconnectors, current transformers and voltage transformers in switchyards to control rooms. The measurement technology used for protection and control has not fundamentally changed since the early twentieth century. Non-Conventional Instrument Transformers (NCITs), such as capacitive voltage sensors and optical current transformers, which are safer and pose less risk to the environment, are now commercially available. Digital transmission of current and voltage signals from NCITs in the switchyard to substation control rooms significantly reduces the cabling required, with a single fibre optic cable capable of replacing more than 100 copper circuits. Ethernet process bus networks simplify system-wide automated testing, and facilitate innovative test methodologies such as the testing of in-service protection relays. The IEC 61850 family of substation communication systems standards were released in the early 2000s. These standards include Ethernet based process-level connections between switchyards and control rooms

however, their in-service performance is largely unknown. High voltage power systems are critical infrastructure, and therefore substation automation systems must be extremely reliable and proven to meet performance requirements before going into service. This research examines two aspects of a shared multifunction process bus: precision synchronising of sampling for analogue to digital conversion using the IEEE Std 1588 Precision Time Protocol (PTP) over Ethernet, and the behaviour and interactions of the various protocols that share the data network. This thesis takes an experimental approach to the assessment of performance, rather than adopting analytical or purely simulation techniques. A novel multi-vendor test bed was constructed from commercially available and late-stage prototype timing and protection equipment. Test protocols were developed throughout the research to thoroughly evaluate the performance of the devices in the test bed. This was a component-based approach, building from the bottom up. Results were validated with system level testing, from the top down, using real-time simulation of a power system and ‘hardwarein-the-loop’ The suitability of PTP process bus synchronisation was assessed in a number of steady state and transient performance studies. The results showed PTP meets the synchronisation requirements of sampled value process buses, with the additional benefit of compensation for path delays. A performance test was developed for transparent clocks (Ethernet switches with specific support for PTP) using a precision Ethernet card, rather than specialist PTP test equipment. The tests identified flawed implementations of PTP, which confirmed the need for vendor-independent conformance testing. A reliability model for PTP timing systems, based on Fault Tree Analysis, was created to provide meaningful comparisons of timing topologies. Detailed network performance tests were conducted with controlled transmission and capture of sampled value (IEC 61850-9-2) messages, and transduced analogue and digital events using Generic Object Oriented Substation Event (GOOSE, IEC 61850-8-1) messages. The precision Ethernet card used for PTP performance tests was used for this protocol testing, which reduced the cost of the test instrumentation. The results from this hardware-based test bed differ from previously published simulations, and show that sampled values, GOOSE and PTP do not unduly influence each other. This thesis presents an Ethernet addressing scheme for sampled values and GOOSE based on the IEC 81364 standard for plant identification that is used in the power industry. Field measurements from a live substation and a real-time simulator with merging unit capability were used to characterise the nature of process bus traffic. Real-time simulation with hardware-in-the-loop (the test bed) was used to link the time synchronisation and real-time networking results back to power system protection performance. The use of a Real Time Digital Simulator with sampled values, GOOSE and PTP in one system is novel, and provides a greater understanding of the factors that affect process bus performance. It accounts for unknown factors that cannot be modelled with software alone. Transformer differential protection was used to assess the influence of synchronising error and Ethernet network loading on the response time to faults. This ‘closed loop’ approach showed that the results produced by the component level test procedures presented in this thesis are an accurate predictor of protection performance. System level testing with real-time simulation enables the protection design to be proven, rather than validating that settings have been correctly entered into the protection relay with secondary injection tests. The research presented in this thesis provides a suite of tests that inform the design of multifunction process bus substation automation systems, and can assess the ongoing conformance of systems placed into service. This should give confidence to utilities considering adoption of NCITs or substation automation system refurbishment that the real-time Ethernet networks and time synchronising systems that underpin a digital process bus will meet their needs and reduce their exposure to risk.