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DESCRIPTION
Teleprotection signals from protective relays are among the most critical data transmitted across utility networks, as they help manage the power grid load, as well as to protect equipment within the power network from severe damages resulting from faulty HV lines. By enabling load-sharing, grid adjustments and immediate fault clearance, Teleprotection has a decisive role in ensuring uninterrupted power supply and therefore requires special attention with regards to network performance and reliability. Specifically, protection commands must be assured immediate delivery when problems are detected, so that faulty equipment can be disconnected before causing a system-wide damage.TRANSCRIPT
Teleprotection over Packet Slide 2
Agenda
• Power Utility Communications: Networks in Transition
• Teleprotection Connectivity and Delay Considerations• Ensuring Communications Performance for Teleprotection over
Packet
• Teleprotection over Packet Use Case
• Conclusion
• Appendix: Pseudowire EmulationLatency Sources in Teleprotection
Teleprotection over Packet Slide 4
Networks in Transition
• Power utility networks are mostly self-owned, privately operated
• Require SDH/SONET-level reliability for mission-critical communications
• Slow migration to IP, but Ethernet transport and IP/Packet-based networks gradually gain traction for higher throughput and lower OpEx
Upgrades to Smart Grid foster transformationNew applications: Substation automation (IEC 61850), NG-SCADA systems, WASA synchrophasors, IP video surveillance
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• Control CapEx and avoid over-burdening network operations and management
Especially where SDH/SONET and PSN co-exist
• Ensure smart communications over packet and service assurance for mission critical apps in PSN environment:
Low end-to-end delay High availabilitySDH/SONET-level resiliencyTeleprotection, in particular, has stringent communications performance requirements !
Migration Challenges
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What is Teleprotection
• Used for power line protection• Protect equipment from severe damages resulting from faulty HV
lines • Common schemes:
Distance (impedance) protectionCurrent differential protectionDirect Transfer TripCombination
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Teleprotection Communications
• Distance Protection: Trips breakers when impedance measurements vary from those taken under normal conditions
Traditionally, no communication was required Pilot-aided distance relays use a communication channel to improve fault clearance
• Differential Protection: Disconnects faulty line segments if differential current measurements on both ends of the protection zone are higher than a setpoint
Requires communication between the end-point relays
Teleprotection over Packet Slide 9
• Traditionally, relays communicated (via a separate comm channel or a multiplexer) over the SDH/SONET backbone, power line carrier (PLC) or a dedicated fiber optic connection
• Communication channel interfaces: X.21, E1/T1, V.35, E&M; modern relays use IEC C37.94 fiber optic
Teleprotection Connectivity
Teleprotection over Packet Slide 10
Two options when migrating to packet communications:
• Continue using TDM connectivity for Teleprotection in parallel to new packet network installations for non-critical substation traffic
Hybrid TDM/PSN multiplexers and access nodes save on network equipment costs
• Use Ethernet or packet network for Teleprotection, provided it can guarantee required performance
Delivery of TDM-based Teleprotection signals over packet requires pseudowire emulation (see appendix I)
Teleprotection Connectivity (Cont’)
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Teleprotection Communications –Key Performance Criteria (IEC 60834)
• Between the moment of change of state at the transmitter input and the receiver output
Transmission Time
• Valid commands in the presence of interference and/or noise, by minimizing the probability of missing command (Pmc)
Dependability
• Preventing false tripping due to a noisy environment, by minimizing the probability of unwanted commands (Puc)
Security
• Bandwidth consumption and resiliency also impact performance
Other
Performance criteria pose a challenge over non-deterministic packet transport and require enhanced, carrier-grade capabilities
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Teleprotection Communications Performance: Latency Budget
• Most power line equipment can withstand a brief shortage/irruption
Typical requirement for total fault clearance time = 100ms
• Actual operation time of protection systems = 70-80% of this period
Including fault recognition, command transmission and line breaker switching
Large electromechanical switches take up the majority of time
• In modern applications, contact transfer is expected in 10ms or less
• For latency sources in Teleprotection communications, see Appendix II
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Teleprotection Communications Performance: Asymmetric Delay
• Differential protection requires same channel delay in transmit and receive paths
Requires special attention in jitter-prone packet networks
Typical relays can tolerate discrepancies of up to 250 μs
• The main tools available for lowering delay variation:A jitter “buffer” at each end of the line for queuing sent and received packets
Traffic management: Ensure highest transmission priority for Teleprotection
Standard PSN-specific synchronization technologies maintain stable networks by disciplining the communications elements to a highly accurate clock source
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Communications Channel Resiliency
• Hardware redundancy:No single point of failure (NSPF) design with redundant, hot-swappable power supplies
Redundant control plane and switch fabric cards
• Link redundancy:1+1 protection topology with automatic switchover between links
Link aggregation group (LAG) per IEEE 802.3-2005 LACP (link aggregation control protocol) for Ethernet-based services
• Path protection:Ethernet Linear protection Switching (G.8031) , AKA “EVC (Ethernet Virtual Connection) protection”
Ethernet Ring Protection Switching (G.8032 ERPS) to provide Five Nines (99.999%) availability
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Traffic Management and Quality of Service
Provide deterministic quality of service and priority for protection signals with multi-level Ethernet traffic management for predictable latency and jitter performance across the service path:
• Classification of incoming traffic into flows
• Metering and policing to regulate traffic with different bandwidth profiles
• Advanced scheduling and queue management to ensure minimal latency and jitter
• Shaping to smooth out bursts and avoid buffer overruns in subsequent network elements
• Packet editing and marking to signal proper handling instructions for subsequent network elements
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Performance Monitoring and Testing
ServiceTurn-up
On-going Monitoring
Fault Management& Recovery
Connectivity Verification
Diagnostic Loopbacks
Performance Verification
& Testing
Performance Monitoring
Threshold Reporting
Statistics Collection Reporting
Fault Detection & Isolation
Fault Propagation & Notification
Resiliency & Repair
• A wealth of carrier-grade Ethernet tools to remotely test, monitor and troubleshoot the communications links operation
• Utility network operators anticipate service degradation ahead of time, as well as cut down truck-rolls and on-site technician calls
Teleprotection over Packet Slide 19
Teleprotection over Packet Proof of Concept Program
• RAD’s Megaplex-4100 multiservice access platform was successfully tested by a major energy utility
• TDM data received from protection units was converted into packets, then transmitted over an MPLS network employing static routing
• The line differential protection equipment featured a variety of TDM communications interfaces, including G.703, X.21, RS-232, E&M, C37.94, Native E1
• End-to-end communication delay requirement of 8-10ms in a packet network environment experiencing a jitter of 2.5ms
Also required symmetrical latency with maximum tolerance of 100-250μs
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Teleprotection over Packet Test Results
RAD’s Teleprotection multiplexers have successfully met requirements:
• Up to 5ms delay with quality of service for signal priority via shaping and traffic engineering tools
• Clock accuracy was rigorously maintained throughout transmission
• High degree of resiliency through various protection schemes, including DS1-level redundancy
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Conclusion
• Critical Teleprotection applications require special attention in the move towards Smart Grids and next-generation networks
• Viable alternatives to existing deployments need to meet exacting performance criteria of minimal transmission time, reliability and security
Extremely low, symmetrical delay, robust clock accuracy, QoS assurance, resiliency, and on-going performance monitoring are “must have” elements for any Teleprotection over packet system
• Hybrid TDM/Packet solutions allow utility operators the freedom to choose the migration path that best suits their needs and budgets
Download comprehensive Teleprotection over Packet Solution Paper
Teleprotection over Packet Slide 23
Appendix I:What is Pseudowire Emulation?
• The synchronous bit stream is segmented
• Headers are added to each segment to form the Packet
• Packets are forwarded to destination over the PSN network
• At destination, the original bit stream is reconstructed by removing headers, concatenating frames and regenerating the timing
• The most common pseudowire emulation standards are CESoPSN, SAToP, TDMoIP
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Appendix II: Latency Sources in Teleprotection
•Includes the relay’s fault identification, command initiation and decision time
Teleprotection Equipment Delay
• Minimized via optimal design of ICs, DS0 xconnect, and• High-performance buffering and forwarding technology
Substation Multiplexer (TDM interface)
• 1-5ms, depending on packet size and # of TDM frames/packet• Smaller packets increase bandwidth overhead, but reduce
latency
Pseudowire Encapsulation and Packetization Delay
• Each element adds processing and queuing delay• Variable delay poses a greater threat and requires advanced
traffic managementPSN Network Elements