WASA and the Roadmap to WAMPAC at SDG&E

by Eric A. Udren, Dr. Damir Novosel, Dan Brancaccio, Quanta Technology, USA, and Tariq Rahman, SDG&E, USA

PMU System Infrastructure

Figure 4 shows a typical PMU system architecture.  Elements of the system include:
Phasor Measurement Units (PMUs) - stand-alone PMUs are available, and PMU functionality may be included in other measurement platforms such as digital fault recorders.  Today, many protective relay types from multiple manufacturers are available with synchrophasor data streaming capability.  The advantage of using relays is that an array of status inputs and breaker trip and control outputs are also available, including communications protocol support that makes the relays available for high-speed wide-area control execution.  Also, relays are installed on all system apparatus, often in redundant pairs, so that a complete and redundant set of measurements can be available for the full range of future WAMPAC applications.  All phasor measurements are performed in compliance with synchrophasor standards – the latest single international standard is the joint IEC-IEEE Standard 60255-118-1 Edition 1 of 2018.  Widely used in existing PMUs is IEEE C37.118.1-2011, or the measurement specifications in the older IEEE C37.118-2005.

Phasor Data Concentrators (PDCs) - PDCs are data-handling platforms that can merge data streams from many PMUs in a substation into a concise data format for transmission to the control center or remote location over a single communications channel.  PDCs can locally archive recent data for post-mortem analysis when the communications path fails, although this may become less important with redundant high-reliability data transmission required for WAMPAC functions like backup fault protection.  To avoid a single point of failure, a substation installation will include at least two PDCs as well as redundant isolated PMU sources for critical points, and separate isolated communications connections.

Communications network - WAMPAC requires robust reliable communications between PMUs or PDCs and control centers or central processing locations.  Many utilities are building these systems for teleprotection and other mission-critical operational needs.  Today, many PMU users are streaming serial PMU data over SONET (DS0 on T1 or higher-bandwidth) paths.  Some utilities, including SDG&E, are now deploying MPLS Ethernet wide-area communications networks with multiple redundant routers and a mesh of alternative communications paths leading to the central control and monitoring location of the WASA or WAMPAC system as shown in Figure 5.  MPLS can transport synchrophasor streams in Ethernet-packet or serial-simulation formats.  Transport latencies are dropping well below 10 ms and are well-suited to WAMPAC protection and control function implementation.  Redundant PMUs and PDCs operating over highly reliable redundant channels are practical today – it is feasible to invest to build the infrastructure to support WAMPAC.

Control center data processing and computing systems - Super PDCs collect single or redundant PMU data streams from each substation or field location. Each Super PDC can in turn supply customized data streams to multiple redundant computing platforms that isolate applications, as well as to remote clients such as neighbor utility system WASA/WAMPAC systems needing boundary information, or to independent system operators.  Redundant or dual tri-mode redundant computing platforms at the central control locations gather data, make evaluations and decisions, carry out protection and control via downward command messaging, generate displays and reports for operators and maintenance personnel, and archive data for disturbance monitoring or post-mortem analysis of system model accuracy.  Going forward, the synchrophasor data processing infrastructure comprises a private cloud computing platform whose components may not be physically co-located as is familiar today.

Communications protocols - most synchrophasors are streamed today in accordance with IEEE C37.118.2-2011, or the similar communications specifications of the older IEEE C37.118-2005.  Both serial and Ethernet transport options are supported, with the requirement that users map or configure data streams and transport paths manually.  WASA used by operators for decisions, or WAMPAC, require NERC CIP-compliant security via secure networking appliances or other data protection tools in an organization-wide security program.  While some utilities use non-routable serial transport as a cybersecurity protection means today, we do not expect this to be effective or acceptable in the future.  Solutions to these issues are now provided by the latest edition of IEC 61850-8, which includes the Routable Sampled Values (R-SV) service for synchrophasor transport using key-based authentication of packets for strong cybersecurity, and IT WAN standard services for automated detection and configuration of publisher-subscriber network connections.  Using IEC 61850-8 transport of synchrophasor streams calls for a new generation of PMU communications interfaces, and will yield easiest application, highest security, and compatibility with IEC 61850 function-level engineering methods that eliminate manual static point mapping. Other new communication protocols such as Grid Protection Alliance’s Streaming Telemetry Transport Protocol (STTP) are being developed to provide additional choices for delivering high speed synchronized measurements to client systems.

Time distribution - PMUs require precise synchronized time, accurate to microsecond levels or better across the region of time-tagging of synchrophasor measurements.  GPS satellite time receivers have been the standard tools that have enabled wide-area precise synchronization of time tags for regional observation, and thus raised interest in WASA and WAMPAC.  However, looking forward, GPS is increasingly vulnerable to disruption or attack as it supports mission-critical protection and control functions or operator decision-making.  GPS receiver manufacturers have greatly enhanced the robustness of the clock function against spoofing or disruption using local and system-wide monitoring algorithms and multiple satellite array reception.

Even more promising for security is the distribution of precision time over the same Ethernet WAN used for transport of phasor streams and control messages.  Redundant coordinated master clocks distribute time signals across the grid to PMUs using IEEE 1588 Precision Time Protocol (PTP) and power utility application profiles IEC 61850-9-3 and IEEE C37.238.  The master clocks can still be synchronized to GPS time, yet power-system-wide time synchronization continues even if the GPS reception is disrupted.  Redundant GPS synchronized clocks at each substation can prevent disruptions that are the result of hardware failure.  However, if the disruption is the result of GPS spoofing an additional trusted time source is required.  SDG&E is planning to deploy a network PTP solution as shown in Figure 6.

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