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

San Diego Gas & Electric Company (SDG&E) is among many users who are deploying Wide-Area Situational Awareness (WASA) systems in which PMUs distributed across the grid stream data to control centers for situational awareness of dynamic wide-area behavior of the grid to system operators, as shown in Figure 1. 

The WASA deployment is evolving from an earlier grid observation system built over the last decade, now extending to most transmission lines and other system elements.  The existing system serves as a tool for engineers to observe system behavior, and to perform post-mortem analysis of disturbances and operating events.  These uses are fundamental but are just a part of a broader roadmap for many important new applications using PMU measurements, as we will explain. 
Over time and across the industry, synchrophasors will serve as the new unified wide-area data-gathering platform serving system observation, operating data collection, control, automation, market functions, asset management and maintenance support, and even high-speed wide-area fault and failure protection functions that supplement and back up local fault protection.  A full-function PMU-based deployment is described as a Wide-Area Monitoring, Protection, Automation, and Control (WAMPAC) system.

WASA and WAMPAC Functions

System Operator Situational Awareness: WASA includes control center displays for operators – voltage and phase profiles and variations, stability warnings, and normal versus unusual flow patterns.  Figure 2 shows an example.

Future SCADA and EMS Support: SCADA and EMS and familiar legacy grid-monitoring production systems on which operators rely have been separate from PMU-based WASA systems, which have been typically placed alongside and not incorporated into these systems.  However, we note that PMUs installed on all lines and apparatus comprise a whole new parallel data-gathering platform.  In place of transducers, RTUs, and IED data concentrators sending uncorrelated magnitude measurements every few seconds, the network of PMUs are streaming ac values with high accuracy at 30 or 60 (or even 120) value sets per second, with exact time and angle relationships across the grid defined by precision synchronized time tags in each value set.  PMUs gather phase and sequence values of voltages and currents, power flows, frequency, and rate of change of frequency (ROCOF).  Binary and analog point values can be included in the data streams. 
With the addition of a control messaging transport service on the same network, a dense PMU infrastructure can be the future data platform for EMS and SCADA.  All of today’s EMS and SCADA applications will perform equally well or better with the vastly richer, denser, and more precise sources of data.  In fact, some traditional EMS are now integrating down-sampled synchrophasor measurements.

Disturbance Monitoring and Analysis: WASA also includes the functions of Disturbance Monitoring Equipment (DME) used today for post-event analysis of system behavior and disturbances.  Records of streamed measurements – the envelopes of the raw power-system signals tracked and recorded continuously with frequency and ROCOF – yields records of disturbed-system oscillations, swings, and propagation of disturbance fronts across the system.   These records are very useful to understand system outages (including wide-area blackouts) and operating problems but are also used to validate system models as the response of the model to the monitored triggering event is compared to real system behavior.  Some system analysis tool suppliers are working on automation of model tuning from event data.  Good models, in turn, are key to accurate and safe determination of operating conditions and limits. 

System State and Condition Monitoring:  Conventional EMS state estimation aims to adjust erroneous measurements and fill in approximations of missing values.  A dense PMU-based data-gathering system can go far beyond this, reporting its holistic assessment of power apparatus states and pinpointing inaccurate measurements for improved state estimation and alarming of failures.  Using related adjacent or redundant measurement comparisons, the condition-monitoring function of a WASA or WAMPAC system to substitute accurate values and to dispatch maintenance activities.

Automated Wide-Area Remedial Action or System Integrity Protection Schemes:  The tuned, accurate system models serve as a basis for Remedial Action Schemes (RAS) or System Integrity Protection Schemes (SIPS) which can be carried out on the WAMPAC platform by including a high-speed wide-area control messaging service.  Today’s RAS and SIPS are typically based on planning studies of potential system failures, carried out in advance, and may not work as required if the system is in a different state than that used in the study. 
With real-time, high-speed state measurements, the protective scheme can respond directly to disturbance indicators like phase angle across a transmission corridor, rather than only on a load flow higher than a set index value after a line trip.  Separately, other advanced users are already demonstrating the capability of combining multiple RAS in a single centralized system so logic is easily updated and so that behavior for complex events is coordinated (e.g. Southern California Edison Centralized RAS or C-RAS.)

Fire and Hazard Risk Reduction: SDG&E has been deploying, for several years, installations of its distribution Falling Conductor Protection (FCP) system based on PMU measurements along the protected circuit communicating synchrophasor voltage and current streams back to a substation phasor data concentrator (PDC) and a controller platform.  The phasor data collection and control system comprises a distribution-scale WAMPAC. The controller is programmed to analyze voltage and current phase angle relationships across the length of the circuit and can detect if a phase conductor has broken and is in the process of falling.  The FCP system detects and locates a conductor break in a typical time of 200 to 300 ms and sends trip commands to circuit switching devices on both sides of the break via IEC 61850 GOOSE messaging. 
A typical distribution conductor at a height of 30 feet or 9 m takes 1.37 s to reach the ground; FCP deenergizes the failing conductor by the time it has fallen about 4 feet or 1.2 m.  A difficult-to-detect high-impedance ground fault is completely circumvented; deenergizing the conductor while still high in the air avoids the risk of human contact or vegetation fire ignition.  The FCP concepts are being more broadly deployed across the SDG&E transmission grid.

Wide-Area Fault and Swing Protection:  A recent work has addressed how phase-current synchrophasor streams gathered from a high-density PMU deployment can be compared in a wide-area current differential scheme, detecting transmission faults a few cycles more slowly than local high-speed primary relays, but much faster than intentionally-delayed remote-zone backup relays. 
A centralized PMU-based backup fault protection controller observes current summations across the region and backs up local relays.  After observing prolonged fault duration, the backup scheme surgically removes faults left by failed relays or stuck breakers, tripping only the breakers required to deal with the specific local protection malfunction.  The scheme does not inhibit the performance of local high-speed relays and isolates the fault with less time delay and more specifically focused tripping than would be carried out by distance backup relays. 
Non-communicating distance relay zones can be left in service as a safety net – they will not get a chance to operate while the PMU-based wide-area protection scheme is in service.
Wide-area synchrophasor-based backup protection is sensitive and effective even for systems with low or unpredictable fault current caused by major penetration of inverter-based generation, and it does not trip for stable or unstable system swings.  Voltage phasors, which play little or no role in wide-area fault protection, can be used to detect swings to execute out-of-step tie tripping or islanding schemes with predictable and planned separation points.

Wide-Area Voltage Stability Management: PMUs have been incorporated in various voltage stability monitoring and contingency analysis tools and deployed by users such as SDG&E. Voltage stability issues often occur as a result of an increase in load demand. If the increase is slow, an EMS model-based Voltage Stability Assessment (VSA) will detect the problem and alert operators.  If faster, dynamic voltage instabilities occur, PMU-based VSA yields quicker and more accurate detection of the potential unstable contingencies requiring faster response. 
Furthermore, if the unforeseen system instability events develop very fast, measurement-based voltage instability detection tools, such as Real-time Voltage Instability Indicator (RVII), could be a complement to model-based VSA tools, detecting active power margins and voltage instability indicators in real time. RVII tools do not require a complex power system model; they are thus not as sensitive to model inaccuracies and can detect voltage instability in less than a second. These tools are also suitable for implementation in SIPS, or instead of undervoltage protection in simpler cases. RVII is able to identify unstable cases even in the case of high system voltage (for which undervoltage load-shedding relays would not operate),and distinguishes stable cases in the presence of very low voltages (for which an undervoltage relay should not operate).

Wide-Area Control: Wide-area holistic control of the grid becomes feasible when the PMU system includes mission-critical high-reliability data collection, high-speed control communications, and accurate system models to predict the result of control actions.  Voltage profiles can be adjusted in response to variable renewable generation via integration of measurements across a wide area and evaluation in a control model, to achieve the best or safest overall result rather than optimizing for one measurement point or a fixed combination of measurements.  Fixed and variable (FACTS) reactive devices can be continuously switched or adjusted for the best holistic profile.  Load flow control and dynamic stability enhancement are expected in the future.

PMU System Self-Monitoring:  While many users do not think of self-monitoring of a protection and control system as a power-system application, it is critical to a sustainable deployment, and supports the mission-critical roles that a WAMPAC system will serve.  The complete PMU system, whose components are reviewed below, is built from self-monitoring IEDs, controllers, and communications channels using repetitive or heartbeat communications services whose integrity can be monitored and alarmed.

The overlapping of the IED and communications service monitoring means that an entire WAMPAC system can monitor its integrity and accuracy in every branch, alarm specific failures for maintenance attention, and engage backup measurements from available data.  Figure 3 shows a contemporary display of PMU and PDC data stream availability.  Maintenance testing is never required, except for peripheral breaker tripping, control, and status input interfaces which can be validated by remote control actions or system state observations.  If protective relays with PMU measurement capability are used, the fault protection operations and protection system maintenance checks of these interfaces can serve WAMPAC P&C with no additional maintenance.

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