CRC Electric Power Engineering Handbook Chapter 11: Power System Dynamics and Stability Section 8: Direct Analysis of Wide Area Dynamics J. F. Hauer W. A. Mittelstadt R. Adapa M. K. Donnelly W. H. Litzenberger Pacific Northwest National Laboratory Bonneville Power Administration Electric Power Research Institute (PNNL) (BPA) (EPRI) Richland, Washington Portland, Oregon Palo Alto, California The material to follow deals with the direct analysis of power system dynamic performance. By “direct” we mean that the analysis is performed on the physical system, and that any use of system models is secondary. Many of the tools and procedures are as applicable to simulated response as to measured response, however. Comparison of the results thus obtained is strongly recommended as a means to test model validity. The resources needed for direct analysis of a large power system represent significant investments in measurement systems, mathematical tools, and staff expertise. New market forces in the electricity industry require that the “value engineering” of such investments be considered very carefully. Many guidelines for this can be found in collective utility experience of the Western Systems Coordinating Council (WSCC), in western North America. 1. Dynamic Information Needs: The WSCC Breakup of August 10, 1996 Large power systems are very rich in information that can be developed from direct measurements of dynamic behavior. Progressive electrical utilities are developing comprehensive data acquisition facilities. The emerging critical path challenge is to extract essential information from this data, and to distribute the pertinent information where and when it is needed. Otherwise system control centers will be progressively inundated by potentially valuable data that they are not yet able to fully utilize. New factors are rapidly compounding this problem. Utility restructuring promises to sharply increase the need for measurement based information while shrinking the time frame in which it must be produced and distributed. In addition, financial pressures dictate that cost recovery for the requisite technology investments be prompt and low risk. CRC Monitor Section – 2 400 500 600 700 800 300 1100 1200 1300 1400 1500 Malin-Round Mountain #1 MW Time in Seconds (see detail) 0.264 Hz, 3.46% damping 0.252 Hz 0.276 Hz 15:48:51 Out-of-Step separation Reference time = 15:35:30 PDT 15:47:36 Ross-Lexington line trips/McNary generation drops off Dittmer Control Center, Vancouver WA sample rate = 20 per sec. 15:42:03 Keeler-Allston line trips Fig. 1. Oscillation buildup for the WSCC breakup of August 10, 1996. These issues were brought into sharp and specific focus by the massive breakup experienced by the Western North American Power System on August 10, 1996. The mechanism of failure (though perhaps not the cause) was a transient oscillation, under conditions of high power transfer on long paths that had been progressively weakened through a series of seemingly routine transmission line outages. Later analysis of monitor records, as in Fig. 1, provides many indications of potential oscillation problems. Verbal accounts also suggest that less direct indications of a weakened system were observed by system operators for some hours, but that there had been no means for interpreting them. It is very likely that, buried within the measurements already at hand, lay the information that system behavior was abnormal and that the system itself was vulnerable. Utility restructuring, through several mechanisms, is making it impossible to predict system vulnerabilities as accurately or as promptly as the increasingly volatile market demands. It is quite possible that standard planning models could not have predicted the August 10 breakup, even if the conditions leading up to it had been known in full detail. This situation has deep roots and many ramifications [1,2,3,4,5,6]. Correcting the situation will require many years of concerted effort, on many fronts. An interim solution is to reinforce capabilities for predicting system vulnerability with the capability to detect and recognize its symptoms as evidenced in dynamic measurements. This can also be viewed as a form of measurement based dynamic security assessment (DSA). In the Western Power System, the technology and infrastructure that this requires are being developed as extensions of the DOE/EPRI Wide Area Measurement System (WAMS) Project and related efforts [7,8,9]. 2. Background Comprehensive monitoring of a large power system is a long step beyond the monitoring of local devices or even regional performance. Developing a suitable investment plan calls for close attention to emerging information needs and information technologies. Even more important–perhaps decisively so–are the interutiilit practices and infrastructure through which dynamic information resources are collectively reinforced and operated. Timeliness of the information is becoming an increasingly important consideration. The pace of utility restructuring strongly encourages more aggressive use of power delivery assets, which translates into CRC Monitor Section – 3 progressive encroachment upon customary operating margins. This can greatly increase the need for direct evidence concerning the proximity and nature of safe operating limits. The Western North America power system provides a useful example of the agencies at work. Driven by stability considerations, and in an earlier regulatory environment, the member utilities of the Western North America power system have made significant progress in the development of monitor facilities for examining system behavior. This development, coordinated through the WSCC, is a collective response to their shared needs for measurement based information about system characteristics, model fidelity, and operational performance [1,2,4]. Utility restructuring is now carrying these and all utilities toward the future in large abrupt steps. Infrastructure development for acquiring wide area dynamic information started in response to technical needs in a cost based environment. Its value in the new price based environment is likely to be much higher, through direct services such as • Real time determination of transmission capacities, assuming necessary progress with mathematical tools. • Early detection of system problems. This enables cost reductions through performance based maintenance, and provides a safety check on network loading. • Refinement of planning, operation, and control processes essential to best use of transmission assets. Such an infrastructure can also provide indirect benefits as an enterprise network [10,11], or "people net." It is a proven means through which a wide range of technical skills can be accessed and shared among the utilities as a virtual technology staff. This may be very important to the future power system, especially in situations where difficult stability problems constrain the use of transmission assists. 3. An Overview of WSCC WAMS Collectively, in response to the needs that utilities share in operating their facilities, these efforts are leading toward an integrated dynamic information network that spans the entire WSCC system. WSCC WAMS is evolving as a hierarchical network of dynamic monitors, plus the information tools and general infrastructure necessary for effective use of the acquired data. Data sources are of many kinds, and they may be located anywhere in the power system. This is also true for those who need the data, or those who need various kinds of information extracted from the data. It would not be practical or sufficient to just collect all WAMS data at one location and then permit users to retrieve it as needed. Data volume, and information demands, call for distributed storage and management. Central to this is a WAMS Information Manager having enough "intelligence" to route data and manage archives on the basis of the information contained. This issue is pursued in a later section. 4. Direct Sources of Dynamic Information There are a variety of means by which dynamic information can be extracted from a large power system. These include • Disturbance analysis • Ambient noise measurements -spectral signatures -open-loop/closed-loop spectral comparisons -correlation analysis CRC Monitor Section – 4 • Direct tests with -low level noise inputs -mid-level inputs with special waveforms -high level pulse inputs -network switching Each has its own merits, disadvantages, and technical implications [8,12,13,14,15,16]. For comprehensive results, at best cost, a sustained program of direct power system analysis will draw upon all of these in combinations that are tailored to the circumstances at hand. Some of these operations–such as network switching tests–are outside the usual scope of power system monitoring. All of them represent required functionalities for the WAMS infrastructure, however, and for the monitor facilities within it. Monitoring is a subset of measurement operations. Even so, it is the monitor facilities that provide the backbone for the dynamic information infrastructure. From a functional standpoint, wide area monitoring for a large power system involves the following general functions: • Disturbance monitoring: characterized by large signals, short event records, moderate bandwidth, and straightforward processing. Highest frequency of interest is usually in the range of 2 Hz to perhaps 5 Hz. Operational priority tends to be very high. • Interaction monitoring: characterized by small signals, long records, higher bandwidth, and fairly complex processing (such as correlation analysis). Highest frequency of interest ranges to 20–25 Hz for rms quantities but may be substantially higher for direct monitoring of phase voltages and currents. Operational priority is variable with the application and usually less than for disturbance monitoring. • System condition monitoring: characterized by large signals, very long records, very low bandwidth. Usually performed with data from SCADA or other EMS facilities. Highest frequency of interest is usually in the range of 0.1 Hz to perhaps 2 Hz. Core processing functions are simple, but associated functions such as state estimation and dynamic or voltage security analysis can be very complex. Operational priority tends to be very high. These functions are all quite different in their objectives, priorities, technical requirements, and information consumers. At many utilities they are supported by separate staff structures and by separate data networks. 5. What is a Monitor? The power system contains many devices that can serve as monitors for some processes and purposes. This document narrows the field somewhat, through the following definition: "A monitor is any device that automatically records power system data, either selectively or continuously, according to some mechanism that permits the data to be retrieved later for analysis and display." With exceptions as noted, present attention is further restricted to monitors that record one or more of the following: • dynamic "swing" interactions among generators and loads through an interconnecting electrical transmission system. • the performance of specific facilities involved in swing interactions. This includes generators, loads, and control systems. CRC Monitor Section – 5 This focus excludes most digital fault recorders and SCADA systems, since their typical record lengths and data rates are (respectively) not adequate for capturing swing dynamics. Network condition monitors might be included, depending upon the specifics of the device. Overall monitor facilities often contain a wide range of recording devices that are not swing monitors. The ubiquitous (and under utilized) digital fault recorder, or DFR, is a particulary good example of this. The facilities may also include devices that have little or no direct recording capability, but that do provide essential "intelligence" to overall facility performance. It is becoming established practice to classify all of the aforementioned devices as intelligent electronic devices, or IED's [17,18]. Drawing upon this terminology, just about any modern measurement system reduces to an IED network. 6. Monitor System Functionalities The primary rule in power system measurements is this: "Record good data, and keep them safe!" These are two distinct functions and both call for close attention. If monitor facilities are used to support direct tests then an even more important rule enters the picture: "Do no harm to the power system!" In both cases it is the measurement of low-level interactions that define the most rigorous functionality requirements to the overall measurement system. This is due, in part, to long recording periods plus the need to resolve very small changes in large signals [12]. There is a more demanding need, however: the evaluation of system dynamic performance in nearly real time (NRT). This is particularly critical during staged tests, when a close balance must be maintained between system security and the quality of test results. It is also important during routine monitor operations, as a means for identifying important data and for generation of operator alerts. Storage Time Domain Display Interactive Recording Freq Domain Display Storage Archival Recording test signal u(t) response y(t) Signal Conditioning System Fig. 2. Information functions in measurement of low level interactions Fig. 2, with the test signal deleted for the case of passive monitoring, represents core information functions that are needed in both situations. The functions are • Signal conditioning, to assure the measurement quality and prompt observability of important signal features. • Archival recording, to assure safekeeping of important results. This should be as comprehensive as possible, and include all phases of testing that involve switching operations or application of test signals. CRC Monitor Section – 6 • Interactive recording, to permit prompt examination of data that cannot be fully assessed in real time. This also provides backup recording of high priority signals. • Time domain display, to permit frequent review of signal waveforms for evidence of data quality and emerging trouble on the power system. • Frequency domain display, to permit frequent review of signal spectra for evidence of data quality and possible trouble on the power system. The NRT displays permit appropriate actions to be take by the test team or monitor operations staff. In some cases the analysis tools underlying these displays are also used in event detection logic (EDL) to trigger automatic functions such as accessory data capture, information routing, or operator alerts. Fig. 2 represents a paradigm that is fundamental to low level measurements of power system dynamics. Not all monitors are interaction monitors, and many interaction monitors lack some of the functions shown. The required functionality resides in the overall measurement system. Fig. 2 will reappear in many expanded forms throughout the Sections to follow. 7. Event Detection Logic Data recording in an individual monitor is either selective or continuous. In selective recording, data "snapshots" are collected upon command of "trigger logic." This requires that dynamic event signatures be detected in real time–otherwise useful data is lost. Continuous recording requires very similar logic, but uses it to sort records already captured according to their information content. Triggers for initiating data capture can be classified in several ways. A "manual vs. automatic" classification provides • manual – local – remote • automatic – pre-scheduled – internal event detection – cross-trigger (from another monitor or IED) Another classification is • external – local manual – remote manual – pre-scheduled (clock initiated) – cross-trigger (from another monitor or IED) • internal event detection This highlights the fact that EDL is fundamentally different from some other triggers. EDL is also a core issue in monitor design and operation, whether recording be selective or continuous. There are four basic factors involved in detecting the onset of a dynamic event. They are • magnitude CRC Monitor Section – 7 • persistence • frequency content • context A simple disturbance trigger might examine just magnitude and persistence, in tests of form "Do the latest M samples each exceed threshold T(M)?" [19]. It is useful to think of the context factor as adjusting such thresholds to system conditions, such as network stress or the operational status of key system resources. A partial list of signatures through which events can be detected, and perhaps recognized, includes the following: • Steps or swings in tieline power. • Large change, or rate of change, in bus voltage or frequency. • Sustained or poorly damped oscillations, perhaps in conjunction with some other event. • Large increase in system noise level. • Increase of system activity in some critical frequency band. • Unusual correlation or phasing between fluctuations in two given signals. The tools needed to extract useful signature information from measured data range from straightforward heuristics to very advanced methods of signal analysis. Recognition of the underlying events calls for pattern recognition logic to match extracted signatures against known event templates. 8. Monitor Architectures The vast majority of dynamic monitors only capture data for disturbances that are strongly observable in the monitor inputs. The "trigger to archive" or "snapshot" monitor in Fig. 3 is typical. A circulating prehistory buffer retains the most recently acquired data, assuring that a certain amount of information will be provided about system conditions before the disturbance is detected. Trigger logic causes the disturbance to be recorded as one or more data snapshots in the motor archives. The device shown also has the following accessory features: • External trigger inputs for initiating data capture. • Flags and alarms sent out to indicate the triggering condition. • External control of monitor settings via a modem port. • Retrieval of archive data via a modem port. The triggered monitor uses a rather conservative architecture that is necessary and appropriate when substantial amounts of data must be captured at high rates (as in digital fault recording). The reliance upon triggered recording–together with a general tendency toward short records and even shorter prehistories–are serious handicaps in wide area measurements, however. CRC Monitor Section – 8 Monitor Archives Modem Port Digi tal Interface Trigger Logic Flags and alarms External triggers Circular buffer dial-up access Trigger selections Signal selections Archive access Gate Analog Signals A/D C onve rte r Signal Merging and Routing Fig. 3. A simple "snapshot" disturbance monitor WSCC experience suggests that triggered data capture does not provide an adequate basis for wide area measurements. Even rather large events may not be sensed by trigger logic that is remote from the site of the disturbance. Records for a cascading failure that develops slowly, from some fairly small initiating event, are very unlikely to present a comprehensive view of the mechanism by which the small failure propagated into a very large one. 100 80 120 60 40 0 20 180 160 140 1000 1400 1200 1800 2000 1600 SECONDS MW 50 MS MALIN-R MOUNTAIN 1+2 DATE 3/6/87 TIME 23:10:39 BPA SYS OPS Fig. 4. Western system oscillations of March 6, 1987 (sum for two parallel circuits) Fig. 4 illustrates the point. The record there, collected on BPA’s earlier Power System Disturbance Monitor, indicates peak-to-peak 0.7 Hz swings of roughly 900 MW on the Pacific AC Intertie (PACI). However, as is usual with triggered monitors, it failed to capture the all-important interval during which the oscillations started. Without this, whatever indications there may have been to warn system operators of pending trouble remain unknown. Some triggered monitors can support interaction measurements under manual control, provided that their storage capabilities and analog to digital (A/D) resolution are sufficient. Their functionality in this mode of operation is similar to that of a high quality tape recorder. The more "intelligent" functions expected of a monitor are lost, and sorting through the data can become a very laborious manual exercise. Monitors that are explicitly designed to operate in a continuous recording mode are a far better option. CRC Monitor Section – 9 Trigger Logic Flags and alarms high capacity ethernet File Tags Controls External trigger Shared Data Analog Signals A/D Converter Signal Merging and Routing Digital Interface Ethernet Port Modem Port Archive access Monitor Archives Observation & control (all func Data sharingInternal Analysis and Display Manual trigger Fig. 5. Basic architecture for a continuously recording monitor Fig. 5 shows a basic "stream to archive" monitor that, typically, will maintain a continuous data record for periods ranging to several weeks. While triggers are used, their role is rather different than for a triggered monitor. Data capture in a continuous monitor does not depend upon trigger logic. Instead, trigger logic is used to • tag the acquired data in ways indicating its likely value. Factors in this are – the magnitude, duration, and frequency content of dynamic activity. – the context in which the data were acquired–e.g., event specific external triggers and system stress levels (from condition monitoring). • generate external flags or alarms when data values reach critical levels. The value tags will also determine such matters as the priority, routing, archiving level, compression, and retention time for the associated data. In a fully developed monitor system the value tags will be amended by an Archive Scanner that automatically reviews and restructures collective monitor archives. This continuous monitor also has some accessory features beyond those in Fig. 3. These are • Real time analysis and display, with an option for manual triggering. • NRT data sharing through the computer network. The next architecture takes the functionality progression several steps further. CRC Monitor Section – 10 C onv erter/Inter face Signals external triggers Event Archives Continuous Archives Gate Signal Selection Raw Display Filters & Decimation FFT Tools Other Analysis Time Domain Displays Freq. Domain Displays Other Displays Trigger Coordination Signal Processing Buffer (SPB) storage control & status flags manual t rigger internal triggers operator alerts cross triggers *event detection logic Long EDL* Short EDL* Long EDL* Short EDL* Fig. 6. A continuous monitor combining short and long EDL Fig. 6 shows a configuration that is representative for a mature interactions monitor that is operating in a manned substation or in a system control center. Absence of an indicated perimeter for the monitor reflects the fact that this might well be a locally networked facility, containing several computers or other IED's plus linkages to the energy management system (EMS). The indicated triggers are both external and internal, manual and automatic. The internal automatic triggers are classified as short or long (fast or slow), depending upon length of the data segment needed by the associated event detection logic. Short EDL can work with a short block of recent data, and is usually sufficient for disturbance monitoring. A distinguishing feature in this architecture is the signal processing buffer (SPB) used for advanced triggers (in the long EDL) and in special displays. SPB functionality is essential for extracting interaction signatures, and for presenting those signatures to operations staff for their interpretation and review. At hardware level, however, this functionality can be distributed among one or more buffers internal to the monitor itself plus external buffers for shared access to the record stream at file level. A next step in monitor refinement is to enhance the EDL and trigger coordination functions of Fig. 6 through artificial intelligence. Fig. 7 represents a Diagnostic Event Scanner (DES) suitable for this purpose, and for the Archive Scanner mentioned earlier. CRC Monitor Section – 11 Operator Alerts Cross Triggers Display Controls Data Retention Requests Record-Value Tags analysis controls Ambient Features Ringdown Features Ambient Analysis: • "Mode Meter" • FFT spectra • FFT correlations • Filter banks* Ringdown Analysis: • Prony analysis • FFT spectra • FFT correlations pause start Fast Edge Analysis: • Levels • Steps • Ramps Signal Processing Buffer (SPB) Signal Selection Dataa Stream Edge Timing Functions: • Detection • Classification • Recoginition Tools: • Interactive review • Heuristic rulesets • Expert systems • ANN's • Others Signature Recognition Logic Edge Features Other data & information analysis controls Feature Extraction Logic (short data blocks) *NOTE: Filter banks can include wavelets and BPA oscillation triggers data routing & processing controls Step/Ramp Detector Fig. 7. Diagnostic Event Scanner (DES) within a continuous monitor 9. Monitor Network Topologies With respect to their architecture, monitor facilities consist of • Central monitors that continually scan signals from transducers, and similar instruments, that are communicated from remote sites. • Distributed monitor networks, which are found in a variety of general forms • Local monitors, for which remote access is usually weak and manually initiated. BPA operates a centralized monitor system, based at the Dittmer control center, that is interlaced with an expanding network of remote monitors and secondary recording devices. Many of the remote monitors provide local or regional surveillance over some important parts of the control system. The general strategy is to collect essential signals (including alarms) from such sites on the central monitors, at modest data rates, and to perform comprehensive high-speed monitoring locally. In most cases the high speed data is immediately available to site operators, but is selectively forwarded to the control center or to system analysts. BPA has three central monitors at or near the Dittmer Control Center. The newest of these, based upon BPA's rapidly evolving Phasor Data Concentrator (PDC), is discussed a bit later. The other two, the Power System Analysis Monitor (PSAM) and the Dittmer PPSM, are indicated in Fig. 9 The PSAM and PPSM have access, in parallel, to several hundred analog signals. Most of these signals represent power flowing within the BPA service area and contain useful information at frequencies up to perhaps 2 Hz. Signals associated with control projects are an increasingly common exception to this. They ordinarily use 20 Hz channels and the corresponding transducers, if any, tend to be conventional electronic units modified for either a 2 Hz or 20 Hz bandwidth. CRC Monitor Section – 12 SEATTLE PORTLAND to Los Angeles (dc) to San Francisco (ac) AREA AREA COULEE BOUNDARY DITTMERCELILO GRAND CUSTER MALIN MONTANA IDAHO communications link to Dittmer control center Dittmer control center MALIN interchange metering point HVDC Terminal Fig. 8. BPA central monitor coverage In this arrangement, the 2 Hz signals present a comprehensive view of interarea "swing" dynamics visible in BPA interchanges, plus information about voltages, reactive flow, important loads, and automatic generation control (AGC). The 20 Hz signals convey a necessary minimum of essential information about controller behavior. They also facilitate alignment and cross analysis of central monitor records with more comprehensive local recordings. The total communications burden for this mixture of central plus distributed monitoring is much less than for a fully centralized monitor system. Furthermore, much of the data can be moved with adequate speed and reliability using general purpose computer networking technology. CRC Monitor Section – 13 A/D Power System Analysis Monitor (PSAM) A/D Portable Power SystemMonitor (PPSM)Data Distribution System (analog) RMS Xducers RMS Xducers Controller Signals RMS Xducers GPS Synchronization & Timing Security Zone Security "Firewall" Fig. 9. Topology of BPA central monitor system for analog signals PMU PMU PMU Phasor Data Concentrator (BPA) PMU PMU PMU Phasor Data Concentrator (Southwest) Phasor Data Concentrator (WAPA) PMU PMU PMU Fig. 10. Partial topology of the emerging WSCC phasor measurements network This deployment of remote monitors on a general network represents an evolutionary step in the gradual transition from analog technology to digital. Direct replacement of BPA's present point-to-point analog communications by digital channels of comparable bandwidth and resolution (about 14 bits, after filtering) would be needlessly expensive at this time. Best use of digital technology will call for new architectures, which should readily accommodate the rapid product enhancements so characteristic of the "information age." CRC Monitor Section – 14 The third monitor system at the Dittmer Control Center is entirely digital. The system consists of multiple Phasor Measurement Units (PMUs) that are linked together by one or more PDC units. The PMUs are synchronized digital transducers that stream their data, in real time, to the Dittmer PDC(s). The general functions and topology for this network resemble those for the Dittmer PSAM and PPSM. Data quality for the phasor technology appears to be very high, however, and secondary processing of the acquired phasors can provide a broader range of signal types. Phasor networks provide their best value in applications that are mission-critical, and that involve truly wide area measurements. Both factors encourage real time data links among regional phasor networks. This can be accomplished at both the PMU and the PDC levels. Connecting a PMU to multiple PDC units is straightforward and has already been done. Selective forwarding of PDC signals to other PDC units seems feasible and is under very active development. A copy of the BPA PDC became operational at Southern California Edison (SCE) facilities in mid 1998, and another copy became operational at WAPA in November 1998. The resulting network is evolving toward a topology of the sort indicated in Fig. 10. Central monitor systems, though very effective, are not a complete answer: • They do not serve information needs at local or regional level. • The dedicated communications can be very expensive. • Communication failures may cause important data to be lost. • Data rates and data volumes for some dynamic processes are so high that continuous transmission to central facilities is not practical. This is especially likely for high performance control systems. • Under normal circumstances, a lot of data are too mundane to merit the costs of continuous transmission. Transmission should be selective, and based upon information value. In many situations better performance and better "value engineering" can be obtained through distributed monitor networks, in which local storage and selective messaging minimize reliance upon dedicated communications. Distributed monitor networks can take many forms, ranging from dial-up access among sites to NRT communication of data and information on full time computer networks. A general computer network permits a much wider range of functions and technologies than those of the central monitors represented in Fig. 9 and Fig. 10. It is particularly valuable for broadening the staff support base for monitor operations. CRC Monitor Section – 15 Field Site A local ethernet Field Site B Field Site C Central Facilities (Operations Center & Technical Support) general ethernet Analysis Workstation (Matlab) PPSM PPSM Core PPSM Analysis Workstation (Matlab) PPSM Central PPSM Analysis Workstation (LabVIEW) Analysis Workstation (FORTRAN) MODEM MODEM Analysis Workstation (Matlab) *PDADS Monitor * Fig. 11. A distributed network for dynamic information Fig. 11 shows a dynamic information network that represents the topology and functionalities associated with distributed regional monitoring. The nomenclature there is based upon BPA/PNNL elements of the WAMS technology package. A "monitor" in this context generally means a measurements workstation, consisting of • a "core monitor" or data capture unit (DCU). • one or more analysis toolsets . Most of this functionality is applied to data already captured but, as indicated in Fig. 2 and Fig. 5, some may be used in near real time. The analysis toolsets , when operated separately from the DCU, provide an analysis workstation. The following comments apply to the network of Fig. 11: • All monitor units and most workstations can communicate with one another. • PPSM data capture (in the DCU) follows the logic shown in Fig. 5. Continuous recording is customary but optional. • PPSM units normally include LabVIEW® analysis station capabilities suitable for initial analysis of captured data. For in-depth analysis and design this is augmented or replaced by a more advanced package based upon Matlab® (plus imbedded FORTRAN®). • The diagram for Field Site B shows a PPSM variant in which LabVIEW analysis tool have been replaced by a more extensive Matlab toolset for combined analysis and design. The result is a Portable Dynamic Analysis and Design System (PDADS). • Analysis in the FORTRAN environment is often performed on larger computers that do not communicate readily with the PPSM or other workstations. Then the communications may be limited to data transfers only. In point of fact there are two monitor types shown in Fig. 11. The standard PPSM is entirely a LabVIEW based virtual instrument [20,21]. It finds its best uses in field analysis or in pre-defined environments where CRC Monitor Section – 16 "pushbutton" analysis is appropriate. The other monitor is an extended PPSM that uses the standard LabVIEW DCU, while turning to Matlab to support its more advanced functions [7,22]. This network topology is suitable when security requirements are modest. Dial-in modems can be particularly inviting points for external attack, however, and they may not be suitable for applications that need a high level of data security [23]. 10. Networks of Networks A well evolved monitor system will necessarily involve a mixture of technologies, data sources, functions, operators, and data consumers. In broad terms, • Required functionalities are determined by who must see what, when, and in what form. • System configuration is strongly influenced by geography, ownership, selected technology, and the technology already in service (legacy systems). • Investment value is strongly enhanced through – selective use of IED's to supervise, integrate, or replace legacy systems. – organization of IED's (including monitors) into local, regional, and wide area networks appropriate to their functions and technologies. The choices that a particular utility will make are strongly colored by its operating and business requirements and, more generally, by the value it places upon information. Overall, the forces at work strongly favor wide area measurement systems that evolve as "networks of networks." There are a lot of advantages to this. Interleaving networks that have different topologies and different base technologies can make the overall network much more reliable, while broadening the alternatives for value engineering. It also permits networks to be operated on the basis of ownership. The ability of a utility to retain data until it is no longer sensitive (delayed release) will almost certainly prove necessary for information sharing in the new power system. CRC Monitor Section – 17 Site A Site C Site B Regional Dynamic Information Center PMU PPSM PMU PPSM Regional PPSM ethernet extended PPSM Regional PMC Regional PMC Monitor To Central PMC PMU Microwave System Analysis Workstation (Matlab) dial-up link Analysis Workstation (Matlab) DSM Terminal dial-up link DSM Nomenclature : PMU Phasor Measurement Unit PMC Phasor Measurement Concentrator PPSM Portable Power System Monitor DSM Dynamic System Monitor Fig. 12. Interleaved networks of PMU, PPSM, and DSM units (plus analysis workstations) Fig. 12 shows interleaved networks of PMU, PPSM, and DSM units (plus analysis workstations). These devices are proprietary to Macrodyne, BPA/PNNL, and Power Technologies Inc. In this case • The PMU network has dedicated microwave communications in addition to dial-up links. • The DSM "network" consists of individual units, accessed from a DSM base unit via dial-up links. • The PPSM network serves in several roles. In addition to advanced monitor functions, it also provides the PMU and DSM units with – alternate communication paths, via the computer network. – local, high volume archiving. – analysis and display functions available both on site and through remote teleoperation. Typical network details are shown in Fig. 13 and Fig. 14. Note that Fig. 13 indicates yet another layer of networking, for a digital transducer network). This could range from a basic PMU/PMT configuration to the highly versatile Integrated Object Network produced by Power Measurement Ltd. [18]. CRC Monitor Section – 18 Digital Transducer Network (DTN) Portable Power System Monitor (PPSM) Data Snapshots Cross-Triggering Commands Precise Clock Time Enhanced Xducers Network Wide Area Data Files and PPSM Control Potential Xformers Current Xformers Continuous Link(s) Real-Time Data Dial-Up Link(s) DTN control and data retrieval Local Archives Interface Electronics Other signals Analysis, Alarms, and Display Other Instruments Network Local Area Archive Files Fig. 13. PPSM interconnections to local transducers Scientific Atlanta SD 390 Dynamic Signal Analyzer (DSA) PTI Dynamic System Monitor (DSM) [wide area network links] PPSM control and data transfer Interface Electronics Enhanced Analog Xducers Potential Xformers Current Xformers Microwave Transmitter Central Monitor Facilities TCSC Controller Portable Power System Monitor (PPSM) Electronic Instruments BEN–5000 Digital Fault Recorder (DFR) LAN communications local analysis and display Phasor Processing Direct Input SSR System Dynamics Fig. 14. Monitor organization local to the Slatt TCSC Fig. 14 represents a local measurement network developing for the 500 kV Thyristor Controlled Series Capacitor (TCSC) that was installed at BPA's Slatt substation under an EPRI FACTS project [24,25,26]. The functionalities there are highly desirable for any large control project. Guidelines for network organization are that • Any monitor (DFR, DSM, PPSM), through the local area network (LAN), trigger data collection on the others according to its own rules. • The PPSM can supervise and control all other monitors or major instruments. All measurements are available to the PPSM for local display and analysis, for archiving, or for routing to other locations on the wide area network (WAN). • The WAN permits remote observation and control of all devices in the measurement LAN through the PPSM. The PPSM can extract real time data from any other PPSM on the WAN and incorporate that data into its own processing. CRC Monitor Section – 19 • Every monitor, and the majority of major instruments, can be accessed through a telephone connection as a (lower performance) backup to WAN failure. In addition to its usual functions, the DFR is used for waveform recording in controller performance tests. The DSA, by itself, is not quite a monitor. It does have good recording capabilities and it is readily networked. It is quite clearly a rather advanced IED and a valuable addition to overall monitor facilities. Fig. 15 provides an expanded view of the Slatt measurement network, and of the regional network containing it. Information facilities of this kind are particularly valuable for certification and operation of major control systems such as the TCSC. For example, control engineers can perform wide area performance tests without the cost and delay of special purpose communication links. Staged tests can be supported remotely, with a minimum of travel, and technical staff can time-share that support across several projects while remaining in their normal work areas. Workteam functionality is a major requirements source in the proper design of dynamic information networks. Signal Generation modulat ion t est signal cos ( ωt ) optical link TCSC Controller Control House PPSM 178320465 ch Chart Recorder Buckley MW Isolation Amplifiers Dynamic Signal Analyzer #1 Slat t V TCSC ohms δδδδω ( synt hesized) damping signal PDADS Workstation TCSC CONTROL HOUSE operat or order internal order Dynamic Signal Analyzer #2 Highpass Filters Optical Receiver Enhanced Transducers Optical Transmitter Microwave Receivers PTI Monitor (DSM) DFR Extension (BEN-5000) optical link DFR Main Unit (BEN-5000) BPA Hq (Portland) PGE Hq (Portland) BPA Labs (Vancouver) Pacific NW Labs (Richland) ethernet RELAY HOUSE Relay House PPSM Dittmer PPSM Boardman PPSM Fig. 15. Slatt measurement network for TCSC modulation tests of June 6-7, 1994 The final network to consider in this "network of networks," then, is the "people net." As indicated in Fig. 15, the TCSC Project draws upon remote technical support from three institutions (BPA, PNNL, and Portland General Electric) at four different locations. Some of this support was provided interactively during commissioning tests [8], and all of it is available as needed. Developing the proper interfaces between the "people net" and the monitor network(s) is critical to value engineering of the dynamic information system. The information users determine who must know what, when, and in what form. The information providers must deal with who must do what, when, and with what tools. Various aspects of these questions are treated in the Sections immediately below. The very important issue of placing a value on the information itself is reserved for separate discussion. 11. WSCC Experience in Monitor Operations A competent monitor network is the "backbone" of the dynamic information infrastructure and it is a fundamental requirement for wide area control [27,28,29]. We should expect (or hope) that most of measurement functions will be mundane ones, performed unobtrusively under routine operating conditions. CRC Monitor Section – 20 However, the network must stand ready to provide mission-critical services with little or no warning. Some examples of high value support are • Early warnings of trouble arising on the system, or in specific equipment. • Integrated recording and information sharing for major system disturbances. • Real-time recording and analysis during tests of controller performance, or of wide area system dynamics. • Recording of anomalous system behavior that is to intermittent for scheduled examination. The reader is advised that these are performance objectives, and that they might not be fully mature operational realities. The WSCC utilities, individually and collectively, have been comparatively aggressive in their development of monitor facilities [8,30,31,32,33,34]. Operation of those facilities has revealed a number of general problems: • Of the triggered monitors installed on the system, expect no more than 50% to provide good records for a major disturbance. Leading problems are – failure to detect the event (i.e., to trigger data capture) – failure to trigger soon enough, or to "retrigger" often enough during protracted disturbances. – inadequate data storage. – overwriting of stored data before they have been downloaded to secure archives. – monitor out of service. – monitor failure from loss of supply power. • Of the continuous monitors installed on the system, expect about 90% to provide good records for a major disturbance. Leading problems are – monitor out of service. – monitor failure from loss of supply power. • The value of a particular event record may be higher for some other utility than for its owner. This situation may not be recognized for several days after the event, in which case the record may well have been deleted from the data system. • Determination of pre-disturbance conditions, though fundamental to subsequent model studies, is seriously hampered by sparse monitor coverage plus failure to retain relevant EMS data. • Operations staff are very cautious about high-level staged tests, and becoming more so (see also [2] and comments by Scottish Power in [27]). Better use must be made of chance disturbances, plus low level ("non-intrusive") tests and measurements. • Very few triggered monitors are designed for low level tests and measurements. These are the defining tasks for a continuous interactions monitor such as BPA’s PPSM and PDC. • Necessary tools and skills for conducting staged tests, and extracting dynamic information from measured data, are not evenly distributed among the utilities. Some mechanism is needed for sharing these resources (and their costs) to meet shared utility needs. CRC Monitor Section – 21 • An emergency response plan is needed to assure safe retention and prompt integration of measured data following major disturbances. It is not realistic to task primary operations staff with this function, and it is one that should be automated anyway. The WAMS effort and the WAMS technologies are directly rooted in this experience. 12. Database Management in Wide Area Monitoring Deployment of new monitors–and the proliferation of IED's in general–are overcoming many of the problems in acquiring raw data. It is clear that this emerging abundance is producing a new generation of challenges in monitor operations. Chief among them are • Timely extraction and routing of information resident in the data. • Avoiding premature deletion of valuable data, but without inundating data facilities. EPRI and BPA have initiated exploratory research into a generic WAMS Information Manager to deal with such matters. A key element in this is a WAMS Database Manager (DBM). Conceptually, its functions include the following: • Automatic Routing: – Operator alerts – Cross triggers to local & remote monitors – Event-driven control of local displays – Data retention requests to local & remote monitors • Servicing of Staff Requests: – Data transfers – Special data operations & displays – External triggering of local or remote monitors – Special log entries • Background Directory Operations: – Exchanges among DBM units – Integration, annotation, & indexing – Posting on EMS, OASIS, WWW • Background Data Operations: – Launching & supervision of the Archive Scanner – Content based compression & archiving – Logging of events & summary features • Utility Functions: – File merging & compression – Hardcopy generation The monitors involved in this include SCADA, DFR, and other IED units in addition to the usual swing monitors. CRC Monitor Section – 22 13. Placing a Value on Information It is apparent that the utilities face a massive investment in information technology. Planning these investments encounters a very basic question: just how do you place a value on information? A partial answer is this: The value of information is precisely that of the decisions derived from it. Fig. 16, another paradigm, is useful for expanding upon this statement. Observed response Power System Unobserved response Information Automatic control System planning System operation Disturbances Decision Processes Measurement Based Information System Fig. 16. The cycle of measurement, information, and decisions TABLE I. Decision time scales • Automatic Control: – Protection: milliseconds – Stability: milliseconds to seconds – AGC: seconds to minutes • System Operation: – minutes to weeks • System Planning: – Operations: hours to weeks – Expansion: weeks to years Decision processes in a power system range from the very rapid ones pre-programmed into protective control equipment to the very slow ones associated with expansion planning. In all cases the decisions are derived, with varying degrees of immediacy, from measurement based information. In some cases the information is encapsulated in a model, or perhaps in operating policies. In others the data is processed immediately–e.g., a controller input or as a signal to system operators. Accumulated over time, information provides a knowledge base that permeates utility practices and, often enough, those of the industry. Such long term effects, together with the many paths by which information enters utility decision processes, will defeat any direct attempt to place a value upon it. More constructive results follow from considerations of affordability and risk management. CRC Monitor Section – 23 Some suggestions in the matter are these: • Consider information an insurance policy against operational uncertainty: – How much insurance is enough? – How much risk is too much? • Distinguish between value, cost, and affordability. • Consider all cost elements, especially lead time and staff demands. Another factor, one that may pre-empt many of these considerations, is regulatory mandates issued at various levels of government. It is very likely that an infrastructure for developing and exchanging dynamic information will be found necessary for assuring power system reliability and, thereby, the public interest. 14. Monitor Placement There can be no simple rule for determining what to monitor, or how to configure particular monitor systems. WSCC recommendations in [1] are still a reasonable guide with respect to functionalities. They state that the overall monitor facilities should support the following applications: • analysis of system disturbances and oscillation incidents • performance evaluation for major control systems • early detection of poor damping or other unusual system behavior • validation of computer models These same guidelines also state that the overall facilities should provide a comprehensive report of line flows, bus conditions, and control actions for • major and/or critical interchanges and load areas • major control systems, especially those for HVDC line flow • major and/or critical generation projects Regarding technical performance, the guidelines emphasize long-term recording capabilities with an overall signal resolution equivalent to 14-16 binary digits and sample rates at or above 20 samples per second. Some general guidelines on technology choices are provided in [8,15] and in Appendix A of this Chapter. The underlying perspective there is that the rationale for power system performance monitors should parallel that for flight recorders on commercial aircraft. CRC Monitor Section – 24 jf h ME XICO Major interaction path "Index" generator SUNDANCE KEMANO MICA COLSTRIP PALO VERDE HOOVER GRAND COULEE MEAD FOUR CORNERS SHASTA Fig. 17. Major interaction paths for the western North America power system Surveillance of major interactions is fundamental to the objectives of wide area monitoring, and to the broader objectives of a dynamic information infrastructure. Fig. 17 indicates the more important interaction paths in the WSCC system, plus "index" generators where the associated dynamic modes are particularly observable and/or controllable. Monitoring may well call for different technologies in different regions. The western power system has an underlying loop structure, with important radial extensions into British Columbia, Alberta, and eastern Montana. Remote imports into the Los Angeles area often approach 10% of system capacity. Power flow along the western side of the loop is concentrated on a few major interties. Along the east side of the loop power flow is diffused across a web of more numerous and weaker lines, eventually concentrating along a major transmission corridor that approaches Los Angeles from the east. Monitor needs on the west side of the system has the following characteristics: • the dominant problem is interarea "swing" dynamics and related control systems. • measurements of dynamic state are the key indicator for system performance. CRC Monitor Section – 25 • fairly good signals for the needed quantities are available from enhanced conventional transducers for power, frequency, and voltage magnitude. • critical information is concentrated at a fairly small number of sites. • substantial monitor facilities are already in place. However, on the eastern side • the dominant problem is overloading of weak links through power surges or through inadvertent "loop" flows. • measurements of static state (powerflow variables) are the key indicator for system performance. • competent measurement of complex voltage and current requires precise time synchronization technology that has just now reaching maturity [13,16,8]. • critical information is broadly dispersed across the network, and the appropriate measurement points are not obvious.. • monitor facilities are in the early stages of development. The natural "state variables" for the west side are, for the most part, those associated with generator and control system dynamics. The natural state variables on the east side are those associated with powerflow on the network. 15. Access to Monitor Data In-depth analysis of monitor data is often done at locations well removed from the original point of data collection, and some other computing environment. Overall monitor facilities must accommodate many different users for the data collected. Some users may need regular access to records from a number of monitors and monitor types. It is highly desirable that • overall data access be provided within a consistent, well integrated framework. • the users process the data with the same tools that they use in other work of the same general nature. Fig. 18 indicates three of the more common analysis environments for monitor data. The LabVIEW environment is typical of normal field operations, the FORTRAN/C environment is typical for system planning, and the Matlab environment is typical for advanced analysis and for control system design. Fig. 19 shows the various means of access to PDC data at BPA. Special Matlab toolsets indicated there include the BPA/PNNL Power System Identification (PSI) Toolbox, of which the Power System Monitor (PSM) Tools are a subset. Toolsets coded in Visual Basic are also in use, but not indicated in this figure. CRC Monitor Section – 26 FORTRAN o r C en vironment Data gen era tion en vironments MATLAB env ironmen t (Po rta ble Dy anmic Ana ly sis a nd Design S ystem, u sed for Slatt TCSC te sts) measur ed respon se mod eled r espon se PSI Toolbo x Lo cal An alys is & Displa y Co ntrolle r De sig n To ols MATLAB Gener ic Toolse ts Ringd own GUI PSM T ools Power Sy stem Mo nitors (PSMs) Pow er Sy stem Mode ling Code s EMTP Too ls Powe r Sys tem Inden tifi cation (PSI ) Too lbox Tra ns lators Tr anslator s Fig. 18. Typical analysis environments for power system response records Custom Menus Add-On Toolsets Result Files Automatic Documentation Displays Hardcopy Outputs Matlab Tools • Data repair & restructuring • RMS calculations • Signal extraction & linking • Filtering & smoothing • Fourier & Prony analysis L abVIEW Vie wer/T rans lato r Translated Files Hardcopy Outputs External Archiver ether net Local Display PMU PMU Message Integrator Internal Archive (binary files) PDC Remote Viewing Tools Mat lab Tr an s lat o r Fig. 19. Surrounding network for the Phasor Data Concentrator at BPA’s Dittmer Control Center 16. Monitor Application Examples This Section presents a cross section of PPSM data collected on the WSCC system. The examples are organized according to the conditions under which they were obtained–that is, during ambient conditions, system disturbances, and staged tests. A. Examples for Ambient Conditions Major modes for the WSCC system are conspicuous in ambient noise activity under just about all conditions. The activity shown in Fig. 20, on BPA interchanges into Canada and California, is typical. Power flows there are represented as north to south; slow trends have been removed by a highpass filter set at 0.1 Hz, and lowpass filtering at 1.0 Hz was used to reduce local-mode activity plus extraneous noise. Fig. 21 and Fig. 22 show that ambient swings are strong, highly coherent, and sharply focused near 0.29 Hz. (See [35,36,37,38] for analysis principles.) Those on the Canada interchanges (Ingledow and Boundary) are in phase, and slightly offset from those with California (at Malin). Such results are a constant reminder that the "AC Intertie" mode associated with the Pacific AC Intertie (PACI) is very much alive. CRC Monitor Section – 27 -40 -20 0 20 40 (recorded 02/09/92) -40 -20 0 20 40 -40 -20 0 20 40 Malin #1 MW BCH Boundary MW (reversed) BCH Ingledow MW (reversed) Time in Seconds 0 10 20 30 40 50 Fig. 20. Signals for ambient noise on major BPA interchanges, 02/09/92 0 1 2 Frequency in Hertz -100 10 20 30 40 BCH Ingledow Malin #1 nfft = 1024, overlap = 0.950 pu (Dittmer Control Center, 02/09/92) BCH Boundary Autospectra in dB Fig. 21. Spectra for ambient noise on major BPA interchanges, 02/09/92 Squared Coherency 0 0.2 0.4 0.6 0.8 1.0 0 1 2 Frequency in Hertz nfft = 1024, overlap = 0.950 pu (Dittmer Control Center, 02/09/92) BCH Boundary MW vs BCH Ingledow MW Malin #1 MW vs BCH Ingledow MW Fig. 22. Coherencies for ambient noise on major BPA interchanges, 02/09/92 CRC Monitor Section – 28 A next issue is whether warning signs such as those conspicuous in Fig. 1 can be extracted from prior ambient activity. Comparison of Fig. 20 and Fig. 23 indicates that such warnings, if present, are not obvious in the time domain records. Fig. 24 and Fig. 25 show Fourier analysis results for thirty minutes prior to tripping of the Keeler-Allston line on August 10, 1996. (See also Fig. 1). At first inspection, the highly structured spectrum in Fig. 25 is very similar to what is usually observed. Closer inspection, with more subtle tools, may show otherwise. The abundance of data collected for this very important disturbance may contain a number of hidden clues concerning proximity to the limits of safe operation. -40 -20 0 20 40 reference time = 08/10/96 15H35m30s -40 -20 0 20 40 -40 -20 0 20 40 Malin #1 MW 0 10 20 30 40 50 BCH Boundary MW (reversed) BCH Ingledow MW (reversed)Time in Seconds Fig. 23. Sample of Ambient Noise on Major BPA Interchanges, starting 400 sec. before Keeler-Allston line trip on August 10 1996 0 500 1000 1500 2000 -2000 200 400 600 800 1000 1200 1400 1600 1800 Time in Seconds Spectral analysis interval Keeler-Allston line trip Reference time = 15H12m10s PDT jfh Fig. 24. Record for prehistory analysis of conditions preceding Keeler-Allston line trip. CRC Monitor Section – 29 0 0.2 0.4 0.6 0.8 1 -20 -100 Frequency in Hertz jfh PACIAlberta 0.7 Hz mode cluster Fig. 25. Prehistory analysis for conditions preceding Keeler-Allston line trip. Fig. 26 and Fig. 27 show an example of wide are correlation involving two widely spaced continuous monitors. The signals represent bus frequency at Kyrene substation in Phoenix, AZ, and bus frequency at Tacoma substation (near Seattle WA but recorded on the central PPSM at the Dittmer Control Center). The substations are about 1000 miles apart. The signals were obtained under ambient conditions, from ordinary frequency transducers and without direct synchronization. Even so the activity peaks (Fig. 27) and the correlations between them (Fig. 28) provide dynamic signature information up to roughly 0.8 Hz. The records used in this analysis are 25 minutes long, at 20 samples per second. In other applications, such as the commissioning of major feedback controllers, the recording periods would span many hours and much of the correlation analysis would be done at the test control site in real time. 0 5000 Frequency Deviation in Hz 10000 15000 -0.04 -0.02 0 0.02 Tacoma (x 0.75) Kyrene jfh Time in Seconds (signals offset for clarity) Tacoma Fig. 26. Ambient fluctuations in bus frequency, Tacoma and Kyrene substations (Tacoma WA and Phoenix AZ) April 15, 1996. CRC Monitor Section – 30 -90 -80 -70 -60 -50 -40 -30 -20 Pxx (db) and Pyy (db) Tacoma Kyrene nfft = 1024, overlap = 0.900 pu samples = [ 02641 : 33000] Kyrene System Frequency vs 005 Tacoma 230 kV -Bus Frequency jfh May 15, 1996 Frequency in Hertz 0 1.0 2.0 Autospectrum in dB Fig. 27. Autospectra for ambient fluctuations in bus frequency, Tacoma and Kyrene substations (Tacoma WA and Phoenix AZ) April 15, 1996. 0 1.0 2.0 0 0.2 0.4 0.6 0.8 1.0 coherency nfft = 1024, overlap = 0.900 pu samples = [ 02641 : 33000] Kyrene System Frequency vs 005 Tacoma 230 kV -Bus Frequency jfh May 15, 1996 Coherency Frequency in Hertz Fig. 28. Coherency of ambient fluctuations in bus frequency, Tacoma and Kyrene substations (Tacoma WA and Phoenix AZ) April 15, 1996. B. Examples from Major Disturbances "Ringdown" signals from major disturbances are a good source of information concerning the behavior of oscillatory dynamics (and of the control systems that affect them). Generator trips units at the Palo Verde plant (near Phoenix AZ) are particularly useful for determining current frequency and damping of the PACI mode. Fig. 29 shows limiting cases among 5 trips during the winter of 1992–1993. Implicit in this figure we find the following information: • Frequency and damping of the PACI mode vary considerably, during normal conditions. • Damping of the PACI mode can reach values low enough to be considered potentially dangerous. CRC Monitor Section – 31 • Frequency content of the ringdown signal is dominated by the PACI mode. This "mode shape" information – provides an excellent modeling benchmark for time domain simulations and for eigenanalyis. – indicates that Palo Verde is a good location for metering the effectiveness of PACI damping controls. – suggests that Palo Verde itself may be a good point for damping the PACI mode. Fig. 30 is just one among many examples gathered through collective efforts of the WSCC system oscillation work groups and, more recently, through the WAMS Project. 12/08/92 03/14/93 TIME IN SECONDS 0 100 200 300 400 5 10 15 20 25 30 35 40 0 (Circuit #1 only) 0.28 Hz @7.5% damping 0.33 Hz @4.5% damping MALIN -ROUND MTN MW SWINGS Dittmer Control Center, Vancouver WA Fig. 29. AC Intertie Response to Palo Verde Unit Trips, 12/08/92 vs. 03/14/93. The events of August 10, 1996 provided ample opportunity (and incentive) to examine the PACI mode under more extreme conditions. Development of these conditions and the subsequent separation are shown in Fig. 30 and Fig. 31. Tripping of McNary generation seems to have been a major factor [4,6] in the breakup. Whether this major loss of system support actually caused the oscillations, or merely made bad conditions worse, cannot be determined without very exacting studies based upon fully realistic models. This work is still in progress. Fig. 30. McNary plant generation during WSCC breakup of August 10, 1996. CRC Monitor Section – 32 790 792 794 796 798 800 1100 1200 1300 1400 1500 measured response model response Malin MW Time in Seconds model fitting window jfh Line Power in MW Mode Frequency in Hz Damping Ratio PACI trend? Alberta Kemano 0.216 0.0596 0.448 0.615 -0.0628 -0.0216 -0.0234 -0.0234 Fig. 31. Oscillation modes just prior to final separation on August 10, 1996 -300 -200 -100 100 200 300 400 WSCC PDC Units, 12/08/98 reference time = 08:14:00 sample rate = 30 sps 0 Summary Plot for Bus Angles 59.96 59.98 60 60.02 60.04 60.06 60.08 60.10 60.12 80 85 90 95 100 105 110 115 120 Time in Seconds Summary Plot for Bus Frequencies Fig. 32. Bus angle and frequency swings for San Francisco trip on December 8, 1998) (Combined data for WSCC phasor measurement system) CRC Monitor Section – 33 On December 8, 1998, a major portion of the electrical services to San Francisco were lost through a chain of unintended breaker actions. All three PDC units collected good data (as did the Dittmer PPSM). Table II describes the data obtained, and Fig. 32 is a sample of the bus angles and frequencies. The primary rms signals indicated there are those conventionally produced in the PSI Toolbox, which was used to align and process the PDC records. TABLE II. Data Collected by the WSCC Phasor Measurements Network for the San Francisco Load Trip of December 8, 1998 Organization PDC Units PMUs Phasors Primary RMS Signals BPA 1 7 45 153 SCE 1 4 24 86 WAPA 1 3 15 32 WSCC Total 3 14 84 271 Major disturbances triggers a number of actions among the operating utilities. A high priority is to determine what happened, why it happened, and how to avoid it in future operations. Key steps in this include the following: • Integration of operating records – initiating and trans-disturbance events – pre-disturbance activity & conditions – system performance and consequences • Search for warning signs – dynamic signatures in prior swing activity – information from model-based stability tools (DSA, voltage stability) • Support for countermeasures – engineering review – model development and analysis – operator alerts & guidelines A general process and toolsets for this are indicated in Fig. 33 (see appendices of [39] plus [40,41,42,43,44,45]). Table III, which tracks behavior of the PACI mode before and during the August 10 breakup, provides good eigenvalue benchmarks for development of the necessary models. CRC Monitor Section – 34 Model Data Eigenvalue Analysis Time-domain Simulation Time/Frequency Analysis Measurements Data System Tests and Measurements Criteria & Models for System Engineering Model-Based Analysis xxoo x σ jω Eigenshape Measurement-Based Analysis DISTURBANCE Fig. 33. Integrated Use of Measurement and Modeling Tools TABLE III. Observed behavior of the PACI mode PACI mode before August 10, 1996 Date/Event Frequency Damping 12/08/92 (Palo Verde trip) 0.28 Hz 7.5 % 03/14/93 (Palo Verde trip) 0.33 Hz 4.5 % 07/11/95 (brake insertion) 0.28 Hz 10.6 % 07/02/96 (system breakup) 0.22 Hz 1.2 % PACI mode on August 10, 1996 Time/Event Frequency Damping 10:52:19 (brake insertion) 0.285 Hz 8.4% 14:52:37 (John Day-Marion) 0.264 Hz 3.7% 15:18 (ringing) 0.276 Hz 15:42:03 (Keeler-Allston) 0.264 Hz 3.5% 15:45 (ringing) 0.252 Hz 15:47:40 (oscillation start) 0.238 Hz -3.1% 15:48:50 (oscillation finish) 0.216 Hz -6.3% C. Examples From Staged Tests Fig. 34 and Fig. 35 summarize results from 1991 test insertions of BPA's 1400 MW Chief Joseph dynamic brake. They demonstrate that the 500 kV connection to Alberta strongly influences overall system dynamics, and that its operational status may be a factor in tuning of future stability controls such as modulation of HVDC, TCSC, or SVC equipment. CRC Monitor Section – 35 Fig. 36 was produced during staged tests of the Slatt TCSC. For subsynchronous resonance (SSR) tests a TCSC modulation signal was produced at the nearby Boardman generator and transmitted to Slatt. PPSM acquisition rates at these sites were set to 400 sps and 900 sps respectively. The basic procedure was to sinusoidally modulate the TCSC at a selected shaft torsional frequency, remove the stimulus while switching the TCSC to the desired operating condition, and then record the ringdown in shaft speed. Fig. 37 is typical. Prony analysis produced values that agree closely with those produced by the General Electric test team with other instruments [24,25,26]. Fig. 36 shows composite autospectra for sequential testing of the four shaft modes. Dittmer Control Center, Vancouver WA 0 200 400 600 800 0 5 10 15 20 25 Pulse #1 * Pulse #2 Pulse #3 *Alberta separated [11/19/91] TIME IN SECONDS AC INTERTIE RESPONSE (MW) Fig. 34. PACI response to energization of the 1400 MW Chief Joseph dynamic brake. 05/08/85 11/19/91, Pulse #1 * 11/19/91, Pulse #2 11/19/91, Pulse #3 *Alberta separated 0.0 0.5 1.5 2.0 1.0 -100 10 -20 -30 -40 -50 FREQUENCY IN HERTZ GAIN IN DB Fig. 35. Effect of Alberta 500 kV connection upon PACI response CRC Monitor Section – 36 Gain Frequency in Hz 05 10 15 20 -60 -40 -20 020 40 60 Gain Phase 0.0 0.5 1.0 1.5 2.0 2.5 open loop, Ross line open 1944-1947 h, June 7 1994 Phase Fig. 36. Frequency response of Slatt-Buckley line MW to TCSC modulation signal TIME IN SECONDS 0 1 2 3 4 5 6 7 8 -7 -6 -5 -4 -3 -2 Test Event 74: 1743 h on August 6, 1994 5 modules in vernier, 1 module in E-fire LN OF POSITIVE SPEED DEVIATION Fig. 37. Boardman generator shaft speed ringdown, TCSC decoupled from shaft -60 -40 -200 0 10 20 30 40 50 60 Test Events 71-74: 1732-43 h on August 6, 1994 5 modules in vernier, 1 module in E-fire FREQUENCY IN HZ AUTOSPECTRUM IN DB Fig. 38. Composite autospectrum for Boardman generator shaft speed deviations (rear sensor) CRC Monitor Section – 37 In April 1999 BPA resumed the testing of WSCC dynamics by modulation of the Pacific HVDC Intertie. These had been suspended for more than a decade, due to lack of immediate need and to lack of a suitable measurement system. The breakup of August 1996 and progressive integration of the WSCC phasor measurement system significantly altered these factors. For this new test program the objective is to safely examine the frequency and damping of a single mode, with a direct testing procedure that is minimally intrusive and that can be automated. The adopted procedure is similar to the shaft ringdown tests illustrated in Fig. 37. Fig. 39 shows typical response for a two different applications of a two-cycle HVDC probing signal, with a base frequency of 0.33 Hz and a level of ±125 MW. If the level of probing were much lower then some kind of time averaging would be needed during modal analysis. In the present case, spatial averaging across multiple signals should be done when Prony analysis is applied to the ringdown. Another option is to perform “extended” Prony analysis, using input/output tools that extract information from the forced response as well as the ringdown response [44,46,47]. These tools require an accurate record of the input signal. Though rarely available in the general case, such records are available for well instrumented tests. The results shown in Table IV were produced with a tool of this class, one that is contained in the Matlab System Identification Toolbox®. It was selected on the basis of immediate convenience rather than full comparisons against similar products or algorithms of the same kind, and its use here is exploratory. To this point the results produced with this tool have been internally consistent, and consistent with those produced with Fourier and Prony methods (see [29], chapter 7). Sharper tradeoffs among the test procedure, the analysis methods, and the design of the probing signal will evolve with as this BPA project continues. 2 40 2 50 2 60 2 70 2 8000000 0 5 10 15 20 25 Malin bus voltage (kV) Pacific HVDC Intertie power (MW) 552 550 548 546 544 Time in Seconds Fig. 39. Power system response to two applications of two cycle probing signal CRC Monitor Section – 38 Table IV Modal parameters identified at various stages of the probing signal test on April 27, 1999. Test Mode 1 Mode 2 Frequency Hz Damping Ratio % Damping neper/sec Frequency Hz Damping Ratio % Damping neper/sec B6 –probe 0.303 10.15 % 0.193 0.416 9 % 0.235 B7 – brake 0.295 9.3 % 0.172 0.417 7.9% 0.207 B8 –probe 0.296 10.02 % 0.186 0.416 8% 0.209 17. Conclusions The material here advances two key points, that • Timely access to dynamic information is essential to successful grid operation in the emerging power system. • Lack of such information is a particularly acute threat to system reliability during the transition to a deregulated industry. The presentation examines these needs and the technology options for meeting them, drawing upon WSCC experience as a guide to the challenges that many other utilities and systems will encounter. It is reasoned that the dynamic information infrastructure will draw upon a hierarchical monitor network that provides integrated and fairly comprehensive measurements of wide area dynamics. In a large power system this is a long step beyond the monitoring of local devices or even regional performance. Reliable data capture, for example, implies sub-networks of continuously recording monitors plus "intelligent" information tools that can cope with the very high data volumes. Even more important–perhaps decisively so–are the inter-utility practices and infrastructure through which dynamic information resources are collectively reinforced and operated. CRC Monitor Section – 39 APPENDIX A. A One Page Perspective on Wide Area Monitoring 1) Purpose: the role of monitor facilities is to avoid system disturbances, not just to record them. Measured information is a major input to the many decision processes that planning and operation involve. 2) Value: information has the same value as the decisions based upon it. 3) Level of investment: consider measured information as insurance against operational uncertainty. How much insurance is enough? How much information is too much? 4) Monitor applications: in addition to event recording, monitor facilities should support a) direct tests and analysis of the power system. b) real time system operations (operator alerts, cross triggers, etc.) 5) Monitor recording options: a) evaluate data in real time, using event detection logic (EDL) to trigger "snapshot" recordings of special activity or conditions. b) record all data continuously, then apply EDL to archived records. 6) EDL considerations: a) EDL tuneup is ongoing: automatic detection of significant abnormal system behavior requires comprehensive examination and knowledge of normal behavior. b) Real time EDL rarely detects the onset of distant events. “Snapshot” monitors often miss the conditions contributing to major disturbances (e.g., cascading outages). c) EDL for scanning continuously recorded archives can be simplified and/or desensitized for – use in real time, as a refinement to the "triggers" now in use. – scanning and characterization of snapshot archives. 7) Value engineering: a) Functionality is the key issue: Who needs to see what, where, when, in what form? Why – what decisions hinge upon the information? b) Staff costs (engineering, operations, maintenance) are dominant in power system monitoring. Compared to these, hardware is free – configure the hardware to minimize staff costs. c) Continuous recording, in combination with a competent archive scanner, provides the best value engineering for wide area measurements of power system dynamics. d) The archive scanner required for continuous recording can and should be designed for the overall monitor facility. Better integration and more timely access increases information value while lowering staff costs. e) Ease of use hastens workflow and reduces direct staff costs, but generally requires that data be translated into multiple work environments. f) User participation is essential to development and operation of high performance monitor facilities. It is highly desirable that custom data interfaces and EDL be directly accessible for modification by the user, CRC Monitor Section – 40 and that this require a minimum of special skills. Refinement of measurement tools is part of the system engineering process. CRC Monitor Section – 41 REFERENCES [1] Evaluation of Low Frequency System Response: Study Results and Recommendations. Report of the WSCC 0.7 Hz Oscillation Ad Hoc Work Group to the WSCC Technical Studies Subcommittee, September 1990. [2] J. F. Hauer and J. R. Hunt, in association with the WSCC System Oscillations Work Groups, "Extending the Realism of Planning Models for the Western North America Power System," V Symposium of Specialists in Electric Operational and Expansion Planning (SEPOPE), Recife (PE) Brazil, May 19-24, 1996. [3] K. E. Stahlkopf and M. R. Wilhelm, "Tighter Controls for Busier Systems," IEEE Spectrum, Vol. 34, No. 4, pp. 48-52, April 1997. [4] D. N. Kosterev, C. W. Taylor, and W. A. 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