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G Controlling Power Systems CONTROL EQUIPMENT AND PRACTICE practices. Kundur et al. (2007) describes best practices in detail, listing over 50 best practices. Terrorist attacks and other disturbances can evolve into instability in a few seconds or tens of seconds--too fast for control room operator actions. Operators may act within a Automatic Controls few minutes during relatively familiar events with alarms, but Additional information is provided below on the fol- in new situations, 15 to 30 minutes may be required to make lowing means of automatic controls that are listed but not assessments and act, especially if load shedding is required. described in Chapter 6. Thus, various types of automatic controls are required. Improving the control of voltage and reactive power may also require relatively low-cost high-voltage equipment additions Techniques for Shedding Load and Generation to Enhance such as shunt capacitor banks. Power System Dynamic Response Capabilities Automatic controls constitute one or more layers of the Power system dynamic response following disturbances defense in depth or multiple layers of defense principle for can, to some degree, be separated between real (active) preventing or mitigating blackouts. In comparison to the power phenomena and reactive power/voltage phenomena. addition of new transmission lines, control improvements Real power (measured in MW) is always held in balance can be rapidly implemented. when the system is operating normally. A disturbance upsets This appendix provides details on automatic controls for this balance and initiates dynamic response from the rotating electric power systems, including new technology and best synchronous generators in the system. An important aspect practices. Such best practices help power systems to survive of the real power balance deals with the availability of spin- major disturbance events, both at power plants and in the ning reserve (unloaded generation synchronized and ready to transmission network. Besides the more conventional con- serve additional demand) and the activation of such reserves trols, emergency controls often termed "special protection following islanding. This would also include measures such systems" are applied to mitigate extreme disturbance events. as load and generator shedding. Activation of reserves at Information technologies hold promise to advance control power plants by prime mover/energy supply system control capabilities in the near future. is limited. The tendency to carry reserves on fewer units, with In short, power system robustness, resilience, and surviv- many units base-loaded, reduces performance. The response ability in the face of major disturbances, including terrorist of units is difficult to predict because power plant operators attacks, can be increased significantly, economically, and can select from several control modes such as traditional rapidly by the use/addition of automatic controls. How- governor control of speed and system frequency, MW control ever, there are several necessary requirements, namely, (1) override of speed control, or coordinated boiler/turbine con- implementation of industry best practices, (2) prioritized trol with limited speed/frequency control. System frequency upgrading of old analog controls (and actuators such as regulation by secondary control (automatic generation con- generator field circuit exciters), and (3) development and trol) or operator actions often takes tens of minutes for large implementation of wide-area controls. North American upsets. Operator-directed or automatic demand-side actions Electric Reliability Council (Electric Reliability Organiza- are potential aids during emergencies. tion) reliability standards for automatic controls, including With automatic underfrequency load shedding and performance monitoring, should evolve to better reflect best with proper coordination between power plant control and 137
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138 TERRORISM AND THE ELECTRIC POWER DELIVERY SYSTEM protection as described below, power system survivability a wide variety of sophisticated features, such as deadbands following real-power imbalances is quite probable. System and control mode shifting. frequency excursions are typically limited to 1 to 2 percent of 60 Hz. One continuing concern, however, is unnecessary Transmission-level Power Electronic Devices and tripping of generation during frequency excursions because Mechanical Devices of boiler upsets and other problems. Prioritized control and protection improvements and modernization would reduce Transmission-level power electronic devices such as static tripping and improve system survivability following events volt-ampere reactive (var) compensators are employed to with load-generation imbalance. provide continuous voltage control, similar to a generator There are, however, relatively simple and low-cost voltage regulator, and/or other functions. Mechanically con- practices that greatly improve reliability. However, these trolled shunt capacitor/reactor banks are switched by local practices are not always followed--the August 14, 2003, voltage relays, by SCADA operators, and sometimes by cascading failure providing a prime example (Nedwick et emergency controls. With digital technology, there is room al., 1995; U.S.Canada Power System Outage Task Force, for more sophisticated control similar to that possible with 2004). Best practices for voltage reactive power require power electronic devices. modern excitation equipment at generators. Replacement of very old equipment with modern thyristor exciters and Local Load-shedding Practices and Techniques digital voltage regulators will improve generator reliabil- ity. Generator voltage regulator controls including limiter Local underfrequency load shedding is commonly circuits should be coordinated with protective relaying. A employed at bulk power delivery substations. Underfre- lack of coordination has contributed to the severity of black- quency load shedding generally requires islanding of a outs. Automatic voltage regulator line drop compensation portion of the interconnection with large generation-load or automatic transmission-side voltage control should be imbalance. In a growing number of power companies, local considered for better regulation of the transmission network undervoltage load shedding is also employed (Taylor, 2007). voltage profile. Also, to avoid possible blackouts during lightning storms or other transient events, automatic reclosing or single-pole switching is employed. Since most terrorist actions are likely Techniques for Maintaining Proper Transmission Network to cause permanent outages, however, automatic reclosing Voltage Profiles will likely be unsuccessful. Voltage should be near the maximum of the allowed volt- age range and should be fairly uniform at all locations. This Special Protection Systems or Remedial Action Schemes high, flat voltage profile reduces losses that cause heating and sagging into trees. Extensive use of relatively low cost Another widely used class of controls is termed special shunt capacitor banks in both transmission and distribution protection systems (SPSs) or remedial action schemes (Tay- systems allow a high and flat voltage profile, with substantial lor, 2007). These are emergency controls that initiate pow- reactive power reserves at generators for emergencies. Volt- erful discontinuous actions, such as controlled separation/ age and reactive power are more complicated with separate islanding, load tripping, or generator tripping at the sending ownership of generation and transmission systems. Rigorous end of an inter-tie. Other possible actions are steam-turbine standards with performance monitoring are required. Overly fast valving, capacitor/reactor bank switching, HVDC fast complex payments for reactive power or reactive power power changes, and dynamic braking. At present, most of markets should be avoided. The section titled "Examples these controls directly detect single or multiple outages and of Voltage/Reactive Power Practice" below in this appendix then make logic decisions about whether to initiate feedfor- describes how poor voltage/reactive power practice played a ward action. The event-based controls are often implemented critical role in the August 14, 2003, blackout (U.S.Canada to prevent cascading for multiple outages, but are sometimes Power System Outage Task Force, 2004). implemented even for N1 outages. Many SPSs are wide area with outage detection at several sites, binary transfer trip signals to logic computers perhaps at control center(s), Primary Automatic Controls to Prevent Cascading Instability and then transfer trip signals to power plants and substations Primary automatic controls, which are located mainly for control action. Reliability for the mission-critical actions at power plants, include automatic voltage regulators and must be at least as high as primary protective relaying, requir- prime mover controls such as speed governors. Automatic ing as a minimum redundancy so that no single component voltage regulators include functions such as power system failure will cause overall control system failure. A large-scale stabilizers, excitation limiters, and possibly connection of SPS implementation is described below in this appendix. line-drop compensation. Prime mover controls include speed and power regulation. Modern controls are digital, allowing
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APPENDIX G 139 Wide-area Feedback/Response-based Controls discontinuous controls are often wide area, but could be local (e.g., underfrequency or undervoltage load shedding). A promising alternative or complement to local controls or to SPS is wide-area feedback/response-based controls. Two types of these controls are continuous feedback control, Sophisticated Control Algorithms and discontinuous control, which take actions similar to Sophisticated control algorithms use various techniques those taken by SPSs. Compared to local controls, wide-area such as adaptive or "intelligent" control as part of digital controls provide greater observability and controllability. control and communication capabilities. Integration with Positive sequence, synchronized phasor measurements are the energy management system (EMS) functions, such as the preferred sensors for control inputs. High-speed digital/ dynamic security assessment, is possible to adapt control to optical communications are required. present operating conditions. The description of wide-area controls above focuses on actions to prevent instability and Continuous Wide-area Control controlled or uncontrolled separations and islanding. If these actions fail, controlled separations could be initiated. This is Continuous wide-area control is being studied by many relatively easy for well-defined inter-ties between areas, but utilities, vendors, and universities. Perhaps the most serious more difficult in a highly meshed system. Adaptive islanding work is that by Hydro Quebec for power system stabilization is a research area. Some aspects of this concept have been (oscillation damping improvement) through generator exci- demonstrated recently in simulation on a large, realistic test tation control, and through the use of static var compensators system (Yang et al., 2006). and other power electronic devices. Example of Impact of Voltage/Reactive Power Practice Wide-area Discontinuous Feedback Control An example of the impact of voltage/reactive power Wide-area discontinuous feedback control is based on practices on system performance from the August 14, 2003, power system response to disturbances rather than on direct blackout is presented (U.S.Canada Power System Out- detection of only certain outages, as in most SPSs. Control age Task Force, 2004). The initial outage of the Eastlake 5 action occurs for outages anywhere in the interconnection generator on August 14 was related to excitation equipment that causes a threatening response. Notable is the Wide-Area problems during production of high reactive power. (The out- Stability and Voltage Control System (WACS) in develop- age likely would have been avoided with modern equipment.) ment at BPA (Taylor et al., 2005). As an example of poor voltage/reactive power practice, Fig- Figure G.1 shows a block diagram of power system stabil- ures G.2 and G.3 show conditions on August 14, 2003. Figure ity controls. The SPS path is feedforward. The continuous G.2 shows the 345 kV voltage profile that many engineers feedback controls are normally local and mainly at genera- would regard as terrible, especially considering that the load tors, but could be wide area. The feedback (response-based) was less than 80 percent of peak summer load and that the Power System Disturbances switch capacitor/reactor banks direct detection Power System y (SPS) trip generators/loads Dynamics Continuous Discontinuous Feedback Controls controlled separation Controls (generators) response detection FIGURE G.1 Power system stability controls. (WACS)
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140 TERRORISM AND THE ELECTRIC POWER DELIVERY SYSTEM 365 13:00 EDT 360 14:00 EDT Desired Voltage Profile 15:00 EDT 355 16:00 EDT 350 345 kV 100% 340 335 330 95% 325 320 Chamberlin (ITC) (ITC) (FE) Lemoyne (FE) (FE-CAA) Harding (FE-CAA) (FE-CAA) (FE-CAA) Star (FE-CAA) South Canton (AEP) Sammis-East (FE) Allen Junction Juniper Brownstown St. Clair Avon Lake FIGURE G.2 August 14, 2003, voltage profile from west to east across northern Ohio. SOURCE: U.S.Canada Power System Outage Task Force (2004). Fig G-2.eps FIGURE G.3 August 14, 2003, reactive power production and reserves. SOURCE: U.S.Canada Power System Outage Task Force (2004). fig G-3 this is a "fixed image", not easy to make changes.
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APPENDIX G 141 13:00 voltage profile was before any outages. Figure G.2 also BPA's two control centers. Fault tolerant (triple-redundant) shows a more desired voltage profile of 103 percent (which programmable logic controllers are at the control centers. could be even higher: standard voltage range is 345 kV ± 5 Each logic computer has the equivalent of around 1,000 logic percent). Voltage at the west (left) end near Detroit is very gates to detect the many combinations of single, double, and good. Voltage at a large Ohio River power plant on the east triple line outages in the series/parallel transmission line end is relatively low. Despite substantial reactive power path. Commands are then sent to generating plants. Besides reserves in the American Electric Power area (Figure G.2) hydro generation tripping in the Northwest sending end and a 765 kV infeed, voltage at the South Canton bus is poor. to reduce power transfer, the controls also switch 500 kV Figure G.3 shows the very low reactive power reserves at capacitor/reactor banks. If an intertie separation does occur, power plants in the Cleveland area. Again, the correspond- controlled separations of the northern and southern portions ing high reactive power output combined with old excita- of the western interconnection into two electrical islands tion equipment caused the initial Eastlake 5 outage. The is initiated. Following a severe outage, control actions are poor voltage profile contributed to lines sagging into trees executed in less than a second. (with heating and sagging inversely proportional to voltage squared). Although inadequately discussed in the reports REFERENCES on the August 14, 2003, blackout, the disaster would likely have been avoided with many more capacitors banks in the Kundur, P., C. Taylor, and P. Pourbeik. (co-chairs and secretary). 2007. Blackout Experiences and Lessons, Best Practices for System Dynamic Cleveland/Akron area. The power system would have been Performance, and Role of New Technologies. IEEE Special Publication much more robust and resilient. 07TP190, July. Nedwick, P., A.F. Mistr Jr., and E.B. Croasdale. 1995. "Reactive Manage- ment: A Key to Survival in the 1990s." IEEE Transactions on Power Example of Special Protection System Implementation Systems 10(2): 10361043. Taylor, C.W. 2007. "Power System Stability Controls." Chapter 12, Power Bonneville Power Administration (BPA) may have the System Stability and Control volume of The Electric Power Engineering world's largest implementation of SPSs. The most important Handbook. Boca Raton, Fla.: CRC Press/IEEE Press. SPSs involve the Pacific AC and DC inter-ties, where the Taylor, C.W., D.C. Erickson, K.E. Martin, R.E. Wilson, and V. main action is tripping of up to 2,700 MW of hydro genera- Venkatasubramanian. 2005. "WACS: Wide-Area Stability and Volt- tion. This is for high power transfer from the Pacific North- age Control System: R&D and On-Line Demonstration." Proceedings of the IEEE [special issue on energy infrastructure defense systems] west to California, where the generator tripping prevents 93(5): 892906. instability (loss of synchronousness among generators). U.S.Canada Power System Outage Task Force. 2004. Final Report on the Load tripping at the California end would have a similar August 14, 2003, Blackout in the United States and Canada: Causes and benefit for stability. Recommendations. Natural Resources Canada and the U.S. Department The most complex scheme involves preventing separa- of Energy. April. Yang, B., V. Vittal, and G.T. Heydt. 2006. "Slow-Coherency-Based Con- tion of the 4,800 MW Pacific AC inter-tie where high-speed trolled Islanding--A Demonstration of the Approach on the August outage detection of around fifty 500 kV lines is installed 14, 2003, Blackout Scenario." IEEE Transactions on Power Systems (detection at both line ends). Outage detection is transmitted 21(4): 18401847. over redundant microwave or fiber-optic communications to