protection as described below, power system survivability following real-power imbalances is quite probable. System frequency excursions are typically limited to 1 to 2 percent of 60 Hz. One continuing concern, however, is unnecessary tripping of generation during frequency excursions because of boiler upsets and other problems. Prioritized control and protection improvements and modernization would reduce tripping and improve system survivability following events with load-generation imbalance.

There are, however, relatively simple and low-cost practices that greatly improve reliability. However, these practices are not always followed—the August 14, 2003, cascading failure providing a prime example (Nedwick et al., 1995; U.S.-Canada Power System Outage Task Force, 2004). Best practices for voltage reactive power require modern excitation equipment at generators. Replacement of very old equipment with modern thyristor exciters and digital voltage regulators will improve generator reliability. Generator voltage regulator controls including limiter circuits should be coordinated with protective relaying. A lack of coordination has contributed to the severity of blackouts. Automatic voltage regulator line drop compensation or automatic transmission-side voltage control should be considered for better regulation of the transmission network voltage profile.

Techniques for Maintaining Proper Transmission Network Voltage Profiles

Voltage should be near the maximum of the allowed voltage range and should be fairly uniform at all locations. This high, flat voltage profile reduces losses that cause heating and sagging into trees. Extensive use of relatively low cost shunt capacitor banks in both transmission and distribution systems allow a high and flat voltage profile, with substantial reactive power reserves at generators for emergencies. Voltage and reactive power are more complicated with separate ownership of generation and transmission systems. Rigorous standards with performance monitoring are required. Overly complex payments for reactive power or reactive power markets should be avoided. The section titled “Examples of Voltage/Reactive Power Practice” below in this appendix describes how poor voltage/reactive power practice played a critical role in the August 14, 2003, blackout (U.S.-Canada Power System Outage Task Force, 2004).

Primary Automatic Controls to Prevent Cascading Instability

Primary automatic controls, which are located mainly at power plants, include automatic voltage regulators and prime mover controls such as speed governors. Automatic voltage regulators include functions such as power system stabilizers, excitation limiters, and possibly connection of line-drop compensation. Prime mover controls include speed and power regulation. Modern controls are digital, allowing a wide variety of sophisticated features, such as deadbands and control mode shifting.

Transmission-level Power Electronic Devices and Mechanical Devices

Transmission-level power electronic devices such as static volt-ampere reactive (var) compensators are employed to provide continuous voltage control, similar to a generator voltage regulator, and/or other functions. Mechanically controlled shunt capacitor/reactor banks are switched by local voltage relays, by SCADA operators, and sometimes by emergency controls. With digital technology, there is room for more sophisticated control similar to that possible with power electronic devices.

Local Load-shedding Practices and Techniques

Local underfrequency load shedding is commonly employed at bulk power delivery substations. Underfrequency load shedding generally requires islanding of a portion of the interconnection with large generation-load imbalance. In a growing number of power companies, local undervoltage load shedding is also employed (Taylor, 2007). 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 to cause permanent outages, however, automatic reclosing will likely be unsuccessful.

Special Protection Systems or Remedial Action Schemes

Another widely used class of controls is termed special protection systems (SPSs) or remedial action schemes (Taylor, 2007). These are emergency controls that initiate powerful discontinuous actions, such as controlled separation/ islanding, load tripping, or generator tripping at the sending end of an inter-tie. Other possible actions are steam-turbine fast valving, capacitor/reactor bank switching, HVDC fast power changes, and dynamic braking. At present, most of these controls directly detect single or multiple outages and then make logic decisions about whether to initiate feedforward action. The event-based controls are often implemented to prevent cascading for multiple outages, but are sometimes implemented even for N-1 outages. Many SPSs are wide area with outage detection at several sites, binary transfer trip signals to logic computers perhaps at control center(s), and then transfer trip signals to power plants and substations for control action. Reliability for the mission-critical actions must be at least as high as primary protective relaying, requiring as a minimum redundancy so that no single component failure will cause overall control system failure. A large-scale SPS implementation is described below in this appendix.



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