The National Academy of Sciences founded The National Academies Press (NAP) with the goal of publishing reports of all four national academies. Annually, NAP publishes more than 200 books from the fields of science, engineering and medicine and offers more than 4000 titles in PDF on its website (http://www.nap.edu/) free of charge.

/pdf/the-national-academies-press-3oqc1apmt7.pdf

The National Academies Press

On August 14, 2003, a cascading outage of transmission and generation facilities in the North American Eastern Interconnection resulted in a blackout of most of New York state as well as parts of Pennsylvania, Ohio, Michigan, and Ontario, Canada. On September 23, 2003, nearly four million customers lost power in eastern Denmark and southern Sweden following a cascading outage that struck Scandinavia. Days later, a cascading outage between Italy and the rest of central Europe left most of Italy in darkness on September 28. These major blackouts are among the worst power system failures in the last few decades. The Power System Stability and Power System Stability Controls Subcommittees of the IEEE PES Power System Dynamic Performance Committee sponsored an all day panel session with experts from around the world. The experts described their recent work on the investigation of grid blackouts. The session offered a unique forum for discussion of possible root causes and necessary steps to reduce the risk of blackouts. This white paper presents the major conclusions drawn from the presentations and ensuing discussions during the all day session, focusing on the root causes of grid blackouts. This paper presents general conclusions drawn by this Committee together with recommendations based on lessons learned.

Causes of the 2003 major grid blackouts in North America and Europe, and recommended means to improve system dynamic performance

It is widely known that corrective switching, including transmission line switching, bus-bar switching, and shunt element switching, may change the states of the power systems, and consequently, affect the distribution of power flows, transmission losses, short circuit currents, voltage profiles as well as transient stability of power systems. In this paper, a new algorithm is developed to find the best line and bus-bar switching action for relieving overloads and voltage violations caused by system contingencies based on a sparse inverse technique and fast decoupled power flow with limited iteration count. A general model of bus-bar switching action is also presented such that the new algorithm can simulate any kind of complicated bus-bar switching action. Furthermore, on the basis of a newly proposed voltage distribution factor by multiple iterations in power flow calculation, a novel algorithm for corrective voltage control by shunt switching is developed. These two algorithms are then integrated into a corrective switching algorithm. Simulation results on the WECC 179-bus system indicate that the new corrective switching algorithm proposed in this paper can effectively solve certain problems of line overloads and voltage violations. The computation time required is also satisfactory.

Corrective switching algorithm for relieving overloads and voltage violations

Voltage collapse is a critical problem that impacts power system operational security. Timely and accurate assessment of voltage security is necessary to detect post-contingency voltage problems in order to prevent a large scale blackout. This paper presents an online voltage security assessment scheme using synchronized phasor measurements and periodically updated decision trees (DTs). The DTs are first trained offline using detailed voltage security analysis conducted using the past representative and forecasted 24-h ahead operating conditions. The DTs are also updated every hour by including newly predicted system conditions for robustness improvement. The associated synchronized critical attributes are obtained in real time from phasor measurement units (PMUs) and compared with the offline thresholds determined by the DTs to assess security. This approach is tested on the American Electric Power (AEP) system and properly trained DTs perform well in assessing voltage security. Several new ideas to improve DT performance are also introduced.

/pdf/decision-tree-based-online-voltage-security-assessment-using-t1kkt26ibi.pdf

Decision Tree-Based Online Voltage Security Assessment Using PMU Measurements

A method of calculating the communication delay (latency) for measurements and control signals in a power system is shown. The basis of the calculation is a dedicated communication channel for control signals. The time delay calculation is examined using a dynamic equivalent of the Western Electricity Coordinating Council transmission system. The impact on control system response is discussed. The application given is a wide area control system for interarea mode damping. Results demonstrate that control signal latency can degrade the performance of controls in a wide area control system.

Latency Viewed as a Stochastic Process and its Impact on Wide Area Power System Control Signals

This paper provides the analytical basis for an application of slow coherency theory to the design of an islanding scheme, which is employed as an important part of a corrective control strategy to deal with large disturbances. The analysis is conducted under varying networks conditions and loading conditions. The results indicate that the slow coherency based grouping is almost insensitive to locations and severity of the initial faults. However, because of the loosely coherent generators and physical constraints the islands formed change slightly based on location and severity of the disturbance, and loading conditions. A detailed description of the procedure to form the islands after having determined the grouping of generators using slow coherency is presented. The verification of the islanding scheme is proven with simulations on a 179-bus, 29-generator test system.

Slow coherency-based islanding

Slow coherency-based generator grouping in power systems is primarily dependent on the participation of generators in the slow modes. In the literature, generator coherency has been typically obtained at a specific operating point. This paper presents a new approach using the continuation method to trace the loci of the coherency indices of the slow modes in the system with respect to variation in system conditions to obtain the updated coherency information between generators. The approach has been applied to a 10-generator 39-bus New England system, and a 29-generator 179-bus model of the WECC system and the impact of load variation on generator coherency indices has been investigated.

Tracing generator coherency indices using the continuation method: a novel approach

In order to investigate power system security and design appropriate control strategies, power systems can be conceptually classified into five operational states: Normal, Alert, Emergency, In Extremis, and Restorative. Various preventive and corrective control strategies have been studied to cope with power systems in the different operational states. Our approach primarily focuses on the corrective control strategies for power systems under emergency and in extremis states. In this paper, two cor- rective control approaches (transmission system reconfigu- ration and controlled system islanding) have been investi- gated and new algorithms have been developed. When the system is in the emergency state, two reconfiguration strategies, line switching and bus-bar switching, have been presented to solve the problems of overloads or voltage violations. Furthermore, to prevent a total system black out, a slow coher-ency grouping based adaptive power system islanding approach using minimal cutsets has been presented to deal with catastrophic events when power system vulnerability analysis indicates that the system is approaching the in extremis state. A controlled under frequency load shedding scheme with the rate of frequency decline is also proposed to balance the real power within each island and aid restoration. The verification of our approach is proven with simulations on a 179-bus, 29- generator WECC system.

Xiaoming Wang

Papers

Slow coherency-based islanding

Tracing generator coherency indices using the continuation method: a novel approach

Adaptive corrective control strategies for preventing power system blackouts