A broad overview of on-line power system security analysis is provided, with the intent of identifying areas needing additional research and development. Current approaches to state estimation are reviewed and areas needing improvement, such as external system modeling, are discussed. On-line contingency selection has become practical, particularly for static security. Additional work is necessary to identify better indices of power system stress to be used in on-line screening filters for both static and dynamic security analysis. Use of optimal power flow schemes to recommend optimal preventive and corrective strategies is presented on a conceptual level. Techniques must be further developed to provide more practical contingency action plans, which include real-world operating considerations and use a reasonably small number of control actions. Techniques must be developed for costing operating variables which are not easily quantified in dollars. Soft or flexible constraints and time variables must be included in the preventive and corrective strategy formulation. Finally, the area of on-line transient and dynamic security analysis is presented. >

On-line power system security analysis

The change in the loading margin to voltage collapse when line outages occur is estimated. First a nose curve is computed by continuation to obtain a nominal loading margin. Then linear and quadratic sensitivities of the loading margin to each contingency are computed and used to estimate the resulting change in the loading margin. The method is tested on a critical area of a 1390 bus system and all the line outages of the IEEE 118 bus system. The results show the effective ranking of contingencies and the very fast computation of the linear estimates.

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Contingency ranking for voltage collapse via sensitivities from a single nose curve

The analysis of the effects of hundreds of outages on line flows and bus voltages is required for the real-time security analysis and contingency enumeration. This increases the demands on the speed, accuracy and adaptability of the solution methods. Ideally, an ac power flow should be solved for each contingency, followed by a check for limit violations and major shifts from the initial system conditions. Such an approach is not feasible for practical systems consisting of hundreds of buses. To cope with this computational barrier, various approximate methods have been developed based on the idea that the vast majority of outages does not cause major shifts/violations. There are two classes of such methods, explicit and implicit techniques, which ease the computational burden by identifying cases with severe system limit violations. The explicit methods do not identify or solve for specific violations. Rather, they quantify the severity of each outage by a scalar index by which all the contingencies can be ranked. The explicit methods are not completely reliable since they are prone to masking errors. Specifically, a contingency with a few severe violations can be ranked equally with one with many minor violations or even worse, with one without violations. The implicit methods, though more demanding in CPU resources, permit the identification of actual violations/ major shifts and, therefore avoid masking errors. This paper describes a new contingency analysis technique which overcomes the deficiencies of existing methods. This technique has been implemented in a production-grade program designed for real-time applications.

Complete bounding method for AC contingency screening

The authors describe a fast security-analysis technique for voltage security assessment in an energy-management system. The proposed method identifies the location of buses with potential voltage problems and thereby defines a voltage-sensitive subnetwork for contingency screening. This allows the evaluation of a large number of contingencies. The efficiency of this method is derived from the use of a voltage subnetwork to drastically reduce the number of bus voltages to be solved; and subsequently from the use of compensation techniques and sparse-vector methods (including adaptive reduction) for screening and final solution of contingencies. Results demonstrating the effectiveness of the method on a small and a large power system are presented. >

Fast contingency screening and evaluation for voltage security analysis

This paper presents a method for determining the double outage contingencies that threaten the system without solving the full contingency set. Two methods for contingency screening with complementary properties are presented. The results of the algorithms are compared to the full double outage contingency analysis results for a large North American case. The results show that the screening algorithms are able to detect nearly all of the contingencies that will result in violations, while requiring only a small fraction of the contingencies to be solved.

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Multiple Element Contingency Screening

On-line steady-state security analysis requires the evaluation of the effects of a large number of possible contingencies on a power system. Almost all contingency analysis programs have had to rely on a list of selected contingencies to be studied. The brute force approach of testing all contingencies every time an evaluation of the system performance is needed, becomes prohibitive due to its computational burden. A particularly attractive technique is to utilize a fast, approximate method to select the few contingencies that actually affect the security of the power system and then analyze only these cases in more detail. The contingency ranking and selection methods presented in the literature have all ranked outage cases on the basis of transmission line overloads. This limits the usefulness of these technqiues as contingencies that cause only voltage problems are totally overlooked. This paper shows that there is little correlation between the contingencies that produce line overloads and those that result in unacceptable voltage profiles, further establishing the need for a contingency selection method that is also voltage sensitive. Such a method, using the first iteration of an AC power flow, is presented. It is shown that the method meets both the speed and accuracy requirements for on-line use.

B. Heath

Papers

Reactive Power Considerations in Automatic Contingency Selection