Complete Bounding Method for AC Contingency Screening

88 SM 726-2 May 1989

Complete Bounding Method for AC Contingency Screening V. Brandwajn, Member Systems Control, Inc. Palo Alto, CA M. G. Lauby, Member Electric Power Research Institute Palo Alto, CA

Summary 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. All the important requirements for real-time security analysis have been incorporated. The method combines rigorous contingency evaluation with flexibility, small computer memory requirements, and no off-line setup time. The performance of the contingency selection technique was evaluated using a test system, representing a major midwestern power pool, that contained 650 buses and 1 100 branches. All network buses were monitored for voltage limit violations and voltage shifts. Approximately 900 branches were monitored for flow limit violations, and more than 100 generators for reactive power limit violations and shifts. The complete bounding method is significantly faster than full AC security analysis. The speed advantage of the complete bounding method increases with both the sytstem size and stress. This increases the method's desirability. The speed of the complete bounding method results from the fact that only a partial P-0 solution and Q-mismatch calculation are required and performed. The average number of buses for which the P-0 solution and Q-mismatch calculations were performed, was approximately 200 for both the heavy and light load cases. The "incremental angle" criterion and the bounding of the Q-mismatch calculations are the keys to the method's efficiency. For each contingency, the complete bounding method establishes an upper bound on the changes in the angular spread using a small subnetwork solution. This bound is used to establish both the set of endangered branches and the set of buses for which the Q-mismatches have to be calculated. The bounding approach is reliable because it correctly selects the set of buses which may have major Qmismatches. The reliability of the complete bounding method 7070

is further enhanced by: 1) separate processing for different types of violations and 2) use of severity indices for major shifts in system quantities. Discussers: Y. Chen and A. Bose

88 SM 725-4 May 1989

Discrete-mode Automatic Generation Control of a Two-Area Reheat Thermal System with New Area Control Error M. L. Kothari, J. Nanda, D. P. Kothari, and D. Das Indian Institute of Technology New Delhi- 1001 6, INDIA

Summary Most of the work reported in the literature pertaining to Automatic Generation Control (AGC) of interconnected power systems is centered around tie-line frequency bias control strategy. Supplementary controllers are designed to regulate the area control errors to zero effectively. Several modern design techniques have been used to optimize the parameters of the supplementary controllers. Supplementary controllers regulate the generation to match the load variation. As the generation change chases the load variation, the frequency and tie-power deviate from the scheduled values. This would result in accumulations of time error and inadvertent interchange. They would also occur due to various measurement errors or intentional offsets in scheduled settings. It is expected that individual areas will make all reasonable efforts to minimize time error and inadvertent interchange accumulations by minimizing or eliminating source causes. There is a need for correcting these accumulations. Such corrections are achieved by making appropriate offsets in system frequency schedules to compensate for time error accumulations and offsets in area net interchange schedules to compensate for inadvertent interchange accumulations. Detailed literature survey clearly shows that the two-step correction scheme, mentioned above, has been used by utilities inspite of some practical difficulties. To avoid those difficulties they are looking forward for a suitable control strategy that not only maintains constancy of system frequency and desired tie-power flow but also achieves zero steady state time error and inadvertent interchange. It is essentially in this area, the investigations are carried out in this paper. Nathan Cohn [1] had proposed a new modified expression for area control error for coordinating system-wide correction of time error and inadvertent interchange. Not much work is available in the literature using the modified expression for ACE. The AGC problem of a two-area reheat thermal system has been investigated considering new area control error (ACEN). For the mth area new area control error is defined as A CENm = A Ptiem + BmA Fm +

= A Ptiem + Bm A Fm +

m

amEm + axmIm

Cam (/m

am

)

is set equal to 60 Bm.

am

Hence ACENm = APtiem + BmA Fm + am

(APtiem + BmA Fm) dt

IEEE Power Engineering May 1989 Engineering Review, May