The main disadvantage o f performance index based contingency selection algorithms consists in the fact, t h a t e f f icient techniques f o r evaluating the PI have been found f o r a second o r d e r formulation only. This causes the so called masking e f f e c t , which r e s u l t s in a missclassification of a possibly large number o f contingencies. In this paper a systematic analysis o f the masking e f f e c t is presented f i r s t . The main p a r t describes a new concept w i t h the aim t o compensate the masking e f f e c t considering the specific situation in an on-line environment. The developed adaptive procedure is founded on the idea, t o verify in the period between successive security checks the ranking a f t e r w a r d s w i t h the aim t o estimate updating c o e f f i c i e n t s f o r a c o r r e c t i v e function. This function is applied t o modify the conventional second o r d e r PI evaluation a t the following contingency ranking.Extensive t e s t s f o r realistic European power systems considering actual load and topology changes indicate a subs t a n t i a l reduction o f the masking e f f e c t and t h e r e f o r e m o r e reasonable contingency ranking. KEYWORDSPower system control. security analysis, automatic contingency selection.0885-8950/90/0500-0539$01.00 0 1990 IEEE

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