Industry experience with special protection schemes

Special protection schemes are those designed to detect one or more predetermined system conditions that have a high probability of causing unusual stress on the power system, and for which preplanned remedial action is considered necessary. The failure of these schemes to accurately detect the defined conditions, or their failure to carry out the required preplanned remedial action, can lead to very serious and costly power system disturbances. It is natural, therefore, that a concern for the reliability of these schemes has been expressed. This paper reports on a joint IEEE-CIGRE survey to determine the experience with special protection schemes, and to question the designers and operators of these schemes regarding the reliability of performance. Questions were also directed to the design process itself, the reliability criteria used for the design, and the analytical methods used to assure reliability in the design.