Reliability considerations of multifunction protection

Most electric utilities have embraced numeric multifunction protection technology as a means of surviving in an industry that has changed dramatically in the last ten years. Led by restructuring and shrinking resources, protection engineers are continuing the move to communicating, multifunction protection technology as a means of reducing cost and maintaining operating performance with fewer personnel. We are at a point in the evolution of the technology where we need to step back and ask some questions about protection reliability. Our application and maintenance philosophies must be reviewed on a regular basis to ensure that they are meeting long-term protection reliability as well today as they did with predecessor technologies such as electromechanical and solid state devices. The purpose of this paper is to evaluate the reliability of numerical multifunction protection systems against predecessor technologies. We define reliability as related to protective relaying, identify a selected group of reliability factors that impact both predecessor and numerical multifunction technologies, discuss each factor in detail, and draw conclusions about the overall reliability of multifunction protection.