Justifying pilot protection on transmission lines

Pilot protection schemes use communication channels to send information from the local relay terminal to the remote relay terminal, thereby allowing high-speed tripping for faults occurring within 100% of the protected line. This document is intended as a tool for protection engineers to assist in determining when pilot protection should be installed for transmission line protection, in addition to a communications independent system. The emphasis is not on which pilot scheme to use, but rather if pilot protection is necessary. (Note: The document does not purport to provide a comprehensive list of all considerations that may be used in making this determination or in determining redundancy requirements.) It is evident that it would be less expensive to only install non-pilot step-distance protection since no communication equipment would be necessary (estimates as high as $150K per terminal for the addition of pilot protection). It is prudent to provide engineering considerations that would aid in justifying the installation and future maintenance costs. This report explores this issue by providing the following: (i) Considerations to determine the need for, and benefits of, pilot protection such as high-speed reclosing, improved system stability and power quality, easier coordination, better resistive coverage, and regulatory issues. (ii) Alternatives to pilot protection and fall back strategies when the channel is lost or degraded, or when temporary configurations are entered. Options include stepped distance, ground inverse time overcurrent, Zone 1 extension, and taking the line out of service. (iii) Considerations to determine pilot system redundancy for a given application depending on the voltage level, regulatory issues and economics, role of voting schemes and redundant channels, dependability vs. security, etc.