Application of Digital Radio for Distribution Pilot Protection and Other Applications

Networked distribution lines, sub-transmission lines, and industrial facility incoming supply lines have always presented an interesting protection challenge. As the number of distributed generators and cogeneration facilities increase, directional overcurrent protection and distance protection may not be selective enough for reliable protection without the implementation of pilot protection schemes such as permissive over-reaching transfer trip and directional comparison blocking. Pilot-wire relaying has been the traditional solution at distribution voltages. Pilot-wire relaying sends a voltage signal between relays at each end of the line across copper wire. These voltages are used for differential protection. This scheme was used because of availability of copper pairs from the phone company and/or the low cost of installation of the communications wire. Today, however, copper pairs are no longer available from the phone company and the cost of installing new copper is increasingly expensive. When fiber access is available (typically at a premium cost), communication based digital protection solutions such as current differential relays and directional overcurrent relays in a pilot protection scheme are used. The modern challenge is a method to provide digital communications for pilot protection that is reliable and affordable. Digital radio is an inexpensive method to provide digital communications for pilot protection at the distribution level. Digital radio has the ability to send permissive, blocking, and transfer trip signals over short to medium distances. Relay to relay messaging protocols have now become standardized through the IEC 61850 GOOSE profile and can provide not only protection information but also metering, monitoring, and control. Practical concerns for the protection engineer include the reliability, security, and latency of digital radio communications, as this has a strong influence on selecting and setting the protection scheme. To address- these concerns, this paper presents actual field data for radio signal reliability and latency. Based on this actual data, some recommendations for pilot protection schemes at the distribution level are presented. In addition, the paper also reviews the application requirements for digital radio, including design for redundancy, path concerns, antenna selection and site evaluation, and use of licensed and spread spectrum radios. Since modern digital radio also support higher communications bandwidth, the paper will explore some other innovative applications that can operate in concert with pilot protection communications.