Digital communications

Abstract

Many protection arrangements require some means of communication. Examples are, distance schemes with some form of intertripping and phase comparison in differential current protection schemes. For a long time, communications were based on lines leased from the Post, Telephone and Telecommunications (PTT) authorities, Power-Line Carrier (PLC) systems, or microwave channels. Advances in information technology in recent years have meant that it is now possible to employ digital communication networks of high capacity for protection control as well as data and voice channels. In fact, technological changes mean that the differences between voice, data and video communications are blurred as are the differences between data communications and data processing in single- and multiprocessor systems. There is now a plethora of information about the power system, potentially of importance to protection, which can now share data and communications with the control and management functions of a network. The rate at which this sharing and interdependence will increase and what final level it will reach is difficult to foresee. Undoubtedly, long term reliability of protection schemes relying on communications has depended on the communication system and therefore the introduction of new communications technologies deeper into protection procedures has to be examined carefully so that system security and dependability is maintained at high levels. It is now essential that the protection engineer is aware of digital communications issues so that he or she can influence the development of the communications network, take advantage of the possibilities that it offers and maintain the high integrity of the protection relays designed according to the principles presented in previous chapters. The purpose of this chapter is to introduce the basic principles of digital communications and their use in protection.