Recent developments in the protection of three-phase transmission lines and feeders

Abstract

The problems associated with the protection of transmission lines and feeders have in recent years become of great moment, and much development of the art has resulted. Some of the difficulties which have arisen in practice are emphasized and some of the methods of overcoming them are described. The design and lay-out of distribution systems, the earthing of the neutral point and the application of arc-suppression coils are briefly discussed, more particularly in their relation to problems of protection. Factors which influence the choice of protective systems are considered and the requirements of schemes to meet the exacting conditions of modern transmission systems are surveyed. The several methods of obtaining discriminative protection are then reviewed. Commencing with modern design of time-discriminative overcurrent relays, emphasis is laid upon the importance of a definite minimum time characteristic and upon correct methods of obtaining directional features. The special requirements of such relays when applied to give sensitive earth-fault protection are indicated, and means of obtaining a suitable polarizing voltage are explained; a description of sensitive earth-fault relays of the wattmeter type is also given. The theory and application of impedance relays is discussed, with reference to definite-impedance and impedance-time relays. A method of compensation for voltage-drop across the arc resistance of a fault is indicated. The field of application of low-voltage release relays is defined and the method of their use illustrated. Attention is then directed to the advantages of pilot-wire feeder protection, and the limiting features of older forms are mentioned. Typical modern opposed-voltages and circulating-current methods which are giving satisfactory service on modern systems are described in some detail. The new field of direct-current pilot-wire protection is discussed; the main classifications of this type of gear are defined and examples are given. The paper concludes with a description of a novel and flexible method of protection for duplicate and multiple parallel feeders.