9291_c003 - 3 Transmission Line Protection 3.1 3.2 The...

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3 Transmission Line Protection Stanley H. Horowitz Consultant 3.1 The Nature of Relaying. ...................................................... 3 -2 Reliability . Zones of Protection . Relay Speed . Primar y and Backup Protection . Reclosing . System Configuration 3.2 Current Actuated Relays . .................................................... 3 -5 Fuses . Inverse-Time Delay Overcurrent Relays . Instantaneous Overcurrent Relays . Directional Overcurrent Relays 3.3 Distance Relays . ................................................................... 3 -8 Impedance Relay . Admittance Relay . Reactance Relay 3.4 Pilot Protection . ................................................................ 3 -10 Directional Comparison . Transfer Tripping . Phase Comparison . Pilot Wire 3.5 Relay Designs. .................................................................... 3 -11 Electromechanical Relays . Solid-State Relays . Computer Relays The study of transmission line protection presents many fundamental relaying considerations that apply, in one degree or another, to the protection of other types of power system protection. Each electrical element, of course, will have problems unique to itself, but the concepts of reliability, selectivity, local and remote backup, zones of protection, coordination and speed which may be present in the protection of one or more other electrical apparatus are all present in the considerations surrounding transmission line protection. Since transmission lines are also the links to adjacent lines or connected equipment, transmission line protection must be compatible with the protection of all of these other elements. This requires coordination of settings, operating times and characteristics. The purpose of power system protection is to detect faults or abnormal operating conditions and to initiate corrective action. Relays must be able to evaluate a wide variety of parameters to establish that corrective action is required. Obviously, a relay cannot prevent the fault. Its primary purpose is to detect the fault and take the necessary action to minimize the damage to the equipment or to the system. The most common parameters which reflect the presence of a fault are the voltages and currents at the terminals of the protected apparatus or at the appropriate zone boundaries. The fundamental problem in power system protection is to define the quantities that can differentiate between normal and abnormal conditions. This problem is compounded by the fact that ‘‘normal’’ in the present sense means outside the zone of protection. This aspect, which is of the greatest significance in designing a secure relaying system, dominates the design of all protection systems. ß 2006 by Taylor & Francis Group, LLC.
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3.1 The Nature of Relaying 3.1.1 Reliability Reliability, in system protection parlance, has special definitions which differ from the usual planning or operating usage. A relay can misoperate in two ways: it can fail to operate when it is required to do so, or it can operate when it is not required or desirable for it to do so. To cover both situations, there are two components in defining reliability: Dependability —which refers to the certainty that a relay will respond correctly for all faults for which it is designed and applied to operate; and Security —which is the measure that a relay will not operate incorrectly for any fault.
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9291_c003 - 3 Transmission Line Protection 3.1 3.2 The...

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