Power System ProtectionsThe steady-state operation of a power system is frequently disturbed by various faults on electrical equipment. To maintain the proper operation of the power system, an effective, efficient and reliable protection scheme is required. Power system components are designed to operate under normal operating , due to any reason, say a fault, there is an abnormality, it is necessary that there should be a device which senses these abnormal conditions and if so, the element or component where such an abnormalityhas taken place is removed, . deleted from the rest of the system as soonas possible. This is necessary because the power system component can never be designed to withstand the worst possible conditions due to the fact that this will make the whole system highly uneconomical. And therefore, if such an abnormality takes place in any element or component of the power system network, it is desirable that the affected element/component is removedfrom the rest of the system reliably and quickly in order to restore powerin the remaining system under the normal condition as soon as possible.The protection scheme includes both the protective relays andswitching circuits, . circuit breakers. The protective relay whichfunctions as a brain is a very important component. The protective relay is a sensing device, which senses the fault, determines its location and then sends command to the proper circuit breaker by closing its trip coil. The circuit breaker after getting command from the protective relay disconnects only the faulted element. this is why the protective relay must be reliable, maintainable and fast in operation.In early days, there used to be electromechanical relay of induction , very soon the disk was replaced by inverted cup, cylinder and the newrelay obtained was known as an induction cup or induction cylinder relay. This relay, which is still in use, possesses several important featuressuch as higher speed; higher torque for a given power input an more uniform torque.However, with the advent of electronic tubes, electronic relays having distinct features were developed during 1940s. With the discovery of solidstate components during 1950s, static relays with numerous advantages were developed. The use of digital computers for protective relaying purposes has been engaging the attention of research and practicing engineers since layer 1960s and 1980s. Now, the microprocessor/mini computer-based relaying scheme, because of its numerous advantages such as self –checking feature and flexibility, has been widely used in power system all over the world.The overall system protection is divided into following sections:(i)Generator protection,(ii)Transformer protection,(iii)Busprotection,(iv)Feeder protection,(v)Transmission line protection.Basic Requirements to Protective RelaysAny protection scheme, which . required to safeguard the power system components against abnormal conditions such as faults, consists basically of two elements(i)Protective relay and (ii) Circuit breaker .The protective relay which is primarily the brain behind the whole scheme plays a very important role. Therefore proper care should be taken in selecting an appropriate protective relay which is reliable, efficient and fast in operation. The protective relay must satisfy the following requirements:⑴ since faults on a well designed and healthy system are normallyrare, the relays are called upon to operate only occasionally. Thismeans that the relaying scheme is normally idle and must operatewhenever fault occurs. In other words, it must be reliable.⑵ Since the reliability partly depends upon the maintenance, therelay must be easily maintainable.⑶ The palpation of the relay can be in two ways. One is the failureto operate in case a fault occurs an second is the relay operationwhen there is no fault. As a matter of fact, relay must operate ifthere is a fault and must not operate if there is no fault.⑷Relaying scheme must be sensitive enough to distinguish betweennormal and the faulty system.Protective RelaysThe function of the protective relay is to sense the fault and energize the trip coil of the circuit breaker. The following types of the protective relays are used for the apparatus such as synchronous machines, bus bar, transformer and the other apparatus and transmission line protection.(1) Over current relays,(2) Under voltage relays,(3) Under frequency relays,(4) Directional relays,(5) Thermal relays,(6) Phase sequence relays such as(i)negative sequence relays and,(ii)zero sequence relays,(7) Differential relays and percentage differential relays,(8) Distance relays such as (I)plane impedance relays,(ii)angleimpedance relay, . Ohm or reactance relays,(iii)angleadmittance relays,. Mho relays and ,(iv)offset and restrictedrelays,(9)Pilot relays such as (i) wire pilot relays,(ii)carrier channelpilot relays,(iii)microwave pilot relays. There are different types of the relaying scheme based on construction. Theyare:(i)electromechanical type,(ii)thermal relays,(iii) transduction relays,(iv)rectifier bridge relay,(v)electronic relays,(vi)digital relaying schemes.电力系统继电保护电力系统的稳态运行经常会因各种电力设备配故障原因而被扰乱。