RELAY DESIGN

RELAY DESIGN RELAY: DISCRIMINATES BETWEEN A FAULT WITHIN ITS PROTECTION ZONE AND ALL OTHER SYSTEM CONDITIONS. PREVENTS FALSE TRIPPING FOR FAULTS OUTSIDE THE ZONE. DEPENDABLE FOR FAULTS WITHIN ITS PROTECTION ZONE AND ENERGIZE THE TRIP COIL OF ITS ASSOCIATED C.B. THIS CAN BE ACHIEVED BY DESIGNING THE RELAY INTO A LOGICAL DECISION-MAKING CAPABILITY. SUCH THAT FOR ANY GIVEN CONDITION OF ITS INPUT SIGNALS, IT IS ABLE TO PRODUCE THE CORRECT OUTPUT FOR EVERY POSSIBLE STATE OF ITS INPUT SIGNALS. LOGICAL PERFORMANCE DEPEND ON THE NATURE OF INPUT AND OUTPUT SIGNALS INPUT ≡ CURRENT AND VOLTAGE THE RELAY OUTPUT IS THE INPUT TO THE BREAKER TRIP COIL. CONSEQUENTLY THE OUTPUT STATE OF THE RELAY WILL BE , WITH ITS CONTACTS CLOSED, CALLED, TRIP, OR WITH ITS CONTACTS OPEN CALLED BLOCK OR BLOCK TO TRIP. CLASSES: 1) MAGNITUDE RELAY 2) DIRECTIONAL RELAY 3) RATIO RELAY 4) DIFFERENTIAL RELAY 5) PILOT RELAY MAGNITUDE RELAY CURRENT MAGNITUDE RELAYS OR OVERCURRENT RELAYS RESPOND TO MAGNITUDE OF THE INPUT CURRENT. OPERATE TO TRIP WHENEVER THE CURRENT MAGNITUDE EXCEEDS A CERTAIN VALUE WHICH IS ADJUSTABLE. IF A VALUE |Ip|EXPRESSED IN TERMS OF THE SECONDARY WINDING OF THE CT CAN BE FOUND FROM SYSTEM SHORT CIRCUIT STUDIES SUCH THAT FOR ALL FAULT WITHIN THE ZONE OF PROTECTION OF THE RELAY, THE FAULT CURRENT MAGNITUDE |If| ALSO EXPRESSED IN TERMS OF THE SECONDARY WINDING WILL BE GREATER THAN |Ip| THE FUNCTIONAL DESCRIPTION BELOW WILL PRODUCE A SECURE AND DEPENDABLE RELAY. |If| > |Ip| |If| < |Ip| LOGICAL DESCRIPTION OF AN OVERCURRENT RELAY |Ip| PICK UP VALUE OF THE RELAY AN ARBITRARY PHASOR IS ASSUMED TO BE REFERENCE. HENCE THE PHASE ANGLE OF THE FAULT CURRENT CAN BE ANYWHERE B/W 0O TO BE REFERENCE. HENCE THE PHASE ANGLE OF THE FAULT CURRENT CAN BE ANYWHERE B/W 0O - 360O PHASORS REPRESENTING FAULT CURRENTS WITH MAGNITUDE SMALLER THAN |Ip| WILL LIE WITHIN THE CIRCLE AND WILL RESULT IN A BLOCK DECISION BY THE RELAY. OTHERWISE A TRIP DECISION WILL RESULT. (RELAY WILL TRIP) THIS SIMPLE METHOD IS HOWEVER NOT SUFFICIENTLY VASATILE IN MANY CASES. A TIME DELAY PARAMETER IS INTRODUCED TO IMPROVE THE SITUATION. IE. THE TIME IT TAKES THE RELAY TO OPERATE AFTER |If| EXCEEDS |Ip| T = Φ(|If| - |Ip|) IF |If| > |Ip| T IS THE RELAY OPERATING TIME Φ IS A FUNCTION WHICH DESCRIBES ITS (DEPENDENCY) DEPENDENCE ON THE FAULT CURRENT LEVEL. CONSIDER TIME CIRCLES T1 AND T2 |If| FALLS ON A TIME LINE OR BETWEEN TWO TIME LINES WHICH REPRESENTS THE OPERATING TIME OF THE RELAY FOR THAT FAULT CURRENT. THE PICKUP SETTING |Ip| OF A RELAY IS ADJUSTABLE THROUGH THE TAPS ON ITS INPUT WINDING. THE FUNCTION Φ IS USUALLY ASYMPTOTIC TO THE PICKUP VALUE AND DECREASES AS SOME INVERSE POWER OF THE CURRENT MAGNITUDE FOR |If| > |Ip| ASYMTOTIC – APPROACHES BUT NEVER REACHES ASYMTOTE THE TIME DIAL SETTING OF ½ GIVES THE FASTEST RESPONSE; 10 GIVES THE SLOWEST OPERATION FOR A GIVEN. THE CIRCLE INDICATES THAT EVERY FAULT CURRENT HAS A CORRESPONDING DELAY AND THIS DELAY IS WHAT IS INDICATED IN THE CIRCLES. THIS REPRESENTS THE SPEED OF THE RELAY FOR A GIVEN FAULT CURRENT LEVEL OR THE RESPONSE TIME OF THE RELAY FOR A GIVEN FAULT CURRENT LEVEL. THE CIRCLE INDICATES THAT EVERY FAULT CURRENT HAS A CORRESPONDING DELAY AND THIS DELAY IS WHAT IS INDICATED IN THE CIRCLES. THE CIRCLE INDICATES THAT IT IS THE MAGNITUDE THAT THE RELAY IS SENSITIVE TO (RESPONDS TO) AND NOT THE ANGLE HENCE THE REFERENCE ANGLE IS ARBITRARY. SUBSEQUENT ASSYMPTOTIC CURVES SHOW THE BEHAVIOUR OF THE RELAY WITH RESPECT TO TIME AND PICK UP CURRENT AND FAULT CURRENT. DIFFERENT CURVES ARE GIVEN FOR DIFFERENT DIAL SETTINGS WHICH SPEEDS UP THE RESPONSE FOR A GIVEN FAULT CURRENT DEPENDING ON THE OPERATING CHARACTERISTIC CURVE.