ELECTRICAL POWER SYSTEM PROTECTION

ELECTRICAL POWER SYSTEM PROTECTION ELECTRICAL POWER SYSTEM PROTECTION IS VERY IMPORTANT TO SAFEGAURD LIFE AND PROPERTY ELECTRICAL POWER SYSTEM INSTALLATIONS ARE VERY EXPENSIVE AND SHOULD BE PROTECTED IN THE EVENT OF A FAULT A PROTECTION SCHEME WOULD HAVE - A TRANSDUCER - A RELAY - A CIRCUIT BREAKER CIRCUIT BREAKING PROCESS WHEN THE TRIP CIRCUIT OF A CIRCUIT BREAKER IS ENERGIZED BY THE RELAY, THE CONTACTS OF THE CIRCUIT BREAKER WHICH IS IN SERIES WITH THE TRANSMISSION LINE BEGIN TO MOVE APART VERY RAPIDLY. AS THE CURRENT THROUGH THE BREAKER CONTACTS (IE. THE FAULT CURRENT) PASSES THROUGH ZERO. THE SPACE B/W THE CONTACTS BECOME A DIELECTRIC AND IS ABLE TO PREVENT THE FAULT CURRENT FROM FLOWING AGAIN THROUGH THE CIRCUIT BREAKER. THIS TAKES BETWEEN 30 – 100 MILISECONDS THERE ARE FASTER SYSTEMS IN PLACE NOW. CONSIDER THE RELAY R23 OF LINE 2-3 AT TERMINAL 2. THE CURRENT AND VOLTAGE INPUT TO IT CHANGE DUE TO FAULT AT P IE. R23 ALSO SEES THE FAULT. HOWEVER, R23 MUST BE SELECTIVE SO THAT IT DOES NOT OPERATE FOR FAULT AT P, IF P IS OUTSIDE THE REACH OF THIS RELAY. REACH – THIS IS THE AREA OF RESPONSIBILITY OF A RELAY THOUGH IT SEES IT (IE THE FAULT) IE. CAUSES SOME CHANGES IN CURRENT AND VOLTAGE INPUT TO THE RELAY. RELAY RELAY CHARACTERISTICS FAST RELIABLE A GOOD RELAY MUST BE FAST AND RELIABLE. Fast RELIABLE (DEPENDABLE AND SELECTIVE) RELIABLE DEPENDABLE SELECTIVE THIS MEANS THAT WHEN THERE IS A FAULT, IT SHOULD OPERATE. AGAIN ON THE OTHER HAND, WHEN THERE IS NO FAULT, IT SHOULD NOT. THIS MEANS THAT THE SMALLEST POSSIBLE PORTION OF THE SYSTEM SHOULD BE ISOLATED; FOLLOWING A FAULT. SPEED AND RELIABILITY CONFLICT THERE SHOULD BE A COMPROMISE KEEP A REASONABLE MEASURE OF THESE TWO WHY DO WE HAVE TO PROTECT THE ELECTRICAL POWER SYSTEM? WHAT ARE THE CONSEQUENCIES OF FAULTS? FAULTS AND SHORT CIRCUITS CAUSE 1) REDUCTION IN STABILITY MARGINS FOR THE POWER SYSTEM 2) DAMAGE TO EQUIPMENT. 3) EXPLOSIONS IN OIL USED FOR INSULATION. 4) DISRUPTIONS IN THE ENTIRE POWER SYSTEM DUE TO SUCCESSION OF PROTECTIVE ACTIONS BY DIFFERENT PROTECTIVE SYSTEMS. (CASCADING) IN THE PROTECTION OF THE ELECTRICAL POWER SYSTEM, SPEED IS IMPORTANT SPEED IS IMPORTANT. 1 CIRCUIT BREAKER – PROVIDES THE INPUT TO THE RELAY (T) 2 TRANSDUCER – PROVIDE THE INPUT TO THE RELAY (T) 3 RELAY – DEVICES WHICH SENSE THE FAULT AND CAUSE THE CIRCUIT-BREAKER TRIP CIRCUIT S TO BE ENERGIZED AND THE BREAKERS TO OPEN THEIR CONTACTS (R) NOTATION LINE 1-2 HAS BREAKER B12 AT END 1, OR A, B AT FAULT (CONDITIONS) (THERE IS) INCREASED CURRENT FLOW TOWARDS THE FAULT POINT. ( ASSUME SOURCE POWER BEYOND THE ENDS OF THE BUSES. WHEN THIS IS NOT THE CASE, THE ANALYSIS IS SIMPLER) INCREASE IN CURRENT AT LINE TERMINAL IS ACCOMPANIED BY REDUCTION IN VOLTAGE. kA, kV THE FAULT CURRENTS AND VOLTAGES ARE IN kA AND kV HENCE TRANSDUCERS ARE USED TO REDUCE THESE QUANTITIES TO TENS AND SMALLER VALUES FOR RELAYING PURPOSES IN THE PROTECTION SCHEME. HENCE TRANSDUCERS REDUCE THESE TO TENS. THE RELAY TAKES THIS REDUCED SIGNAL, DETERMINE IF A FAULT HAS OCCURRED AND RESPONDS ACCORDINGLY. WITHIN 8 – 40 MILLISECONDS OR LESS (FOR MORE MODERN SYSTEMS) DEPENDING ON DESIGN AND THE MAGNITUDE OF THE FAULT CURRENT. IF THE RELAY DECIDES THAT IT IS A FAULT, THEN IT WILL TRIP THE CIRCUIT BREAKER. SIGNALS CAN BE SENT USING ANY OF THE FOLLOWING MEDIA FOR PROTECTION PURPOSES: TELEPHONE CIRCUIT HIGH FREQUENCY SIGNALS ON POWER LINES POWER LINE CARRIERS MICROWAVE CHANNEL PROTECTION ZONE OF PROTECTION