TRANSDUCERS IN ELECTRICAL POWER SYSTEMS ENGINEERING PROTECTION

TRANSDUCERS CONVERT HIGH VOLTAGES AND CURRENT TO LOWER VALUES FOR THE FOLLOWING REASONS 1) THE LOWER LEVEL INPUT TO THE RELAYS ENSURE THAT THE PHYSICAL HARDWARE USED TO CONSTRUCT THE RELAYS WILL BE QUITE SMALL AND THUS LESS EXPENSIVE. 2) THE PERSONNEL WHO WORK ON THE RELAYS WILL BE WORKING IN A SAFE ENVIRONMENT IN PRINCIPLE, THE TRANSDUCERS ARE SAME AS THE POWER TRANSFORMERS; PUT TO SPECIFIC USE. The current transformer is expected to produce a current in the secondary winding which duplicates the primary current waveform as faithfully as possible. THE AMOUNT OF POWER DELIVERED BY THESE TRANSFORMERS IS RATHER MODEST SINCE THE LOAD ON THEM IS EITHER RELAY AND METER AND USED ONLY WHEN NEEDED. THIS LOAD ON CT AND VT IS REFERRED TO AS THEIR BURDEN. BURDEN : THE IMPEDANCE CONNECTED TO THE TRANSFORMER SECONDARY WINDING. IT MAY ALSO BE REFERRED TO AS THE VOLTAMPERE DELIVERED TO THE LOAD. IE. IT MAY ALSO REFER TO THE VOLTAMPERE DELIVERED TO THE LOAD. EG. A TRANSFO RMER DELIVERING 5 AMPERES TO A RESISTIVE BURDEN OF 0.1 OHM CAN BE SAID TO HAVE A BURDEN OF 2.5VA VOLTAMPERES AT 5A, 5AMPERES. CT CURRENT TRANSFORMER CURRENT TRANSFORMERS HAVE RATIO ERRORS WHICH CAN AND ARE CALCULATED IN SOME CASES AND IN OTHER CASES HAVE TO BE MEASURED. IF IMPEDANCE BURDEN IS LARGE, THIS ERROR CAN BE HIGH. IF THE CT IS PROPERLY SELECTED WITH RESPECT TO THE BURDEN, THIS ERROR CAN BE MINIMIZED. CT BUILT WITHIN THE BUSHING THROUGH WHICH A TERMINAL OF THE POWER EQUIPMENT IS BROUGHT OUT……………ARE ALSO AVAILABLE HENCE CALLED BUSHING CT’S EQUIPMENT ENCLOSED IN GROUNDED TANKS FILLED WITH OIL INSULATING MEDIUM…….. ARE ALSO AVAILABLE THIS TYPE IS AVAILABLE IN DEAD - TANK SYSTEMS. EHV (- SWITCHING STATIONS) HERE LIVE – TANK CB ARE IN USE, FREE STANDING CURRENT TRANSFORMERS CT CURRENT RATING STANDARDISED AT 5A SECONDRIES. SECOND STANDARD 1A IN EUROPE FOR SHORT PERIODS, YOU CAN HAVE 10-20 TIMES THAT DURING SHORT CIRCUIT IN SECONDARY. CT RATIOS RANGE FROM 50: 5 - 1200: 5 CURRENT RATIO CURRENT TRANSFORMER RATIOS CURRENT TRANSFORMER RATIOS A:A 50A:5A 100A:5A 150A:5A 200A:5A 250A:5A 300A:5A 400A:5A 450A:5A 500A:5A 550A:5A 600A:5A 650A:5A 700A:5A 750A:5A 800A:5A 850A:5A 900A:5A 950A:5A 1000A:5A 1050A:5A 1100A:5A 1150A:5A 1200A:5A BURDEN BECAUSE IF FOR EXAMPLE 5 AMPERES IS DELIVERED BY A TRANSFORMER TO A RESISTIVE BURDEN OF 0.1OhM(Ω) THE TRANSFORMER MAY ALSO BE SAID TO HAVE A BURDEN OF 2.5 VOLTSAMPERES AT 5 AMPERES. THIS IS BECAUSE GIVEN V=IR V = .5V FOR I = 5A AND R =0.1Ω(Ohms) HENCE VI = .5X5 = 2.5VA VOLTAGE TRANSFORMER TWO TYPES EXIST 1) FOR LOW - VOLTAGE APPLICATION SYSTEM VOLTAGES 12 Kv or LOWER PRIMARY WINDING AT SYSTEM VOLTAGE AND SECONDARY WINDING AT 67V (THIS REPRESENTS SYSTEM LINE TO NEUTRAL VOLTAGE ) 67V*√3 = 116V REPRESENTING THE SYSTEM LINE TO LINE VOLTAGE ) ARE AN INDUSTRIAL STANDARD. THIS IS SIMILAR TO A MULTIWINDING POWER TRANSFORMER AND BECOMES EXPENSIVE AT HIGHER SYSTEM VOLTAGES. AT HV AND EHV LEVELS, A CAPACITANCE POTENTIAL – DIVIDER CIRCUIT IS USED. C1 AND C2 ARE ADJUSTED SO THAT A VOLTAGE OF A FEW kV IS OBTAINED ACROSS C2 WHEN TERMINAL A IS AT SYSTEM POTENTIAL. IN SUCH A COUPLING CAPACITOR VOLTAGE TRANSFORMER (CVT) THE TAPPED VOLTAGE IS FURTHER REDUCED TO RELAYING VOLTAGE LEVEL BY A TRANSFORMER. CIRCUIT DIAGRAM OF A CAPACITOR – COUPLED VOLTAGE TRANSFORMER CVT WITH ITS TUNING INDUCTANCE L. IN SUCH A COUPLING CAPACITOR VOLTAGE TRANSFORMER (CCVT) (CVT) THE TAPPED VOLTAGE IS FURTHER REDUCED TO RELAYING VOLTAGE LEVEL BY A TRANSFORMER. THE VOLTAGE AT POINT A CAN BE CONSIDERED TO BE THAT OF AN INFINITE BUS SO FAR AS THE CONNECTED CAPACITORS ARE CONCERNED.