Power transformers

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Power transformers

  1. 1. TRANSFORMER  Transformer is a static device used for transferring of power from one circuit to another without change in frequency.  Operates on the principle of mutual induction between two circuits linked by a common magnetic field.  EMF induced in a winding is proportional to the flux density in the core, cross section of the core, frequency and no. of turns in the winding.
  2. 2. WORKING PRINCIPLE OF A POWER TRANSFORMER  A Transformer consists of two mutually inductive coils that are electrically separated but magnetically linked through a low reluctance path .  When one coil is connected to a source of alternating voltage ,an alternating flux is set up in the laminatrd core ,most of which links with the second coil wound on the same core ,in which it induces EMF according to Faraday ‘s laws of electro magnetic induction  If the second coil is connected to a load , a current flows through it and thus the electric energy is transfered completely magnetically from 1st coil to 2nd coil at voltage depending upon the no.of turns in these coils.
  3. 3. TRANSFORMER BASICALLY CONSISTS OF:  Magnetic Circuit comprising Limbs, yokes, clamping structures  Electrical circuit comprising primary, secondary windings  Insulation comprising of transformer oil and solid insulation viz. paper, pressboard, wood etc. and bracing devices  Main tank housing all the equipment  Radiators, Conservator tank  On or Off load tap changer  Vent pipe, Buchholz relay, Thermometers  Fans, Cooling pumps connected piping  Terminals i.e. connecting leads from windings to bushing with supporting arrangements
  4. 4. FEATURES OF POWER TRANSFORMERS  Single Phase  Three phase  Star or Delta connected Primary  Star or Delta connected Secondary  With or without Tertiary winding  Provided with Off-circuit tap switch or On-load Tap Changer for voltage regulation
  5. 5. H.V Winding L.V Winding Core Fundamental equation of transformerFundamental equation of transformer
  6. 6. EMF EQUATION OF A POWER TRANSFORMER  Induced EMF Primary (rms) = E1 =4.44 f N1 Bm A Induced EMF Secondary (rms) = E2 = 4.44 f N2 Bm A E1/ N1 =E2 /N2 = 4.44 f Bm A E2 / E1 = N2 /N1 =K ,Transformation ratio. if K is more than 1 , it is stepup transformer if K is less than 1 ,it is step down transformer. for an ideal tr. Input VA = output VA V1 I1 = V2 I2.
  7. 7. Codes and Standards • Codes or Regulations are mandatory requirements stipulated to ensure the safety of the product during testing and service. • Standards are the basis of agreement and can be used for limited scope or even restricted. Standards also promote interchangeability. Standards exist for material, product, process, testing, calibration etc. • Specifications are based on mandatory requirements of the purchaser and agreed requirements of the standard.
  8. 8. DESIGN PARAMETERS – FROM USER POINT Voltage Ratio No. of phases Flux density Rated capacity Current density Insulation& cooling medium Insulation levels Tap changer Vector group Cooling arrangement Percentage Impedance Oil preservation system Short circuit withstanding Operating conditions capacity
  9. 9. NORMALLY FLUX DENSITY IS CHOSEN NEAR KNEE POINT OF MAGNETIZATION CURVE LEAVING SUFFICIENT MARGIN TO TAKE CARE OF VOLTAGE AND FREQUENCY VARIATIONS. CRGO STEEL WITH SILICON CONTENT OF APPROX. 3% IS USED FOR MAGNETIC CIRCUIT. CHARACTERISTICS OF GOOD CORE ARE I. MAX. MAGNETIC INDUCTION TO OBTAIN A HIGH INDUCTION AMPLITUDE IN AN ALTERNATING FIELD. II. MINIMUM SPECIFIC CORE LOSS AND LOW EXCITATION CURRENT. III. LOW MAGNETOSTRICTION FOR LOW NOISE LEVEL. IV. GOOD MECHANICAL PROCESSING PROPERTIES. MAGNETOSTRICTION IS CHANGE IN CONFIGURATION OF A MAGNETIZABLE BODY IN A MAGNETIC FIELD WHICH LEADS TO PERIODICAL CHANGES IN THE LENGTH OF THE BODY IN AN ALTERNATING MAGNETIC FIELD. DUE TO MAGNETOSTRICTION OF LAMINATIONS IN AN ALTERNATING FIELD CORE VIBRATES GENERATING NOISE IN THE CORE.
  10. 10. CURRENT DENSITY IS AN IMPORTANT PARAMETER TO DESIGN THE SECTION OF THE CONDUCTOR FOR A SPECIFIED TEMPERATURE RISE, RATED CAPACITY AND SHORT CIRCUIT WITHSTAND CAPACITY OF THE TRANSFORMER. DIFFERENT TYPES OF WINDINGS : DISTRIBUTED CROSSOVER WINDING SPIRAL WINDING HELICAL WINDING CONTINUOUS DISC WINDING INTERLEAVED DISC WINDING SHIELDED LAYER WINDING
  11. 11. Transformer oil serves as an electrical insulation and also as a coolant to dissipate heat developed in the transformer. CHARACTERISTICS OF TRANSFORMER OIL: PHYSICAL Appearance The oil shall be clear, transparent and free from suspended matter. If color of oil is a) Light - indicates degree of refining b) Cloudy or foggy - Presence of moisture c) Greenish tinge - Presence of copper salts d) Acid smell - Presence of volatile acid. Can cause corrosion
  12. 12. Density At 27deg. c is 0.89gm/cu.cm. This ensures that water in the form of ice present in oil remains at the bottom and does not float up to a temp. of about – 10 deg. c. Viscosity Is a measure of oil resistance to continuous flow without the effect of external forces. Oil must be mobile in transformers to take away heat. Viscosity shall be as low as possible at low temperatures. Flash point is the temperature at which oil gives so much vapor, which when mixed with air forms an ignitable mixture and gives a momentary flash on application of a flame. Minimum flash point of a good oil shall be 140 deg. C.
  13. 13. Pour point is the temperature at which oil will just flow under prescribed conditions. If oil becomes too viscous or solidifies it will hinder the formation of convection currents, thus cooling of equipment will be affected. Maximum pour point shall be -9 deg. C Interfacial Tension Is the measure of resultant molecular attractive force between unlike molecules like water and oil at the interface. Presence of soluble impurities decrease molecular attractive force between oil and water. This gives an indication of degree of sludging of oil. Minimum value 40 dynes/M or 0.04 N/M .
  14. 14. CHEMICAL Neutralization Number Is a measure of organic and inorganic acids present in the oil. Expressed as mg. of KOH required to neutralize the total acids in one gm. Of oil. Limits for fresh oil - 0.03 mg KOH/gm - maximum Limits for used oil - 0.05 mg KOH/gm - maximum It leads to formation of sludge, metal surface corrosion and lowering of di-electric strength. Corrosive Sulphur It indicates the presence of sulphur, sulphur compounds, which are corrosive in nature and corrode the copper surface.
  15. 15. Oxidation Stability This is measured by ageing the oil by simulating actual service condition of a transformer. Covers the evaluation of acid and sludge forming tendency of new mineral oils. For used oil, should be minimum to minimize electrical conduction and corrosion Water Content By moisture entry into oil. a) By accidental leakage b) Breathing action c) During oil filling or topping up d) By chemical reaction In unused oil - Maximum 30 ppm Oil in transformer 145 KV & above - Maximum 15 ppm Oil in transformer below 145 KV - Maximum 25 ppm It reduces electrical strength and promotes degradation of oil as well as paper.
  16. 16. ELECTRICAL Electric Strength Is the voltage at which arc discharge occurs between two electrodes when oil is subjected to an electric field under prescribed conditions. New oil unfiltered - 30 KV minimum (rms) New oil filtered - 60 KV minimum (rms) Resistivity It is numerically equal to the resistance between opposite faces of a centimeter cube of oil. Insulation resistance of the windings of transformer is dependant on the resistivity of oil. A low value indicates the presence of moisture and conducting contaminants. Values for a new transformer are (12) At 27 deg. c 500x 10 ohm.cm (12) At 90 deg. c 30x 10 ohm.cm
  17. 17. Dielectric Dissipation Factor (Tan Delta & Loss Tangent) Is measure of dielectric losses in oil & hence the amount of heat energy dissipated. It gives an indication as to the quality of insulation. A high value indicates presence of contaminants or deterioration products such as water, oxidation products, soluble varnishes, and resins. 1) Tan delta at 90° for unused oil - maximum 0.2 2) Tan delta at 90° for oil before charging transformer - maximum 0.005 (1/2%) Low value of tan delta indicates low losses
  18. 18. TWO WINDINGS IS SAME. THIS IS CALLED SUBTRACTIVE POLARITY. WHEN THE INDUCED EMFS ARE IN OPPOSITE DIRECTION , THE POLARITY IS CALLED ADDITIVE. PRI. AND SEC. WINDINGS ON ANY ONE LIMB HAVE INDUCED EMFS THAT ARE IN TIME PHASE. DIFFERENT COMBINATIONS OF INTERNAL CONNECTIONS AND CONNECTIONS TO TERMINALS PRODUCE DIFFERENT PHASE DIVERGENCE OF SEC. VOLTAGE. VECTOR GROUP OR CONNECTION SYMBOL OF A TRANSFORMER DENOTES THE METHOD OF CONNECTION OF PRI. AND SEC. WINDINGS AND THE PHASE ANGLE DIVERGENCE OF SEC. WITH RESPECT TO PRIMARY.
  19. 19. VECTOR GROUPS 1 3 4 U VW u w v u w v 1 & 3 – YNd1 2 & 4 – YNd11 U VW 2
  20. 20. U VW u v w u v w 1 2 3 4 Vector Groups U V W 1 & 3 – Dyn11 2 & 4 – Dyn5
  21. 21. NECESSARY TO REDUCE THERMAL DEGRADATION OF INSULATION TO ENSURE LONGER LIFE. HEAT GENERATED IN THE TR. IS TRANSMITTED TO ATMOSPHERE THROUGH OIL. DIFFERENT TYPES OF COOLING: ONAN TYPE – OIL NATURAL AND AIR NATURAL. HOT OIL IS CIRCULATED BY NATURAL MEANS DISSIPATING HEAT TO ATMOSPHERE BY NATURAL MEANS. ONAF TYPE – OIL NATURAL, AIR FORCED. HERE AIR IS BLOWN ON TO THE COOLING SURFACES. FORCED AIR TAKES AWAY HEAT AT A FASTER RATE. OFAF TYPE – OIL FORCED, AIR FORCED. IF THE OIL IS FORCE CIRCULATED WITHIN THE TR.AND RADIATOR BY MEANS OF AN OIL PUMP, IN ADDITION TO FORCED AIR, STILL BETTER RATE OF HEAT DISSIPATION IS ACHIEVED OVER ONAF
  22. 22. OFWF TYPE – OIL FORCED, WATER FORCED. HERE WATER IS EMPLOYED FOR COOLING OIL INSTEAD OF AIR. AMBIENT TEMP. OF WATER IS LESS THAN ATMOSPHERIC AIR. HENCE BETTER RATE OF COOLING IS OBTAINED. IN THIS TYPE OIL TO WATER HEAT EXCHANGERS ARE EMPLOYED. DIFFERENTIAL PRESSURE BETWEEN OIL AND WATER IS MAINTAINED. OIL IS CIRCULATED AT A HIGHER PRESSURE. ODAF/ODWF TYPE – OIL DIRECTED, AIR/WATER FORCED. IF THE OIL IS DIRECTED TO FLOW PAST THE WINDINGS, LARGE QUANTITIES OF HEAT CAN BE TAKEN AWAY BY OIL. COOL OIL IS DIRECTED TO FLOW THROUGH THE WINDINGS IN PREDETERMINED PATHS. OIL IS CIRCULATED BY A FORCED OIL SYSTEM LIKE OIL PUMPS. THIS ENSURES FASTER RATE OF HEAT TRANSFER.
  23. 23. ABSORBS MOISTURE. PRESENCE OF MOISTURE REDUCES DIELECTRIC STRENGTH OF OIL. DIFFERENT METHODS ARE AVAILABLE TO REDUCE CONTAMINATION OF OIL WITH MOISTURE. 1. SILICAGEL BREATHER: IT IS CONNECTED TO THE CONSERVATOR TANK. IT CONSISTS OF A CARTRIDGE PACKED WITH SILICAGEL DESSICANT AND A SMALL CUP CONTAINING OIL. AIR IS DRAWN INTO THE CONSERVATOR THRO. OIL CUP AND BREATHER WHERE MOST OF THE MOISTURE IS ABSORBED. 2. BELLOWS AND DIAPHRAGM SEALED CONSERVATORS: A BELLOW TYPE BARRIER OR A DIAPHRAGM TYPE BARRIER IS FITTED IN THE CONSERVATOR. AIR ENTERING THE CONSERVATOR TANK PUSHES THE DIAPHRAGM DOWNWARDS. AS OIL EXPANDS THE DIAPHRAGM IS PUSHED UPWARDS. POSITION OF DIAPHRAGM IS INDICATED BY OIL LEVEL INDICATOR. DIAPHRAGM ACTS AS A BARRIER.
  24. 24. 3. GAS SEALED CONSERVATORS: IN THIS METHOD A CUSHION OF AN INERT GAS LIKE NITROGEN IS PROVIDED OVER OIL SURFACE IN THE CONSERVATOR. GAS PRESSURE IS ALWAYS MAINTAINED HIGHER THAN ATMOSPHERIC PRESSURE. NITROGEN GAS PRESSURE INSIDE THE CONSERVATOR IS REGULATED BY NITROGEN CYLINDER AND PRESSURE REDUCING VALVE WHICH ADMIT NITROGEN TO THE CONSERVATOR WHEN THE PRESSURE FALLS. EXCESSIVE PRESSURE DEVELOPED INSIDE THE CONSERVATOR IS RELIEVED THROUGH A RELIEF VALVE. 4. REFRIGERATION BREATHERS: AN AIR DRYER IS FITTED TO THE CONSERVATOR. AIR BREATHED THRO. THE UNIT IS DRIED IN PASSING DOWN A DUCT COOLED BY A SERIES OF THERMOELECTRIC MODULES BASED ON PELTIER EFFECT. TOP AND BOTTOM ENDS OF THE DUCT ARE TERMINATED IN THE EXPANSION SPACE ABOVE OIL LEVEL IN THE CONSERVATOR AND AIR IS CONTINUOUSLY CIRCULATED THRO. THE DUCT BY THERMOSYPHON FORCES.
  25. 25. SHORT CIRCUIT WITHSTAND CAPACITY: EFFECTS OF SHORT CIRCUIT: ENERGY IN THE SYSTEM GETS RELEASED IN THE FORM OF HEAVY FLOW OF CURRENT WHEN FAULT OCCURS. EVERY FAULT FED BY THE TRANSFORMER STRESSES THE WINDINGS. THE STRESS DEVELOPED IN THE WINDING IS RELATED TO THE INTENSITY OF FAULT. EACH FAULT CAUSES SHARP RISE IN TEMPERATURE AND PRODUCES MECHANICAL FORCES IN THE WINDING. THESE FORCES ACT IN THE AXIAL AND RADIAL DIRECTIONS OF THE WINDING, AND CAUSE COMPRESSIVE OR TENSILE STRESSES ON THE WINDING AND TEND TO DEFORM IT.
  26. 26. Radial forces: are due to flux in the space between coils. Tend to burst coils and crush on the core. Strengthening of winding Axial forces: are due to radial component of flux which crosses the winding at the ends and gives rise to axial compressive force tending to squeeze the winding in middle. Proper drying, compression and clamping
  27. 27. Thermal effect: rapid rise of temperature causes I) mechanical weakening of insulation due to thermal ageing – long term effect. Ii) decomposition of insulation to produce gases – short term effect. Iii) conductor annealing – becomes brittle & cracks will be formed. Limit of max. Average temperature after short circuit is 2500 c for oil immersed transformer using copper winding.
  28. 28. TAP CHANGERS ARE DEVICES FOR REGULATING THE VOLTAGE OF TRANSFORMER. OFF CIRCUIT TAP CHANGER : TAP CHANGING IS EFFECTED WHEN TR. IS OFF. THESE ARE CHEAPER. THEY ARE USED WHERE FREQUENCY OF TAP CHANGING IS VERY LESS. ON LOAD TAP CHANGER : HERE TAP CHANGING IS EFFECTED WITHOUT INTERRUPTING LOAD. ON LOAD TAP CHANGER NORMALLY CONSISTS OF TRANSITION RESISTORS WHICH BRIDGE THE CIRCUIT DURING TAP CHANGING OPERATION. TWO TYPES OF OLTCS : SINGLE COMPARTMENT TYPE – IN THIS TYPE SELECTION OF TAPS AND SWITCHING ARE CARRIED OUT ON THE SAME CONTACTS. DOUBLE COMPARTMENT TYPE – IN THIS TAP SELECTION IS DONE SEPARATELY AND SWITCHING IS DONE IN A SEPARATE DIVERTER SWITCH.
  29. 29. TYPES OF TAP CHANGERS  Based on applicationBased on application Off-Circuit tap changerOff-Circuit tap changer On Load Tap Changer (OLTC)On Load Tap Changer (OLTC)  Based on mounting (for OLTC)  Internal  External
  30. 30. Internally Mounted OLTC
  31. 31. EXTERNALLY MOUNTED OLTC
  32. 32. EXTERNALLY MOUNTED OLTC
  33. 33. MAXIMUM THROUGH CURRENT INSULATION LEVEL TO GROUND AND BETWEEN VARIOUS CONTACTS NO OF STEPS AND BASIC CONNECTIONS TEMPORARY OVERLOADS AND SHORT CIRCUIT STRENGTH AUTOMATIC VOLTAGE REGULATING RELAYS ARE USED FOR AUTOMATIC CONTROL OF BUS BAR VOLTAGE. OUTPUT OF VOLTAGE TRANSFORMER CONNECTED TO CONTROLLED VOLTAGE SIDE OF THE TR. IS USED TO ENERGIZE AVR RELAY. WHEN VOLTAGE DEVIATION EXCEEDS A PRESET LIMIT, A CONTROL SIGNAL TO RAISE OR LOWER TAP OPERATION IS GIVEN. A TIME DELAY UNIT IS CONNECTED IN THE CIRCUIT TO PREVENT UNNECESSARY OPERATION OR HUNTING OF TAP CHANGER DURING TRANSIENT VOLTAGE CHANGE.
  34. 34. BASIC CONDITIONS OF OPERATION  Load current must not be interrupted during tap change operation.  Tap change must occur without short-circuiting the tap winding directly.  Positive change of tap position. It means ‘make-before-break’ mechanism to be used. This calls for a transition impedance. Also the mechanism should be fast acting type – spring loaded.
  35. 35. GENERAL DESIGN CONSIDERATIONS  Capable to normal load/overloads on transformer.  Maximum system voltage  Step voltage & no. of steps  Test voltage to earth and across tapping range  Maximum surge voltage to earth and across range.  Maximum test voltages between phases (where applicable)  Current rating – normal and overload
  36. 36. PARTS OF TAP CHANGER  Selector switch Tap selection takes place in this switch  Diverter Switch Make –before-break mechanism with transition impedance. Arcing takes place and hence housed in a separate compartment.  Surge relay  Conservator with oil level gauge.
  37. 37. TRANSITION IMPEDANCE  Reactor type  Resistor type
  38. 38. REQUIREMENTS OF TRANSITION IMPEDANCE  No voltage fluctuations during switching cycle  Circulating currents should not be excessive  Duration of arc should be minimum to minimize contact erosion and reduce contamination of oil.
  39. 39. TAP CHANGER CONTROLS  Manual / Electrical  Local / Remote  Manual / Automatic  Independent Operation  Parallel Operation Group Control  Master  Follower
  40. 40. R2 R1 7 5 3 1 8 6 4 2 N PRINCIPLE OF TAP CHANGER OPERATION M2 T2 T1 M1
  41. 41. M2 T2 T1 M1 M2 T2 T1 M1 M2 T2 T1 M1 M2 T2 T1 M1 M2 T2 T1 M1 M2 T2 T1 M1 1 2 3 4 5 6
  42. 42. FEATURES OF TAP CHANGER  Motor drive mechanism Should rotate in both the directions Step-by-step operation Tap change in progress indication Tap change complete indication Sequence contact Remote Tap position control & indication
  43. 43. TAP CHANGER OIL QUALITY Use of tap changer Water content Dielectric strength At neutral point of windings < 40 ppm > 30 KV At positions other than neutral end < 30 ppm > 40 KV Standard values for transformer oil testing according to CIGRE 12 – 13 (1982) apply to tap changer oil at service temperature.
  44. 44. OPERATING CONDITIONS The environment in which a transformer works and the quality in design and construction play a role on its performance. A transformer working under normal operating conditions, in all probability, gives satisfactory performance throughout its life . NORMAL OPERATING CONDITIONS 1. Rated voltage and rated current with permissible margins. 2. Temperatures of oil and windings not exceeding the prescribed values. 3. Availability of auxiliary and control supply and proper functioning of accessories and protective devices. 4. Free from external faults such as line breakdowns and equipment breakdowns.
  45. 45. USER SHOULD SPECIFY THE CONDITIONS UNDER WHICH TRANSFORMER IS EXPECTED TO WORK VIZ. QUALITY AND NATURE OF LOAD, TEMPERATURE LIMIT, VOLTAGE CONDITIONS, SHORT CIRCUIT WITHSTAND CAPACITY CONSIDERING PRESENT AND EXPECTED FAULT LEVELS. PARAMETERS SPECIFIC TO LOCATIONS ARE TO BE EVALUATED AND SPECIFIED TO ASSESS THE OPERATING REQUIREMENT. MANUFACTURERS SHOULD ENSURE THAT FACTORY TESTS AS REQUIRED UNDER STANDARDS AND THE USER SPECIFICATIONS ARE DONE TO VERIFY THE QUALITY AND ABILITY OF THE TRANSFORMER TO WITHSTAND ALL SERVICE STRESSES DURING LIFE TIME OF THE TRANSFORMER.
  46. 46. Design Basis • Life-time cost of transformer = Initial cost of transformer + Operational cost for its life period This is called the “Capitalized cost of transformer”.
  47. 47. DESIGN BASIS - CAPITALIZATION Rationalized CBIP Capitalization Formula: Capitalized Cost = Initial Cost (IC) + Capitalized { No-load Loss (Wn) + Load Loss (Wl) + Auxiliary Losses (Wa) } Capitalized cost = IC + Xn.Wn +Xl.Wl + Xa.Wa Factors affecting Xn; Xl and & Xa Rate of Interest Rate of Electrical Energy Life of Transformer
  48. 48. DESIGN BASIS  The design of a transformer aims at achieving lowest capitalized cost.  Low No-load Loss means higher magnetic material cost and vice-versa  Low Load Loss means higher copper (material) cost and vice-versa.  Several iterations are made to optimize the total cost before freezing the design and drawings are made.  Extensive use of CAD programs is needed for finalizing design.
  49. 49. CONSTRUCTION FEATURES
  50. 50. Core
  51. 51.  Higher the number of steps in cross section, better is space utilization and smaller is the core diameter. 90 to 95 % utilization factor is optimal.  Core area (A) is determined by the Flux Density (B) which in turn depends on many factors - like loss capitalization and overall design economics.  As the no load losses attract very high capitalization, attempts are continuously made to reduce them.  Improved manufacturing techniques like core building with 2-lamination packets, step-lap joints, v-notched laminations, bolt-less cores are used.  Hi-β core steels like M0H, ZDKH, etc are available in which the specific core losses are lower than normal grades.
  52. 52. A A V ie w A - A C o n v e n tio n a l S te p la p
  53. 53. WINDINGS- L.V WINDING  L.V Windings in Transformers are either  Spiral OR layer wound for low current ratings  Helical Wound with radial cooling ducts for higher ratings.  Disc type wound  Distributed Cross-over (Run-over) coils  The conductor used is paper insulated rectangular copper (PICC)  For higher currents, transposed conductors are used, to uniformly distribute the current across the cross section of the wire of coil.
  54. 54. SPIRAL/LAYER TYPE WINDING Mandrel/Press-board cylinder Cooling Duct Conductor – Layer 1 Conductor – Layer 2 Conductor – Layer 3
  55. 55. Helical Coil (Single layer) Helical coil (Double Layer) Start Finish
  56. 56. TRANSPOSED CONDUCTORS Transposed conductors (CTC) are used to improve current distribution in the cross section of the winding wire. Individual cable can be coated with epoxy so that the cured and finished conductor is mechanically stronger and withstand short circuit forces better.
  57. 57. H.V WINDING/1 HV winding invariably uses PICC or CTC. Type of winding used is - Layer winding or - Disc winding up to 132 kV and/or - Interleaved winding or - Rib shielded winding
  58. 58. T em porary O ver-voltage s S w itchin g O ver-voltage s O ver-vo ltages d ue to lightning . P o w e r S yste m s O ve r vo lta g e s POWER SYSTEM OVER VOLTAGES
  59. 59. TEMPORARY OVER-VOLTAGES  Typically due to faults  < 1.2 pu  ms to tens of second or even minutes  Not dangerous to insulation
  60. 60. SWITCHING OVER-VOLTAGES  Due to system switching operations  1.5 pu – 5 pu dépends on system voltage  mostly damped asymmetric sinusoids  front time of first peak – tens of µs to a few ms.  decides external insulation in EHV/UHV systems
  61. 61. OVER VOLTAGES DUE TO LIGHTNING  Due to ‘direct’ or ‘indirect’ lightning strokes.  known to contribute to ≅ 50% of system outages in EHV & UHV systems  few hundred kV to several tens of MV.  Few kA to 200 kA  very short duration : time to front : 1 to few tens of µs  time to tail : few tens to hundreds of µs.  Decides line insulation (BIL)  Severely influences Transformer insulation.
  62. 62. Cg Cs α = K √ Cg/Cs IMPULSE VOLTAGE DISTRIBUTION
  63. 63. IMPULSE VOLTAGE DISTRIBUTION α = 0 α = 1 0 α =5 X V
  64. 64. Disc Type Winding Paper Insulated Conductor Press-board Cylinder
  65. 65. 1234 5 6 7 8 9 1526 3 7 4 8 9 1S23 4 5 S 6 78 Conventional Shielded Interleaved DISC WINDING CONCEPTS
  66. 66. Impulse Voltage Distribution 1. Plain Disc Winding 2. Rib Shield Winding 3. Inter-leaved Disc Winding Number of discs from line end V O L T A G E G R A D I E N T P u
  67. 67. TERTIARY WINDING/1 In Star-Star Connected Transformers and Auto transformers, Tertiary Winding is used to stabilize phase to phase voltages in case of unbalanced load - Suppressing third harmonic currents in earthed neutral - reducing zero sequence reactance - for supplying auxiliary load or for connecting capacitors.
  68. 68. TERTIARY WINDING/2  Tertiary is required to be designed for a power rating equal to one-third the rated power, it increases the cost of the transformer by 10- 12 percent.  Tertiary winding is known to fail due to transferred surges and Short circuits  Present practice is to do away with tertiary up to 100 MVA for 3 phase 3 limbed core transformers.
  69. 69. DESIGN PROCESS  Design should meet Requirements of customer specification Relevant National or International standards Statutory and regulatory requirements Manufacturer’s Plant Standards  Optimized design
  70. 70. OPTIMIZATION  Objective of Optimization  To arrive at a design that yields minimum capitalized cost.  It is a function of the following:  Core diameter  Core height  Flux Density  Current Density
  71. 71. COMPUTER AIDED DESIGN  Improve productivity of design personnel Release of Engineering information may be 25 – 40% of delivery cycle.  Reduce delivery cycle  Better analysis and arriving at a most optimum design  To solve electro-static, electro-magnetic problems and to provide a robust structural and thermal design.
  72. 72. WHY IT IN DESIGN  More precise calculations  Tailor made designs  No standard ratings specified above 1 MVA  Change of specification parameter  Relative change of material cost  Ongoing development of technology
  73. 73. COMPUTER AIDED DESIGN  Design optimization  Design analysis “fem” 2d / 3d engineering  Analysis  Electromagnetic  Electrostatic  Structural  Thermal  Computer aided drafting  Database and Data Management
  74. 74. WHAT IS QUALITY?  Conformance Quality  Performance Quality  Appearance Quality  Functional Quality  Esteem Quality  ‘Ability’ Quality QUALITY OF DESIGN/GRADE FITNESS FOR USE
  75. 75. POOR QUALITY RESULTS IN FAILURES. TYPES OF FAILURES  Infant failures: Early life failures are the result of latent defects. - Latent defects are abnormalities that cause failure, depending on degree of abnormality and amount of applied stress. - Delivered defects are those that escape test / inspection within the factory - They are directly proportional to total defects in the entire processes.
  76. 76.  Mid life failures: These are results of – - Freak system disturbances - Wrong specifications - Poor maintenance  Old age failures: These are results of – - Ageing of insulation system - Wear & tear
  77. 77. TRANSFORMER END FRAMES CORE INSULATION WINDINGS TANK OIL RAW MATERIAL FABRICATION FITTINGS MAJOR ASSEMBLIES COMPONENT ELEMENT
  78. 78. Electrical * Power frequency * Over-voltages (External & Internal) * Part winding resonance * Partial Discharge contd.. 82 MAIN FACTORS CAUSING STRESSES IN THE WINDING
  79. 79. 83  Mechanical * Core Vibration * Force due to Short Circuit or Faults * Inrush Current * Over-fluxing  Thermal * Winding Temperature * Core loss * Core Shorting * Malfunctioning of Cooling System * Hot Spot (Local overheat) * Arcing
  80. 80. CHALLENGES IN TRANSFORMER DESIGN & MANUFACTURING  Structures design (tank etc.) To be designed for: Lifting & Jacking Full or partial vacuum Internal Pressure Seismic Load Tests conducted: Leakage test Vacuum test Radiography (if specified) DPT on load bearing items Contd.. 84
  81. 81.  Short-circuit withstand capability  Adequate radial supports  Use of pre-compressed press-board to minimize shrinkage in service  Proper stabilization of coils  Use of glued conductors  Springs or hydraulic dampers if required Contd.. 85
  82. 82. Stray losses control:  Stray losses due to linkage of high magnitude of flux with magnetic materials  Stray losses form a large part, more than 20% of total load losses  These may cause hot spots  Measures for stray loss control  Use of laminated material  By breaking the magnetic path  By providing non-magnetic shield  By providing parallel low reluctance magnetic path contd.. 86
  83. 83. High Voltage stresses Design of Insulation system to ensure withstand capability for  Lightning Impulse and Switching Surges.  Long duration high voltage system disturbances  Internal Partial Discharges This is done by -  Choosing proper type of windings  Calculation/plotting of impulse / switching surges and long duration voltage stress distribution  Provision of adequate major and minor insulation by using angle rings, moulded components etc.  Corona shielding where required 87
  84. 84. QUALITY DESIGN PERFORMANCE  Following are prerequisites for a long trouble-free service of the transformers:  A well designed insulation system. Good mechanical strength to withstand the inevitable short- circuit forces.  Proper design review by a team of engineers from Design, Quality, Marketing, Production etc to ensure that the design is meeting customer’s specification.  Good manufacturing practices to ensure conformation of the final product to the design documents.  Proper erection & commissioning and subsequent maintenance. 88

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