PROTECTIVE DEVICES & MAINTENANCE.ppt
- 1. Welcome to the Presentation on Specifications Protective devices and Maintenance
- 2. WHAT IS A TRANSFORMER? Transformer is a static device which transforms A.C. Electrical power from one voltage to another voltage keeping the frequency same by electromagnetic induction.
- 3. Power Transformers Transformers Capacity In A System: Generally 6 to 8 times the installed capacity Working Principle: Static piece of apparatus used for transferring power from one circuit to another without change in frequency. It can raise or lower the voltage with a corresponding decrease or increase in current.
- 4. Ideal Transformer : No Losses EMF equation of a Transformer : E1=4.44fN1øm E2=4.44fN2øm where N1=No. of turns in Primary N2=No. of turns in Secondary øm = Maximum flux in the core in Webers
- 5. Voltage Transformation ratio (k) : E2/E1 = N2/N1 = k (a)Step up Transformer : k>1 (b)Step down Transformer : k<1 Regulation : %Regulation = ((E2-V2) / E2) * 100 where, E2 = Secondary Terminal Voltage at no load V2 = Secondary Terminal Voltage at load
- 6. Losses in Transformers : (a)No load losses = Hysteresis loss + Eddy current loss How can we minimize these losses ? By using steel of high silicon content with thin laminations . The input power of a transformer, when on no load measures the core loss. (b)Load Losses : I2R losses + Stray losses(losses occurring in the mechanical structure and winding conductor due to the stray fluxes). These are measured by short circuit test.
- 7. Efficiency: % efficiency = ((output)/ input) * 100 Condition of maximum efficiency : Iron loss = copper loss. Transformers are among the most efficient machines. In lower capacity ranges: Around 95% efficiency In higher capacity ranges : 99% is achievable. Auto Transformer : Has a single continuous winding which is used for the input and output voltages. Used where transformation ratio differs a little from unity.
- 8. Components of Transformer 1. Windings 2. Core 3. Bushings 4. Conservator 5. Breather 6. Buchholz Relay 7. OLTC 8. Oil level Indicator 9. Thermometers 10.Various protective devices
- 9. Bushings Two types: (1)Porcelain : Upto 36 kv (2)Condenser
- 10. Various Types of Cooling in Transformers Various Types of Cooling in Transformers: (1)ONAN : OIL NATURAL AIR NATURAL (2)ONAF : OIL NATURAL AIR FORCE (3)OFAF : OIL FORCE AIR FORCE (4)OFWF : OIL FORCED WATER FORCED Type of cooling vs Rating : OFAF :100% ONAF :60% ONAN :40%
- 11. Parallel Operations of Transformers Satisfactory performance requires that they have : The same voltage ratio. The same per unit(or percentage) Impedance. The same polarity. The same phase sequence and zero relative phase displacement.
- 12. (3) and (4) are absolutely essential. (1) Must be satisfied to a closer degree. (2) The more near , better will be the load division. Tap Changers : Two types: (1) Off-Load Tap Changer. (2) On-Load Tap Changer.
- 16. DISTRIBUTION TRANSFORMER TRANSFORMER WHICH IS USED FOR THE PURPOSE OF DISTRIBUTION OF POWER. 11KV/433V is the standard voltage rating. STANDARD KVA ratings are 25,63,100,160,200,250,315,400,500,630,750,1000, 1250,1500,2000,2500 KVA. IS-2026 is the INDIAN NATIONAL STANDARD.
- 17. MAIN FEATURES Outdoor,oil cooled, 3 phase,50hz Primary is delta connected and secondary is star connected. Naturally cooled (ONAN type). Amongst all the types of transformers this is the most required and most used type.
- 18. PARTS OF TRANSFORMER MAIN TANK RADIATORS CONSERVATOR EXPLOSION VENT LIFTING LUGS AIR RELEASE PLUG OIL LEVEL INDICATOR TAP CHANGER WHEELS HV/LV BUSHINGS FILTER VALVES OIL FILLING PLUG DRAIN PLUG CABLE BOX
- 30. ROUTINE TESTS Oil BDV TEST. Oil breakdown voltage is checked as per IS-335. 100 mm L X 70 mm B X 80 mm Ht. glass pot. 500ml Oil sample. Spherical electrodes with gap of 2.5 mm Recommended value : 60KV Equipment used : OIL BDV TEST SET.
- 31. MAINTENANCE OF TRANSFORMER Transformer is the heart of any power system. Hence preventive maintenance is always cost effective and time saving. Any failure to the transformer can extremely affect the whole functioning of the organization.
- 32. MAINTENANCE PROCEDURE OIL : 1. Oil level checking. Leakages to be attended. 2. Oil BDV & acidity checking at regular intervals. If acidity is between 0.5 to 1mg KOH, oil should be kept under observation. 3. BDV, Color and smell of oil are indicative.
- 33. MAINTENANCE PROCEDURE 1. Sludge, dust, dirt ,moisture can be removed by filtration. 2. Oil when topped up shall be of the same make. Otherwise it may lead to sludge formation and acidic contents. 3. Insulation resistance of the transformer should be checked once in 6 months. 4. Megger values along with oil values indicate the condition of transformer.
- 34. MAINTENANCE BUSHINGS: Bushings should be cleaned and inspected for any cracks. Dust & dirt deposition, salt or chemical deposition, cement or acid fumes depositions should be carefully noted and rectified.
- 35. MAINTENANCE Periodic checking of any loose connections of the terminations of HV & LV side. Breather examination. Dehydration of Silica gel if necessary. Explosion vent diaphragm examination. Conservator to be cleaned from inside after every three years. Cleanliness in the transformer yards with all nests, shrubs removed.
- 36. PROTECTION OF TRANSFORMERS HT fuse & D.O. fuse. LT circuit breaker. HORN GAPS & Lightning Arrestor. Breather.
- 37. FAILURES & CAUSES Insufficient Oil level. Seepage of water in oil. Prolonged Over loading. Single Phase loading. Unbalanced loading. Faulty Termination (Improper sized lugs etc) Power Theft. Prolonged Short Circuit. Lack of installation checks.
- 38. FAILURES & CAUSES Faulty design Poor Workmanship Improper formation of core. Improper core bolt insulation. Burr to the lamination blades Improper brazing of joints. Burr /sharp edges to the winding conductor. Incomplete drying. Bad insulation covering. Insufficient cooling ducts in the winding.
- 39. FAILURES & CAUSES Bad Quality of raw material. Transit damaged transformers. After failure, transformer is removed and replaced with new/repaired one without removing the cause of failure which results in immediate or short time failure.
- 40. Specifications for Distribution Transformers up to 100 KVA The distribution transformers shall comply with the latest versions of Indian Standard Specifications IS 1180(part1) and IS 2026 The Standard Ratings shall be 16, 25, 63 and 100 KVA No load voltage ratios shall be as follows: (a) 11000/433-250 volts for 16, 25, 63 and 100 KVA (b) 10450/433-250 for 100 KVA only, whenever specified No taps are to be provided in these transformers
- 41. Winding Connections and Vectors The primary winding shall be connected in delta and the secondary winding in star ( Vector symbols Dy 11), so as to produce a positive displacement of 30 deg from the primary to the secondary vector of the same phase. The neutral of the secondary winding shall be brought to a separate insulated terminal. The following standars fittings shall be provided (a) Two earthing terminals (b) Oil level gauge indicating three positions of oil, marked as follows: Minimum( -5 deg C) 30 deg C Maximum (98 deg c) (c) Lifting lugs
- 42. The distribution transformer forms a link between the consumers and primary distribution Dielectric strength of transformer oil shall be checked once in a year Acidity of transformer oil shall be checked once in a year Oil level shall be checked once in a month Over loading should be avoided by checking Tong Tester Readings.
- 43. Distribution Transformers (DTRs) Most rugged of all electrical equipment All parts are tightly clamped No rotating or moving parts inside Shall be least prone to failure In a number of Indian Power Utilities, the rate of failure is high (20 to 30% annually)
- 44. Equipment Life Cycle Specification – Design – Manufacture – Transport – Erect – Commission – Operate – Protect – Maintain
- 45. MAINTENANCE TECHNIQUES The maintenance techniques to be adopted should meet the following criteria: Field staff should be able to carry out such tests and interpret the test results Testing procedure to be evolved after detailed discussions with the supplier of the equipment and testing equipment Written procedure to be made available to the operating staff
- 46. Maintenance of Equipment should Include Diagnostics Overhauls Painting Mechanical Aspects Use of correct lubricants as specified by the manu- facturer Availability of adequate spare parts
- 47. Responsibilities for Maintenance of Distribution Transformers S.No Name of the work to be carried out Person responsibl e to do the work Persons responsible to ensure that it is done 1. Monthly maintenance a. Maintaining distribution transformer yard and keeping the earth pits tidy and watering of earth pits Area L.M Area L.I b. Cleaning the Transformer including bushings Area L.M Area L.I c. Checking up of oil level and reporting if it is low Area L.M Area L.I
- 48. Responsibilities for Maintenance of Distribution Transformers S.No Name of the work to be carried out Person responsible to do the work Persons responsible to ensure that it is done 1. Monthly maintenance e Checking of earth connection Area L.M Area L.I F Reconditioning breather by reactivating silica gel or replacement and maintaining oil seal Area L.M Area L.I g Checking the LT Fuses and renewing them Area L.M Area L.I h Topping up oil, where necessary Area L.M Area L.I
- 49. S. N o Name of work to be carried out Person responsible to do the work Persons responsible to ensure that it is done 2. Quarterly Maintenance a. Renewing of H.G. fuses Area L.M Area L.I b. Measurement of insulation resistance and recording HV to earth, LV to earth and HV to LV with 1000 v megger and recording along with temperature of the winding AE ADE Responsibilities for Maintenance of Distribution Transformers
- 50. S. N o Name of work to be carried out Person responsible to do the work Persons responsible to ensure that it is done 2. Quarterly Maintenance c. Measurement of load currents R- ph,Y-ph, B-ph and neutral AE ADE d Measurement of voltages at the transformers and at tail end of LT feeders and noting down RY YB BR RN YN BN AE ADE Responsibilities for Maintenance of Distribution Transformers
- 51. S.No Name if work to be carried out Person responsibl e to do the work Persons responsible to ensure that it is done 3 Annual Maintenance F Lubricating AB switch and checking for proper operation LM LI g Checking line and earth connection at AB switches LM LI h Checking line and earth for lightning arrestor LM LI Responsibilities for Maintenance of Distribution Transformers
- 52. S.No Name if work to be carried out Person responsibl e to do the work Persons responsible to ensure that it is done 3 Annual Maintenance d Checking connections for HV and LV bushings including LV neutral LM LI e Getting oil samples tested for breakdown and acidity and recording LI AE f Measurement of earth resistance and recording LI AE Responsibilities for Maintenance of Distribution Transformers
- 53. Maintenance Schedule of Distribution Transformer 1. Cleaning of bushings and external surface of tank Monthly 2. Checking of oil levels in the conservator and gauge glass Monthly 3. Checking of silicagel in the breather and replacement if necessary Montlhly 4. Checking of oil level in the oil seal of breather & top up if necessary Monthly 5. Checking of HG fuse & LT fuse and renew if necessary (correct gauge shall be maintained) Monthly 6. Checking of vent pipe diaphragm Monthly 7. Checking of terminal loose connections if any and tightening the same Monthly
- 54. Maintenance Schedule of Distribution Transformer 8. Checking for any oil leaks & rectification(including replacement of oil seals if required) Monthly 9. Taking tong tester readings during peak load hours and remedial action Quarterly 10 . Noting down neutral currents and load balancing in all the three phases Quarterly 11 . Measurement of IR values Half yearly 12 . Testing of oil for BDV, acidity Half yearly 13 . Checking of lightning arrestors and replacement if required (once before monsoon) Half yearly
- 55. Maintenance Schedule of Distribution Transformer 14 . Measurement of earth resistance, checking of earthing system and rectification, if required Half yearly 15 . Overhaul of transformer Once in 5 years
- 56. COMMON DEFECTS NOTICED AND THE CAUSES S.L No Part Defects Causes 1. Tank a. Leakage of oil b. Deformati on c. Overheati ng Corrosion/mechanical damage- Gaskets worn out excessive internal pressure-Improper circulation of cooling oil and /or inadequate ventilation. 2. Radiators a. Leakage of oil b. Deformati on c. Overheati ng Corrosion/mechanical damage- Gaskets worn out excessive internal pressure-Improper circulation of cooling oil and /or inadequate ventilation.
- 57. COMMON DEFECTS NOTICED AND THE CAUSES S.L No Part Defects Causes 3. Conservat or a. Leakage of oil b. Deformati on c. Overheati ng Corrosion/mechanical damage- Gaskets worn out excessive internal pressure-Improper circulation of cooling oil and /or inadequate ventilation. 4. Breather Ineffective Inlet choked-Silicagel saturated 5. Explosion vent Glass Broken Mechanical
- 58. COMMON DEFECTS NOTICED AND THE CAUSES S.L No Part Defects Causes 6. Core a. Loose b. Increased Losses c. Excessive Noise Bolts loosening up-change in characteristics due to heating, vibration of stampings 7. Winding a. Short circuited b. Loosening c. Insulation Brittle d. Open circuited Overloading-Air bubbles-loss of insulation-shrinkage displacement, overheating, decomposition, burn out
- 59. COMMON DEFECTS NOTICED AND THE CAUSES S.L No Part Defects Causes 8. Oil a. Discolorati on b. High acidity c. Low BDV d. Sludge Contamination-Increased moisture-Decomposition, chemical action with other parts 9. Terminal Bushing a. Breakage b. Leakage of oil Strain-Gasket worn out-Loose fit
- 60. DISTRIBUTION TRANSFORMER FUSE RATINGS LT MAINS AND CABLES Capacity of Transformer in KVA Full load Amps 11Kv side Current LT side Size of To fuse wire in SWG Size of single crore PVC A1 cable in sq mm Size of LT fuse units to be used 11KV side LT Side For mains For feeders For main(a mps) For feede r (amp s) 1. SØ 10 ---- ---- -- -- 4 25 2. SØ 15 5 25 3. 25 1.31 33.4 38 20 50 1x50(2x25) 3x100 3x10 0
- 61. DISTRIBUTION TRANSFORMER FUSE RATINGS LT MAINS AND CABLES Capacity of Transforme r in KVA Full load Amps 11Kv side Current LT side Size of To fuse wire in SWG Size of single core PVC A1 cable in sq mm Size of LT fuse units to be used 4. 50 2.62 66.7 33 2x20 70 1x70(1x50 +1x25) 3x200 3x10 0 5 63 3.31 84.0 33 2x13 90 1x70 3x200 3x10 0 6. 75 3.94 100.0 33 2x1(1 x14) 95 1x95 (1x70+1x1 50) 3x200 3x20 0 7. 100 5.25 133.5 33 2.16 150 1x150(1x9 5+1x75 3x300 3x30 0 (3x2 00+3 x100 )
- 62. DISTRIBUTION TRANSFORMER FUSE RATINGS LT MAINS AND CABLES Capacity of Transformer in KVA Full load Amps 11Kv side Current LT side Size of To fuse wire in SWG Size of single core PVC A1 cable in sq mm Size of LT fuse units to be used 8. 150/160 6.4 213.4 30 2x14 2x120 2x120(1x1 50+1x95) X300 6x30 0 9. 200 10.5 236.7 28 3x14 2x150 2x130(1x1 50+1x120 6x130 3x30 0 10. 250 13.12 334.4 28 4x14 3x120 1x120 9x300 9x30 0 11. 316 16.53 420.0 20 Note: 2x25 means 2 feeders of 25sq mm Cable
- 63. Permissible Values for Transformer Oil characteristics Sl.N o Characteristic Voltage Class Permissible Values 1. Dielectric Strength across 2.5 mm gap 145 KV and above 72.5 KV to 145KV below 72.5 KV 60 KV (min) 50KV(min) 40KV (min) 2. Moisture Content 145 KV and above 72.5 KV to 145KV below 72.5 KV 15 PPM 20 PPM 25PPM 3. Specific resistivity at 90oc 1.0x1012 ohm Cm (IS 6103) 4. Dielectric dissipation factor (tan δ) at 900C 0.005 (I.S 6262) 5. Interfacial tension 0.030 (I.S 6104) 6. Acidity 0.03 to 0.05 mg KOH/g of oil
- 64. Some of the shortcomings in operation and maintenance of Distributions Transformers: • Transformers oil level is not maintained which may lead to flash over. • Not filtering the oil. • Breaker for L.V. Protection removed or not provided at the time of installation. • Inadequate protection from overloads and short circuits • Non-Maintenance of silica gel breather leading to ingress of moisture. • No factual data is collected on loading of the transformers.
- 65. • Transformer is not properly earthed. • Broken bushings are not replaced. • Unbalanced loading on different phases due to single phasing for load management or otherwise. • Long LT lines, loose LT lines, trees touching LT lines cause frequent short circuits and over load. • Non provision of lightning arrestors or removal of L.As by staff. • Improper HV protection. • Frequent blowing off of H.G. fuses used for HV protection due to improper fitting(including wrong size of fuse wire)and wind action resulting in mechanical damage.
- 66. The power transformers are usually provided with adequate protection and also skilled staff is available to look after the periodical maintenance and day to day operations. Distribution transformers are located in far flung areas of small villages spread through out the country, which do not receive any maintenance. The large scale electrification of villages and massive energisation of agricultural pump sets have resulted in installation of over 17 – 25 lakhs distribution transformers, which have very little or no protection and practically no maintenance. The overloading of the transformer goes unnoticed till the D.T. fails.
- 67. A well planned maintenance schedule will bring down the failure to a great extent. OVERLOADING:- The Distribution transformers are generally designed to carry 10% overload continuously. But it is not advisable. When the load in a D.T.reaches 80% ,it should be strengthened by replacing with higher capacity or new transformer installed. Unbalance and overloading may cause the neutral to be cut. The loads should properly be balanced and overloading strictly avoided.
- 68. INADEQUATE / POOR DESIGN 1. Transformer tank size Inadequate clearances for free circulation of oil 2. Percentage Impedance (Mechanical strength of the coil) Designing the transformers with increased impedance to increase the short circuit withstand capacity. Percentage impedance depends on two factors: (a) size of wire used in H.V. coils (b) Radial distance between H.V. & L.V. coils Impedance affects the short circuit stresses. 3. Improper use of Aluminium Wires 4. Improper use of inter layer papers. 5. Use of inferior quality materials
- 69. The old standards specify that a transformer should clear a short circuit within 2 seconds, if the % impedance is 4 %. But the present standards specify the short circuit withstand time of 1 sec only irrespective of capacity. The Design of a transformer to counteract mechanical stresses under short circuit conditions, ultimately makes it more costly and hence a compromise will have to be struck between the capital cost and the short circuit capability of the equipment.
- 70. INTELLIGENT DTRS (A TECHNOLOGY SOLUTION) • Failures of DTRs are being explained away stating that they are scattered and unattended • Performance of DTRs can still be monitored remotely by retrofitting intelligence • Even load survey is possible for accurate Distribution Planning • Provide an Electronic meter with/without load survey but with remote reading facility
- 71. Obtain instant data of 3Phase currents 3Phase voltages Phase Angle Active power & energy Reactive power Power factor, etc. Where meters have load survey facility data logging and storage for last 30/45/60 days, obtain data remotely Conduct load survey Data very accurate and quickly transferable (portable) (contd..) INTELLIGENT DTRS (A TECHNOLOGY SOLUTION)
- 72. SPECIFICATIONS FOR TRANSFORMER OIL-IS335-1983 Sl.N o CHARACTERISTI CS UNITS LIMIT IS335 1983 1. Appearance Clear free of sediments and Suspende Matter 2. Density at 270C g/cm3 Max. 0.890 3. Kinematic Viscosity 270C mm2/ s Max. 27.0 4. Flash Point(PM) 0C Min. 130 5. Pour point 0C Max. -6 6. Interfacil Tension 270C N/M Min. 0.04 (contd…
- 73. SPECIFICATIONS FOR TRANSFORMER OIL-IS335-1983 Sl.N o CHARACTERISTICS UNITS LIMIT IS335 1983 7. Neutralisation Number mgKOH /g Max. 0.03 8. Water Content Ppm Max. 50 9. Corrosion Sulfur Non corrosive 10. Inhibitor Not Present 11. Breakdown voltage Unfiltered /Filtered KV Min. 30 60 12. Dielectric Dissipation factor tanδ at 90 0C - Max. 0.005 (contd…
- 74. The D.T. Repair and Maintenance centers should be set up wherever necessary for maintenance and rectification of the failed units. The following need to be checked at regular intervals. Connections Fuses Oil level, oil leakage, and condition Bushings, Loadings, Earth resistance Rod gaps Protective equipment –LTCB, LAs, and A.B. Switches etc. Breather and silica gel.
- 75. PREVENTIVE MAINTENANCE: Ensures • Proper Tree clearances • Proper condition of LT lines (like avoiding loose spans, maintaining clearances etc.) • Oil level. • Oil dielectric strength. • Earth Resistance value (Less than 5 ohms) • Effective operation of fuses • No oil leaks • No moisture ingress Switching on and off operations during widely varying load conditions cause disturbed movements in the windings.
- 76. Purchase procedures: SEBs adopt the method of lowest tender for ordering transformers. Vendor rating, by aiming at total minimum cost and not merely the minimum initial cost is to be done. Specification and Tests: To procure high quality, reliable and long life transformers, there is need to tighten the specifications and provide some tests which would result in good quality transformers. Planning of Protection of DTs. Failures due to overloading and inadequate protection are large and REC has suggested the use of CSP transformers and fail safe distribution transformers.