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Transformer & OLTC


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Transformer & OLTC

  1. 1. Wednesday, February 17, 2010 Rohit Dave
  2. 2. Maintenance may be defined as the upkeep of the sub station’s electrical equipments in proper working condition and efficient to derive the following : # # # # # # Reliable and efficient operation Optimum utilisation Availability Reduced down time Detection of premature faults Minimizing revenue loss etc. To meet the above requirement, the equipment has to be checked, attended, to trouble-shot and operated under specified conditions.
  3. 3. The need for maintenance During the operational life of the electrical equipment, ageing occurs due to various stresses which is incident on the equipment both under normal and fault conditions and result in deterioration of physical and chemical properties of components making up the electrical equipment. The expected performance can not be obtained from the equipment once it is aged.
  4. 4. The electrical equipment in service are subject to the following stresses : • Electrical stresses • Thermal stresses • Mechanical stresses • Environmental stresses • Combined stresses
  5. 5. Electrical Stress: The Insulation of electrical equipment experience the following voltages : • Continuous normal power frequency rated voltage • Temporary power frequency over voltages due to voltage regulation, Ferranti effect, and long duration power frequency over voltages. • Lightning Impulse Voltage Waves (Surges) • Switching Impulse Waves (Surges)
  6. 6. Thermal Stress Elevated temperatures may be reached during operation due to dielectric losses, increased I2R loss or by heat absorption from surroundings. Temperature may increase abnormally due to sustained short circuit current. The cooling system failure in power transformers also stress the windings thermally.
  7. 7. Mechanical Stress There may be permanent mechanical stress due to improper installation Circuit breakers may experience vibration stress during normal closing/opening and during making / breaking under fault The bus bar vibration in rigid bus bar stresses the bus bar support mechanically
  8. 8. Environmental stress Environmental factors include pollution, Radiation, Humidity, dust particles, moisture etc,. Proximity to chemical industry, sea coast and intentional damage caused by humans contribute to ageing and deterioration Combined stress In most of the electrical equipment, normally some of the stress factors as above will be present
  9. 9. Maintenance Schedules for Power Transformers 1. 2. 3. 4. Checking the Color of silica gel in the breather and also oil level of the oil seal. If silica gel Color changes from blue to pink by 50% the silica gel is to be reconditioned or replaced. Observation of oil levels in (a) main conservator tank (b) OLTC conservator (c) bushings and examining for oil leaks if any from the transformer Daily Visual check for overheating if any at terminal connections (Red hots) and observation for any unusual internal noises. Checking for noice, vibration or any abnormality in cooling fans & oil pumps of power transformers standby pumps & fans are also to be run condition to be observed. Daily in each shift Daily Daily 5. 6. 7. Observation of oil & winding temperatures & recording Visual check of explosion vent diaphragm for any cracks Checking for any water leakage into cooler in case of forced cooling system. Hourly Daily Daily 8. Physical examination of diaphragm of vent pipe for any cracks Monthly 9. Cleaning of bushings, inspect for any cracks or chippings of the porcelain and checking of tightness of clamps and jumpers Monthly 10. Measurement of IR values of transformer with 2.5 KV megger upto 33KV rating and 5.0 KV megger above 33KV rating. Recording of the values specifying the temperature which measurements are taken. Monthly 11. 12. Cleaning of Silicagel breather Checking of temperature alarms by shorting contacts by operating the knob. Monthly Monthly 13. 14. 15. Testing of main tank oil for BDV and moisture content Testing OLTC oil for BDV & moisture content Testing of Bucholtz surge relays & low oil level trips for correct operation Quarterly Quarterly Quarterly 16. Checking auto start of cooling fans and pumps Quarterly
  10. 10. 17. Checking of Bucholtz relay for any gas collection and testing the gas collected 18. Checking of operation of Bucholtz relay by air injection ensuring actuation alaram & trip Noting the oil level in the inspection glass of Bucholtz relay and arresting of oil leakages if any. Checking of all connections on the transformer for tightness such as bushings, tank earth connection Lubricating / Greasing all moving parts of OLTC mechanism Half yearly or during shutdown Monthly Testing of oil for dissolved gas analysis of EHV transformers upto 100KVA capacity Overhauling of oil pumps and their motors also cooling fans & their motors. Once in a year Once in a year 29. 30. 31. Testing of oil in main tank for acidity, tan delta, interface tension specific resisitivity Bushing testing for tan delta Calibration of oil & winding temperature indicators Measurement of magnetizing current at normal tap and extreme taps 32. 33. 34. Measurement of DC winding resistance Turns ratio test at all taps Inspection of OLTC mechanism and contacts its diverter switch 19. 20. 21. 22. 23. 24. 25. 26 27. 28. Quarterly or during fault Quarterly Quarterly or as given in the manufacturers manual Checking of control circuitry, interlocks of oil pumps and cooling fans for auto Half yearly or during start and stop operation at correct temperatures and also for manual operation shutdown Testing of motors, pumps and calibrating pressure gauge Half yearly Pressure testing of oil coolers Half yearly Testing of oil samples for dissolved gas analysis (for 100MVA transformers) Half yearly Once in a year Once in a year Repeats One in a year Once in a year Once in a year Once in a year or number of operation as recommended by manufacturers are completed whichever is
  11. 11. 35. 36. Overhaul of tap changer and mechanism Replacement of oil in OLTC 37. 40. Calibration of thermometers (temperature indicators) and tap position indicator. Remaining old oil in thermometer pockets, cleaning the pockets and filing with new oil. Checking oil in the air cell (for transformers of 100 MVA & above capacity) Bushings partial discharge test and capacitance (EHV transformers) 41 Filtration of oil / replacement of oil and filtration 38. 39. 42. One in a year Once in year or whenever number of operations as recommended by manufacturer are completed whichever is earlier. Yearly Yearly Yearly Yearly Whenever the IR values of transformer are below permissible limits and oil test results require filtration / replacement of oil One in 10 years General overhaul (consisting 1) Inspection of core & winding (2) Through washing of windings (3) Core tightening (4) Check-up of core bolt insulation (5) Replacement of gaskets (6) Overhaul of OLTC
  12. 12. Maintenance Schedule of Distribution Transformers 1. Cleaning of bushings and external surface of tank cooling pipes Monthly 2. Checking of oil levels in the conservator and gauge glass Monthly 3. Checking of silicased in the breather and replacement is necessary Monthly 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 guage shall maintained) Checking of vent pipe diaphragm Checking of terminal loose connections is any and tightening the same Checking for any oil leaks & rectification (including replacement of oil seals if required) Taking long tester reading during peak load hours and remedial action Noting down neutral currents and load balancing in all the three phase Measurement of IR Values Testing of oil for BDV, activity Checking of lightening arrestors and replacement is required once before monsoon) Measurement of earth resistance checking of earthing system and rectification if required. Overhaul of transformer Monthly 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. Monthly Monthly Monthly Quarterly Quarterly Half yearly Half yearly Half yearly Half yearly Once in 5 years
  13. 13. Common Defects noticed and the cause Sl.No Part 1. Tank 2. Radiators 3. Conservator 4. 5. 6. Breather Explosion Core 7. Winding 8. Oil 9. Terminal Bushing Tap Switch 10 a. b. c. a. b. c. d. a. b. c. d. a. b. Defects Leakage of oil Deformation Overheating Leakage of Oil Deformation Overheating Leakage of Oil Deformation Overheating Ineffective Glass broken Loose Increased Losses Excess Noise Short Circuited Loosening Insulation Brittle Open circuited Discoloration High Acidity Low BDV Sludge Breakage Leakage of Oil a. b. c. Inoperative Broken lever Maloperation – Insulation failure–Failure Burnt Contact operation mechanism – overheating Short Circuit a. b. c. a. b. c. a. b. c. Causes Corrosion / mechanical damage – Gaskets worn out excessive internal pressure – Improper circulation of cooling oil and / or inadequate ventilation. Corrosion / mechanical damage – Gaskets worn out excessive internal pressure – Improper circulation of cooling oil and / or inadequate ventilation. Corrosion / mechanical damage – Gaskets worn out excessive internal pressure – Improper circulation of cooling oil and / or inadequate ventilation. Inlet choked – Silica gel saturated Mechanical Bolts loosening up – change in characteristics due to heating vibration of stampings Overloading – Air bubbles – loss of insulation – shrinkage displacement Overheating decomposition burn out. Contamination – Increased moisture Decomposition chemical action with other parts. – Strain – Gasket Worn out – Loose fit. of
  14. 14. MAINTENANCE Approach Strategies Run to failure Maintenance ( Corrective Actions Only ) Time directive maintenance ( Preventive Maintenance ) Repair and restoration of equipment/component that have failed or malfunctioning and are not performing their intended function Periodic and Planned maintenance actions taken to maintain a piece of equipment within the expected operating conditions. It extends the equipment life and is performed prior to equipment failure to prevent it. This includes technical specification surveillance, in service inspection and other regulatory forms of equipment maintenance. The continuous or periodic monitoring and diagnosis in order to forecast component Condition Directive Maintenance degradation so that as needed planned maintenance can be performed prior to ( Predictive ) equipment failure. Not all equipment conditions and failure modes can be monitored therefore, predictive maintenance must be selectively applied.
  15. 15. Time directive maintenance ( Preventive Maintenance ) Inspection of Transformer O/H of OLTC Checking of Protection Circuit DGA Earth Test BDV of Oil Cleaning Working of Fire Protection Equipment Painting Transformer Testing
  16. 16. Run to failure Maintenance ( Corrective Actions Only ) Failure of gasket  Failure of Bushing  Failure of PT / CT  Oil Leakage Burning of Jumpers Winding / insulation Failed OLTC Problem
  17. 17. Condition Directive maintenance ( Predictive ) Based on DGA / Transformer Testing Filtration of Oil Drying of Transformer
  18. 18. During Inspection Unwanted materials Oil Level Silica Jell degraded All indicating lamps Oil Leakage from any Parts of Transformer Painting/Cleaning Required All Doors & inspection window glass Condition of Radiator Hot/Cold
  19. 19. Major Failures  Core Over heating breakdown in core plate insulation leads to circulating currents & usually sparking at fault.  Winding Overheating due to poor joints is common fault in any parts of the electrical circuit. Breakdown of inter-strand insulation results in circulating current causing overheating of insulation and hot spots at point of fault Poor joints
  20. 20. Most common problem observed in OLTC OLTC 1. Mechanical Problem 1. Failure of AC Supply Tripping Motor MOCB LT Cable Fault Motor Failure Failure of declutching switch 2. Mechanical Problem Braking of shear Pin Failure in friction device 2. Electrical Problem Wear & Tear on Fixed/Moving contact Resistance Open Tracking on phase board Jumper burnt Braking of Barrier board
  21. 21. The OLTC provides uninterrupted voltage regulation of transformers under load. The voltage is regulated by changing the voltage ratio. This is done in steps. The transformer is equipped with a tap winding whose tapping are connected with the tap selector of the OLTC
  22. 22. Changing Voltage Ratio a. Linear Voltage Regulation c. Coarse & fine Voltage regulation Step Voltage is 200 Volt/tap b. Reversing Voltage Regulation
  23. 23. Normal Taping from Winding
  24. 24. Linear Voltage Regulation We have two different make OLTC 1. Crompton 1. F33 2. AT 2 CTR
  25. 25. Reversing Voltage Regulation We have different makes OLTC Flange Mounted OLTC Crompton AT In Tank (A) With Pre-Selector Switch (B) Without Pre-Selector Switch Easun NGEF Diverter switch BHEL (A) (B) Pre-Selector Switch
  26. 26. Compartment in OLTC Flange Mounted OLTC Driving Mechanism High Speed Selector switch Selector switch is capable of making & Breaking load in addition to selecting tap A chamber houses the motor and driving mechanism
  27. 27. Compartment in OLTC Top Plate In Tank V type Selector Switch Inside the Main Tank Transmission Shaft Driving Mechanism
  28. 28. Compartment in OLTC In Tank D type Top Plate Diverter switch inner OSR Diverter switch Oil Vessel Pre-Selector Switch Main Tank
  29. 29. Operation in OLTC Pennant Cycle Tap is changing from No.1 to No.2
  30. 30. Operation in OLTC Flag Cycle Normal Main moving contact left fixed contact. Load is carried by resistor R1 R1 leave fixed contact 3, R2 contact fixed contact 4 Load is carried by resistor R2 R2 contact fixed contact 4. R1 contact fixed contact 3 Main moving is floating Tap change is completed
  31. 31. OLTC with Pre-selected switch (NGEF, Esson) Flag Cycle
  32. 32. Parts in OLTC Current Plate Main Shaft Phase Board Resistance Jumpers connected to Tapping Roller Contact Fixed Contact
  33. 33. Barrier Plate Jumper coming from Xmer Resistance Phase Board Fixed Contac Main Moving Contact Roller Contact
  34. 34. Driving Mechanism DSS Counter Raise Lower Contactor Motor Protection Relay Tap indicator Motor
  35. 35. AVR mounted on RTCC Automatic Voltage Regulating Relay (AVR): AVR is used to maintain secondary voltage (11KV) of power transformer by operating primary side (22/33KV) OLTC Tap. Input to AVR is secondary side of 11000/110V Potential transformer directly connected on secondary side of Power Transformer (Circuit PT or Transformer PT). EMCO, NMC, Pradeep and accord make AVRs are in service. Typical settings of AVRs – Auxiliary supply = 110V AC Nominal Voltage = 110V AC (From 11000/110V PT) Raise Operation : V <108V Lower Operation : V >112V Time Delay = 120 Sec.
  36. 36. Directional Sequence Switch (DSS): DSS is cam operated multiple contact switch used to maintain OLTC motor supply to during operation and cut-off after completion of OLTC operation. Switron, Shirke Electricals, Recom make DSS are in service. Typical DSS contacts and their application is as follows.
  37. 37. Electrical Interlocks And Protection: Directional Raise Limit switch – Blocks electrical raise operation when tap at maximum position (Tap15 or 16). Directional Lower Limit switch – Blocks electrical lower operation when tap at minimum position (Tap1). Motor Protection Relay (MPR) – Driving motor is protected for overload by MPR which isolates it under fault condition. Hand Interlock Switch (HIS) – HIS isolates electrical supply during manual crank handle is in. Ensures safety to operating personal. OLTC Timer Scheme – Timer scheme will operate if motor supply is continuous for more than set time (Set time=2*time for one operation) and avoids high/lower voltages during DSS and contactor mal-operation .
  38. 38. Protection for OLTC • OSR • Motor Stuck up Alarm • Motor Supply Failed Alarm OLTC PRV On RTCC
  39. 39. Over hauling of OLTC Washing
  40. 40. Over-hauling of OLTC Cleaning
  41. 41. CONDITION MONITORING OF TRANSFORMER OIL Parameters for condition monitoring of oil service Two ways are available to an operating engineer 1.To make periodic oil tests to establish trends and classify them. 2.To conduct dissolved gas analysis to assess the internal condition of transformers
  43. 43. PHYSICAL CONTAMINATION 1.Dust, fiber, metallic, particles, other solid impurities. 2.Dissolution of varnish. 3.Free and dissolved water. CHEMICAL DETERIORATION Oxidation resulting in acids sludge and polar impurities. CONTAMINATION OF GASES a) Dissolved air from atm. Nitrogen, co2 b) Generated in oil, methane, ethane, acetylene, ethylene etc. Before the oil is put in the transformer, its properties should be fully ensured.
  44. 44. Sample of transformer oil illustrating seven color classification Interpreting Transformer Oil Test Data There is classic relationship between the transformer insulating oil tests neutralization or acid number (NN) and interfacial tension (IFT). Several independent studies have shown that an increase in NN should normal be followed by a characteristic drop in IFT. When test results for a given oil sample do not fall between the range shown on either side of median line, further investigation is necessary.
  46. 46. SERVICE OIL TESTS The service oil tests to be conducted are furnished below: Service oil tests as per IS: 1866 –2000 Sr. No. TESTS INFORMATION PROVIDED BY TESTS 1 Interfacial Tension Sludge present in the oil. 2 Neutralisation Number Acid present in the oil . 3 Moisture content (ppm) Reveals total water content or cellulosic deterioration. 4 Flash point Sudden drop in flash point indicates of unsatisfactory working condition of transformer. 5 Sludge Indicated deterioration. 6 Dielectric Dissipation Reveals presence of moisture, resins, varnishes or their products of oxidation in oil. 7 Dielectric strength Conductive contaminants and moisture present in the oil. 8 Resistivity Indicative of conducting impurities. 9 Dissolved Gas Analysis Reveals ppm of combustible gases dissolved in the oil to assess the internal condition of the transformer.
  47. 47. GROUP III OILS: To categorize under group III, the parameters should be well beyond the limits proposed in table IV. Such oils should be initially filtered under vacuum and temperature to verify whether the properties improve or not. If properties like Dielectric Dissipation Factor, Interfacial Tension do not improve on filtration, then there is a case for oil to be replaced.
  48. 48. Transformer Oil Filtration Transformer oil filtration is carried out with oil filter machine of adequate capacity. The oil filtration plant is designed to remove dissolved moisture, dirt, air and other gases from the transformer oil. This two stage plant operates on the principle of Low Temperature and High Vacuum. This term contains in general: the heatup of oil, removal of solid particles of more than 3 µm size from the oil, vacuum degasification and drying of oil.
  49. 49. Unfiltered oil is taken in to the system through the inlet valve. Then the oil is pumped to the system through positive displacement pump. Preliminary filters protect the pump by entering solid and magnetic particles up to 1 mm size. The oil is heated up to 60°C. After heating the oil passes through the cartridge filters where the particles up to 5 microns are filtered. Then the oil is passed through a specially designed vacuum chamber, where the moisture, air and other gases are extracted from the oil. The vacuum chamber is designed in such a way that the oil is spread out and is allowed to fall by gravity over the media inside, forming a thin film of oil providing a large surface area exposed to vacuum. With this exposure, the dissolved moisture and gases are evacuated to improve the insulating properties of transformer oil.
  50. 50. In the course of oil filtration clean oil in the transformer vessel becomes mixed with the original filling and, consequently, a few cycles are necessary for the oil filling to achieve the performance required. The filtration, however, is carried through for a period until it achieves the parameters necessary (which happens already after four circulations, but sometimes 10 circulations are necessary).
  51. 51. During the filtration solid particles are removed from the transformer oil which entered it as a consequence of wear, chemical and heat decomposition of solid particles, but also by loosened rust and suspended sludges.  During the oil drying it is to reckon with water contained in fixed insulation material. It is known that of the total volume of water in the transformer up to 97 per cent is stored in the wood pulp. When drying a water affected transformer with the help of oil the curve of water removal from the oil is a significantly different one.
  52. 52. Heater Vacuum Chamber Compressor After Filtrations Before Filtrations
  53. 53. Drying of Transformer  This includes reconditioning of transformer windings, insulation, gasket sealing and removal of moisture.  Such overhauling is carried out at site with adequate lifting and handling facility. Dehydration of Core Coil assembly of the transformer. Active part of the transformer is heated and then evacuated for removal of moisture from the insulation. Core coil assembly is heated by hot oil circulation or induction heating system.
  54. 54. Drying of transformers Transformers are dried using the oil-spray method. The principle consists in heating up the transformer by spraying hot transformer oil on the internal parts of the transformer structure. As soon as the required temperature of the active part of transformer is achieved the vacuum degassing follows. Vapours of water are sucked off from the machine at the operating pressure of 5 -10 mbar and the temperature of 60 - 80 °C. Normally the capacity of vacuum pumps is 100 - 500 m3 of gases and vapours per hour, which for the initial efficiency of 30 per cent means that up to 2 liters of water are removed from the transformer in one hour. The efficiency, however, drops gradually down to approx. one tenth of the value, depending on the moisture degree. The process is then stabilized by hot oil which continues to be sprayed into the internal area of the transformer. Since the temperature at the point of spraying decreases considerably due to the evaporation of water, and the propagation of heat in vacuum is heavily constrained, it is necessary to heat up the internal parts of the transformer. This is done by ventilating of the transformer interior and heating up the same with hot oil mist. In such a way about 50 to 100 litres of water are removed from the transformer during the drying. The time necessary to dry out a transformer is about 10 days.
  55. 55. Transformers are dried using the oil-spray method.
  56. 56. Dissolved Gas Analysis Dissolved Gas Analysis is widely accepted as the most reliable tool for the earliest detection of incipient faults in transformers and tap selector units. Hydrocarbon (mineral-based) oils and silicones are used as insulation fluids in transformers because of their high dielectric strength, heat transfer properties and chemical stability. Under normal operating conditions very little decomposition of the dielectric fluid occurs. However, when a thermal or electrical fault develops, dielectric fluid and solid insulation will partially decompose. The low molecular weight decomposition gases include hydrogen, methane, ethane, ethane, acetylene, carbon monoxide and carbon dioxide. These fault gases are soluble in the dielectric fluid. Analysis of the quantity of each of the fault gases present in the fluid allows identification of fault processes such as corona, sparking, overheating and arcing.
  57. 57. DGA ANALYSIS
  58. 58. Structure of insulating oil & fault gases Relative Solubility as a function of Temperature
  59. 59. Flag Point ( Key Gas) Method Gas Normal(<) Abnormal(>) Interpretation H2 150ppm 1500ppm Corona, Arcing CH4 25ppm 80ppm Sparking C2H6 10ppm 35ppm Local Overheating C2H4 20ppm 150ppm Severe Overheating C2H2 15ppm 70ppm Arcing CO 500ppm 1000ppm Severe Overheating CO2 10,000ppm 15,000ppm Severe Overheating
  60. 60. Rogers Ratios
  61. 61. Duval Triangle Diagnostic results using Duval Triangle Method Area Diagnosis PD D1 Partial Discharges D2 T1 T2 High Energy Discharge. T3 Thermal fault greater than 700 deg.C. DT low energy dischargingsparking Thermal Fault (< 300deg.C) Thermal Fault ( 300deg.C – 700deg.C) Indeterminate-thermal fault or electrical discharge
  62. 62. The accuracy of the methods used has been evaluated % Correct Diagnoses % Unresolved Diagnoses % Wrong Diagnoses IEEE Key Gas Method 42 0 58 IEEE Rogers Ratios 62 33 5 Doernenburg Ratios 71 26 3 IEC Basic Gas Ratios 77 15 8 IEC Duval Triangle 96 0 4
  63. 63. CH4/H2 1092/95 11.4 1 C2H6/C H4 576/1092 0.527 0 C2H4/C 2/H6 2033/576 3.526 1 C2H2/C 2H4 0.21/2033 0.0001 0 1 0 1 0 Circulating / Overheated Joint
  64. 64. C2H2/C2H4 33.04/750.76 0.044 0 CH4/H2 530.22/99.6 5.323 2 C2H4/C2H6 750.76/253.24 2.965 1 0 2 1 indicate thermal fault(>700*C);Over heating of Copper
  65. 65. Rogers Ratio 10 1 0 CH4/H2 844.58/482.92 1.75 1 C2H6/CH4 219.3/844.58 0.26 0 C2H4/C2H6 1528.31/219.3 6.97 1 C2H2/C2H4 66.53/1528.31 0.044 0 Circulating currents and / or overheating joints.
  66. 66. Bucholz gas analysis (IS 3638) Colour of gas Identification Colorless Air White Gas of decomposed paper and cloth insulation Yellow Gas of decomposed Wood insulation Grey Gas of overheated oil due to burning of iron Black Gas of decomposed oil due dielectric arc Combustibility Combustible gas indicates decomposed insulation and oil vapour
  67. 67. Chemical test Method 1 Method 2 Solution 1 5 g AgNO3 dissolved in 100 ml of distilled water Aqueous solution of Ammonia Solution 2 A weak solution of Ammonia in water is slowly added to 100 ml of solution 1 until a white ppt which forms first, disappears in mixture 100 mg Palladous chloride (PdCl) dissolved in 100 ml of distilled water Method 1 Observation Identification Both solution clear. No PPT The Gas is Air Solution 1 White ppt turning brown on exposed to sunlight Gas of Oil Dissociation Solution 2 Dark brown ppt Gas of decomposed paper, cotton or wood insulation Both solution Method 2 Observation Identification Both solution clear. No PPT The Gas is Air Solution 1 Brick red ppt Gas of Oil Dissociation Solution 2 Black ppt in 2 to 3 minutes Gas of decomposed paper, cotton or wood insulation Mere darkening Small concentration but positive presence of gas of decomposed solid insulation. Both solution