Trans & Swg maint.ppt
- 3. Cause % of cases Design 36 Manufacturing problem 28 Material defects 13 Poor maintenance 5 Lightning surges 4 Short circuits 2 Components % of cases Winding 29 Terminal 29 Tank and Di-electric fluid 13 Magnetic Circuit 11 Other accessories 5 The main causes of failures of transformers in service (CIGRE Survey)
- 4. Causes % Failures Design defects 35.7 Manufacturing problems 28.6 Material defects 13.1 Transport or storage problems 1.2 In-correct maintenance 4.8 Abnormal overload Less than 1 Over-fluxing 1.2 Lightning 3.6 External short circuit 2.4 Loss of cooling 1.2 Unknown 7.1 main causes of failure as pertaining to our country
- 5. Cause Effect Remedial Measures Failure of yoke bolt in insulation Causes local short circuit in the lamination resulting in intense local eddy currents Insulated yoke bands preferred or yoke bolt insulation should be class ‘B’ insulation or higher. High flux density in core Causes large amount of force at time of switching and repeated switching damage winding insulation Flux density should not exceed 1.9 Tesla at maximum operating voltage Narrow oil duct in winding Results in improper cooling and damages insulation Adequate duct from point of effective cooling Improper transpositions Results in more loss and more heating Adjust the transpositions so that all conductors should have equal reactance Inadequate clearance between phases May result in short circuit Provide adequate clearance as per the voltage class Clamping ring not properly designed May fail during short circuit condition Thickness of clamping ring should be designed such as to withstand short circuit forces Insufficient bracing of leads May fail during short circuit condition Strong supports are required for bracing of leads Radiators not properly designed Result in improper cooling causing higher temperature for oil/windings Proper calculation of radiators is necessary Failure Due to Defective Design
- 6. Cause Effect Remedial Measure Loose winding and improper sizing Result in interturn or interdisc short circuit Proper sizing for keeping winding under clamping condition Burrs on lamination Result in local short circuit and result in heating Burr free condition to be ensured by good manufacturing facility Burrs on spacers and blocks Result in damaging conductor insulation Burr free condition to be ensured by good manufacturing facility Bad brazed joints Damage the conductor insulation and winding may fail Adopt good brazing procedures Metallic parts left over during manufacture May cause partial discharge Better house keeping to ensured Insulation surface contamination Results in insulation failure Cleanliness to be ensured All metal components not earthed Partial discharge may start and oil quality may get affected All metal components are to be properly earthed and this is to be added in check-list Bad and porous welding of transformer tank Result in oil leakage Surface cleanliness to be ensured and adopt good welding procedures Improper drying process Winding and insulation are not fully stabilized due to moisture leading to failure Extensive drying and oil impregnation process should be strictly followed as per voltage class Failure due to manufacturing deficiencies
- 7. Cause Effect Remedial measure Sharp edges in copper conductors Produce partial discharge and damage the conductor insulation The surface finish should be smooth Improper conductor insulation Deteriorate under influence of high voltage stress and damage insulation Check the incoming conductor insulation and also no. of layers for conductor covering Poor oil quality Insulation failure Maintain BDV & PPM as per manufacturer’s recommendations Particles in oil held in suspension Temporary breakdown Maintain oil cleanliness Bare copper for connection Formation of oxidation and sludges Provide enamel coating or paper covering on bare copper Defective accessories like OLTC, Bushing Results in transformer failure These accessories to be procured from well established supplier in view of high service reliability Failure due to defective materials
- 8. Life expectancy of transformer will get diminished through inadequate protection while operating in the abnormal conditions such as: • Sustained overload conditions • Switching surges • Lightning surges • Transferred surges
- 9. Some of the reported failures for transformers have been attributed due to either of the following causes: Failure of the winding insulation due to short circuit stresses Failure of winding insulation due to surge voltages and transient surges Failure of magnetic circuit Failure of OLTC Failure of bushings and other accessories Failure due to poor insulation and poor cooling arrangements
- 10. Failure of tertiary windings generally have been experienced because of: Overstressing and inadequate cooling Improper implementation of protective schemes Frequent switching ON/OFF of capacitive reactive load Improper short circuit withstand capability
- 11. On –load tap changers are the second largest reason for trouble in power transformers after short circuit. The defects in OLTC are of the following type: 1.Burning of transition resistance Burning and damage of rollers and fixed contacts Misalignment of the tap changer assembly Error in time sequence operation Defect in tap changing driving gear i.e. mal operation of limit switches and step-by-step contractors etc.
- 12. What is a circuit breaker? • General definition by the International Electrotechnical Commission (IEC): • “Circuit breakers are mechanical switching devices, capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying for a specified time and breaking currents under specified abnormal circuit conditions such as those of a short circuit. A circuit breaker is usually intended to operate infrequently, although some types are suitable for frequent operation."
- 15. standardised definitions of the main types of switchgear.
- 16. 1 Stem / terminal 2 Twist protection 3 Metal bellows 4 Interrupter lid 5 Shield 6 Ceramic insulator 7 Shield 8 Contacts 9 Stem 10 Interrupter lid Principle structure of a vacuum interrupter 1 2 3 4 5 6 7 8 9 10
- 17. vacuum properties as a breaking medium.
- 19. axial magnetic field contacts
- 20. contacts creating an axial magnetic field.
- 21. a) Arrangement of the ferromagnetic circuit b) Ferromagnetic circuit in open position c) Contact system without slots incorporated in the electrodes d) Incorporated slots in the electrodes e) Principle of the complete hybrid quadrupolar contact system
- 22. contact structures used to create the RMF
- 23. contacts creating a radial magnetic field. The arc obeys electromagnetic laws, therefore it moves from the center to the outside of the “petals”.
- 24. contact structures used to create the RMF (spiral and "contrate").
- 25. types of breaking devices used according to voltage values.
- 30. types of breaking devices used according to voltage values.
- 31. Sulphur hexafluoride fully satisfies the valency requirements of the sulphur molecule. Its molecular structure is octahedral with a fluorine molecule at each apex. The effective collision diameter of the SF6 molecule is 4.77 Å. The six bonds are covalent which accounts for the exceptional stability of this compound. • SF6 can be heated without decomposition to 500°C in the absence of catalytic metals. • SF6 is non-flammable. • Hydrogen, chlorine and oxygen have no action on it. • SF6 is insoluble in water. • It is not attacked by acids. In its pure state SF6 has no toxicity and this is regularly confirmed on new gas prior to delivery, by placing mice in an atmosphere of 80% SF6 and 20% oxygen for a period of 24 hours
- 32. Arc decomposition products In an electric arc, the temperature can reach 15,000 K and a small proportion of SF6 is decomposed. The decomposition products are formed in the presence of: • an electric arc formed by the opening of contacts normally comprising alloys based on tungsten, copper and nickel, containing residual quantities of oxygen and hydrogen, • impurities in the SF6 such as air, CF4 and water vapour, • insulating components comprising plastic materials based on carbon, hydrogen and silica. • other metallic or non-metallic materials from which the equipment constructed.
- 33. • Hydrofluoric acid HF • Carbon dioxide CO2 • Sulphur dioxide SO2 • Carbon tetrafluoride CF4 • Silicon tetrafluoride SiF4 • Thionyl fluoride SOF2 • Sulphuryl fluoride SO2F2 • Sulphur tetrafluoride SF4 • Disulphur decafluoride S2F10. Arc decomposition products
- 34. Certain of these by-products can be toxic, but it is very easy to adsorb most of them using materials such as activated alumina or molecular sieves.Certain also are formed in extremely minute quantities (S2F10). If the adsorbent (molecular sieve or activated alumina) is present in the equipment in sufficient quantity, then the level of corrosion due to SF6 decomposition products (HF in particular) is very slight if not negligible. This is due to the fact that the adsorbents have a very rapid and effective action such that the corrosive gases do not have sufficient time to react with other materials present.
- 36. Chromatogram with absorbent (molecular sieve)
- 37. results of analysis of SF6 from circuit-breakers with and without molecular sieves.
- 38. maximum permitted impurity levels
- 39. comparison of three SF6 decomposition products from power arc
- 40. Definition of TLV – Threshold Limit Value – Potentially toxic gases are assigned a value known as TLV, which is expressed as a concentration in the air, normally in parts per million by volume (ppmv). The TLV is a time- weighted average concentration at which no adverse health effects are expected, for exposure during 8 hours per day, for up to 40 hours per week.
- 41. Assessment of risk to health posed by arced SF6 The level of risk to health due to exposure to used SF6 depends on a number of factors: • the degree of decomposition of the SF6 and the types of decomposition products present, • the of dilution of used SF6 in the local atmosphere, • the time during which an individual is exposed to the atmosphere containing used SF6.
- 42. Assessment of toxicity using SOF2 concentration Although used SF6 contains a multi-component mixture of chemical agents, one particular constituent has been shown to dominate in determining the toxicity. This is the gaseous decomposition product thionyl fluoride SOF2. The dominance of this component results from its high production rate (formed volume in l per arc energy in kJ) relative to those of the other decomposition products, combined with its toxicity rate. The TLV for SOF2 is 1.6 ppmv. SOF2 may further react with water, leading to the generation of sulphur dioxide SO2 and hydrofluoric acid HF: however due to similar concentration and TLV values, the overall toxicity effect is similar, for SOF2 or the products of its hydrolisis.
- 43. Thionyl fluoride SOF2 Sulphuryl fluoride SO2F2 Disulphur decafluoride S2F10 The first two are the most abundant decompositionproducts of arcing in SF6 whereas thelatter is estimated to be the most toxic.To have a toxic effect, a chemical agent must bepresent in sufficient quantity relative to its TLV. The “risk index” in the table gives an indicationof the relative contributions of the three decompositionproducts to overall toxicity. In a typical sample of arced SF6 , the contribution to toxicity due to SOF2 outweighs that due SO2F2 by about200 times, and that due to S2F10 by about10,000 times. S2F10 can clearly be neglected, ascan SO2F2.
- 44. SF6 and the global environment Atmospheric pollutants produced by human activity are divided into two major categories according to their effects: * stratospheric ozone depletion (hole in the ozone layer), * global warming (greenhouse effect). SF6 does not contribute significantly to stratospheric ozone depletion because it contains no chlorine which is the main agent in ozone catalysis, or to the greenhouse effect because quantities present in the atmosphere are very small.
- 52. Double motion of contacts
- 54. Thank You
- 56. The oil in OCBs serves two purposes. It insulates between the phases and between the phases and the ground, and it provides the medium for the extinguishing of the arc. When electric arc is drawn under oil, the arc vaporizes the oil and creates a large bubble that surrounds the arc. The gas inside the bubble is around 80% hydrogen, which impairs ionization. The decomposition of oil into gas requires energy that comes from the heat generated by the arc. The oil surrounding the bubble conducts the heat away from the arc and thus also contributes to deionization of the arc. OCB (Oil Circuit Breaker)
- 57. Figure 12 Dead Tank Oil Circuit Breaker 1 bushing 6 plunger guide 2 oil level indicator 7 arc control device 3 vent 8 resistor 4 current transformer 9 plunger bar 5 dashpot All three phases are installed in the same tank. The tank is made of steel and is grounded. This type of breaker arrangement is called the dead tank construction. The moving contact of each phase of the circuit breaker is mounted on a lift rod of insulating material. There are two breaks per phase during the breaker opening. The arc control pots are fitted over the fixed contacts. Resistors parallel to the breaker contacts may be installed alongside the arc control pots. It is customary and convenient for this type of breakers to mount current transformers in the breaker bushings.
- 58. Vacuum circuit breakers are used mostly for low and medium voltages. Vacuum interrupting heads were developed for up to 36 kV per break. For higher voltages, the interrupters are connected in series. The interrupting chambers of vacuum breakers are made of porcelain and are sealed. They cannot be open for maintenance. The contact life is expected to be about 20 years, provided the vacuum is maintained. Because of the high dielectric strength of vacuum, the interrupters are small. The gap between the contacts is about 1 cm for 15 kV interrupters, 2 mm for 3 kV interrupters.
- 60. Following are some of the important condition based maintenance techniques being adopted for assessing the condition of circuit breaker: Operating timings measurement •Contact resistance measurement •Contact travel measurement •Dew point measurement of SF6 gas/air •Tan delta measurement of grading capacitors •Vibration measurement •Operational lock-out checks •Trip/close coil current measurement •SF6 gas/hydraulic oil •Air leakage monitoring
- 61. Pressure spectrogram • Spectral analysis of the heating volume pressure reveals large fluctuations over the entire bandwidth of the detection system (3 dB point at 30 kHz). • The two vertical lines are due to initiation of high current (left) and spark gap discharge (right).
- 62. Pressure cross correlation • Two pressure sensors are used, displaced 180° in the heating volume. • The cross correlation between those measurements shows both stationary and propagating structures. • Negative timelag: Sensor 1 detects signal first.
- 68. THANK YOU
- 70. Conclusions • Synthetic tests in power lab allow to mimic a real circuit breaker environment • A comprehensive set of diagnostics is used to (i) derive scaling laws of circuit breaker performance and (ii) build an improved physical understanding of the arc interruption processes • On the diagnostics side, we will focus on the testing of new pressure sensors, providing more mechanical stability and a higher frequency response • Links to computational fluid dynamics simulations: – Reproduce observed correlations between pressure sensors using 3D simulations (ongoing) – Quantify connection between fluctuations in the arcing zone and heating volume (future) – Validate turbulence models used in the simulations (future)
- 73. The induction disk unit operates on the same principle as induction motor. The metal disk is mounted on a shaft that can freely rotate. The current coils are fixed. They create magnetic field that induces eddy currents in the metal disk. The magnetic field of the eddy currents interacts with the magnetic field of the stationary coils and produce torque on the disk. The disk and its shaft rotate and bring the moving contact towards the fixed contact into a closed position. The motion of the shaft is opposed by a spring that returns the disk and the moving contact into the open position when the current drops below a preset value. The time to close the contact depends on the contact travel distance which is set by a time dial. The pick-up current is adjustable by selecting current taps on the current coil. The relays are normally available with three ranges of current taps: 0.5 to 2.0 A, 1.5 to 6.0 A, and 4 to 16 A. The time dial has usually positions marked from 0 to 10, where for 0 setting the contact is permanently closed.
- 76. Voltage from a potential transformer is applied to the lower pole and induced into the upper poles. The upper poles induce eddy currents in the disc. The torque is produced by the interaction of the eddy currents and flux from the lower pole. Voltage settings are adjusted by voltage coil taps. The time settings are adjusted by time dial that adjusts the travel distance of the moving contact. The moving contact rotates in the horizontal plane. The return torque is provided by the spring acting on the shaft. Electromechanical relays
- 77. 87B High Impedance Bus Differential Relay 87G Generator Differential Relay 87T Transformer Differential Relay
- 78. Where does one use circuit breakers? 12...24 kV 6000...24000 A 50...500 kA generation transmission distribution transformer HV substation MV substation 72...800 kV 2500...4000 A 25...63 kA 12...40 kV 400...2500 A 20...50 kA 110...660 kV 10...1250 A 25...100 kA LV MV ~ Values above diagram: • Top: System voltage • Center: Rated current • Bottom: Maximal short-circuit current
- 79. Circuit breaker geometry 1. Current flows through contacts 2. Plug is mechanically separated from fingers 3. Arc forms between the separated contacts 4. Arc is extinguished at a current zero (CZ) crossing using a combination of flow and turbulence • Gas: Sulfur hexafluoride (SF6), base pressure 6 bar • Nozzle material: Poly tetra fluoro ethylene (PTFE), i.e. Teflon® • Finger and plug contact material: Copper- Tungsten (20% Cu, 80% W by weight)
- 80. Circuit breaker testing • Weil-Dobke synthetic test circuit: 1. On the left-hand side of the gas circuit breaker (GCB) to be tested is the high current part of the circuit. The current peak is typically 60 kA, frequency 50 Hz. 2. On the right-hand side of the GCB is the high voltage part of the circuit. The voltage peak is typically 30 kV, frequency 1 kHz.
- 81. Pressure band autopower Top right figure: To enable a systematic analysis of the pressure fluctuations, we use the band autopower, i.e. the frequency integrated spectrogram vs. time. For our scaling studies we use the average band autopower amplitude. Bottom left figure: Relative fluctuation level vs. maximum heating volume pressure. The red curve shows the fit δp/p pmax 0.4.
- 82. 1. High current phase Top right figure: Current (blue) and arc voltage (red). The current is terminated by the vacuum circuit breaker (VCB) after two half cycles. The arc voltage displays a positive extinction voltage and a negative re-ignition voltage close to the first CZ crossing. Bottom left figure: Plug travel (blue) and heating volume pressure (red). Contact separation occurs at 5 mm, vplug = 5.5 m/s. The early pressure oscillations are due to travelling waves in the heating volume.
- 83. 2. High voltage phase 1. The spark gap (SG) is fired just before CZ and injects a high frequency current. 2. When the GCB interrupts the injected current, it is stressed by the transient recovery voltage (TRV) oscillating in the high voltage circuit across the GCB. The figure shows a „fail/hold“ sequence: The first CZ is a failure to interrupt, whereas the second CZ is a successful interruption (or hold).
- 84. Circuit breaker performance evaluation Using the empirical scaling formula di/dtlimit = di/dtmeasured (Rmeasured/Rcritical)1/m, where Rmeasured is the arc resistance 500 ns before CZ, m = 2.8 and Rcritical is a constant, one can map di/dtmeasured at holds and fails to di/dtlimit. The figure shows di/dtlimit as a function of heating volume pres- sure. The red curve shows the fit di/dtlimit p1.0. However, we expect a p0.4 scaling based on previous experiments. Additional measurements will be added to our analysis to clarify this issue.
- 85. 230 kV, 15 GVA, SF6 Double Pressure Breaker The principle of operation is similar to the air blast breakers, except that the SF6 gas is not discharged into the atmosphere. A closed circuit completely sealed and self- contained construction is used. The equipment consists of a compressor, a storage container, a blast valve that admits gas to the interrupting chamber, and a filter through which the exhaust gas is returned to the compressor. This is called the double pressure breaker design.
- 88. Following condition assessment techniques may be adopted. Dynamic contact resistance measurement Dew point measurement of SF6 gas. Contact travel measurement Operating timings Tan delta measurement of grading capacitors Trip/close coil currents measurement SF6 gas/hydraulic oil/air leakage monitoring.