Transformer Design: Economics & Reliability

Transcription

1 Transformer Design: Economics & Reliability Weidmann-ACTI Inc. Technical Conference New Diagnostic Concepts for Better Asset Management November 8 10, 2004 Holiday Inn Capitol Plaza, Sacramento, CA H. Jin Sim Waukesha Electric Systems

2 A lot of good information is provided for your information. We will cover just highlights today. Let s make this interactive! Please stop me and ask questions during the presentation if you have them.

3 Failure Survey Information From ANSI/IEEE C37.91, Table 1 Table 1 summarizes failure statistics for a broad range of transformer failure causes reported by a group of U.S. utilities over a period of years. Failures Number % of Total Number % of Total Number % of Total Winding Tap changer Bushing Terminal board Core Miscellaneous Total

4 Thermal Dielectric Mechanical Operation Issues for Reliability

5 Thermal Cooling Internal Cooling determines Watts/Sq. In Gradient Losses within the winding Surface area of windings exposed to the cooling oil Flow of the oil within the winding (Guided or natural) External Cooling determines Oil Temperature Rise Radiators Fans Pumps Other heat exchangers Ambient temperature Gradient plus Average Oil Temperature rise = Wdg. Temp. Rise Accurate Calculation and Proper Execution determines the ability of the Equipment s reliable operation

6 Thermal Hot-Spot Temperatures are calculated by using; Loss distribution Heat transfer Hot-Spot Temperatures determine; Rate of insulation aging Bubble formation (along with moisture content and gas saturation level) Maximum safe loading level

7 Example of Leakage Field Plot for Hot-spot Temperature Calc. 28 kv, kv 28 kv, kv 28 kv, kv 28 kv, kv

8 Result of the analysis indicating specific needs for improvements Hot-Spot kv Hottest Inner Winding Winding Winding Winding Outer Winding

9 Thermal (Cont.) Insulation Life (Based on Tensile Strength of Paper) 9.80 x x EXP [15000/(θ H + 273)] Follows an adaptation of the Arrhenius reaction rate theory 6 C and 8 C rule (doubling of aging rates)

10 Thermal (Cont.) Normal Insulation Life for well-dried and oxygen-free insulation systems with the Hot-spot temperature maintained at 110 C (See table below) Many transformers last much longer than Normal life since the temperature does not stay this high constantly Basis 50 % retained Tensile Strength of Insulation (Former C57.92 Criterion) 25 % retained Tensile Strength of Insulation 200 retained DP in insulation Interpretation of Distribution Transformer Functional Life test data (Former C57.91 Criterion) Normal Insulation Life Hours Years 65, , , ,

11 Thermal (Cont.) Standards and Specifications ONAN/ONAF/ONAF offers at least the base MVA with all external coolers out of service OFAF (Formerly FOA) does not have base rating If the unit is expected to be loaded beyond the maximum rating, specify the capability required with as much information as available Familiarize with the loading guide (such as IEEE C57.91) and manufacturer s instruction manual Specify maximum loading parameters (Top Oil and Hot-spot temperatures) if the industry guide is not suitable for your applications

12 Dielectric Tools used to analyze stresses and design margin FEA field plots Stressed oil volume theory Cumulative creep curve Volt-time curve Oil Duct theory

13 Dielectric (Cont.) FEA field plots used to develop minimum clearances Minimum Design Margin is acceptable!

14 Dielectric (Cont.) Major insulation design

15 Dielectric (Cont.) Stressed Oil Volume v.s. Breakdown Stress

16 Dielectric (Cont.) Creep Curve

17 Dielectric (Cont.) Volt Time curve Relative strength C B A Insulation Coordination time (µs) Oil or pressboard breakdown relative strength vs time - schematic

18 Dielectric (Cont.) Oil Duct Theory E d The narrower the duct, the higher the average breakdown stress Must be manufacturable Barriers must stay in place through all mechanical stresses (Shipping, operational forces, maintenance, etc.)

19 Dielectric (Cont.) Minimum Design margin is based on process capabilities to remove moisture from the insulation and free of contaminants in the total insulation system Vapor Phase drying is the current state-of-the-art process to achieve consistently uniform dryness Proper application of the insulation theories and consistent execution provides maximum design margins

20 Dielectric (Cont.) Partial Discharge and Design Rules Limit the design stress (E) below the PD inception EHV Weidmann - Partial Discharge-Inception 100 E (kv/mm) Oil Duct Width (mm) Non-Insulated Electrodes Gas-Saturated Oil

21 Dielectric (Cont.) Partial Discharge and Design Rules (creep breakdown curve d c = creep distance in mm)

22 Dielectric (Cont.) Typical requirements for Power Transformers beyond the current industry standards Dissolved Gas in Oil before and after tests Particle counts in oil Moisture content of oil and paper Partial Discharge in µv and pico Coulomb (Some of these will become IEEE requirements soon.)

23 Dielectric (Cont.) Standards and Specification Standards for Class I power transformers do NOT require Impulse testing and one hour long Induced test with RIV and PD measurements Specify if the user feels these are necessary Some manufacturers treat them same as Class II and perform tests Specifying Reduced BIL may lower the initial cost Must do the Insulation Coordination carefully Consider higher steady-state stresses during normal operation Some designers will design what s considered minimum level

24 Mechanical Short-Circuit Force Calculation and Management Tools used for the analysis Leakage Flux Field Classical Force calculations Classical Material strength analysis Failure Mode and Design Margin analysis Model testing to verify Design Margins

25 Leakage Flux Field and Resultant Forces in a Core Form Transformer: Text book (Next slide shows a real life example.) Lorentz force law f = J X B J = current density (SI units are used) B = magnetic field (leakage field) surrounding the coils

26 Failure Modes Studied Radial Forces Inner Winding Forced buckling (Beam bending) Free buckling (Hoop buckling) Outer Winding Hoop tension (Stretching) Relaxation buckling Axial Forces (All windings) Conductor tipping Conductor beam bending Clamping system Vibration Tap 3 N Tap 1 8R HV@ Tap 5 16L Combined Forces Spiral motion Telescoping Examples of Leakage Field Plots

27 Short-Circuit Force Calculation and Management The maximum radial short circuit forces (Fr) is calculated using Fr 6.274* π *( r Fr = maximum radial short circuit force (N). r Lm = medium radius of the LV winding (m). r Hm = medium radius of the HV winding (m). I * N = Ampere*Turn value of the transformer. I SC /I R = ratio of symmetrical short circuit current to rated current defined by (1). K = asymmetrical short circuit factor defined by (3). α = length of leakage path (m) = 0.5 * (L + H) + (a + b + c)/3. The maximum axial short circuit forces (Fa) is calculated using Fa = = 4* Fr * X X Lm + rhm) * 1000 * α [ I * N * ( ISC / IR) * K] Fa = maximum axial short circuit force (N). X = displacement of winding electrical centers (m) = (H + L)/4 + (G H + G L )/4 + Tolerance D = a/2 + b + c/2 2 + D 2 2

28 Mechanical Short-Circuit Force Calculation and Management (Cont.) Classical Material strength specification and analysis

29 Mechanical Short-Circuit Force Calculation and Management (Cont.) Critical Buckling Stress

30 Mechanical Short-Circuit Force Calculation and Management (Cont.) Critical Tipping Stress Conductor Modulus of Elasticity E = 16,000 ksi for Annealed Copper E = 18,000 ksi for Hard Copper E = 9,500 ksi for EC Aluminum Key Spacer Embedding Constant C = 9,000 ksi

31 Mechanical Short-Circuit Force Calculation and Management (Cont.) Model testing to verify Design Margins

32 Mechanical Short-Circuit Force Calculation and Management (Cont.) Failure Mode and Design Margin analysis (Example) We calculate and manage all failure modes DETC Tap 5 LTC Tap 8R HV winding 0.1 DETC Tap 5 LTC Tap 16L HV winding 0.1 Stress DETC Tap 5 LTC Tap 16L DETC Tap 1 LTC Tap 8R HV winding 0.1 Failure Mode Allowable HV Radial Hoop 3,854 13,000 psi Beam Bending 7,110 13,000 psi Axial Tipping 164, ,830 lb LV Radial Buckling 4,655 7,800 psi Beam Bending 20,412 26,000 psi Axial Tipping 155,254 >1,000,000 lb Clamping Structure 139, ,000 lb

33 Mechanical (Cont.) Structural Design Tanks for full vacuum and pressure Corners are formed rather than welded Braces are box-beam style rather than angles Welding both inside and outside Flanged openings are raised bosses Full pressure and leak tested Core clamps Designed to withstand full short-circuit forces Supports all internal components and assemblies

34 Transf. Operational Considerations Transformer Operation Issues for Reliability Loading Switching Protection/Coordination Service Environment Monitoring Maintenance Life Extension

35 Transf. Operational Considerations Transformer Operation Issues for Reliability Loading Limits used for loading? Hot-Spot Temperature? Top Oil Temp.? Other? How are these monitored? Alarm and Trip settings IEEE C57.91 Loading guide for transformers Harmonics Potential overheating of windings and leads IEEE 519 Causes, limits, mitigation IEEE C Recommended Practice for transformers Impact (High magnitude, short duration) loading Needs to be properly designed for thermal and mechanical stress Connected directly to generator? UT/UAT/SST application guide IEEE C57.116

36 Transf. Operational Considerations Transformer Operation Issues for Reliability Loading Summer/Winter Ambient Temperature Profiles Max Top Oil Temp Max Hot Spot Temp Max Loss of Life Higher Current Rating Components

37 Transf. Operational Considerations Transformer Operation Issues for Reliability Loading Harmonics Higher than Normal (<0.05 per unit) User must specify entire harmonic profile Affects temperature rise, eddy and stray losses Larger equivalent KVA

38 Transf. Operational Considerations Transformer Operation Issues for Reliability Loading Impact Loading Mechanical and Thermal Performance Considerations Full Voltage Start Reduced Voltage Start How many starts per hour? Bumpless Transmission with Variable Frequency Drives

39 Transf. Operational Considerations Transformer Operation Issues for Reliability Loading Impact (High current for short duration) loading Transformer MVA > Full Load MVA Transformer MVA > RMS MVA or Combined Starting and Load Cycle Transformer MVA > Starting MVA // C 1000 N Pulse Per Hour C Per Unit Pulse

40 Transf. Operational Considerations Transformer Operation Issues for Reliability Loading GSU Good reference C Guide Wye Delta for UT Delta-Wye for UAT and SST Overexcitation Voltage Regulation considerations Breaker Protection

41 Transf. Operational Considerations Transformer Operation Issues for Reliability Switching Break / Transformer Interaction TRV frequency close to Transformer s natural resonance frequency System parameters (largely the Capacitance) are different today Cables v.s. Overhead wires High series capacitance built into the transformers The industry lacks Understanding of the problem Standards and guides What should we do? IEEE working on the Guide Users and equipment manufacturers work together to mitigate the problem for existing installations Users and manufacturers work together to PREVENT the problem for new installations

42 ENGINEERING ANALYSIS for Br. / Tr. Interaction Circuit Breaker Transformer Lightning Arrester Load Lg Circuit Breaker e3 Recovery Voltage Transients L R Contact e2 Reignition Cg C l L l e1 V bd Lg = Generator Inductance System Equivalent circuit Cg = Generator Capacitance L = Stray Inductance R = Resistance L l = Load Inductance C l = Load Capacitance V bd = Min. Breakdown voltage for Breaker contacts Rignition Current Cycles Arc Interruption Time

43 Transf. Operational Considerations Transformer Operation Issues for Reliability Protection/Coordination ; Voltage Insulation coordination Volt-time characteristics of equipment Surge arrester characteristics Fast-front transient surges in GIS installations Capacitor switching up or down the system Installation of coordinated devices How close are the surge arresters from the equipment being protected? Devices to avoid resonance? Snubber, Reactor, Capacitor, etc System study? EMTP, SPICE, TNA, etc

44 Transf. Operational Considerations Transformer Operation Issues for Reliability Protection/Coordination ; Current Proper selection and setting of the relay protection (C37.91) Directional relays Differential relays Overcurrent relays Impedance relays Proper implementation of protection scheme Trip v.s. Alarm Reclosure settings (Time delay and number of reclosings) Protection Zone and combination of electrical/mechanical/thermal protective relays to determine appropriate action Training operators for newer relays Connections Settings Testing Maintenance

45 Transf. Operational Considerations Transformer Operation Issues for Reliability Protection/Coordination ; Others Mechanical Gas accumulation Gas detection Pressure (Sudden gas, Sudden gas/oil, Sudden oil, Static) Thermal Winding temperature Top oil temperature Fuses or Overcurrent relays Excellent material for system engineers: IEEE C37.91, Guide for Protective Relay Applications to Power Transformers

46 Transf. Operational Considerations Transformer Operation Issues for Reliability Service Environment High altitude Thermal Dielectric Ambient temperatures Below 20 C or above 40 C Many other Unusual service conditions (Next slide)

47 Other unusual service conditions: a) Damaging fumes or vapors, excessive or abrasive dust, explosive mixtures of dust or gases, steam, salt spray, excessive moisture, or dripping water, etc. b) Abnormal vibration, tilting, shock, or seismic conditions. c) Ambient temperatures outside of normal range. d) Unusual transportation or storage conditions. e) Unusual space limitations. f) Unusual maintenance problems. g) Unusual duty or frequency of operation, impact loading. h) Unbalanced ac voltages, or departure of ac system voltages from a substantially sinusoidal waveform. i) Loads involving abnormal harmonic currents such as those that may result where appreciable load currents are controlled by solidstate or similar devices. Such harmonic currents may cause excessive losses and abnormal heating. j) Specified loading conditions (kva outputs, winding load power actors, and winding voltages) associated with multiwinding transformers or autotransformers. k) Excitation exceeding either 110% rated voltage or 110% rated volts per hertz. l) Planned short circuits as a part of regular operating or relaying practice. IEEE/ANSI C , Clause m) Unusual short-circuit application conditions differing from those described as usual in Clause 7. n) Unusual voltage conditions including transient overvoltages, resonance, switching surges, etc., which may require special consideration in insulation design. o) Unusually strong magnetic felds. It should be noted that solar magnetic disturbances may result in the flow of telluric currents in transformer neutrals. p) Large transformers with high-current isolated-phase bus ducts. It should be noted that high-current isolated-phase bus ducts with accompanying strong magnetic fields may cause unanticipated circulating currents in transformer tanks and covers, and in the bus ducts. The losses resulting from these unanticipated currents may result in excessive temperatures when corrective measures are not included in the design. q) Parallel operation. It should be noted that while parallel operation is not unusual, it is desirable that users advise the manufacturer when paralleling with other transformers is planned and identify the transformers involved.

48 Transf. Operational Considerations Transformer Operation Issues for Reliability Monitoring What to monitor? How to monitor? What to do with these information? This is a subject for another two-day long seminar!

49 Transf. Operational Considerations Transformer Operation Issues for Reliability Maintenance Periodic schedule? Preventive? Predictive? Condition-based? User s experience with specific equipment History of specific equipment performance OEM s recommendations (Instruction Manuals)

50 Transf. Operational Considerations Transformer Operation Issues for Reliability Life Extension Evaluation of the Condition Risk Assessment Reconditioning

51 Transf. Operational Considerations Transformer Operation Issues for Reliability Life Extension Evaluation of the Condition External DGA, Oil Quality, Furan Analysis Electrical measurements (PF, TTR, Megger, FRA, Doble Excitation, etc.) Components (Bushings, Arresters, Tap Changers, Conservator, etc.) Monitoring devices (Gages, Relays, Detectors, etc.) Leaks / Rusts Internal Core & Coil inspection Leads Components (BCT s, Tap Changers, Bushing connections, etc.) Insulation materials (Degree of Polymerization)

52 Transf. Operational Considerations Transformer Operation Issues for Reliability Life Extension Risk Assessment Value to User Vintage of the equipment Operational history (Loading and Through faults) Operational environment Availability of Spares (Transformer or parts) Forced or Planned outage?

53 Transf. Operational Considerations Transformer Operation Issues for Reliability Life Extension Reconditioning External Arrester Replacement (old style? MOV?) Auxiliary Cooling replacement (Fans, pumps) Test and replace defective devices (Gauges, PRD, pressure relays) Gaskets Components in control box Oil (Dry out / Recirculation) Internal Core and Coil reclamping Tap Changer maintenance / upgrade Leads (positions, clamping) Electrical and mechanical connections General cleanliness

54 Summary & Conclusions Specifications, Design, Execution of these at OEM, and Verification testing build in robustness Monitoring, Maintenance, Proper implementation of the Operating strategy, and taking appropriate actions in time protect your investment and provide maximum return on that investment

55 Questions?