Design Review
Traditional InterpretationAccording to IEEE a transformer is considered to consist of six components:
1.Windings2.Oil3. Core 4.Bushings5.Tap Changer6.Tank & associated devices (including conservator, preservation system,cooling system)
Typical Functional subsystems Electromagnetic circuit Current carrying circuit Dielectric system Mechanical structure Cooling system Bushings LTC Oil preservation and expansion system Protection and Monitoring
Basic parameters Magnetic core arrangement Windings arrangements Major Insulation Winding / Core clamping Oil/materials compatibility LTC arrangements Cooling Bushings: selection and arrangements Preservation systemSpecific subjects of design review
Specific subjects of design review
Unit Composition and functional purpose Losses Temperature profile Dielectric safety margin / sensitivity to deterioration.Short circuit stabilitySensitive points in the bushings, LTC Controllability and testability
Basic parameters
Basic parameters (nameplates)Rated power at all tapsNo-load rated voltageCurrent valuesWinding connections and vector relationshipTapchangerWinding impedanceInsulation levelTemperature limits
B flux densityS cross sectional area of magnetic circuitf FrequencyVolts per turn
Parameters of geometrically similar transformersFlux density=Const; Current density=Const.
224 MVA, 240 kV730 MVA, 420kV
:
X =I r rated currentW- number of turnsE-volts per- turnH- average height of the windings, =
Leakage Reactance-parameter of winding geometry
2Geometrical parameters of the windings arrangement= f (R1,R2,b1,b2)
Impedance Capacitance-Volt per turn-RatioBasis to advise design feature (geometry)
Equivalent circuit of a transformer
Magnetizing currentActive component
Harmonic content of magnetizing current
The average total iron loss can be decomposed into hysteretic, classical (Eddy current) and excess components.Core losses
Loss relationship
Stray Losses
Core form design
Core constructionSteel LaminationsArrangementJoint patternInsulationClampingGrounding
Flux density at rated voltage and 110%Flux density- maximum valueNo-load current at 100 and 110 % excitationNo-load losses at 100 and 110 % excitationNo-load current harmonics at 100% rated voltage and 110%Core parameters
Core configuration
Core grounding
Winding arrangementArrangementConstructionConductor configurationCooling arrangement
Arrangement
Insulation Level
Major Insulation
Windings arrangementMajor Insulation
Winding ConstructionDisc: continious/plain disc; interleaving; innershielded;Combined continious-interleaving
Helical: multiple conductor strands in a simple helixRectangular or CTC conductor
Layer: turns are wound like thread on a spool;Typically CTC conductors
Shell Form design
Flat pancake coils to make up winding groups,connected in series
The magnetic circuits goes through opening window of the winding groups and around of groupsShells are built with rectangular cross sections
Mechanical support for core and coils:The phases are installed in the tank bottomVarious type of wedging Beams through the window
Shell form.Main features
Typical winding arrangement in shell form autotransformer
Critical Insulation Spaces
First (line) coil:maximum impulse stressesBetween pairs of Coils:impulse & power frequency overvoltageAcross Taps: impulse oscillationPhase to Ground,HV-LV:switching surge; power frequencyPhase-to phase: between leadsDielectric Design
Typical Insulation construstion
Estimation of mechanical margin
Electrodynamic stability of the windings under the action of radial electromagnetic forces;
Conductors strength in bending with axial and radial electromagnetic forces;
Critical tilting forces of windings conductors;Rigidity factors of windings under axial deformations;Rigidity factors of pressing structure and reduced masses of yoke beams;
Electrodynamic stability of windings at axial oscillations,caused by the action of axial electromagnetic forces Assessment of Short-circuit stability performance
Estimation of electrodynamic stability of windings under short-circuit stresses Actual Permissible Critical Average stress from radial forces, MpaResidual (plastic) radial displacement, mmBending moment due to axial forces, NmBending moment due to radial forces, NmThe maximum axial force, kH
Mechanical margin under 3-Phase Short Circuit on LV (generator) Side 417 MVA, 750 kV, GSU
Estimation of dielectric margin
Insulation levelImpulse full wave (BIL)Impulse chipped waveSwitching surge Short duration 1 min testShort duration 2 min testLong duration 1 hourVery Fast Frequency Surge (Resonance)
224 MVA 240/110 kV autotransformer
224 MVA 240/110 kV autotransformer
Dielectric margin off GSU 417 MVA, 750 kV
Dielectric margin of Shunt Reactor 400kV
Temperature profile
Typical thermal modelThermal Model. Natural convection
Mean temperature rise of a coil surface is a function of heat flux density W/m2, width of vertical cooling channels, oil viscosity and ratio of radial duct between coils h and radialdimension of the coil
The mean winding temperature rise above air is a sum of the mean temperature rise of conductor surface above the oil and temperature drop in insulation
Thermal model suggested by IEC
I effect of bottom yoke II - effect of windingIII-oil leaving winding IV mixed oilTemperature profile
Thermal model considering local conductivityForced convection
Temperature profile of winding
Reason for temperature estimation error
Winding time constant considerations
Temperature rise test at the factoryWinding-oil, 3minBulk, 109 min
Design testability
Design factors. diagnostic accessibility Internally grounded electrostatic shield between the windings reduces the sensitivity of tests. A waterproof dielectric in the oil barrier space prevents estimation of water content in pressboard A dielectric material with elevated dielectric losses in the winding support insulation masks the change in the condition of the main insulation. Presence of resistors in the circuit of the core causes distortion of dielectric characteristics
Design factors. diagnostic accessibility Internal connection of tertiary windings and neutral ends of star windings prevents the evaluation of the condition of inter-phase insulation
The sensitivity of detection of hoop buckling by leakage reactance or capacitive measurements reduces with increasing voltage rating (increasing inter-winding gap).
AT monofsico Marelli, serial 40430, 1969Columna;Cambiador de tomas: Bajo carga, trece posicionesImpedancia AT/MT5.12%
AT monofsico Westinghouse, # 7002870; Acorazado ,1981Sin tensin, cinco posiciones Impedancia, % AT/MT = 15,7 %
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