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 %