AI Skill Report Card
Designing Power Transformers
Quick Start13 / 15
Given a transformer specification (power, voltages, vector group, application), produce a full design package:
Input: 250 MVA, 220/110 kV autotransformer, ONAF/ODAF cooling
1. Select topology: 3-leg core, autotransformer winding scheme
2. Maxwell 3D: core + windings → flux density, no-load losses
3. Compute load loss (I²R + eddy + stray) at rated current
4. Fluent: oil flow + winding hot-spot temperature
5. Mechanical: radial/axial forces at through-fault current
6. Insulation: BIL level selection, impulse distribution along winding
7. Output: efficiency, mass, cost estimate, test protocol
Recommendation▾
Add a second worked example with a different transformer type (e.g., instrument CT or furnace transformer) to show methodology variance across types
Workflow14 / 15
Progress:
- Step 1: Select transformer type — power, instrument (CT/VT), or special (furnace, rectifier, phase-shifting)
- Step 2: Electromagnetic design (2D/3D FEM) — core geometry, steel grade, winding arrangement, turns ratio
- Step 3: Loss calculation — no-load (core) loss and load (short-circuit) loss, stray loss in tank/clamps
- Step 4: Thermal analysis — oil/winding gradients, hot-spot temperature, cooling mode (ONAN/ONAF/OFAF/ODAF)
- Step 5: Mechanical strength under short-circuit — radial buckling, axial compression, clamping force
- Step 6: Insulation design — clearances, BIL/BSL, impulse voltage distribution, partial discharge margin
- Step 7: Type and special tests — temperature rise, impulse, induced/applied voltage, short-circuit withstand
Step details
1. Type selection drives everything downstream:
- Power transformer: 2-winding or autotransformer, core-form vs shell-form
- Instrument transformer: CT (burden, accuracy class), VT (capacitive or inductive)
- Special: furnace (high current, low voltage), rectifier (harmonic duty), phase-shifting
2. Electromagnetic design
- Core: select steel (e.g., M165-35A grain-oriented), set flux density ~1.6-1.7 T, stepped-leg cross-section
- Windings: disc winding for HV (better impulse distribution), helical/screw winding for LV (high current)
- FEM (Maxwell/Flux 2D for axisymmetric core, 3D for tank/clamp stray fields) to verify flux density and leakage reactance
3. Loss calculation
- No-load loss: from core steel loss curves × flux density × core weight, corrected for joints/building factor
- Load loss: I²R (DC resistance) + eddy loss in conductors + stray loss in tank/structural steel (from 3D FEM)
- Compare against guaranteed values; iterate conductor transposition (CTC) to limit eddy loss
4. Thermal analysis
- Oil circulation model (natural ONAN or forced ODAF) — CFD in Fluent for duct flow and hot-spot location
- Winding hot-spot temperature rise per IEC 60076-2, target margin below insulation thermal class limit
- Verify top-oil rise and average winding rise against guarantee
5. Mechanical strength
- Compute through-fault current from system impedance
- FEM/analytical: radial forces (hoop stress, buckling), axial forces (end-thrust on clamping structure)
- Verify against short-circuit withstand per IEC 60076-5
6. Insulation coordination
- Select BIL/BSL per system voltage class (IEC 60076-3)
- Model impulse voltage distribution along disc winding (capacitive network) — check entry turns and line-end stress
- Set oil gap clearances, barrier placement, creepage distances
7. Type and special tests
- Routine: resistance, ratio, polarity, impedance, induced/applied voltage, no-load/load loss
- Type: temperature rise, lightning impulse, switching impulse (if applicable)
- Special: short-circuit withstand test, sound level, PD measurement, DGA baseline
Recommendation▾
Include concrete formulas or numeric ranges (e.g., typical hot-spot limits, BIL tables by voltage class) rather than only qualitative guidance
Tools & Standards
- ANSYS Maxwell, Flux 2D/3D (Cedrat) — electromagnetic FEM, leakage field, forces
- COMSOL AC/DC Module — alternative multiphysics EM solver
- FLUENT — oil/air CFD thermal analysis
- TransformerDesign, ELCUT — specialized/lighter EM solvers
- IEC 60076 series (design, loss, insulation, short-circuit) and IEEE C57 — governing standards
Example
Input: Power transformer 250 MVA; 220/110 kV; autotransformer
Process:
- Maxwell: magnetic system — 3-leg core, M165-35A steel
- Windings: HV disc type, LV screw type
- Losses: no-load 85 kW, load 580 kW
- Fluent: oil cooling ODAF, hot-spot verified below class limit
- Impulse: BIL 950 kV, verified entry-turn stress
- Tank: FEM mechanical analysis under short-circuit force
Output: Efficiency 99.65%; mass 180 t; cost $4.2M; type tests passed; delivered to 220 kV substation
Best Practices
- Always iterate EM design and loss calculation together — flux density choice affects both core loss and winding window size
- Use 3D FEM for tank/clamp stray loss — 2D underestimates eddy currents in structural steel near leads
- Design impulse distribution with extra inter-disc capacitance at line entry; this is the most common insulation failure point
- Size cooling with margin for altitude/ambient derating per site conditions
- Cross-check mechanical short-circuit withstand against actual system fault level, not just nameplate impedance assumption
Common Pitfalls
- Don't rely only on 2D axisymmetric FEM for stray loss — tank hot spots require 3D
- Don't ignore conductor transposition — uncompensated eddy loss in high-current LV windings causes local overheating
- Don't set BIL based on voltage class alone — verify actual impulse distribution, especially for autotransformers with tapped windings
- Don't skip short-circuit mechanical verification for repeat/similar designs — small geometry changes shift force distribution significantly
- Don't underestimate oil duct blockage risk in ODAF designs — CFD must check actual duct velocity, not just average flow rate