π Case Study
Substation Design: 345/138 kV Auto-Transformer Load Flow & Tap Optimization
Excessive circulating current between parallel windings causing 12Β°C above nameplate winding temperature
ποΈ Project Overview
Upgraded interconnection substation linking two RTOs in Midwest with 1200 MVA auto-transformer
π― Challenge
Excessive circulating current between parallel windings causing 12Β°C above nameplate winding temperature
π§ Design Approach
Load flow-driven tap schedule optimization + harmonic-aware OLTC control logic
π Design Diagram
AI-generated project design illustration
π Key Calculations
Circulating Current Due to Tap Mismatch
(ΞTaps Γ V_primary) / (Z_series Γ N)
Result: 412 A
Accounted for 68% of total winding loss
Optimal Tap Delta for Min Loss
argmin Ξ£(IΒ²R + CoreLoss)
Result: Β±0.25%
Reduced hotspot temp by 9.4Β°C
π Results
Winding temperature normalized to 65Β°C rise; extended transformer life by 18 years per IEEE C57.91π‘ Lessons Learned
- β’Auto-transformer circulating currents scale non-linearly with tap difference β small mismatches cause large losses
- β’OLTC algorithms must integrate real-time load flow feedback, not fixed schedules
β Key Takeaways
- 1Auto-transformer circulating currents scale non-linearly with tap difference β small mismatches cause large losses
- 2OLTC algorithms must integrate real-time load flow feedback, not fixed schedules