Optimized Efficiency to Minimize Transmission Losses and Reduce Overall System Operating Costs of 1100 kV Converter Transformers for Power Transmission System

Converter transformers are the largest single source of controllable loss in a line-commutated ultra-high-voltage direct-current (UHVDC) converter station, yet they are usually specified with loss-capitalization rules developed for sinusoidally loaded power transformers. This paper develops and applies a three-layer framework that minimizes the transmission losses and the overall operating cost of 1100 kV converter transformers. Layer 1 is an analytical electromagnetic–thermal–economic design model of a single-phase, two-wound-limb unit whose total owning cost (TOC) is minimized by differential evolution and cross-checked by particle swarm optimization over seven variables, including the strand thickness of continuously transposed conductors, the core-steel grade and magnetic tank shunts, subject to impedance, over-excitation, hot-spot, transport-height and 600 t transport-mass limits. Layer 2 is a harmonic-aware loss model that computes the valve-winding spectrum from the commutation overlap at every load and corrects the IEC h 2 eddy-loss law for skin effect. Layer 3 coordinates the firing-angle and on-load tap-changer set-point and stages the coolers for minimum loss over an 8760-h load and ambient profile. Applied to a ± 1100 kV, 12 GW bipole with 611.8 MVA units, the TOC-optimal design cuts rated losses from 1288 kW (conventional design) and 1725 kW (minimum-first-cost design) to 1077 kW, raises full-load efficiency to 99.79% and lowers the TOC by 2.75% and 9.54%, respectively. At station level, the three layers together reduce annual converter-transformer energy losses from 152.0 GWh to 122.9 GWh (− 19.2%), saving about USD 1.75 million and 16.6 kt of CO₂ per year. The study also shows that rated-point loss factors underestimate the annual load-loss energy by 4.4–7.1% and that tap tracking lowers the no-load energy by 11.7%.