Experimental Technology and Management

[Objective] As a key component of ultrahigh-voltage (UHV) transmission systems， the operational reliability of UHV transformers directly affects power grid security， stability， and power quality. With China’s rapid development of hybrid UHV AC/DC power grids and the increasing occurrence of geomagnetic disturbances， the issue of DC bias has become highly important. The resulting surge in core and structural component losses， along with increased local temperature rises， poses a serious threat to grid safety. Therefore， an in-depth investigation into the magneto–thermal behavior of UHV transformers under DC bias conditions is essential. [Methods] This study focuses on a 1000 kV UHV main transformer. A three-dimensional magneto–thermal coupling model is developed based on actual product parameters， including structural components such as the tank， belly plates， tie plates， support plates， footings， and magnetic shields， along with a segmented-frame core. First， the magnetic characteristic curves of the ferromagnetic core material under different DC bias currents are obtained through single-sheet measurements， then corrected and extrapolated for accuracy. Second， under high to medium operating conditions， different levels of DC excitation are applied to the high-voltage side to calculate excitation currents for various DC bias conditions， followed by a comparative analysis of magnetic flux density and loss characteristics of the core and structural components at maximum excitation. Third， the calculated losses under various DC bias levels are directly coupled to the thermal field—ignoring oil flow effects—to perform magneto–thermal simulations and assess temperature distribution and hotspot formation. Finally， to verify the accuracy and reliability of the proposed model and method， experiments are conducted on a 10 kV transformer model to examine excitation currents and loss behavior under different DC bias conditions. [Results] The findings indicate that (1) without DC bias， the excitation current exhibits a symmetric peaked waveform with mainly odd harmonic components; as the DC component increases， the waveform becomes distorted， the amplitude increases significantly， and harmonic components emerge. (2) Introducing a 4 A DC bias current causes a 49.1% increase in core loss compared to the unbiased state. At the same time， the longitudinal leakage flux in the main channel between the high- and medium-voltage windings increases by 27.4%， significantly intensifying eddy current losses in structural parts， especially near winding ends. (3) Magneto–thermal coupling simulations show that with a 4 A DC bias， the overall core temperature rises notably， with the middle core limb experiencing the most significant increase， reaching 66.28 K. Distinct local hotspots appear in structural components， particularly near winding ends. (4) Experimental validation confirms consistent trends in excitation current and a gradual rise in no-load loss， with the growth rate decreasing over time. Although some numerical deviations occur， they remain within acceptable limits， validating the accuracy of the proposed approach. [Conclusions] Simulation and experimental results demonstrate that DC bias causes significant increases in losses and temperature， worsens local overheating， and accelerates insulation aging risks. Therefore， the negative effects of DC bias must be considered in the design， evaluation， and long-term operation of UHV transformers.