Complex-Network-Guided High-Risk Substation Identification and Targeted Mitigation of Transformer DC Bias in Multi-Infeed UHVDC Receiving-End Grids

In multi-infeed UHVDC receiving-end grids, transformer DC-bias risk is affected not only by the magnitude of grounding-electrode current, but also by the AC-grid topology, substation grounding condition, voltage-level-dependent current limits, and polarity coordination among grounding electrodes. Conventional single-electrode or magnitude-only assessments may therefore fail to identify the substations that require priority mitigation. This paper proposes a full-registry high-risk substation identification and targeted mitigation framework for transformer DC bias in multi-infeed UHVDC receiving-end grids. A field–circuit coupling model is established using the earth-resistivity model, grounding-electrode parameters, substation grounding parameters, transformer winding DC resistances, and AC-grid topology. To address earth-resistivity uncertainty, a measurement-based correction procedure is introduced and verified by an engineering field-measurement case. On this basis, the receiving-end grid is represented as a weighted complex network, and high-risk substations are identified by jointly considering network importance, voltage-dependent limits, multi-mode DC-bias exposure, and over-limit severity. A case study with 898 substations is carried out under four representative grounding-electrode operating modes. The results show that CJ single-electrode operation produces no over-limit substation, whereas YZ single-electrode, same-polarity two-electrode, and opposite-polarity two-electrode operation produce 2, 3, and 2 over-limit substations, respectively. Polarity coordination changes the risk pattern: same-polarity operation mainly aggravates the UHV substations NY UHV and ZMD UHV, whereas opposite-polarity operation relieves them but concentrates the 500 kV risk at SMPP and ZT. Guided by the high-risk ranking, installing 2 Ω neutral-point resistors at only two over-limit 500 kV substations reduces the currents at ZT and SMPP from 5.26 A and 5.68 A to 2.55 A and 0.58 A, respectively, bringing all evaluated substations within their limits and avoiding system-wide retrofitting.