The calculation and suppression of the AC Grid DC bias current under the monopole operation of UHVDC system

This paper take the receiving-end power grid of +/- 800kV Xiluodu-Zhexi (Xizhe) UHVDC project as an example, According to the topological structure of Zhejiang power grid and wide-earth model, a DC bias model of Xizhe UHVDC receiving-end power grid was established based on field-circuit coupling method. The scope considered in this simulation model: regard the grounding electrode as the center, for substation of 500kV and 220kV with earthed neutrals within 100km and substation of 110kV with earthed neutrals within 50km, the simulate calculation model is constructed. The soil structure was measured by Magneto-telluric method. Based on the measured data, the soil structure was considered as a horizontal multi-layer soil model. An experiment was conducted to test the validity of simulation model: measure the DC current flow through neutral point of each transformer of all sites while Jinhua grounding electrode was working in monopole operation and ground current was 500A DC. Experiment data shows that theoretical analysis results consistent with the experimental results. Suppression strategy was proposed and proved to be effective.