Calculation and control of DC bias current distribution in an AC power system around a typical ±800 kV DC grounding electrode

When the ultra-high-voltage direct current (UHVDC) transmission system operates in a monopolar ground return mode, an enormous DC current flows into earth via the DC grounding electrode in the converter station and then comes into the AC system around the converter station through the neutral earthed transformers, which will cause DC bias in AC systems and harm the transformers' safe operation. In this study, the DC current distribution in the AC systems around the Linyi converter station of the Shanghaimiao-to-Shandong UHVDC Transmission Project was studied. Based on the direct field-circuit coupled method, the DC bias calculation model was established in CDEGS. By using the model, the DC potential in the earth and the DC current distribution in the AC systems were calculated. Then, the DC current distribution in the AC systems was calculated and analysed after adopting a blocking capacitor and a resistor limiting method in a comprehensive way, and the effect of this controlling method was evaluated further. The research results have some practical engineering guiding significance for solving the DC bias problem of AC systems around the UHVDC grounding electrode.