Study On Dc Bias And Its Suppression Measures When HVDC System Under Monopolar Ground Operation

With the implementation of the policy of “West-to-East Power Transmission,NorthSouth Mutual Supply,and National Networking”,HVDC transmission systems are widely used due to their advantages in long-distance transmission and power system networking.When the HVDC transmission system is operated during monopole operation,the DC current of the grounding pole to the ground will change the surface potential of the surrounding substation,generate DC current in the substation with line interconnection,trigger the DC bias phenomenon and affect the power grid's safety and stability.At present,domestic and foreign scholars have carried out a series of researches on DC bias phenomenon and its treatment scheme,and proposed a variety of DC bias related calculation and treatment methods.However,there are still some problems in the calculation of DC bias current and the research on suppression measures.In view of the deficiencies of related research,this paper studies the occurrence,influence and governance of DC bias phenomenon.The main contents are as follows:Firstly,a high-voltage direct current transmission system is taken as the research object,and the DC bias phenomenon caused by the monopole operation is studied.And the DC path network model based on the structure characteristics of autotransformer is established.The effects of soil resistivity and grid topology on DC current distribution are further analyzed.Finally,according to the excitation characteristics of transformer under DC bias,the operating conditions of transformer under different DC current are studied.Secondly,the two most important DC bias magnetic treatment methods are studied,and the suppression principle and suppression effect are analyzed.The solution of DC bias magnetic single point treatment and multi-point treatment is proposed.By comparing the effects of different suppression methods and the influence of different treatment schemes on DC current distribution in the power grid,the limitations of current DC bias magnetic treatment schemes are analyzed.Finally,an optimal configuration method of DC bias suppression measures based on NSGA-II algorithm is proposed.And the factors considered in optimizing the configuration problem are analyzed.The DC bias suppression measures are optimally configured by the resistance method and the capacitance method.Based on the optimized configuration effect of resistance method and capacitance method,an optimal configuration scheme of RC mixing is proposed.According to the characteristics of RC mixing method,the variable setting and algorithm operator are improved,the optimal scheme is determined,and the suppression effect of each method is compared to prove its superiority.