The Impact Of Converter Transformer Marshalling And Receiving-end Grid Structure On DC Bias Current

As a higher voltage level, ± 800kV HVDC has obvious advantages in terms of transmission distance, transmission capacity, economy, etc., but the corresponding DC bias problem is more serious.± 800kV converter station has more converter transformers and the receiving-end grid is more complex. Marshalling of 24 converter transformers and receiving-end grid structural parameters will both impact the size of bias current. This paper presents research of converter transformer marshalling and receiving-end grid structure’s impact on bias current, view to find a method to reduce the bias current or suppression measures, provide security guarantees for converter transformer and AC transformer.Firstly, by analyzing the converter transformer marshalling and operation mode, an analysis model for converter station bias current is established, the bias current of converter station and converter transformer under different marshalling mode is calculated and compared. Secondly, combining the bias current flow path equivalent model, we put forward that the main receiving-end grid factors affecting bias current are distance from substation to grounding electrode, voltage level, length and loop number of the transmission line. Using the control variable method, the impact analysis model of each factor is established. Based on the analysis of each model, we get the influence mechanism of each factor. Then, taking the receiving-end grid of Xiluodu-Zhexi ±800 kV HVDC project as an example, the bias current is calculated, by analyzing the bias current distribution under the influence of each factor, the correctness of each factor’s influence mechanism is verified. Finally, to reduce the bias current of converter transformer and AC transformer, we put forward some methods and measures.This paper makes up the lack of research on converter transformer marshalling and receiving-end grid structure’s impact on bias current, the obtained results explain causes of large bias current in HVDC receiving-end grid. Revealing the influence mechanism of each affecting factor, providing a reference for transformer DC bias risk assessment nearby the grounding electrode and the method to reduce and inhibit bias current.