916 / 2019-04-30 16:39:51

Research on the DC-bias Problem from the Internal Dynamic Behavior of MMC-HVDC

Modular multilevel converter,Asymmetric arm impedance,Asymmetric operation,DC bias

With the extensive construction of HVDC projects, the problem of DC bias becomes more and more serious. Different from the traditional HVDC transmission systems, MMCs have unique topology and control strategy. As a result, when the system is asymmetric, the internal dynamic behavior will lead to a DC leakage into the AC side. Based on the average switching function model, the bias current is affected by the abnormal fundamental frequency component in the circulating current. In this paper, the asymmetry of the upper and lower bridge arm parameters and the asymmetric operation are considered to analyze the corresponding 1st harmonic in the circulating current。Then, the detailed characteristics of the bias current under the two unbalanced conditons can be achieved. A simulation model is built with PSCAD/EMTDC, which verifies the influence on the AC side of MMCs under the asymmetric conditions.