The Improvement of Multi-infeed DC system by MMC-HVDC connection

MMC-HVDC can provide voltage support for the receiving power grids. The Multi-infeed interaction factor (MIIF) between the converter buses can be reduced in a multi-infeed DC system by the connection of the MMC-HVDC system. Consequently, the MIESCR (Multi-infeed Effective Short Circuit Ratio) of the converter bus can be increased and the stability of the multi-infeed DC system can be improved. However, this improvement is related to the operation mode and the connection location of the MMC. Therefore, it is important to study the proper operation mode and the connection location of the MMC. Based on the nodal impedance matrix, the calculation formula which shows the improvement of the MIIF by the MMC-HVDC connection is obtained. From the formula, the best operation mode and connection location of the MMC station is given. Then the correctness of the calculation formula is verified in a typical case. Finally, the simulation results of an actual power grid verify the improvement of the transient stability by the MMC connection.