To deal with the stability and accuracy problem caused by the power interface

an improved damping impedance interface algorithm of power hardware-in-the-loop simulation for modular multilevel converter based high-voltage direct current(MMC-HVDC)transmission systems is proposed. According to the characteristic of the power device in MMC

where off-resistance is generally much larger than on-resistance

the process of the impedance calculation is simplified. And the trapezoidal integral method with absolute stability and high precision is used to disperse the sub-module capacitance to realize highly efficient impedance matching for DIM under normal operating conditions of MMC. According to the characteristic of MMC blocking operation

a real-time impedance matching method for MMC blocking mode is proposed. A delay compensation control method based on Fourier decomposition for reconstructing the voltage signal is proposed to improve the accuracy of the simulation system. A digital simulation system of the two-terminal MMC-HVDC transmission system is developed in PSCAD/EMTDC based on the proposed interface algorithm to simulate different operating conditions. The excellent performance of stability and accuracy is seen in simulation results that show the improved damping impedance interface algorithm is able to keep the hybrid simulation system stable under different kinds of disturbances and the maximum relative error of the active power less than 1. 5%. This work is supported by Science and Technology Development Plan of State Grid Corporation of China(No. SGRIZLKJ〔2015〕457).