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Subsynchronous Damping Control (SSDC) compensation damping plays a significant role in power system stability in HVDC system. This paper builds a linear model to analysis SSDC compensation damping characteristics based on an EPRI proposed double-ended DC test system. Calculation results show that electrical damping is obviously relevant to some factors, such as generator capacity, DC system transmission capacity and DC/AC system connection strength. The ratios of the electrical damping to the Unit Interaction Factor (UIF) and other two specific constants, which are part of UIF, don't change significantly. The two constants can be used to indicate characteristics of the electrical damping and represent the `shaft to converter station' electrical damping transfer characteristic. In this paper, the two constants are called Backward Unit Interaction Factor and Forward Unit Interaction Factor. Therefore, electrical damping characteristics can be represented by UIF and the newly proposed two constants.