Subsynchronous resonance between DFIG-based wind farms and series compensated transmission system has been reported and studied comprehensively, which is named as subsynchronous control interaction (SSCI). Usually, static synchronous compensators (STATCOMs) are installed in wind farms, which can provide quick-response of reactive power requirement in power systems. This paper focuses on the SSCI mitigation optimization of STATCOM with a new proposed supplementary damping controller (SDC). Subsynchronous voltage distributes differently in the system. Therefore, selections of feedback signals and locations of the proposed mitigation algorithm could result in different damping performance. Hence, impacts of these two factors are examined, explained and discussed. Finally, the effectiveness of SDC and optimal selection are verified by simulations with PSCAD/EMTDC.