Selective Modal Analysis (SMA) has been used in the design of WAMS based damping of inter-area oscillations. In conjunction with a Linear Matrix Inequality (LMI) formulation and a polytopic model a single control law can be found to guarantee pole placement for multiple contingencies. The previous attempts have used a model containing all the ε and ω modes, with SMA used to eliminate all other states. In practical applications the resulting system is still too large to use in a polytopic model. This paper presents an algorithm to further reduce the system size to the relevant modes of oscillations. A 16 machine, 68 bus system is used as the test case with PSSs, DC lines and SVCs acting as the control.