In this study, a hybrid technique is employed for ameliorating the voltage stability of test system with thyristor controlled series compensator (TCSC) installation. Improved gravitational search algorithm is used to first set the optimal location of TCSC to generate optimal power flow solution under normal and contingency condition while firefly algorithm is utilised to improve voltage stability and TCSC cost sizing, respectively. The proposed technique is implemented in MATLAB working platform and the power flow security and voltage stability is evaluated with IEEE 6, 14 and 30 bus transmission systems. The optimal TCSC placement is governed by new proposed index of apparent power flow index. The performance of the proposed method is deduced to be superior when results are compared with traditional gravitational search algorithm (GSA) and other hybrid methods of fuzzy-GSA and radial basis function neural network techniques.