An L^₂ gain disturbance attenuation algorithm and a parameter estimation method are investigated for nonlinear thyristor controlled series capacitor(TCSC). The restriction of assumption inequality is eliminated by introducing the minimax method into the dissipative Hamiltonian system. With the test function formed, the worst-case disturbance can be calculated, accordingly the conservativeness brought by conventional disturbance treatment is significantly reduced. Meanwhile, the parameter projection mechanism is used to design adaptive law to enhance the efficiency of the parameter tracking error convergence. By considering the mechanical power perturbation and short-circuit ground fault, simulation results show that the proposed scheme can effectively improve the transient performance of the system.