The paper proposes a Frozen Riccati Equation based approach for the construction of Lyapunov function to present a control law for the terminal voltage regulation in a post-fault transiently stable situation in a multi-machine power system. The test system is modelled in nonlinear singularly perturbed manner. Without linearization or complex solution of Hamiltonian-Jacobian Bellman equation, the gain matrix is obtained using Frozen Riccati Equation method. The positive definite matrix is identified as the quadratic Control Lyapunov function. The existence of this Lyapunov function ensures the stability of the system. IEEE-14 bus system is used as the test system. The performance of the proposed controller for the test system is compared with Linear Quadratic Regulator. The tracking error comparison done using IAE and ISE shows that the suggested controller offers superior performance and at the same time keep nonlinearities of the system intact.