Improving transient stability of power systems by using passivity-bassed nonlinear STATCOM controller

Abstract

This report presents a novel nonlinear control scheme for designing Static Synchronous Compensators (STATCOM). A passivity-based approach is proposed for designing robust nonlinear STATCOM controller. The mathematical model of STATCOM will be represented by a Euler-Lagrange (EL) system corresponding to a set of EL parameters. The STATCOM modeled in the a–b–c reference frame are first shown to be EL systems whose EL parameters are explicitly identified. The energy-dissipative properties of this model are fully retained under the d-q axis transformation. By employing the Park’s transformation, the differential geometry approach is used to investigate the power system dynamics with considering STATCOM under the synchronous d-q frame. Based on the transformed d-q EL model, passivity-based controllers are then synthesized using the technique of damping injection. Two possible passivity-based feedback designs are discussed, leading to a feasible dynamic current-loop controller. Motivated from the usual power electronics control schemes, the internal dc-bus voltage dynamics are regulated via an outer loop proportional plus integral (PI) controller cascaded to the d-axis current loop. The STATCOM controller based on passivity is obtained with a feedback control law from linear system models. The STATCOM controlled by the proposed passivity-based current control scheme with outer loop PI compensation has the features of enhanced robustness under model uncertainties, decoupled current-loop dynamics, guaranteed zero steady-state error, and asymptotic rejection of constant load disturbance. Digital computer simulation for a large operation point variations have been studied the STATCOM controller design. For analysis of the system performance, the PSCAD/EMTDC programme was applied. Simulation results show that the proposed STATCOM controller can effectively enhance transient stability of the power system even in the presence of large operation point variations.