Application of an NLPID controller on a UPFC to improve transient stability of a power system

To put flexible AC transmission systems (FACTS) devices into realistic use, it is necessary to consider the whole picture of the application of FACTS devices. Hence, components-based simulation should be studied, including the whole system design and the power electronics parts, such as switching schemes and converter controls. These issues are essential to the practical usage of FACTS devices. With all these components included, especially the presence of GTO and power electronics devices, the nonlinearity characteristics in the system are dominant. Thus, the acquisition of system state-space equations becomes time consuming and unrealistic. This leads to complexity in the modelling and designing the control mechanism. For example, the conventional A, B, C mechanism-based control theory becomes impractical and less effective. It becomes necessary to seek a new control method, which will be independent of system model. In the paper, a components-based unified-power-flow-controller (UPFC) model is developed. A novel tracker of differential (TD) and a nonlinear PID (NLPID) control system are proposed to be the main control of the UPFC. Digital simulation studies have been conducted and the results show that the proposed NLPID is very effective and efficient in enhancing the transient stability of a power system. In addition, the results also show that the proposed controller is adaptive and robust.