Development of state space model and study of performance characteristics of digital based excitation control system ST4B with single machine connected to infinite bus

Digital based excitation control systems (ECSs) with microprocessors or PCs are coming up very fast and replacing analog controllers for excitation. The ECS model ST4B is a fast responding digital based ECS incorporating proportional integral controls, recently proposed by IEEE. Detailed state space model of ST4B with a single machine connected to infinite bus (SMIB) has been developed to provide a complete description and a thorough insight of the model. SMIB has been modelled in terms of K-constants arrived at based on a detailed analysis by the author (1998). The chosen significant parameters of ECS, voltage regulator gains K_pr and K_ir and time constant T _A, have been varied over a wide range and time response and frequency response characteristics have been derived through simulation and computation, using specially developed programmes along with the software package MATLAB-SIMULINK, and the best values of the parameters arrived at. As K_pr and K_ir increase up to an optimum value, the time responses improve and for higher values of gains the responses tend to become uneven and oscillatory