Transient Stability Analysis of a System of Two Machines Connected to an Infinite Bus

Abstract The term large disturbance is often used in place of system collapse. It is the process by which the series of events accompanying voltage instability, frequency instability, and transient instability leads to a blackout or abnormally low voltages in a significant part of the power system. Large disturbance may be due to tripping of the lines on an account of fault or increase in load than the available supply. Transient stability deals with the effect of large and sudden disturbances in the system, while steady-state Stability deals with the effect of small disturbances in the system. Small disturbance can be a change in the gain of the automatic voltage regulator in the excitation system of a large generating unit. This paper studies the effect of a large disturbance in a power system. The system is represented by a two-machine system connected to an infinite bus. This study involves the effects of the fault on the speed of the rotor of the synchronous generator. Two cases were investigated: firstly, when fault occurred on a bus, and secondly, when fault occurred on the middle of a line. The conditions of the network before, during and after the fault are established in this paper. The equal area criterion was applied to the swing curves obtained to predict the critical clearing time of the system. The critical clearing time was found to lie between 0.20s and 0.225s if the fault occurred on the bus, and lies between 0.245s and 0.355s if the fault occurred at the middle of a line. Thus the system is more transiently stable in the second case than in the first. Machine 1 was less stable in the first case, while machine 2 was less stable in the second case. The critical clearing time is essential to the design of proper relaying schemes for fault clearing.