This paper investigates the effect of line reclosing on transient stability assessment (TSA) for multi-machine power systems. The critical clearing time (CCT) is selected as the key parameter to estimate the TSA. Line reclosing has a direct effect on increasing the critical clearing time (i.e. improving the relative transient stability) for two-machine equivalent system (using the equal area criterion). However, the effect is not straightforward for a multi-machine system. A three-machine nine-bus system is used to investigate the effect of line reclosing on critical clearing time. The classical model of generators has been used in this study. The characteristics of line reclosure time versus critical clearing time have been studied for 3-phase faults at each end of each line in the power system sample used. The effect is compared with that when line reclosing is not applied. The results show that in some cases, faster line reclosing improves the relative transient stability (increases CCT). However, in some other cases, faster line reclosing may not improve the relative transient stability (decreases CCT) and leaving the line without reclosing may be better than reclosing it at some reclosing times. These cases should be considered in line reclosure time selection. Some explanations for the results are introduced using stability regions and transient energy function method.
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