Critical Clearing time calculation

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For computing, critical clearing time for generating station in a grid network. I am applying bolted fault at generator bus in an iterative manner until the angle of the generator fails to reach its pre-fault value i.e. loses synchronism. The generator speed also reflects the same in the simulation. Any other factor to be considered for evaluation ? How does critical clearing angle comes into picture in this process ? It was also observed that even if generation in the nearest generators is varied in load flow. The ciritical clearing time still remains the same , Any observations in this regard ? Thanks in advance