If two 250 mile long transmission lines are exporting excessive MW and one trips, what is the most likely cause of system collapse?

The situation described involves two long transmission lines exporting excessive megawatts (MW), and if one of these lines trips, the system is likely to experience angle instability.

Angle instability typically refers to the system’s inability to maintain the synchronized operation of various generators and their associated loads after a disturbance. In this case, the tripping of one line reduces the capacity for power transfer which can lead to a mismatch between generation and load. If too much power is being exported and one line fails, the remaining line may become overloaded and unable to maintain the necessary angle between the voltages of the generators and the loads. This can cause generators to lose synchronism with the grid, which is a key characteristic of angle instability.

Voltage collapse could occur as a secondary effect if the overall stability of the system deteriorates due to angle instability, but the primary concern in this scenario is the immediate threat posed by losing synchronism. Current overload is also a concern but is a consequence of sustained excessive MW flow, and frequency instability pertains more to generation/load balance rather than the angle between voltage phasors in this context. Thus, angle instability is the most relevant response to the scenario described.