What is the major cause of voltage collapse in a transmission line?

The main reason for stability is that producing is equal to consumption, if in some moment this is not fulfill than we have problem with P-V and Q-f stability so voltage and frequency go out of set limits and we have collapse in system.

Reactive power demand may cause voltage collapse. Also demand is more than generation.

increasing the load is greater then time response of system

system means :generation,transmission and distribution

The major cause of voltage collapse in a transmission line

A blackout in an electric system means that the complete system collapses. Such a blackout affects all electricity consumers in the area. It can originate from several causes. One example is the loss of generation, e.g. the trip of a power plant that causes a mismatch between production and load.  This puts a strain on other generators, resulting in under-frequency in the system while it “catches up”, and may result in the further loss of other generators. Another example is an overload of the transmission system caused by congestion, forcing an overloaded power line to trip, causing increased loading of other lines those results in additional trips, and – in the end – a voltage collapse due to the high impedance in the weakened grid. In general, one initial event that might even is considered as minor, leads to a second event, a third and so forth, with increased stresses on the system which finally collapses. The course of events is an example of the well-known “Domino Effect”.

Heavy Load

Damage of the Generator Shaft

Swing Rotor

the very important and main thing in transmission in the balance of reactive power . if it exceeds it value or it decrease from its value in both the cases the voltage profile will be collapsed. so we have to maintain the proper value of reactive power.

the main cause is load increse specially with long transmission lines and the increase of reactive power demand