What is cascaded failure in terms of transmission lines?

A cascading failure is a process in a system of interconnected parts in which the failure of one or few parts can trigger the failure of other parts. Such a failure may happen in many types of systems, including power transmission, computer networking, finance, human body systems, and bridges.
Cascading failures may occur when one part of the system fails. When this happens, other parts must then compensate for the failed component. This in turn overloads these nodes, causing them to fail as well, prompting additional nodes to fail one after another.

Large blackouts are often caused by a series of events called as a cascade failure. Cascade failures are initiated by a disturbance of the energy flow in the system. If a fault occur on a transmission line and its trips (disconnected from the other section) than Failure of transmission line puts additional stress on the on the other part of the system and thus weakens the system.

 Additionally, the trigger event may cause voltage or frequency swings. Increased stress on the network may cause relays to trip other lines in the system. Voltage and frequency swings are dangerous for generator equipment and may cause protection devices to shut down generation. Both types of events cause further instability. Once these failures follow up rapidly after one other, one speaks of a cascading failure. ( cascading means ‘one after the another’ )

The uncontrolled successive loss of system elements triggered by an incident at any location. Cascading results in widespread electric service interruption that cannot be restrained from sequentially spreading beyond an area predetermined by studies.