Which phenomenon describes the loss of a generating unit or plant after a close-in fault that is not quickly cleared?

The phenomenon that describes the loss of a generating unit or plant after a close-in fault that is not quickly cleared is transient instability. Transient instability occurs when there is a sudden disturbance in the power system, such as a fault, and the system experiences an inability to maintain its stability due to rapid changes in power flow and system conditions. If the fault is not cleared quickly, it may lead to an imbalance in the system, ultimately resulting in the generating unit or plant tripping off to protect itself from damage or unsafe operating conditions.

In the context of power systems, transient instability is particularly critical because it reflects the system's dynamic response during and after a fault. A close-in fault causes immediate disturbances in voltage and current levels, and if these are not addressed quickly, it can cause the generators to lose synchronism and lead to a system collapse.

The other phenomena listed, such as voltage collapse, frequency deviation, and thermal overload, pertain to different aspects of system stability and operation. Voltage collapse typically refers to a gradual deterioration in voltage levels across the system, frequency deviation deals with fluctuations in system frequency usually due to changing loads or generation, and thermal overload refers to conditions where equipment operates beyond its thermal limits due to excessive current, which can lead to