Under what grid condition would a hydro plant’s fast-acting governor be crucial for preventing a cascading failure?

A hydro plant’s fast-acting governor is crucial for preventing a cascading failure during a grid frequency disturbance, specifically during a sudden loss of generation or a large load increase. A governor is a control system that regulates the speed and power output of a turbine. A fast-acting governor can quickly adjust the turbine's wicket gates (or needle valve in a Pelton turbine) to increase or decrease power generation in response to changes in grid frequency. Grid frequency is a measure of the balance between generation and load. When generation is lost (e.g., a large power plant trips offline) or load suddenly increases, the grid frequency will drop. If the frequency drops too low, it can trigger under-frequency relays at other generators and loads, causing them to disconnect from the grid. This further reduces generation and can lead to a cascading failure, where the system collapses in a chain reaction. A fast-acting governor can quickly increase the hydro plant's power output to compensate for the lost generation, helping to arrest the frequency decline and prevent the cascading failure. The governor acts as a primary frequency response mechanism, buying time for slower, secondary control systems to respond and restore the generation-load balance.