Loading & Load Rejection Response
After the Breaker Closes
Module 7.3 ended at synchronization. This module covers what happens next — the loading sequence that brings the unit to normal operation — and the opposite scenario: what happens when load is suddenly and unexpectedly lost.
The Loading Sequence
Initial load pickup happens immediately after the generator breaker closes, transitioning the governing system from purely speed control to a blend of speed and load control — connecting directly to Module 3.1's droop and load reference concepts, now actually being applied as loading begins.
Loading continues at a thermally-limited ramp rate, governed by the same thermal stress considerations that shaped the roll-up sequence (Module 7.3) — ramp rate limits (Module 3.5) apply during loading for the same underlying reason they applied during speed acceleration.
Thermal stress concerns don't end once the unit is synchronized and at rated speed. Increasing steam flow and pressure as load rises still drives temperature changes throughout the turbine, so Module 1.5's thermal stress principles remain directly relevant throughout the entire loading process, not just during roll-up.
Once loading completes, the unit settles into base load / normal operation — the steady-state condition every other module in this course has implicitly assumed as its backdrop, where vibration baselines (Module 4.6), oil condition trending (Module 6.6), and heat rate tracking (Module 1.4) are all continuously assessed.
Load Rejection — The Reverse Scenario
A load rejection occurs when electrical load is suddenly lost, most seriously from a generator breaker opening unexpectedly while producing significant power — removing the load that was previously absorbing the turbine's mechanical output. This is precisely the scenario Module 5.1 identified as uniquely time-critical for overspeed risk: with load suddenly gone, speed can rise very rapidly.
The governing system's speed control loop (Module 3.1) is the first line of defense, rapidly closing governor valves in response to the sudden speed increase to reduce steam flow and limit the overspeed excursion. This is the governor doing exactly the job described in Module 3.1 — responding to a speed deviation from reference — but under far more demanding, time-critical conditions than normal load-following adjustments.
Sudden load loss → rapid speed rise → governor valve closure (Module 3.1) attempting to limit the excursion → outcome depends entirely on whether that response was fast enough to stay below the emergency trip threshold (Module 5.1).
Runback and Recovery
Following a load rejection, the unit typically executes a runback — rapidly reducing to a lower, stable load level — and then either resynchronizes to pick load back up gradually, or, if the overspeed excursion triggered the emergency trip (Module 5.1), the unit trips entirely. Whether the unit recovers via runback or fully trips depends directly on whether the governor's response was sufficient to keep speed below that emergency trip threshold — the practical dividing line between a controlled recoverable event and a full trip.