Load Control Modes
Different Ways of Deciding "How Much Load"
Modules 3.1 through 3.4 covered how the control system executes a load reference target once it has one. This module covers where that target actually comes from — the different modes and mechanisms that determine what the unit's load reference setpoint should be at any given moment, from steady base load through automatic grid-driven adjustments.
Base Load and Load Following
In base load mode, the unit holds a fixed load reference and stays there continuously — no operator or dispatch system is actively moving the setpoint. This doesn't mean the governor is inactive: droop response (Module 3.1) still automatically makes small output adjustments in response to grid frequency deviations, even while the underlying setpoint itself stays fixed.
In load following mode, the load reference is actively moved according to a schedule or dispatch instructions, ramping output up and down over the course of a day to track changing system demand. This puts real additional demands on the unit compared to steady base load — more frequent valve position changes and more thermal cycling on components, connecting directly to Module 1.5's low-cycle fatigue discussion. Load-following duty is tracked differently than steady baseload hours precisely because it accumulates that cycling-related component wear differently.
Primary Frequency Response vs. AGC
Primary frequency response is the automatic, near-instantaneous load change a unit's droop characteristic produces the moment grid frequency deviates from normal — this is droop mode (Module 3.1) actually doing its job during a real event. It happens within seconds, entirely automatically, with no operator action or external signal required — it's the fastest layer of grid frequency support available.
AGC (Automatic Generation Control) operates differently: it's a centralized system run by the grid operator that sends automatic load reference adjustment signals directly to participating units' control systems, coordinating output across many generators over a timescale of minutes to correct frequency and maintain scheduled power interchange between regions. When a unit is under AGC control, an external signal is adjusting its load reference — but the local EHC control system still executes the resulting valve movements through exactly the same logic covered in Modules 3.1–3.3.
Primary frequency response is fast (seconds) and local, happening automatically through droop at the unit level. AGC is slower (minutes) and centrally coordinated across many units. Both matter to grid stability, but at very different timescales and through different mechanisms.
Constraints on Load Reference — Ramp Rate, Limit, and Runback
Regardless of what mode is setting the load reference target, the control system enforces protective constraints on how that target is actually pursued. A load ramp rate limit caps how quickly load reference — and therefore actual output — is allowed to change, protecting against excessive thermal stress from too-rapid load swings, directly connecting to Module 1.5's thermal stress and fatigue concepts. A load limit caps the maximum load reference the system will accept at all, for reasons ranging from degraded auxiliary equipment to temporary steam condition restrictions, overriding whatever a dispatch instruction might otherwise request.
A runback is different from both: it's an automatic, control-system-driven reduction in load reference triggered by a specific developing abnormal condition — a feedwater pump trip, a condenser vacuum problem — reducing load rapidly to protect equipment before the condition potentially worsens into something requiring a full protection trip.
A runback sits in an important middle ground: it's neither routine load control nor a full trip. It's the control system taking automatic corrective action on a developing problem, distinct from the independent protection system's trip logic covered in Module 3.4 — but both exist for the same underlying reason, preventing a worse outcome.