Capstone: Diagnosing a Control System Anomaly

The Scenario

Unit 2 is under AGC control (Module 3.5) and has been asked to increase load. You notice actual load output lagging noticeably behind the AGC-commanded load reference — about 40 seconds slower than the unit's documented normal response. Nothing has alarmed or tripped. This module walks through the diagnostic reasoning to find the actual root cause, applying the concepts covered across this entire track.

Step 1 — Resist the Instinct to Blame the Digital Logic

A sluggish response to an AGC signal looks, at first glance, like it could be a software or digital control problem. But Module 3.2 specifically warned against this instinct: a meaningful share of real EHC issues trace back to the hydraulic side of the system — HPU pressure, fluid condition, actuator mechanical performance — not the digital controller. The correct next step isn't assuming a software fault; it's checking the rest of the control chain systematically.

Step 2 — Rule Out the Digital Control Loop and Protection System

A manual droop response check shows normal behavior matching specification — this rules out a fundamental problem with the speed/load control loop logic from Module 3.1. The protection system shows no active trips, alarms, or abnormal permissive states — per Module 3.4's system independence principle, this confirms the issue isn't the protection system withholding a permissive or asserting an interlock. Two major categories are now eliminated.

Step 3 — Follow the LVDT Discrepancy

Checking individual valve LVDT feedback (Module 3.2) turns up the key finding: valve 3's digital command is correctly set to 100%, but its actual LVDT-reported position sits at only 87%. This is the critical distinction — the digital command itself is correct. The gap is downstream of the command, in the physical actuation chain: something is preventing the actuator from completing its commanded travel.

Checking HPU pressure confirms it: pressure is trending below normal spec. Recall from Module 3.2 that HPU pressure is what supplies the actual force behind valve movement — the digital controller and servo valve only direct where that force goes, they don't generate it. Degraded HPU pressure directly limits how much force is available to fully drive valve 3's actuator to its commanded position.

Step 4 — Understanding the Secondary Symptom

The valve sequencing pattern (Module 3.3) shows valve 4 beginning to open before valve 3 has actually reached fully open — which initially looks like a sequencing logic error. But this is a downstream consequence, not an independent problem: the sequencing curve is likely still commanding the correct pattern based on load reference, but valve 3 physically stalling short of 100% throws off the pattern the sequencing logic assumes it's achieving. One root cause (HPU pressure) is producing two visible symptoms (sluggish AGC response and an apparent sequencing anomaly).

Step 5 — What This Diagnosis Points Toward

The evidence consistently points to HPU pressure degradation as the root cause, with the sluggish AGC response and sequencing anomaly both being downstream consequences rather than separate problems. The appropriate next step is investigating the hydraulic power unit itself — pump condition, relief valve settings, fluid level and condition — rather than digital control logic or valve mechanical inspection, which the evidence doesn't point toward. This is the same kind of correct root-cause identification the Track 1 and Track 2 capstones built toward: following the evidence systematically rather than fixing the first visible symptom.