Roll-Up, Critical Speeds & Synchronization
The Sequence Itself
With every permissive from Module 7.2 satisfied, the actual roll-up sequence begins — the process of accelerating from turning gear speed to full rated speed and connecting to the grid. This module walks through that sequence in detail, drawing together vibration diagnostics, thermal stress management, and control system concepts from across the entire course.
From Turning Gear to Rated Speed
Initial roll-off turning gear begins once steam admission starts, transitioning the rotor from the slow, continuous rotation of turning gear (Module 2.6) toward full operating speed — the same auxiliary oil pump that supported turning gear (Module 6.2) now supports this broader acceleration phase.
As speed climbs, the rotor must pass through critical speed — the resonant frequency introduced in Module 2.1 and covered in vibration diagnostic detail in Module 4.2. Vibration amplitude naturally rises as the rotor briefly operates near resonance. This is the same rise-and-fall amplitude pattern described in Module 4.2's Bode plot discussion, now being managed live during an actual startup rather than analyzed after the fact.
Not every vibration rise during startup indicates a problem. A temporary amplitude increase while passing through critical speed is expected and normal (Module 4.2) — the actual concern would be if amplitude were abnormally high, or failed to drop back down after passing through.
For cold and warm starts particularly, acceleration pauses at defined rated speed hold points, allowing rotor and casing metal temperatures to equalize before continuing — directly implementing the automatic startup speed hold concept from Module 3.6. These holds are driven by the same thermal stress evaluator logic covered there, essentially waiting for the rotor's internal temperature gradient to relax (Module 1.5) before it's safe to keep accelerating.
Synchronization — A Fundamentally Different Operating Regime
Once at rated speed and stable, synchronization matches frequency, voltage, and phase angle precisely before closing the generator breaker — the automatic synchronization concept from Module 3.6 put into practice. This moment marks the turbine's transition from an isolated, speed-controlled machine to one electrically connected to the grid; after this point, grid frequency itself becomes a major influence on the unit's behavior, a fundamentally different regime than everything before it.
Managing the Critical Speed Transit
Critical speed is deliberately passed through with a rapid, non-lingering transit rather than approached slowly or held at, even though the amplitude rise itself is expected and normal. Sustained near-resonant operation is the exact mechanism behind oil whip's danger (Module 4.5) — rapid transit minimizes exposure to that risk even during a routine, expected event.
Throughout this transit, vibration monitoring confirms the rotor is behaving exactly as expected — a live, real-time application of Module 4.1's frequency analysis and Module 4.2's Bode plot concepts, watching for the expected pattern rather than an abnormal or sustained excessive amplitude.
Critical speed at rest (Module 2.1) → Bode plot identification technique (Module 4.2) → live monitoring during actual transit (this module) → rapid pass-through to avoid oil whip risk (Module 4.5). The same concept appears across four different modules, each adding a layer of practical application.
Synchronization itself requires precision within tight sync window tolerances for frequency, voltage, and phase angle — closing the breaker outside these tolerances risks severe electrical and mechanical shock to both generator and grid. This precision requirement is exactly why automatic synchronization systems (Module 3.6) are generally preferred over manual synchronization for routine starts — matching these tolerances consistently and reliably is difficult to achieve through manual operator judgment alone.