Electro-Hydraulic Control (EHC) Systems

Same Loop, Different Technology

Module 3.1 established the fundamental control loop every turbine governing system implements: measure speed, compare to reference, adjust valve position. This module covers how modern turbines actually implement that loop — through electro-hydraulic control (EHC) — and briefly contrasts it with the mechanical-hydraulic governors that came before, since the underlying logic is identical even though the technology is completely different.

From Digital Command to Physical Valve Movement

An EHC system's digital controller executes the entire speed/load control loop in software: reading speed feedback, applying droop logic, calculating the target valve position. Because this logic lives in software rather than mechanical linkages, EHC systems can implement far more sophisticated control than mechanical-hydraulic governors ever could — valve sequencing curves, load limiting functions, and multiple simultaneous control modes running together.

That digital output has to become physical motion somehow. The electro-hydraulic servo valve converts the controller's electrical signal into a proportional hydraulic flow — it's the bridge between digital logic and the physical hydraulic system. That hydraulic flow is only possible because of the hydraulic power unit (HPU), which supplies high-pressure hydraulic fluid to the whole system; the digital controller and servo valve only direct where that force goes, they don't generate it. Finally, the valve actuator — a hydraulic cylinder connected to the governor valve stem — converts that hydraulic pressure into the linear force that actually opens or closes the valve, the same governor valve concept from Modules 1.1 and 3.1, now finally physically executed.

Closing the Loop with LVDT Feedback

A Linear Variable Differential Transformer (LVDT) mounted directly on the valve actuator provides continuous, precise electrical feedback of actual valve position back to the digital controller. This creates a nested control structure: the outer loop controls speed and load by commanding a target valve position (Module 3.1's logic), while an inner loop uses LVDT feedback to ensure the actuator actually reaches and holds that commanded position accurately. Two feedback loops are working together here, not just one — speed control at the outer level, position control at the inner level.

Troubleshooting Across the Whole Chain

Because EHC combines digital electronics with hydraulic power and mechanical actuation, real control system problems can originate anywhere in that chain. HPU pressure degradation, hydraulic fluid contamination, or servo valve mechanical wear can all produce symptoms that look, at first glance, like a digital logic or software problem — a valve responding sluggishly or inaccurately doesn't automatically mean the digital controller is at fault.

What Came Before — Mechanical-Hydraulic Governors

Older mechanical-hydraulic governors implemented this same fundamental control loop using spinning flyweights as the mechanical speed sensor, connected through mechanical linkages to hydraulic relays and valves — with droop and other control characteristics built directly into spring tension and linkage geometry rather than software. Some plants still operate units with this older governing technology. Understanding it matters less for its own sake and more because it clarifies exactly what EHC actually replaced: not the underlying control concept, which is unchanged, but the physical means of implementing it.