Q3Power System 2
Question
Q.3. How speed governing system of a steam turbine works? What are the different components used for this purpose? Draw a diagram showing arrangement of this system.
Answer
The speed governing system of a steam turbine senses shaft speed deviation and adjusts steam valve position (via a speed governor, hydraulic amplifier/servomotor, linkage, and steam control valve) to regulate turbine power output and maintain speed/frequency close to its scheduled value, incorporating a deliberate droop characteristic for stable load sharing among parallel generators.
The speed governing system of a steam turbine is a closed-loop feedback control system that continuously senses the turbine-generator shaft's rotational speed and automatically adjusts the steam flow admitted to the turbine to maintain speed (and hence system frequency, since frequency is directly proportional to synchronous speed) close to its scheduled value, while allowing controlled, proportional load sharing among multiple generators operating in parallel.
Speed governor: the primary sensing element (traditionally a centrifugal flyball mechanism, or in modern systems an electronic speed sensor/transducer) that detects the actual turbine shaft speed and compares it against the reference (scheduled) speed corresponding to system frequency, generating an error signal proportional to any speed deviation.
Speed changer / load reference: an operator- or automatic-generation-control-adjustable input that allows the reference speed/load setpoint to be deliberately shifted (used to adjust the generator's output at a given system frequency, or to participate in secondary/tertiary frequency control).
Hydraulic amplifier and servomotor: since the mechanical force available directly from the governor's flyball (or an electronic transducer's output signal) is far too small to directly move the large, high-pressure steam control valve, a hydraulic amplifier (pilot valve and hydraulic servomotor) amplifies this small error signal into the substantial mechanical force needed to reposition the steam valve, with feedback linkage providing local position feedback to ensure stable, proportional valve positioning rather than an on/off bang-bang response.
Linkage mechanism: mechanical (or electro-hydraulic, in modern systems) linkages transmit the governor's speed-error signal to the hydraulic amplifier input and, in turn, connect the servomotor's output to the actual steam control valve stem, with a feedback link providing the proportional (droop) relationship between valve position and speed error that gives the overall speed-governing system its characteristic steady-state droop.
Steam control valve: the final control element that physically regulates the flow rate of high-pressure steam admitted into the turbine, directly determining the turbine's mechanical power output for a given steam condition.
Overall operation: if system load increases (causing generator speed/frequency to momentarily sag below the reference value), the governor detects this speed deficit and, through the hydraulic amplifier and linkage, opens the steam valve further, admitting more steam and increasing turbine mechanical power output to help restore the speed/frequency balance; conversely, a load decrease (causing speed/frequency to rise above reference) causes the governor to close the steam valve somewhat, reducing power output. This governing action is deliberately designed with a small proportional droop characteristic (rather than attempting to hold speed perfectly constant at every load), which is essential for achieving stable, well-defined load sharing among multiple generators operating in parallel on the same system, as discussed in the corresponding droop-sharing numerical problem elsewhere in this paper.