Hydraulic Governor is basically a speed controller which is used to perform all the functions of turbine control, viz speeding up to rated speed, loading the machine to full load, governing action of loading or unloading during grid frequency variations. As the name specifies it operates on the hydraulic oil pressure difference which is used in older steam turbines before electro hydraulic governor. However in later steam turbine control hydraulic governor is provided in parley with the Electro-Hydraulic governing as a back up control which readily assumes the turbine control in the event of EHC failure.
Block Diagram of Hydraulic governing system
The schematic representation of hydraulic governor control is shown in figure. The major parts of hydraulic governor are Hydraulic speed sensor, Hydraulic speed Governor, Hydraulic convertor (Amplifier) and load shedding relay solenoid. The hydraulic governor is connected with SLLD (start ad load limiting device).
Principle components of Hydraulic governor
Hydraulic Speed Transmitter:
This is the speed sensor component in the hydraulic governor which converts the speed signal in to hydraulic pressure signal i.e. primary oil pressure signal. The normal value of primary oil pressure is in the range of 1.3 to 2.4 kg/cm2.
Starting and load limiting device
This is an initial operating device which is used to open the HP stop valves. It also provides auxiliary trip oil for latching of hydraulic protection devices & Stop valves.
To raise the speed of the turbine up to turbine synchronization speed. And offers load limiting function to hydraulic governing by restricting the speed governor main transfer arm travel.
Hydraulic speed governor:
This is an important component of hydraulic governor which generates the auxiliary secondary oil pressure which adjusts the control valve lift to match the given set speed.
- This controls the speeding up the turbine at synchronization speed and provides fine control of speed (95% t0 102%) during synchronization.
- Controls the machine load as per the setting after synchronization.
- Provides complete loading and unloading of the machine remotely from control room by adjusting the speeder gear.
Load shedding relay solenoid valve
This is used to drain the auxiliary secondary oil circuit which initiates the momentary close of control valves to avoid over speeding of steam turbine or machine when the load shedding relay is activated. When a bulk rejection takes place in power plant then by sensing it load rejection relay activates and opens the solenoid valve which drains the auxiliary secondary oil.
Hydraulic convertor or Hydraulic amplifier
The auxiliary secondary oil pressure (2.3 to 4.3 kg/sec) is not sufficient to operate the control valves. So the auxiliary secondary oil is amplified or converted as Secondary oil which directly fed to the control valve to open/close. This is nothing but hydraulic piston arrangement where the auxiliary secondary pressure acts on pistons which alters the trip oil and generates secondary oil.
Limitations of Hydraulic Governor
- Hydraulic governor is inherently a speed controller which controls the speed of the machine during normal and grid fluctuations. It is most suitable for peak load power plants (Gas and Hydro).
- It is not suitable to base load power plants like nuclear power plants s the nuclear power plant cannot respond immediately to fast changes of load due to the effect of thermal imbalance in nuclear power plant.
- Turbine Stress elevator (TSE) influence which protects the turbine from overstress during undue loading is not incorporated in hydraulic governing system due to absence of electrical signals. TSE can be easily connected to EHC.