A D-EHC system and an emergency trip system provided overspeed protection in the original design. The D-EHC system opened a drainpath for the hydraulic fluid in the overspeed protection control header, if the turbine exceeded 103 percent of rated speed. As a result of the loss of fluid pressure in the header, the control and intercept valves closed.
The emergency trip system tripped the turbine if speeds exceeded 110 percent of the rated speed. The emergency trip system has an emergency trip control block, trip solenoid valves, a mechanical overspeed device, three test trip blocks with pressure sensors and test solenoid valves, rotor position pickups, speed sensors, and a test panel. The mechanical overspeed trip device consisted of a spring-loaded trip weight mounted in the rotor extension shaft.
In the new design, the two systems that provide overspeed protection, the D-EHC control system and the electrical overspeed trip system, differ from those in the original.
The new digital electrohydraulic control (D-EHC) has three modes of operation to protect the turbine generator against overspeed. The first is the intercept-valves control function, which initiates closure of the intercept valves when the error between the demand position signal and the actual position signal of the intercept valve exceeds the setpoint. The second mode is load unbalance, which operates at greater than or equal to 30-percent load rejection. It causes all control and intercept valves to close quickly. The third mode is the emergency overspeed trip. All control valves and intercept valves are fully closed quickly by the actuation of each fast-acting solenoid valve of the electrical overspeed trip system.
The electrical overspeed trip system consists of redundant processors, three speed channel circuits, and trip relays. An independent electrical overspeed system replaces the mechanical overspeed device of the original design. A trip of the system opens a drainpath for the hydraulic fluid in the emergency trip supply. A loss of fluid pressure in the trip header causes the main stop and reheat valves to close. Also, a relay trip valve in the connection to the emergency trip supply opens to drop the pressure in the relay emergency trip supply and cause the control and intercept valves to close.