ACADs (08-006) Covered Keywords Turbine overspeed , turbine overpressure, turbine valves, turbine trip signals Descri

1. Turbine Protection and Control System ACADs (08-006) Covered Keywords Turbine overspeed, turbine overpressure, turbine valves, turbine trip signals Description Supporting Material

2. Purpose 1. Protects turbine against following: a. Overspeed b. Overpressure c. Mechanical damage d. Thermal damage

3. Purpose 2. Most important prevention is for overspeed • Energy of rotating parts of turbine can be disastrous • Turbine rotation held at 1800 rpm when connected to grid (turbine blading moves at 500 mph) c. Turbine load determined by position of control valves d. Inadvertent loss of load at full power with control valves full open 1) Caused by generator breakers opening 2) Turbine tends to accelerate to excessive speeds 3) Protection system closes valves to prevent overspeed 3. Controls position of turbine valves based on load

4. Turbine Valves • Review steam flowpath to turbines 2. Main steam flow to HP turbine • Stop valves (4) • Control valves (4) c. Stop and control valves located on level 2 of turbine building just below front end of HP turbine d. Outlets of stop valves interconnected e. No. 2 stop valve has internal bypass valve

5. Turbine Valves 3. HP turbine exhaust sends two lines to each of four MSRs 4. MSR outlets to LP turbines a. Combined Intermediate Valve (CIV) -- 6 total b. CIVs located on turbine deck level as close to turbine as possible c. CIVs are two valves in one body, separate controls: 1) Intercept valve 2) Intermediate stop valve d. CIVs function to protect turbine from overspeed e. Full open during normal at-power conditions 5. Point out arrangement of turbine valves in relation to HP and LP turbine

6. Turbine Valve Control and Trip System 1. Valves controlled by two 1600 psig hydraulic fluid systems a. EHC -- controls position during normal operation b. ETS -- provides rapid closure of all 20 valves should an emergency arise 2. EHC fluid pressure sent to piston/cylinder arrangement via servo valve to force turbine valve open against spring pressure with displaced fluid going to a drain 3. Separate ETS fluid seats an internal valve disc which blocks EHC fluid from draining beneath the valve piston (surface area below disc larger than above disc)

7. Turbine Valve Control and Trip System 4. If a turbine trip is required during an emergency, a solenoid valve would operate to dump ETS fluid to a drain 5. Once the ETS fluid is dumped, the EHC fluid drains out of cylinder and spring pressure closes valve 6. Some valves are either open or closed (not modulated), therefore, do not have servo valves but instead have solenoid valves to supply pressure to open or dump fluid to close valve 7. All 20 valves close in 0.3 seconds

8. Mechanical Turbine Trip Signals 1. Overspeed trip device a. HP turbine front b. Consists of a separate control shaft c. Consists of unbalanced ring around shaft d. At 110% rated speed (1980 rpm), ring is forced to travel outward 7/16 inch by centrifugal force e. Ring strikes a mechanical trip finger f. Trip finger dumps ETS hydraulic fluid and all 20 valves close in 0.3 seconds 2. Remote trip switch in control room 3. Local trip switch in front of HP turbine

9. Electrical Trip Signals • Reactor trip • Safety injection 3. Low condenser vacuum 4. High SG level 5. MSR high level 6. Low main lube oil pressure 7. Excessive vibration

10. Electrical Trip Signals (cont’d) 8. Excessive thrust bearing wear 9. Loss of generator stator coding 10. High exhaust hood temperature 11. Generator breakers trip 12. Low ETS hydraulic fluid pressure (1100 psig) 13. Control system (EHC) a. Loss of speed signals b. Loss of power to system c. Low EHC fluid pressure (800 psig)

11. EHC System 1. Governs steam flow to turbine 2. Automatically positions turbine valves based on turbine speed and load signals 3. Controls modulating valves and valves which are either open or closed 4. Modulating valves a. Main stop valve No. 2 (internal bypass valve modulated during turbine warm-up) b. Control valves (all four) c. Intercept valves (1, 2, and 3)

12. EHC System 5. Non-modulated valves (open or closed) a. Main stop valves 1, 2, 3, and 4 b. Intermediate stop valves (all six) c. Intercept valves (4, 5, and 6) 6. 47% of all turbine trips caused by EHC System 7. EHC System consist of hydraulics and electronics

13. EHC System 8. Hydraulics a. Located on level 1 of turbine building at east end b. Supplies 1600 psig hydraulic fluid for valve control and ETS c. Reservoir d. EHC pumps e. Accumulators f. EHC coolers g. Transfer and filtering unit

14. Electronics a. Four major subsystems 1) Speed control unit 2) Load control unit 3) Flow valve control unit 4) Standby control unit b. Speed Control Unit 1) Produces speed error based on following: a) Comparison of actual speed to desired dialed in by operator (1800rpm)-used during at-power b) Comparison of desired acceleration rate set by operator to actual acceleration rate-used during startup

15. Electronics b. Speed Control Unit (cont’d) 2) Acceleration rate requires turbine shell temperature above 350F a) SLOW -- 60 rpm/min. for 30 minutes b) MEDIUM -- 90 rpm/min. for 20 minutes c) FAST -- 180 rpm/min. for 10 minutes 3) Speed error sent to load control unit

16. Electronics c. Load Control Unit 1) Supplies flow demand signals to flow control units for stop, control, and combined intermediate valves 2) Demand signals based on following: a) Speed error b) Comparison of first stage pressure c) Comparison of actual load rate to, desired loading rate during load ramps d) Load limiter 3) Turbine runback closes valves for two seconds at 133% per minute rate 4) Failure of turbine runback signal to reset causes 2 second valve closures every 30 seconds until condition clears

17. Electronics d. Flow Control Units • Uses solenoid valves, servo valves, and hydraulics to position turbine valves • Non-modulated valves do not have flow control units 3) Demand position of valve compared to actual position from valve 4) Position error signal sent to servo valves to increase or decrease hydraulic pressure in order to reposition turbine control valve and eliminate the position error

18. Standby control a. Provides capability of manual turbine control upon failure of speed control and/or load control units b. Completely independent from main EHC System c. No loading rate limit

19. Summary A. Purpose of turbine protection system B. Main Turbine Valves 1. Main stop valves 2. Control valves 3. Intercept valves 4. Intermediate valves C. Turbine Valve Control and Trip System D. Mechanical Trip Signals E. Electrical Trip Signals F. EHC System 1. Purpose 2. Hydraulics 3. Electronics a. Speed control unit b. Load control unit and limits c. Flow control unit d. Startup control unit