2x250MW steam turbine interlocks and protections

1. B.PARAMESWARA RAO
ASST.ENGINEER
TSGENCO

3. What is an Interlock?
 An interlock is a feature that makes the state of two
mechanisms or functions mutually dependent.
 It prevents incorrect operation to avoid possible
damage of equipment.
 Interlocks can be considered as start permissives of
any equipment.

4. What is Protection?
 The state of action to prevent possible damages of
any equipment or system.
 It is necessary to safeguard the equipment against
abnormal deviation of process parameters to
unacceptable values .

5. TURBINE PROTECTION SYSTEM
HYDRAULIC
Protection
ELECTRICAL
Trip System
Generator
Mechanical Protection

6.  The function of the turbine protection system is to
trip the turbine by instantly closing stop & control
valves of HP & IP turbine thereby cutting of the
steam supply.
 Trip circuit mainly consists of the following items
Main Trip valves MAX 51 AA005 & MAX 51 AA006
 Two solenoid valves for trip initiation
MAX 51 AA001 & MAX 51 AA002
TURBINE PROTECTION SYSTEM

7.  During the trip initiation
The pistons of Main Trip valves MAX 51 AA005 & MAX 51
AA006 move to downward by the spring force. There by trip
oil circuit is connected to drain and Trip oil pressure is falls to
zero, causing the stop valves and control valves to close by
spring force.
TURBINE TRIP INITIATION

8. Turbine Trip Gear
Remote Trip Solenoid Valves
MAX51 AA 001,002
Main Trip Valves
MAX51 AA 005,006
Drain
Trip oil
Tripoil
Drain

9. 1.Bearing Lube Oil Pressure very Low
(<1.8Kg/cm2)
 The protection acts if the lube oil pressure
decreases to 1.8Kg/cm2 .
 It is incorporated to safeguard the bearing from
overheating.
 The trip signal is initiated by three pressure
switches in 2-out-of-3- logic.

10. 2.Drum Level Very high (>225mm)
• The protection acts if Drum Level raises
to > 225mm.
• carryover of silica in to the turbine causes
scaling on turbine blades.
• The trip signal is initiated by drum level
transmitters in 2-out-of-3- logic with a time
delay of 10sec.

11. 3.MS Temp Very Low (< 460 Deg C)
 Sudden drop of MS temperature leads to
serious thermal shocks which in turn induces
surface stresses on HP turbine thereby reducing
the life of the turbine.
 The protection gets armed after the MS temp
crosses 510 Deg C.
 The trip is acted when MS temp. drops to
460Deg C.

12. 3.MS Temp Very Low (< 460 Deg C)
 The temperature is measured by means of Six
thermocouples and protection criteria is derived
in 2 out of 2 logic.
MS TEMP-
LEFT(>510)
MS TEMP-
RIGHT(>510)
&
PROTECTION
ARMING

13. 3.MS.TEMP V. LOW TRIP LOGIC
MS TEMP-
LEFT(<460)
MS TEMP-
RIGHT <460
&
TRIP TO
TURBINE

14. 4.HP EXHAUST STEAM TEMPERATURE HIGH :
 The HP exhaust steam temperature protection circuit
protects the exhaust sections of the turbine, the blading
and the extraction points against overheating.
 Under extreme operating conditions the HP turbine can
be run at low flow rate and simultaneous relatively high
back pressure. This prevents the steam from expanding,
which causes the exhaust steam temperature to rise.
 The temperature is measured by means of three
thermocouples and protection criteria is derived in 2 out
of 3 logic.
 HPT Exhaust temp High Alarm (>485 Deg C)
 HPT Exhaust temp High Trip (>510 Deg C)

15. 5.Fire protection Operated or M.O.T Level Low
 Tripping of the turbine during the fire hazard can be
initiated either manually by means of fire protection
push buttons 1 &2.
or
 M.O.T level Very low (- 115 mm) .
The trip signal is initiated from 2 out of 3 level
transmitters mounted on Main Oil Tank.

16. 5.Fire Protection Push Buttons on BTG

17. 5.Fire protection Operated or M.O.T Level Low

18. Emergency Shut off valve
GateValve
5.Fire protection Operated or M.O.T Level Low

19. 6.AXIAL SHIFT HIGH/LOW ( ±1 MM) TRIP
• The Measurement is used to monitor the
position of the shaft in the Thrust Bearing as
Axial Shift.
• The turbine is tripped in case, the permissible
limit exceeds beyond 1 mm to avoid the
surface rubbing between stationary and
rotating components.
• The Measuring Range Is -1.5 To + 1.5 MM.
Towards
Generator
Thrust
Bearing
Towards
HPT
PositiveNegative

20. 6.AXIAL SHIFT HIGH/LOW ( ±1 MM) TRIP
3 Nos Axial Shift Sensors
At Bearing No.2

21. 7. Low Vacuum Electrical Trip (< -0.7Kg/cm2)
•The protection acts if the Condenser pressure
falls to -0.7Kg/cm2 .
• it is provided to protect the condenser against
a non-permissible pressure increase.
• The trip signal is initiated by three Vacuum
switches in 2-out-of-3- logic.

22. Condenser
Pressure (S-1)
Above
-0.7 kg/cm2
PROTECTION
ARMING
2/3
Logic
Condenser
Pressure (S-2)
Above
-0.7 kg/cm2
Condenser
Pressure (S-3)
Above
-0.7 kg/cm2
Low Vacuum Condenser Electrical Trip Arming

23. Condenser
Pressure (S-1)
Below
-0.7 kg/cm2
Trip To turbine
2/3
Logic
Condenser
Pressure (S-2)
Below
-0.7 kg/cm2
Condenser
Pressure (S-3)
Below
-0.7 kg/cm2
Low Vacuum Condenser Electrical Trip Protection

24. 8.H2 Cold Gas Temp Very High
 If the cold gas temperature of the Generator increases, it
will increase the stator winding &core temperatures.
 The possible reasons :
Loss of BCW water to the H2 coolers.
• The temperature is measured by means of four RTDs
and protection criteria is derived in 2 out of 2 logic.

25. 8.H2 Cold Gas Temp Very High-Logic
TEMP SIGNAL AT
COOLER-C
AND
TEMP SIGNAL AT
COOLER-D
NORMAL VALUE 35 to 40 DEG C
ALARAM VALUE > 45 DEG C
TRIP VALUE > 50 DEG C
TEMP SIGNAL AT
COOLER-A
AND
TEMP SIGNAL AT
COOLER-B
OR
TRIP TO
TURBINE

26. 9.Hot Air Temp After Main Exciter Very High
 If the Hot air temperature of the Exciter increases, it will
lead to damage of Diode Wheels located in the main
exciter.
 The possible reasons :
Loss of BCW water to the coolers.
• The temperature is measured by means of Two RTDs &
Two Temp. Switches and protection criteria is derived in 2
out of 2 logic.

27. 9.Hot Air Temp After Main Exciter Very High
TEMP SIGNAL
(SWITCH)
AND
TEMP SIGNAL
(SWITCH)
NORMAL VALUE 50 to 55 DEG C
ALARAM VALUE > 75 DEG C
TRIP VALUE > 80 DEG C
TEMP SIGNAL
(RTD)
AND
TEMP SIGNAL
(RTD)
OR
TRIP TO
TURBINE

28. 10.Liquid in Terminal Bushing High
 Liquid in Generator Terminal Bushing high
signifies entry of oil or water due to generator
cooler leakage.
 Due to entry of oil or water, insulation value may
go down and earth fault or stator winding failure
may occur.
 The liquid level is measured by means of Three
level Switches and protection criteria is derived
in 2 out of 3 logic.

29. 10.Liquid in Terminal Bushing High
Arrangement of Level Detectors
Level Detector
Sight Glass
Shutoff Valve

30. Generator Mech. Trip to Turbine (Logic)
Cold Gas Temp High
>50 Deg
Hot Air Temp After
M.Exciter >80 Deg
Liquid in terminal
Bushing Top High
(2 sec Delay)
OR
gate
Generator
Mechanical
Trip to Turbine

31. Unit Trip Interlock Block Diagram
Generator
Trip
Turbine
Trip
Low Forward
power Relay
Reverse
Power
Relay
Boiler
MFT
11.Master Fuel Trip (M.F.T)
12.Generator Electrical protection Operated

32. Turbine Interlocks & Protections
13.Emergency Turbine Trip From Control Room.

34. Over Speed Trip Device
• When ever a preset over speed is reached, the eccentric fly bolt(8) activates the piston(4)
and limit switch(10) via a pawl(7).
•This connects the Auxiliary Trip oil to drain thereby depressurizing it(X to C).
•The loss of Auxiliary Trip medium pressure causes the Main Trip valve to drop, which
in turn causes the trip oil pressure to collapse.

35. TURBINE OVERSPEED TRIP DEVICE
FLY BOLT
Aux Trip Oil
AUX START-UP
OIL TO RESET
AUX TRIP OIL
Drain
TURBINE
OVERSPEED TRIP
DEVICE

36. Low Vacuum Trip device
•The purpose of the low vacuum trip is to
operate the trip valve by draining the aux.
trip fluid whenever the condenser back
pressure increases beyond the permissible
limits.
•In the hydraulic low vacuum trip device, a
compression spring set to a specific tension
pushes downwards against diaphragm, the
topside of which is subject to the vacuum.
•Deterioration of vacuum inside condenser
causes the pilot valve in the low vacuum trip
to be moved from its upper position
downwards by the pretension spring,
resulting in depressurization space of below
right hand valve.
•The right hand valve is moved to the lower
end position by spring, hence opening
auxiliary trip fluid circuit. This causes a
reduction in the pressure of fluid below
differential pistons of main trip valves, and
their tripping by the springs. When trip
fluid circuit is open to drain, all stop and
control valves close.

37. Drain
Control Oil
Atmosphere
Vacuum
Aux Trip Oil
Drain
LOW VACUUM TRIP DEVICE
Resetting Of Low
Vacuum Trip Device
Tripping Of Low
Vacuum Trip Device
Low Vacuum
Trip Device

40. Turbine Trip Gear
Remote Trip Solenoid Valves
MAX51 AA 001,002
Main Trip Valves
MAX51 AA 005,006

41. Generator
Trip
Turbine
Trip
Low Forward
power Relay
Reverse
Power
Relay
Boiler
MFT

42. Turbine Trip Conditions
Discussed Above
MFT Trip

43. Tripping Conditions
Due to GT & UAT faults