Safe operation of plant
Lowest cost of generation
Long equipment life
Minimum environmental effects
The basic requirements of an interlock
system for a unit shall accomplish the following:
(1) Protect personnel from injury.
(2) Protect equipment from damage.
(3) Protect boiler/turbine/generator operation by
limiting actions to a prescribed operating
sequence or by initiating trip devices when
approaching an out-of-range or unstable operating
Operating safety ,
To prevent uncontrolled fire ,
To prevent implosions and explosions in the
Less than fireball and loss of 240 V AC any elevation in service.
Drum level very low
Drum level very high
Both ID fan off
Both FD fan off
Air flow is less than 30%
Furnace press VLO
Furnace press VHI
Loss of all fuel trip
Unit flame failure trip
Both emergency PB trip
Reheater protection trip
Loss of 220 V DC more than 2 sec
Shut/close main fuel valve (LDO)
Shut/close main fuel valve (HFO)
Trip all running mills
Trip all running P A fans
Trip all running SA fans
Close HPLP bypass( if open)
Shut/close SH attemp block valve
Shut/close RH attemp block valve
Trip Electrostatic Precipitators (ESP)
1. Drum boilers, a MFR trip on low drum level
is required by ASME Boiler Code to
prevent loss of natural circulation and
to protect the drum against thermal shock.
2. Turbine trip on high drum level of boiler to prevent
water carryover to Turbine.
3. For controlled circulation boilers, pumps are
utilized to ensure circulation.
Adequate circulation is determined by
differential pressure measurements,
and tripping of the boiler fuel(MFR) is required
when adequate circulation is not verified.
When spray attemperators are used for
regulating superheat and reheat
temperatures, the spray control valves
and block valves should be automatically
closed when the MFR is tripped to prevent
spray water from entering the turbine
through the Main Steam /cold reheat line.
Excessive negative pressure (draft) can
produce implosions of the boiler
walls, precipitator, or the ducts
connected to them.
The most likely time for a furnace implosion is at t
he time of a boiler-fuel trip (MFR).
This causes a sudden
drop in temperature, which in turn causes a
sudden negative pressure within the furnace.
C B trip
C B trip
C B trip
Low condenser vacuum
Low lube oil press
Hi axial shift
HPSU emergency OFF
Bearing temperature protection optd
Main steam temp VLO
Elect generator protection optd class A
Elect generator protection optd class B
Fire protection optd
Turbine emergency PB optd
Trip command initiated from ATRS
Overspeed of turbine
Loss of vacuum in the condenser is
detected and initiates a turbine trip to
prevent over heating damage to the low
pressure turbine section .
Overspeed detection is a basic turbine
protection to prevent damage from high
An excessive axial motion of the rotor spindle at th
e thrust bearing initiates a turbine trip
to prevent or reduce mechanical damage .
Turbine trip results in the closing of the main and r
eheat stop valves.
The generator should not be tripped until the
turbine-generator unit reaches a no load
or negative load condition.
A reverse power relay (short timer) is used in
the sequential trip logic scheme where motoring is
allowed for a short time to ensure the prime mover
has lost sufficient energy to prevent overspeed
following a turbine trip .
Turbine vacuum breaker will open to reduce coasting down
time of turbine :
If Turbine tripped on
1. axial shift tripped,
2. Lube oil press VLO,
3. Fire protection operated,
2 sec Field
C B trip
C B trip
C B trip
Class A tripping• This trip will operate
when the fault needs to isolate the generator
immediately. • This gives the command to trip
turbine and generator.
Class B tripping – • This trip will operate when
the fault is not serious view of generator or it is
not directly harmful to generator • This gives the
command to trip turbine.
UT LV side ref protection -64UAT
Earth fault protection-51NUAT
UAT buchholz relay
UAT PRV trip
UAT oil temp HI
UAT winding temp HI
Liquid main leads top max
Cold gas cooler temp A+B more than 55 c deg
Cold gas cooler temp C+D more than 55 c deg
Hot air main exciter more than 85 c deg
When generator is connected to the grid system and
power input to turbine is insufficient to drive
generator ,motoring will occur.
Motoring is harmfull to turbine blades if it sustained
for extended period.
This is due to loss of the cooling effect of steam flow
which can result in overheating the last stage blading.
Anti motoring detection include exhaust hood
temperature monitoring and reverse power relay(32G)
with long timer.
1. Reverse power relay ( long timer eg 30 sec ) logic for
( it is a protection of turbine )
2.Reverse power relay (short timer eg 3 sec ) logic for turbine trip to
Generator trip to prevent overspeed of Turbine
Trip generator and
C B trip
Generator ,GT,UAT protection : class A
C B trip
C B trip
C B- circuit breaker
1. NFPA 85 -Boiler and Combustion Systems
2. IEEE Guide for Protective relay applications to
Power Transformer (IEEE Std C37.91) .
3. IEEE Guide for AC Generator Protection (IEEE Std
BOILER TRIP Conditions ,FSSS, master fuel
BHEL (T) Boiler ,500 MW
1• Less than fire ball and loss of +24V DC(N) at any elevation in service.
2• Drum level very low for more than 5 seconds.
3• Drum level very high for more than 10 seconds.
4• Both FD Fans off.
5• Both ID Fans off.
6• Air flow is less than 30%
7• Furnace pressure very low.
8• Furnace pressure very high.
9• Loss of all fuel trip
10• Unit flame failure trip
11• Both emergency trip Push Buttons pressed (Operator initiated boiler trip)
12• Loss of reheater protection.
13• Loss of +24V DC(T) for more than 2 seconds
14• All Secondary Air Heaters off.
15• Water wall circulation not correct for more than 5 seconds
16• Boiler feed pumps tripped for more than 10 seconds
KWU Steam Turbine – 500 MW
1. Turbine overspeed ( hyraulic)
2. Manual trip device (hydrualic)
3. Turbine overspeed (electronic)
4. Axial shift
5. Trip switch optd (Desk)
6. HP exh temp high
7. TG brg temp high
8. Condenser press trip
9. Fire protection trip
10. MOT Level VLO
11. Low MS temp trip
12. Trip command from ATRS
13. Gen electrical protection
14. Gen protection (mech)
15. MFT Relay optd
16. HP casing top-bot diff HI
17. IP Casing top-bot diff HI
18. LP casing top bot diff HI
BOILER TRIP Conditions ,FSSS, master fuel relay( MFR)
Doosan Boiler ,660 MW ,Supercritical boiler
1.Both ID Fans Off
2.Both FD Fans Off
3.Unit Air Flow < 30 % TMCR
4.All Feed Water Pumps Are Off For More Than 40 Sec
5. the furnace pressure High / Low for more than 8 sec ( 150 mmwc / -180
6. the furnace pressure High – High / Low - Low ( 250 mmwc / - 250 mmwc)
7.Loss Of Re-heater Protection
9.All SAPH Off
10.Economizer Inlet Flow Low For More Than 10 Sec
11.Furnace Vertical Wall Temperature High For more than 3 Sec (479 C)
12.SH Pressure High On Both Side
13.SH Temperature High For More Than 20 Sec ( 590 C)
14.RH O/L Temperature High For More Than 20 Sec ( 590 C)
15.Separator Level Low-Low During Wet Mode
16.Separator Level High-High During Wet Mode
17. MFT Relay Tripped
18.Loss Of Fuel Trip
19.Unit Flame Failure Trip
Boiler trip -FSSS MFR conditions
-Shanghai electric co ,China- boiler 600 MW
(1) Manually operate MFR
(2) IDF trip
(3) FDF trip
(4) All preheater trip
(5) MCS power loss
(6) Drum water over max.
(7) Boiler light-up failed
(8) Drum water below min.
(9) Boiler pressure excess max limitation (choose two from the three)
(10) Boiler pressure below min limitation (choose two from the three)
(11) Whole furnace loses fuel
(12) Whole furnace loses flame
(13) Loss flame test cooling air (delayed) (choose two from the three)
(14) Air flow below min. value (choose two from the three)
（ 15 ） All boiler water recycle pump trip
（ 16 ） Reheater loses protection.
（ 17 ） All water feed pump trip
（ 18 ） Air flow of economizer inlet is lower than min set value (choose two from the three)
Steam Turbine emergency trip conditions ( electric)
Harbin Power Engg Co Ltd –China ;turbine- 600 MW
1. When turbine is over speeding to 3300rpm.
2 .When condenser pressure VHIGH
3 .When axial displacement is more than +1.0mm or less than -1.0mm.
4. When lube oil pressure is as Vlow
5 .HP /LP casing expansion difference reachs to +11.1mm or -5.1mm.
6 .LP casing expansion difference reachs to +27.8mm or -4.3mm.
7. When generator trips.
8. When boiler MFT.
9 .When EH oil pressure is as low
10. DEH is in fault (power off).
11. ETS is in fault (power off).
12 .X or Y bearing vibration of any bearing of #1 ～ 8 is HIGH
13. Stop unit manually by button on standby manual operation panel.
14 .Stop turbine remotely.
15. Local STOP button in turbine hall.