Interlocks: Are the programmed
or hardwired control systems to protect systems and improve the operation
reliability.
Protections: Are the programmed or hardwired
control systems to protect the equipments, man power and systems from
failure/harm.
The interlock and protection system is used to
ensure safety of equipment and personnel as well as smooth & trouble free
operation of the plant
This system initiates automatic corrective
actions to stabilize the unit quickly. The protection scheme is developed to
trip the equipment automatically with or Class A trip involves a serious
electrical fault like differential, stator earth fault etc. and is considered
to be the most dangerous in terms of the shock on the unit. Since it involves
serious electrical faults, connections from both generator and the HV bus is
immediately switched off to limit the damage at the fault point and also to
isolate the healthy system. Hence the unit (turbine, generator and boiler) has
to be tripped without time delay. Alarm & buzzers are generally used to
alert the operator.
POWER PLANT PROTECTIONS & INTERLOCKS AND THEIR SIGNIFICANCE
|
Sl No. |
Interlock description |
Significance |
|
A |
Boiler |
|
|
1 |
FD & SA fan trip/stop on tripping of ID fans |
To avoid furnace pressurizing |
|
2 |
Fuel feeding system trip/stop on trip/stop of SA fans |
To avoid jamming of fuel feeding system due to no spreading air |
|
3 |
FD fans trip/stop on high furnace pressure (>25 MMWC) |
1-To avoid furnace leakage |
|
2-To avoid furnace explosion |
||
|
3-To avoid buck stay damage |
||
|
4 |
ID fans trip/stop on low furnace pressure (-25 MMWC) |
1-To avoid carryover of fuel at secondary combustion zone |
|
2-To avoid back end flue gas ducts explosion due to accumulation of
unburnt (Unburnt results into formation of CO gas) |
||
|
5 |
FD fan trips on low drum level (On tripping ID fans, boiler all
systems like FD,SA & fuel feeding system trip) |
To avoid boiler pressure parts over heating & failure |
|
B |
Steam Turbine |
|
|
1 |
Turbine trips on high main steam pressure |
To protect turbine internals & casing from high pressure damage |
|
2 |
Turbine trips on low main steam pressure |
To protect turbine internals
from saturated steam (water particles in steam) |
|
3 |
Turbine trips on high main steam temperature |
To protect Turbine internals from creep failure (Turbine internals
fail on prolonged exposure to temperature more than recommended) |
|
4 |
Turbine trips on low temperature |
1-To protect Turbine from uneven expansion |
|
2-To protect Turbine internals from water particles in steam (Low
pressure & temperature steam will have water particles in it) |
||
|
5 |
Turbine trips on high bearing temperature (>110 deg C) |
To protect turbine bearing failure & other secondary
system/operation interruption for long time |
|
6 |
Turbine trips on high vibration (>5 mm/sec or >110 microns) |
To protect turbine bearing failure & other secondary
system/operation interruption for long time |
|
7 |
Turbine trips on high axial displacement |
To protect turbine internals from rubbing & damages |
|
8 |
Turbine trips on high differential expansion |
To protect turbine internals uniform thermal expansion & from
rubbing & damages |
|
9 |
Turbine trips on low control oil pressure |
To ensure reliable operation of HP & LP actuators |
|
10 |
Turbine trips on low lube oil pressure |
To avoid damages to the bearings |
|
11 |
Turbine trips on low trip oil pressure |
|
|
12 |
Turbine trips on low vacuum or high exhaust pressure |
To avoid damages to the rotor blades |
|
Note: High back pressure on rotor creates reaction force to rotation
of turbine rotor |
||
|
13 |
Turbine trips on high back pressure |
|
|
14 |
Vacuum breaker valve opens on activation of trip interlocks like |
To reduce the speed of rotor within minimum time to avoid damages to
the bearings & internal parts. |
|
1.High bearing temperature |
Note: High back pressure on rotor creates reaction force to rotation
of turbine rotor |
|
|
2-High bearing vibration |
|
|
|
3-High axial displacement |
|
|
|
4-High differential expansion |
|
|
|
5-Low lube oil pressure |
|
|
|
15 |
High hot well level |
To avoid entry of water into Turbine |
|
C |
Fuel handling |
|
|
1 |
Belt conveyor trips on operation of Zero speed switch (ZSS) |
1-To avoid the further damage to the belt conveyor |
|
2-To avoid system disturbance & major damages to the conveyor
structure |
||
|
Note: ZSS operates when belt gets cut or slips on pulley |
||
|
2 |
Belt conveyor trips on operation of belt sway switch (BSS) |
1-To avoid swaying of belt |
|
2-To avoid belt side edges damage |
||
|
3-To avoid fuel spillage |
||
|
3 |
Belt Pull cord Switch (PCS) |
To stop the belt conveyor during emergency situations to avoid damages
to the man & system |
|
D |
Boiler feed pumps |
|
|
1 |
Pump trips on high bearing temperature |
To avoid bearing damage & secondary system damage/disturbance |
|
2 |
Pump trips on high bearing vibrations |
To avoid bearing damage & secondary system damage/disturbance |
|
3 |
Pump trips on low suction pressure |
To avoid pump cavitation |
|
4 |
Pump trips on high differential pressure |
To avoid pump cavitation |
|
5 |
Pump trips on high balance leak off pressure |
To avoid further damages to the balance & counter balance discs |
|
6 |
Pump trips on lower cooling water temperature |
To avoid failure of pump's bearings & seal |
|
7 |
Pumps trips on over load |
To avoid damages to the pump internals |
|
8 |
BFP trips on Deaerator level low |
|
|
E |
Boiler fans |
|
|
1 |
Fan trips on high bearing temperature |
To avoid bearing damage & secondary system damage/disturbance |
|
2 |
Fan trips on high bearing vibrations |
To avoid bearing damage & secondary system damage/disturbance |
|
F |
Motor |
|
|
1 |
Motor trips on higher bearing temperature |
To avoid bearing damage & secondary system damage/disturbance |
|
2 |
Motor trips on higher winding temperature |
To protect winding |
|
3 |
Motor trips on over load |
To protect winding |
|
G |
Generator |
|
|
1 |
Over current protection |
Protects the generator from over load, short circuit & earth
faults |
|
2 |
Earth Fault Protection |
To protect the generator from earth faults & short circuits |
|
3 |
Generator Differential Protection |
To protect the generator from winding faults or unbalance currents in
winding |
|
4 |
Reverse Power Protection |
To avoid motoring of generator during reverse flow of power to
generator from other source |
|
5 |
Low Forward Power Protection |
To protect the generator running under load |
|
6 |
High bearing temperature |
To avoid bearing damage & secondary system damage/disturbance |
|
7 |
High bearing vibrations |
To avoid bearing damage & secondary system damage/disturbance |
|
8 |
Higher winding temperature |
To protect winding |
|
9 |
Higher core temperature |
To protect core |
|
10 |
High air temperature |
To limit winding temperature |
|
|
Other protections |
|
|
11 |
High & Low voltage protections |
|
|
12 |
High & low frequency protection |
|
|
13 |
Rotor earth fault protection |
|
|
14 |
Loss of excitation |
|
Read Power plant standard operating procedures
Class A trip
This involves serious electrical faults and is
considered to be the most dangerous in terms of the shock on the unit. Since it
involves serious electrical faults, connections from both generator and the EHV
bus is immediately switched off to limit the damage at the fault point and also
to isolate the healthy system. Hence the whole unit need to be tripped.
Class B trip
Class B primarily relates to mechanical problems.
This results in tripping of turbine followed by generator.
Class C
Read Generator and Turbine inter tripping
Class C involves basically external system
related problems like frequency, overvoltage etc. This does not involve instant
tripping of the unit. CPP unit operates on house load
Classes of Generator protections
|
SL NO. |
CLASS A |
CLASS B |
CLASS C |
|
1 |
Generator Differential Protection |
Loss of Excitation |
Under Frequency |
|
2 |
100% Stator Earth Fault Protection |
Rotor Earth Fault |
Over Frequency |
|
3 |
Generator Over Voltage Protection |
Over excitation |
Pole Slipping Protection |
|
4 |
95% Stator Earth Fault Protection |
|
Tripping of unit transformer |
|
5 |
Starting Over Current Protection |
|
|
|
6 |
Over fluxing Protection of Generator |
|
|
|
7 |
Differential Protection of GT |
|
|
|
8 |
Buchholz Relay of GT |
|
|
|
9 |
Trip from oil & winding temperature of generator transformer |
|
|
|
|
These protection when operated initiate tripping of Generator Circuit
Breaker, Field Circuit Breaker, Generator Transformer Circuit Breakers &
Unit Transformer LV Circuit Breakers and turbine. |
This results in tripping of turbine followed by generator. |
Class C involves basically external system related problems like
frequency, over voltage etc. This does not involve instant tripping of the
unit. |
