What are the different interlock and protection used in power plant

 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 1.High bearing temperature 2-High bearing vibration 3-High axial displacement 4-High differential expansion 5-Low lube oil pressure	To reduce the speed of rotor within minimum time to avoid damages to the bearings & internal parts. Note: High back pressure on rotor creates reaction force to rotation of turbine rotor
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

 

Classes of STG Trips:

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

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