Interlock and Protection
The interlock and protection system is used to ensure safety of equipment and personnel as well as stable operation of a unit. This system initiates automatic corrective actions to stabilize the unit quickly. The protection scheme is developed to trip the equipment automatically with or without time delay. Audio-visual alarm annunciation, supplementing interlock, and protection are provided to alert the operator. Pulverized fuel-fired steam generators are prone to explosion hazards due to unstable burning and fire-outs occurring in burners. Implosion in a furnace may take place if the furnace negative pressure exceeds the structural stability of the furnace. In order to eliminate or to minimize boiler-furnace explosion/implosion, the NFPA has developed the Burner Management System. Before admitting fuel to an unfired boiler, it is essential to ensure the furnace is purged. Tripping of a generator essentially means disconnecting it from the grid by opening the generator circuit breaker.
Keywords
interlock; protection; trip; explosion; implosion; purge; fault
11.1 Introduction
The interlock and protection system ensures the safety of equipment and personnel as well as the stable operation of a unit within permissible limits at all times. Many abnormalities can occur during normal operation of units, most of which can be corrected by operator intervention. However, operation of modern large power plants at times requires instant decision-making and corrective actions that may not be possible by operator action since human reaction time is inherently slow. A sluggish response on the part of operator may lead to catastrophic damage resulting from equipment failure, inadvertent error, and mishandling. To handle such situations the interlock and protection system initiates automatic corrective actions to stabilize the system quickly. This system also handles operations such as automatic cutting in and out of auxiliaries, systems, etc., in sync with changing load conditions. There are many examples of abnormalities that require immediate isolation of a unit for safety of equipment and/or minimizing damage to equipment. The protection scheme is usually designed to trip the equipment automatically with or without time delay, depending on the nature of emergency.
Protection to personnel and major equipment may be achieved by resorting to an intrinsically safe design and trained conduct of individual. However, individual conduct can not be predicted, since it is influenced by ignorance, fatigue, or negligence. Hence, it is essential that a design is intrinsically safe as far as possible so that no single failure in the system would prevent a mandatory shut-down. This aspect of design is called fail-safe. From this perspective, measurement, control, and protection systems are designed such that critical protection and control meet the “2 out of 3 logic” condition (which means if 2 inputs from a combination of 3 similar inputs of the same parameter are satisfied, then the logic will allow the output command to proceed). For less critical protection and control adopting “1 out of 2” (1 input from a combination of 2 similar inputs of same parameter) logic condition would suffice.
For guaranteeing safe operation various equipment, particularly larger equipment, of a power plant are provided with different type of interlock and protections. Most major equipment is provided with a start-interlock that prohibits it from starting until its all start permissive are satisfied. Similarly, trip-interlocks are provided for the protection of running equipment in the event of one or more off-normal operating conditions and/or failure of associated equipment. Occurrence of any of these trip-interlocks and/or any off-normal condition, which may damage equipment and/or impair safety, should immediately cause the equipment to stop automatically.
In addition to incorporating protective devices, since not all conditions that cause damage are detected by any of the mandatory automatic trip devices, operating personnel must be knowledgeable of the limitations of automatic protection systems. Audio-visual alarms, supplementing interlocks, and other protections are provided to alert the operator when any equipment behaves abnormally or any of the operating parameters go beyond the preset permissible limits. Whenever any abnormal operating conditions occur and/or any equipment trip, corresponding “failure” alarm would alert the operator to take the applicable corrective actions.
The three major pieces of equipment in a steam power plant are the steam generator, steam turbine, and electric power generator or alternator. Each piece of equipment is provided with a dedicated hand-reset type lock-out relay through which tripping of associated equipment is initiated. Each of these three protection systems is independent of each other, but intertripping interlock among lockout relays is also provided to initiate isolation of the equipment, even if they are not directly affected, but isolation of which is necessary to ensure safety. A similar philosophy is adopted in gas turbine and diesel engine power plants.
The following sections cover the interlock and protection of a steam generator, steam turbine, gas turbine, diesel engine, and associated electric power generator or alternator. Interlock and protection of fuel firing equipment is also discussed, since they are integrally tied to start-up and shut-down of steam generators. No attempt, however, has been made to cover all types of protection, since that would be difficult to accommodate in a chapter. The scope of this book also does not cover interlock and protection of other auxiliary equipment associated with steam generator, steam turbine, gas turbine, diesel engine, and alternator.
11.2 Steam Generator
Steam generators today are large in size with a high steam-generating capacity and highly sophisticated firing system. These types of steam generators are more costly and vital to the power grid system than other types. With the increase in unit size and capacity, forced outages have greater significance not only for the larger loss of revenue but also for greater risk of injury and more damage to plants. Additionally, all steam generators, especially pulverized fuel-fired steam generators, are prone to explosion hazards due to unstable burning and fire-outs occurring in burners. All efforts should be made at both the design stage as well as during operation to ensure fail-safe operating conditions to prevent furnace explosion as well as furnace implosion.
An explosion in a container may occur when there is rapid pressure increase accompanied by combustion that propagates pressure waves. This pressure increase gets released by finding the least-resistant path in vent holes, explosion doors, etc. In the worst case scenario the pressure increase gets released by bursting the container. The primary reasons for such pressure build-up may be the result of one or a combination of the following:
i. Release of gases as a result of combustion
ii. Inadequate escape facilities for ventilation
iii. When the crest of pressure waves gets superimposed on the crest of fresh waves, the result is an accentuated effect
Explosion in a furnace may occur due to a variety of reasons. Some of the frequently occurring causes are [1]:
i. Improper purging of furnace, air, and gas paths
ii. Inaccurate ignition procedure
iii. Inadequate ignition procedure
iv. Fuel supply to the furnace is continued without ensuring proper combustion
v. Following a flame-out burners re-lit hurriedly
vi. Introduction of main fuel without ensuring adequate ignition energy
Statistics reveal that most furnace explosions are attributed to operator error, necessitating improvement in control and detection equipment. It is also known that operator actions are not fast and effective enough to execute parallel activities of multiple operating steps to ensure safe and proper supply and burning of fuel in the furnace. In large steam generators, where the fuel input rate is also high, major furnace explosions can result from the ignition of accumulated fuel even for 1–2 s. Statistics further reveal that the majority of explosions occur during start-up, shut-down, and low-load operations.
Implosion in a balanced draft fossil-fired furnace may occur if furnace negative pressure exceeds the structural stability of the furnace. From field experience it is found that implosions in some plants have occurred without main fuel firing, in some other plants prior to light-up of main fuel, and in some instances implosion occurred following a master fuel trip (MFT). From these experiences it may be concluded that during normal running a furnace is free from occurrence of any implosion.
Furnace implosion is usually induced by incorrect use of ID fan dampers, and/or rapid decay of furnace pressure either due to abrupt loss of supply of fuel to furnace or due to steep decay of furnace temperature following MFT [2].
(Note: Following an MFT the fuel supply to the furnace is stopped instantly, then the furnace gas temperature decreases rapidly to the air inlet temperature within approximately 2 s. However, due to reserve heat in the furnace, the gas temperature does not drop to that low a value but to an intermediate temperature of say, 813 K, rated temperature of steam.
During normal running of a steam generator pressure and temperature in the furnace are usually maintained at about (−) 0.1 kPa (g) {or 101.2 kPa (abs)} and 1673 K, respectively. Hence, after an MFT, applying ideal gas law, the pressure in the furnace reduces to
which is greatly higher than the recommendation of NFPA 85: Boiler and Combustion Systems Hazards Code discussed below.
The furnace pressure (draft) control system is designed to control furnace pressure at the desired set point in the combustion chamber by accurately positioning draft-regulating equipment (draft fan inlet vane/fan blade pitch/fan speed changing drive, etc.). Operating speed of furnace draft control equipment should not be less than that of air-flow control equipment to prevent negative pressure in the furnace from exceeding the danger limit.
Additional facilities, e.g., flame-scanning system, automatic burner control, and monitoring system including interpretation of adequacy of ignition energy, air and fuel metering, etc., are also provided to facilitate operators to secure safe and satisfactory performance.
In order to prevent permanent deformation of furnace structural members of all boilers, except fluidized bed boilers, due to yield or buckling arising as a result of either furnace explosion or furnace implosion, NFPA 85 recommends the following at the design stage:
i. The positive transient pressure should not be more than +8.7 kPa or the test block pressure of FD fan at ambient temperature, whichever is less
ii. The negative transient pressure should not be more than (−)8.7 kPa or the test block negative pressure of ID fan at ambient temperature, whichever is less
For fluidized bed boiler the NFPA 85 recommendation to prevent permanent deformation of furnace structural members is:
i. The positive transient pressure should be 1.67 times the predicted operating pressure or +8.7 kPa, whichever is greater, but the pressure should be restricted to the maximum head capability of the air supply fan at ambient temperature
ii. The negative transient pressure should not be more than (−)8.7 kPa or the test block negative pressure of ID fan at ambient temperature, whichever is less.
11.2.1 Burner management system (BMS)
In order to eliminate or to minimize boiler-furnace explosion/implosion hazards, the National Fire Protection Association (NFPA) of the United States has formulated some standards, a key feature of which is broadly identified as the Burner Management System (BMS).
The BMS should permit starting of equipment in sequence, when a set of preset conditions, necessary for the safety of equipment, are satisfied. When the equipment is started, the system will continuously monitor safe operating conditions determined in advance, warn operating personnel, and remove equipment from service in sequence again when the preset safe conditions are not met in practice.
BMS logic is designed so that no single failure in the system would prevent a mandatory boiler shut-down. The logic supervises overall furnace conditions, monitors all critical parameters of fuel firing system, and cuts out all fuel input to the furnace whenever dangerous conditions occur. Other requirements of the BMS are as follows:
1. An interlock system consisting of sequence interlocks, dictating the order in which the equipment must be operated, and also safety interlocks including mandatory trips
2. Dependable flame-monitoring and scanning devices both for oil and coal flame. In the event of complete/partial loss of ignition in the furnace, immediate action to be taken to stop fuel supply
3. A reliable start-up including furnace purge
4. Fuel oil start-stop control and oil firing supervision
5. Pulverizer and coal-feeder start-stop control and coal-firing supervision
6. Maintain minimum total air-flow supply to boiler at 30%, even when fuel load is less than 30%
7. Prevent accumulation of unburned fuel in furnace, which can ignite and lead to furnace explosion
The flame scanner sighting head may be sensitive either to broadband ultraviolet radiations peculiar to hydrocarbon (coal, oil and gas) combustions or to the brightness level in the red band of the spectrum of oil and coal flame or combination of these two, but must not respond to radiations from hot refractory or slag. Visible light flame scanners are also popular.
A comprehensive BMS of a pulverized coal fired boiler includes control, protection, and intertripping interlocks of the following firing equipment and their associated equipment:
11.2.2 Sequence of operation
Earlier safety was ensured by sequential operation of various boiler auxiliaries. The sequence of starting and stopping used to be designed to ensure that unless the conveying system, which removes the products of combustion from the furnace, was in service, those feeding fuel and air to the furnace would not run. As such, the starting sequence of various boiler auxiliaries is configured as follows:
The stopping sequence of the above boiler auxiliaries should be in the reverse order of the starting sequence. The failure of any equipment in the above order should cause immediate stop of all succeeding equipment. Selective interlocking should ensure that with only one of the two ID fans running one FD fan could be operated. Similarly, failure of one of the two FD/PA fans should initiate preferential cut out of running pulverizers as recommended by the equipment manufacturer.
11.2.3 Furnace purge interlock
Prior to admitting fuel to an unfired boiler, it is essential to ensure the furnace is free from gaseous or suspended combustible matters. The sequence of activities of furnace purge cycle is as follows:
i. Establish AC and DC power supply to BMS and associated equipment
ii. Ensure master fuel relay (MFR) is in tripped condition
iii. Verify that none of the MFR tripped commands (described below) are present
iv. Open air and flue gas path isolating dampers, wind box dampers, and other control dampers to ensure open-flow path from the inlet of FD fans, through the furnace, to ID fans and the chimney
v. Place all burner air registers in purge position or ensure all auxiliary air damper positions maintain adequate wind box to furnace differential pressure
vi. Ensure all burner headers, igniter header, individual burner, and individual igniter shut-off valve are closed
vii. Verify that all PA fans are not running
viii. Check all flame scanners show “no flame” condition
ix. Establish a minimum of 25% of design full load mass air flow through the furnace
x. For pulverized coal-fired steam generators ensure mass air flow through the furnace is not greater than of 40% of design full load mass air flow
If all above conditions are satisfied “Purge Ready” indication comes ON. Initiate command for “Purge Start.” Such action will maintain above air flow for a period of at least 5 minutes. On completion of above time period, “Purge Complete” indication switches ON. The furnace may now be treated to be free from any combustible matter and is ready for fuel firing.
11.2.4 Boiler light-up
After the purge is complete, the “MFR Reset” may be initiated provided all operating conditions of the boiler are normal and “MFR Trip” conditions, presented below, are absent. The boiler is now ready for light-up. (Detailed treatment of start-up and shut-down of steam generators is given in Chapter 12.)
11.2.5 Master fuel trip (MFT)/boiler trip
In accordance with NFPA 85, there are certain minimum required interlocks that should be provided to ensure basic furnace protection of a multiple burner boiler. Furthermore, in order to establish integrated operation of boiler-turbine-generator, these minimum required interlocks are supplemented by some additional interlocks for the protection of the furnace. Occurrence of any one of emergency conditions, as follows, will trip automatically master fuel relay (MFR, industrially known as 86MF relay):
iii. Total air flow decreases below purge rate air flow by 5% of design full load air flow
(Whenever total air flow falls below purge rate air flow, removal of combustibles and products of combustion from the furnace gets impaired.)
iv. Furnace pressure in excess of prescribed operating pressure (In the event of excessive increase in furnace pressure MFR has to be tripped so as to obviate furnace explosion.)
v. Furnace negative pressure in excess of prescribed operating negative pressure
(In case of excessive fall in furnace pressure MFR should be tripped in order to avoid furnace implosion.)
vi. Boiler drum level (applicable to drum boiler) in excess of prescribed operating level
(Excessive increase in boiler drum water level may interfere with the operation of the internal devices in the drum, which separate moisture from the steam, resulting in carryover of water into superheater or steam turbine causing mechanical damage.)
vii. Boiler drum level (applicable to drum boiler) falling below prescribed operating level
(Excessive fall in boiler drum water level may uncover boiler riser wall tubes and expose them to furnace heat without adequate water cooling, resulting in riser tube burn-out unless action is taken either to restore the supply of feedwater or to kill the fire, i.e., trip the MFR.)
ix. First pulverizer burners fail to ignite
x. Last pulverizer in service tripped
xi. All fuel inputs to furnace are shut off
xii. Loss of all flame
(Admission of fuel in the furnace at any time without ensuring proper combustion is a potential hazard as such fuel input to the furnace must be immediately cut out in case of loss of flame, i.e., loss of fire.)
xiii. Partial loss of flame that results in a hazardous accumulation of unburned fuel
xiv. Loss of energy supply for combustion control, burner control, or interlock systems
xv. All source of 6.6kV (HT power to major auxiliaries) supply tripped
xvi. In a reheat boiler if the following conditions occur simultaneously:
a. One or both ESV and IV of steam turbine are closed
Tripping of MFR will initiate the following:
i. Oil burner tripping sequence
iii. Seal air fan tripping sequence
iv. Pulverizer tripping sequence
v. Closing fuel oil shut-off valve
vi. Maintaining air register position
vii. Closing HP/LP bypass station
viii. Closing superheater attemperation spray water supply
ix. Closing reheat attemperation spray water supply
x. Tripping high voltage supply to electrostatic precipitator
xi. Rapid unloading of alternator if its load is above unloading limit. If the alternator load remains above the unloading limit even after expiry of a preset time, initiate tripping of alternator.
When the steam generator is tripped follow these steps:
1. Check and ensure that all fuel to furnace has been completely cut off
2. Maintain drum level (DO NOT take water to drum if level in drum is not visible. It may happen when water wall tube ruptures, that will damage the tubes.)
3. Purge the furnace and shut down draft fans (except in the case there is change of furnace getting pressurized, e.g., due to tube rupture, etc., keep one ID fan running with furnace maintained in suction)
4. Check and ensure SH, RH attemperator spray water supply is completely cut off and follow shut-down procedure of steam generator (see Chapter 12)
11.3 Fuel-Firing Equipment
11.3.1 Fuel oil shut-off valve
This valve may be opened provided the following conditions are satisfied:
The shut-off valve will close if any one of the following conditions occurs
11.3.2 Pulverizer
Pulverizer may be started provided all of the following conditions are fulfilled:
ii. No pulverizer trip command is present
iii. Pulverizer ignition energy is available
iv. Pulverizer discharge gate/valve open
v. Pulverizer seal air valve open
vi. Pulverizer cold air damper open
vii. Tramp iron hopper valve open
viii. Coal feeder inlet and coal-bunker outlet gates are open
ix. Pulverizer outlet temperature less than a high limit recommended by the manufacturer
x. Pulverizer lubricating oil pressure, if provided, is healthy
xi. Primary air pressure adequate
xii. Differential pressure between pulverizer under bowl and seal air header is adequate
xiii. Air flow is adjusted between 30% and 40% of boiler maximum continuous rating (BMCR) flow
xiv. For tangential fired steam generators, either air and fuel nozzle tilts are horizontal or any coal feeder is ON
For wall-fired steam generators, the corresponding pulverizer air registers are in oil firing position
Starting of pulverizer should permit associated coal feeder to start
Pulverizer should trip automatically in the event of any of the following conditions:
ii. Pulverizer discharge gate/valve is not open
iii. Loss of elevation/unit DC power
v. Lubricating oil pressure, if applicable, falls below a permissible limit
vi. Loss of one of the two running PA fans should cause preferential pulverizer tripping recommended by the manufacturer
Stopping/tripping of pulverizer should initiate tripping of associated coal feeder.
11.3.3 Coal feeder
The coal feeder may be started provided all of the following conditions are satisfied:
i. Elevation DC power is available
ii. Ignition energy is available
iii. Associated pulverizer is running
iv. Pulverizer hot air damper/gate is open
v. Pulverizer inside temperature is adequate as recommended by the manufacturer
vi. Fuel air damper is closed (tangential fired steam generators)
Starting of the coal feeder should initiate the following:
i. Release associated feeder speed and fuel air to auto control
ii. Check pulverized fuel flame after expiry of trial time
iii. If the coal flame is established, that ensures the pulverized fuel burner is established which in turn allows oil burner administrative shut-down
iv. If coal flame is not established within a preset time, initiate the pulverizer trip sequence.
The coal feeder should trip automatically in the event of any of the following conditions:
i. Failure of any of the start permissive
ii. Associated pulverizer is tripped
iii. Coal flow has not been established within a preset time of starting of the coal feeder
Stopping/tripping of the coal feeder should initiate the following:
i. Cut out auto signal to associated feeder speed and fuel-air
ii. Emergency shut-down of pulverizer and coal firing system (see Chapter 12)
11.4 Steam Turbine
Prior to admitting steam to turbine, it is essential that following minimum conditions are met:
i. AC and DC power supply to turbine protection, control and associated equipment are available
ii. None of the turbine-tripped commands (described below) are present
iii. Condensate extraction pumps are running
iv. Boiler feed pumps are running
v. Manufacturer’s recommended steam conditions, i.e., pressure and temperature of steam at turbine inlet are available
vi. Turbine lube oil pressure is adequate
vii. Condenser vacuum meets manufacturer’s recommended value
Turbine is now ready for admitting steam.
(Detail treatment on the start-up and shut-down of steam turbines is given in Chapter 12.)
A running turbine may trip due to variety of reasons, e.g., malfunction or trouble in one or more of its auxiliaries and/or operating conditions, abnormal conditions in the boiler, or trouble in the unit electrical system. Tripping of a turbine is initiated through the Turbine Lockout Relay (industrially known as 86T relay).
Turbine protection system is independent of other protection systems of the plant. The governor and the protection system are constructed in such way that failure of any component will not prevent the turbine from being safely shut-down.
The turbine protection system may be actuated either electrically or hydraulically. In the event of electric trip, the turbine lockout relay should actuate and energize turbine trip solenoid. Both trip solenoid and hydraulic trip system act on the hydraulic control system, which causes fall in control/trip oil pressure leading to closure of HP turbine emergency stop valves (ESVs), HP and IP turbine control valves and IP turbine interceptor valves (IVs), thereby shutting off steam supply to turbine.
When steam supply to turbine is insufficient to drive the generator, motoring will follow, which is detrimental to the health of turbine blades because of loss of ventilation effect of steam flow with consequent overheating of blades. Under such condition it is essential to trip the unit lockout relay (industrially known as 86U relay).
In order to ensure safe running of the turbine following automatic trip conditions of turbine are generally provided:
1. Axial shift high
(In case of excessive axial shift of the rotor due to thrust, blades and glands of turbine will get subjected to mechanical damage.)
2. Lube oil pressure very low
(In the event of lube oil system failure, bearings would quickly lead to wiping of the journal and thrust bearings).
4. Condenser vacuum very low
(If the condenser vacuum falls below a lower set value, turbine exhaust hood temperature will increase, which may cause damage to the last stage blades due to stress from centrifugal force juxtaposed with thermal stress. Condenser tubes will also get damaged since under the influence of temperature they will expand and may get buckled.)
6. Boiler drum level very high
(Excessive increase in boiler drum water level results in carryover of water into steam turbine. Water droplets accompanying steam impinge high speed turbine blades and damage the blades severely impairing efficiency of the turbine. Hence, on occurrence of carryover of water, steam supply to turbine should be cut off.)
7. Main steam pressure very low
8. Main steam temperature very low
(On occurrence of conditions 7/8, there is an apprehension of moisture carryover to turbine, leading to mechanical damage to turbine blades.)
9. Main steam/hot reheat steam temperature very high
(To prevent creep failure, this protection is provided.)
10. Turbine/generator bearing temperature very high
(This protection is provided to protect bearing babbit metal from wiping.)
11. Turbine/generator bearing drain oil temperature very high
(Turbine lube oil, being a mixture of hydrocarbons, has a tendency to oxidize. If the oxidation rate is high due to higher temperature sludge will form, which gets deposited in various parts of lubrication system, creating blockage and leading to oil starvation and overheating. Furthermore, with increase in oil temperature, viscosity of the oil will become low resulting in discontinuities in the bearing oil film with consequent damage to bearings.)
12. Turbine/generator bearing vibration very high
13. Excessive differential expansion
14. Frequency low
(Continuous operation of a turbine with a frequency lower than a recommended value may reduce the life of some of the blades, since an under-frequency condition may be the resonance condition for some of the stages resulting in higher stresses in these blades. Nevertheless, tripping a turbine under low frequency is not desirable since this will cause further stress to other running units, which may lead to total power failure.)
15. HP feedwater heater level very high
(In the event the level in any of the HP feedwater heaters exceeds a preset very high limit, there may be a possibility of water entering the turbine through associated extraction steam line causing serious water damage necessitating complete shut-off of steam supply to turbine.)
16. Main oil tank level very low (optional)
Tripping of Turbine should initiate the following:
i. Close emergency stop valves and HP control valves
ii. Close interceptor valves and IP control valves
iii. Close turbine inlet isolating valve/s (if any)
iv. Close extraction steam non-return valves (NRVs)
v. Close heater inlet extraction steam valves
vi. Open HP casing drain valve/s
vii. Open extraction steam line drain valves
viii. Open HP-LP bypass valves
ix. Trip 6.6 kV (HT supply to major auxiliaries) unit bus incomer breakers
x. Trip generator field breaker
Immediately after the turbine trip, check and ensure that
1. Turbine speed is actually falling down
2. Lube oil supply to turbine/generator bearings is adequate
3. Auxiliary oil pump has taken start automatically. If this pump fails to start then start the same manually
4. All extraction line NRVs and isolating valves in the extraction lines near feedwater heaters have closed
If the cause of the trip is such that the unit can be brought back shortly, it is not needed to break condenser vacuum. However, if it is necessary to bring the rotor to a quick stop, break the vacuum by opening vacuum breaker valve and admitting atmospheric air to the condenser. Initiate emergency shut-down of boiler (discussed in Chapter 12). When the turbine is coasting down before it comes to stand still put it on turning gear for uniform cooling of hot turbine rotor.
11.5 Gas Turbine
The interlock and protection system of gas turbine performs following functions prior to startup, during startup, running, shut-down and cool-down processes:
i. Prior to starting, it checks all vital systems of turbine, generator, and associated auxiliaries are in a healthy condition
ii. During start-up operation, this system maintains all checks and permits to start the turbine
iii. The system monitors various protective circuits, and permits the unit to continue if the operating parameters are within limits
iv. At any stage of the process, if a vital system fails and indicates that the turbine is not within safe run up parameters, it will trip the machine
v. During normal operation, the system provides an overall protection to the turbine against any failure of protective circuits and trips the machine.
Prior to starting the gas turbine the following minimum conditions must be met:
i. AC and DC power supply to turbine protection, control and associated equipment are available
ii. None of the gas turbine tripped commands are present
iii. Fuel gas pressure at gas turbine fuel gas skid inlet is adequate
iv. Fuel oil pressure at gas turbine fuel oil skid inlet is adequate
v. Lube oil flow through sight glasses are within normal limit
vi. Lube oil pressure is equal to or above the manufacturer’s recommended value
vii. Electrical control and protection systems are in service
viii. Annunciations, instruments, all indicating lamps are healthy
ix. Auxiliary power supply is at the rated voltage and frequency
Once above conditions are satisfied the gas turbine is purged with at least 8% of full-load mass air flow for a period of not less than 5 minutes. The gas turbine is now ready for starting.
(Further details on start-up and shut-down of gas turbine are given in Chapter 12.)
To prevent damage to gas turbine and auxiliaries from abnormal operating conditions arising out of malfunction of equipment or system, a number of protective devices are provided. The protective function supervises parameters of the gas turbine in all phases of operation. The sensors are monitored continuously for malfunctions. Whenever such a malfunction does occur, an alarm is given. The protective system includes over-speed, over temperature, flame detection, high vibration, etc., as described below:
Over-speed protection
The over speed protection actuates whenever speed of the rotor exceeds a preset limit causing closure of fuel gas/fuel oil trip shut-off valve.
Over-temperature protection
The gas turbine unit is protected against any possible damage caused to the machine in the event turbine outlet gas temperature exceeds a predetermined limit. In such case the over-temperature protection system trips the turbine to arrest any further temperature increase.
Flame detection and protection
Flame detection device avoids accidental flooding of combustors with unburned fuel due to loss of flame or improper combustion. This device is used to detect flame in the combustors and to trip the turbine in case of flame failure.
Vibration protection
The vibration protection system of the gas turbine unit comprises vibration transducers fitted on various bearing housings and relative shaft vibration transducers at journal bearings. In the event the vibration becomes excessive, the unit will trip to avoid any damage.
Lube oil pressure and temperature protection
In the case gas turbine lubricating oil pressure drops below a preset low value, the protective system trips the unit. This is to prevent the turbine from being run with inadequate oil supply to bearings and other accessories. Likewise, bearing temperature also trips the unit should the temperature of bearings exceed preset limit.
Hydraulic and pneumatic protection
In the event of collapse in control oil/air pressure in piping systems, the hydraulic/pneumatic protection system brings vital system components to a safe operating condition and trips the turbine.
Electrical interlocks and protections
The protections provided for generator, transformers, motors, etc., are against unhealthy system conditions or to isolate faulty equipment from the system to minimize the extent of damage through the fault. In the event any of the electrical protections, linked with the connected generator or connected electrical system, operates it trips the turbine.
Immediately after the gas turbine trip, check and ensure that
11.6 Diesel Engine
Like all prime movers diesel engines are also not free from emergencies, occurrence of which should cause starting-up and/or running of engines to stop.
Before the diesel engine is started the following checks should be made:
i. AC and DC power supply to diesel engine protection, control, and associated equipment are available
ii. None of the tripped commands are present
iii. Fuel oil pressure at diesel engine inlet is adequate
iv. Lube oil pressure is equal to or above the manufacturer’s recommended value
v. Starting air cylinders air pressure is at or above the recommended value
vi. Water to the external cooling circuit is adequate
vii. Shaft barring arrangement is engaged
viii. Electrical control and protection systems are in service
ix. Annunciations, instruments, all indicating lamps are healthy
x. Auxiliary power supply is at the rated voltage and frequency
The diesel engine is now ready for starting.
(Detail treatment on the start-up and shut-down of diesel engines js given in Chapter 12.)
Emergency conditions that trip the diesel engine are discussed below:
Incomplete oil circulation (at starting time)
When the engine start command is initiated, the lube oil and fuel oil pumps for pre-start priming will operate. If the lube oil pressure does not build up to the preset value during priming period, starting-up of engines is to stop.
Air starting fault
After the oil pressure reaches the preset value air is admitted to diesel engine pipeline. If air pressure fails to reach the preset value start-up of engine is to be aborted.
Lube oil pressure drop
While the engine is running, if the lube oil pressure drops below a preset value running engine has to be stopped.
Fuel oil pressure drop
In the event fuel oil pressure drops below a preset value while the engine is running, the engine has to be stopped.
Inadequate cooling water pressure
During normal engine operation, if the cooling water pressure drops below the low limiting value, the engine should stop.
Speed of the started engine fails to increase
Once command for starting the engine has been given, and normal priming is achieved, the engine speed should increase automatically to warming up speed. If the speed fails to increase the supply of starting air as well as the lube oil and fuel oil priming pumps should be stopped.
Engine over-speed
If the speed of the engine reaches or exceeds its over-speed limit starting-up and/or running of the engine needs to be stopped.
Tripping of generator electrical protections
In the event of operation of any of the electrical protections, pertaining to the connected generator or connected electrical system, starting-up and/or running of the engine needs to be stopped.
Immediately after the tripping of diesel engine, check and ensure that
11.7 Generator (Alternator)
As discussed in Chapter 9, Systems Of Large Power Station, power generated by the generator is supplied to the grid/bus-bar through the generator transformer and generator circuit breaker. By closing the generator circuit breaker the generator gets connected to the grid – this is called synchronization. Prior to synchronizing minimum following conditions should be fulfilled:
i. AC and DC power supply to generator protection, control and associated equipment are available
ii. Generator seal oil system, if there be any, is in service
iii. Generator hydrogen cooling system, if there be any, is in service
iv. Generator stator cooling water system, if there be any, is in service
v. None of the generator-tripped commands are present
vi. Excitation system provides permissive
vii. Voltage developed by generator matches with that of the grid,
viii. Phase sequence of voltage of the generator must be same as that of the grid
ix. The speed (revolutions per minute or more precisely revolutions per second) of the prime mover (steam turbine/gas turbine/diesel engine) to be connected must match the frequency (cycles per second/Hertz) of the grid.
Tripping of the generator essentially means disconnecting it from the grid by opening the generator circuit breaker.
The generator is tripped for faults, which may cause severe damage to it and/or the connected prime mover (steam turbine/gas turbine/diesel engine) and needs immediate isolation of the unit from the grid. These faults generally include the fault inside and outside the protective zone of the generator, generator transformer, and unit auxiliary transformer, and in the case of a steam turbine, low lubricating oil pressure, high axial shift, etc. The tripping of the generator, initiated through generator lockout relay (industrially known as 86G relay), in turn ensures complete shutting down of the prime mover and the boiler (applicable to steam power plant only). The generator is tripped automatically on occurrence of any of the following typical conditions, but may not be restricted to these only:
vi. Generator volts/Hertz high (over flux)
vii. Generator differential protection
viii. Generator loss of excitation
xi. Generator stator earth fault
xii. Generator rotor second earth fault
xiii. Generator split phase protection for inter turn fault
xiv. Generator pole-slip protection
xv. Generator back up impedance protection
xvi. Generator negative phase sequence protection
xvii. Generator rotor high temperature protection
xviii. 132/220/400/700 kV over current protection
xx. Bus differential protection
xxi. Overall differential protection
xxii. Generator transformer oil temperature high
xxiii. Generator transformer winding temperature high
xxiv. Generator transformer negative sequence current very high
xxv. Generator transformer over current protection
xxvi. Generator transformer standby earth fault
xxvii. Generator transformer restricted earth fault
xxviii. Generator transformer Buchholz protection
xxix. Unit auxiliary transformer on-line tap changer (OLTC) Buchholz protection
xxx. Unit auxiliary transformer Buchholz protection
xxxi. Unit auxiliary transformer differential protection
xxxii. Unit auxiliary transformer over current protection
xxxiii. Unit auxiliary transformer restricted earth fault protection
xxxiv. Generator reverse power relay operation
xxxv. Steam turbine trips causing generator low forward power relay to operate (applicable to steam power plant only)
xxxvi. MFR trips with generator load above unloading limit (applicable to steam power plant only)
Tripping of generator should initiate the tripping of:
i. Prime mover (steam turbine/gas turbine/diesel engine) lockout relay
ii. Master fuel relay (applicable to steam power plant only)
iii. HT (6.6kV) switchgear unit incomer breakers
Immediately after the generator trip, check and ensure that
1. Generator circuit breaker is open
2. Field breaker is switched off
3. Unit HT (6.6 kV) switchgear breaker is open
4. Seal oil supply, if applicable, to generator seals is adequate
5. Generator hydrogen cooling system and stator cooling water system are in service, if these systems are provided
6. Prime mover (steam turbine/gas turbine/diesel engine) speed is falling down
7. Lube oil supply to bearings of generator/prime mover (steam turbine/gas turbine/diesel engine) is adequate
8. Auxiliary oil pump has taken start automatically, if not, start the same manually
9. All the extraction line NRVs and isolating valves in the extraction lines near feedwater heaters have closed (applicable to steam power plant only)
10. Check and ensure that all fuel to furnace/gas turbine/diesel engine (as applicable) has been completely cut off
11. Maintain condenser vacuum (applicable to steam power plant only)
12. Keep condensate extraction pump, boiler feed pump, circulating water pump running (applicable to steam power plant only)
13. Maintain drum level (applicable to sub-critical steam power plant only)
14. Purge the furnace and shut-down draft fans (applicable to steam power plant only)
15. Check and ensure SH, RH attemperator spray water supply is completely cut-off (applicable to steam power plant only).