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Electrical Systems in a
Thermal Power Plant
Presented
By:
Group B4
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Electrical SystemElectrical system one of the most important components of apower plant. With the help of this system the mechanical energyis converted to electrical energy and is distributed to variousconsumers according to their requirements.
Components:
Air Cooled generators
Digital voltage regulator Numerical protection relays for fastest response
Fault data recorder
SF6 circuit breaker for high voltage applications
Energy management system
Real time display of generation, dispatch and auxiliaryconsumption
SCADA (supervisory control and data acquisition) for remoteswitchyard operation and data transfer of load dispatch centre
Centralized monitoring of HT motor protection relays
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Contents
Boiler
Turbine
Generators
Transformers
Distributed Control System
Illumination
Fire Barriers Earthing
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Control System Operation of the power plant must be coordinated to meet the
electricity demand and the production process must be regulatedso that the cycle and the equipment are operated with design
limits
Control system provides integrated interface that is responsible
for monitoring and controlling its boiler and turbine-generator,and also for all other tasks as well, such as data management
and technical-service support.
Control system functions:
Acquisition & Processing of process data.
Open loop and closed loop control.
Open loop system is a non feedback type system relying only on the
current and the model of the system
Closed loop system gets feedback from the system and adjusts itself
accordingly
Calculation and optimization of the plant performance
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Control System
A distributed control system is one in which the controller
elements are not central in location but are distributed
throughout the system with each component sub-system
controlled by one or more controllers
Entire system of controllers is connected by networks for
communication and monitoring
The plant's information and control systems are determined
based on requirements of mechanical and electrical engineering
To ensure maximum operational surveillance and ease of
operation a basic system plan is created;
the system's configuration is drawn up;
the system's input output information is collected;
control logic diagrams are prepared;
logic of the plants automation systems is examined carefully.
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Operator Work Station
Alarm Monitoring
Mimic/graphic display
Trend Display
Operator guidancedisplay
System and diagnostic
display
Control Bar chart
Sequence display
Fault analysis display
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Types of Control Systems
On-Off control systems produces a controlaction that varies in discrete steps.
Motor driven rotating equipment such as pumps,
fans, compressors and conveyors;
Motor operated shutoff valves and dampers; and
Solenoid operated equipment such as pneumatic
shutoff valves.
Modulating control systems produces a control
output system that varies smoothly from one
value to another
Mainly found in the Boiler Regulation Control,
Steam Turbine Automation Control, and CCS
(Coordinated Control System)
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Data Acquisition System (DAS)
Main task is to acquire the data from any kind ofinputs such as sensors via amplifiers,
multiplexers, and any necessary analog-to-digital
converters
The data is used for
Plotting: Bar Chart, Curve plotting, etc.
Alarm Monitoring
Recording of SOE (Sequence of Event)
quick identification of the cause of faults.
Display Real-time or Historical Trends
Provides information and analysis of different
parameters.
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Boiler A Boiler or steam generator essentially is a container into
which water can be fed and steam can be taken out at desiredpressure, temperature and flow.
Boiler should have a facility to burn a fuel and release the heat.
Functions of a boiler:-
To convert chemical energy of the fuel into heat energy To transfer this heat energy to water for evaporation as well
to steam for superheating.
The basic components of Boiler are
Furnace and Burners, Steam and
Superheating Low temperature superheater
Platen superheater
Final superheater
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Boiler fittings and accessories
Safety valve: to relieve pressure and prevent possible
explosion of a boiler Water level indicators: show the operator the level of fluid in
the boiler
Bottom blow down valves: provide a means for removingsolid particulates that condense and lie on the bottom of a
boiler Continuous blow down valve: allows a small quantity of
water to escape continuously. Its purpose is to prevent thewater in the boiler becoming saturated with dissolved salts Saturation would lead to foaming and cause water droplets to
be carried over with the steam - a condition known as priming Flash Tank: High pressure blow down enters this vessel
where the steam can 'flash' safely and be used in a low-pressure system or be vented to atmosphere while the ambientpressure blow down flows to drain
Automatic Blow down/Continuous Heat Recovery System:This system allows the boiler to blow down only when makeup
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Hand holes: for inspections & installation of tubes and inspection ofinternal surfaces
Steam drum internals, A series of screen, scrubber & cans (cyclone
separators)
Low- water cutoff: It is a mechanical means (usually a float switch) that isused to turn off the burner or shut off fuel to the boiler to prevent it from
running once the water goes below a certain point
Surface blow down line: It provides a means for removing foam or otherlightweight non-condensible substances that tend to float on top of the
water inside the boiler
Circulating pump: It is designed to circulate water back to the boiler after ithas expelled some of its heat
Feedwater check valve : A non-return stop valve in the feedwater line. Desuperheater tubes: A series of tubes or bundles of tubes in the water
drum or the steam drum designed to cool superheated steam.
Chemical injection line: A connection to add chemicals for controllingfeedwater pH.
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Steam turbines Steam turbines have been used predominantly as prime
mover in all thermal power stations. The steam turbines aremainly divided into two groups: - Impulse turbine Impulse turbines change the velocity of a
water jet. The jet pushes on the turbine's curved blades which
changes the direction of the flow. The resulting change inmomentum (impulse) causes a force on the turbine blades.
Impulse-reaction turbine Reaction turbines are acted on bywater, which changes pressure as it moves through the turbineand gives up its energy.
The turbine generator consists of a series of steam turbinesinterconnected to each other and a generator on a commonshaft. There is a high pressure turbine at one end, followedby an intermediate pressure turbine, two low pressureturbines, and the generator.
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AC Generator
Alternating voltage may be
generated by rotating a coil in the
magnetic field or by rotating a
magnetic field within a stationary
coil.The value of the voltage generated
depends on-
the number of turns in the coil,
strength of the field,
the speed at which the coil ormagnetic field rotates
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Stator electrical faults
Overload Over Voltage
Unbalanced Loading
Over fluxing
Rotor Electrical faults Loss of excitation
Loss of Synchronization
Failure of prime mover
Lubrication Oil failure
Over-speeding Rotor Distortion
Difference in expansion between rotating and stationary parts
Excessive Vibration
Core Lamination faults
Problems faced with Generators
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Generator protection
Designed to run at a high load factor for a large number of
years
The generator circuit protection should be designed in such a
way that Damage is kept to a minimum,
Machine is not subjected to abnormal condition
The machine and its auxiliaries are supervised by monitoring
devices Electrical and mechanical faults may occur, therefore
generators may be provided with protective relays which in
case of faults, quickly initiate a disconnection of the machine
from the system and , if necessary, initiate a completeshutdown of the machine.
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Various methods of Generator ProtectionOver and Under Voltage Protection
The under voltage relays are mainly installed for the purpose ofidentifying loss of PT voltage
Overvoltage relays are also used as backup to the over excitationduring normal operation
Synchronizer and Sync-Check Relays
Unit can be brought up to speed automatically and synchronized to the
system Before doing so, the amplitude of the voltages of the system and
generator terminal must be within a narrow margin
frequency difference between the machine and the system, must belower than a given value
Generator Differential Protection
It protects generator winding against internal faults like excess currentflow
Short-Circuit Protection
Voltage control is provided to differentiate between a low-current fault
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Reverse Power Protection
This protective function trips the unit when power flows from the
system to the generator.
Voltage Balance Protection
The main function of the voltage balance relay is to avoid false
tripping of other protection relays due to a loss of secondary voltage
feed
Volts/Hertz Protection
when a severe over excitation occurs, the most probable
result is partial or complete destruction of the cores
insulation
over excitation occur during run-up, prior to synchronization
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Over/Under Frequency Protection
results from full or partial load rejection or overloading condition
Load rejection can be caused by a fault in the system or load shedding
Overload conditions may arise from tripping a large generator or a
transmission line
Loss-of-Field Protection
removal of the source of excitation to the generator can happen by
unplanned opening of the field breaker, a failure of the exciter and a
short-circuit in the field winding
in a loss-of-excitation condition its speed increases by some amount
of up to 3 to 5% of normal.
A fully loaded unit that loses its field may experience serious damage
every quickly under these conditions. Therefore the protection against loss-of-field occurrences is set to
alarm and trip the unit relatively quickly
Stator and Rotor Thermal Protection
Generators have a number of RTDs (resistance temperature detectors)
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Breaker Failure Protection
Most faults involving the generator require tripping the line breakers.
Failure of any such breaker to operate properly results in loss of
protection and other abnormal conditions, such as motoring
breaker failure scheme is carried out by a combination of triggering
signals from the generator protective relays, over current relays and
breaker auxiliary switches, via a timer
Loss of Synchronism Protection based on the fact that the apparent impedance at the
generators terminals changes in a predicted manner during an unstable
condition
To minimize any harmful effects, the protection should separate the
generator from the system as soon as possible
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TransformerAccording to winding it can be core type orshell type
According to cooling it can be classified as AN, ON, ONAN, ONAF, OFAF,OFWF
It can be liquid filled (oil), air cooled or resin cast type
Transformer core:
provides continuous path for electromagnetic flux
made of cold rolled grain oriented alloy steel(CRGO), which reducedhysteresis loss and enabled the core to operate at higher flux density
specific loss in watt/kg is lowest in the direction of rolling
Transformer oil:
protects the core assembly from chemical attack
provides dielectric strength of the transformer system
provides efficient cooling system of the transformer
Transformer oil is a pure hydrocarbon mineral oil and generally paraffin base
Oil temp. indicator:
Operates on the principle of expansion of liquid with temperature Fitted with max pointer and two mercury switches to provide alarm and trip
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Relief vent:
Terminal pressure may be built up to very high value by a severe fault
Diaphragm breaks open and relieves the pressure if the pressure is morethan 35 KN/m2
Circuit breakers:
capable of making and breaking of electric circuit under normal and abnorma
conditions
DG system:
Turbo generator sets takes about 20-25 min for coasting down to barring
speed
essential to provide emergency oil supply to the turbine and generated
bearings
DG sets gives oil supply to scanners in steam generators and power
generation
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Illumination SystemThe auxiliary building shall be provided with:
Main Lighting system for full illumination under normal power supply
conditions and shall operate from 415/ 240V AC Power supply tapped from
respective 415V switchgear
Emergency lighting system for reduced illumination operated by DG supply
feeders during failure of main power supply. It will cover only 20% of fixtures
in the building and associated area
Minimum emergency lighting system for reduced illumination during failure
of main power supply with the help of 220V DC batteries/ supply feeders
Suitable no. of Portable lighting units
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Luminous Performance Characteristics of commonly usedLuminaries
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Illumination Levels
Depending on the size of the area, no. of luminaries can becalculated.
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Design of Illumination System
Illumination Calculations
Lighting Layouts of Control
Room
Lighting layout of Switchyard.
Single Line Diagrams
Bill Of Materials
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Fire Barrier Envelope system installed around electrical componentsand cabling that are rated by test laboratories in hours of
fire resistance and are used to maintain safe-shutdownfunctions free of fire damage
Encase the components they are protecting to reduce thethermal exposure to the protected component during
elevated fire conditions. Fire barriers are often employed to ensure that the plant
can safely shut down in the event of a fire.
Barriers protect important equipment until the fire detectionand automatic fire suppression systems operate.
In the unlikely event that an automatic fire protectionsystem fails to operate, the barriers continue to providepassive fire protection.
Fire rating is defined as the endurance period of a firebarrier or structure, which relates to the period ofresistance to a standard fire exposure before the firstcritical point in behavior is observed
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Materials for fire barriers Insulation materials limit the exposure to the heat transfer rate by
reducing the conductive heat transfer rate to the protected
cable/circuits in accordance with Fouriers Law. Example :
A.B.S., ACETATE, CERAMIC, EPOXY/FIBERGLASS
Intumescences materials reduce the heat transfer rate to the
protected cable/circuits by chemically absorbing heat energy.
This endothermic reaction causes the material to swell,
increasing in volume and decreasing in density. Example:
Graphite, Sodium silicate
Ablation materials reduce the heat transferred to the protected
cable/circuits by sublimation. When heated, the ablation material
is consumed (sacrificed) through sublimation and mass loss
which provides cooling and forms a thermal shield. Example:
Silicone
Hydrate materials contain chemically bound water that is used
up during a fire exposure by an endothermic reaction, which
maintain temperatures near 100C until the hydrate (water) is
converted into steam. Example: Hydrated alkali metal silicate
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Swelling of Intumescences materials
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EARTHING SYSTEM
Following basic requirements are to be satisfied so as to ensure a properand sound earthing system.
The earth resistance for the switchyard area should be lower than a
certain_limiting value in order to ensure that a safe potential gradient
is maintained in the switchyard area
The grounding conductor material should be capable of carryingthe maximum earth fault current without-overheating and mechanical
damage.
The maximum fault level in the 400 kV system has been estimated to
be 40 kA and this value of fault current to used is the design of earth
mat for the 400 kv substation.
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REQUIREMENTS OF A GOOD
EARTHING SYSTEM
All metallic objects which do not carry current andinstalled the substation such as structures, parts
of electrical equipments, fences, armouring
and sheaths of the low voltage power and control
cables should be connected to the earthingelectrode system..
The design of the ground conductor should take
care of the effect of corrosion for the total life
span of the plant
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Material
The material of the earthing conductors shall be asfollows :
a) Conductors above ground level and in built up
trenches Galvanized steel
b) Conductors buried in earth Mild steel
c) Earth electrodes Mild steel rod