INDIAN INSTITUTE OF SPACE SCIENCE AND TECHNOLOGY,

THIRUVANANTHAPURAM

PROCESS INSTRUMENTATION AND CONTROL

INTERNAL PROJECT REPORT

Submitted By

ROHAN DUTTA

VASU DUBEY

IN

AVIONICS

IN

Singrauli Super Thermal Power Station, NTPC Ltd.

August, 2010

ABSTRACT

With increase in demand of power in India, new power projects are being

constructed with higher capacity and advancement technology for the better economy and

reliability of operation. Compared to lower capacity power plant, 500 MW capacity power plants

have incorporated certain special technical features, which are of importance from control point

of view. 500 MW Turbine at Singrauli Super Thermal Power Station (SSTPS) is of Kraftwork

Union, West Germany design and Boiler is supplied by BHEL. This dissertation work is a case

study of various control aspects in thermal power plant. It describes starting from fundamentals

of a thermal power plant up to the latest technology in control called DDCMIS. Implementation

of controllers like P, PI, and PID is also studied in this dissertation.

2

TABLE OF CONTENTS

CERTIFICATE

ACKNOWLEDGEMENT

ABSTRACT

LIST OF SYMBOLS & ABBREVIATIONS USED

CHAPTER – 1 INTRODUCTION 7

a. INTRODUCTION 7

CHAPTER – 2 OVERVIEW OF THERMAL POWER PLANT 9

a. BOILER 9

b. FUEL & AIR SYSTEM 10

c. TURBINE 10

d. CONDENSATE WATER SYSTEM 11

e. FEEDWATER SYSTEM 11

f. GENERATOR 11

CHAPTER – 3 NEED OF AUTOMATION IN THERMAL POWER PLANT

12

CHAPTER – 4 IMPORTANT PARAMETERS & INSTRUMENTATION

a. PRESSURE MEASUREMENT 14

b. STRAIN MEASUREMENT 16

c. TEMPERATURE MEASUREMENT 17

d. TURBO-SUPERVISORY INSTRUMENTS 18

e. LOCATION OF DETECTORS 22

3

CHAPTER – 5 CONTROL LOOPS IN POWER PLANTS 23

a. FIRING RATE CONTROL 23

i. FUEL FLOW CONTROL 23

ii. AIR FLOW CONTROL 23

b. INLET VANE POSITION CONTROL 24

c. PRIMARY AIR HEADER PRESSURE CONTROL 24

d. MILL AIR FLOW CONTROL AND OUTLET TEMPERATURE

CONTROL 24

e. FEEDWATER CONTROL 25

f. SUPER HEAT STEAM TEMPRATURE CONTROL 25

g. REHEAT TEMPERATURE CONTROL 26

h. DEAERATOR PRESSURE CONTROL 26

i. DEAERATOR LEVEL CONTROL 26

j. HOTWELL LEVEL CONTROL 26

i. HIGH LEVEL CONTROL 27

ii. LOW LEVEL CONTROL 27

k. REHEATER TEMPERATURE CONTROL 28

a. BURNER TILT 28

b. RH SPRAY SYSTEM 28

CHAPTER – 6 ELECTRO HYDRAULIC GOVERNING SYSTEM 29

a. ELECTRO HYDRAULIC CONTROLLER 29

b. SPEED CONTROLLER 29

c. SPEED CONTROL LOOP 30

d. DN/DT MONITORING 31

e. LOAD CONTROLLER 32

f. LOAD CONTROL LOOP 32

g. PRESSURE CONTROLLER 33

h. VALVE POSITION CONTROL 33

CHAPTER – 7 DDCMIS 34

a. COMPARISION 34

4

b. GEOGRAPHICALLY DISTRIBUTING THE HARDWARE 35

c. FUNCTIONALLY DISTRIBUTING THE HARDWARE 35

d. OPERATOR STATION 36

e. PLANT MONITORING SYSTEM 36

f. ENGINEERING STATION 36

CHAPTER – 8 FURNACE SAFEGUARD SUPERVISORY SYWSTEM (FSSS)

37

a. SYSTEM DESCRIPTION 37

CHAPTER – 9 CONCLUSION & FUTURE DIRECTION 38

a. CONCLUSION 38

b. FUTURE DIRECTION 38

i. SLIDING SET POINT OPERATION 38

ii. MAX DNA 38

APPENDIX 39

REFERENCES 45

5

LIST OF SYMBOLS & ABBREVIATIONS USED

: Valve

: Pump

: Gauge

: OR Gate

: Turbine

MW: Mega Watt D/A: De-Aerator

DAS: DATA Acquisition System TSE: Turbine Stress Evaluator

HPT: High Pressure Turbine PI: Proportional Integral

IPT: Intermediate Pressure Turbine PID: Proportional Integral Differential

LPT: Low Pressure Turbine VDU: Visual Display Unit

PA: Primary Air DC: Direct Current

FD: Force Draft RH: Re-Heater

ID: Induced Draft MFT: Master Fuel Trip

SH: Super Heater UCB: Unit Control Board

BFP: Boiler Feed Pump CRH: Cold Re-Heat

CEP: Condensate Extraction Pump HRH: Hot Re-Heat

HEA: Heavy Energy Arc

6

CHAPTER - 1

INTRODUCTION

1.1 INTRODUCTION:

The 500 MW unit of Singrauli Super Thermal Power station is itself an institution where

learning is unlimited. As the power plant has different units like Boiler, Turbine, Generator etc.,

different logics have been used to meet the requirements. A case study is done about the different

control systems in the different areas of a power plant. It is of great importance to know that how

the implemented control system is achieving its target.

The control system being used in Singrauli Super Thermal Power station is centrally located as it

is very tedious to maintain a good environment for electronic hardware locally, because of the

conditions there.

As shown in the fig 1.1 equipment room is the heart of the control system. It receives the

commands from control room panel and also from local/ field. It also receives feedback and the

inputs, from fields. The signals are processed here and send either to control room panel (for

alarm or indication) or field to actuators, or to DATA Acquisition System (DAS) and printers.

A View of NTPC-SSTPS

7

I

P MOTOR VALVE

8

DAS

COMMAND FROM LOCAL

FE

E

D

B

A

K

FIELD

SINGRAULI SUPER THERMAL PLANT

TRANSDUCERS

PANEL

EQUIPMENT ROOMI

N

P

U

T

F

R

O

M

Fig . 1.1 CONTROL SYSTEM

CHAPTER - 2

OVERVIEW OF A THERMAL POWER PLANT

2.1 INTRODUCTION:

Thermal power plant, used to generate electricity, can be coal based or oil based. This fuel is

burnt in boiler through which steam is generated. This steam is taken to turbine which is coupled

with Generator. When turbine rotates, Electricity is produced through generator.

Here is discussed a brief case study of 500 MW unit of Singrauli Super Thermal Power Station.

The three essential units of any Thermal Power plant as shown in the figure 2.1 are:

Boiler

Turbine

Generator

2.2 BOILER:

The boiler is Radiant, Reheat, Controlled circulation, Single drum, and Dry bottom type unit.

Each boiler corner is fitted with tilting tangential burner boxes comprising of 4 high Energy Arc

Igniters, 4 light–up heavy oil fired burners & eight pulverized coal burners.(Refer to Appendix)

9

2.3 FUEL & AIR SYSTEM:

The system for firing pulverized coal utilizes bowl mills to pulverize the coal and a tilting

tangential firing system to admit the pulverized coal, together with the air required for

combustion to the furnace. As crushed coal is fed to each pulverizer by its feeder, Primary Air

supplied from Primary Air (PA) Fan, which dries the coal as it is being pulverized and transports

the coal through coal piping system to the coal nozzles in wind box assembly of furnace. Fully

Preheated Secondary Air for combustion enters the furnace around the pulverized coal nozzles.

Combustion is completed as the gases spiral up in the furnace.

2.4 TURBINE:

The turbine is a reaction, condensing type, tandom compound with throttle governing and

regenerative system of feed water heating. It is coupled directly to the generator. Te turbine is

single shaft machine with separate HP, IP & LP turbines. The HP turbine is single flow cylinder

where as IP & LP is a double flow cylinder as shown in fig. 2.4.

10

2.5 CONDENSATE WATER SYSTEM:

The condensate extraction pumps deliver the condensate through the three low pressure

feedwater heaters, the Deaerator to the Feed storage tank Storage Tank., which is start point to

Feedwater System. Low pressures Feedwater Heaters receive extraction steam from the turbine.

Condensate absorbs heat from the extraction steam as it passes through the heaters.(Refer to

Appendix)

2.6 FEEDWATER SYSTEM:

The purpose of feedwater system is to provide adequate flow of properly heated & conditioned

water to the boiler and maintain boiler drum level compatible with boiler load. Feedwater flows

from Deaerator storage tank to Boiler Feed Pumps. The flow continues through High Pressure

Feedwater Heaters string to the boiler Economizer inlets. Finally feedwater enters the boiler

drum.

2.7 GENERATOR:

Turbo Generator for 500 MW units is two pole water and Hydrogen gas cooled generators. The

primary water flows through the stator winding and picks up heat losses arising in generator

directly, which are transferred to the Secondary cooling water. Core losses, windage losses and

rotor winding losses are picked up by circulating Hydrogen and transferred to Hydrogen cooling

water in the gas coolers. A brushless exciting system with rotating diodes is provided to excite

the machine. It consists of pilot exciter which is a 3 phase permanent magnet, 16 pole revolving

field, the output of which is rectified and controlled by thyristor voltage regulator to provide a

variable dc current for main exciter. The main exciter is 6 pole, revolving armature unit. The

three phases are induced in the rotor of the main exciter and is rectified by the rotating diodes

and to the field winding of generator rotor through DC leads fed in the rotor shaft. Since the

rotating Rectifier Bridge is mounted in the rotor, the slip rings are not required and the output of

rectifier is connected directly to the field winding through the generator rotor shaft.

11

CHAPTER - 3

NEED OF AUTOMATION IN THERMAL POWER PLANT

The word ‘Automation’ is widely used today in relation to various applications such as office

automation, plant or process automation etc. This dissertation presents the application of a

control system for the Automation of a process/plant, such as a power station. Complete plant

automation provides the operating personnel with the necessary tool to increase plant availability

& efficiency.

Automation actively controls the plant during the three phases of operation.

Plant start up,

Power generation in stable or transient conditions

Plant shut down.

1) The automation system is of a particular importance when the plant behaves in an

unpredictable way. The automation will then automatically and immediately respond

without time delay to bring plant into a safe operating state with respect to environment

process components and human beings.

2) During stable generation of power, the analog portion of the automation system keeps the

actual generated power value within the limits of the desired load demand.

3) During major load changes the automation system automatically redefines new set points

and switches ON or OFF process pieces to automatically bring the individual processes in

an optimally coordinated way to the new desired load demand. This load transfer is

executed according to preprogrammed adaptively controlled load gradient and in safe

way.

4) In thermal power plant it has become necessary to pay special attention to the additional

thermal stresses which results from thermal changes. These stresses damage heavy

masses of turbine and rotor. Turbine Stress Evaluator (TSE) takes care of this in an

automated plant. It stops further increasing of load or lowers main steam temperature or

flow if excessive stress in the metal temperature are indicated

12

5) For start up, acquisition and analysis of a wide variety of information pertaining to

various parameters of steam turbine, demand quick decisions and numerous operations

from the operating personnel. In order to reduce the tedious task of monitoring various

parameters and effect sequential start up, we have to minimize the possible human errors.

And to achieve start up in minimum time in optimum way Automatic Run Up System

(ATRS) is introduced.

6) Furnace Safeguard &Supervisory System (FSSS), is the system, which has been designed

to protect Boiler and its auxiliaries. It offers maximum protection, minimum nuisance

trips, minimum power consumption and max life of the components used. It includes oil

guns, HEA Igniter, Flame scanner, Heavy oil trip valve, Heavy oil recirculation valve etc.

7) Availability, reliability and efficiency of a boiler, turbine unit hinge around the close

control of the chemical regimes of the working fluids, i.e. water / steam in the circuits as

well as the combustion in the boiler. To monitor this Analytical Instruments and their

control system has been incorporated. Which continuously monitors O2, CO2 and CO

formation in boiler, and conductivity, cation conductivity, pH, sodium, silica, Hydrogen

in water/steam circuit.

13

CHAPTER – 4

IMPORTANT PARAMETERS AND INSTRUMENTATION

This chapter is about the various parameters that are to be measured in a thermal power plant and the respective instrumentation that is required for the same.

1. PRESSURE:

Pressure measurement not only is critical to the safe and optimum operation of such industrial process as steam generation, hydraulic equipment operation, air and gas compression, vacuum processing (measurement), hydrostatic pressure, differential pressure etc.

Pressure measuring devices:

• Manometers using water ,mercury and other liquids of known density for low pressure measurement.

• Diaphragm, capsule bellows for measuring medium pressures.

• Bourdon tube gauges for measuring medium and high pressures.

• Transducers of different types for measuring pressure of all ranges for telemetering purposes.

14

Absolute – pressure Manometer

Bellows : This is thin wall metal tubes with deeply convoluted side walls which permits axial

expansions and contraction.

15

U-tube manometer

p2 p1

h

Area A1

Area A2

P2

h

P1

Well Manometer

h

Absolute Pressure - P

Vacuum

Common form of bellows used in pressure gauge

2. STRAIN:

• If a wire is held under tension, it gets slightly longer and its cross-sectional area is reduced.

• This changes its resistance (R) in proportion to the strain sensitivity (S) of the wire's resistance.

• The strain sensitivity is also called the Gauge factor (GF).

16

The Pigtail is used on horizontal pipelines where there is sufficient space above the pipe.

'U' type is used when mounting the gauge on a vertical pipeline, or on horizontal pipelines where there is not sufficient space for a ring type siphon.

3. TEMPERATURE :

• Assess the material fatigue, heat balance, heat transfer etc.

• Assessing the process condition

• Efficient and economic operation at determined load.

• Vital information display for safe operation of the plant.

• Analysis

Instruments

• PRIMARY INSTRUMENTS

17

Strain Gage

Typical metal-foil strain gages

Thermocouple

Resistance thermometer (Pt100,Cu53)

• SECONDARY INSTRUMENTS

Instruments that use electronic bridges. (For alarm, protection)

Selection of instruments at a particular place in a power plant is done on the following basis :

• The accuracy required

• The range of temperature

• Process media on sensing element

• The layout conditions and restrictions

• Facilities available for calibration of the instrument

4. TURBOSUPERVISORY INSTRUMENTS (TSI)

Hallprobes:

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Stainless Steel Shealth

Ceramic-EncapsulatedResistance Element Internal Lead Wire

(0.05 to 0.07mm Dia)

High Purity Ceramic insulator

High temp.Seal

ExtensionLead wire

THERMOMETER ASSEMBLY

Principle of a proximity sensor:

The oscillator in the matching unit supplies (RF signal) the coil in the sensor with a load Independent, high frequency alternating current. If the sensor is brought near to a metal object, the eddy currents are induced in surface of the object and the power is absorbed thus alternating the voltage across the coil. The alternating voltage is proportional to the distance between the

19

(USED FOR SPEED MEASUREMENT)

PRINCIPLE :

Ix

Ix

Fy

EyA B

(+) (-)

Ex

F = q (E + V*B)

In Y direction the

force is= q( Ey – Vx Bz )

Bz

Ey = Vx Bz

Ey = Hall Effect

VAB = EY WHALL VOLTAGE

Magnetic Field Applied Perpendicular To The Direction Of Drift

w

OSCILLATOR

PROBE GAP

VO

LT

S

0 100

EXTENSION CABLEAND PROBE

OBSERVEDMATERIAL PROXIMITOR

VOLTAGEDETECTOR

CONDUCTIVEMATERIAL

PROXIMITOR

Eddy Current

Radio FrequencySignal

PROXIMITY SENSOR :

measured Object and the sensor. The output amplifier forms a load independent D.C. Voltage signal.

SIESMIC SENSOR:

20

LVDT : Linear variable Differential Transformers (Casing expansion)

AC voltmeterAC

Secondary coil connected in series Opposition,In Null position secondary voltage will Cancelthere will be no o/p voltage.When core is Displaced from null Position,two voltage noLonger cancel.A net o/p voltage will now result.

( + )

( - )

Nominal range

50 10050100

LVDT voltage as a function of core position

Core position (%)

Voltage out

Measuring relative expansion :

21

N

S

1

2

3

4

56

Exciter of vibrationPermanent MagnetPlunger CoilMagnetic Return PathSpringSensor Case

SIESMIC MASS : Plunger Coil principle

Natural frequency of spring mass system ω = 2πf = √c/m

m = mass of plunger coil with springC = spring constantAbove ω,coil stands still due to its mass inertia

When coupling the siesmic sensor to vibrating structure,a relative movement isgenerated between permanent magnet & plunger coil.a voltage is induced in the coilWhich is proportional to the vibration velocity :

e = B . l . Vl= Length of conductorsB= magnetic inductionv =speed of vib.

22

BASIC ARRANGEMENT OF HALL PROBES AT BEARING 1

SENSOR

WIRE

BRACKET

PERMANENT MAGNETSN-POLE

S-POLE

DISK

CHAPTER - 5

CONTROL LOOPS IN POWER PLANT

In this chapter various control loops in a Thermal Power Plant have been discussed with

reference to 500MW unit of Singrauli Super Thermal Power Plant, NTPC.

5.1 FIRING RATE CONTROL:

Firing rate control is achieved by controlling the flow of fuel as well as air going in the

furnace.

Fuel flow to the boiler is controlled by varying the speed of coal feeders or by variation

of the fuel oil flow.

Airflow has to be controlled in proportion to the fuel flow to maintain a good

combustion at all load conditions.

5.1.1 FUEL FLOW CONTROL:

The boiler demand signal, produced in the boiler demand control is fed through a minimum

selector and compared with a signal, representing the total fuel flow and the control deviation is

fed to a PI controller The output signal of on be controller is the set point signal for the eight coal

feeders. To speed up mill response a derivative signal of boiler demand is added to the controller

output, via an analog memory which is tracking the feeder setpoint, when on auto. A bias setter

in the setpoint signal to each feeder allows different loads of the mills on auto.

The calibration of feeder speed signal representing coal flow, can be achieved by manually via

an analog memory, when the calorific value of coal varies.

5.1.2 AIR FLOW CONTROL:

The boiler demand signal and the fuel flow signal (delayed according to mill response time) are

fed to a maximum selector and the higher of this signal is compared with the calibrated air flow

signal. The control deviation is fed to two controllers which position the two electric actuators

for control of the Force Draft (FD) Fan blade pitch.

23

5.2 INLET VANE POSITION CONTROL:

To save energy and to keep the FD fan inlet vanes always in a good control position, the fan

speed is controlled. The average inlet van position of the fans on auto is compared with the

desired set point, for instance 80%, and the control deviation fed to the three individual

controllers. These controllers supply more or less pulses and position the scoop tubes of the

hydraulic couplings between motors and fans.

5.3 PRIMARY AIR (PA) HEADER PRESSURE CONTROL:

Primary air supply to all pulverizes is maintained by controlling the primary header pressure at

the common discharge duct of two PA Fans. PA header pressure is controlled by regulating the

blade pitch of constant speed, axial reaction type PA Fans.

5.4 MILL AIRFLOW CONTROL & OUTLET TEMPRATURE CONTROL:

Coal air temperature at the mill outlet and PA flow to each mill is controlled by modulating hot

air and cold air dampers in PA line to mill as shown in Fig 5.2. Measured value signal is

compared with a desired value signal generated by characterization of feeder speed signal. The

output of proportional plus reset action PA flow controller acts on the hot air damper to regulate

the primary air flow. Provision is given to ensure a preset minimum flow through the mill at all

times. Coal air temperature at the mill output is compared with the manually selected desired

value signals in a proportional, reset and rate action controller. The output of this controller shall

act on the cold air damper for regulating coal air temperature. Interlocks are provided for

automatically opening the cold air damper and closing the hot air damper on a pulverizer trip,

based on the signals from, FSSS (discussed later).

24

5.5 FEED WATER CONTROL:

The boiler drum level has to be kept constant under all load condition. Under normal load

conditions ( above 20% load) the controller system acts as three element control utilizing steam

flow as feed forward signal, feedwater flow as feedback signal and the drum level control

deviation as main control signal. This control varies the speed of the two Turbine Driven feed

pumps or through a hydraulic coupling of one electric motor driven feed pumps. On low load

control is achieved by positioning a low load control valve with drum level as the only influence.

5.6 SUPER HEAT (SH) STEAM TEMPERATURE CONROL:

This control loop has a task to maintain the SH temperature at a constant value. The boiler has

two SH lines; each line has two injection coolers for temperature control. The final super heaters

are placed after injection coolers. Temperature is adjusted by two circuit controllers. The dual

loop controllers are composed of a PI controller, which actuates the injection valve in

25

DAMPERS

HOT PA AIR

AIR & COAL MIXTURE

TOFURNACE

COAL FROM FEEDER

BOWL MILL

AIR & COAL MIXTURE

TO FURNACE

COLD AIR

Fig. 5.2 BOWL MILL

ROLLERS

dependence of the control deviation of the boiler outlet temperature and in dependence of the

rate of change the temperature behind the injection cooler. The rate of change is raised by an

integrator. The input signal of the integrator is consisting of the difference between its output

signal and the actual value of the temperature.

5.7 REHEAT TEMPERATURE CONTROL:

The reheat steam temperature is controlled on the one hand by changing the burner tilt position

on the other hand by means of injection coolers. The burner tilt position is precontrolled

according to the load. This signal is corrected by the output of the PID temperature controller. In

case of fast load changes an additional D-part of the boiler load index is superimposed. Required

set point is set through a setter. There are two injection control valves available for one cooler;

one valve is in operation; and other is standby.

5.8 DEAERATOR PRESSURE CONTROL:

The pressure in deaerator is normally controlled from the cross over pipes between IP and LP

turbines. During low load and turbine bypass operation, cold reheat steam is used for deaerator

heating as shown in Fig. 5.3. Auxiliary steam is used to maintain the pegging steam pressure at

3.5 atm during bypass operation, hot start up and warm start up and a minimum pressure of 1.5

atm during a cold start up.

5.9 DEAERATOR LEVEL CONTROL:

The D/A control loop maintains the water level in the D/A by regulating the condensate flow to

D/A with help of two 100% capacity flow control valves C and D located in condensate line.

Transmitters are provided to measure:

Deaerator level

Condensate flow

HP heater 5A drain flow

HP Heater 5B drain flow

26

5.10 HOTWELL LEVEL CONTROL:

Steam after LP Turbine gets condensed and gets collected in Hotwell, from where Condensate

Extraction Pumps (CEP) take suction as shown in Fig.5.5. For healthy operation of CEP and

condenser adequate level has to be maintained in Hotwell. For this two different control loops

regulate the flow of water.

5.10.1 HIGH LEVEL CONTROL:

The condenser hotwell high level is controlled by regulating the flow of excess condensate fro

CEP discharge to condensate storage tank.

5.10.2 LOW LEVEL CONTROL:

Condenser hot well low level is controlled by regulating the make flow to condenser. The make

up can be provided from Deminaralised water plant. The additional make up is given from

condensate storage tank.

27

5.11 REHEATER TEMPERATURE CONTROL

Reheater are the tubes in boiler near Goose Neck area in which CRH from HP Turbine exhaust

enters At 350oC and again gets reheated to 535oC and goes to IP Turbine inlet via HRH lines.

For controlling the temperature of outlet there are two arrangements provided:

1) Burner tilt

2) RH spray system

5.11.1 BURNER TILT:

In the tangential firing system the furnace itself constitutes the burner. Fuel and air are

introduced to the furnace through four wind box assemblies located in the furnace corners, which

can be vertically adjusted by means of movable air deflectors and nozzle tips. These can be tilted

upward or downward through a total of approx 60o. This movement is effected through

connecting rods and tilting mechanism in each wind box compartment, all of which are

connected to a drive unit each corner operated by open loop control. By tilting arrangement the

flame gets shifted hence temp can be varied.

5.11.2 RH SPRAY SYSTEM:

A direct cooling method is achieved by spraying water to steam in CRH line (RH inlet) by two

injection coolers, one in each CRH line. The water is taken from BFP. Control valves are

provided to control the amount of spray required.

Actual temperature of RH outlet (HRH lines) is sensed by four thermocouples two in LHS and

two in RHS. Thermocouples convert temperature into current.

28

CHAPTER - 6

ELECTRO HYDRAULIC GOVERNING SYSTEM

The turbine is equipped with electro hydraulic governing system to facilitate the operation of the

Turbo-set in an inter connected grid system measuring and processing of signal after the

advantages such as, dynamic stability and simple representation of complicated functional

relationship. The processed electrical system is introduced at a suitable point in the hydraulic

circuit through electro hydraulic converter. The hydraulic control provides the advantage of

continuous control of large positioning forces for control valves. The integration of electrical and

hydraulic system offers the following advantages:

Exact load frequency drop with high sensitivity.

Reliable operation in case of isolated power grids.

Dependable control during load rejection.

Low transient and low steady state speed deviations under all operational conditions.

Excellent operational reliability and dependability.

Safe operation of the Turbo-set in conjunction with the TSE.

6.1 ELECTRO- HYDRAULIC CONTROLLER:

The parameters to be controlled i.e. speed, load and turbine throttle pressure are measured and

converted into electrical signals by means of suitable transducers.

6.2 SPEED CONTROLLER:

The speed controller is used for starting the turbine up to synchronization and block loading. It

can also operate over the full range during emergency such as generator tripping from full load to

house load operation or during rapid load throw-offs or severe frequency fluctuation. The speed

controller is always kept in the readiness for operation even when it is not directly controlling the

turbine by tracking the signal by the other controller in service.

29

SELECTION CIRCUITSPEED CONTROLLER

Fig. 6.1 SPEED CONTROL LOOP

6.3 SPEED CONTROL LOOP:

Speed reference value is set by means of potentiometer operated from remote UCB control

Panel) or manually (control cabinet) normally in the range of 0-3000 rpm (0-9V).above a speed

of 47Hz (2830 rpm) a reducing gear box lowers the speed of potentiometer to 1/4 th, to facilitate

exact adjustments, of speed.

NR NRTD

ACTUAL SPEED

The control device for the speed reference value generates the time dependent

speed reference value NRTD which influences the speed controller as shown in Fig. 6.2.. NR is fed

to a high gain DC amplifier. The subsequent integrator responds to a very small imbalance of

the inputs of DC amplifier. Output of the integrator is NRTD which changes like a ramp.

30

6.4 DN/DT MONITORING:

During rolling of turbine, if between the speed 600 rpm to 2829 rpm the rate of speed rise is very

slow i.e. less than 108 rpm/min, then DN/DT monitoring operates giving appropriate alarms in

UCB. It blocks further rise in speed and brings back the speed reference to 600 rpm, this is

incorporated to avoid low acceleration rates when the turbine speed lies in the critical speed

range 2850 rpm.

31

6.5 LOAD CONTROLLER:

The load controller used for controlling turbine output during load operation. The loading and

unloading gradients are influenced by TSE and Rate of change of reference.

This controller is designed for two modes of operation:

a) Power operation in conjunction with power system with PI control characteristics.

b) Isolated grid operation with P control characteristics.

6.6 LOAD CONTROL LOOP:

Load reference value is set from reference value setter module on the turbine panel (PR).

The device for the load reference value contains P channel parallel to I channel. On account of

this the response of the device is Proportional to small change and PI for large changes of load

reference value. Control device for load reference value generates PRTD/PLIM which influences the

load controller. This signal rises during start-up at a rate (MW/Min) selected through load

gradient setter until the final load reference value PR has been reached. PLIM is also subjected to

the additional limits because of TSE margins (if the same is in switched on). If the rate of P RTD is

limited by load gradient setter the proportional channel is automatically switched off and the

response of PRTD is purely integral.

PR PRTD

TSE LOAD GRADIENT LOAD LIMITER ACTUAL LOAD

32

SEL CKT

FREQUENCY INFLUENCE

MIN.. LOAD CONTRO

LLER

Fig. 6.3 LOAD CONTROL LOOP

6.7 PRESSURE CONTROLLER:

Pressure controls the turbine load with respect to the main steam pressure deviation, and prevents

(during quick load increase) large pressure drops. There are two modes of operation of pressure

controller:

In initial pressure mode, pressure controller tries to maintain initial pressure (turbine inlet

throttle pressure). It reduces the difference between reference pressure and the actual pressure to

zero by sacrificing load.

In limit pressure mode the boiler storage capacity is utilized. The pressure controller influences

the turbine CV’s to support boiler pressure control only if a preset main steam pressure deviation

is exceeded. This provision allows the load controller to handle small, quick load variation until

pick up limit pressure control is reached. This pick up limit is seldom reached in the usual

frequency supported load control mode since boiler pressure control holds pressure deviation

within narrow limits.

6.7 VALVE POSITION CONTROL:

The output from the speed controller and the load controller are compared in a MAX –

MIN selector, and the output from this is again compared with the pressure controller

output in a minimum selector. The output from this is fed to the valve position controller.

Therefore the signals from the speed, load and pressure control are superimposed and

selected to give an output to the valve position control.

33

THROTTLE PR.

ACTUAL PR.

SEL CKT

PRESSURE CONTROLLER

Fig 6.5 PRESSURE CONTROL LOOP

CHAPTER - 7

DDCMIS

DDCMIS stands for Distributed Digital Control and Monitoring and Information System.

Distributed control implies that the actual control and management functions are in fact

distributed throughout the entire plant in several processing units. In such configuration plant is

divided in many small groups and each group is controlled by a dedicated set of processors and

other hardware. The tasks of measurement, control, operation, communication, sequence control

etc. are distributed amongst a number of processing units, each incorporating a microcomputer.

These microcomputers are linked via a common hiearchy and are configured in a hierarchical

common structure.

7.1 COMPARISION:

A basic difference between hardwired and distributed system is given below:

HARDWIRED DISTRIBUTED

A) Control system not flexible Control system programmable

B) Signal transmission via numerous Signal transmissions via coaxial cable

cable

C) Component drift introduces error Drift doesn’t effect in performance

in performance

D) Distribution of control not Geographical & functional

possible distribution of control

E) System tuning/optimization System tuning/optimization

not easy easy with friendly man machine interface

F) Limited diagnostic feature Extensive self diagnostic features

G) Many varieties of modules Fewer varieties of modules

34

H) Data acquisition is a separate Combined data acquisition &

system control system can be offered

i) All sub systems are not available Single source of supply

from single source

The above mentioned function can be achieved in two ways:

Geographically distributing the hardware

Functionally distributing the hardware

7.2 GEOGRAPHICALLY DISTRIBUTING THE HARDWARE:

In this concept the hardware is distributed geographically in the plant, i.e. the electronics module

are not placed in a central; location but segregated in small groups and kept near the respective

systems being controlled. These systems have the advantage of saving a lot of cabling costs, as

the signals need not be routed all the way to the central equipment room. However this system is

not used in NTPC because of environment condition present in power plants, due to which it is

difficult to maintain required environment for microprocessor base hardware at many places in

plant.

7.3 FUNCTIONALLY DISTRIBUTING THE HARDWARE:

In this concept the hardware is kept in a centralized control equipment room but the electronic

hardware is functionally divided to perform the function independently, i.e. failure of group/sub

group does not affect or jeopardize another group/sub group. This concept is adopted by NTPC,

as this needs only one centrally air conditioned room for the electronic module.

7.4 OPERATOR STATION:

The operator station can consist of two different techniques: Conventional stations with push

buttons, lamps and indicators or VDU stations. The VDU station provide the possibility to tune

individual control loops and perform interface monitoring functions, such as simple indication

function, tabulations, bar charts, trends, mimic displays or simple protocols.

35

7.5 PLANT MONITORING SYSTEM:

The plant monitoring system informs the plant personnel on overall plant behavior and historical data. This data allows the plant management, operator and maintenance personnel to take decision in regard to:

Scheduling of further plant output Operation of the plant Recording of plant operational data Scheduling of plant maintenance outage

The plant monitoring system provides via VDU printer, hard copy unit or plotter, plant real or non real time data or calculated data such as:

Plant efficiency Life time calculation and monitoring Early detection of deterioration of process components

7.6 ENGINEERING STATION:

The engineering station allows diagnosing and recording control system internal disturbances.

This station also allows developing programs, control schemes directly via VDU keyboard.

Fig. 7.3 A typical DDCMIS screen depicting parameters in the IPT & LPT

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CHAPTER - 8

FURNACE SAFEGUARD SUPERVISORY SYSTEM (FSSS)

8.1 SYSTEM DESCRIPTION:

The Furnace Safeguard Supervisory System (FSSS) is designed to ensure the execution of a safe,

orderly operating sequence in the start up and shut down of firing equipment and to prevent

errors of omission or commission in the following such as safe operating procedure.

To maintain the decentralized and hierarchical structure of FSSS the design envisages 3 local

buses each for control of each mill elevation. Separate local bus provided for each oil elevations.

Further three local buses are assigned for boiler trip functions (MFT), mill protection & oil

protection each in 2 out of 3 mode. Another local bus controls the unit functions like Boiler

Purge, etc. Master fuel trip initiation is shown in fig. 8.1. It trips the boiler, if any of the

conditions are met.

CHAPTER – 9

37

LOSS OF UNIT 220 V DC

ALL FD FAN OFF

ALL ID FAN OFF

DRUM LEVEL VERY HIGH

DRUM LEVEL VERY LOW

FURNACE PRESSURE VERY HIGH

FURNACE PRESSURE VERY LOW

REHEAT PROTECTION TRIP

FLAME FAILURE

ORMASTER

FUEL TRIP

Fig 8.1 MASTER FUEL TRIP

CONCLUSION & FUTURE DIRECTION

9.1 CONCLUSION:

The described Distributed Digital Control & Monitoring System uses high speed microprocessor

to perform all tasks in a power plant using one type of hardware. The special features of this

system are its extended partitioning and Intra plant Bus. The control and automation function can

be partitioned in various levels and island with each level to comply with particular process

requirements. The Intra Plant Bus replaces signal cabling by distributing the control system

hardware through the plant and making marshaling racks superfluous.

9.2 FUTURE DIRECTION:

The new technologies in the field of power plant are Sliding Set point operation & MAX DNA.

9.2.1 SLIDING SET POINT OPERATION:

In this concept the electric power is controlled using the close loop control equipment of the

turbine and the steam pressure using the close loop control equipment of the steam generator

(boiler). The basic set point for the unit power is set according to the schedule transmitted via a

subsequent set point control circuit. This concept is applicable to run the plant in Availability

Based Tariff regime, in which the power demand frequently increases or decreases. In the

original concept fuel is fixed to raise the load, which takes a significant time. But in sliding set

point concept the turbine valves are throttled in such a manner that the capacity of the boiler is

utilized. But to restore the capability of boiler, fuel is fired in such a way that it gives least

stresses to boiler.

9.2.2 MAX DNA:

MAX DNA is Distributed control system for new generation. The system allows user flexibility

to operate a small stand alone control system to a mega control system with plant automation. Its

open architecture permits the integrity of process control, management information system, local

& wide area network. It provides for high resolution graphics utilizing 32 bit graphical user

interface. Customer has flexibility when it comes to expanding the system or linking to third

party system. It also has on-line simulation and testing facility.

APPENDIX

38

Coal to Electricity:

Why Coal?

39

Advantages of Coal Fuel

Abundantly available in India

Low cost

Technology for Power Generation well developed

Easy to handle, transport, store and use

Coal55%

Gas10%

Diesel1%

Hydel26%

RES5%

Nuclear3%

Boiler:

Water System:

40

Hotwell CEP D/ALPH

Steam System:

Air System:

41

DRUM LTSHDIVISIONAL

SHPLATEN

SH

HPT

REHEATERIPTLPTCONDENSER

FD FANS SAPH WIND BOX

PA FANS PAPH MILL

To MILL

Parameters Measured:

Parameter Instrument Used Control Circuit Contains

Display Units Used

Equipment Example

Temperature

Thermometer

Temperature Gauge

Thermo couple

RTD

Optical Pyrometer

Infrared Thermometer

Direct Contact

Direct ContactMounted on equipment

Direct ContactMounted on equipment

Direct ContactMounted on equipment

Direct Non Contact

Direct Non Contact Type

Instrument Itself

Instrument Itself

Indicator Recorder CRT

Indicator Recorder CRT

Instrument Screen

Computer CRT

Room Temp

Bearing, Casing, Fluid. Gas, Air, Steam Temp

Bearing, Casing, Fluid. Gas, Air, Steam Temp

Bearing, Casing, Fluid. Gas, Air, Steam Temp

Furnace, Hot Pipe Temp

42

High Tension Lines & Transformers

Pressure Manometer / Kenetometer

Pressure Gauge

Pressure Transmitter

Directly Connected to SystemDirectly MountedDirectly Mounted

Instrument Itself

Instrument Itself

Indicator Recorder CRT

Condenser Vacuum

Fluid. Gas, Air, Steam

Fluid. Gas, Air, Steam

Flow OrificeVenturi

Installed inside Pipe or duct & pr drop across measured & converted into flow

Indicator, Recorder, CRT

Fluid. Gas, Air, Steam

Axial Shift Indicator, Recorder, CRT

Turbine Shaft

Thermal Expansion

Indicator, Recorder, CRT

Turbine casing & Rotor

Speed Taco meterStroboscobeHallprobe

Instrument ItselfInstrument ScreenIndicator, Recorder, CRT

Turbine, Pumps Fans rotor speed in rpm

Vibration Vibration Pickup Directly Mounted on equipments Bearings

Indicator, Recorder, CRT

Turbine, Pumps, Fans, Generators

Flame Intensity

Flame ScannersDiscremenating Type

Fire Ball Scanners

Directly Mounted Cooled by air

Directly Mounted Cooled by air

Intensity Meter

Intensity Meter

Oil Gun Flame

Coal Flame

Level Float

Level Indicator

Dial Gauge

Float mounted inside Tank

Connected across Impulse lines

Mounted at top tank, works on the principle of

Scale Gauge Mounted at tank

Seen physically (Glass Tube)

Instrument Itself

Fuel oil, DM water, Condensate storage tanksAll Tanks

Main & Control (FRF) oil tank

43

Sonarprobe

Hydrastep

float displacement

Mounted at opposite sides of tankConnected across Impulse lines. Works on the principle of density difference

Indicator, Recorder, CRT

Indicator, Recorder, CRT

Lube & Governing oil tanks

Boiler Drum & High Pressure Heaters

REFERENCES

“Fuel Firing System”, NTPC Singrauli, 500MW Unit, Stage II, Instruction Manual, BHEL

“Automatic Turbine Run UP System”, Operation And Maintenance Manual vol.1, KWU

“Control & Instrument Package”, NTPC Singrauli 2*500 MW Stage II, Operation and

Maintenance Manual, vol. XIV

“Boiler 500 MW”, Operation and Maintenance Manual, vol.1

“Turbine 500 MW”, Operation and Maintenance Manual, vol.1

“Generator 500 MW”, Operation and Maintenance Manual, vol.1

“Turbine & Generator Set”, Singrauli Power Station, Instrumentation & Control Manual,

vol. 1, KWU

“Electro hydraulic Turbine Control”, Singrauli Power Station, Control & Instrument

Package manual, vol. 3

44

“Advances in Power Station construction”, Pergamon Press, 1988

“Large Power Steam Turbines Operation” vol. 1&2, Pennwell Publishing Company

Tulsa, Oklahoma

“Modern Power plant Engineering”, Prentice Hall of India, 1997

45