AVR Study Report

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VOCATIONAL TRAINING REPORT

DAHANU THERMAL POWER STATION

BRIEF STUDY OF BRUSHLESS EXCITATION

SYSTEM AT DAHANU THERMAL POWER STATION

Pankaj Gupta

Harikumar ChariVikrant Chaudhari

Electrical Engineering Dept


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INDEX

1. INTRODUCTION

2. Salient Features

3. Main Plant Overview

4. Thermal Power Plant Cycle And Its Operation

5. Special Features

6. Power Generation Process

7. Main Plant

8. Generator

9. Transformers

10. Motor

11. Electrical System at Dahanu TPS

12. Safety Precautions

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CHAPTER 1

IntroductionLocation: Dahanu Thermal Power Station is about 120 km north of Mumbai on

Mumbai-Delhi Western Railway route and is situated at 3 km away from Dahanu

Railway station. It is suitably linked by Railway as well as road. The Western

Express highway is 26 km east of Power station.

Basic Requirement of the Project:

The estimated requirements of coal, water and land for two units of 250 MWcapacities are as follows:

Coal per year 2.5 million tonnes based on an average calorific value of 3300

kcal/kg.

Cooling water of 84,000 cu 3 /hr for once through cooling

Fresh water for boiler 300 cu 3 /hr make-up

Land for power plant 351 hectares

Land for Ash disposal 370 hectares area for 25 years.

Land:

Total land - 821.58 hectares

Land for Plant - 351.58 hectares

Land for Ash disposal area - 370.00 hectares

Land for colony - 100.00 hectares

Water:

Sea Water - 84,000 cu 3 /hr

Fresh Sweet Water - 300 cu

3

/hr

Coal:

1.85 Million Tonnes / Year based on blended average calorific value of 4200

kcal/kg. i.e. 1.60 million tonnes Indian coal and 0.40 million tonnes imported

coal.

2.5 million tonnes / year based on average calorific value of 3300 kcal/kg.

Dahanu Plant has started receiving washed coal from SBCWL Coal Washery

project at Korba, MP. This Washery project is a joint venture between BSES

Ltd & Spectrum Technologies, USA.

Coal is imported from Indonesia by Sea.

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Major Milestones at Dahanu TPS:

Start of Land Development - 01/04/1990

Finalization of Main Plant Contract - 29/01/1991

Boiler Unit # I start of foundation work - 11/04/1991 Station Commercial Operation - January 1996

Activity Unit # I Unit # II

First Synchronization 06/01/1995 29/03/1995

Commercial Operation July 1995 January 1996

Special Features:

Complete automatic control & monitoring of the turbine, boiler andauxiliaries by Digital Distributed Control, Monitoring and Information

System (DDC-MIS).

Utilization of concrete volute pumps for CW System.

Erection of the tallest chimney (275.3 mtr.) in the country.

State-of-the-art Electronic Precipitators (ESP) with efficiency of 99.9% and

technologies for complete environment safeguard.

Completely Hydrogen cooled Generators.

Supervisory Control and Data Acquisition (SCADA) system for transmission

and distribution. Microprocessor based fire detection system.

Advanced air pollution monitoring system.

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CHAPTER 2

Excitation Systems

The basic function of the excitation system is to provide direct current to the

synchronous machine field winding.

Excitation systems perform control and protective functions essential to the

satisfactory performance of the power system.

2.1 Control functions: -

1) Control of voltage and reactive power flow.

2) Enhancement of system stability.

The protective functions ensure that the capability limits of the synchronous

machine, excitation systems and other equipment are not exceeded.

Generator consideration: -

1. Maintain terminal voltage as output varies

To supply and adjust field current to maintain terminal voltage as

the output varies within the continuous capability of the generator

(V curves).

2. Must be able to respond to transient disturbance with field forcing.

To respond to transient disturbances with field forcing consistent

with generator instantaneous and short term capabilities.

Power systems consideration: -

Effective control of voltage and enhancement of systems stability-

1. Rapid response to improve transient stability.

2. Modulation of field to enhancing small signal stability. (PSS).

To fulfill these roles satisfactorily, the excitation system must satisfy the

following requirements:

a) Meet specified response criteria;

b) Provide limiting and protection to prevent damage to itself, generator and

other equipment;

c) Meet specified requirements for operating flexibility, reliability and

availability.

Excitation systems are extremely important as a first line of defense in

maintaining system stability and their role has continually been growing.

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These systems have continued to evolve improved speed of response and

capabilities, particularly in 1960s; also began to include power system stabilizers.

Modern exciters are capable of practically instantaneous response and very high

ceiling voltages, with a wide array of control and protective circuits.

2.3 Characteristics of modern excitation systems

1) Fast response.

2) High ceiling voltage

Exciter ceiling voltage is the maximum voltage that may be attained by an

exciter with the specified condition of load. For rotating exciters ceiling should be

determined at rated speed and specified field temperature.

2.4 Dynamic performance measures:

Following are the few important parameters on which dynamic performanceof an excitation system is judged.

1. Nominal exciter ceiling voltage

Nominal exciter ceiling voltage is the ceiling voltage of an exciter loaded

with the resistor having an ohmic value equal to the resistance of the field winding

to be excited.

This resistance shall be determined at a temperature of

a) 75 C for field winding designed to operate at rating with a temperature rise

of 60C or less.

b) 100C or field winding designed to operate at rating with a temperature rise

greater than 60C.

For rotating exciters the temperature of the exciter field winding should be

considered to be 75C.

2. Excitation system ceiling voltage

Maximum direct voltage that excitation system is able to supply from its

terminal under specified conditions.

3. Excitation systems ceiling current

It is the maximum current that the excitation system is able to supply from its

terminal for a specified time.

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4. Excitation system voltage time response

Excitation systems output voltage expressed as the function of time under

specified condition.

5. Excitation systems voltage response timeThe time in seconds for the excitation systems to attain 95%of the difference

between the ceiling voltage and the rated load field voltage under specified

conditions.

6. High terminal response excitation systems

It is an excitation system, having voltage response time of 0.1s or less. It

represents a high response and fast acting excitation systems.

2.5 Elements of Power System

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2.6 Types of excitation systems:

Classification based on the power source.

1. DC excitation systems.

2. AC excitation systems.3. Static excitation systems

1. DC excitation systems

Use dc generators as source of power, driven off main generator shaft or separate

motor, self or separately excited. Typical of early systems and have been slowly

disappearing since 1960s; still exist and need to be modeled in stability studies,

often with modern voltage regulators fitted.

DC excitation system

2. AC excitation systems

Use ac machines (alternators) as source of power, with exciter usually on same shaft

as turbine-generator. Either controlled or non-controlled rectifiers rectify AC output

of the exciter, which may be stationary or rotating (brush less excitation systems).

Early systems used combination of magnetic and rotating amplifiers as regulators;

new systems use electronic amplifier regulators.

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AC excitation system

3. Static excitation system

All components are static (i.e. stationary) and dc is supplied to the generator field

via slip rings.

Power supply to rectifiers is from main generator or the station auxiliary bus.

Three main types: -

Potential-source controlled-rectifier systems

Compound-source rectifier systems

Compound-controlled rectifier excitation systems

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2.7Comparison between Brush less & Static Excitation systems:

Brush less excitation system Static excitation system

1) Exciter response limited to excitermachine time constant.

1) Fast response

2) Field discharge with natural time

constant or inverter operation

2) Field discharge by field discharge

resistor

3) Supply from PMG hence no initial

built up circuit required.

3) Initial built up circuit is required.

4) Easy supporting of short circuit

currents

4) Supporting of short circuit current

requires special

compounding transformer.

5) No slip ring and brush gears henceexcitation quantities are

approximated.

5) Direct measurement of fieldquantities is possible.

6) Since no slip ring and brush gears

are involved, wear & tear is less

hence the maintenance cost is low.

6) Wear & tear of slip rings and brush

gears requires more frequent

maintenance.

7) Power requirement are smaller

hence uses small Thyristor,

rectifiers

7) Power requirements are higher. No

limitation on the degree of

redundancy of thyristor bridge.

8) Less space required for necessary

cubicles

8) More space required for cubicles.

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CHAPTER - 3

Study of Excitation System at DTPS

3.1 Basic mode of operation

THYRISIME-04- voltage regulator is designed for excitation and control of

brush less generators the machine sets consists of the generator and a direct coupled

exciter unit with a 3phase main exciter rotating rectifiers and a permanent magnet

auxiliary exciter the main component of voltage regulator are two close loop control

systems each followed by a separate gate control unit and Thyristor sets and de-

excitation equipment.

The control system (1) for an automatic generator voltage control comprisesthe following one

1. Generator voltage control.

2. Excitation current regulator controlling the field current of main exciter.

3. Circuit for automatic excitation build up during start up and field suppressing

during shut down.

4. Limiter for the under excited range.

5. Delayed limiter for over excitation range.

Field forcing limits (practically un-delayed)

The output current of the thyristor sets to the maximum permissible value when

the voltage regulation calls for maximum excitation. Normally, the maximum

permissible value is 1.5times the rated excitation. The over excitation limiter

ensures delayed reduction of the excitation current to the rated value in the over

excited range that is between rated excitation and maximum excitation. The delay

time depends on the amount by which the rated value has been exceeded. These

limiters protect Thyristor sets and machine against over excitation with two high

values or to long duration.

In the under excited range the under excitation limiter ensures that the minimum

excitation for stable parallel operation of the generator with the system is availableand that the under excited reactive power is limited accordingly. The response

characteristics is formed on the basis of the generator reactive current active current

and terminal voltage and can be matched to the generator and system data.

The power system stabiliser (PSS) serves to dampen rotor oscillations of

synchronous generator by additional influence on the excitation systems and

acceleration as well as slip signal is required for stabilisation, are derived fro the

active power and terminal frequency as supplied by the generator after suitable

amplification the stabilising signal generated which acts on the voltage regulator of

the synchronous machine.

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Control system (2) manual it mainly comprises a second excitation current

regulator with separate sensing for the actual value. This control system is also

called manual control system, because for constant generator voltage manual

readjusting of the excitation current set point is requires when changing thegenerator load.

The excitation current regulator permits plotting of generator characteristics

and setting of protective relays during no load and short circuit runs of the generator

during communing and maintenance work. The system can also be used for the

setting of generator excitation during normal operation when the AVR is defective.

Normally the AVR is in-service even during start up and shutdown of the

generator set. Set-point adjuster of the excitation current regulator for manual is

track automatically so that the event of fault change over to manual control is

possible without delay. Automatic changeover is initiated by some special faultcondition. Correct operation of the follow up control circuit is monitored and can be

observed on the matching instrument.

Either control system is co coordinated with a separate gate control and

Thyristor set. Separate equipment is also provided supplying power to either control

system. The two separate Thyristor set for automatic voltage regulation and

excitation current control have the same ample dimensioning regarding current and

blocking voltage. Each Thyristor is fused separately. The Thyristor set for

automatic regulation can be switched off by means of an isolated contacts in the

gate control, power supply and output side. This isolator in conjunction with

corresponding arrangement and design of Thyristor set enables an exchange of

Thyristor and fuse during operation if necessary whilst operation is continued by

means of the excitation current regulator (manual). In addition the thirstier set for

automatic voltage regulation is equipped with current flow monitoring system for

detecting failure of firing pulses or fuses. Automatic changeover to the manual

current regulator is initiated by this system.

On the input side the Thyristor set are fed with auxiliary power from 220V,

400Hz permanent magnet auxiliary exciter. The output side of the Thyristor set

feeds the field windings of the main exciter with variable DC current.

To de excite the generator during shut down or when the generator protectionsystem has picked up. A command is transmitted to the output of both control

system, driving the Thyristor set being in service to minimum negative output

voltage (inverter operation) de-excites the main exciter in less than 0.5 s. the

generator de-excitation following is a function of the relevant effective generator

time constant.

The off command is issued to the field breaker via its tripping coil. In the

event of failure of electronic field suppression by inverter operation de-excitation

would be achieved with a delayed via the field discharge resister.

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3.2 Excitation control, start up and shut down, field breaker control and

De-excitation.

Excitation and voltage closed loop control are not necessary for speeds under

approximately 0.95 times rated speed. Further more closed loop control of thegenerator voltage to the rated voltage would not be permissible at low speeds, since

the generator and unit transformers would become saturated. For this reason,

functions are provided for enabling excitation during start up and for blocking

excitation during shut down of the generator.

The speed is detected via the largely speed proportional voltage of the

auxiliary exciter. The field breaker is to be switched on after reaching the speed

limit required by a manual command or from a functional group control system.

Closing of the field breaker is inter locked with the criterion ramp function

generator lower limit to insure that the generator voltage builds up slowly with out

over shooting. During excitation current control (MANUAL), the lower limit of the

set point adjuster is interred locked instead to ensure that zero excitation is obtained

after closing. With the field breaker being closed and the speed limits exceeded the

pulse blocking signal to the generator runs up, thus building up the generator

excitation if automatic voltage control (AUTO) has been selected. The run up

command is stored by a memory with remnant relay.

When the speed drops below that limit values during shut down this initiates

together with the status Generator not loaded

Field breaker off command

Pulse blocking signal to the gate control set.

Run down of function generator.

Run down of function generator may also be initiated during rated speed by the

off state of the field breaker, when the breaker is tripped from the generator

protection system. The speed criteria are monitored with respect to their

importance. Presence of the criterion n< or absence of the criterion n> while the

generator is loaded will be alarmed.

Under excitation current control (manual) no automatic excitation is build-up is

affected during start up. When the field breaker is closed the excitation is at itslowest possible value = zero value approx. the desired excitation can be set on the

set point adjuster (lower/ rise push button in the control room). During shut down of

generator the field, current set point adjuster receives continuous lower command

on tripping of the field breaker so that the set point adjuster is set to the lower limit

position.

The tripping circuit for the de-excitation are provided twice for redundancy

reason. This should be complemented by corresponding safety in the power supply

for the trip circuits.

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The field breaker is automatically tripped during generator shut down. In

emergencies, the field breaker can also be tripped manually via the generator

protection system. In this case, turbine-tripping command is transmitted through

turbine control equipments.

During short circuit operation of the generator for setting of the generatorprotective equipment, the degree of excitation is adjusted by means of the excitation

current regulator (manual). During this mode of operation, a manual faulted criteria

available in the alarm system of the regulator can be provisionally be used for

tripping the field breaker.

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3.3 Automatic voltage regulator (AUTO)

The design of the amplifier circuit for the close loop control of the generator

voltage is shown in the fig. The potentiometer R1 fed from the +10V stabilised

voltage presets the base reference value, the standard corresponding 85% ratedvoltage.

Added to this is the variable value from the set point adjuster, as standard 0-

20%, corresponding to a total setting range of 85 to 105%. The output voltage of the

set point adjuster is applied through amplifier N1 to the potentiometer R2 that

enables the setting range width to be changed. The position of the set point adjuster

is available at the output of the amplifier N1 for position indication and as a

variable for the under and over excitation limiters.

The generator voltage actual values present on the potentiometer R3; it is set

such that the out put voltage of the subsequently arranged amplifier N2 amounts to8V at rated generator voltage.

The comparison of the set point with the actual value takes places at the input

of the potential amplifier N4.furtahe more the out put of the amplifier N3 which

sums up the influencing factors of the compensation (reactive current effect) and of

the under and over excitation limiters, is also switched to this junction point. The

result of these influences is that of an additional set point. The inputs for the limiters

as well as further free inputs of the amplifiers N3 can be individually switched to

this point. The compensation can be set to between 0 and approx. 10% by a

potentiometer.

A cascade of a proportional integral voltage regulator (N4, N7) and a

following excitation current regulator (N14, N15) serves for dynamic evaluation of

the deviation of between set point and actual value. The deviation is determined and

amplified by amplifier N4; the gain is set at potentiometer R4. The integral function

is provided by amplifier N7 and adjusted at potentiometer R7. The feedback resistor

R8 determines the static gain.

The amplifier N9, N10 with high proportional gain limit the +/- out put

voltage

Have the amplifier N8 and thus the input signal to the excitation current regulator,

depending on the setting of the potentiometers R9, R10.Negative out put voltage of the PI voltage regulator results in a positive set

point value to the input of the excitation current proportion (P-) regulator N15. This

P-regulator compares the positive set point value against a negative actual value

signal from amplifier N14 and amplifies the difference. The gain of the P- amplifier

is adjusted by potentiometer R15. Under steady state condition, the set point and

actual value the signal have approx the same amount, the difference setting to a

value which multiplied by the gain of N15v results in the required signal going to

the gate control set.

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Negative voltage to the gate control set generates firing angle infinity < 900

thus supplying power to the field winding of the main exciter. Positive voltage

generates firing angles infinity > 900 thus drawing power from the field winding of

the exciter resulting in current falling towards zero.

The relay K15 switches a positive voltage to a limiting input of amplifier N15during the de-excitation resulting in equal positive voltage to gate control circuit.

The Thyristor set current drops < 0.5 sec to zero the generator voltage or generator

current in the case of a short circuit follows with a delay corresponding to the

relevant generator time constant. As soon as the current reaches zero Thyristor turns

off and field winding looses excitation.

The excitation current actual value = output current of the Thyristor is sensed

by two transducer one combination is provided for automatic voltage regulator and

other for manual control system.

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3.4 Field forcing

With the automatic voltage regulation calling for maximum excitation of the

generator the thyristor set initially supplies a higher voltage to field winding of the

main exciter than that actually required under steady state conditions. Until the

actual current in the main exciter, field winding has reached the excitation currentset point coming from the PI voltage regulator. Over driving, the voltage to the field

winding of the main exciter reduces the time required for building up the

corresponding current in same field winding and thus improves the exciter response.

The over driving function also referred to as field forcing also becomes effective in

the case of smaller control operations. The proportional excitation current regulator

N15 together with proper setting of the control limits achieves this effect. The

maximum possible out put voltage of the thyristor set UA max is determined by a

limit set as required in the gate control unit that is the minimum delay angle for

rectifier operation. Because of the high gain of the current regulator, N15 smallincrease of the set point is sufficient to reach UA max.

The maximum output current IA max is determined by the maximum

possible excitation current set point from the PI voltage regulator. This set point is

limited by amplifier N10 with respect to a reference voltage coming from ramp

function generator N11 the function of which is explained below. This reference

voltage is corresponding with the setting on potentiometer R11.

Over driving also becomes effective in case of down ward control action. The

mean possible reversed output voltage gain is determined by limit set in the gate

control unit i.e. the maximum delay angle for inverter operation. The positive signal

from the PI voltage regulator to the excitation current regulator is limited by

amplifier N9 according to setting of the potentiometer R9 to a small value.

At the beginning of a generator start up cycle the output voltage of the ramp

function generator N11 is zero so that the thyristor set output current is limited to

zero. When a speed value just short of synchronization is reached the ramp function

generator gets its input voltage to run up to its maximum value with approximately

20 sec. thus due to the gradual enabling of the current the generator receives its

voltage with in a few seconds with out over shooting. On shut down of the machine

the ramp function generator runs back to the output zero after revolving the run up

command. Failure of diodes in the rotating rectifier between the main exciter andthe generator field winding reduces its load capacity. The control panel in the

regulator cubical includes a switch by means of which the normal field forcing

values ZA max of 1.5 times rated excitation can be limited to 1.1 times the rated

excitation. This is effected via a voltage divider R10 in the ramp function generator

output by means of which the field forcing limitation is reduced. Field forcing

limitation I.e. limitation of thyristor set output current to ZA max is monitored by a

limit monitor which senses output current of the thyristor set and its pickup value

being adjusted o 1.1 IA max. response of this limit monitor initiates automatic

change over to the manual control system.

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3.5 Under excitation limiter

Operation of the generator requires its capability curve to be observing from

which the permissible active and reactive power combinations can be seen as wellas the ranges in which the generator must not be operated or for limited period only.

In the basic capability diagram below or permissible points inside the area A-B-C-

D-E-F.

The section A-B-C is obtained on the assumption of a rotor displacement

angle, which, because of the increase of this angle on short circuit conditions in the

systems, is to provide a safety margin from the actual stability limit. D-E is a

selection of the circuit rated apparent power equals constant sections C-D results

from stator end heating, if applicable.

The limit line in the under excited range may be exceeded slowly when the

system voltage slowly increases during light load operation. With the generator

voltage kept constant by the voltage regulator, this causes the over excited reactive

power to be reduced or the under excited reactive power to be increased. Exceeding

of the limit line can in this case be prevented before the under excitation limiter is

activated by changing the limit. Transformer tap changer position early enough or,

if no tap changer is provided by increasing the generating voltage or by switching

measures in the systems.

The limit line may also be exceeded quickly for example due to switching

procedures in the systems (connection of lines) or incorrect operation of the unit

Xmer tap changer. This requires automatically initiated measures on the voltageregulator equipment to returned in to the permissible range by increasing the

generator voltage. This must be performed quickly enough to prevent generator

tripping via the under voltage operation systems. Increasing of the generator voltage

can then be cancelled by switching measures such as changing the unit Xmer tap

changer position.

Figure.1 shows the basic circuits required to form a response

characteristics which corresponds to the limiting characteristics in the under excited

range of the capability curve and which further more permits representation of a

straight line to take in to consideration the stator and heating in the under excitedrange if required. For this purpose, three line to line generator voltage actual value

in the form of a dc signal which both signals are required.

The generator current is converted across the resistor R3 in to a proportional

ac voltage and this applied via the potentiometers R1/R2 to the phase controlled (by

the generator voltage) rectifies V1/V2. These form a signal proportional to the

reactive current Ixsin or active current Ixcos; the potentiometers R1/R2 are used

to standardise the signals.

The response characteristics of the under excitation limiter is shown

deriving from the reactive capability curve in reactive/ active current co ordinates.

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The reason is, that pole angle of stability observations the foot A on the reactive

current axis must be shifted proportional with the generator voltage. The curve A-

B-C shifts proportionally away from the active current axis when the voltage rises;

similarly, this distance drops when the voltage drops.

The foot pond A is forced by comparing the generator reactive current andthe generator voltage. The reactive current is applied through a fixed resistor, the

generator voltage through potentiometer R4 to the summation amplifier N3; the

setting of the potentiometer R4 determines the position of the point A. point B

(potentiometer R5) is formed by comparing the active current with the generator

voltage. As soon as the active current exceeds the value according to point B, the

amplifier N1 issues an additional signal to the sum amplifier N3. The

proportionality factor for this signal depends on the setting of potentiometer R6,

which sets the angle B. the limit characteristics A-B-C of the reactive capability

diagram, is thus simulated. The active current value is smoothened before beingused in forming the response characteristics to a degree, preventing response of the

under excitation limiter due to pole angle oscillations.

If further bend in the characteristics is necessary due to stator end heating the

amplifier N2 supplies the signal, potentiometer R7 determines the point C,

potentiometer R8 angle . Since stability observations are not important for this

bend, the active current is not compared with the variable generator voltage but it is

a fixed voltage value. The limitations signal to amplifier N2 limits the length of the

bend to the part C-D with the purpose of avoiding and undesired rise in excitation in

the over excitation range. Behind point D the response characteristics runs parallel

to the steeper bend.

The out put voltage of amplifier N3 is proportional to horizontal distance =

the reactive current distance on the operating point from the response

characteristics. When the response characteristics is exceeded its output polarity

changes from minus in the permissible range to plus in the non-permissible range.

Proportional amplifiers N1 refer to fig-2 limits the first correcting action on the

voltage regulator required when the response characteristics are exceeded quickly.

This action is limited by the amplifier N2 to increasing the deviation from the

response characteristics on the response side.

Integrator N4 that is also controlled by the deviation from the responsecharacteristics i.e. directly and in addition via the output voltage of proportional

amplifier N1 ensures the permanent and accurate setting.

The out put signal of proportional amplifier N1 and of integrator N4 (both

negative) are added in voltage regulator with reserved signal then influence the

amplifier input for comparing the positive voltage set point and the negative actual

value. The result is that the excitation and generator voltage increases until the

deviation from the response has been eliminated.

Remaining increase of the generator voltage is limited by the limitation of the

integrator, which depends on the position of the potentiometer R9 and via

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potentiometer R8 on the position of the set point adjuster for the generator voltage.

The setting can for example be carried out in a way to ensure that independently of

the position of the generator voltage is set point adjuster on increase up to 1.1 times

rated voltage is possible the limitation thus is shifted with the position of the set

point adjuster.The adjustable limit monitor gives a signal to the control room on response of

the integrator. The pick up value of the limit monitor may be chosen, example to

indicate the occasion for changing the unit transformer tap with the aim to reset the

generator voltage to normal value.

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3.6 Over excitation limiter

The operating state of the generator with rated excitation current determines

the section EF in the over excitation range of the capability curve. The rated

excitation current may only be temporarily exceeded.If the system voltage drops due to reactive power requirements switching

procedure or faults, the voltage regulator increases the generator excitation in order

to keep the generator voltage constant. Thermal overloading of the exciter and

generator rotor can arise due to large drop in the system voltage if the operator does

not reduce the set point for the generator voltage or if the ratio of unit transformer is

not re adjusted.

The purpose of the over excitation limiter in this case is to automatically limit

the generator excitation by reducing the generator voltage. The limitation of the

generator excitation can afterwards be replaced by changing the ratio of the unittransformer provided it has an on load tap changer.

From the circuit diagram of the over excitation limiter the actual values for

the over excitation limiter is the field current of the main exciter coming from a

shunt via transducer.

The amplifier N4 compares the actual value with a set point value preset by

potentiometer R2 normally to 100-105% rated excitation. The gain is set by

potentiometer R4 such that 140% rated excitation produce a 10 Volt signal at the

output of N4. The shortest response time of the over excitation limiter is assigned to

this signal. The limiting voltage from potentiometer R3 prevents a larger value.

The output voltage of the amplifier N4 is normally positive (excitation

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1. Thermal protection of the generator field winding (by limitation of the

excitation) on the one side, by adoptions of the generator voltage to a reduced

grid voltage level.

2. Support of the system and station supply voltages (by over excitation) in

case of system voltage drops on the other side. Integrator N5 allows for aresponse time, which is inversely proportional to the control deviation.

The shortest response time using standard settings amounts to 10 seconds.

This value ensures that the over excitation limitation does not influence the

effectiveness of over current relays within the scope of the generator protection

system.

The over excitation limiter is again made inoperative if excitation drops

below the response value for a short period of time by 3.3% for 2 minutes.

The main exciter is equipped with a measuring coil for the generator

excitation current. The AC signal supplied by this coil is rectified and watched andtransmitted to an instrument, which indicates the generator excitation current in the

control room.

The over excitation limiter is supplemented by a stator current limiter, which

in principle works the same way. It is operative only in the overexcited part of the

capability diagram i.e. in the part of the curve D-E. Lowering of the generator

excitation in the under excited area of the diagram would increase the stator current

instead of limiting it. Therefore a limit monitor disables the limiter, when the

reactive current falls short often-adjustable reactive current limit.

The output signal of the over excitation and stator current limiter go through

diodes to the comparator point of the generator voltage regulator. If both limiters

respond, only the larger signal will influence the voltage regulator.

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3.7 Actual voltage feedback signal

This module is mainly used in automatic voltage regulators of large

alternators for forming, adjusting and amplifying the actual voltage feedback signal,

for adjusting and amplifying the voltage reference signal, for summing andamplifying additional control signals and for providing components for shaping the

response of amplifiers in the automatic voltage regulators.

Mode of operation

Voltage transformers are used to provide six voltages (24volts, 50/60Hz),

which are proportional to the alternator voltage. This form the input signals to the

module and is rectified by the six diodes. The rectified and summed voltages are fed

to a decoupling amplifier via a potential divider. The decoupling amplifier output

the actual value feedback signal of the alternator voltage; this is used for control andmonitoring functions.

A further decoupling amplifier used to feed the voltage reference signal to the

various control functions, the reference signal being obtained from a remote

motorised setting potentiometer. The module also contains potentiometers for the

presetting the basic voltage reference and the bandwidth of the variable voltage

component.

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3.8 Fault indication and automatic change over to manual

Following alarms are issued from the voltage regulator to the control room.

1. AVR fault,

2. AVR, automatic changeover to MANAUL,3. AVR, loss of alarm voltage.

The group alarm AVR fault collects the individual alarms; refer to the

functional diagram. The initiating individual signal (s) is (are) stored and can be

identified locally (in the voltage regulator cubicle) by means of LEDs.

The initiations-

1. Power supply AUTO,

2. Generator voltage actual value,

3. Thyrister set AUTO,

4. Faulty over excitation.Causes automatic changeover to MANUAL, unless changeover is not blocked

due to a fault in the excitation current control (MANUAL). Changeover can also be

blocked in either direction by switches provided locally in the voltage regulator

cubicle if components of MANUAL or AUTO are not ready for operation. This

blocking takes effect both for automatic and manual commands.

Fault conditions initiating automatic changeover also cause the alarm AVR,

automatic changeover to MANUAL to be given. The Power supply AUTO

alarm initiating automatic changeover occurs in the event has under voltage in the

stabilised 15 volts power supply for the automatic voltage control and the

associated gate control set. The alarm Alternator voltage actual value is initiated

either by tripping of the voltage transformer MCB for the generator voltage actual

value, or by the generator voltage actual value monitor. Response of the current

flow monitoring system initiates the thyristor set AUTO alarm. The alarm faulty

over excitation is with only a short delay initiated by the field forcing limitation

monitor and additionally by a generator reactive current monitor as follows:

The above-mentioned field forcing limitation monitor responds above 1.65*

Rated excitation approx. at operating temperature of the machine, at somewhat

higher value if the machine being cold. Lower excitation values are limited with

delay by the over excitation limiter.

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3.9 Compensation

The purpose of the compensation is to generate an increase in the controlled

generator voltage proportional to the reactive current to partly compensate for the

voltage drop in the unit transformer. For this reason signal available from the underexcitation limiter which is proportional to the reactive current is applied to the

comparator point for the generator voltage set point and actual value. The effect

referred to a reactive current = rated current of the generator is adjustable between 0

and 0.1 X-rated voltage.

As a result of partial compensation of the voltage drop across the unit

transformer the voltage regulator reacts more pronounced to changes in the voltage

drop between the generator and the grid. Change n reactive power associated with

the change in voltage drop becomes noticeably bigger.

For this it is irreverent whether the voltage change occurs on the grid side orwhether it is purposely introduced by set point changes or by tap change in the unit

transformer. The change in the controlled generator voltage with the reactive

current should be noted with respect to a station supply possibly connected to the

generator terminals and because of the voltage limits of the generator.

If the generator is operating in parallel with other generator via the unit

transformer on to a common bus bar then the voltage drop may be compensated to a

maximum residual of 4 to 6% otherwise changes in the set point or of the unit

transformer tap would result in substantial cross currents between the generator.

The same applies with the parallel operation of individual generator with the grid.

The voltage regulator of large generators is preferably operated with out

compensation. The decision whether compensation is suitable requires individual

consideration or local operation experience.

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3.10Monitoring of generator voltage actual value

For the automatic voltage regulation the generator voltage actual value is

sensed as the average of the three line to line voltages. Drop of this actual values by

blowing of a voltage transformer fuse or by an interruption in the inter connectionin the voltage regulator or in the circuits of the intermediate voltage transformer and

associated rectifier would cause the generator to increase the excitation and

generator voltage to improper and dangerous values.

An adequate action in this case is immediate change over to manual that is

the separate regulator for the exciter field current as this current regulator is

automatically tracked according to the automatic voltage regulator normal

excitation is restored and tripping of the machine is avoided.

For this purpose the three phase secondary voltage from a second set of

voltage transformer is required. After reducing the voltage level by intermediatevoltage transformers the average of the voltage is formed and compared to the

corresponding signal from the first set of voltage transformers by means of

differential amplifiers.

Voltage drops incoming from the grid cause both signals to change by small

amount and the output of the differential amplifier will remain zero. But a failure

arising in the circuit of the first set of voltage transformers only will produce

positive output voltage on the differential amplifier output. This will be detected by

a connected limit monitor if the response limit adjusted is exceeded. The response

limit is to be set so that interruption of the phase will be determined safely.

A failure in the circuit of the second set of voltage transformer will produce a

negative output voltage signal on the differential amplifier output, which is not

evaluated. The response of the limit monitor is evaluated for a single alarm

Generator voltage actual values, automatic change over to manual, a group alarm

AVR fault, and an alarm AVR automatic change over to MANUAL.

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3.11 Volts per Hertz limiter (V/F Limiter)

The voltage regulator keeps the generator voltage constant independent of the

generator frequency. Excitation of the generator with excessive under frequency

value is prevented by speed dependent enabling of the excitation at speed values of0.95 pu or by blocking speed values of less than 0.9 pu approximately. This means

that the excitation equipment permits excited operation of the generator with

frequency deviation up to 0.1 pu below normal frequency.

The magnetic flux of the unit transformer is directly proportional to the

terminal voltage and inversely proportional to the frequency i.e. proportional to the

ratio V/Hz.

Excessive magnetic flux increases thermal stressing of the unit transformer

and of the generator. The function of the V/Hz limiter is to issue a signal when a

present V/Hz limit value is exceeded and to reduce this value to the permissiblelimit.

The ac generator voltage is transferred to level of 8V approximately by an

intermediate transformer and then fed to frequency voltage converter. The dc output

voltage of this converter is proportional to frequency and amounts to +8V at rated

frequency.

At the proportional amplifier N1 the frequency proportional signal is

compared against a dc signal proportional to the generator voltage. The generator

voltage proportional signal is adjustable to potentiometer R1 the setting of which

determines the pickup value of the V/Hz limiter. The setting range is 7.2 to 8.0

volts approximately corresponding to V/Hz limit pickup value setting of 1 pu to 1.1

pu.

The V/Hz limiter responds when the output of the negative generator voltage

signal exceeds the amount of positive frequency signal. The output voltage of the

amplifier N1 then changes from negative to positive side. The normally negative

output voltage is fed to the input of the integrator N2, the output voltage of the latter

then being +11V app.. The out put voltage of the integrator N2 changes to negative

values when the pickup level of the V/Hz limiter is exceeded.

The output of the integrator N2 is reversed by reversing stage N3 the output

of which being positive on pickup of V/Hz limiter. Positive output voltage ofamplifier N3 biases the comparator between the set point and actual value of the

generator voltage in the regulator. This simulates a too high voltage to the voltage

regulator and returns V/Hz ratio to the set response value.

When integrator N2 changes its initial output potential the capacitor across

the integrator becomes charged in the opposite direction, the rate of change at the

output of N2 depending on the input current of N2.

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The V/Hz limiter thus has a response time which is shorter to greater the

V/Hz ratio deviates from the set value. It depends also on the size of the capacitor

and setting of the potentiometer R3. the setting of potentiometer R2 determines on

which deviations of the V/Hz ratio from the set level. Output voltage of +11V willappear at amplifier N1 determining the shortest responsible possible.

When the output of amplifier N3 changes to positive voltage an electronic

limit switch will give a signal to the control room to draw the attention of the

operator to the abnormal situation.

Normally when the unit transformer is more vulnerable with respect to V/Hz

value than the generator is. It must be observed that deviation response time

characteristics is of the type

T= C/deviation.

Where C is the constant value but with a constant response time value aftercertain deviation is exceeded.

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CHAPTER - 4

Power System Stabiliser (PSS)

Application of power system stabilizer (PSS) is to improve the dynamic

performance of a power system. The most widely employed PSS is the lag-lead type

where the gain setting is fixed at certain values, which are determined under

particular conditions. The operating point of power system drifts as a result of load

changes or major disturbances such as 3-phase faults. To take care of machinery

during such critical conditions PSS is necessary.

In presence of AVR oscillations of the rotor can be de-stabilized due to negative

damping torque. To make the system stable it is required to provide some other

means of damping so as to make the total damping torque positive, modulating

some control quantity does this.The necessary condition for damping are

1) The oscillations should be seen in the quantity, which is used, as the

modulating signal. For example speed deviation and power output of the

generator-modulating signal should also be easily measured and should be

less susceptible to noise.

2) Variation of the controllable quantity should be able to cause an adequate

variation in the torque. E.g. Input mechanical torque, however it is not

practically feasible due to slow response of the turbine control system.

Another option is to control the voltage reference of AVR or of HVDC andSVC control in the systems.

3) The controller should modulate the controllable quantity approximately using

the modulated signal so that damping torque is produced at the rotor

oscillations frequency.

The most widely accepted and inexpensive way to achieving stabilizer is to

modulate the voltage reference of the AVR using speed/power/bus frequency

signals. The controller in each case will be different since these oscillations are not

in phase with each other.

29

SYSTEM

CONTROLLER

MODULATED

SIGNAL

MODULAT

ED QUANTITY


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4.1Structure of PSS

The main objective of the PSS is to increase the power transfer in the

network, which would otherwise be limited by oscillatory instability. The PSS must

also function properly when the system is subjected to large disturbances.

WASHOUT CIRCUIT: -

The washout circuit is provided to eliminate steady state bias in

the output of the PSS, which will modify the generator terminal

voltage. The PSS is expected to responds only to transient

variations in the input signal and not to the dc offsets in the

signal. This is achieved by subtracting from it the low frequency

component of the signal obtained by passing the signal through a

low pass filter.

Gw(s)= sTw / (1+s Tw) = 1- 1/(1+s Tw) = 1- Glp(s)

DYNAMIC COMPENSATOR: -

The compensator should modulate the controllable quantity

appropriately using the modulating signal so that damping

torque is produced at the rotor oscillation frequency.

TORSIONAL FILTER: -

The torsional filter in the PSS is essentially a band reject filter to

attenuate the first torsional mode frequency. The transfer

function of the filter can be expressed as

FILT(s) = Wn2 / (s2 + 2 Wns+Wn

2)

Torsional filter is necessitated by the adverse interaction of PSS

with the torsional oscillations. This can lead to shaft damage,

particularly at light generated loads when the inherent

mechanical damping is small even if shaft damage does not

occur; stabilizer output can go in to saturation (due to torsionalfrequency components) making it ineffective.

LIMITER: -

The output of the PSS must be limited to prevent the PSS acting

to counter the action of AVR. For example when the load

rejection takes place the AVR acts to reduce the terminal voltage

when PSS action calls for higher value of the terminal voltage

(due to increase in speed/frequency). It may even be desirable to

trip the PSS in case of load rejection

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The negative limit of PSS output is of importance during

the back swing of the rotor (after initial acceleration is over).

The AVR action is required to maintain the voltage (and thus

prevent loss of synchronism) after the angular separation has

increased. PSS action in the negative direction must be curtailedmore than in the positive direction. Recent studies show that

higher negative limit can impair first swing stability.

31

sTw

1+sTw

Gc (s) Gf(s)

Vs max

Vs min

LIMITER

TORSIONAL

FILTER

COMPENSATORWASH OUT

CIRCUIT

MODULATING

SIGNAL


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CHAPTER 5

New Trends -Digital Voltage Regulator

(DVR)Control of the voltage and reactive power of synchronous machine has been

so far achieved mainly with analog control amplifier in excitation equipment. In

these, the control signal is formed by continuous comparison of the measured actual

value with required reference values available in the form of dc voltage. The control

signal varies the output of thyristor-controlled rectifiers, provided as power stage,

by shifting the firing pulses. The pulses are generated in gate-controlled units

provided with discrete components.

The analog control amplifiers having general met the requirements of controlspeed and adaptability to control systems. However automatic diagnostics features

such as self-monitoring and fault detection, often required in present-day systems,

are difficult to be incorporated. Also the number of modules required for

configuring systems with complex control such as limiters, stabilizers etc are many.

With the introduction of fast microprocessor, modules are now available,

which can meet the highly demanding task in the field of AVR. These modules can

also be used for various sub systems in excitation equipment such as monitoring,

protection and control thus forming the decentralize digital voltage regulator.

Features of digital voltage regulator are:-

1] Self-monitoring of the modules.

2] Ease of setting and measuring of the variables, using local micro terminal.

3] Settings which are digital i.e. they are exactly reproducible and are not subject to

variation over long period of time.

4]. Digital firing and control module having stable behaviour even with distorted

synchronous voltage.

5]. A significantly small no of different modules.

6]. Easy adaptation of customers individual requirements.

7]. The maximum converter out put voltage can be reached in less than 20mssignifying a very fast control response.

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5.1 The digital voltage regulator philosophy.

For voltage regulation of synchronous machines several special conditions

have to be met. An important factor is the measurement of the active and reactive

power and derives variables such as load angle and power factor. The actualmeasurement is made by measuring the instantaneous values of the current as the

A.C voltage passes through zero.

As the phase voltage passes through zero from positive to negative, the

instantaneous value of the current corresponds to the value of the reactive current.

As the line voltage passes through zero from negative to positive the instantaneous

value of the current corresponds to the value of active current.

Thus reactive current can be measured six times in each period as the phase

voltage passes through zero and the active current can be measured six times as the

line voltage passes through zero. An interrupt generator is used to produce 12 pulsesin each period, which among other functions, are used in determining the active and

reactive components of the current.

When short circuit occur close to the generator. The response time of the

regulator must be very short. This means that after the short circuit event, the

converter out put voltage must reach 95% of its maximum value within 20ms. In the

digital voltage regulator the control signal is computed periodically from the

measured actual value and the reference values, there fore the calculation must be

repeated at every short interval of time in order to meet the requirement.

The transfer function of the voltage regulator represents an important

parameter while studying the stability of the network. The transfer function of the

digital voltage regulator remains the same as that for analog voltage regulator.

Amplification factor and time constant, which are known from the analog regulator,

can be used as initial input parameters during commission.

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5.2 Functional layout Description: -

It consists of a single channel regulator. Heart of the system is C.P.U and is

connected to various peripherals through address bus, serial I/O and control bus.

Data processing is synchronized by clock pulses of 12MHz derived from internalfrequency generator. Voltage and current measuring modules measures the

generated voltage, current and field current. An important function of this module is

to produce interrupt pulses in relation to the point where the generator voltage

passes through zero. These pulses are required for measuring active and re active

currents.

Analog and digital conversions of measured values is done through A/D

conversion module. This module lies before the voltage and current measuring

module. Digital I/O module is used for D/A conversion. Central processing unit

accepts the measured values in digital forms in synchronism with interrupt pulses.The pulses are synchronized by input A/C voltage of thyristor converter equipment.

Central Processing Unit

C.P.U uses 16-bit microprocessor, which can be used for any application. The

module has 64 KB of memory. The memory sub divided into to different blocks.

1) EPROM with basic programs

2) EPROM program to suit the customer requirement.

3) Free for future use.

4) EE-PROM for setting reference values.5) RAM as working memory.

The digital voltage regulator is capable of performing wide range of function which

are as follows:-

a) Control of generator voltage.

b) Limitation of field current.

c) Limitation of load angle.

d) Limitation stator current.

e) Damping of active power oscillations.

f) Number of supplementary functions.The following subroutines are called at every second interrupt pulses:-

a) Formation of generator voltage actual value signal.

b) Reference and actual value difference.

c) P.I.D algorithm.

Digital pulse generator

Regulator described above. In addition to the main function of generating

double pulses the module has a number of auxiliary function to meet specific plant

requirement .

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1) Filtering of heavily distorted synchronizing voltages due commutation in

thyristor. This is done with data registers.

2) Correction of gate triggering angle proportional to changes in frequency with

respect to the rated frequency.

3) Blocking of the gate pulses to the inverter region through an external signal.4) Monitoring and limiting of field current. To prevent slip rings short circuit

5) Gate pulses can be adjusted any one of the following of the methods

1) The voltage regulator control output.

2) The internal reference value.

3) Internal field current controller.

Additional feature with microprocessor hardware within the excitation

equipment.

The central processing unit describe earlier can also be used for another sub

systems within the excitation systems. In the additional to the excitation sequence

control and excitation monitoring both describe below.

Rotor temperature measurement.

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Excitation sequence control

Excitation systems depend on the individual plant requirements and often

require to be modified during commissioning. As result software has been designed

in a simple way so that it is easy to program and specialized programmer isrequired.

The arrangement of the excitation systems is simple as shown in the fig. The CPU

communicates with the digital I\O module and with the alarm indication module.

Data exchange with plant systems is done with digitally isolated contacts for inputs

and potential fed contacts for outputs.

When CPU is used for sequence control the memory allocation is as follows.

1) Block 1 EPROM containing the operating systems, which interprets the

functional blocks of the programming language.

2) Block 2 EPROM contains the user program

3) Block 3 reserved for future use.

4) Block 4 EEPROM for intermediate storage of auxiliary values so as to reduce

computation time.

5) Block 5 RAM working memory.

Program cycle is 20ms.

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5.4 Excitation monitoring-:

The analog signals of generator voltage and current are converted into digital

form by A\D converter and are fed to the CPU. Through the data and address bus.

Signals from the current monitoring CTs go through the central processing moduleunit through another current flow monitor module. Data exchange with the control

sub systems takes place through the digital I\O module.

With this subsystem following functions can be performed.

Monitoring the actual value signals of the generator voltage and field current.

Monitoring the Thyristor current flow.

Protection of Thyristor against the over load.

Generation of voltage reference.

Monitoring if temperature reference values for stator and rotor current in gas turbine

application.

5.5 Excitation protection:-

The field current is converted in to the digital form by A\D converted module

and then fed to the CPU.

The module performs the following functions.

Over current protection. (Instantaneous)

Over current time delay form an inverse time characteristics.

i) Local operation.

The micro terminal is provided with the keys for setting the reference values

and with the plug in connections. Which can be connected to the memory blocks. A

LCD provides simultaneous displays of four addresses and their corresponding data.

The most important data and setting appear directly in alphanumeric form. With

keyboard the data can be modified.

ii) Self monitoring: -

The self monitoring function in the various modules represents important

advantage over the analog equipment. Faults within the controller and the pulse

generation module are detected immediately. With this feature and with redundant

equipment automatic change over to the stand by channel is done before the fault

can affect the excitation current.

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Self monitoring includes the following functions.

a) Program sequence check.

This consists of an independent counter which is periodically reset to zero by

internal pulses in the program. If the counter runs over a limit, it indicates aprogram fault.

b) Interrupt pulse monitor: the 12 interrupt pulses per period are counted and at

the same time the internal clock timer responsible for producing interrupt

pulses, is also monitored during under frequency or over frequency

conditions.

c) The A\D and D\A converters checked periodically by feeding the analog out

puts of the D\A converter back to the A\D converter during the short pauses

in the program. The converted values are then compared with the input

values.d) The digital input and output modules are checked periodically.

e) The pulse out put from the digital firing and control module are checked for

short circuit.

f) The out put voltages of the power supply modules are monitored.

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CHAPTER 6

Distributed Digital Control System

(DDC)DDC is an integrated solution for control and monitoring of power plants. A

complete plant automation system provides the operating personnel with necessary

tool to increase plant availability and efficiency. The automation system

automatically controls and monitors plant operating systems, plant start up and shut

down. The operation is improper when the plant operates in unpredictable manner

the DDC helps in quick recovery of plant in such critical conditions. It prevents

chain tripping of the power plant.

The complete goal of the plant automation can only be achieved whenoperating personnel can rely on the automation system and are released from the

cumbersome control duties. Thus the operating personnel can concentrate on the

over all plant behaviour and on the preventive maintenance. Enunciation concept

ensures the operator that the automation system actions are correctly achieved based

on plant data.

Applications of DDC: -

1. During plant shut down and start up

2. During stable generation of power3. During major changes

Features of DDC: -

1. Modular electronics

2. Programmable processor

3. Data acquisition

4. Remote multiplexing

5. High data transmission speed

6. Redundant multi channel configuration.

DDC is an automation system with microprocessor based intelligent

multiplexing system. The system is designed on a modular basis and allows

tightening the scope of control hard ware to particular control strategy. The DDC

provides signal conditioning and transmission, modulating controls, on off logic

sequential control, individual and process protection, man machine process

interface.

Communication: -

1. Serial bus technique is utilised.

2. Very high data transmission speed for real time control.

3. Purely cyclic mode of operation.

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4. Redundant fault tolerant.Control of the plant: -

1) Functional distribution of control functions.

2) Geographical distribution of control hard ware.

Data transmission: -

Data transmission takes place over two buses.

1) Intra plant bus (IPB).

2) Local bus.

Local bus: - Local bus connects all the I/Os and processing electronic modules

which are part of the station. Each local bus is independent of other local bus and

intra plant.

Intra plant bus: - connects local buses coaxial cable. IPB and local busconnected to each other through bus couplers.

Local bus and intra plant bus use serial communication with time division

multiplexing. Message format is based on broadcasting state information in secure

address message.

The features of this technique are as follows.

1) No side effects of function execution because each message is addressed

exclusively by its source address.

2) Convenient monitoring of running system because each message identifies its

source.

Hardware: -

The system has two types of processing modules.

1) Individual control module.

2) Universal processing module.

Individual control module: -

The module implemented to control supervise monitor and protect one

individual final control element of any type such as valves, pumps, fans etc. the

module equipped with microprocessor. The input allows gathering data on the

position status of final control element as well as the status of a related poweramplifier.

The output allows ON/OFF or 4-20 mA control signals. A serial I/O inter-

phase to local bus is available. This link is mainly used to receive process signal

required for interlock and permissive logic within the control logic.

Universal processing module: -

Module is utilized for two different applications.

Automation unit, functionally superimposed on individual control module.

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Combining individual control and automatic function the universal processing

module has larger and fast programming capability. This later allows to perform

long and complex functions.

Universal processing module can perform various arithmetic and logic

functions like PID, AND, OR, Summation, Subs traction etc.

Man process interface: -

Overall man process interface is the actual interface between the plant

management operating and maintenance personnel. The function includes.

1) Operator station.

2) Plant monitoring system.

3) Engineering station.

Operator station: -Operator station consists of conventional station like push buttons; lamp

indicators or CRT based station. Both this techniques can be implemented. The

station gives the operator possibility to give orders to individual control loops. The

CRT based station provides the possibility to tune individual control loops. The

simple function like mimic displays, bar charts, and trends of the process variable is

possible.

Plant monitoring system: -

PMS informs the plant personnel of overall plant behaviour and historical

data. This data allows the plant management, operator and maintenance personnel

to take decision in regard to scheduling of plant, a maintenance outage, and

operation of plant and recording of plant data.

The PMS system via CRT provides printouts, hardcopies of plant real and non-

real datas. The various datas observed through PMS are: -

1) Plant efficiency.

2) Lifetime calculation and monitoring.

3) Early detection of deterioration of process components.

These datas are indicated bar charts and mimic displays.

Engineering station: -

The engineering station allows to dialogue and record control systems

internal disturbance. Station also allows programme control schemes directly via

CRT and keyboards.

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CHAPTER 7

TECHNICAL SPECIFICATIONS OF

D.T.P.S

Salient features

i. Type of station : Thermal

ii. Station Capacity : 500 MW (2x250 MW)

iii. Fuel : 100% coal

(100% gas firing

arrangement for future)iv. Coal Source

MCL : LOCM ,LOCM II, LOCM III

BOCM, NEW BOCM,

SECL : GEWARA, NEW GEWARA, NEW

KUSMUNDA, MANIKPUR.

PRIVATE : THALCHAR (A GRADE)

IMPORTED : AUSTRALIAN COAL

Transportation : By rail & sea

Consumption : 2.56 million tones per year

assuming 8000 Hrs. of operation

v. Cooling water, Source : From Arabian Sea

vi. Consumptive Requirement : 76000 M/Hr

vii. Ash disposal : Wet Ash Disposal system.

viii. Chimney : RCC multiflue (2 Flue) chimney

with flue height 275 M high.

ix. Design Heat Rate : 2250 Kcal/Kwh at 3% make up

x. HP/LP Bypass capacity : 60 % MCR

xi. Land Requirement (In Acres)a. Plant area : 351.58 Hectors

b. Ash disposal area : 370 Hectors

c. Colony area : 100 Hectors

---------------

TOTAL : 821.58 Hectors

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Boiler

i. Manufacturer : BHEL (C.E. Design) TIRUCHIRAPALLY

ii. Type : Natural circulation, Balance draft, Double

pass, Single drum, Single reheat, DirectPulverised Coal Fired Water Impounded

Bottom. (Gas firing & Gas regulating

system, provision for future).

iii. Boiler Design Pr & Temp. : 182.5 Kg/cm2 & 540O C

iv. Boiler Designation : 15240 199 63 .5

11506 142 63 .5

v. Type of Firing : Tilting Tangential

Steam Turbine

i. Make : BHEL, KRAFT WERK UNION

DESIGN (GERMANY)

ii. Rated Load : 250 MW

iii. Max. Load under Valve Wide Open : 262 MW

(VWO) condition

iv. Overall Length (Meter) : 17.75

v. Overall Width (Meter) : 12.20

Rated speed :50.0 Hz

Generator

i. Make : BHEL, HARDWAR

ii. Type : THRI 108/44

iii. Code : IS: 5422, IEC - 4

iv. Stator winding cooling : Indirectly hydrogen cooledRotor Winding cooling : Directly hydrogen cooled

v. MW rating : 250

vi. MVA rating : 294.1

vii. Rated terminal voltage : 16.5 KV

viii. Rated Stator current : 10286 amps

ix. Rated power factor : 0.85 Lag

x. Rated speed / frequency : 3000 rpm / 50 Hz

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Excitation system

i. Make : BHEL, HARDWAR

ii. Type : Brush less Excitation system

With Rotating Diodesiii. Main Exciter

a. Type : ELR70 /62-30/ 6-10

b. Active Power : 1350 Kw

c. Rated current : 3200 amps

d. Rated voltage : 420 Volt

e. Frequency : 50 Hz

iv. Pilot Exciter

a. Type : ELP50 /29-30/ 16b. Apparent power : 35 KVA

c. Rated current : 105 amps

d. Rated voltage : 220V 22V

e. Frequency : 400 Hz

f. Speed : 50 cycles /sec

BRUSHLESS EXCITATION SYSTEMTECHNICAL SPECIFICATION

a.) Pilot Exciter

i. Manufacturer BHEL

ii. Type ELP 56/29-30/16

iii. Type of drive Direct coupled

iv. Normal speed (r.p.m.) 3000

v. Rated Voltage (Volts) 220

vi. Rated frequency (Hz.) 400

vii. Rated Current (amp.) 105

viii. Type of insulation SPUN GLASS

b) Main Exciter


ii. Type ELR 70/62-30/6-10

iii. Type of drive Direct Coupled

iv. Normal Speed (rpm) 3000

v. Rated rectified Voltage (Volts) 420

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vi. Rated rectified current (amp.) 3200

vii. Ceiling rectified voltage (Volts) 600

viii. Ceiling rectified current (amps) 4500

ix. Exciter field voltage with manual control

(Rectified)

Maximum (Volts) 74.0

Minimum (Volts) 15.0

x. Field Current at generator MCR and rated power

factor lagging (amp) 2388

xi. Nominal exciter response ration 72.0

xii. Class of insulation F

xiii. Insulation material on rotor winding MICALASTICxiv. Insulation material on field winding SPUN GLASS

xv. Type of end winding support STEEL BANDAGE

xvi. Guaranteed maximum temperature with secondary

cooling water temperature as per specification

As per IEL

Exciter stator winding in contact with insulation

Rotor winding

c) Exciter rectifier assembly


ii. Type of rectifier SSOL/2120

iii. Total number of rectifier cells per ring 30

iv. Number of parallel paths per bridge arm 10

v. Number of bridge arms 6

vi. Maximum number of exciter cells/bridge without

which the rectifier can give output corresponding to

generator MCR 2

vii. Max. peak inverse voltage rating of cell (volts)

2000

viii. Number of rectifier cells in series in each bridge

arm 1

ix. Method of over voltage protection RC-NETWORK

x. Overload rating of the rectifier cell 370 A for 10 sec.

xi. Ceiling output voltage of rectifier assembly (Volts) 600

xii. Ceiling output current of rectifier assembly (Amp.) 4500

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xiii. Maximum junction temperature of rectifier cell 90O

xiv. Method of mounting cells to cooling fins SCREWD

xv. Make and type of cooling fins HEAT SINK

xvi. Type of visual indication provided for faulty

rectifier cells STROBOSCOPE

xvii. Ceiling duty duration (Sec.) 10 Sec.

d) Air/Water Coolers (For exciter)

i. Number of coolers 2 x 50%

ii. Material of

Tubes 90/10 Cu Ni Base tube

Fines Copper

iii. Material of tube plates Carbon Steel

iv. Material of water boxes --do--

v. Quantity of circulating water required per cooler 130 M3 /hr

vi. Maximum allowable water 38

e) Gas system data

i. Volume of hydrogen space in generator 69 m3

ii. Cooling gas flow 1800 M3 min

iii. Volume of CO2 at NTP required for displacing

hydrogen 138 m3

iv. Volume of H2 at NTP required for displacing

carbon di-oxcide and to bring the casing to the

rated pressure380

v. Volume of CO2 at NTP required for displacing air 138 m3

vi. Purity of H2 required (%)

Normal 98

Minimum allowable 96

vii. Leakage of H2by volume at NTP per day at rated

H2pressure in the generator


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Carbon dioxide 40

Nitrogen --

xii. Standard to which the cylinders conform

Hydrogen IS:7285

Carbon dioxide ----do--

Nitrogen --

xiii. Internal volume of the Cylinders

Hydrogen (m3) 80

Carbon dioxide (m3) 40

Nitrogen (m3) --

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CHAPTER 8

References

1. BHEL manuals on AVR THYRISEM 04

2. BHEL manuals on DDC PROCONTROL-P

3. Power system stability by Prabha Kundur

4. Reports on PSS.

AVR Study Report

Documents

Transcript of AVR Study Report