summer internship report indian oil corporation limited (guwahati)

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SUMMER INTERNSHIP REPORT

INDIAN OIL CORPORATION LIMITED (GUWAHATI)

(1st June to 30

th June 2016)

Submitted by: BISHAL SARMA

¾ B. TECH

ELECTRICAL & ELECTRONICS ENGINEERING

NIT WARANGAL

Submitted on: 4th

July,2016

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PREFACE

The knowledge of any subject is incomplete until it is done practically.

Electrical & Electronics particularly requires a thorough knowledge of practical

training for a comprehensive understanding. The progress is certainly based on

the discovery of the new facts.

The science of computers has grown tremendously over the last few

decades and day-by-day new technologies are being added to this ever

growing vast field. The young scientists and field scholars must be appreciated

for their training and fieldwork.

This report describes the work carried out by me during one month

internship at I.O.C.L., Noonmati (Guwahati Refinery). During this period, I

have understood a lot of things related to the working of a refinery in its

different divisions under Electrical dept. and Instrumentation Dept. This has

developed a sense of confidence in me. I perceive as this opportunity as a big

milestone in my career development. This internship is proved to be a good

practical experience and has also enhanced my technical knowledge. A lot of

credit goes to my instructors who helped me all the way from the very

beginning.

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ACKNOWLEDGEMENT:

The internship opportunity I had with IOCL Guwahati was a great chance for

learning and gaining practical knowledge. Therefore, I consider myself as a very

lucky individual as I was provided with an opportunity to be a part of it. I am

also grateful for having a chance to meet so many wonderful people and

professionals who led me through this internship period.

I would like to acknowledge my profound and sincere gratitude Ms. Padmashri

Sarma (Asst. Manager ,T & D), Mr. Amit Roy(CMNMEL), Mr. A.S.

Chowdhury(CITM), Mr. A. Jamir (SPUM/TPS), Mr. S.C. Saini (SM, Electrical

Maintainance), Mr. S. Saharia(DMIT) for allotting me in different areas during

the internship period. It is my radiant sentiment to place on record my best

regards, deepest sense of gratitude to Mr. P Pathgiri, Mr. A. Prajapati, Mr. K.

K. Verma, Mr. Gaurav Alok, Mr. R. Minz, Mr. DR Boro for their careful and

precious guidance which were extremely valuable for my study both

theoretically and practically. I offer thanks and gratitude to all the respondents

who extended so earnestly their co-operation answering the queries on time

and helping me in the internship period.

I perceive as this opportunity as a big milestone in my career development. I

will strive to use gained skills and knowledge in the best possible way and I will

continue to work on their improvement, in order to attain desired career

objectives.

Bishal Sarma

¾ B. Tech

Electrical & Electronics Engineering

NIT Warangal

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ABBREVIATIONS

Abbrv. Full meaning Abbrv. Full meaning

IOCL Indian Oil Corporation Limited Pf Power factor

TPS Thermal power station DPT Differential pressure transmitter

MW Mega-watt Pa Pascal(unit for pressure)

DM De-mineralization Hg Mercury

TG Turbo-generator MLG Magnetic level gauge

STG Steam turbo generator I/P Current to pressure

MP Medium pressure OC Open circuit

LP Low pressure SC Short circuit

TPH Tons per hour RTD Resistance temperature

detector

IJT Isgec John Thompson HV/HT High Voltage/High tension

CDU Crude distillation unit LV/LT Low voltage/Low tension

DCU Delayed coking unit CT/VT/PT Current/Voltage/Potential

Transformer

HGU Hydrogen generation unit IDMTL Inverse definite minimum time

lag

HDT Hydrotreater unit

RPM Rotation per minute

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CONTENTS:

Preface 1

Acknowledgement 2

Abbreviation 3

Contents 4

1. Overview of I.O.C.L Guwahati 5-6

2. Thermal Power Station(TPS)

2.1 Introduction

2.2 Major electrical equipment details

2.3 General details of boiler

2.4 General Description of generators

2.5 Problems associated with operation of generator

7-19

7-8

8-12

13-14

15-17

18-19

3. Protection of electrical equipments in Guwahati refinery

3.1 Importance of protection

3.2 PSP- basic components

3.3 Transformer protection

3.4 Generator protection

3.5 Bus-bar protection

3.6 Motor protection

3.7 Relays

3.8 Circuit breakers

20-45

20-21

22

22-25

25-30

30-33

33-41

42

43-45

4. Instrumentation

4.1 Importance and relevance

4.2 Different types of instruments in refinery

4.2.1 Flow measurement

4.2.2 Pressure measurement

4.2.3 Level measurement

4.2.4 Temperature measurement

4.2.5 Other miscellaneous instruments

46-60

46

46-60

46-49

46-52

52-54

55-58

58-60

5. Conclusion 61

6. Bibliography 62

1. OVERVIEW OF IOCL GUWHATI :

Indian Oil Corporation Limited

corporation with its headquarters in

largest public corporation, according to the

corporation in India when ranked by revenue.

petroleum company in the world

national oil companies in Asia

49% share in the petroleum products market, 31% share in refining capacity and 67%

downstream sector pipelines capacity in India. The Indian

operates 11 of India's 23 refineries with a co

tons per year. In FY 2012 IOCL sold

PBT of 37.54 billion, and the

and tax of 10.68 billion. The company is mainly controlled by Government of India which

owns approximately 58.57% shares in the company.

companies of India, apart from

Corporation, Steel Authority of India Limited

Authority of India Limited.

Indian Oil operates the largest and the widest network of fuel stations in the country,

numbering about 20,575 (16,350 regular ROs & 4,225 Kisan Seva Kendra). It has also started

Auto LPG Dispensing Stations (ALDS). It supplies

households through a network of 5,934 Indane distributors. In addition, Indian

and Development Center (R&D) at

technology solutions to the operating divisions of the corporation and its customers within the

country and abroad.

Guwahati Refinery is the country’s first

Refinery serving the nation since 1962

Rumanian assistance, the initial crude processing capacity at the time of commissioning of

this Refinery was 0.75 MMTPA

ONGC crude.

OVERVIEW OF IOCL GUWHATI :

Indian Oil Corporation Limited, or Indian Oil, is an Indian state-owned

corporation with its headquarters in New Delhi, India. The company is the world's

largest public corporation, according to the Fortune Global 500 list, and the largest public

in India when ranked by revenue. It has also earned reputation as

petroleum company in the world and No. 1 petroleum trading company

national oil companies in Asia-pacific region. Indian Oil and its subsidiaries account for a

re in the petroleum products market, 31% share in refining capacity and 67%

downstream sector pipelines capacity in India. The Indian Oil Group of Companies owns and

refineries with a combined refining capacity of 80.7

tons per year. In FY 2012 IOCL sold 75.66 million tons of petroleum products and reported a

, and the Government of India earned an excise duty

The company is mainly controlled by Government of India which

owns approximately 58.57% shares in the company. It is one of the seven

, apart from Coal India Limited, NTPC Limited, Oil and Natural Gas

Steel Authority of India Limited, Bharat Heavy Electricals Limited

Oil operates the largest and the widest network of fuel stations in the country,

g about 20,575 (16,350 regular ROs & 4,225 Kisan Seva Kendra). It has also started

Auto LPG Dispensing Stations (ALDS). It supplies Indane cooking gas to over 66.8 million

etwork of 5,934 Indane distributors. In addition, Indian

and Development Center (R&D) at Faridabad supports, develops and provides the necessary

operating divisions of the corporation and its customers within the

Guwahati Refinery is the country’s first Public Sector Refinery as well as Indian

serving the nation since 1962. It is known as GONGOTRI of Indian

, the initial crude processing capacity at the time of commissioning of

0.75 MMTPA and the Refinery was designed to process a mix of OIL and

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owned oil and gas

. The company is the world's 119th

list, and the largest public

It has also earned reputation as 18th

largest

petroleum trading company among the

Oil and its subsidiaries account for a

re in the petroleum products market, 31% share in refining capacity and 67%

f Companies owns and

mbined refining capacity of 80.7 million metric

tons of petroleum products and reported a

earned an excise duty of 232.53 billion

The company is mainly controlled by Government of India which

one of the seven Maharatna status

Oil and Natural Gas

icals Limited and Gas

Oil operates the largest and the widest network of fuel stations in the country,

g about 20,575 (16,350 regular ROs & 4,225 Kisan Seva Kendra). It has also started

cooking gas to over 66.8 million

etwork of 5,934 Indane distributors. In addition, Indian Oil's Research

supports, develops and provides the necessary

operating divisions of the corporation and its customers within the

as well as Indian Oil’s first

of Indian Oil. Built with

, the initial crude processing capacity at the time of commissioning of

and the Refinery was designed to process a mix of OIL and

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The refining capacity was subsequently enhanced to 1.0 MMTPA and with INDMAX, the

pilot plant for first in-house technology of Indian Oil, the ISOSIV and Hydrotreater the

Refinery has been able produce eco-friendly fuels. The Refinery produces various products

and supplies them to North eastern India as well as beyond, upto Siliguri end through the

Guwahati-Siliguri Pipeline, spanning 435 KM, which was the first Pipeline of Indian Oil and

commissioned in 1964. Most of the products of Guwahati Refinery are evacuated through

pipeline and some quantity also through road transportation.

Quality LPG, Motor Spirit, Aviation Turbine Fuel, Superior Kerosene Oil, High Speed Diesel,

Light Diesel Oil and Raw Petroleum Coke are the products of this Refinery.

In line with Indian Oil’s responsibility towards the society, Guwahati Refinery has

contributed yeomen service towards developing the community, which exists around it. The

CSR agenda of the Refinery focuses on three broad areas of education, health care and

providing water supply. Initiatives taken under these heads are participative in nature with

community participation in a partnership model for ensuring sustainable development of the

community.

Guwahati Refinery is also known for its sincere efforts on development as well as

implementation of effective Safety, Health & Environment management systems and

procedures along with good performance in occupational health and safety.

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2. THERMAL POWER STATION AT GUWHATI REFINERY:

2.1 INTRODUCTION:

Thermal power station(TPS) of Guwahati Refinery was commissioned in 1962 with the

collaboration of Romanian Govt. Initially, it was having 4 Romanian boilers of 20 tons/hr

capacity each, two nos. of Romanian Generators of 3 MW capacity each and water

softening plant for supplying the treated water to the boilers and refinery. During the

course of 40 years, following changes were made –

1985: Generator # 3 was commissioned of 8 MW capacity.

1986: Once through cooling water system has been converted to re-circulating system.

1992: Boiler #5 was commissioned of capacity 40 tons/hr.

1993: Water softening plant has been replaced by DM plant.

1998: TG #4 was commissioned.

2002: 3rd

chain of DM plant commissioned.

2004: TG #3 and TG #4 MMI upgraded from ECIL make to ABB make.

2005: TG #5 was commissioned (12 MW capacity)

2005: TPS cooling tower reinnovated.

2006: STG #5 commissioned.

2006: Deaerator 1 and 2 condemned and removed.

2007: Rumanian boiler #2 condemned and removed.

Guwahati refinery uses thermal power station (TPS) for the generation of electricity and

process steam for the units. It uses fuel oil, refinery oil, gas, MRN as fuel for the generation

of heat energy. As the feed water in the boiler evaporates due to intense heat, it becomes high

pressurized steam (≈37.5 kg/m²). The steams passes through steam headers to the turbines, it

forces its way through the turbine thus rotating the turbine. The turbine is now connected to

generator (together turbogenerator) via a coupler. As the turbine is rotating, electrical enegy

is produces from the generator. Part of the steam supplied to the turbine is also extracted at

two sections i.e. controlled MP extraction and uncontrolled LP extraction.

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List of major equipments of Thermal Power Station (TPS) Guwahati Refinery:

Boilers:

• 2×20 TPH Romanian Boilers (Boilers #3, #4).

• 1×40 TPH IJT boiler (Boiler#5).

• 2×50 TPH Thermax Boilers (Boilers #6 , #7).

Total installed capacity : 180 TPH

Steam Turbine:

• 2×8.0 MW BHEL make extraction cum condensing steam turbines.

• 1×12.0 MW BHEL make extraction cum condensing steam turbine,

Total installed capacity : 28 MW

DM plant (De mineralization plant):

• 2×50 M3/hr DM water chains.

• 1×60 M3/hr DM water chains.

Total capacity: 160 M3/hr

Cooling Towers:

• TPS cooling tower for TPS.

• Unit cooling tower for CDU,DCU.

• Process cooling tower for HGU,HDT, INDMAX and nitrogen.

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2.2 MAJOR ELECTRICAL EQUIPMENT DETAILS:

• Turbogenerator #3:

MAKE : BHEL

TYPE : EK-1000

Continuous rating at Generator Terminals : 8 MW

Turbine Speed : 8000 RPM

Generator Speed : 3000 RPM

Coupling : Speed Reduction Gear

Live Steam Pressure : 35 ATA

Live Steam Temperature: 435 D Centigrade

Max. Live Steam Flow: 63 MT/hr

Speed Governor : Hydro-dynamic

Governing System : SRI II

Pressure Governor : ASKANIA

Protection : Over speed, Axial Displacement, Low lube

oil pressure, Low vacuum, High Vibration.


MAKE : BHEL

TYPE : EK-1000_2







Max. Live Steam Flow: 63 MT/hr

Speed Governor : Hydro-dynamic

Governing System : SRIV




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MAKE : BHEL

TYPE : EK-100-2







Max. Live Steam Flow: 94.7 MT/hr

Speed Governor : Electronic

Governing System : Electronic




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• Typical Technical Data of Turbogenerator( #3 or #4)

Type: TGQ21822612

Apparent output: 10000 KVA

Active output: 8000 MW

Rated power factor: 0.8 pf lag

Rated voltage: 6.3 kV±5% Volts

Rated current: 916 Amps

Rated speed: 3000 rpm

Rated frequency: 50±1% Hz

SC ratio: 0.512

Generator field resistance: 0.3288 Ohm at 20 D. Cent.

Critical speed : 1900 rpm

MI of rotor: GD²=1.221 Jm²

No. of generator terminals: 6

Generator phase connections: Y

Generator brushes:

Number-

Size-

Grade-

6 per ring (2 rings)

25×32 mm

HMCR

Minimum permissible diameter of slip-rings 360 mm

Maximum output with one cooler out of

service:

7000 kVA

Type of cooling: Closed circuit air cooling

Volume of air cooling: 28800 m³/hr

Designed for: Tropical climates

Insulation class (stator, rotor) : Class ‘B’

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• Typical Technical Data of exciter of TG:

Type: EX17316614

Rated output: 90 KW

Rated voltage: 220 V

Rated current: 405 A

Rated speed: 3000 rpm

Type of excitation: Separate

Ceiling Voltage: 352 V/15 sec

Type of drive: Direct

Nominal excited response: 1 min

Main pole air gap: 8 mm

Interpole air gap: 12 mm

Exciter brushes: Reaction type

Number: 6×4 sets

Grade: EG14

• Typical General Characteristics of Generator #3 / #4

Open Circuit characteristics

(Rated voltage : 6.3 kV)

Short circuit characteristics

(Rated current : 916 Amps)

Voc

(100%=6.3 kV)

If(Amps)

Isc( Amps) If(Amps)

31% 40 146.6 40

47% 60 293.12 80

63% 80 440 120

93% 120 586.2 160

100% 130 721.3 200

120% 180 865.5 240

126% 200 916 254

131% 220 1010 280

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2.3 GENERAL DETAILS OF BOILERS :

A boiler is a closed vessel in which the heat produced by the combustion of fuel is transferred

to water for its conversion into steam at the desired temperature and pressure.

To achieve this, the boiler has to perform the following functions-

� Serve as a furnace where air is mixed with fuel in a controlled combustion process to

liberate large quantity of heat.

� Provide a pressure tight enclosure which includes metal tubes, heaters and pressure

parts in which steam is produced as a result of the heat from combustion of fuel.

� Provides a mean for raising the temperature of the steam produced to a degree of

superheat.

Boiler unit consists of Furnace, Superheater, Economizer, Air-preheater, De-superheater and

Stack.

The Guwahati refinery has 5 active boilers. Two boilers have become obsolete ( boiler #1,

boiler #2). The details of the working boilers are furnished below-

Boiler #3, #4 data

MAKE: ROMANIAN

Maximum rating: 20 TPH

Peak Output: 22 TPH

Steam Drum Pressure: 39 kg/cm²

Steam temperature: 450±5 D Cen

Fuel: Oil and fuel gas

No. of oil burners: 4

No. of gas burners: 4

Oil capacity per burner(MCR): 545 kg/hr

Fuel gas capacity( MCR): 425 NM³/hr

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Boiler #5 data

MAKE ISGEC JOHN THOMPSON

MCR of the boiler 40 Tons/hr

Peak evaporation 44 Tons/hr

Steam Drum Pressure 39 kg/cm²

Steam Temperature 450±5 D Cend.

Fuel MRN/LSHS and fuel gas

No. of burners 2

Boiler #6, #7 data

MAKE THERMAX

Max. Continuous generation

capacity(100%MCR)

57700 kg/hr

Max. Allowable working pressure and design

pressure

50.0 kg/cm²

Steam outlet pressure 39.0 kg/cm²

Steam outlet Temperature 450±5 D Cend.

Feed water inlet temperature 105 D Cend.

Hydrostatic test temperature 75.0 kg/cm²

Boiler Accessories and mountings(for all the five boilers) :

Name of accessories/mountings Numbers

Deaerators 2

Boiler feed water pumps 5

Fuel oil pump 3

Induced draft fan + forced draft fan 3+5

Air preheater 3

Safety valve 10

De-superheater 5

Economizers 5

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2.4 GENERAL DESCRIPTION OF GENERATOR:

In Guwahati refinery, continuous generation is mandatory, otherwise a huge amount of loss

will incur. So the generators are designed for continuous operation with voltage variation of

±5% of the rated voltage and frequency variation of ±1% in general. Maximum air

temperature 45 Deg. Centigrade with cooling water temperature 38 Deg. C .

Different parts of Generator:

a) Stator:

1) Stator frame: The stator frame is fabricated structure made out of mild steel plates. It

houses and supports the stator core together with the winding.

2) End Curves: The end covers are castings of aluminium alloy bolted to side plates of stator

frame. The inlet passage is specially designed with built in guide vans which ensures uniform

distribution of air to the fans.

3) Stator core: Stator core is made up of segments of insulated punching of non-grain

oriented high quality Si – Sheet steel to give minimum electrical loss.

4) Stator windings: The stator winding is a double layer multiturn/Roebel bar type lap

winding. The half coils are made of electrolytic copper strips, insulated with mica based

epoxy insulation of suitable thickness to give a long and uninterrupted service. The straight

part of the half bar is coated with a conductive varnish to prevent corona discharges in the

slot. Resistance thermometer elements are placed in the core teeth and in the windings ar

carefully selected points to measure the temperature rise of the machine. The end windings

are supported by epoxy glass laminate spacers to give a rigid structure to withstand the short

circuit forces of the winding. Six output terminals are brought out from the rings through

insulated cover.

b) Rotor: The rotor is forged from a homogeneous steel of special alloy steel properly heat

treated to meet the required metallurgical and magnetic properties. The slots are milled

throughout the active length of the rotor body. The slots are dovetailed at the top for housing

the wedges. At bottom of the slots, sub-slots are provided for entry of cooling air.

1) Rotor winding: The rotor coils are continuously wound multi turn coils made from silver

alloyed copper of rectangular cross-section. Radial cooling arrangement has been made by

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providing suitable holes in the rotor conductor strips, strip insulation and wedges. The

winding is insulated from rotor body with L-shaped troughs made of epoxy glass laminate.

The windings are kept in position by bronze wedges. In addition to keeping the winding in

position, the wedges also act as a short circuited damper windings in addition with the shrunt

fit retaining rings.

2) Retaining rings: These are made from high tensile, non-magnetic steel and shrunk onto

the spigot on the rotor body. At the other end, they are supported by forged steel hubs.

Ventilating holes are drilled for circulation of air for cooling the end windings of the rotor.

3) Rotor balancing: The rotor is balanced with the help of sophisticated balancing machines.

The balancing weights are fitted in dovetail grooves provided in the hubs and fans. The rotor

is dynamically balanced and subjected to an over speed of 20% for two minutes.

4) Slip rings: These are made of forged steel and shrunk on either side of the rotor between

the end cover and the main pedestal bearings. Mica splittings are used to insulate the slip-

rings from the rotor body. The excitation to the rotor winding is taken from these slip-rings.

The connecting leads are suitably insulated and taken through slots milled on surface of the

rotor. Wedges are provided to keep these leads in position. Class ‘B’ type of insulation is

used.

5) Brushes: Brushed used for turbogenerator are made form a combination of graphite and

other binding materials in suitable composition to have low friction co-efficient and self

lubricating properties. A pair of flexible Cu-leads is provided for each brush for carrying the

required current. The contact pressure is applied on the centre line of the brush by means of

radially mounted helical spring. The brush pressure is nearly 180 gms/cm². In order that the

wearing of the brushes is uniform, the slip ring polarities may be interchanged once n a three

months.

6) Ventilation arrangement: The turbogenerator is cooled by air circulated by means of

two axial fans. The air after circulation is cooled by air coolers. The air is drawn through

suction ducts by axial fans mounted at either side of the rotor. The warm air flow out through

exhaust at the bottom of the rotor frame.

7) Resistance temperature detector: The RTDs are made of platinum resistance elements.

The detectors are placed in a groove cut in rectangular glass laminate and embedded at

different positions like stator teeth, stator core and slots. The leads from these resistance

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thermometers are brought out and connected to a terminal board. These resistances

temperature detector operates on the principle that the resistance of the elements of the

elements will change with temperature depending on their temperature co-efficient of

resistance. The change in resistance can be accurately measured in a bridge circuit. A graph

can be drawn showing the variation of resistance with temperature, which can be used to

know the temperature rise under different operating conditions of the turbogenerator

Fig: RTD characteristics

8) Fire detectors: For the protection of the turbogenerator against any possible fire hazards.

Six protectostat fire detector relays are provided on either side of the stator windings. These

relays have two sets of normally open contacts. One set of contacts will close when

temperature surrounding the relays exceeds 80 D. Cent. The other sets of contacts closes

when temperature exceeds 100 D. Cent. Both the sets of contacts are used for automatic fire

alarm and fire extinguishing systems.

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2.5 PROBLEMS ASSOCIATED WITH OPERATION OF GENERATOR:

• Faults in Generator:

The accidental short circuit faults could be symmetrical or asymmetrical. Short circuit

currents which are many times higher than the rated phase currents are dangerous due

to the associated dynamic forces. Especially short circuit directly at the generator

terminals in the bus duct is more dangerous when fault occurs, and the breaker trips. It

is necessary to impact the Turbo generator after removing end cover for deformation if

any. Winding faults within the generator will need repairs and testing of the machines.

Usually an inter-turn short circuit within the rotor is difficult to locate since generator

may continue to operate satisfactorily. Multiturn shorts however manifest in increased

vibrations of rotor due to uneven magnetic fluxes.

• Bearing vibration:

The double amplitude vibration at the turbogenerator and exciter bearings at rated

speed must not be more than 50 micron. The maximum permissible value even in the

worst case however is 0.1 mm (100 microns). Such operation is considered only during

emergency, under operating authority risk. Vibrations exceeding the above limits needs

a careful study by a balancing expert, who if necessary may rebalance the rotor in case it

is found that the alignment of the bearing does not need any adjustment.

• Abnormal operating conditions:

a) Short Circuit: The short circuit may be symmetrical or asymmetrical ( 3 phase or 2

phase/ single phase). The short circuit current is dangerous due to dynamic forces. If it

lasts for a long period, it is dangerous due to its thermal effects also. Especially

dangerous are the heavy short circuit occurring directly at the terminals of the generator

or on the bus bars and the unsymmetrical short circuit. After such short circuit, an

inspection of turbogenerator must be performed during which it is necessary to

measure the insulation resistance of the stator and the rotor to dismantle the end

covers and examine the state of the stator and rotor windings. Defects found out must

be rectified. Then only the machine may be put into operation again.

b) First earth fault in rotor: Generator is permitted to operate with one earth fault in

rotor. However this earth fault should be cleared at the earliest.

c) Loss of excitation: Operation of a generator without field will cause excessive

heating. The degree to which this heating will occur depends on several conditions

including the initial load on the machines, the manner in which the generator is

connected to the system. When the excitation is lost, the generator tends to over speed

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and operates as an Induction generator. This over speed generally results in reduction of

load due to the characteristics of turbine governor, an increase in stator current and

possible low voltage at the generator terminals and is accompanied by high rotor

currents. The rotor body currents will cause extremely high and possible dangerous

temperature rise in short time. If the machine is found to be operated without field for

an unknown interval of time, it should be immediately tripped. If line and shut down for

an insulation to determine the degree of rotor damage from heating. Loss of excitation

relay can take care of this hazard.

• Effect of leading and lagging pf load:

The reactive capability curve gives the limits of loading at various power factor of a

synchronous generator. Beyond the limits of the curve will result in overheating the field

winding due to excessively high field current.

Operation with leading power factor with reduced field current will result in overheating

the ends of the stator core and the end structure of the machine due to eddy current set

up due to armature reaction and leakage flux which rotates at synchronous speed. The

heating effect of leakage flux increases with decrease in saturation of the retaining rings

resulting from the lower values of field current.

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3. PROTECTION FOR THE VARIOUS ELECTRICAL COMPONENTS OF

GUWAHATI REFINERY

3.1 IMPORTANCE OF PROTECTION:

A power system is not only capable to meet the present load but also has the flexibility to

meet the future demands. A power system is designed to generate electric power in

sufficient quantity, to meet the present and estimated future demands of the users in a

particular area, to transmit it to the areas where it will be used and then distribute it within

that area, on a continuous basis.

To ensure the maximum return on the large investment in the equipment, which goes to

make up the power system and to keep the users satisfied with reliable service, the whole

system must be kept in operation continuously without major breakdowns.

This can be achieved in two ways:

� The first way is to implement a system adopting components, which should not fail

and requires the least or nil maintenance to maintain the continuity of service. By

common sense, implementing such a system is neither economical nor feasible,

except for small systems.

� The second option is to foresee any possible effects or failures that may cause long-

term shutdown of a system, which in turn may take longer time to bring back the

system to its normal course. The main idea is to restrict the disturbances during such

failures to a limited area and continue power distribution in the balance areas.

Special equipment is normally installed to detect such kind of failures (also called

‘faults’) that can possibly happen in various sections of a system, and to isolate faulty

sections so that the interruption is limited to a localized area in the total system

covering various areas. The special equipment adopted to detect such possible faults

is referred to as ‘protective equipment or protective relay’ and the system that uses

such equipment is termed as ‘protection system’.

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In Guwahati refinery, since the failure or damage of important equipments may cause a

huge loss to the company, hence the protection of various components viz motors,

generators, transformer, bus bar etc are of high importance here.

In a nutshell, main functions of power system protection are –

• To safeguard the entire system to maintain continuity of supply.

• To minimize damage and repair costs.

• To ensure safety of personnel.

The basic requirements of power system protection are-

• Selectivity: To detect and isolate the faulty item only.

• Stability: To leave all healthy circuits intact to ensure continuity of supply.

• Speed: To operate as fast as possible when called upon, to minimize damage,

production downtime and ensure safety to personnel.

• Sensitivity: To detect even the smallest fault, current or system abnormalities and

operate correctly at its settings.

The protective system should act fast to isolate faulty sections to prevent-

• Increased damage at fault location. Fault energy = I² × Rf × t, where t is time in

seconds.

• Danger to the operating personnel (flashes due to high fault energy sustaining for a

long time).

• Danger of igniting combustible gas in hazardous areas, such as methane in coal

mines which could cause horrendous disaster.

• Increased probability of earth faults spreading to healthy phases.

• Higher mechanical and thermal stressing of all items of plant carrying the fault

current, particularly transformers whose windings suffer progressive and cumulative

deterioration because of the enormous electromechanical forces caused by multi-

phase faults proportional to the square of the fault current.

Sustained voltage dips resulting in motor (and generator) instability leading to extensive

shutdown at the plant concerned and possibly other nearby plants connected to the system.

23

3.2 POWER SYSTEM PROTECTION-BASIC COMPONENTS:

1. Voltage transformers and current transformers: To monitor and give accurate feedback

about the healthiness of a system.

2. Relays: To convert the signals from the monitoring devices, and give instructions to open

a circuit under faulty conditions or to give alarms when the equipment being protected, is

approaching towards possible destruction.

3. Fuses: Self-destructing to save the downstream equipment being protected.

4. Circuit breakers: These are used to make circuits carrying enormous currents, and also to

break the circuit carrying the fault currents for a few cycles based on feedback from the

relays.

5. DC batteries: These give uninterrupted power source to the relays and breakers that is

independent of the main power source being protected.

3.3 TRANSFORMER PROTECTION:

Here is a brief summary of the types of faults that can occur in a power transformer:

• HV and LV bushing flashovers (external to the tank)

• HV winding earth fault

• LV winding earth fault

• Inter-turn fault

• Core fault

• Tank fault

Different protection relay that are used for transformer fault detection are as –

1) Differential protection: Differential protection, as its name implies, compares currents

entering and leaving the protected zone and operates when the differential current

between these currents exceed a pre-determined level.

The type of differential scheme normally applied to a transformer is called the current

balance or circulating current scheme as shown in Figure below.

Fig: Differential protection using current balance scheme (external fault conditions)

The CTs are connected in series and the secondary c

relay is connected across the midpoint where the voltage

current passes through the relay, hence no operation f

Under internal fault conditions (i.e. faults between the CTs) the relay operates,

the CT secondary currents add up and pas

Fig: Differential protection using current balance scheme (internal fault conditions)

2) Buchholz protection: Failure of the winding insulation will

which can decompose the oil

also precipitate a breakdown of oil into gas.

Severe arcing will cause a rapid release of a large volume of gas as well as oil vapor.

action can be so violent that the build

the conservator.

The Buchholz relay can detect both gas and oil surges as it is mounted in the pipe to the

conservator. The figure is shown below


The CTs are connected in series and the secondary current circulates between them.

relay is connected across the midpoint where the voltage is theoretically nil, therefore

current passes through the relay, hence no operation for faults outside the protected

Under internal fault conditions (i.e. faults between the CTs) the relay operates,

the CT secondary currents add up and pass through the relay as seen in figure below.


Failure of the winding insulation will result in some form of arcing,

decompose the oil into hydrogen, acetylene, methane, etc. Localized heating can

precipitate a breakdown of oil into gas.

Severe arcing will cause a rapid release of a large volume of gas as well as oil vapor.

action can be so violent that the build-up of pressure can cause an oil surge from the


conservator. The figure is shown below-

24


urrent circulates between them. The

heoretically nil, therefore no

or faults outside the protected zone.

Under internal fault conditions (i.e. faults between the CTs) the relay operates, since both

in figure below.


result in some form of arcing,

into hydrogen, acetylene, methane, etc. Localized heating can

Severe arcing will cause a rapid release of a large volume of gas as well as oil vapor. The

an cause an oil surge from the tank to


Fig: Details of Buchholz rel

Fig: Mounting of Buchholz relay

Fig Fig

Fig: Details of Buchholz relay construction

25

26

The unit contains two mercury switches. The production of gas in the tank will eventually

bubble up the pipe to be trapped in the top of the relay casing, so displacing and lowering

the level of the oil. This causes the upper float to tilt and operate the mercury switch to

initiate the alarm circuit. A similar operation occurs if a tank leak causes a drop in oil level.

The relay will therefore give an alarm for the following conditions, which are of a low order

of urgency:

• Hot spots on the core due to shorted laminations

• Core bolt insulation failure

• Faulty joints

• Inter-turn faults and other incipient faults involving low power

• Loss of oil due to leakage.

The lower switch is calibrated by the manufacturers to operate at a certain oil flow rate (i.e.

surge) and is used to trip the transformer HV and LV circuit breakers.

3.4 GENERATOR PROTECTION:

A generator is the heart of an electrical power system, as it converts mechanical energy into

its electrical equivalent, which is further distributed at various voltages.

It will be appreciated that a modern large generating unit is a complex system, comprising

of number of components:

• Stator winding with associated main and unit transformers

• Rotor with its field winding and exciters

• Turbine with its boiler, condenser, auxiliary fans and pumps.

Many different faults can occur on this system, for which diverse protection means are

required. The various types of electrical faults are:

Stator insulation failure, Overload, Overvoltage, Unbalanced load, Rotor faults, Loss of

excitation, Loss of synchronism.

27

Below is the detailed explanation of various faults and protection measure for

corresponding faults-

1) Stator earthing and earth faults: The neutral point of the generator stator winding is

normally earthed so that it can be protected, and impedance is generally used to limit earth

fault current.

The stator insulation failure can lead to earth fault in the system. Severe arcing to the

machine core could burn the iron at the point of fault and weld laminations together. In the

worst case, it could be necessary to rebuild the core down to the fault necessitating a major

strip-down. Practice, as to the degree of limitation of the earth fault current varies from

rated load current to low values such as 5 A.

Generators connected direct to the distribution network are usually earthed through a

resistor. However, the larger generator–transformer unit (which can be regarded as isolated

from the EHV transmission system) is normally earthed through the primary winding of a

voltage transformer, the secondary winding being loaded with a low ohmic value resistor. Its

reflected resistance is very high (proportional to the turns ratio squared) and it prevents

high transient overvoltages being produced as a result of an arcing earth fault.

When connected directly through impedance, overcurrent relays of both instantaneous and

time-delayed type are used. A setting of 10% of the maximum earth fault current is

considered the safest setting, which normally is enough to avoid spurious operations due to

the transient surge currents transmitted through the system capacitance. The time delay

relay is applied a value of 5%

2) Overload protection: Generators are very rarely troubled by overload, as the amount of

power they can deliver is a function of the prime mover, which is being continuously

monitored by its governors and regulator. Where overload protection is provided, it usually

takes the form of a thermocouple or thermistor embedded in the stator winding. The rotor

winding is checked by measuring the resistance of the field winding.

28

3) Overcurrent protection: It is normal practice to apply IDMTL relays for overcurrent

protection, not for thermal protection of the machine but as a ‘back-up’ feature to operate

only under fault conditions. In the case of a single machine feeding an isolated system, this

relay could be connected to a single CT in the neutral end in order to cover a winding fault.

With multiple generators in parallel, there is difficulty in arriving at a suitable setting so the

relays are then connected to line side CTs.

4) Overvoltage protection: Overvoltage can occur as either a high-speed transient or a

sustained condition at system frequency.

The former are normally covered by surge arrestors at strategic points on the system or

alternatively at the generator terminals depending on the relative capacitance coupling of

the generator/transformer, and connections, etc.

Power frequency overvoltages are normally the result of:

• Defective voltage regulator

• Manual control error (sudden variation of load)

• Sudden loss of load due to other circuit tripping.

Overvoltage protection is therefore only applied to unattended automatic machines, at say

a hydroelectric station. The normal setting adopted are quite high almost equal to 150% but

with instantaneous operation.

5) Rotor faults: The rotor has a DC supply fed onto its winding which sets up a standing flux.

When this flux is rotated by the prime mover, it cuts the stator winding to induce current

and voltage therein. This DC supply from the exciter need not be earthed. If an earth fault

occurs, no fault current will flow and the machine can continue to run indefinitely, however,

one would be unaware of this condition. Danger then arises if a second earth fault occurs at

another point in the winding, thereby shorting out portion of the winding. This causes the

field current to increase and be diverted, burning out conductors.

In addition, the fluxes become distorted resulting in unbalanced mechanical forces on the

rotor causing violent vibrations, which may damage the bearings and even displace the

rotor by an amount, which would cause it to foul the stator. It is therefore important that

rotor earth fault protection be installed. This c

below-

� Potentiometer method

center tap. The tap point is

earth fault in the field winding

voltage occurs for faults at end of the windings. However, there are

faults at the center of the winding may get undetected. Hence, one lower

provided in the resistance. Though normally, the center tap

pushbutton or a bypass switch is used to check

winding. A proper operating procedure shall be

changeover is done at least once in a day

� AC injection method:

the field circuit through a coupling capacitance. The capacitor prevents the chances

of higher DC current passing through the transformer. An earth fault at any part of

the winding gives rise to the field current, which is detected by the sensitive relay.

Care should be taken to ensure that the bearings are insulated, since there is a

constant current flowing to the earth through the capacitance.

rotor earth fault protection be installed. This can be done in a variety of ways as explained

Potentiometer method: The field winding is connected with a resistance having

center tap. The tap point is connected to the earth through a sensitive relay R. An

earth fault in the field winding produces a voltage across the relay.

voltage occurs for faults at end of the windings. However, there are

faults at the center of the winding may get undetected. Hence, one lower

provided in the resistance. Though normally, the center tap is connected, a

pushbutton or a bypass switch is used to check for the faults at the center of

A proper operating procedure shall be established to ensure that this

at least once in a day.

Fig: Potentiometer method

: This method requires an auxiliary supply, which is injected to



es rise to the field current, which is detected by the sensitive relay.


constant current flowing to the earth through the capacitance.

29

in a variety of ways as explained

The field winding is connected with a resistance having

connected to the earth through a sensitive relay R. An

across the relay. The maximum

voltage occurs for faults at end of the windings. However, there are chances that the

faults at the center of the winding may get undetected. Hence, one lower tap is

is connected, a

for the faults at the center of

established to ensure that this

This method requires an auxiliary supply, which is injected to



es rise to the field current, which is detected by the sensitive relay.


� DC injection method:

injection voltage adopted

bias the field voltage to be

the fault current to flow

operate under fault conditions

Fig: AC injection method

: This method avoids the capacitance currents by rectifying the

injection voltage adopted in the previous method. The auxiliary voltage is used to

bias the field voltage to be negative with respect to the earth. An earth fault causes

to flow through the DC power unit causing the sensitive relay to

operate under fault conditions.

Fig: DC injection method

30

This method avoids the capacitance currents by rectifying the

in the previous method. The auxiliary voltage is used to

negative with respect to the earth. An earth fault causes

through the DC power unit causing the sensitive relay to

31

6) Loss of excitation: If the rotor field system should fail for whatever reason, the generator

would then operate as an induction generator, continuing to generate power determined by

the load setting of the turbine governor. It would be operating at a slip frequency and

although there is no immediate danger to the set, heating will occur, as the machine will not

have been designed to run continuously in such an asynchronous fashion. Some form of

field failure detection is thus required, and on the larger machines, this is augmented by a

mho-type impedance relay to detect this condition on the primary side.

7) Loss of synchronization: A generator could lose synchronism with the power system

because of a severe system fault disturbance, or operation at a high load with a leading

power factor. This shock may cause the rotor to oscillate, with consequent variations of

current, voltage and power factor. If the angular displacement of the rotor exceeds the

stable limit, the rotor will slip a pole pitch. If the disturbance has passed, by the time this

pole slip occurs, then the machine may regain synchronism otherwise it must be isolated

from the system. Alternatively, trip the field switch to run the machine as an asynchronous

generator, reduce the field excitation and load, then reclose the field switch to

resynchronize smoothly.

3.5 BUS BAR PROTECTION:

Buses exist throughout the power system and, particularly, wherever two or more circuits

are interconnected. The number of circuits that are connected to a bus varies widely. Bus

faults can result in severe system disturbances, as high fault current levels are typically

available at bus locations and because all circuits supplying fault current must be opened to

isolate the problem. Thus, when there are more than six to eight circuits involved, buses are

often split by a circuit breaker (bus tie), or a bus arrangement is used that minimizes the

number of circuits, which must be opened for a bus fault. There are many bus arrangements

in service dictated by the foregoing and by the economics and flexibility of system

operation. In Guwahati refinery, two buses system is present to ensure more reliability and

continuity. The general protection schemes for double bus system is explained below-

SINGLE BREAKER–DOUBLE BUS

Fig: Typical four circuit single breaker doub

This arrangement (shown in figure)

can be operated from either of the buses

independently, and one bus can be used as a

The disadvantage is that it requires complicated switching of the protection for both the bus

differential and line protection. Two different

Figure, lines 1 and 2 are shown connec

For this operation, the differential zones are outlined dashed for bus 1, and dash

2.

DOUBLE BUS:

circuit single breaker double bus and bus differential protection

This arrangement (shown in figure) provides high flexibility for system oper

can be operated from either of the buses, the buses can be operated togethe

ntly, and one bus can be used as a transfer bus if a line breaker is out of service.

requires complicated switching of the protection for both the bus

and line protection. Two differential zones for the buses are required. In

Figure, lines 1 and 2 are shown connected to bus 1, with lines 3 and 4 connect

For this operation, the differential zones are outlined dashed for bus 1, and dash

32

protection zones.

ility for system operation. Any line

operated together, as shown, or

reaker is out of service.

requires complicated switching of the protection for both the bus

ial zones for the buses are required. In the

connected to bus 2.

For this operation, the differential zones are outlined dashed for bus 1, and dash-dot for bus

Faults on either buses or associated circui

bus at that time. Faults in the bus tie breaker

protection are required for each bus, as

avoid switching if voltage is required for line protectio

need to be applied to reduce complicat

programmable logic, which are provided for such devi

DOUBLE BREAKER–DOUBLE BUS

Fig: Typical four-circuit double breaker

This is a very flexible arrangemen

protected by a separate different

from paralleled CTs, and this provides prot

overlapping the two breakers. Line protection operates to trip both breakers

With all disconnected switches no

buses does not interrupt service on the lines. All swit

ults on either buses or associated circuits require tripping of all circuits connecte

lts in the bus tie breaker must trip both buses and all circuits. VTs for

d for each bus, as shown. However, line-side VTs are preferable to

avoid switching if voltage is required for line protection. Modern microprocessor rel

need to be applied to reduce complications by using the flexibility of such relays and

hich are provided for such devices.

DOUBLE BUS:

circuit double breaker–double bus and the bus differential protection zones.

arrangement that requires two circuit breakers per circuit. Each bus is

protected by a separate differential, with zones as illustrated. The line protection operates

provides protect ion for the bus area between the two zones

two breakers. Line protection operates to trip both breakers

With all disconnected switches normally closed (NC), as shown, a fault on

service on the lines. All switching is done with break

33

require tripping of all circuits connected to the

both buses and all circuits. VTs for

side VTs are preferable to

Modern microprocessor relays

such relays and the

double bus and the bus differential protection zones.

circuit. Each bus is

zones as illustrated. The line protection operates

ect ion for the bus area between the two zones

two breakers. Line protection operates to trip both breakers.

either of the

done with breakers, and either

34

bus can be removed for maintenance. Line-side voltage, either VTs o r CCVT s, is necessary if

required by the line protection.

Differential protection for buses: Complete differential protection requires that all circuits

connected to the bus be involved, because it compares the total current entering the zone

with the total current leaving the zone. Except for a two-circuit bus, this means comparisons

between several CTs that are operating at different energy levels and often with different

characteristics. The most critical condition is the external fault just outside the differential

zone. The CTs on this faulted circuit receive the sum of all the current from the other

circuits. Thus, it must reproduce a potential high-current magnitude with sufficient accuracy

to match the other CT secondary currents and avoid mis-operation. Therefore, CT

performance is important. The relays and CTs are both important members of a ‘‘team’’ to

provide fast and sensitive tripping for all internal faults, at the same time, restrain for all

faults outside the differential zone. Two major techniques are in use to avoid possible

unequal CT performance problems: (1) multirestraint current and (2) high- impedance

voltage. A third system employs air-co re transformers to avoid the iron-core excitation and

saturation problems. All are in practical service. They exist with various features, depending

on the design. Each feature has specific application rules. These should be followed

carefully, for they have been developed to overcome the inherent deficiencies of

conventional CTs on both symmetrical and asymmetrical fault currents.

3.6 MOTOR PROTECTION:

The protection of motors varies considerably and is generally less standardized than the

protection of the other apparatus or parts of the power system. This results from the wide

variety of sizes, types, and applications of motors. The protection is principally based on the

importance of the motor, which usually is closely related to the size.

35

The potential hazards normally considered are-

1. Faults: phase or ground

2. Thermal damage a. From Overload (continuous or intermittent)

b. From Locked rotor (failure to start or jamming)

3. Abnormal conditions-

a. Unbalanced operation

b. Undervoltage and overvoltage

c. Reversed phases

d. High-speed reclosing (reenergizing while still running)

e. Unusual ambient or environmental conditions(cold, hot, and damp)

f. Incomplete starting sequence

These are for induction motors, which represent the large majority of all

motors in service. For synchronous motors, additional hazards are

4. Loss of excitation (loss of field)

5. Out-of-step operation (operation out of synchronism)

6. Synchronizing out of phase

These can be reclassified relative to their origins:

A. Motor induced -

1. Insulation failure (within motor and associated wiring)

2. Bearing failure

3. Mechanical failures

4. Synchronous motors: loss of field

B. Load induced-

1. Overload (and underload)

2. Jamming

3. High inertia

36

C. Environment induced -

1. High ambient temperature

2. High contaminant level: blocked ventilation

3. Cold, damp ambient temperature

D. Source or system induced-

1. Phase failure (open phase or phases)

2. Overvoltage

3. Undervoltage

4. Phase reversal

5. Out-of-step condition resulting from system disturbance

E. Operation and application induced-

1. Synchronizing, closing, or reclosing out of phase

2. High duty cycle

3. Jogging

4. Rapid or plug reversing

The various protection schemes for important abnormal operating conditions are described

below –

• OVERVOLTAGE PROTECTION:

� The overall result of an overvoltage condition is a decrease in load current and poor

power factor.

� Although old motors had robust design, new motors are designed close to saturation

point for better utilization of core materials and increasing the V/Hz ratio cause

saturation of air gap flux leading to motor heating.

� The overvoltage element should be set to 110% of the motors nameplate unless

otherwise started in the data sheets.

• UNDERVOLTAGE PROTECTION:

� The overall result of an undervoltage condition is an increase in current and motor

heating and a reduction in overall motor performance.

37

� The undervoltage protection element can be thought of as backup protection for the

thermal overload element. In some cases, if an undervoltage condition exists it may

be desirable to trip the motor faster than thermal overload element.

� The undervoltage trip should be set to 80-90% of nameplate unless otherwise stated

on the motor data sheets.

� Motors that are connected to the same source/bus may experience a temporary

undervoltage, when one of motors starts. To override this temporary voltage sags, a

time delay set point should be set greater than the motor starting time

• UNBALANCE PROTECTION:

� Indication of unbalance -> negative sequence current / voltage

� Unbalance causes motor stress and temperature rise

� Current unbalance in a motor is result of unequal line voltages

o unbalanced supply, blown fuse, single-phasing

� Current unbalance can also be present due to:

o Loose or bad connections

o Incorrect phase rotation connection

o Stator turn-to-turn faults

� For a typical three-phase induction motor:

o 1% voltage unbalance relates to 6% current unbalance.

o For small and medium sized motors, only current transformers (CTs) are

available and no voltage transformers (VTs). Measure current unbalance and

protect motor.

o The heating effect caused by current unbalance will be protected by enabling

the unbalance input to the thermal model

o For example, a setting of 10-15% x FLA for the current unbalance alarm with

a delay of 5-10 seconds and a trip level setting of 20-25% x FLA for the

current unbalance trip with a delay of 2-5 seconds would be appropriate.

• GROUND FAULT PROTECTION:

� A ground fault is a fault that creates a path for current to flow from one of the

phases directly to the neutral through the earth bypassing the load

� Ground faults in a motor occur:

o When its phase conductor’s insulation is

stress, moisture or internal fault occurs between the

� To limit the level of the ground fault current

supplies neutral and ground. This

grounding transformer sized to ensure

Zero Sequence CT Connec

o Best method.

o Most sensitive & inherent noise immunity

� All phase conductors are passed through the window of the same CT referred to

the zero sequence CT.

� Under normal circumstances, the three phase currents will sum to zero

an output of zero from the Zero Sequence CT’s secondary.

� If one of the motors phases were to

currents would no longer equal zero causing a current to flow in the secondary

the zero sequence. This current would be detecte

fault.

Ground faults in a motor occur:

When its phase conductor’s insulation is damaged for example due to voltage

moisture or internal fault occurs between the conductor and ground

limit the level of the ground fault current connect impedance between the

supplies neutral and ground. This impedance can be in the form of a resistor

grounding transformer sized to ensure maximum ground fault current is limited.

Zero Sequence CT Connection

Most sensitive & inherent noise immunity.

All phase conductors are passed through the window of the same CT referred to

.

Under normal circumstances, the three phase currents will sum to zero

t of zero from the Zero Sequence CT’s secondary.

If one of the motors phases were too shorted to ground, the sum of the phas

currents would no longer equal zero causing a current to flow in the secondary

the zero sequence. This current would be detected by the motor relay as a

38

damaged for example due to voltage

conductor and ground.

connect impedance between the

impedance can be in the form of a resistor or

maximum ground fault current is limited.

All phase conductors are passed through the window of the same CT referred to as

Under normal circumstances, the three phase currents will sum to zero resulting in

shorted to ground, the sum of the phase

currents would no longer equal zero causing a current to flow in the secondary of

d by the motor relay as a ground

Residual Ground Fault Connection

o Less sensitive.

o Drawbacks due to asymmetrical starting

� For large cables that cannot be fit through the zero sequence CT’s window, the

residual ground fault configuration can be used.

� This configuration is inherently less sensitive than that of the zero sequence

configurations owing to the fact that the CTs are not perfectly matched.

� During motor starting, the motor’s phase currents typically rise to mag

excess of 6 times motors full load current and are asymmetrical.

� The combination of non

magnitudes produce a false residual current. This current will be

the motor relay as a ground fault unless the ground fault

high enough to disregard this error during startin

• DIFFERENTIAL PROTECTION:

� Differential protection may be considered the first line of protection

phase-to-phase or phase

response of the differential element may

occurred to the motor.

Residual Ground Fault Connection

Drawbacks due to asymmetrical starting current and un-matched CTs

For large cables that cannot be fit through the zero sequence CT’s window, the

fault configuration can be used.

This configuration is inherently less sensitive than that of the zero sequence


During motor starting, the motor’s phase currents typically rise to mag

excess of 6 times motors full load current and are asymmetrical.

The combination of non-perfectly matched CTs and relative large phase current

magnitudes produce a false residual current. This current will be misinterpreted by

ground fault unless the ground fault element’s pickup is set

high enough to disregard this error during starting.

DIFFERENTIAL PROTECTION:

Differential protection may be considered the first line of protection

phase-to-ground faults. In the event of such faults, the quick

response of the differential element may limit the damage that may have otherwise

occurred to the motor.

39

matched CTs.

For large cables that cannot be fit through the zero sequence CT’s window, the

This configuration is inherently less sensitive than that of the zero sequence


During motor starting, the motor’s phase currents typically rise to magnitudes

perfectly matched CTs and relative large phase current

misinterpreted by

element’s pickup is set

Differential protection may be considered the first line of protection for internal

of such faults, the quick

limit the damage that may have otherwise

Core balance method:

o Two sets of CT’s, one at the beginning of

the neutral point

o Alternatively, one set of three core

o The differential element subtracts the

the current going into each phase and

the differential pickup level.

Summation method with six CTs

� If six CTs are used in a summing

the two CTs on each phase

and asymmetrical currents may

outputs.

� To prevent nuisance tripping in this

be set less sensitive, or the

through the problem period d

� The running differential delay can then be

responds very fast and is sensitive to low

Two sets of CT’s, one at the beginning of the motor feeder, and the

neutral point

Alternatively, one set of three core-balance CTs can also be used

The differential element subtracts the current coming out of each phase from

current going into each phase and compares the result or difference with

erential pickup level.

Summation method with six CTs:

If six CTs are used in a summing configuration, during motor starting, the

the two CTs on each phase may not be equal as the CTs are not perfectly identical

currents may cause the CTs on each phase to have different

To prevent nuisance tripping in this configuration, the differential level may

be set less sensitive, or the differential time delay may have to be

period during motor starting.

The running differential delay can then be fine tuned to an application such that it

responds very fast and is sensitive to low differential current levels.

40

the motor feeder, and the other at

CTs can also be used

current coming out of each phase from

compares the result or difference with

configuration, during motor starting, the values from

perfectly identical

to have different

configuration, the differential level may have to

extended to ride

fine tuned to an application such that it

differential current levels.

Biased differential protection

� Biased differential protection m

the neutral CT’s.

� This method has a dual slope characteristic.

characteristic is to prevent a mis

during external faults. CT

saturation.

� Characteristic allows for very sensitive

sensitive settings when the fault current is

incorrect operating signals.

Biased differential protection - six CTs:

Biased differential protection method allows for different ratios for system/line and

This method has a dual slope characteristic. Main purpose of the percent

characteristic is to prevent a mis-operation caused by unbalances between CTs

external faults. CT unbalances arise as a result of CT accuracy errors or CT

Characteristic allows for very sensitive settings when the fault current is low and less

sensitive settings when the fault current is high and CT performance may produce

ting signals.

41

for different ratios for system/line and

Main purpose of the percent-slope

caused by unbalances between CTs

result of CT accuracy errors or CT

settings when the fault current is low and less

high and CT performance may produce

42

• Short circuit protection:

� The short circuit element provides protection for excessively high overcurrent faults.

� Phase-to-phase and phase-to-ground faults are common types of short circuits.

� When a motor starts, the starting current (which is typically 6 times the Full Load

Current) has asymmetrical components. These asymmetrical currents may cause one

phase to see as much as 1.7 times the RMS starting current.

� To avoid nuisance tripping during starting, set the short circuit protection pick up to

a value at least 1.7 times the maximum expected symmetrical starting current of

motor.

� The breaker or contactor must have an interrupting capacity equal to or greater than

the maximum available fault current or let an upstream protective device interrupt

fault current.

• Stator RTD Protection:

� A simple method to determine the heating within the motor is to monitor the stator

with RTDs.

� Stator RTD trip level should be set at or below the maximum temperature rating of

the insulation.

� For example, a motor with class F insulation that has a temperature rating of 155°C

could have the Stator RTD Trip level be set between 140°C to 145°C, with 145° C

being the maximum (155°C - 10°C hot spot).

� The stator RTD alarm level could be set to a level to provide a warning that the

motor temperature is rising.

43

3.7 RELAYS:

Relay is a protective device which closes the contacts of trip circuit and thereby sends a

signal to respective circuit breaker, if any abnormal condition occurs in that protected

circuit where the relay operation is specified. Earlier in Guwahati refinery, various

electromechanical relays were installed, but with the advancement in technology,

numerical relays are now being installed in the refinery to enhance reliability , speed,

sensitivity etc. The following numerical relays are installed in Guwahati Refinery-

Siemens make relay (installed in new units HT substation and 33kV Switch Gear)

• 7SJ600 Over Current Relay

• 7RW600 Over/Undervoltage relay

• 7VK512 Check synchronization relay

• 7UT512 Differential relay

ABB make relay: (installed in new unit substation)

• SPI30UC Over/undervoltage relay

• SPAD346 C2 differential relay

Alstom make relay (installed at 6.3 kV generation bus alstom section)

Micom P-121,127,221,921,111,211

CS PC

PL300NC (CDV relay)

Easun Reyrolle

RHO-3 motor protection relay

Under frequency relay

Circuit breaker (CB) is a switchgear, which can make or break a circuit manually and breaks

the circuit automatically under fault conditions. The CB has two contacts

contact other one is termed as

energized and this trips the breaker by moving the contacts apart. The arc produced

between the contacts is extinguished by air, oil and vacuum medium. Based on this

classification, in Guwahati refinery, the following types of breakers are use

1) AIR CIRCUIT BREAKER: Interrupting contacts situated in air

of small arcs by the Arc-chute as it rises due to

breakers are normally employed for 380

These circuit breakers are used in LT breakers in Guwahati refinery.

2) OIL IMMERSED CIRCUIT BREAKER:

and the oil acts as the ionizing medium between the contacts. The oil is mineral type, with

high dielectric strength to withstand the voltage across the contacts under normal

conditions.

3.8 CIRCUIT BREAKER:


the circuit automatically under fault conditions. The CB has two contacts-

other one is termed as moving contact. Under fault condition, the



classification, in Guwahati refinery, the following types of breakers are use

nterrupting contacts situated in air. Arc is chopped into a n

hute as it rises due to heat and magnetic forces. The air circuit

s are normally employed for 380-480 V range.

Fig: Air Circuit breaker

These circuit breakers are used in LT breakers in Guwahati refinery.

OIL IMMERSED CIRCUIT BREAKER: In this design, the main contacts are immersed in oil



44


one is fixed

. Under fault condition, the trip coil is



classification, in Guwahati refinery, the following types of breakers are use-

Arc is chopped into a number

agnetic forces. The air circuit

In this design, the main contacts are immersed in oil



Fig: Single break oil circuit breaker

Oil has the following advantages:

• Ability of cool oil to flow into the

• Cooling surface presented by oil

• Absorption of energy by decomposition of oil

• Action of oil as an insulator lending to more compact design of switchgear.

And Disadvantages are:

• Inflammability (especially if

• Maintenance (changing and purifying).

In Guwahati Refinery, Oil circuit breakers are used in the following substations

i) NE panel breakers in Thermal Power station (TPS).

ii)12/02 substation HT breakers.

iii) DM&S substation HT breakers.

: Single break oil circuit breaker Fig: Double break oil circuit breaker

Oil has the following advantages:

• Ability of cool oil to flow into the space after current zero and arc goes out

• Cooling surface presented by oil

• Absorption of energy by decomposition of oil


• Inflammability (especially if there is any air near hydrogen)

• Maintenance (changing and purifying).

In Guwahati Refinery, Oil circuit breakers are used in the following substations

) NE panel breakers in Thermal Power station (TPS).

)12/02 substation HT breakers.

ation HT breakers.

45

Double break oil circuit breaker

space after current zero and arc goes out


In Guwahati Refinery, Oil circuit breakers are used in the following substations-

46

3) VACUUM CIRCUIT BREAKERS:

A vacuum circuit breaker is suitable for mainly medium voltage application circuit breaker

where the arc quenching takes place in vacuum. The major parts of vacuum circuit breaker

are breaker contacts, vapour condensing shields, metallic bellows, end flanges and

enclosure. The pressure of vacuum inside vacuum CB is normally maintained at 10⁻⁶ bar.

Fig: Vacuum Circuit Breaker

In Guwahati refinery, the following substations have Vacuum CB-

i) All HT breakers of new substation (MAKE Siemens).

ii) All HT breakers in TPS HT generating section (MAKE Areva and Alstom).

iii) All HT breakers at old HT substation (MAKE Jyoti).

iv) All HT breakers in new intake substation (MAKE Areva).

4.1

� Instrumentation technology is provided to optimize the Plant efficiency without

compromising the safety and environment around working area.

� It provides control to restrict things to go beyond operator control. If somehow

things go beyond contr

� Leads to a safer life in an explosive environment.

4.2 DIFFERENT TYPES OF INSTRUMENTS IN GUWAHATI REFINERY

4.2.1 FLOW MEASUREMENT:

• Local indicator ( Rotameter , differential pressure

• Remote indicator ( Differential pressure transmitter , orifice, venturimeter,

Ultrasonic flow meter)

LOCAL INDICATOR:

• ROTAMETER:

Rotameter (also known as variable area flow meter) are typically made from a

tapered glass tube that is positioned vertically in the fluid flow. A float that is the

same size as the base of the glass tube rides upward in relation to the amount of

flow. Because the tube is

bottom, the float resides at the point where the differential pressure between the

upper lower surfaces balance the weight of the float. In most rotameter applications

4. INSTRUMENTATION

4.1 IMPORTANNCE AND RELEVANCE:

Instrumentation technology is provided to optimize the Plant efficiency without

compromising the safety and environment around working area.

It provides control to restrict things to go beyond operator control. If somehow

go beyond control, it automatically shutdown the plant in a safe way.

Leads to a safer life in an explosive environment.

DIFFERENT TYPES OF INSTRUMENTS IN GUWAHATI REFINERY

FLOW MEASUREMENT:

Local indicator ( Rotameter , differential pressure gauge)

Remote indicator ( Differential pressure transmitter , orifice, venturimeter,

Ultrasonic flow meter)

Fig: Rotameter

known as variable area flow meter) are typically made from a



flow. Because the tube is larger in diameter at the top of the glass than at the



47

Instrumentation technology is provided to optimize the Plant efficiency without

It provides control to restrict things to go beyond operator control. If somehow

ol, it automatically shutdown the plant in a safe way.

DIFFERENT TYPES OF INSTRUMENTS IN GUWAHATI REFINERY:

Remote indicator ( Differential pressure transmitter , orifice, venturimeter,

known as variable area flow meter) are typically made from a



larger in diameter at the top of the glass than at the



48

the flow rate is read directly from a scale inscribed on the glass; in some cases an

automatic sensing device is used to the float and transmits a flow signal.

These transmitting rotameters are often made from stainless steel or other

materials for various fluid applications and high pressures. Rotameter may range in

size from ¼ inch to greater than 6 inch. They measure a wider band of flow(10 to 1)

than an orifice plate with an accuracy of ±2% and a maximum operating pressure of

300 psi when constructed of glass. Rotameters are commonly for purge and levels.

• DIFFERENTIAL PRESSURE GAUGE:

Differential pressure gauges are often found in industrial process systems and yet,

they are easily overlooked or misunderstood. In fact, a differential pressure gauge

can often times provide multiple solutions to everyday problems.

Differential (Dp or Δp) is the difference between two applied pressures. For example,

the pressure at point ‘A’ equals 100 psi and the pressure at point ‘B’ equals 60 psi.

The differential pressure is 40 psi (100 – 60=40 psi).

A differential pressure gauge is a visual indicator, designed to measure ad display the

difference in pressure between two pressure points in a process system. They

typically have two inlet ports, each connected to the pressure points that are being

monitored. In effect, the differential pressure gauge performs the mathematical

operation of subtraction through mechanical means. This eliminates the need for an

operator or control system to watch two separate gauges and determine the

difference in readings.

Differential pressure gauges are also used to measure the flow of a liquid inside a

pipe. Utilizing an orifice plate, venture, or flow nozzle to reduce the diameter inside

the pipe; the differential pressure gauge measures the pressure before and after

orifice. The pressure drop across the orifice is then mechanically translated by the

difference pressure gauge into the flow rate. Differential pressure gauges are an

uncomplicated solution for a visual indicator when measuring process flow.

• DIFFERENTIAL PRESSURE TRANSMITTER(DPT):

The most common and useful industrial pressure measuring instrument is the

differential pressure transmitter. This equipment will sense the difference in

pressure in two ports and produce an output signal with reference to a calibrated

pressure range.

The industrial DPTs are made of two housings. Pressure sensing element is housed

in the bottom half, and the electronics are housed at the top half. It will have two

pressure ports marked as ‘High’ and ‘Low’. It is not compulsory that the high port will

be always at high pressure and low port will be always at low pressure. This labelling

has its relation to the effect of the port on the output signal.

• ORIFICE FLOW METER:

An orifice flow meter is the most common head type flow measuring device. An

orifice plate is inserted in the pipeline and the differential pressure across it is

measured.

The orifice plate inserted in the pipeline causes an incr

corresponding decrease in pressure. The flow pattern shows an effective decrease in

cross section beyond the orifice plate, with a maximum velocity and minimum

pressure at the vena contracta.

A concentric orifice plate is the si

acting as a primary device, the orifice plate constricts the flow of a fluid to produce a

differential pressure across the plate. The result is a high pressure upstream and a

low pressure downstream that is pro

orifice plate usually produces a greater overall pressure less than other primary

devices. A practical advantage of this device is that cost does not increase

significantly with pipe size.




e always at high pressure and low port will be always at low pressure. This labelling

has its relation to the effect of the port on the output signal.

ORIFICE FLOW METER:



Fig: Orifice flow meter

The orifice plate inserted in the pipeline causes an increase in flow velocity and a



pressure at the vena contracta.

A concentric orifice plate is the simplest and least expensive of the head meters



low pressure downstream that is proportional to the square of the flow velocity. Ann



significantly with pipe size.

49




e always at high pressure and low port will be always at low pressure. This labelling



ease in flow velocity and a



mplest and least expensive of the head meters



portional to the square of the flow velocity. Ann



• VENTURI TUBES:

Venturi tubes are differential pressure producers, based on Bernoulli’s Theorem.

General performance and calculations are similar to those of orifice plates.

It consists of a cylindrical inlet section equal to the pipe diameter; a converging

conical section in which the cross

increase with a corresponding increase in velocity head and a decrease in the

pressure head; a cylindrical throat section where the velocity is constant so that the

decreased pressure head

velocity decreases and almost all of the original pressure head is recovered. The

unrecovered pressure head is commonly called as head loss.

In the venturi meter, velocity is inc

upstream cone.

• ULTRASONIC FLOWMETER:

It provides volumetric flow rate. We typically use the transmit

sounds wave transmitted in the direction of fluid flow travels faster than those

travelling upstream. The transmit

velocity. Ultrasonic flow meter have negligible pressure drop, have high turn down

capability, and can handle a wide range applications. Crude oil production,

transportation and processing

4.2.2 PRESSURE MEASUREMENT:

When a fluid is in contact with a boundary, it produces a force at right angles to that

boundary. The force per unit area is called the pressure. (

There are three basic m

The simplest method involves balancing the unknown pressure against the pressure

produced by a column of liquid of known density.

The second method involves allowing the unknown the unknown pressure to act on

a known area and measuring the resultant force either directly or indirectly.

nturi tubes are differential pressure producers, based on Bernoulli’s Theorem.



in which the cross-sectional area decreases causing the velocity to



decreased pressure head can be measured; and a diverging recovery cone where the


unrecovered pressure head is commonly called as head loss.

Fig: Venturi tube

In the venturi meter, velocity is increased and the pressure is decreased in the

ULTRASONIC FLOWMETER:

It provides volumetric flow rate. We typically use the transmit-time method, where


upstream. The transmit-time difference is proportional to the fluid



transportation and processing are typical applications for this technology.

PRESSURE MEASUREMENT:


boundary. The force per unit area is called the pressure. (� � �/�

There are three basic methods for pressure measurement:


produced by a column of liquid of known density.


nd measuring the resultant force either directly or indirectly.

50

nturi tubes are differential pressure producers, based on Bernoulli’s Theorem.



sectional area decreases causing the velocity to



can be measured; and a diverging recovery cone where the


reased and the pressure is decreased in the

time method, where


time difference is proportional to the fluid



are typical applications for this technology.


��



nd measuring the resultant force either directly or indirectly.

51

The third method involves allowing the unknown pressure to act on an elastic

membrane of known area and measuring the resultant stress or strain.

• Pressure measurement by balancing a column of liquid of known density:

The simplest form of instrument for this type of measurement id U-type manometer.

Consider a simple U-tube containing a liquid of density D as shown in the figure. The

points A and B are at the same horizontal level, and liquid at C stands at a height h mm

above B. Then,

Pressure at A=Pressure at B=atmospheric pressure +pressure due to column of liquid

BC=atmospheric pressure +hDg

If the liquid is water the unit of measure is mm water, and if the liquid is mercury then

the unit of measure is mm Hg. The corresponding SI unit is Pascal and

1 mm water=9.80665 Pa

1 mm Hg=133.322 Pa

52

• Pressure measurements by allowing the unknown pressure to act on a known area

and measuring the resultant force:

The simplest method for determining a pressure by measuring the force that is

generated when it acts on a known area in Dead weight tester but this system is used for

calibrating instruments rather than measuring unknown pressures.

• Pressure measurement by allowing the unknown pressure to act on a flexible

member and measuring the resultant motion:

The great majority of pressure gauges utilize a Bourdon, tube, stacked diaphragms, or a

bellows to sense the pressure. The applied pressure causes a change in the shape of the

sensor that is used to move a pointer with respect to a scale.

The Bourdon tube is a hollow tube with an elliptical cross section. When a pressure

difference exists between the inside and outside, the tube tends to straighten out and the

end moves. The movement is usually coupled to a needle on a dial to make a complete

gauge. It can also be connected to a secondary device such as an air nozzle to control air

pressure or to a suitable transducer to convert it into an electric signal. This type can be

used for measuring pressure difference.

PRESSURE GAUGE: Pressure gauges are based on the principle of Bourdan tube(C

type).The bourdan tube is a non-circular elliptical cross sectional C shaped hollow tube.

When a pressure difference exists between inside and outside the tube, it tends to

straighten out and the end moves. The movement is usually coupled to a needle on a dial

53

to make a complete gauge. It can also be connected to a secondary device such as an air

nozzle to control air pressure or to a suitable transducer to convert it into an electric

signal. This type can be used for measuring pressure difference.

4.2.3 LEVEL MEASUREMENT:

• LEVEL GAUGE: A level gauge is a device which is used to indicate the level of

liquid in a chamber. Level gauge may be of different types. Some of them are:

Plastic tube type

Glass tube type

Magnetic type

• MAGNETIC LEVEL GAUGE: Magnetic Level Gauges provides clear, high clarity

indication of liquid level. Magnetic Level Gauges are principally designed as an

alternative to glass level gauges. MLGs are now widely used in all industries as they

avoid direct contact with indicator system; it eliminates need of glass for direct level

indication and prevents chemical spillage due to breakage of glass.

A magnetic level gauge includes a “floatable” device that can float both in high

density and low density fluids. They can also be designed to accommodate sever

environmental conditions up to 210 bars at 370 degree Celsius.

In a magnetic level gauge, its level gauge body colour changes with the level of fluid.

This is due to the magnetic property of the float inside the level gauge.

Magnetic Level Gauges operates on the principle of magnetic field coupling to

provide fluid level information. Float chamber is typically constructed with non

magnetic pipe having process connections that matches to the vessel connections.

Float size and weight is determined by the process fluid, pressure, temperature and the

specific gravity of the process fluid. Float contains magnets to 0 provide 360 magnetic

flux field.

• MAGNETIC LEVEL GAUGE – FLAPPER: Indicator system is consists of

bicolour rollers equipped with magnets mounted on rail inside the housing. As the

level starts rising or falling magnetic float also travels with liquid level in non

54

magnetic chamber. The magnetic interaction between magnets in float and bicolour 0

rollers causes each roller to rotate 180.

• MAGNETIC LEVEL GAUGE - CAPSULE SHUTTLE: Indicator system consists

of capsule huttle housed in the glass tube inside the housing. As the level starts rising

or falling magnetic float also travels with liquid level in non magnetic chamber. The

magnetic interaction between magnets in float and capsule shuttle causes capsule to

travel along the magnetic float.

• LEVEL TROLL: Level troll works on the principle of feeling of weightless or loss

of weight when some object is immersed in the liquid level. This is due to the

buoyancy force exerted by the liquid surface on the object. Buoyancy force depends

on the volume of the object immersed in the liquid.

The variation in level of buoyancy resulting from a change in liquid level varies the

net weight of the displacer increasing or decreasing the load on the torque arm. This

change is directly proportional to change in level and specific gravity of the liquid.

The resulting torque tube movement varies the angular motion of the rotor in RVDT

providing a rotor change proportional to the rotor displacement, which is converted

and amplified to a D.C. current.

RADAR TYPE LEVEL: Radar level measurement is based on the principle of

measuring the time required for the microwave pulse and its reflected echo to make a

55

complete return trip between the non-contacting transducer and the sensed material

level. Then, the transceiver converts this signal electrically into distance/level and

presents it as an analogue and/or digital signal. The transducer’s output can be

selected by the user to be directly or inversely proportional to the span.

GWR transmitter sends low power pulses guided along a probe immersed in the

process media. When the pulse reaches the surface of the material to be measured,

part of the energy is reflected back to the transmitter and the time difference between

the generated pulse and reflected pulse is converted into a distance from which the

total level is calculated.

The benefits of radar as a level measurement technique are clear.

� Radar provides a non-contact sensor that is virtually unaffected by changes in

process temperature, pressure or the gas and vapour composition within a

vessel.

� The measurement accuracy is unaffected by changes in density, conductivity

and dielectric constant of the product being measured or by air movement

above the product.

� The echoes derived from pulse radar are discrete and separated in time. This

means that pulse radar is better equipped to handle multiple echoes and false

echoes that are common in process vessels and solids silos.

56

4.2.4 TEMPERATURE MEASUREMENT:

Measurement of temperature is done with the help of various devices. The devices

which are being used in IOCL guwahati refinery for the measurement of temperature

are as follows:

Temperature gauge

Thermocouple

RTD

Temperature transmitter

None of these devices are connected directly to the line for it may damage the

instrument. An additional device known as thermo well is necessary to be installed in

order to use these instruments for the measurement of temperature of various process

fluids.

A description of these instruments is given below:

• TEMPERATURE GAUGE: This is a local indicator of temperature. The principle

behind the working of this instrument is bimetallic strip. A bimetallic strip consists of

two strips of different metal which expand at different rates as they are heated, usually

steel and copper is used as the two metals and sometime brass is used in place of

copper. The strips are joined together throughout their length by riveting, brazing or

welding. The different rate of expansion of the two metals forces the flat strips to

bend one way if heated and the opposite way if cooled below its initial temperature.

Fig: Temperature gauge Fig: A bimetallic strip

57

• THERMOCOUPLE: This is a remote indicator of temperature. Here the

temperature measurement is done by exploiting the principle called ‘Seeback Effect’.

According to this effect when two dissimilar metal are joined together to form

two junction are the junctions are kept at different temperatures then there is a

current flow occurring across the loop and if any portion is chipped off then

there will be a potential developed across the two ends of the chipped off portion.

Some signal conditioning is required to be done for using the thermocouple as a

temperature measuring device in an industry. These are as follows:

Amplification of the emf generated as it is very low.

Cold junction compensation(Since the emf produced is dependent on the

temperature difference of the two junctions hence the cold junction needs to be at zero

for ensuring that the temperature shown by the device is same as the temperature

which is being measured. Since it is not possible to provide a physical zero hence this

is provided by signal conditioning of the output).

Fig: A thermocouple

Fig: EMF due to Seeback effect

58

• RESISTANCE TEMPERATURE DETECTOR (RTD): This is a remote indicator

of temperature. RTD stands for Resistance Temperature Detector. This device is

based on the principle that resistance of a wire is dependent on its temperature.

Resistance of a metal strip is given by the equation R=DL/A, where D is the

resistivity of the material of the metal strip, L is the length of the metal strip and A is

the cross-sectional area of the metal strip. As the temperature is changed the

dimensions of the metal strip i.e. the length L and the cross sectional area A changes

(D does not change appreciably) due to which the value of resistance R changes. By

measuring this change in resistance we can measure the change in temperature. The

material used for RTD is Platinum (Pt 100) and it is used in such dimensions so that at

00 the resistance is 100Ω.

Fig: RTD

• TEMPERATURE TRANSMITTER: It is a remote indicating type instrument. The

temperature which is measured is transmitted to the control room. The device consists

of a temperature sensor and an inbuilt signal conditioning circuit. The sensor can be a

Thermocouple or an RTD. If it is a thermocouple then the change in voltage is

converted to 4-20 ma signals and then transmitted and if it is an RTD the change in

resistance is converted to 4-20ma signal and then transmitted.

Fig: Temperature transmitter

59

• THERMO WELL: This is a device which is used to avoid direct contact of the

temperature measuring device with the process fluid. Direct contact of the fluid with

the device may result in corrosion of vital parts of the device. Thus a thermo well acts

as a protective device and an interface between process medium and temperature

measuring device.

Fig: Thermowell

4.2.5 OTHER MISCELLANEOUS INSTRUMENTS USED IN REFINERY:

I/P CONVERTER: A typical I/P transducer is a force-balance device in which a coil

suspends and hang in the field of a permanent magnet. Current flowing through the coil

makes it an electromagnet and causes a force of repulsion between the electromagnet and

permanent magnet. An increase in current through the coil increases the repulsive force,

thereby moving the link connected to the flapper upward. It reduces the gap between the

flapper and nozzle. The relative position of flapper to the nozzle results in an Air Gap that

causes leakage of air. The remaining of supply pressure after leakage is the back pressure

which acts as a pilot pressure to control the outlet pressure.

The I/P transducer must be supplied with air usually at a pressure of 20 Psi (Hence supply

pressure = 20 Psi). When the input current is at maximum (20 mA) the repulsion between

permanent magnet and electromagnet will also be the maximum, such that there will be

no gap between the flapper and nozzle. So the entire 20 Psi will be available as back

60

pressure. But the I/P transducer should be linearly calibrated such that 4-20 mA input = 3-

15 Psi output. That is when input current is 20 mA, the output should be only 15 Psi. The

valve plug is the device which helps in restricting the output at 15 Psi. The valve plug

(mechanical equivalent of Zener diode) is designed such as to give a maximum output of

15 Psi. The remaining excess pressure is given out through exhaust. When input current is

minimum (4 mA), the repulsion between the two magnets will be the minimum and it

result in a larger Air Gap. Through this Air Gap 17 Psi pressure will leak out. The

remaining 3 Psi (20 – 17 Psi) will be the output pressure of transducer.

ANALYZERS: Analyzers are devices that measures and transmit information

about chemical composition, physical properties or chemical properties of the sample.

The analyzers in use in IOCL Noonmati are as follows:

1. Gas detector

2. Oxygen analyzer

3. PH meter

4. SOX analyzer

5. NOX analyzer

� Gas detector: Hydrocarbon gas detector works on the principle of the absorption

of infra red rays by hydro carbon molecules present in the atmosphere. The amount of

61

absorption depends on amount of hydrocarbon components present. Higher the

amount of absorption higher will be the concentration of hydrocarbon present.

� Oxygen analyzer: This works on the principle of the difference in the partial

pressure on either side of the cell. On one side of the cell, we have instrument air (as

reference) and the other side faces the air whose oxygen concentration is to be

measured. Due to difference in partial pressure of oxygen across the cell, an EMF is

generated guided by Nernst’s Eqn. The equation is given by:

E=A T log (% of O2 in instrument air/ % of O2 in air to be monitored)

Where, A=R/nF

R = gas constant, which is 8.31 (volt-coulomb)/ (mol-K)

T = temperature (K)

n = number of moles of electrons exchanged in the electrochemical reaction (mol)

F = Faraday's constant, 96500 coulombs/mol

� pH meter: It is an instrument used to measure the pH of a liquid. A pH meter

consists of a glass electrode having a reference pH solution. Whenever a liquid whose

Ph is to be measured comes in contact with the electrodes, a voltage is generated

depending on the pH value of the measured liquid. This voltage is then converted to

universal pH scale by auxiliary circuits associated with the pH meter transmitter.

� SOX analyzer: Works on the principle that sulphur will emit light known as

fluorescence, in presence of UV radiation. A detection chamber is there to detect the

wavelength of emitted light waves. For a specified range this radiation is measured by

a photometer which provides us the required data.

� NOX analyzer: This works on the principle that NO will emit light known as

chemilumeniscence, in presence of highly oxidising ozone molecules. A detection

chamber is there to detect the wavelength of the emitted light waves. For a specified

range, this radiation is measured by a photometer which provides the required data

62

5. CONCLUSION: Guwahati refinery, known as Gangotri of Indian Oil is well known

for its achievements during the last 53 years and all together, IOCL is always holding the

fame of best Public Sector Unit in India. Notwithstanding IOCL Guwhati has incorporated

and installed various extra ordinary and efficient equipments in electrical section, but still

more modernization (specially in Thermal Power Station) is required to keep the pace in the

race. As we observed during the internship session, the overall efficiency of the STG was

around 50%, which is generally a good value, but still it could be increased if the obsolete

components are removed and new instruments incorporated with electronics are installed.

Immediate replacement of the highly priced equipments is not at all possible, but through a

proper plan or arrangement, it could be done within the coming years. Microprocessor relay

should be installed in order to increase the reliability and stability of the power system and

the operational and maintenance instructions for such relays should be given to the people

concerned by the experts. It is obvious that IOCL Guwahati has been striving hard to improve

its efficiency and performance and to give quality products to the consumers, and to

achieve the peak and to maintain its glory; the structural reforms should be coupled with

operational reforms.

63

6. BIBLIOGRAPHY:

1. Practical power system protection; L.G. Hewitson, Mark Brown & Ramesh Balakrishnan;.

2. Protective Relaying-Principles and Applications; J. Lewis Blackburn , Thomas J. Domin; 3rd

edition.

3. Power system analysis; Hadi Sadat; 3rd

edition.

4. Motor Protection Principles; Craig Wester, GE Multilin &Craig. Wester.

5. Operating manuals of STG; IOCL Guwhati, 2011 edition.

6. Power system protection and Switchgear; Badri Ram, D.N. Vishwakarma; 2nd

edition.

7. Electrical power system; C.L. Wadhwa; 6th

edition.

summer internship report indian oil corporation limited (guwahati)

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