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SUMMER INTERNSHIP REPORT INDIAN OIL CORPORATION LIMITED (GUWAHATI) (1st June to 30th 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)
7-19
2.1 Introduction
7-8
2.2 Major electrical equipment details
8-12
2.3 General details of boiler
13-14
2.4 General Description of generators
15-17
2.5 Problems associated with operation of generator
18-19
3. Protection of electrical equipments in Guwahati refinery 3.1 Importance of protection
20-45 20-21
3.2 PSP- basic components
22
3.3 Transformer protection
22-25
3.4 Generator protection
25-30
3.5 Bus-bar protection
30-33
3.6 Motor protection
33-41
3.7 Relays 3.8 Circuit breakers 4. Instrumentation
4.1 Importance and relevance 4.2 Different types of instruments in refinery
42 43-45 46-60 46 46-60
4.2.1 Flow measurement
46-49
4.2.2 Pressure measurement
46-52
4.2.3 Level measurement
52-54
4.2.4 Temperature measurement
55-58
4.2.5 Other miscellaneous instruments
58-60
5. Conclusion
61
6. Bibliography
62
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1. OVERVIEW OF IOCL GUWHATI : Indian Oil Corporation Limited, Limited or Indian Oil, is an Indian state-owned owned oil and gas corporation with its headquarters in New Delhi, India.. The company is the world's 119th largest public corporation, according to the Fortune Global 500 list, and the largest public corporation in India when ranked by revenue. It has also earned reputation as 18th largest petroleum company in the world and No. 1 petroleum trading company among the national oil companies in Asia-pacific Asia region. Indian Oil and its subsidiaries account for a 49% share re in the petroleum products market, 31% share in refining capacity and 67% downstream sector pipelines capacity in India. The Indian Oil Group off Companies owns and operates 11 of India's 23 refineries with a combined combined refining capacity of 80.7 million metric tons per year. In FY 2012 IOCL sold 75.66 million tons of petroleum products and reported a PBT of 37.54 billion,, and the Government of India earned an excise duty of 232.53 billion and tax of 10.68 billion. The company is mainly controlled by Government of India which owns approximately 58.57% shares in the company. It is one of the seven Maharatna status companies of India,, apart from Coal India Limited, NTPC Limited, Oil and Natural Gas Corporation, Steel Authority of India Limited, Limited Bharat Heavy Electricals icals Limited and Gas Authority of India Limited. Indian Oil operates the largest and the widest network of fuel stations in the country, numberingg 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 households through a network etwork of 5,934 Indane distributors. In addition, Indian Oil's Research and Development Center (R&D) at Faridabad supports, develops and provides the necessary technology solutions to the operating divisions of the corporation and its customers within the country and abroad. Guwahati Refinery is the country’s first Public Sector Refinery as well as Indian Oil’s first Refinery serving the nation since 1962. 1962 It is known as GONGOTRI of Indian Oil. Built with Rumanian assistance,, the initial crude processing capacity at the time of commissioning of this Refinery was 0.75 MMTPA and the Refinery was designed to process a mix of OIL and ONGC crude.
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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 : TYPE : Continuous rating at Generator Terminals : Turbine Speed : Generator Speed : Coupling : Live Steam Pressure : Live Steam Temperature: Max. Live Steam Flow: Speed Governor : Governing System : Pressure Governor : Protection :
•
BHEL EK-1000 8 MW 8000 RPM 3000 RPM Speed Reduction Gear 35 ATA 435 D Centigrade 63 MT/hr Hydro-dynamic SRI II ASKANIA Over speed, Axial Displacement, Low lube oil pressure, Low vacuum, High Vibration.
Turbogenerator #4:
MAKE : TYPE : Continuous rating at Generator Terminals : Turbine Speed : Generator Speed : Coupling : Live Steam Pressure : Live Steam Temperature: Max. Live Steam Flow: Speed Governor : Governing System : Pressure Governor : Protection :
BHEL EK-1000_2 8 MW 8000 RPM 3000 RPM Speed Reduction Gear 35 ATA 435 D Centigrade 63 MT/hr Hydro-dynamic SRIV ASKANIA Over speed, Axial Displacement, Low lube oil pressure, Low vacuum, High Vibration.
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•
Turbogenerator #5:
MAKE :
BHEL
TYPE :
EK-100-2
Continuous rating at Generator Terminals :
12 MW
Turbine Speed :
6500 RPM
Generator Speed :
1500 RPM
Coupling :
Speed Reduction Gear
Live Steam Pressure :
35 ATA
Live Steam Temperature:
435 D Centigrade
Max. Live Steam Flow:
94.7 MT/hr
Speed Governor :
Electronic
Governing System :
Electronic
Pressure Governor :
ASKANIA
Protection :
Over speed, Axial Displacement, Low lube oil pressure, Low vacuum, High Vibration.
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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-
6 per ring (2 rings)
Size-
25×32 mm
Grade-
HMCR
Minimum permissible diameter of slip-rings
360 mm
Maximum output with one cooler out of
7000 kVA
service: 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
Short circuit characteristics
(Rated voltage : 6.3 kV)
(Rated current : 916 Amps)
Voc
Isc( Amps)
If(Amps)
(100%=6.3 kV)
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
57700 kg/hr
capacity(100%MCR) Max. Allowable working pressure and design
50.0 kg/cm²
pressure 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 sliprings 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 longterm 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 multiphase 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.
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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.
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Fig: Differential protection using current balance scheme (external fault conditions) The CTs are connected in series and the secondary current current circulates between them. The relay is connected across the midpoint where the voltage is theoretically heoretically nil, therefore no current passes through the relay, hence no operation for for faults outside the protected zone. Under internal fault conditions (i.e. faults between the CTs) the relay operates, since both the CT secondary currents add up and pass pas through the relay as seen in figure below.
Fig: Differential protection using current balance scheme (internal fault conditions) 2) Buchholz protection: Failure of the winding insulation will result in some form of arcing, which can decompose the oil into hydrogen, acetylene, methane, etc. Localized heating can 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. The action can be so violent that the build-up build of pressure can an cause an oil surge from the tank to 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 belowbelow
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Fig: Mounting of Buchholz relay Fig Fig
Fig: Details of Buchholz relay rel construction
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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
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Rotor with its field winding and exciters
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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.
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Below is the detailed explanation of various faults and protection measure for corresponding faults1) 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.
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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
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Manual control error (sudden variation of load)
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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
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rotor earth fault protection be installed. This can c be done in a variety of ways as explained below Potentiometer method: 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. 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 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 winding. A proper operating procedure shall be established to ensure that this changeover is done at least once in a day. day
Fig: Potentiometer method AC injection method:: This method requires an auxiliary supply, which is injected to 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 es 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.
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Fig: AC injection method DC 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 the fault current to flow through the DC power unit causing the sensitive relay to operate under fault conditions. conditions
Fig: DC injection method
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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-
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SINGLE BREAKER–DOUBLE DOUBLE BUS: BUS
Fig: Typical four circuit single breaker double doub bus and bus differential protection zones. This arrangement (shown in figure) provides high flexibility ility for system operation. oper Any line can be operated from either of the buses, buses the buses can be operated together, togethe as shown, or independently, ntly, and one bus can be used as a transfer bus if a line breaker reaker is out of service. The disadvantage is that it requires complicated switching of the protection for both the bus differential and line protection. Two differential differential zones for the buses are required. In the Figure, lines 1 and 2 are shown connected connec to bus 1, with lines 3 and 4 connected connect to bus 2. For this operation, the differential zones are outlined dashed for bus 1, and dash-dot dash for bus 2.
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Faults ults on either buses or associated circuits circui require tripping of all circuits connected connecte to the bus at that time. Faults lts in the bus tie breaker must trip both buses and all circuits. VTs for protection are required d for each bus, as shown. However, line-side side VTs are preferable to avoid switching if voltage is required for line protection. protectio Modern microprocessor relays rel need to be applied to reduce complications complicat by using the flexibility of such relays and the programmable logic, which hich are provided for such devices. devi DOUBLE BREAKER–DOUBLE DOUBLE BUS: BUS
Fig: Typical four-circuit circuit double breaker–double breaker double bus and the bus differential protection zones.
This is a very flexible arrangement arrangemen that requires two circuit breakers per circuit. Each bus is protected by a separate differential, different with zones as illustrated. The line protection operates from paralleled CTs, and this provides protect protect ion for the bus area between the two zones overlapping the two breakers. Line protection operates to trip both breakers. breakers With all disconnected switches normally no closed (NC), as shown, a fault on either of the buses does not interrupt service on the lines. All switching swit is done with breakers, break and either
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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.
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The potential hazards normally considered are1. Faults: phase or ground 2. Thermal damage
a. From Overload (continuous or intermittent) b. From Locked rotor (failure to start or jamming)
3. Abnormal conditionsa. 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 induced1. Overload (and underload) 2. Jamming 3. High inertia
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C. Environment induced 1. High ambient temperature 2. High contaminant level: blocked ventilation 3. Cold, damp ambient temperature D. Source or system induced1. 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 induced1. 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.
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UNDERVOLTAGE PROTECTION:
The overall result of an undervoltage condition is an increase in current and motor heating and a reduction in overall motor performance.
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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
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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.
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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
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Ground faults in a motor occur: o When its phase conductor’s insulation is damaged for example due to voltage stress, moisture or internal fault occurs between the conductor and ground. ground To 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 or grounding transformer sized to ensure maximum ground fault current is limited. Zero Sequence CT Connection Connec o Best method. o Most sensitive & inherent noise immunity. immunity
All phase conductors are passed through the window of the same CT referred to as the zero sequence CT.. Under normal circumstances, the three phase currents will sum to zero resulting in an outputt of zero from the Zero Sequence CT’s secondary. If one of the motors phases were too to shorted to ground, the sum of the phase phas currents would no longer equal zero causing a current to flow in the secondary of the zero sequence. This current would be detected detected by the motor relay as a ground fault.
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Residual Ground Fault Connection o Less sensitive. o Drawbacks due to asymmetrical starting current and un-matched matched CTs. CTs
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 magnitudes mag excess of 6 times motors full load current and are asymmetrical. The combination of non-perfectly non perfectly matched CTs and relative large phase current magnitudes produce a false residual current. This current will be misinterpreted by the motor relay as a ground fault unless the ground fault element’s pickup is set high enough to disregard this error during starting. startin
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DIFFERENTIAL PROTECTION:
Differential protection may be considered the first line of protection for internal phase-to-phase or phase-to-ground phase 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.
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Core balance method: o Two sets of CT’s, one at the beginning of the motor feeder, and the other at the neutral point o Alternatively, one set of three core-balance core CTs can also be used o The differential element subtracts the current coming out of each phase from the current going into each phase and compares the result or difference with the differential erential pickup level.
Summation method with six CTs: CTs If six CTs are used in a summing configuration, during motor starting, the values from the two CTs on each phase may not be equal as the CTs are not perfectly identical and asymmetrical currents may cause the CTs on each phase to have different outputs. To prevent nuisance tripping in this configuration, the differential level may have to be set less sensitive, or the differential time delay may have to be extended to ride through the problem period during d 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.
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Biased differential protection - six CTs: Biased differential protection method m allows for different ratios for system/line and the neutral CT’s. This method has a dual slope characteristic. Main purpose of the percent-slope percent characteristic is to prevent a mis-operation mis caused by unbalances between CTs during external faults. CT unbalances arise as a result of CT accuracy errors or CT saturation. 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 incorrect operating ting signals.
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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.
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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.
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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 RefinerySiemens make relay (installed in new units HT substation and 33kV Switch Gear) •
7SJ600
Over Current Relay
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7RW600
Over/Undervoltage relay
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7VK512
Check synchronization relay
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7UT512
Differential relay
ABB make relay: (installed in new unit substation) •
SPI30UC
Over/undervoltage relay
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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
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3.8 CIRCUIT BREAKER: 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 contactscontacts one is fixed contact other one is termed as moving contact.. Under fault condition, the trip coil is 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 useuse 1) AIR CIRCUIT BREAKER: Interrupting nterrupting contacts situated in air. air Arc is chopped into a number n of small arcs by the Arc-chute hute as it rises due to heat and magnetic agnetic forces. The air circuit breakerss are normally employed for 380-480 380 V range.
Fig: Air Circuit breaker These circuit breakers are used in LT breakers in Guwahati refinery. 2) OIL IMMERSED CIRCUIT BREAKER: In this design, the main contacts are immersed in oil 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.
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Fig:: 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 • Action of oil as an insulator lending to more compact design of switchgear. And Disadvantages are: • Inflammability (especially if there is any air near hydrogen) • Maintenance (changing and purifying). In Guwahati Refinery, Oil circuit breakers are used in the following substationssubstations i)) NE panel breakers in Thermal Power station (TPS). ii)12/02 )12/02 substation HT breakers. iii) DM&S substation ation HT breakers.
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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 CBi) 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).
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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 things go beyond control, control, it automatically shutdown the plant in a safe way. 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 gauge) •
Remote indicator ( Differential pressure transmitter , orifice, venturimeter, Ultrasonic flow meter)
LOCAL INDICATOR: •
ROTAMETER:
Fig: 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 larger in diameter at the top of the glass than at the 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
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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.
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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.
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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 bee 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.
Fig: Orifice flow meter The orifice plate inserted in the pipeline causes an increase increase in flow velocity and a 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 simplest simplest and least expensive of the head meters 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 proportional proportional to the square of the flow velocity. Ann 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.
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•
VENTURI TUBES: Venturi nturi 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-sectional cross sectional area decreases causing the velocity to 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 can be measured; and a diverging recovery cone where the velocity decreases and almost all of the original pressure head is recovered. The unrecovered pressure head is commonly called as head loss.
Fig: Venturi tube In the venturi meter, velocity is increased increased and the pressure is decreased in the upstream cone. •
ULTRASONIC FLOWMETER: It provides volumetric flow rate. We typically use the transmit-time transmit time method, where sounds wave transmitted in the direction of fluid flow travels faster than those travelling upstream. The transmit-time transmit time difference is proportional to the fluid 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 are typical applications for this technology.
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 methods m for pressure measurement: 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 nd measuring the resultant force either directly or indirectly.
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The third method involves allowing the unknown pressure to act on an elastic membrane of known area and measuring the resultant stress or strain.
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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
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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.
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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
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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
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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.
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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
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magnetic chamber. The magnetic interaction between magnets in float and bicolour 0 rollers causes each roller to rotate 180.
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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
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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.
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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
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•
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
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•
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
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• 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
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pressure. But the I/P transducer should be linearly calibrated such that 4-20 mA input = 315 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
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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
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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.
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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.
