Vocational Training Report
Synopsis on Thermal Power Station
In partial fulfilment of the requirements
For the award of the degree
BACHELOR OF TECHNOLOGY
(Roll. No. 1104220026)
Under the Guidance
Mr. A. MARKHEDKAR
Department Of Electrical Engineering
Madan Mohan Malaviya University of Technology, Gorakhpur (U.P.), India
Vocational Training Report
Synopsis on Thermal Power Station
Under Guidance of:
Mr. A. MARKHEDKAR
Additional General Manager
Electrical Maintenance Department
Training MAY-JUNE 2014
With profound respect and gratitude, I take opportunity to convey my thanks to Mr. K.L MAURYA Sir for permitting me to complete my training in NTPC Ltd. and to be the part of this esteemed organization.
I extend my heartfelt thanks to MR. A. MARKHEDKAR Sir for providing me the proper guidance during my training period in Vindhayachal Stage I.
I would also thank Mr. Vikas Gupta, Mr. Mohammad rafi, Mr. Jitendra
kumar Gupta, Mr. Rajesh kumar Verma, Mr. S.P. Kushwaha for helping me with different aspects of the power plant.
I’m extremely grateful to all- the operation and guidance that has helped
me a lot during the course of training. I have learnt a lot under them and
i will always be indebted to them for this value addition in me.
My special thanks to my father Mr. Prem sagar(SPDT. O&M VSTPS) &
alumnus Mr. R.K. Singh(AGM, CHP VSTPS) who helped me at every stage
of training and making this project possible.
At last I would like to convey my thanks to all the members of the Electrical Maintenance Dept., and I am are forever indebted to the omnipotent and to my parents for their cheerful encouragement, unfailing patience and consistent support.
VOCATIONAL TRAINEE 15th May –14th June
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This is to certify that this vocational training report entitled
“Synopsis on Thermal Power Station at NTPC Vindhyachal” from
15th May –14th June 2014, is a bona-fide record presented by Mr. MILIND PUNJ , REG. No. 1104220026 , in fulfillment of summer training under
B-Tech curriculum in Electrical Engineering by the Madan Mohan Malviya
University of Technology, Gorkhpur.
Training In-Charge Mr. A. MARKHEDKAR
HR - EDC AGM- (EMD)
NTPC Vindhyachal NTPC Vindhyachal
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1. Introduction: NTPC Ltd 1
3. Power Plant Basics 4
Coal Handling Plant
5. Working Of Steam Power Plant 11
Basic Cycles of Power Plant
7. Generator & it’s Auxiliaries 26
9. Protection of Generator 32
40 11. Switchyard 47 12. Switchyard Equipment’s 49 13. Switchyard Protection 56 14. Essential Quality of Electrical Protection 56 15. HVDC Back To Back System 57 16. Water Treatment Plant Storage 58 17. Conclusion 59 18. Bibliography 60
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INTRODUCTION: NTPC Ltd.
NTPC Limited (formerly National Thermal Power Corporation) is the
largest Indian state-owned electric utilities company based in New Delhi, India.
It is listed in Forbes Global 2000 for 2012 ranked at 337th in the world.
SERVICE – Electricity generation and distribution natural gas exploration, production, transportation and distribution
PRODUCT - electrical power, natural gas
REVENUE- 690.36 billion (US$13 billion) (2011–12)
NET INCOME - 98.14 billion (US$1.8 billion)(2011–12)
It is an Indian public sector company listed on the Bombay Stock Exchange in which at present the Government of India holds 84.5% (after divestment of the stake by Indian government on 19 October 2009) of its equity.
With an electric power generating capacity of 41,184 MW, NTPC has embarked on plans to become a 128,000 MW company by 2032.
On 21 May 2010, NTPC was conferred Maharatna status by the Union Government of India.
NTPC's core business is engineering, construction and operation of power generating plants and providing consultancy to power utilities in India and abroad.
The total installed capacity of the company is 36,514 MW (including Joint Ventures) with 16 coal-based and 7 gas-based stations, located across the country. In addition under JVs (joint ventures), six stations are coal-based, and another station uses naphtha/ LNG as fuel.
By 2017, the power generation portfolio is expected to have a diversified fuel mix with coal-based capacity of around 27,535 MW, 3,955 MW through gas, 1,328 MW through hydro generation, about 1,400 MW from nuclear sources and around 1,000 MW from Renewable Energy Sources (RES).
NTPC’s share at 31 Mar 2001 of the total installed capacity of the country was
24.51% and it generated 29.68% of the power of the country in 2008–09. Every fourth home in India is lit by NTPC.
By approval of the Central Government under section 21 of the Companies Act, 1956, the name of the Company "National Thermal Power Corporation Limited" has been changed to "NTPC Limited" with effect from 28 October 2005. The primary reason for this is the company's foray into hydro and nuclear based power generation along with backward integration by coal mining.
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Vindhyachal station of NTPC is located at Vindhyanagar in Singrauli District of MP state. The station is situated on the bank of Govind Vallabh Pant Sagar popularly known as Rihand reservoir. Distance of station from Varanasi in UP and Sidhi in MP is about 240 Kms. and 100 Kms. Respectively.
CAPACITY (4260 MW):
6 X 210 = 1260 MW
2 X 500 = 1000 MW
1999 & 2000
(UNIT VII & VIII)
2 X 500 = 1000 MW
(UNIT IX &X)
2 X 500 = 1000 MW
500 MW (under construction)
Stage-I & Stage-II were totally declared commercial in Feb’92 and Oct’2000 respectively.
Stage-III– Unit-9 Synchronized on 27th Jul 2006 and became commercial from 1st Dec 2006. Unit-10 was synchronized on 8th Mar 2007 and unit became commercial from 15th Jul 2007.
Stage-IV-Unit- 11& 12 were totally declared commercial in Mar’13 and Mar’14 respectively
Total land acquired 6178 acres.
Water for NTPC Vindhyachal is drawn from NTPC Singrauli station discharge channel which in turn takes it from Rihand reservoir. A total commitment of 190 cusecs of Make - up water is already available which covers requirement for Stage-I, II & III. Closed loop cooling water system using Cooling Towers has been adopted at Vindhyachal Station.
The coal linkage for this station is from the Nigahi Coal Mines of NCL. From there, the coal is transported by Merry Go Round (MGR) transportation system (22 Kms length with double track) owned and operated by NTPC. The requirement of coal per year for use in stage- I, II and stage-III is around 17.1 million MT.
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STAGE WISE COMMISSIONING
STAGE I - Oct. 1987-Feb.1991 (7th Plan)
STAGE II - Mar. 1999-Feb. 2000 (9th Plan)
STAGE III –Jul. 2006 -Mar.2007 (10th Plan)
STAGE IV-Mar. 2013-Mar.2014(11th Plan)
ASSOCIATED TRANSMISSION SYSTEM:
Power generated by NTPC-Vindhyachal flows to the Western Region Grid. The power is shared between various states of Western Region. Transmission lines connected to Vindhyachal Station are as follows –
400 KV –4 lines to Jabalpur
HVDC –Link between Western –Northern Region
400 KV –Link line to Korba
400 KV –4 Lines to Satna –Bina
132 KV –2 Lines to Waidhan
ALLOCATION OF POWER:
The percentage sharing of power from 2260 MW (Stage-I & II) of Vindhyachal between the beneficiary states is as below –
Goa, Daman & Diu, Dadra, Nagar Haweli
The allocation of power from 1000 MW of stage-III is as below
Goa, Daman & Diu, Dadra, Nagar Haweli
Some of the major performance highlights are as below –
Station qualified for meritorious productivity award of Govt. of India several times.
Vindhyachal stood 4th in the Country in 1998-99 with a PLF of 90 %
Vindhyachal stood 5th in the Country in 1999-2000 with a PLF of 88%
Vindhyachal stood 12th in the country in 2002-03 with a PLF of 85 %
Vindhyachal stood 18th in the country in 2003-04 with a PLF of 82 %
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in the country in 2005-06 with a PLF of 92 %
in the country in 2006-07 with a PLF of 92 %
Uninterrupted running of Vindhyachal Unit #3 (210 MW) for 559 days is a new national record.
Due emphasis has been given on Tree Plantation at NTPC-Vindhyachal. So far more than 17.38 lakh trees have been planted in and around the station. The work of tree plantation is being taken as one of the targets every year and is being monitored regularly.
2006-2007 21, 04632 MT 56.00%
SAFETY & OTHER AWARDS TO STATION:
1987 –National Safety Council Award
1998 –British Safety Council Award
1999 –British Safety Council Award
1999 –National Safety Award
2000 –National Safety Award
2003 –Golden Globe Award for Environment (By NTPC)
2004 –Green-tech Safety award
2005 − CII EXIM Business Excellence Award
2006 –Green-tech Silver Award for Environment
2007- National award for economics of quality (dl shah commendation award) by
Hon’able President of India on 9th Feb’07
2008- National award for economics of quality (dl shah award) on 16th Feb’ 08
2008- National award for economics of quality (dl shah commendation award) on 16th Feb’ 08
POWER PLANT BASICS
1. LOCATING A THERMAL POWER PLANT
(1) SITE REQUIREMENT
Adequate Land–Lease from forest department.
For Each 1000MW = 90 to 200 Acres of land.
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Which can bear Heavy structures.
(3) WATER REQUIREMENT For Boiler 3 to 4 Tons/hr./MW.
For cooling once through -20000m3/hr./100MW.
(4) COAL REQUIREMENT: NIGAHI MINE S- CLOSE TO 60000 TONS/DAY.
(6) DISPOSAL OF ASH- 30% ASH
(7) TRANSMISION OF POWER
(8) PROXIMITY TO AIR FIELD
(9) FISHERIES AND MARINE LIFE. 10C
(10) PERSONNEL REQUIREMENT
2. PLANT LOADING FACTOR (PLF)
It is defined as the ratio of total generation of the power plant to its installed capacity. Since its inception, NTPC has always maintained high values of PLF close to 85% - 95% which has increased the overall power production capability of the plant.
3. POWER PURCHASE AGREEMENT (PPA)
Before setting up of a power plant, MoU’s are interested in purchasing the power from the
plant. A fixed amount is allotted and the concerned state has to pay the cost accordingly even
if the demand is less.
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BASIC LAYOUT OF A COAL FIRED PLANT
The basic building block of a coal fired plant is steam, which is generated from properly treated and De-Mineralized water (DM water), supplied by CW (Circulating Water) pump house. The coal is finely powered to increase its surface area for efficient firing and this process is called pulverization and it is carried out in Coal Handling Plant (CHP) located near MGR (Merry Go Round) system of NTPC. The Pulverized coal is then sent to bunkers via conveyor belts and is then sent to the furnace via a typical air draft system discussed at later stage.
The firing of coal inside furnace maintains it to a very high temperature and this heat energy is converted into pressure energy in the form of super heated steam extracted from the DM water flowing inside the water walls located in the boiler which is in turn embedded into the furnace.
This super heated steam does the actual work on turbine blades which reduces its temperature and pressure and converts the pressure (&heat) energy into kinetic energy of moving blades. The exhaust steam is re circulated via condenser which would be discussed later. Now the rotational kinetic energy of turbine is converted into electrical energy using Turbo Generator (TG) and sent to the distribution sector (Switchyard) via Generator Transformer (GT). The power flow and all the necessary protection regarding load dispatch is commissioned in Switchyard.
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COMPLETE DESCRIPTION OF POWER PLANT
COAL TRANSPORTATION & HANDLING
Coal is a heterogeneous solid fuel and Composed of two types of material Coal = Combustible + Non- combustible
1. Maceral is the coaly matter produces heat on coal combustion C (coal) + O2 (Air) = CO2 +Heat (exothermic reaction)
Heat→ Steam raising → Mechanical energy →Ele
2. Second part mineral is undesirable one and on combustion forms ash.
Ash = thermally transformed minerals (Oxides)
WHY COAL QUALITY?
•Coal Quality affects the performance of a power station starting from Milling system to
•Coal quality has a significant bearing on design of steam generator and on types & size of auxiliaries.
•Knowledge of coal quality is Essential for its Efficient utilization
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•Correct Assessment of Coal quality is Essential for Correct Performance Assessment.
Useful Heat Value
Ash% + Moisture
Value GCV (Kcal/
% at (60% RH &
Kg) (at 5%
Not exceeding 19.5
Exceeding 5600 but
19.6 to 23.8
Exceeding 6049 but
not exceeding 6200
not exceeding 6454
Exceeding 4940 but
23.9 to 28.6
Exceeding 5597 but
not exceeding 5600
Exceeding 4200 but
28.7 to 34.0
Exceeding 5089 but
not exceeding 4940
not Exceeding 5597
Exceeding 3360 but
34.1 to 40.0
Exceeding 4324 but
not exceeding 4200
not exceeding 5089
Exceeding 2400 but
40.1 to 47.0
Exceeding 3865 but
not exceeding 3360
Exceeding 1300 but
47.1 to 55.0
not exceeding 2400
MGR is merry-go-round system of NTPC for captive transportation of coal from the mine end to the power plant. MGR of NTPC-VINDHYACHAL has its own locomotives and wagons. Various rakes are deployed for transportation of around 30,000 MT/day of coal. One rake consists of one locomotive and up to 33 wagons. Due to configuration of tracks at track hoppers end and for safety reason more than 33 wagons are never deployed in a rake. Signalling is also provided for safety of the rake movement. Operational activities of MGR, which are required for bringing coal from mine end to track hopper of NTPC- Vindhyachal are identified and planned in accordance.
For unloading of coal from the locomotive, two methods are deployed-
Manual via track hopper
Track hoppers are conical arrangements for inlet of coal inside the conveyor belt arrangement system via paddle feeders which directs the flow of coal to conveyors. Track Hopper, normally of 200-250m length. Manual labor required for coal scavenging.
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In wagon tippler arrangement the wagons are jacked at once and turned down into the track hopper. No manual labor required.
COAL HANDLING PLANT
The coal is received at track hopper of CHP through the merry go round system used for coal transportation. Coal flows through paddle feeders, belt conveyors, vibro feeder / vibrating screen to the crusher where it undergoes size reduction (-20 mm size). The crushed coal is transported through different conveyors and trippers to the respective unit bunkers. The coal crushed in excess of the requirement is stacked at stockyard of CHP through stacker / reclaimers.
Whenever required by system, the coal is reclaimed and fed to unit bunkers through stacker / reclaimers. Cage bar / cage screen are checked regularly to ensure that crusher output is always maintained as per the requirement stated above. In case bigger size coal is found, that crusher is offloaded and inspected for rectification.
Uncrushed coal from track hopper is fed to crusher, where it under goes a size reduction, and then it is fed to the bunkers through various conveyors and trippers. The sizes of coal feeding to the bunkers are being checked manually by physical inspection.
The coal which comes from mine through the MGR system is crushed and the crushed coal from crusher is diverted to the stacker-cum-reclaimers through concerned conveyors if
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all unit bunkers level is normal. The coal is stacked in 05 nos. available yards in a planned manner by proper dozing and water spraying process.
Coal is reclaimed from stack yard with the help of stacker-cum-reclaimer and fed to the bunkers as per requirement of the process or whenever
The conveyor belt motors are continuously monitored for over/under loading conditions via SCADA interface panel from CHP control room. Magnetic separators and various types of protection limit switches are present inside the conveyor belt region for monitoring. Transfer points (TP) are used for connecting various conveyor regions and Penthouse contains magnetic separators and metal detectors in the raw coal. The
Coal is then crushed in CRUSHER HOUSE and sent to Bunkers via other set of conveyor bel
WORKING OF STEAM POWER PLANT
Coal is burnt in a boiler, which converts water into steam.
The steam is expanded in a turbine used to drive alternator.
The steam expanded is condensed in a condenser to be feed into the boiler again.
SCHEMATIC ARRANGEMENT OF COAL STEAM POWER PLANT
Steam generating equipment includes:
A boiler is closed vessel in which water is converted into steam by utilizing the heat of coal combustion. Steam boilers are broadly classified into following two types:
Water tube boilers
In a water tube boiler, water flows through the tubes and the hot gases of combustion flow over these tubes. Water-tube boilers are used for high- pressure boilers
Fire tube boiler
In a fire tube boiler, the hot products of combustion pass through the tubes surrounded by water. The heated water then rises into the steam drum. Here, saturated steam is drawn off the top of the drum. The steam will reenter the furnace in through a superheater in order to become superheated. Superheated steam is used in driving turbines. Since water droplets can severely damage
turbine blades, steam is superheated to 730°F (390°C) or higher in order to ensure that there is no water entrained in the steam.
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2. SUPER HEATER
A device which removes last traces of moisture.
It helps in reduction in requirement of steam quantity.
Steam being dry reduces the mechanical resistance of turbine.
No corrosion at the turbine blades
3. ECONOMISER AND AIR HEATER
They are such devices which recover the heat from the flue gases on their way to chimney and raise the temperature of feed water.
Economizer raises boiler efficiency.
Air Pre-heaters recover the heat from the flue gases leaving the economizer and heat the incoming air required for combustion
Which condenses the steam at the exhaust of turbine
It creates a very low pressure at the exhaust of turbine, this helps in converting heat energy of steam into mechanical energy in the prime mover.
The condensed steam can be used as feed water to the boiler.
It creates a very low pressure at the exhaust of turbine, this helps in converting heat energy of steam into mechanical energy in the prime mover.
The condensed steam can be used as feed water to the boiler.
5. PRIME MOVER (TURBINE)
A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into mechanical energy.
About 86% of all electric generation in the world is by use of steam turbines.
It has almost completely replaced the reciprocating piston steam engine.
6. COOLING TOWER
Remove heat from the water discharged from the condenser so that the water can be discharged to the river or re circulated and reused.
Air can be circulated in the cooling towers through natural draft and mechanical draft.
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ARRANGEMENT OF STEA M POWER PLANT
Induced Draft Forced Draft
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Indian coal contains about 30% of ash. The hourly consumption of coal of a 200 MW unit is about 110 tons. With this, the hourly production of ash will be 33 tons. If such large amount of ash is discharge in atmosphere, it will create heavy air pollution thereby
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resulting health hazards. Hence it is necessary to precipitate dust and ash of the flue gases.
Precipitation of ash has another advantage too. It protects the wear and erosion of ID fan. To achieve the above objectives, Electrostatic Precipitator (ESP) is used. As they are efficient in precipitating particle form submicron to large size they are preferred to mechanical precipitation.
An ESP has series of collecting and emitting electrons in a chamber collecting electrodes are steel plates while emitting electrodes are thin wire of 2.5mm diameter and helical form. Entire ESP is a hanging structure hence the electrodes are hung on shock bars in an alternative manner.
It has a series of rapping hammer mounted on a single shaft device by a motor with the help of a gear box at a speed of 1.2 rpm. At the inlet of the chamber there are distributor screens that distribute the gas uniformly throughout the chamber.
There are transformer and rectifiers located at the roof of chamber. Hopper and flushing system form the base of chamber.
Flue gases enter the chamber through distributor screen and get uniformly distributed. High voltage of about 40 to 70 KV form the transformer is fed to rectifier. Here ac is converted to dc. The negative polarity of this dc is applied across the emitting electrode while the positive polarity is applied across the collecting electrodes. This high voltage produces corona effect negative (–ve) ions from emitting electrode move to collecting electrode. During their motion, they collide with ash particles and transfer their charge. On gaining this charge, ash particles too move to collecting electrode and stock to them. Similar is the case with positive (+ve) ions that moves in opposite direction.
The rapping hammers hit the shock bars periodically and dislodge the collected dust from it. This dust fall into hopper and passes to flushing system. Here it is mixed with water to form slurry which is passed to AHP.Efficiency of ESP is approximately 99.8%.
1. Side view of electrostatic precipitator
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BASIC CYCLES OF A POWER PLANT
The power plant now can be subdivided into a number of closed process cycles which are as follows-
1. Coal cycle
2. Primary and Secondary Air draft cycle
3. Super Heated Main Steam Cycle
4. Circulating Water and Condensate cycle
5. Flue Gas Cycle.
Now the basic inputs and requirements for proper burning of coal are
1. Pulverization of coal for increased surface area and easy flow into furnace.
2. Proper Ignition temperature and moisture free.
3. Air for complete combustion. Thus the required for burning of coal and conversion of water into steam is covered under the above mentioned cycles.
Raw Coal from
TO FA AND FD
Bunkers are cylindrical storage tanks of crushed coal from CHP. The coal level is always maintained within limits and is handled by conveyors whose rate can be controlled from the CHP control room. For a 210MW stage at BTPS there are 10 bunkers and each bunker is connected to RC feeder which is special type of motor arrangement for feeding properly crushed coal into mill motors. The idea here is to reject any type of metallic or impurity or unwanted substances from coal. A RC feeder consists of three rollers with a conveyor belt in it, the amount once released from this feeder definitely goes into the furnace and the amount is controlled via the speed of conveyor belt and two opening gates, one motorized and one manual. There are 10 set of Mill Motors. Each mill motor contains a bowl and roller arrangement for crushing of coal and this crushed coal is carried away to the furnace via mix PA discussed at later stage.
RC FEEDER SPECIFICATIONS
SECO - 36'' Gravimetric (with Mechanic
Minimum - 11350 Kg/hr.
Maximum - 87820 Kg/hr.
Density of material to be handled
720 to 1280 Kg/m3
Drive Motor (KW)
Clean out motor (KW)
415V, 3-phase, 50 Hz
130.5,1 at 1375 rpm
Free standing control cabinet Tropical
service, Eddy current clutch control,
water sprays Motion monitor, Purge air
MOTOR USED IN BOWL MILL
M/s Siemens West Germany/BHEL Bhopal.
SQ – Motor
Standard continuous rating at 400C
Derated rating for specified normal
Minimum permissible starting voltage
AIR DRAFT CYCLE
The Primary Air, Secondary Air & Flue gas System:
The PA needed to carry away the pulverized coal from the Pulverizes to furnace is supplied by two numbers PA Fans. A portion of PA passes through APH where it picks up the heat from flue gas and becomes hot PA.
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FD SAP WIND B
The SA which ensures the complete combustion of the fuel in the furnace is supplied by two numbers Force Draught (FD) fans. The whole SA passes through APH & picks up heat from flue gases & goes to wind box. From wind box it goes to furnace via SADC comprising of FAD located at coal mill elevation which opens proportional to feeder speed. Auxiliary air dampers which maintains furnace to wind box DP. Oil air dampers, which are open at the oil elevation, are in-service proportional to the oil pressure. In case there is no oil firing, these dampers behaves like Auxiliary air dampers.
In order to avoid the furnace getting subjected to the positive pressures, the flue gases which results from the combustion of the coal are drawn out of the furnace via APH (Air Pre Heaters) using two numbers ID fans, which discharges the flue gas to Chimney which is further discussed in flue gas cycle.
The temperature of the air provided by PA Fan is controlled via cold PA extracted from the PA line before APH, the cold PA is then mixed with Hot PA from PAPH and passed via bowl mill motors creating primary air draft system.
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Seal Air Fan:
These are used for supplying seal air to the mills to prevent ingress of coal dust into gearbox lubrication oil. There are two fans per boiler.
The soot blowers are used for efficient on-load cleaning of furnace, super heaters, Reheaters and regenerative air heaters. There are three types of soot blowers provided in the plant in requisite numbers. They are:
1. Long retractable soot blowers
2. Wall blower
3. Air heater blower
Superheated steam is tapped from the super heater for the purpose of soot blowing.
Boiler can simply defined as the device where any liquid is boiled or Boiler may be defined as a device that is used to transfer heat energy being produced by burning of fuel to liquid, generally water, contended in it to cause its vaporization. Boiler, in simple
terms, can be called“Steam Generator”. The following are fact efficient combustion usually
referred as “The three T’s”.
a) Time –It will take a definite time to heat the fuel to its ignition temperature and having ignited, it will also take time to burn.
b) Temperature –A fuel will not burn until it reaches its ignition temperature.
c) Turbulence –Turbulence is introduced to achieve a rapid relative motion between the air and fuel particles
Block Diagram of Steam Cycle
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COMPONENTS OF A BOILER
2. BOILER DRUM
3. DOWN COMERS
4. WATER WALL
5. PRIMERY SUPER HEATER
6. PLATEN SUPER HEATER
7. FINAL SUPER HEATER
10. OIL GUNS
12. BUCK STAYS
1. FORMED IN TWO STAGES - ECO I & II AND ECO III
2. FORMS PART OF FEED WATER CIRCUIT
3. PRE HEAT BOILER FEED WATER
4. RECOVERY OF HEAT FROM FLUE GAS
5. LOCATED IN BOTTOM OF REAR PASS
6. NO STEAM FORMATION
1. TO SEPARATE WATER FROM STEAM
2. TO REMOVE DISSOLVED SOLIDS
3. TO PROTECT WATER WALLS FROM STARVATION
4. ACTS AS TEMPORARY PRESSURE RESERVOIR DURING TRANSIENT LOADS
1. FORMS FURNACE ENCLOSURE
2. GENERATION OF STEAM
3. PROVIDES SEALING TO FURNACE
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RAISE STEAM TO HIGHER TEMPERATURE ARRANGED IN 3 STAGES
LTSH LOCATED ABOVE ECONOMISER
RADIANT PENDENT TYPE (DIV PANEL) ABOVE FURNACE
CONVECTIVE FINAL SUPER HEATER ABOVE FURNACE IN CONVECTIVE PATH
RE HEAT THE STEAM FROM HP TURBINE TO 540 DEG
COMPOSED OF THREE SECTIONS
RADIANT WALL REHEATER ARRANGED IN FRONT & SIDE WATER WALLS
REAR PENDANT SECTION ARRANGED ABOVE GOOSE NECK
FRONT SECTION ARRANGED BETWEEN UPPER HEATER PLATEN & REAR WATER WALL HANGER TUBES
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The Main steam & Re-Heated steam System :
The heat produced by the combustion of the coal is used to generate the steam. The steam is further heated in the super heaters & forms Main Steam. This main steam is delivered to the Turbine through main steam (MS) lines in a controlled manner by means of Main Steam Stop Valves & Control valves that are regulated by the TG Governing System. In turbine the steam expands & converts Heat energy in to Mechanical energy.
In between the HP turbine & IP turbine the steam is again carried to the Boiler, where it is reheated & delivered to the IP turbine in a controlled manner by means of RH steam stop valves & control valves that are regulated by the TG Governing system
After the turbine, the steam goes to the condenser where it is condensed under deep vacuum using CW. The condensate is collected in the Hot well
CIRCULATING WATER CYCLE
Condensate, Feed Water & Boiler Water system
The condensate from the hot well is extracted by CEP & is pumped, via LP Heaters where it gets heated up, to the Deaerator. The condensate now becomes feed water & gets stored in the Feed Storage Tank just below Deaerator. The BFP takes Suction from this Feed Storage tank & pumps this feed water through HP heaters, feed control station & economizer to the Boiler Drum, where it becomes Boiler Water. This boiler water goes to the boiler water walls where it absorbs heat, transforms in to steam & return back to the Boiler drum. In stage I boiler this circulation of boiler water is natural, whereas in stage III boiler it is assisted by CW pumps.
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CW system consists of CW pump house having CW pumps for supplying CW to the condenser of stage III units through common CW duct & also for supplying raw water to Auxiliary pond for miscellaneous use. Auxiliary Water pump house, which draws water from Auxiliary pond & supplies for various purposes such as fire water system, raw water supply for clarification plant etc. Clarification plant, where the raw water is clarified & is fed to the clarified water tank. Clarified water pump house, where the clarified water is pumped to supply it to generator gas coolers & other BCW system.
At Vindhyachal Station 500 MW capacity turbines are of Kraft Werk Union (KWU - Germany) design and supplied by BHEL. The turbine is condensing, tandem compounded, horizontal, reheat type, single shaft machine. It has got separate high pressure, intermediate and low pressure parts. The HP part is a cylinder and IP & LP parts are double flow cylinders. The turbine rotor is rigidly coupled with each other and with generator rotor.
HP turbine has throttle control. The stream is admitted through two combined stop and control valves. The steam from reheaters is admitted to IP turbine through two combined stop and control valves. Two crossover pipes connect IP and LP cylinder.
The entire turbine is provided with reaction blading. The moving blades of HPT, LPT and front rows of LPT have inverted T roots and shrouded. The last stages of LPT are twisted; drop forged moving blades with fir tree roots. They also have guide blades for proper functioning. The TG unit is mounted on six bearings HPT rotor is mounted on two bearings, a double wedged journal bearing at the front and combined thrust bearing adjacent to front IP rotor coupling.
In the 500 MW KWU turbines, single oil is used for lubrication of bearings, control oil for governing and hydraulic turbine turning gear. During start-ups, auxiliary oil pump (2
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Nos.) supplies the control oil. Once the speed of the turbine crosses 90% of the rated speed, the main oil pump takes over. Under emergency, a DC oil pump can supply lubrication oil. Before the turbine is turned or barred, the Jacking Oil Pump (2 Nos.) supplies high pressure oil to jack up the turbine generator shaft to prevent boundary lubrication in bearing.
The turbine is equipped with a hydraulic turning gear assembly comprising two rows of moving blades mounted on the coupling between IP and LP rotors. The oil under pressure supplied by the AOP strikes against the hydraulic turbine blades rotates the shaft at 110rpm (220 rpm under full vacuum condition).
Turbine shaft glands are sealed with auxiliary steam supplied by an electro-hydraulically controlled seal steam pressure control valve. A pressure of 0.01 Kg / square-cm (g) is maintained in the seals.
Above a load of 80 MW the turbine becomes self-sealing. The leak off steam from HPT/IPT glands is used for sealing LPT glands.
TURBINE PROTECTION SYSTEM
Turbine protection system performs to cover the following functions:-
1. Protection of turbine from inadmissible operating conditions.
2. In case of plant failure, protection against subsequent damages.
3. It restricts occurring failures to minimum.
Standard turbine protection system comprises the following:-hydraulic turbine protection.
The main elements of the Turbine Protection system are as follows:
1. Emergency Governors.
2. Emergency Governor Pilot Valves.
3. Emergency stop valve (ESV) Servomotors.
4. Interceptor valve (I V) Servomotors.
5. Turbine shutdown switch.
6. Electro-hydraulic transducer.
7. Initial steam pressure unloading gear.
H.P. - L.P. BY- PASS SYSTEM
The HP By-pass system in coordination with LP By-pass enables boiler operation and loading independent of the turbine. For matching the live steam and metal temperature for a quick start-up, by-pass stations have been provided, which dumps the steam to the condenser through pressure reducing station and de superheaters, during the period the steam parameters at the boiler are being raised. This allows quick rising of parameters to a level acceptable to the turbine for rolling during start-up. It helps in quick start of turbine and low noise level, also economizes the consumption of DM water. The HP By- pass system consists of two parallel branches that divert steam from the Main Steam line
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to Cold Reheaters line. The steam pressure on the valve upstream side can be maintained at the desired level. The steam is de-superheated in order to keep the steam temperature in cold reheat line within limits, below 345 degree Celsius. The steam downstream of the HP by-pass station is maintained by 2 nos. pf spray water temperature control valves BPE-1 and BPE-2 with valve mounted electro-hydraulic actuators.
With the use of turbine by-pass station it is possible to build up the matching steam parameters at the boiler outlet during any regime of starting, independent of the steam flow through turbine. The steam generated by boiler, and not utilized by the turbine during start-up or shutdown, is conserved within the power cycle and thus losses of steam into the atmosphere are cut down to the barest minimum. By-pass system enables to shorten the start-up time.
CONDENSATE EXTRACTION PUMP (CEP)
The condensate extraction pump (CEP) is a centrifugal type vertical pump, consisting of pump body, can, distributor housing and the driven lantern. The pump body is arranged vertically in the can and is attached to the distributor with the rising main. The rotor is guided in bearings lubricated by the fluid pumped, is suspended from the support bearing, which is located in the bearing pedestal in the driven lantern. The shaft exit in the driver lantern is sealed off by one packed stuffing box. The steam after condensing in the condensing in the condenser known as condensate is extracted out of the condenser hot well by condensate pump and taken to the Deaerator through ejectors, gland steam cooler and series of LP heaters. The function of these pumps is to pump out the condensate to the Deaerator through injector, gland steam cooler, and LP heaters. These pumps have four stages and since the suction is at a negative pressure, special arrangements have been made for providing sealing.
BOILER FEED PUMP (BFP)
These pumps almost consume 2% of total power generation of the power plant. Basically these pumps are meant to supply continuous water to the boiler drum.
There are two- types of BFP’s
1. Turbine Driven (TDBFP)
2. Motor Driven (MDBFP)
Initially the plant is started with MD-BFP, and then after attaining proper steam pressure TD-BFP’s are used to decrease power consumpti
Each BFP is provided with a Booster pump in its suction line which is driven by the main motor of the boiler feed pump. One of the major damages which may occur to a BFP is from Cavitation or vapour bounding at the pump suction due to suction failure. Cavitations will occur when the suction pressure of the pump at the pump suction is equal or very near to the vapour pressure of the liquid to be pumped at the particular feed water temperature. By the use of the booster pump in the main pump suction line, always there will be positive suction pressure which will remove the possibility of Cavitations. Therefore all 3 feed pumps are provided with the main shaft driven booster pump in its suction line for obtaining a definite positive suction pressure.
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GENERATOR AND ITS AUXILLARIES
The 500 MW generator is a 3-phase, horizontally mounted 2-pole cylindrical rotor type, synchronous machine driven by steam turbine. The stator windings are cooled by de- mineralized water flowing through the hollow conductor while the rotor winding is cooled by hydrogen gas. Fans mounted on the generator rotor facilitate the circulation of the H2 inside the machine requiring cooling. 4 coolers mounted inside the machine cool the H2 gas. The generator winding is insulated by epoxy thermo- setting type insulation. It is provided with static excitation system. Two H2 driers are provided to facilitate moisture removal. H2 is circulated through them via the fans in dry condition. Normally one drier is kept in service and other is in standby. Liquid Level Detectors (LLDs) are provided to indicate liquid in the generator casing, to indicate whether oil is leaking or water. It can be drained through drain valves. H2 gas purity is to be maintained at more than 99%.
The cooling water system consists of 2x100% duty AC motor driven pumps, 2x100% duty water coolers, 2x100% duty mechanical filters, 1x100% duty magnetic filter, expansion tank, polishing unit and ejector system. The stator water pump drive the water through coolers, filters and winding and finally discharges into the expansion tank situated at a height of about 5m above the TG floor. It is maintained at a vacuum of about 250mm Hg by using water ejectors. A gas trap is provided in the system to detect any traces of hydrogen gas leaking into the stator water system. To prevent leakage of hydrogen from generator housing, ring type seals are provided at the both ends of the generator. The seal ring is free to adjust its position according to shaft position.
The rotor is a cast steel ingot and it is further forged and machined. The rotor is to be designed very accurately as it has to work on speeds such as 3000 rpm. Also a fairly high current is to be carried by the rotor windings to generate the necessary magnetic field.
2. Rotor winding:
Silver bearing copper is used for the winding with mica as the insulation between conductors. A mechanically strong insulator such as micanite is used for lining the slots. When rotating at high speeds centrifugal force tries to lift the windings out of the slots, so they are screwed to the rotor body. The two ends of the windings are connected to slip rings, usually made of forged steel.
3. Stator core:
The stator is the heaviest load to be transported. The major part of this load is stator core. This comprises of an inner frame and outer frame. The outer frame is a rigid fabricated structure of welded steel plates, within this shell is a fixed cage of girder built circular and
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axial ribs. The ribs divide the yoke into compartments. The inner cage is usually fixed to the yoke by an arrangement of springs to dampen the double frequency vibrations.
The stator core is built from a large number of punchings or sections on thin steel plates. The use of CRGO can contribute to reduction in weight of stator core.
4. Stator windings:
Each stator conductor must be capable of carrying the rated current without overheating. The insulation must be sufficient to prevent leakage currents flowing between the phases to earth.
Windings for the stator are made up from copper strips wound with insulated tape which is impregnated with varnish, dried under vacuum and hot pressed to form a solid insulation bar. These bars are then placed in stator slots and held in with wedges to form the complete winding. The end turns are rigidly braced and packed with blocks to withstand the heavy forces.
Rotor Cooling System:
The rotor is cooled by means of gap pick up cooling, wherein the hydrogen gas in the air gap is sucked through the scoops on the rotor wedges and is directed to flow along the ventilating canals milled on the sides of the rotor coil, to the bottom of the slot where it takes a turn and comes out on the similar canal milled on the other side of the rotor coil to the hot zone of the rotor. Due to rotation of the rotor, a positive suction as well as discharge is created due to which a certain quantity of gas flows and cools the rotor.
The conductors used in the rotor windings are hollow which is done to have internal cooling of the rotor.
Hydrogen Cooling System:
Hydrogen is used as a cooling medium due to its high heat carrying capacity and low density. But it can also form an explosive, or escape out of the generator casing which may result into many catastrophic results. So the pressure of H2 should be maintained properly. The filling in and purging of H2 is to be done safely without bringing in contact with air. To fill H2 inside generator first CO 2 is filled through generator and then H2 is passed since H2 has no reaction with CO2 and while taking H2 out of generator first H2 is taken out then CO2 is passed through generator and then air is allowed to enter.
Stator Cooling System:
The stator is cooled by distillate which is fed from one end of the machine by Teflon tube and flows through the upper bar and returns back through the lower bar of another slot. The stator winding is cooled in this system by circulating DM water through hollow conductors. The DM water should be at 40°C. As it is a closed loop the water that comes out of the generator is again cooled and de mineralized. Water passes through lower bars
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along the length to the other end returns through the upper bars of another slot and drain into drain header.
RATINGS OF TURBO GENERATOR
YY (DOUBLE -STAR)
Rated HB2B Pressure:
Class of Insulation:
Rated H2 pressure:
3.5 bar (g)
Stator winding cooling:
Direct water cooling (DM water)
Stator core and rotor
Direct H2 cooling
Class of Insulation:
Static excitation system
Excitation power from generator via excitation transformer. Protective relays for excitation transformer.
Field forcing provided through 415 v aux supply
Converter divided in to no of parallel (typically4) paths. Each one having separate pulse output stage and air flow monitoring.
Two channels : Auto & manual, provision for change over from Auto to Manual
Limiters: Stator current limiter, Rotor current limiter, Load angle limiter etc.
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Alternate supply for testing.
Brushless Excitation System
EXC TRFR 18KV/700V 1500KVA
Three Phase Main Exciter.
Three Phase Pilot Exciter.
Metering and supervisory equipment.
Eliminates Slip Rings, Brush gear and all problems associated with transfer of current via sliding contacts
Simple, Reliable and increasingly popular system the world over, Ideally suited for large sets
Minimum operating and maintenance cost
Self generating excitation unaffected by system fault/disturbances because of shaft mounted pilot exciter
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FIELD BREAKER R Y ARMATURE B ROTATING DIODES
GENERATOR PILOT MAIN EXCITER EXCITER 3 PHASE EXCITER: Rated power: 2135 kVA; 10.5 kW Rated voltage: 520 V Stator current: 2370 A Rated Speed: 3000 RPM Frequency: 50 Hz Connection: YY (DOUBLE-STAR) Self- excitation required: 65 V, 154 A
YY connection is given to reduce effective impedance of windings and to increase amount of current output since the voltage induced remains constant.
Difference b/w Brushless Exciter and Static Exciter:
S.NO Description Brushless Excitation Static Excitation 1 Type of system. Brushless system gets activated Static excitation system uses with pilot exciter, main thyristors & taking exciter and rotating supply from output of diodes. the generator 2 Dependency on external No external source requirement Field flashing supply required supply. since pilot exciter has for excitation build up. permanent magnet field. 3 Response of the excitation Slower than static type since Very fast response in the order system. control is indirect (on the of 40 ms. due to the field of main exciter) and direct control and solid magnetic components state devices employed. involved. 4 Requirement of additional One additional bearing and an No additional bearing and bearing and increase of increase in shaft length increase in the shaft turbo generator shaft are required. length are required. length. 5 Maintenance. Less since slip rings and brushes More since slip rings and are avoided. brushes are required. Also over hang vibrations are very high resulting in faster wear and tear.
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Ratings of brushless excitation system:
Permanent magnet generator: (pilot exciter):
Class of Insulation:
Excitation system Details:
EXCITATION SYSTEM OF THE TURBO GENRATOR
Excitation energy for the Turbo Generator (TG)is obtained from a separate excitation source using a thyristor exciter, which provides the controlled rectifier current to the field winding. The thyristor (service) exciter consists of an auxiliary A.C. generator mounted on the TG shaft and two thyristor converters cooled by distillate from the TG stator cooling circuit.
Either of the converters is arranged in a 3-φ bridge circuit. The con in parallel and they function simultaneously. Each converter has its own individual
thyristor firing control system, which is interconnected through the circuits for synchronizing of firing pulses applied to the two converter arms. Due to this interconnection, uniform sharing of load current between the parallel–operating converters is provided and besides, each thyristor firing control system duplicates the other if loss of supply voltage occurs.
If the service exciter fails, the TG field excitation can be provided from the standby exciter. For this purpose, the use is made of a separately installed set consisting of a D.C. generator and an A.C. driving motor.
The auxiliary generator is of the self –excitation type. Power to the field winding is obtained from the rectifier transformer connected to generator stator winding and the
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thyristor converter cooled by natural air circulation. Application of the auxiliary generator field flashing is accomplished by means of short- time connection of a 220 V separate power source. When applied, the separate power source provides a buildup of the generator terminal voltage up to 10-20 % of the rated value, there upon the connected thyristor bridge starts to function and promotes a build up generator terminal voltage up to the rated value.
Changing automatically or manually the firing angle of the thyristors in the converters accomplishes control of the TG filed excitation. With a decrease of the auxiliary voltage to 80 % of the rated value or in the case of its complete loss, the regulator and thyristor firing control system are supplied with back up power from a 220 V storage battery. With the restoration of the auxiliary voltage to 84% of the rated value, the back-up power supplies are blocked.
PROTECTIONS OF GENERATOR
The core of an electrical power system is generator. During operating conditions certain components of the generator are subjected to increase stress and therefore, could fail, referred to as faults. It can be internal fault or external fault depending upon whether they are inside or outside of the machine. The machine with fault must be tripped
immediately. The corrective measures agains care by stubborn system.
Task of the protective system:
o Detect abnormal condition or defect.
o Limit its scope by switching to isolate the defect.
o Alarm the operating staff.
o Unload and/or trip the machine immediately.
Requirement of protective devices:
Only that part of the installation containing fault should is disconnected.
Safety against faulty tripping: There should be no trip when there is no fault.
The device must act within the required time.
Lowest signal input value at which the device must act.
There should be a clear a distinction between the tripping time of the device, considering the circumstances such as current and total tripping time for the fault.
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Protective Devices :
The choice of protective equipment for the generator should precisely understand the type of fault and do the necessary preventive measures for avoiding it.
Overhead line differential
G.T. restricted earth fault, Main
Earth fault protection:
Stator earth fault
Stator earth fault, Stand by
Rotor earth fault
Stator Inter turn fault
Negative Phase Sequence Current
Generator Backup Impedance
Loss of excitation
Low forward power
Generator Local Breaker Backup (LLB)
Generator Transformer Protections
Winding Temperature High
Oil Temperature High
Bus Bar Protection
As with electrical motor protection, generator protection schemes have some similarities and overlap. This is advantageous, since not all generators have all of the protection schemes listed in this section. In fact, there are many protection schemes available; only the more common ones are discussed here.
Classes of Turbine Generator Trips
There are different classes of protective trips for generators, each with different actions, depending on the cause and potential for damage. Each of the four Classes of trip (A, B, C, &D) is discussed below.
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trips will disconnect the generator from the grid and shut down the turbine-generator (i.e., it will trip the turbine and the field breaker). Typical causes could be generator electrical protection, main transformer electrical protection, ground faults or any other cause that may directly affect the units safe electrical output.
Trips will disconnect the generator from the grid, but will leave the turbine generator supplying the unit load. Typical initiation of this event is a grid problem, thus resulting in this loss of load.
Trips are generator over-excitation trips and are activated only if the generator is not connected to the grid (it may still be supplying the unit loads). Typical causes of this over- excitation are manually applying too much Notes: excitation or applying excitation current below synchronous speed.
Trips the turbine and then trips the generator after motoring. The causes of this type of trip are associated with mechanical problems with the turbine generator set. Each of these trips, along with their causes and exact effects, will be discussed further in your station specific training.
As discussed in the previous sections, over-currents in the windings due to over-loads or faults will cause extensive damage. The generator must be separated from the electrical system and field excitation removed as quickly as possible to reduce this damage to a minimum. During run-up and shutdown, the field may accidentally be applied while the frequency is below 60Hz. Under these conditions normal protections may not work or may not be sensitive enough. A sensitive over-current protection called supplementary start over-current is usually provided when the frequency is less than about 56Hz.
1. Generator differential protection
Differential protection can be used to detect internal faults in the windings of generators, including ground faults, short circuits and open circuits. Possible causes of faults are damaged insulation due to aging, overheating, over-voltage, wet insulation and mechanical damage. Examples of the application of differential protection are shown in Figure 23 that considers a generator winding arrangement with multiple windings, two per phase (this type of differential protection is also called split phase protection for this reason).
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protection is also called split phase protection for this reason).
In Figure 23 a), the currents in the two windings will be balanced, causing the currents in the protection circuit to be balanced. Hence in this case, the differential relay will not operate.
In Figure 23 b), a ground fault is shown on one of the windings. In this case the fault current direction is shown and it will be unbalanced. This will result in unbalanced secondary currents in the protection circuit, causing the differential relay to operate. Similarly, a short circuit within a winding will cause the two winding currents to be unmatched, causing the differential relay to operate.
In Figure 23 c), an open circuit is shown, resulting in no current in the one winding. Again, the unbalanced currents will cause the differential relay to operate. In generators with single windings per phase, the differential protection (Figure 24) is similar to the transformer protection previously discussed. This arrangement will provide high-speed Tripping of the generator and field breaker plus shutdown of the turbine (class A trip). This minimizes insulation damage due to overheating, as well as damage from arcing if the insulation has already been damaged.
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2. Generator Ground Fault Protection
Generators are usually connected to the delta winding of a delta-star main transformer. This allows the generator to produce nearly balanced three phase currents even with unbalanced loading on the primary of the main transformer. This minimizes stress, vibration and heating of the stator windings during unbalanced system conditions and electrical system faults. However, with the generator connected to a delta winding, a separate protection has to be used to protect against stator faults. Any resistance to ground will pull the delta towards ground and may initially go undetected by the differential relay. The stator ground relay will trip the generator before severe damage results. Often the ground relay has a low-set alarm included to allow possible correction before a trip condition exists.
3. Generator Stator Ground Protection
Figure 25 illustrates ground protection system when the generator neutral connection is done through a neutral grounding transformer. Some locations utilize a grounding resistor and accompanying CT. Possible causes of ground faults are insulation damage due to aging, overheating, over-voltage, wet insulation and mechanical damage. If the faults are not cleared, then the risk of insulation damage will occur due to overheating (as a result of high currents) or damage from arcing if the insulation has already been damaged.
4. Rotor Ground Fault Protection
The windings on the rotor of an AC generator produce the magnetic field at the poles. In four pole generators (typical of 60 Hz, 1800 rpm units), the occurrence of a single ground fault within the rotor generally has no detrimental effects. A second ground fault, however, can have disastrous results. It can cause part of the rotor winding to be bypassed which alters the shape of the otherwise balanced flux pattern. Excessive vibration and even rotor/stator contact may result. A means of detecting the first ground fault provides protection against the effects of a second fault to ground on the rotor. Figure 26 shows a simplified excitation system with a Ground Fault Detection (GFD) circuit. The GFD is connected to the positive side of the exciter source.
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5. Generator Phase Unbalance Protection
If a generator is subjected to an unbalanced load or fault, the unbalance will show up as ac current in the rotor field. With the 4-pole 800 rpm generators used in nuclear stations, this current will be at twice line frequency or 120Hz. Continued operation with a phase imbalance will cause rapid over-heating of the rotor due to the additional induced circulating currents (these currents will also cause heating of other internal components of the generator). This will result in rapid and uneven heating within the generator and subsequent damage to insulation and windings (hence, reduced machine life) and thermal distortion could occur. Also the unbalanced magnetic forces within the generator due to these currents will cause excessive vibration. This may result in bearing wear/damage and reduced machine life and may result in a high vibration trip. A specialized relay to detect these circulating currents, called a negative sequence current relay, is used to detect the phase imbalance within the generator. The term negative sequence is just a mathematical term to describe the effects of unbalancing a symmetrical three phase system. The most critical phase unbalance would come from an open circuit in one of the windings and may not be detected by any other protection. Other causes of phase imbalance include unequal load distribution, grid faults and windings faults.
6. Generator Loss of Field Protection
When a generator develops insufficient excitation for a given load, the terminal voltage will decrease and the generator will operate at a more leading power factor with a larger load angle. If the load angle becomes too large, loss of stability and pole slipping will occur and the turbine generator will rapidly go into over- speed with heavy ac currents flowing in the rotor. A loss of field could be caused by an exciter or rectifier failure, automatic voltage regulator failure, accidental tripping of the field breaker, short circuits in the field currents, poor brush contact on the slip-rings or ac power loss to the exciters (either from the station power
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supply or from the shaft generated excitation current). A relay that sense conditions resulting from a loss of field, such as reactive power flow to the machine, internal impedance changes as a result of field changes or field voltage decreases, may be used for the detection of the loss of field. A field breaker limit switch indicating that the breaker is open also gives an indication that there is no field to the generator.
7. Generator Over-Excitation Protection
If the generator is required to produce greater than rated voltage at rated speed (or rated voltage below rated speed), the field current must be increased above normal (generated voltage is proportional to frequency and flux). The excess current in the rotor and generated voltage will result in over-fluxing of the generator stator iron and the iron cores of the main and unit service transformers. Damage due to overheating may result in these components. Over-voltage may also cause breakdown of insulation, resulting in faults/arcing. This problem may occur on generators that are connected to the grid if they experience generator voltage regulation problems. It may also occur for units during start-up or re- synchronizing following a trip (the field breaker should open when the turbine is tripped). When the field breaker opens, a field discharge resistor is inserted into the rotor circuit to help prevent terminal voltage from reaching dangerous levels. Over-excitation on start-up may be a result of equipment problems or Notes: operator error in applying excessive excitation prematurely (excitation should not be applied to the generator until it reaches near synchronous speed). A specialized volts/hertz relay is used to detect this condition and will trip the generator if excessive volts/hertz conditions are detected.
8. Generator Under-frequency Protection
While connected to a stable grid, the grid frequency and voltage are usually constant. If the system frequency drops excessively, it indicates that there has been a significant increase in load. This could end to a serious problem in the grid and it is of little use to supply a grid that may be about to collapse. In this case, the generator would be separated from the grid. The grid (or at least portions of it) may well collapse. The system can slowly rebuild (with system generators ready to restore power) to proper, pre-collapse operating conditions. As mentioned above, if a generator connected to the grid has sufficient excitation applied below synchronous speed (since grid frequency has dropped) for it to produce rated voltage, the excitation level is actually higher than that required at synchronous speed. Over excitation and the problems described above may result. A specialized volts/hertz relay compares voltage level and frequency and will trip the generator if present volts/hertz levels are exceeded.
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9. Generator Out of Step Protection
This protects the generator from continuing operation when the generator is pole
slipping. Pole slipping will result in mechanical rotational impacts to the turbine, as the
generator slips in and out of synchronism. This can be the result of running in an under
excited condition (see the section on loss of field) or a grid fault that has not cleared.
Relays that detect changes in impedance of the generator can be used to detect the
impedance changes that will occur when the unit slips poles. Another method to
provide this protection is to detect the loss of excitation, using the loss of field protection
and trip the unit if excitation is too low (i.e., trip the generator when pole slipping is
imminent). This has been discussed in the loss of field section of this module.
10. Generator Reverse Power Protection
Motoring refers to the process of an AC generator becoming a synchronous motor,
that is, the device changing from a producer of electrical power to a consumer of it.
Following a reactor trip or setback/step back to a very low power level, it is beneficial to
enter the motoring mode of turbine-generator operation. However, this is not a desirable
mode of operation for standby or emergency generators. They are not designed to
operate in this manner and can be seriously damaged if power is allowed to flow in
the wrong direction. A means of indicating when the transition from exporter to importer
of power occurs is provided by a device known as a reverse power relay. As its name
suggests, it is triggered by power flowing in a direction opposite to that which is normally
desired. This can be used for generator protection, as is the case with standby generators
or as a permissive alarm/interlock for turbine-generator motoring. Figure 27 shows
a typical arrangement of a reverse power protection circuit employing both a CT and a
Voltage Transformer (VT) to power the relay and hence, protect the generator. The relay
will operate when any negative power flow is detected.
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Switchgear is one that makes or breaks the electrical circuit. It is a switching device that
opens& closes a circuit that defined as apparatus used for switching, Lon rolling & protecting
the electrical circuit & equipments. The switchgear equipment is essentially concerned with
switching & interrupting currents either under normal or abnormal operating conditions. The
tubular switch with ordinary fuse is simplest form of switchgear & is used to control & protect
& other equipments in homes, offices etc. For circuits of higher ratings, a High Rupturing
Capacity (H.R.C) fuse in condition with a switch may serve the purpose of controlling &
protecting the circuit. However such switchgear cannot be used profitably on high voltage
system (3.3 KV) for 2 reasons. Firstly, when a fuse blows, it takes some time to replace it &consequently there is interruption of service to customer. Secondly, the fuse cannot
successfully interrupt large currents that result from the High Voltage System. In order to
interrupt heavy fault currents, automatic circuit breakers are used. There are very few types
of circuit breakers in B.P.T.S they are VCB, OCB, and SF6 gas circuit breaker. The most
expensive circuit breaker is the SF6 type due to gas. There are various companies which
manufacture these circuit breakers: ABB, L&T, ALSTOM. Switchgear includes switches, fuses,
circuit breakers, relays & other equipments.
THE EQUIPMENTS THAT NORMALLY FALL IN THIS CATEGORY ARE:-
An isolator is one that can break the electrical circuit when the circuit is to be switched on
no load. These are used in various circuits for isolating the certain portion when required
for maintenance etc. An operating mechanism box normally installed at ground level drives
the isolator. The box has an operating mechanism in addition to its contactor circuit and
auxiliary contacts may be solenoid operated pneumatic three phase motor or DC motor transmitting through a spur gear to the torsion shaft of the isolator. Certain interlocks are
also provided with the isolator
1. Isolator cannot operate unless breaker is open
2. Bus 1 and bus 2 isolators cannot be closed simultaneously
3. The interlock can be bypass in the event of closing of bus coupler breaker.
4. No isolator can operate when the corresponding earth switch is on
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2. SWITCHING ISOLATOR
Switching isolator is capable of:
1. Interrupting charging current
2. Interrupting transformer magnetizing current
3. Load transformer switching. Its main application is in connection with
the transformer feeder as the unit makes it possible to switch gear one
transformer while the other is still on load.
3. CIRCUIT BREAKER
One which can make or break the circuit on load and even on faults is referred to as circuit
breakers. This equipment is the most important and is heavy duty equipment mainly utilized
for protection of various circuits and operations on load. Normally circuit breakers installed
are accompanied by isolators.
4. LOAD BREAK SWITCHES
These are those interrupting devices which can make or break circuits. These are normally on
same circuit, which are backed by circuit breakers
5. EARTH SWITCHES
Devices which are used normally to earth a particular system, to avoid any accident happening
due to induction on account of live adjoining circuits. These equipments do not
handle any appreciable current at all. Apart from this equipment there are a number of
relays etc. which are used in switchgear.
In LT switchgear there is no interlocking. It is classified in following ways:-
1. MAIN SWITCH
Main switch is control equipment which controls or disconnects the main supply. The main
switch for 3 phase supply is available for the range 32A, 63A, 100A, 200Q, 300A at 500V grade.
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With Avery high generating capacity of the modern power stations extremely heavy carnets
would flow in the fault and the fuse clearing the fault would be required to withstand extremely heavy stress in process. It is used for supplying power to auxiliaries with backup fuse
protection. With fuses, quick break, quick make and double break switch fuses for 63A and
100A, switch fuses for 200A,400A, 600A, 800A and 1000A are used.
AC Contractors are 3 poles suitable for D.O.L Starting of motors and protecting the connected motors.
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4. OVERLOAD RELAY
For overload protection, thermal overload relay are best suited for this purpose. They operate
due to the action of heat generated by passage of current through relay element.
5. AIR CIRCUIT BREAKERS
It is seen that use of oil in circuit breaker may cause a fire. So in all circuits breakers at large capacity air at high pressure is used which is maximum at the time of quick
tripping of contacts. This reduces the possibility of sparking. The pressure may vary from 50- 60kg/cm^2 for high and medium capacity circuit breakers.
1. MINIMUM OIL CIRCUIT BREAKER
These use oil as quenching medium. It comprises of simple dead tank row pursuing projection
from it. The moving contracts are carried on an iron arm lifted by a long insulating tension rod
and are closed simultaneously pneumatic operating mechanism by means of tensions but
throw off spring to be provided at mouth of the control the main current within the controlled
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o Type-HKH 12/1000c·
o Rated Voltage-66 KV
o Normal Current-1250A·
o Breaking Capacity-3.4+KA Symmetrical
o 3.4+KA Asymmetrical
o 360 MVA Symmetrical
o Motor Voltage-220 V/DC
o Rated Voltage-12 KV
o Supply Voltage Closing-220 V/DC
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2. AIR CIRCUIT BREAKER
In this the compressed air pressure around
15 kg per cm^2 is used for extinction of arc
caused by flow of air around the moving
circuit . The breaker is closed by applying
pressure at lower opening and opened by
applying pressure at upper opening. When
contacts operate, the cold air rushes around
the movable contacts and blown the arc:
It has the following advantages over OCB:-
i. Fire hazard due to oil are eliminated.
ii. Operation takes place quickly.
iii. There is less burning of contacts since the duration is short and consistent.
iv. Facility for frequent operation since the cooling medium is replaced constantly.
Rated Voltage-6.6 KV Current-630 A
Auxiliary current-220 V/DC
3. SF6 CIRCUIT BREAKER
This type of circuit breaker is of construction to
dead tank bulk oil to circuit breaker but the principle
of current interruption is similar to that of air blast
circuit breaker. It simply employs the arc
extinguishing medium namely SF6. When it is
broken down under an electrical stress, it will
quickly reconstitute itself.
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Standard-1 EC 56
Rated Voltage-12 KV
Insulation Level-28/75 KV
Rated Frequency-50 Hz
Breaking Current-40 KA
Rated Current-1600 A
Making Capacity-110 KA
4. VACUUM CIRCUIT BREAKER
It works on the principle that vacuum is used to save the purpose of insulation and. In regards of
insulation and strength, vacuum is superior dielectric medium and is better that all other
medium except air and sulphur which are generally used at high pressure.
Rated frequency-50 Hz·
Rated making Current-10 Peak KA
Rated Voltage-12 KV
Supply Voltage Closing-220 V/DC
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Switchyard is considered as the HEART of the Power Plant. Power generated can be worthful only if it is successfully transmitted and received by its consumers. Switchyard plays a very important role as a buffer between the generation and transmission. It is a junction, which carries the generated power to its destination (i.e. consumers). Switchyard is basically a yard or an open area where many different kinds of equipments are located
(isolator, circuit breaker etc…), responsible for connect transmission line as per requirement
(e.g. any fault condition).
Power transmission is done at a higher voltage.
(Higher transmission voltage reduces transmission losses). Therefore, the power generated by the Turbo generator of 1 to 6 units is 15.75KV and of 6 to 12 units is 21KV which is further stepped-up to 400KV by the Generating transformer & then transmitted to switchyard.
Switchyards can be of400KV &132KV.
In VSTPS there are two interconnected switchyards:-
(i) 400KV SWITCHYARD
(ii) 132KV SWITCHYARD
There are on total 21 bays in this switchyard.
(A bay is basically a way for the incoming power from generator as well as outgoing power for distribution).
7 for unit Generating Transformer.
7 for various distribution lines such as:
satna line; jabalpur #1 line; korba line;
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jabalpur #2 line; Rihand #1 line; SSTPP line; Rihand #2 line;
2 for Bus coupler. 2 for TBC.
2 for ICT.
1 for the Bus Section.
There are on total 6 buses in 400KV switchyard. There are two transfer buses: Transfer bus-1
Transfer buses are kept spare and remain idle and are used only for emergency purposes. BUS COUPLER-1 & BUS COUPLER-2 interconnects Bus-1 & Bus-2, Bus-3 & Bus-4 respectively. Bus couplers are very beneficial as they help in load sharing between the different buses.
There are 15 bays in 132KV switchyard.
4 for Station Transformer.
4 for C.W. Transformer.
for Colony Transformer.
1 for S-V-R line.
1 for Bus Coupler.
There are only 2 buses in 132KV switchyard. Bus-1 is arranged in U-shape configuration
Whereas Bus-2 is a single straight line inserted in between U-shaped Bus- 1. BUS COUPLER is used to couple Bus-1 & Bus-2.
There are 4 C.W.Transformer connected with 132 KV switchyard. (C.W. #1, C.W. #2, C.W. #3, C.W. #4) rated at 25 MVA.
The main purpose of C.W. Transformer is to step-down the system voltage of 132 KV to 6.9 KV, which is required for C.W. Pump Motor used for condensing the water.
Colony Transformer Bay:
There are 2 Colony Transformer connected with 132KV switchyard. (Colony Transformer#1 & Colony Transformer#2) rated at 12.5 MVA.
The purpose of Colony Transformer is to step-down the system voltage of 132 KV to 11 KV for supplying the electricity to colonies for domestic purposes.
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1. Lightening arrestor
2. Current transformer
3. Voltage transformer (CVT)
4. Power transformers / I.C.T.
5. Bus bar and clamp fittings
6. Support structure
8. Circuit Breaker
9. Wave traps
10. Earthing switch
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• Operates under no load condition
• Interlocked with breakers and earth switches
• Should withstand extreme wind pressures
• Motor driven and hand driven
• Local as well as remote operation possible
• Isolates sections for maintenance
• Used to select bus bars
• CT switching for bus bar protection
400KV HORIZONTAL CENTRAL BREAK TYPE ISOLATOR
• USED TO GROUND SECTIONS REQUIRED FOR MAINTENANCE
• GROUND INDUCTION VOLTAGES
• INTERLOCKED WITH BREAKERS AND ISOLATORS
• CAN OPERATE FROM LOCAL ONLY
• MOTOR DRIVEN AS WELL AS HAND DRIVEN & SAFETY DEVICE
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• Current Transformer is an instrument transformer which transforms current from one level to another level, such as, 1000A/1A (CT ratio) i.e. transforms current from the level of 1000A into current of 1A level
• Direct measurement of high current (in the tune of 100A or more) is not possible as devices used for measurement of current are not designed to handle such huge amount of current
Why Current Transformer is required
• System has two basic requirements
metering of energy sourced or consumed
Protection of the electrical system from faults and disturbances
• Types of Current Transformer (CT)
• Measuring CTs
• Protection CTs
• Protection CTs for special applications
Where Current Transformer is connected:
• For metering and protection of a feeder, CT is connected at the beginning of the feeder
• It has a primary winding and one or more secondary windings wound on core of magnetic material
• Metering and Protection devices are connected to the secondaries of the CT
CAPACITIVE VOLTAGE TRANSFORMER
Primary voltage is applied to a series of capacitors group. The voltage across one of the capacitor is taken to aux PT. The secondary of the aux PT is taken for measurement and protection.
secondary voltages(110 volts ac) for meters and energy meters
• voltages for protective relays
• voltages for synchronizing
• disturbance recorders and event logs
• over flux relay
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To discharge the high voltage surges in the power system due to lightning to the ground. Apparatus to be protected:
Overhead lines………Earth/Ground wires (PA=30DEG)
Substation…………...Lightning Masts, Earth wire
A circuit breaker can either make or break a circuit either automatically or manually, like no load, full load and short circuit conditions. The characteristic feature of a circuit breaker has made it a very useful electrical device in electrical power system.
Types of circuit breakers:-
1) Oil circuit breakers
2) Air blast circuit breakers
3) Sulphur hexafluoride circuit breakers
4) Vacuum circuit breakers
Oil circuit breakers:
As the name suggests, the principle arc quenching medium in this case is oil. Whenever the fixed contacts are separated from the moving contacts, an arc is struck in between the contacts, which cannot be allowed in a circuit. The same needs to be extinguished. Therefore, as soon as the arc is struck, the arcing medium is completely surrounded by oil, and the heat of the arc, thus produced helps in producing hydrogen gas, which helps in quenching the arc.
Advantages of oil circuit breaker:
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1) The hydrogen gas thus produced has excellent cooling properties. Hence it cools the arc.
2) The volume of hydrogen gas is about 1000 times, that of air. Thus it causes turbulence in the arcing region, thus eliminating the arcing products.
Disadvantages of oil circuit breakers:
The oil circuit breakers also have a few limitations. They are:
1) The use of oil as the arc quenching medium increases the risk of fire.
2) Reuse of the oil leads to carbonisation and degradation of the oil.
Air blast circuit breakers:
The air blast circuit breakers are another category of circuit breakers which use air as the arc quenching medium. They, in a similar manner, remove the arcing products from the arcing region, thus extinguishing the arc.
Advantages of air blast circuit breakers:
1) The risk of fire, as in the oil circuit breakers is removed.
2) The problem of carbonisation is also r degrade its quality.
Disadvantages of air blast circuit breakers:
1) Air blast circuit breakers have inferior arc quenching properties.
2) Regular maintenance of the compressor plant is required, which supplies the air blast.
Sulphur hexa-fluoride (SF6 ) circuit breakers:
The Sulphur hexafluoride circuit breakers are the most widely used circuit breakers. They involve the use of sulphur hexafluoride gas, which acts as an arc quenching medium.
Advantages of the sulphur hexafluoride circuit breakers:
1) Due to their superior arc quenching properties, they have very low arcing time.
2) Since the dielectric strength of the gas is very high, they can interrupt high values of current.
Disadvantages of sulphur hexafluoride circuit breakers:
1) Sulphur hexafluoride gas is expensive.
2) Regular conditioning of the gas is required, especially after use.
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PROTECTION OF SWITCHYARD
For EHV lines distance protection are used as Main protection. The measurement principle of the protection is based on measuring the impedance of the lines from the point of relay to the fault. This impedance is proportional to the distance. The relay can be set by calculating the impedance of the total length of the line based on the available data of line i.e. resistance and reactance / Km and length of line.
The relay always measures the impedance of the line when the line voltage & current are connected to the relay. The measured values are compared with the setting. In case of fault on the line the impedance measured shall be less than the set value & fall in the operating characteristic of the relay & the relay will operate to trip the respective breaker. The resistance and reactance of the line are independently set on the relay i.e. the characteristic shall be trapezoidal.
Local Breaker Back up Protection:
All the protections of the Generator transformer and unit auxiliary transformers finally operate the Generator master trip relay. This master trip relay issues tripping command to Generator bay breaker. In the event, the Generator breaker does not open within preset time say 200 ms, the LBB scheme is energized.
Generator Feeder Protection
ESSENTIAL QUALITIES OF ELECTRICAL PROTECTION
Having looked at the fundamental purpose of electrical protection, we should cover the four main building blocks that are used to meet these requirements:
When electrical faults or short circuits occur, the damage produced is largely dependent upon the time the fault persists. Therefore, it is desirable that electrical faults be interrupted as quickly as possible. Since 1965, great strides have been made in this area. High-speed fault detecting relays can now operate in as little time as 10 milliseconds and
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output relaying in 2 milliseconds. The use of protection zones, that will be discussed later, minimized the requirement for time-delayed relaying.
Reliability & Security
The protective system must function whenever it is called upon to operate, since the consequences of non-operation can be very severe. This is accomplished by duplicate A and B protections and duplicate power supplies. Protections must isolate only the faulted equipment, with no over-tripping of unaffected equipment. This is accomplished by the use of over-lapping protection zones.
The protection must be able to distinguish between healthy and fault conditions, i.e., to detect, operate and initiate tripping before a fault reaches a dangerous condition. On the other hand, the protection must not be too sensitive and operate unnecessarily. Some loads take large inrush starting currents, which must be accommodated to prevent unnecessary tripping while still tripping for fault conditions. The ability of relaying to fulfil the sensitivity requirement is improved through the use of protection zones. The basic idea behind the use of protection zones is that every component in an electrical system (i.e., bus, transformer, motor, and generator) has distinct characteristics. Protections can be made to perform faster (speed), with increased security and increased sensitivity if it deals with only that one element. As we start to look at the electrical system in view of protection zones, we have to get a clear picture of where the boundaries of these zones will normally be in any electrical system. For simple systems such as a motor in the diagram below, the fuse or thermal circuit breaker would be the boundary of the zone. The ratings of the fuse would be designed to protect that motor only.
HVDC Back to back System A back-to-back station (or B2B for short) is a plant in which both converters are in the same area, usually in the same building. The length of the direct current line is kept as short as possible. HVDC back-to-back stations are used for coupling of electricity grids of different frequencies (as Northern and western grid) coupling two networks of the same nominal frequency but no fixed phase relationship (as until 1995/96 in Etzenricht, Dürnrohr,Vienna, and the Vyborg HVDC scheme). different frequency and phase number (for example, as a replacement for traction current converter plants) The DC voltage in the intermediate circuit can be selected freely at HVDC back-to-back stations because of the short conductor length. The DC voltage is usually selected to be as low as possible, in order to build a small valve hall and to reduce the number of thyristors connected in series in each valve. For this reason, at HVDC back-to-back stations, valves with the highest available current rating (in some cases, up to 4,500 A) used.
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VINDHYACHAL HVDC: Data sheet of Vindhyachal HVDC are: 1. Date of completion: April 1989 2. Specification: a) Power rating: 2 x 250 MW b) No. of blocks: 2 c) AC voltage: 400KV d) DC voltage: ±70KV e) Converter transformer: 8 x 156 KVA
System Salient Features:
1. It connects Vindhyachal Super Thermal Power Stations (Western Region) to Singrauli Super Thermal Power Stations (Northern Region) in Indian Grid.
2. Each Block power carrying capacity is 250 MW.
3. Bidirectional power flow capability is available.
4. The project achieve load diversity of Northern and Western region in Indian Grid by meeting high demand from surplus power available in either regions.
5. First commercial Back to Back HVDC Station in India
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Water Treatment Plant and Storage
Since there is continuous withdrawal of steam and continuous return of condensate to the boiler, losses due to blow down and leakages have to be made up to maintain a desired water level in the boiler steam drum. For this, continuous make-up water is added to the boiler water system. Impurities in the raw water input to the plant generally consist of calcium and magnesium salts which impart hardness to the water. Hardness in the make-up water to the boiler will form deposits on the tube water surfaces which will lead to overheating and failure of the tubes.
Thus, the salts have to be removed from the water, and that is done by a water de- mineralizing treatment plant (DM). A DM plant generally consists of cation, anion, and mixed bed exchangers. Any ions in the final water from this process consist essentially of hydrogen ions and hydroxide ions, which recombine to form pure water. Very pure DM water becomes highly corrosive once it absorbs oxygen from the atmosphere because of its very high affinity for oxygen.
The capacity of the DM plant is dictated by the type and quantity of salts in the raw water
input. However, some storage is essential as the DM plant may be down for maintenance.
For this purpose, a storage tank is installed from which DM water is continuously
withdrawn for boiler make-up. The storage tank for DM water is made from materials not
affected by corrosive water. The piping and valves are generally of stainless.
Industrial training being an integral part of engineering curriculum provides not only easier understanding but also helps acquaint an individual with technologies. It exposes an individual to practical aspect of all things which differ considerably from theoretical models. During my training, I gained a lot of practical knowledge which otherwise could have been exclusive to me. The practical exposure required here will pay rich dividends to me when I will set my foot as an Engineer.
The training at NTPC Vindhyachal was altogether an exotic experience, since work, culture and mutual cooperation was excellent here. Moreover fruitful result of adherence to quality control awareness of safety and employees were fare which is much evident here.
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PRESENTATIONS FROM EDC DEPARTMENT,BTPS
PRESENTATION FROM SENIORS AND EMPLOYEES OF VSTPP
NTPC VSTPP WEBSITE
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