SUMMER TRAINING REPORT

5th July to 31st July

Submitted By:-

Abhinav Sharma B.Tech 3rd year

Gautam Buddh Technical University

CERTIFICATE

This is to certify that ABHINAV SHARMA (0719221001), student of 2007-

2011 Batch of Electrical & Electronics Branch in 3rd Year of G.L. BAJAJ

Institute of Technology, Greater Noida has successfully completed his

industrial training at Badarpur Thermal Power Station- NTPC, New Delhi for

four weeks from 5th July to 31st July 2010. He has completed the whole

training as per the training report submitted by him.

Training In-charge  Badarpur Thermal Power Station

NTPC, Badarpur New Delhi.

Table of Contents

1. Acknowledgement

2. About the Company

3. Thermal Power Plant

Introduction Operation

4. Electricity Generation Process

5. EMD-I

6. EMD-II

Acknowledgement

With profound respect and gratitude, I take the opportunity to convey my thanks to complete the training here. I express gratitude to the Program Manager and other faculty members of Electrical & Electronics Engineering Department of G.L. BAJAJ of Institute of Technology for providing this opportunity to undergo industrial training at National Thermal Power Corporation, Badarpur, New Delhi.

I do extend my heartfelt thanks to Ms. Rachna Singh Bhal for providing me this opportunity to be a part of this esteemed organization.

I am extremely grateful to Mr. G.D.Sharma, Superintendent of Im-Plant Training at BTPS-NTPC, Badarpur for his guidance during whole training.

I am extremely grateful to all the technical staff of BTPS-NTPC for their co-operation and guidance that helped me a lot during the course of training. I have learnt a lot working under them and I will always be indebted of them for this value addition in me.

Finally, I am indebted to all whosoever have contributed in this report work and friendly stay at Badarpur Thermal Power Station, Badarpur, New Delhi.

ABOUT THE COMPANY

CORPORATE VISION

“A world class integrated power major, powering India's growth with increasing global presence.”

CORE VALUES:

BCOMIT

B- Business ethics

C- Customer focus

O- Organizational & professional pride

M- Mutual respect & trust

I- Innovation & speed

T- Total quality for excellence

NTPC Limited is the largest thermal power generating company of India, Public Sector Company. It was incorporated in the year 1975 to accelerate power development in the country as a wholly owned company of the Government of India. At present, Government of India holds 89.5% of the total equity shares of the company and the balance 10.5% is held by FIIs, Domestic Banks, Public and others. Within a span of 31 years, NTPC has emerged as a truly national power company, with power generating facilities in all the major regions of the country.

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 31134 MW (including JVs) with 15 coal based and 7 gas based stations, located across the country. In addition under JVs, 3 stations are coal based & 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 53000 MW, 10000 MW through gas, 9000 MW through Hydro generation, about 2000 MW from nuclear sources and around 1000 MW from Renewable Energy Sources (RES). NTPC has adopted a multi-pronged growth strategy which includes capacity addition through green field projects, expansion of existing stations, joint ventures, subsidiaries and takeover of stations.

NTPC has been operating its plants at high efficiency levels. Although the company has 18.79% of the total national capacity it contributes 28.60% of total power generation due to its focus on high efficiency. 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. 170.88BU of electricity was produced by its stations in the financial year 2005-2006. The Net Profit after Tax on March 31, 2006 was INR 58,202 million. Net Profit after Tax for the quarter ended June 30, 2006 was INR 15528 million, which is 18.65% more than for the same quarter in the previous financial year. 2005).

Pursuant to a special resolution passed by the Shareholders at the Company’s Annual General Meeting on September 23, 2005 and the 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 October 28, 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.

A graphical overview

NTPC Limited

Type Public

Founded 1975

HeadquartersDelhi, India

Key peopleR S Sharma, Chairman & Managing Director

IndustryElectricity generation

Products Electricity

Revenue▲INR 416.37 billion (2008)

Net income▲INR 70.47 billion (2008)

Employees23867 (2006)

Website http://www.ntpc.co.in

STRATEGIES

Technological Initiatives

Introduction of steam generators (boilers) of the size of 800 MW

Integrated Gasification Combined Cycle (IGCC) Technology

Launch of Energy Technology Centre -A new initiative for development of technologies with focus on fundamental R&D

The company sets aside up to 0.5% of the profits for R&D

Roadmap developed for adopting ‘Clean Development

Mechanism’ to help get / earn ‘Certified Emission Reduction

Corporate Social Responsibility

As a responsible corporate citizen NTPC has taken up number of CSR initiatives

NTPC Foundation formed to address Social issues at national level

NTPC has framed Corporate Social Responsibility Guidelines committing up to 0.5% of net profit annually for Community Welfare

The welfare of project affected persons and the local population around NTPC projects are taken care of through well drawn Rehabilitation and Resettlement policies

The company has also taken up distributed generation for remote rural areas

Partnering government in various initiatives

Consultant role to modernize and improvise several plants across the country

Disseminate technologies to other players in the sector

Consultant role “Partnership in Excellence” Programme for improvement of PLF of 15 Power Stations of SEBs.

Rural Electrification work under Rajiv Gandhi Garmin Vidyutikaran

Environment Management

All stations of NTPC are ISO 14001 certified

Various groups to care of environmental issues

The Environment Management Group

Ash Utilization Division

Afforestation Group

Centre for Power Efficiency & Environment Protection

Group on Clean Development Mechanism

NTPC is the second largest owner of trees in the country after the Forest department.

JOURNEY OF NTPC

NTPC was set up in 1975 with 100% ownership by the Government of India. In the last 30 years, NTPC has grown into the largest power utility in India.

In 1997, Government of India granted NTPC status of “Navratna’ being one of the nine jewels of India, enhancing the powers to the Board of Directors.

NTPC became a listed company with majority Government ownership of 89.5%.NTPC becomes third largest by Market Capitalization of listed companies

The company rechristened as NTPC Limited in line with its changing business portfolio and transforms itself from a thermal power utility to an integrated power utility.

National Thermal Power Corporation is the largest power generation company in India. Forbes Global 2000 for 2008 ranked it 411th in the world.

National Thermal Power Corporation is the largest power generation company in India. Forbes Global 2000 for 2008 ranked it 317th in the world.

1975 1975

1997 1997

2005 2005

20042004

2008 2008

2009 2009

NTPC has also set up a plan to achieve a target of 50,000 MW generation capacity.

NTPC has embarked on plans to become a 75,000 MW company by 2017.

NTPC is the largest power utility in India, accounting for about 20% of India’s installed capacity.

THEMAL POWER PLANT

2017 2017

2012 2012

INTRODUCTION

Power Station (also referred to as generating station or power plant) is an industrial facility for the generation of electric power. Power plant is also used to refer to the engine in ships, aircraft and other large vehicles. Some prefer to use the term energy center because it more accurately describes what the plants do, which is the conversion of other forms of energy, like chemical energy, gravitational potential energy or heat energy into electrical energy. However, power plant is the most common term in the U.S., while elsewhere power station and power plant are both widely used, power station prevailing in many Commonwealth countries and especially in the United Kingdom.

A coal-fired Thermal Power PlantAt the center of nearly all power stations is a generator, a rotating machine that converts Mechanical energy into Electrical energy by creating relative motion

between a magnetic field and a conductor. The energy source harnessed to turn the

generator varies widely. It depends chiefly on what fuels are easily available and the types of technology that the power company has access to.

In thermal power stations, mechanical power is produced by a heat engine, which transforms Thermal energy (often from combustion of a fuel) into rotational energy. Most thermal power stations produce steam, and these are sometimes called steam power stations. About 80% of all electric power is generated by use of steam turbines. Not all thermal energy can be transformed to mechanical power, according to the second law of thermodynamics. Therefore, there is always heat lost to the environment. If this loss is employed as useful heat, for industrial processes or district heating, the power plant is referred to as a cogeneration power plant or CHP (combined heat-and-power) plant. In countries where district heating is common, there are dedicated heat plants called heat-only boiler stations. An important class of power stations in the Middle East uses by-product heat for desalination of water.

CLASSIFICATION

By fuel

• Nuclear power plants use a nuclear reactor's heat to operate a steam turbine generator.

• Fossil fuelled power plants may also use a steam turbine generator or in the case of natural gas fired plants may use a combustion turbine.

• Geothermal power plants use steam extracted from hot underground rocks.

• Renewable energy plants may be fuelled by waste from sugar cane, municipal solid waste, landfill methane, or other forms of biomass.

• In integrated steel mills, blast furnace exhaust gas is a low-cost, although low-energy density, fuel.• Waste heat from industrial processes is occasionally concentrated enough to use for power generation, usually in a steam boiler and turbine.

By prime mover

• Steam turbine plants use the dynamic pressure generated by expanding steam to turn the blades of a turbine. Almost all large non-hydro plants use this system.

• Gas turbine plants use the dynamic pressure from flowing gases to directly operate the turbine. Natural-gas fuelled turbine plants can start rapidly and so are used to supply "peak" energy during periods of high demand, though at higher cost than base-loaded plants. These may be comparatively small units, and sometimes completely unmanned, being remotely operated. This type was pioneered by the UK, Prince town being the world's first, commissioned in 1959.

• Combined cycle plants have both a gas turbine fired by natural gas, and a steam boiler and steam turbine which use the exhaust gas from the gas turbine to produce electricity. This greatly increases the overall efficiency of the plant, and many new base load power plants are combined cycle plants fired by natural gas.

• Internal combustion Reciprocating engines are used to provide power for isolated communities and are frequently used for small cogeneration plants. Hospitals, office buildings, industrial plants, and other critical facilities also use them to provide backup power in case of a power outage. These are usually fuelled by diesel oil, heavy oil, natural gas and landfill gas.

• Micro turbines, Sterling engine and internal combustion reciprocating engines are low cost solutions for using opportunity fuels, such as landfill gas, digester gas from water treatment plants and waste gas from oil production.

FUNCTIONING

In a thermal power plant, one of coal, oil or natural gas is used to heat the boiler to convert the water into steam. The steam is used to turn a turbine, which is connected to a generator. When the turbine turns, electricity is generated and given as output by the generator, which is then supplied to the consumers through high-voltage power lines.

Process of a Thermal Power Plant

Detailed process of power generation in a thermal power plant:

1) Water intake: Firstly, water is taken into the boiler through a water source. If water is available in a plenty in the region, then the source is an open pond or river. If water is scarce, then it is recycled and the same water is used over and over again.

2) Boiler heating: The boiler is heated with the help of oil, coal or natural gas. A furnace is used to heat the fuel and supply the heat produced to the boiler. The increase in temperature helps in the transformation of water into steam.

3) Steam Turbine: The steam generated in the boiler is sent through a steam turbine. The turbine has blades that rotate when high velocity steam flows across them. This rotation of turbine blades is used to generate electricity.

4) Generator: A generator is connected to the steam turbine. When the turbine rotates, the generator produces electricity which is then passed on to the power distribution systems.

5) Special mountings: There is some other equipment like the economizer and air pre-heater.An economizer uses the heat from the exhaust gases to heat the feed water. An air pre-heater heats the air sent into the combustion chamber to improve the efficiency of the combustion process.

6) Ash collection system: There is a separate residue and ash collection system in place to collect all the waste materials from the combustion process and to prevent them from escaping into the atmosphere.Apart from this, there are various other monitoring systems and instruments in place to keep track of the functioning of all the devices. This prevents any hazards from taking place in the plant.

OPERATION

Introduction

The operating performance of NTPC has been considerably above the national average. The availability factor for coal stations has increased from 85.03 % in 1997-98 to 90.09 % in 2006-07, which compares favourably with international standards. The PLF has increased from 75.2% in 1997-98 to 89.4% during the year 2006-07 which is the highest since the inception of NTPC.

Operation Room of Power Plant

In Badarpur Thermal Power Station, steam is produced and used to spin a turbine that operates a generator. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser; this is known as a Rankine cycle. Shown here is a diagram of a conventional thermal power plant, which uses coal, oil, or natural gas as fuel to boil water to produce the steam. The electricity generated at the plant is sent to consumers through high-voltage power lines.The Badarpur Thermal Power Plant has Steam Turbine-Driven Generators which has a collective capacity of 705MW. The fuel being used is Coal which is supplied from the Jharia Coal Field in Jharkhand.Water supply is given from the Agra Canal.

Table: Capacity of Badarpur Thermal Power Station, (BTPS) New Delhi

There are basically three main units of a thermal power plant:

1. Steam Generator or Boiler

2. Steam Turbine

3.Electric Generator

The description of some of the components written above is described as follows:

1. Cooling towers 

Cooling Towers are evaporative coolers used for cooling water or other working medium to near the ambivalent web-bulb air temperature. Cooling tower use evaporation of water to reject heat from processes such as cooling the circulating water used in oil refineries, Chemical plants, power plants and building cooling, for example. The tower vary in size from small roof-top units to very large hyperboloid structures that can be up to 200 meters tall and 100 meters in diameter, or rectangular structure that can be over 40 meters tall and 80 meters long. Smaller towers are normally factory built, while larger ones are constructed on site.The primary use of large , industrial cooling tower system is to remove the heat absorbed in the circulating cooling water systems used in power plants , petroleum refineries, petrochemical and chemical plants, natural gas processing plants and other industrial facilities . The absorbed heat is rejected to the atmosphere by the evaporation of some of the cooling water in mechanical forced-draft or induced draft towers or in natural draft hyperbolic shaped cooling towers as seen at most nuclear power plants.

2.Three phase transmission line

Three phase electric power is a common method of electric power transmission. It is a type of polyphase system mainly used to power motors and many other devices. A Three phase system uses less conductor material to transmit electric power than equivalent single phase, two phase, or direct current system at the same voltage. In a three phase system, three circuits reach their instantaneous peak values at different times. Taking one conductor as the reference, the other two current are delayed in time by one-third and two-third of one cycle of the electrical current. This delay between “phases” has the effect of giving constant power transfer over each cycle of the current and also makes it possible to produce a rotating magnetic field in an electric motor.

At the power station, an electric generator converts mechanical power into a set of electric currents, one from each electromagnetic coil or winding of the generator. The current are sinusoidal functions of time, all at the same frequency but offset in time to give different phases. In a three phase system the phases are spaced equally, giving a phase separation of one-third one cycle. Generator’s output is at a

voltage that ranges from hundreds of volts to 30,000 volts. At the power station, transformers: step-up” this voltage to one more suitable for transmission.After numerous further conversions in the transmission and distribution network the power is finally transformed to the standard mains voltage (i.e. the “household” voltage).

The power may already have been split into single phase at this point or it may still be three phase. Where the step-down is 3 phase, the output of this transformer is usually star connected with the standard mains voltage being the phase-neutral voltage.

3.Electrical generator 

An Electrical generator is a device that converts kinetic energy to electrical energy generally using electromagnetic induction. The task of converting the electrical energy into mechanical energy is accomplished by using a motor. The source of mechanical energy may be a reciprocating or turbine steam engine,water falling through the turbine are made in a variety of sizes ranging from small 1 hp (0.75 kW) units (rare) used as mechanical drives for pumps, compressors and other shaft driven equipment , to 2,000,000 hp(1,500,000 kW) turbines used to generate electricity. There are several classifications for modern steam turbines.Steam turbines are used in all of our major coal fired power stations to drive the generators or alternators, which produce electricity. The turbines themselves are driven by steam generated in ‘Boilers’ or ‘steam generators’ as they are sometimes called.

Energy in the steam after it leaves the boiler is converted into rotational energy as it passes through the turbine. The turbine normally consists of several stage with each stages consisting of a stationary blade (or nozzle) and a rotating blade. Stationary blades convert the potential energy of the steam into kinetic energy into forces, caused by pressure drop, which results in the rotation of the turbine shaft. The turbine shaft is connected to a generator, which produces the electrical energy.

4.Boiler feed water pump

A Boiler feed water pump is a specific type of pump used to pump water into a steam boiler. The water may be freshly supplied or retuning condensation of the steam produced by the boiler. These pumps are normally high pressure units that use suction from a condensate return system and can be of the centrifugal pump type or positive displacement type.

Construction and operation:

Feed water pumps range in size up to many horsepower and the electric motor is usually separated from the pump body by some form of mechanical coupling. Large industrial condensate pumps may also serve as the feed water pump. In either case, to force the water into the boiler; the pump must generate sufficient pressure to overcome the steam pressure developed by the boiler. This is usually accomplished through the use of a centrifugal pump.

Feed water pumps usually run intermittently and are controlled by a float switch or other similar level-sensing device energizing the pump when it detects a lowered liquid level in the boiler is substantially increased. Some pumps contain a two-stage switch. As liquid lowers to the trigger point of the first stage, the pump is activated. I f the liquid continues to drop (perhaps because the pump has failed, its supply has been cut off or exhausted, or its discharge is blocked); the second stage will be triggered. This stage may switch off the boiler equipment (preventing the boiler from running dry and overheating), trigger an alarm, or both.

5. Steam-powered pumps

Steam locomotives and the steam engines used on ships and stationary applications such as power plants also required feed water pumps. In this situation, though, the pump was often powered using a small steam engine that ran using the steam produced by the boiler. A means had to be provided, of course, to put the initial charge of water into the boiler(before steam power was available to operate the steam-powered feed water pump).The pump was often a positive displacement pump that had steam valves and cylinders at one end and feed water cylinders at the other end; no crankshaft was required.

In thermal plants, the primary purpose of surface condenser is to condense the exhaust steam from a steam turbine to obtain maximum efficiency and also to convert the turbine exhaust steam into pure water so that it may be reused in the steam generator or boiler as boiler feed water. By condensing the exhaust steam of a turbine at a pressure below atmospheric pressure, the steam pressure drop between the inlet and exhaust of the turbine is increased, which increases the amount heat available for conversion to mechanical power. Most of the heat liberated due to condensation of the exhaust steam is carried away by the cooling medium (water or air) used by the surface condenser. 

6. Control valves

Control valves are valves used within industrial plants and elsewhere to control operating conditions such as temperature,pressure,flow,and liquid Level by fully partially opening or closing in response to signals received from controllers that compares a “set point” to a “process variable” whose value is provided by sensors that monitor changes in such conditions. The opening or closing of control valves is done by means of electrical, hydraulic or pneumatic systems

7. Deaerator

A Dearator is a device for air removal and used to remove dissolved gases (an alternate would be the use of water treatment chemicals) from boiler feed water to make it non-corrosive. A dearator typically includes a vertical domed deaeration section as the deaeration boiler feed water tank. A Steam generating boiler requires that the circulating steam, condensate, and feed water should be devoid of dissolved gases, particularly corrosive ones and dissolved or suspended solids. The gases will give rise to corrosion of the metal. The solids will deposit on the heating surfaces giving rise to localized heating and tube ruptures due to overheating. Under some conditions it may give to stress corrosion cracking.Deaerator level and pressure must be controlled by adjusting control valves- the level by regulating condensate flow and the pressure by regulating steam flow. If operated properly, most deaerator vendors will guarantee that oxygen in the deaerated water will not exceed 7 ppb by weight (0.005 cm3/L)

8. Feed water heater

A Feed water heater is a power plant component used to pre-heat water delivered to a steam generating boiler. Preheating the feed water reduces the irreversible involved in steam generation and therefore improves the thermodynamic efficiency of the system.[4] This reduces plant operating costs and also helps to avoid thermal shock to the boiler metal when the feed water is introduces back into the steam cycle.In a steam power (usually modeled as a modified Ranking cycle), feed water heaters allow the feed water to be brought up to the saturation temperature very gradually. This minimizes the inevitable irreversibility’s associated with heat transfer to the working fluid (water). A belt conveyor consists of two pulleys, with a continuous loop of material- the conveyor Belt – that rotates about them. The pulleys are powered, moving the belt and the material on the belt forward.

9. Pulverizer

A pulverizer is a device for grinding coal for combustion in a furnace in a fossil fuel power plant.

10. Boiler Steam Drum

Steam Drums are a regular feature of water tube boilers. It is reservoir of water/steam at the top end of the water tubes in the water-tube boiler. They store the steam generated in the water tubes and act as a phase separator for the steam/water mixture. The difference in densities between hot and cold water helps in the accumulation of the “hotter”-water/and saturated –steam into steam drum. Made from high-grade steel (probably stainless) and its working involves temperatures 390’C and pressure well above 350psi (2.4MPa). The separated steam is drawn out from the top section of the drum. Saturated steam is drawn off the top of the drum. The steam will re-enter the furnace in through a super heater, while the saturated water at the bottom of steam drum flows down to the mud-drum /feed water drum by down comer tubes accessories include a safety valve, water level indicator and fuse plug. A steam drum is used in the company of a mud-drum/feed water drum which is located at a lower level. So that it acts as a sump for the sludge or sediments which have a tendency to the bottom.

11. Super Heater

A Super heater is a device in a steam engine that heats the steam generated by the boiler again increasing its thermal energy and decreasing the likelihood that it will condense inside the engine. Super heaters increase the efficiency of the steam engine, and were widely adopted. Steam which has been superheated is logically known as superheated steam; non-superheated steam is called saturated steam or wet steam; Super heaters were applied to steam locomotives in quantity from the early 20th century, to most steam vehicles, and so stationary steam engines including power stations.

12. Economizers

Economizer, or in the UK economizer, are mechanical devices intended to reduce energy consumption, or to perform another useful function like preheating a fluid. The term economizer is used for other purposes as well. Boiler, power plant, and heating, ventilating and air conditioning. In boilers, economizer are heat exchange devices that heat fluids , usually water, up to but not normally beyond the boiling

point of the fluid. Economizers are so named because they can make use of the enthalpy and improving the boiler’s efficiency. They are a device fitted to a boiler which saves energy by using the exhaust gases from the boiler to preheat the cold water used the fill it (the feed water). Modern day boilers, such as those in cold fired power stations, are still fitted with economizer which is decedents of Green’s original design. In this context they are turbines before it is pumped to the boilers. A common application of economizer is steam power plants is to capture the waste hit from boiler stack gases (flue gas) and transfer thus it to the boiler feed water thus lowering the needed energy input , in turn reducing the firing rates to accomplish the rated boiler output . Economizer lower stack temperatures which may cause condensation of acidic combustion gases and serious equipment corrosion damage if care is not taken in their design and material selection.

13. Air Preheater

Air preheater is a general term to describe any device designed to heat air before another process (for example, combustion in a boiler). The purpose of the air preheater is to recover the heat from the boiler flue gas which increases the thermal efficiency of the boiler by reducing the useful heat lost in the fuel gas. As a consequence, the flue gases are also sent to the flue gas stack (or chimney) at a lower temperature allowing simplified design of the ducting and the flue gas stack. It also allows control over the temperature of gases leaving the stack.

14. Precipitator

An Electrostatic precipitator (ESP) or electrostatic air cleaner is a particulate device that removes particles from a flowing gas (such As air) using the force of an induced electrostatic charge. Electrostatic precipitators are highly efficient filtration devices, and can easily remove fine particulate matter such as dust and smoke from the air steam.ESP’s continue to be excellent devices for control of many industrial particulate emissions, including smoke from electricity-generating utilities (coal and oil fired), salt cake collection from black liquor boilers in pump mills, and catalyst collection from fluidized bed catalytic crackers from several hundred thousand ACFM in the largest coal-fired boiler application.

The original parallel plate-Weighted wire design (described above) has evolved as more efficient ( and robust) discharge electrode designs were developed, today focusing on rigid discharge electrodes to which many sharpened spikes are attached , maximizing corona production. Transformer –rectifier systems apply

voltages of 50-100 Kilovolts at relatively high current densities. Modern controls minimize sparking and prevent arcing, avoiding damage to the components. Automatic rapping systems and hopper evacuation systems remove the collected particulate matter while on line allowing ESP’s to stay in operation for years at a time.

15. Fuel gas stack

A Fuel gas stack is a type of chimney, a vertical pipe, channel or similar structure through which combustion product gases called fuel gases are exhausted to the outside air. Fuel gases are produced when coal, oil, natural gas, wood or any other large combustion device. Fuel gas is usually composed of carbon dioxide (CO2) and water vapor as well as nitrogen and excess oxygen remaining from the intake combustion air. It also contains a small percentage of pollutants such as particulates matter, carbon mono oxide, nitrogen oxides and sulfur oxides. The flue gas stacks are often quite tall, up to 400 meters (1300 feet) or more, so as to disperse the exhaust pollutants over a greater aria and thereby reduce the concentration of the pollutants to the levels required by governmental environmental policies and regulations.When the fuel gases exhausted from stoves, ovens, fireplaces or other small sources within residential abodes, restaurants , hotels or other stacks are referred to as chimneys.

ELECTRICITY GENERATION PROCESS

Coal is conveyed (14) from an external stack and ground to a very fine powder by large metal spheres in the pulverised fuel mill (16). There it is mixed with preheated air (24) driven by the forced draught fan (20). The hot air-fuel mixture is forced at high pressure into the boiler where it rapidly ignites. Water of a high purity flows vertically up the tube-lined walls of the boiler, where it turns into steam, and is passed to the boiler drum, where steam is separated from any remaining water. The steam passes through a manifold in the roof of the drum into the pendant super heater (19) where its temperature and pressure increase rapidly to around 200 bar and 540°C,sufficient to make the tube walls glow a dull red. The steam is piped to the high pressure turbine (11), the first of a three-stage turbine process. A steam governor valve (10) allows for both manual control of the turbine and automatic set-point following. The steam is exhausted from the high pressure turbine, and reduced in both pressure and temperature, is returned to the boiler reheater (21). The reheated steam is then passed to the intermediate pressure turbine (9), and from there passed directly to the low pressure turbine set (6). The exiting steam, now a little above its boiling point, is brought into thermal contact with cold water (pumped in from theCooling tower) in the condenser (8), where it condenses rapidly back into water, creating near vacuum-like conditions inside the condensor chest. The condensed water is then passed by a feed pump (7) through a deaerator (12), and pre-warmed, first in a feed heater (13) powered by steam drawn from the high pressure set, and then in the economiser (23), before being returnedto the boiler drum. The cooling water from the condensor is sprayed inside a cooling tower (1), creating a highly visible plume of water vapour, before being pumped back to the condensor (8) in cooling water cycle. The three turbine sets are sometimes coupled on the same shaft as the three-phase electrical generator (5) which generates an intermediate level voltage (typically 20-25 kV). This is stepped up by the unit transformer (4) to a voltage more suitable for transmission (typically 250-500 kV) and is sent out onto the three-phase transmission system (3). Exhaust gas from the boiler is drawn by the induced draft fan (26) through an electrostatic precipitator (25) and is then vented through the chimney stack (27).

ELECTRICITY GENERATION PROCESS(A BASIC OVERVIEW)

HOW ELECTRICITY IS GENERATED?

Thermal power station burns fuel and uses the resultant heat to raise steam which drives the TURBO GENERATOR. The fuel may be ‘fossil’(coal,oil,natural gas) or it may be fissionable, whichever fuel is used, the objective is same to convert the mechanical energy into electricity by rotating a magnet inside a set of winding.

COAL TO STAEM

Its other raw materials are air and water. The coal brought to the station by trains or by other means, travels handling plant by conveyer belts, travels from pulverizing mills, which grind it as fine as the face powder of size upto 20 microns. The finely produced coal mixed with preheated air is then blown into the boiler by a fan called primary air fan where it burns more like a gas than as a solid, in the conventional domestic or industrial grate, with additional amount of air, called secondary air supply, by forced draft fan.

As coal is ground so finally the resultant ash is also a fine powder. Some of it binds together to form pumps, which falls into ash pits at the bottom of the furnace. The water-quenched ash from the bottom is conveyed to pits for subsequent disposal or sale. Most of ash, still in fine partical form is carried out of boilers to the precipitator as dust, where electrodes charged with high voltage electricity trap it. The dust is then conveyed to water to disposal area or to bunker for sale while the clean flue gases are passed on through IP fans to be discharged through chimneys.

The heat released from the coal has been absorbed by the many kilometers tubing which line the boiler walls. Inside the tubes the boiler feed water, which is transformed by heat into staemat high temperature and pressure.. The steam superheated in further tubes (superheaters) passes to turbine where it is discharged through the nozzle on the turbine blades. Just as the energy of wind turns the sail of the windmill, the energy of steam striking the blade makes the turbine rotate.

Coupled to the end of the turbine is the rotor of the generator. The rotor is housed inside the stator having heavy coils of the bars in which electricity is produced through the movement of magnetic field created by the rotor. Electricity passes from stator windings to step-up transformer which increases its voltage so that it can be transmited efficiently over lines of grid.

The staem which has given up its heat energy is cahnged back into water in a condenser so that it is ready for re-use. The condenser contains many kilometers of tubing through which cold water is constantly pumped. The staem passing around the tubes looses heat.Thus it is rapidly changed back into water.

But, the two lots of water, that is, the boiler feed and cooling water must never mix. Cooling water is drawn from river- bed, but the boiler feed water must be absolutely pure, far purer than the water we drink (de-mineralized water), otherwise it may damage the boiler tubes.

SUMMER TRAINING SCHEDULE

EMD-I (Electrical Maintenance Department-I)--- 1 week

EMD-II (Electrical Maintenance Department-II)--- 1 week

EMD – I

In EMD-I we went through following 4 sectors:

• Coal Handling Plant

• Motors

• Switchgear

• High Tension Switchgear

Coal Handling Plant

Coal is delivered by highway truck, rail, barge or collier ship. Some plants are even built near coal mines and coal is delivered by conveyors. A large coal train called a "unit train" may be a kilometres (over a mile) long, containing 60 cars with 100 tons of coal in each one, for a total load of 6,000 tons. A large plant under full load requires at least one coal delivery this size every day. Plants may get as many as three to five trains a day, especially in "peak season", during the summer months when power consumption is high. A large thermal power plant such as the Badarpur Thermal Power Station, New Delhi stores several million tons of coal for use when there is no wagon supply.

Coal Handling Plant LayoutModern unloaders use rotary dump devices, which eliminate problems with coal freezing in bottom dump cars. The unloader includes a train positioner arm that pulls the entire train to position each car over a coal hopper. The dumper clamps an individual car against a platform that swivels the car upside down to dump the coal. Swivelling couplers enable the entire operation to occur while the cars are still coupled together. Unloading a unit train takes about three hours.

Shorter trains may use railcars with an "air-dump", which relies on air pressure from the engine plus a "hot shoe" on each car. This "hot shoe" when it comes into contact with a "hot rail" at the unloading trestle, shoots an electric charge through the air dump apparatus and causes the doors on the bottom of the car to open, dumping the coal through the opening in the trestle. Unloading one of these trains takes anywhere from an hour to an hour and a half. Older unloaders may still use manually operated bottom-dump rail cars and a "shaker" attached to dump the coal. Generating stations adjacent to a mine may receive coal by conveyor belt or massive diesel electric-drive trucks.

Coal is prepared for use by crushing the rough coal to pieces less than 2 inches (50 mm) in size. The coal is then transported from the storage yard to in-plant storage silos by rubberized conveyor belts at rates up to 4,000 tons/hour. In plants that burn pulverized coal, silos feed coal pulverisers (coal mill) that take the larger 2 inch pieces grind them into the consistency of face powder, classify them, and mixes them with primary combustion air which transports the coal to the furnace and preheats the coal to drive off excess moisture content. In plants that do not burn pulverized coal, the larger 2 inch pieces may be directly fed into the silos which then feed the cyclone burners, a specific kind of combust or that can efficiently burn larger pieces of fuel.

Run-Of-Mine (ROM) Coal

The coal delivered from the mine that reports to the Coal Handling Plant is called Run-of-mine, or ROM, coal. This is the raw material for the CHP, and consists of coal, rocks, middling’s, minerals and contamination. Contamination is usually introduced by the mining process and may include machine parts, used consumables and parts of ground engaging tools. ROM coal can have a large variability of moisture and maximum particle size.

Coal Handling

Coal needs to be stored at various stages of the preparation process, and conveyed around the CHP facilities. Coal handling is part of the larger field of bulk material handling, and is a complex and vital part of the CHP.

Stockpiles

Stockpiles provide surge capacity to various parts of the CHP. ROM coal is delivered with large variations in production rate of tonnes per hour (tph). A ROM stockpile is used to allow the wash plant to be fed coal at lower, constant rate.

Coal Handling Division of BTPS, New Delhi

A simple stockpile is formed by machinery dumping coal into a pile, either from dump trucks, pushed into heaps with bulldozers or from conveyor booms. More controlled stockpiles are formed using stackers to form piles along the length of a conveyor, and reclaimers to retrieve the coal when required for product loading, etc. Taller and wider stockpiles reduce the land area required to store a set tonnage of coal. Larger coal stockpiles have a reduced rate of heat lost, leading to a higher risk of spontaneous combustion.

Stacking

Travelling, lugging boom stackers that straddle a feed conveyor are commonly used to create coal stockpiles. Stackers are nominally rated in tph (tonnes per hour) for capacity and normally travel on a rail between stockpiles in the stockyard. A stacker can usually move in at least two directions typically: horizontally along the rail and vertically by luffing its boom. Luffing of the boom minimises dust by reducing the height that the coal needs to fall to the top of the stockpile. The boom is luffed upwards as the stockpile height grows.

Wagon Tripler at Badarpur Thermal Power Station, New Delhi

Some stackers are able to rotate by slewing the boom. This allows a single stacker to form two stockpiles, one on either side of the conveyor. Stackers are used to stack into different patterns, such as cone stacking and chevron stacking. Stacking in a single cone tends to cause size segregation, with coarser material moving out towards the base. Raw cone ply stacking is when additional cones are added next to the first cone. Chevron stacking is when the stacker travels along the length of the stockpile adding layer upon layer of material. Stackers and Reclaimers were originally manually controlled manned machines with no remote control. Modern

machines are typically semi-automatic or fully automated, with parameters remotely set.

Reclaiming

Tunnel conveyors can be fed by a continuous slot hopper or bunker beneath the stockpile to reclaim material. Front-end loaders and bulldozers can be used to push the coal into feeders. Sometimes front-end loaders are the only means of reclaiming coal from the stockpile. This has a low up-front capital cost, but much higher operating costs, measured in dollars per tonne handled.

Coal Storage Area of the Badarpur Thermal Power Station, New Delhi

High-capacity stockpiles are commonly reclaimed using bucket-wheel reclaimers. These can achieve very high rates.

Coal Sampling

Sampling of coal is an important part of the process control in the CHP. A grab sample is a one off sample of the coal at a point in the process stream, and tends not to be very representative. A routine sample is taken at a set frequency, either over a period of time or per shipment.

Screening

Screens are used to group process particles into ranges by size. These size ranges are also called grades. Dewatering screens are used to remove water from the product. Screens can be static, or mechanically vibrated. Screen decks can be made from different materials such as high tensile steel, stainless steel, or polyethylene.

Screening and Separation Unit of Coal Handling Division

Magnetic Separation

Magnetic separators shall be used in coal conveying systems to separate tramp iron (including steel) from the coal. Basically, two types are available. One type incorporates permanent or electromagnets into the head pulley of a belt conveyor. The tramp iron clings to the belt as it goes around the pulley drum and falls off into a collection hopper or trough after the point at which coal is charged from the belt. The other type consists of permanent or electromagnets incorporated into a belt conveyor that is suspended above a belt conveyor carrying coal. The tramp iron is pulled from the moving coal to the face of the separating conveyor, which in turn holds and carries the tramp iron to a collection hopper or trough. Magnetic separators shall be used just ahead of the coal crusher, if any, and/or just prior to coal discharge to the in-plant bunker or silo fill system.

Coal Crusher

Before the coal is sent to the plant it has to be ensured that the coal is of uniform size, and so it is passed through coal crushers. Also power plants using pulverized coal specify a maximum coal size that can be fed into the pulverizer and so the coal has to be crushed to the specified size using the coal crusher. Rotary crushers are very commonly used for this purpose as they can provide a continuous flow of coal to the pulverizer.

Pulverizer

Most commonly used pulverizer is the Boul Mill. The arrangement consists of 2 stationary rollers and a power driven boul in which pulverization takes place as the coal passes through the sides of the rollers and the boul. A primary air induced draught fan draws a stream of heated air through the mill carrying the pulverized coal into a stationary classifier at the top of the pulverizer. The classifier separates the pulverized coal from the unpulverized coal.

An external view of a Coal Pulverizer

Tangential Burners

The tangential burners are arranged such that they discharge the fuel air mixture tangentially to an imaginary circle in the center of the furnace. The swirling action produces sufficient turbulence in the furnace to complete the combustion in a short period of time and avoid the necessity of producing high turbulence at the burner itself. High heat release rates are possible with this method of firing. The burners are placed at the four corners of the furnace. At the Badarpur Thermal Power Station five sets of such burners are placed one above the other to form six firing zones. These burners are constructed with tips that can be angled through a small vertical arc. By adjusting the angle of the burners the position of the fire ball can be adjusted so as to raise or lower the position of the turbulent combustion region. When the burners are tilted downward the furnace gets filled completely with the flame and the furnace exit gas temperature gets reduced. When the burners are tiled upward the furnace exit gas temperature increases. A difference of100 degrees can be achieved by tilting the burners.

Ash Handling

The ever increasing capacities of boiler units together with their ability to use low grade high ash content coal have been responsible for the development of modern day ash handling systems. The widely used ash handling systems are1. Mechanical Handling System2. Hydraulic System3. Pneumatic System4. Steam Jet SystemThe Hydraulic Ash handling system is used at the Badarpur Thermal Power Station.

Hydraulic Ash Handling System

The hydraulic system carried the ash with the flow of water with high velocity through a channel and finally dumps into a sump. The hydraulic system is divided into a low velocity and high velocity system. In the low velocity system the ash from the boilers falls into a stream of water flowing into the sump. The ash is carried along with the water and they are separated at the sump. In the high velocity system a jet of water is sprayed to quench the hot ash. Two other jets force the ash into a trough in which they are washed away by the water into the sump, where they are separated. The molten slag formed in the pulverized fuel system can also be quenched and washed by using the high velocity system. The advantages of this system are that its clean, large ash handling capacity, considerable distance can be traversed, absence of working parts in contact with ash.

ELECTRIC MOTORS

An electric motor uses electrical energy to produce mechanical energy. The reverse process that of using mechanical energy to produce electrical energy is accomplished by a generator or dynamo. Traction motors used on locomotives and some electric and hybrid automobiles often performs both tasks if the vehicle is equipped with dynamic brakes.

A High Power Electric Motor

Categorization of Electric Motors

The classic division of electric motors has been that of Direct Current (DC) type vs Alternating Current (AC) types. The ongoing trend toward electronic control further muddles the distinction, as modern drivers have moved the commutator out of the motor shell. For this new breed of motor, driver circuits are relied upon to generate sinusoidal AC drive currents, or some approximation of. The two best examples are: the brushless DC motor and the stepping motor, both being

polyphase AC motors requiring external electronic control. There is a clearer distinction between a synchronous motor and asynchronous types. In the synchronous types, the rotor rotates in synchrony with the oscillating field or current (eg. permanent magnet motors). In contrast, an asynchronous motor is designed to slip; the most ubiquitous example being the common AC induction motor which must slip in order to generate torque.

Comparison of Motor Types

At Badarpur Thermal Power Station, New Delhi, mostly AC motors are employed for various purposes. We had to study the two types of AC Motors viz. Synchronous Motors and Induction Motor. The motors have been explained further.

AC Motor

Internal View of AC Motors

An AC motor is an electric motor that is driven by an alternating current. It consists of two basic parts, an outside stationary stator having coils supplied with AC current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the rotating field.There are two types of AC motors, depending on the type of rotor used. The first is the synchronous motor, which rotates exactly at the supply frequency or a sub multiple of the supply frequency. The magnetic field on the rotor is either generated by current delivered through slip rings or a by a permanent magnet.The second type is the induction motor, which turns slightly slower than the supply frequency. The magnetic field on the rotor of this motor is created by an induced current.

Synchronous Motor

A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the alternating current and resulting magnetic field which drives it. Another way of saying this is that it has zero slip under usual operating conditions. Contrast this with an induction motor, which must slip in order to produce torque.Sometimes a synchronous motor is used, not to drive a load, but to improve the power factor on the local grid it's connected to. It does this by providing reactive power to or consuming reactive power from the grid. In this case the synchronous motor is called a Synchronous condenser. Electrical power plants almost always use synchronous generators because it's very important to keep the frequency constant at which the generator is connected.

Advantages

Synchronous motors have the following advantages over non-synchronous motors:

• Speed is independent of the load, provided an adequate field current is applied.• Accurate control in speed and position using open loop controls, eg. Stepper motors.• They will hold their position when a DC current is applied to both the stator and the rotor windings.• Their power factor can be adjusted to unity by using a proper field current relative to the load. Also, a "capacitive" power factor, (current phase leads voltage phase), can be obtained by increasing this current slightly, which can help achieve a better power factor correction for the whole installation.• Their construction allows for increased electrical efficiency when a low speed is required (as in ball mills and similar apparatus).

Examples:

• Brushless permanent magnet DC motor.• Stepper motor.• Slow speed AC synchronous motor.• Switched reluctance motor.

Induction Motor

An induction motor (IM) is a type of asynchronous AC motor where power is supplied to the rotating device by means of electromagnetic induction.

Three Phase Induction MotorsAn electric motor converts electrical power to mechanical power in its rotor (rotating part). There are several ways to supply power to the rotor. In a DC motor this power is supplied to the armature directly from a DC source, while in an AC motor this power is induced in the rotating device. An induction motor is sometimes called a rotating transformer because the stator(stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side. Induction motors are widely used, especially polyphase induction motors, which are frequently used in industrial drives. Induction motors are now the preferred choice for industrial motors due to their rugged construction, lack of brushes (which are needed in most DC Motors) and — thanks to modern power electronics — the ability to control the speed of the motor.

Construction

The stator consists of wound 'poles' that carry the supply current that induces a magnetic field in the conductor. The number of 'poles' can vary between motor types but the poles are always in pairs (i.e. 2, 4, 6 etc). There are two types of rotor:1. Squirrel-cage rotor2. Slip ring rotorThe most common rotor is a squirrel-cage rotor. It is made up of bars of either solid copper (most common) or aluminum that span the length of the rotor, and are connected through a ring at each end. The rotor bars in squirrel-cage induction motors are not straight, but have some skew to reduce noise and harmonics.The motor's phase type is one of two types:1. Single-phase induction motor2. 3-phase induction motor

Principle of Operation

The basic difference between an induction motor and a synchronous AC motor is that in the latter a current is supplied onto the rotor. This then creates a magnetic field which, through magnetic interaction, links to the rotating magnetic field in the stator which in turn causes the rotor to turn. It is called synchronous because at steady state the speed of the rotor is the same as the speed of the rotating magnetic field in the stator. By way of contrast, the induction motor does not have any direct supply onto the rotor; instead, a secondary current is induced in the rotor. To achieve this, stator windings are arranged around the rotor so that when energised with a polyphase supply they create a rotating magnetic field pattern which sweeps past the rotor. This changing magnetic field pattern can induce currents in the rotor conductors. These currents interact with the rotating magnetic field created by the stator and the rotor will turn. However, for these currents to be induced, the speed of the physical rotor and the speed of the rotating magnetic field in the stator must be different, or else the magnetic field will not be moving relative to the rotor conductors and no currents will be induced. If by some chance this happens, the rotor typically slows slightly until a current is re-induced and then the rotor continues as before. This difference between the speed of the rotor and speed of the rotating magnetic field in the stator is called slip. It has no unit and the ratio between the relative speeds of the magnetic field as seen by the rotor to the speed of the rotating field. Due to this an induction motor is sometimes referred to as an asynchronous machine.

SWITCHGEAR

The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream.The very earliest central power stations used simple open knife switches, mounted on insulating panels of marble or asbestos. Power levels and voltages rapidly escalated, making open manually-operated switches too dangerous to use for anything other than isolation of a deenergized circuit. Oil-filled equipment allowed arc energy to be contained and safely controlled. By the early 20th century, a switchgear line-up would be a metal-enclosed structure with electrically-operated switching elements, using oil circuit breakers. Today, oil-filled equipment has largely been replaced by air-blast, vacuum, or SF6 equipment, allowing large currents and power levels to be safely controlled by automatic equipment incorporating digital controls, protection, metering and communications.

A View of Switchgear at a Power PlantTypes

A piece of switchgear may be a simple open air isolator switch or it may be insulated by some other substance. An effective although more costly form of switchgear is "gas insulated switchgear" (GIS), where the conductors and contacts are insulated by pressurized (SF6) sulphur hexafluoride gas. Other common types are oil [or vacuum] insulated switchgear. Circuit breakers are a special type of switchgear that are able to interrupt fault currents. Their construction allows them to interrupt fault currents of many hundreds or thousands of amps. The quenching of the arc when the contacts open requires careful design, and falls into four types:

Oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through the arc.

Gas (SF6) circuit breakers sometimes stretch the arc using a magnetic field, and then rely upon the dielectric strength of the SF6 to quench the stretched arc.

Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the contact material), so the arc quenches when it is stretched a very small amount (<2-3 mm). Vacuum circuit breakers are frequently used in modern medium-voltage switchgear to 35,000 volts.

Air circuit breakers may use compressed air to blow out the arc, or alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing out the arc.

Circuit breakers are usually able to terminate all current flow very quickly: typically between 30 ms and 150 ms depending upon the age and construction of the device.

Classification

Several different classifications of switchgear can be made:

By the current rating:

By interrupting rating (maximum short circuit current that the device can safely interrupt)

Circuit breakers can open and close on fault currents Load-break/Load-make switches can switch normal system load currents Isolators may only be operated while the circuit is dead, or the load current

is very small.By voltage class:

Low Tension (less than 440 volts AC) High Tension (more than 6.6 kV AC)

By insulating medium:

Air Gas (SF6 or mixtures) Oil Vacuum

By construction type:

Indoor Outdoor Industrial Utility Marine Draw-out elements (removable without many tools) Fixed elements (bolted fasteners) Live-front Dead-front Metal-enclosed Metal-clad Metal enclose & Metal clad Arc-resistant

High Tension Switchgear at Thermal Power Plant

By IEC degree of internal separation:

No Separation Bus bars separated from functional units Terminals for external conductors separated from bus bars Terminals for external conductors separated from functional units but not

from each other Functional units separated from each other Terminals for external conductors separated from each other

Terminals for external conductors separate from their associated functional unit

By interrupting device:

Fuses Air Blast Circuit Breaker Minimum Oil Circuit Breaker Oil Circuit Breaker Vacuum Circuit Breaker Gas (SF6) Circuit breaker

By operating method:

Manually-operated Motor-operated Solenoid/stored energy operated

By type of current:

Alternating current Direct current

By application:

Distribution. Transmission system

A single line-up may incorporate several different types of devices, for example, air-insulated bus, vacuum circuit breakers, and manually-operated switches may all exist in the same row of cubicles.Ratings, design, specifications and details of switchgear are set by a multitude of standards. In North America mostly IEEE and ANSI standards are used, much of the rest of the world uses IEC standards, sometimes with local national derivatives or variations.

Functions

One of the basic functions of switchgear is protection, which is interruption of short-circuit and overload fault currents while maintaining service to unaffected circuits. Switchgear also provides isolation of circuits from power supplies. Switchgear also is used to enhance system availability by allowing more than one source to feed a load.

Safety

To help ensure safe operation sequences of switchgear, trapped key interlocking provides predefined scenarios of operation. James Harry Castell invented this technique in 1922. For example, if only one of two sources of supply is permitted to be connected at a given time, the interlock scheme may require that the first switch must be opened to release a key that will allow closing the second switch. Complex schemes are possible.

HIGH TENSION SWITCHGEAR

High voltage switchgear is any switchgear and switchgear assembly of rated voltage higher than 1000 volts. High voltage switchgear is any switchgear used to connect or to disconnect a part of a high voltage power system.These switchgears are essential elements for the protection and for a safety operating mode without interruption of a high voltage power system. This type of equipment is really important because it is directly linked to the quality of the electricity supply. The high voltage is a voltage above 1000 V for alternating current and above 1500 V for direct current.

High Tension Switchgear of a Thermal Power Plant

The high voltage switchgear was invented at the end of the 19th century for operating the motors and others electric machines. It has been improved and it can be used in the whole range of high voltage until 1100 kV.

Functional Classification

Disconnectors and Earthing Switches

They are above all safety devices used to open or to close a circuit when there is no current through them. They are used to isolate a part of a circuit, a machine, a part of an overhead-line or an underground line for the operating staff to access it without any danger. The opening of the line isolator or busbar section isolator is necessary for the safety but it is not enough. Grounding must be done at the upstream sector and the downstream sector on the device which they want to intervene thanks to the earthing switches. In principle, disconnecting switches do not have to interrupt currents, but some of them can interrupt currents (up to 1600 A under 10 to 300V) and some earthing switches must interrupt induced currents which are generated in a non-current-carrying line by inductive and capacitive coupling with nearby lines (up to 160 A under 20 kV).

A Vacuum Circuit Breaker (High Tension Switchgear)

High-Current Switching Mechanism

They can open or close a circuit in normal load. Some of them can be used as a disconnecting switch. But if they can create a short-circuit current, they can not interrupt it.

Contactor

Their functions are similar to the high-current switching mechanism, but they can be used at higher rates. They have a high electrical endurance and a high mechanical endurance. Contactors are used to frequently operate device like electric furnaces, high voltage motors. They cannot be used as a disconnecting switch. They are used only in the band 30 kV to 100 kV.

Fuses

The fuses can interrupt automatically a circuit with an over current flowing in it for a fixed time. The current interrupting is got by the fusion of an electrical conductor which is graded. They are mainly used to protect against the short-circuits. They limit the peak value of the fault current. In three-phase electric power, they only eliminate the phases where the fault current is flowing, which is a risk for the devices and the people. Against this trouble, the fuses can be associated with high-current switches or contactors.They are used only in the band 30 kV to 100 kV.

Circuit Breaker

A high voltage circuit breaker is capable of making, carrying and breaking currents under the rated voltage (the maximal voltage of the power system which it is protecting): Under normal circuit conditions, for example to connect or disconnect a line in a power system; Underspecified abnormal circuit conditions especially to eliminate a short circuit. From its characteristics, a circuit breaker is the protection device essential for a high voltage power system, because it is the only one able to interrupt a short circuit current and so to avoid the others devices to be damaged by this short circuit.

EMD – II

In EMD-II went through following 2 sectors:

• Generator

• Transformer

GENERATORS

The basic function of the generator is to convert mechanical power, delivered from the shaft of the turbine, into electrical power. Therefore a generator is actually a rotating mechanical energy converter. The mechanical energy from the turbine is converted by means of a rotating magnetic field produced by direct current in the copper winding of the rotor or field, which generates three-phase alternating currents and voltages in the copper winding of the stator (armature). The stator winding is connected to terminals, which are in turn connected to the power system for delivery of the output power to the system.

A 210 MW Turbine Generator at BTPS, New Delhi

The class of generator under consideration is steam turbine-driven generators, commonly called turbo generators. These machines are generally used in nuclear and fossil fuelled power plants, co-generation plants, and combustion turbine units. They range from relatively small machines of a few Megawatts (MW) to very large generators with ratings up to 1900 MW. The generators particular to this category are of the two- and four-pole design employing round-rotors, with rotational operating speeds of 3600 and 1800 rpm in North America, parts of Japan, and Asia (3000 and 1500 rpm in Europe, Africa, Australia, Asia, and South America). At Badarpur Thermal Power Station 3000 rpm, 50 Hz generators are used of capacities 210 MW and 95 MW. As the system load demands more active power from the generator, more steam (or fuel in a combustion turbine) needs to be admitted to the turbine to increase power output. Hence more energy is transmitted to the generator from the turbine, in the form of a torque. This torque is mechanical in nature, but electromagnetically coupled to the power system through the generator. The higher the power output, the higher the torque between turbine and generator. The power output of the generator generally follows the load demand from the system. Therefore the voltages and currents in the generator are continually changing based on the load demand. The generator design must be able to cope with large and fast load changes, which show up inside the machine as

changes in mechanical forces and temperatures. The design must therefore incorporate electrical current-carrying materials (i.e., copper), magnetic flux-carrying materials (i.e., highly permeable steels), insulating materials (i.e., organic), structural members (i.e., steel and organic), and cooling media (i.e., gases and liquids), all working together under the operating conditions of a turbo generator.

An open Electric Generator at Power Plant

Since the turbo generator is a synchronous machine, it operates at one very specific speed to produce a constant system frequency of 50 Hz, depending on the frequency of the grid to which it is connected. As a synchronous machine, a turbine generator employs a steady magnetic flux passing radially across an air gap that exists between the rotor and the stator. (The term “air gap” is commonly used for air- and gas-cooled machines). For the machines in this discussion, this means a magnetic flux distribution of two or four poles on the rotor. This flux pattern rotates with the rotor, as it spins at its synchronous speed. The rotating magnetic field moves past a three-phase symmetrically distributed winding installed in the stator core, generating an alternating voltage in the stator winding. The voltage waveform created in each of the three phases of the stator winding is very nearly sinusoidal. The output of the stator winding is the three-phase power, delivered to the power system at the voltage generated in the stator winding.In addition to the normal flux distribution in the main body of the generator, there are stray fluxes at the extreme ends of the generator that create fringing flux patterns and induce stray losses in the generator. The stray fluxes must be accounted for in the overall design. Generators are made up of two basic members,

the stator and the rotor, but the stator and rotor are each constructed from numerous parts themselves. Rotors are the high-speed rotating member of the two, and they undergo severe dynamic mechanical loading as well as the electromagnetic and thermal loads. The most critical component in the generator is the retaining rings, mounted on the rotor.These components are very carefully designed for high-stress operation. The stator is stationary, as the term suggests, but it also sees significant dynamic forces in terms of vibration and torsional loads, as well as the electromagnetic, thermal, and high-voltage loading. The most critical component of the stator is arguably the stator winding because it is a very high cost item and it must be designed to handle all of the harsh effects described above. Most stator problems occur with the winding.

STATOR

The stator winding is made up of insulated copper conductor bars that are distributed around the inside diameter of the stator core, commonly called the stator bore, in equally spaced slots in the core to ensure symmetrical flux linkage with the field produced by the rotor. Each slot contains two conductor bars, one on top of the other. These are generally referred to as top and bottom bars. Top bars are the ones nearest the slot opening (just under the wedge) and the bottom bars are the ones at the slot bottom. The core area between slots is generally called a core tooth.

Stator of a Turbo Generator

The stator winding is then divided into three phases, which are almost always wye connected. Wye connection is done to allow a neural grounding point and for relay protection of the winding. The three phases are connected to create symmetry between them in the 360 degree arc of the stator bore. The distribution of the winding is done in such a way as to produce a 120degree difference in voltage peaks from one phase to the other, hence the term “three-phase voltage.” Each of the three phases may have one or more parallel circuits within the phase. The parallels can be connected in series or parallel, or a combination of both if it is a four-pole generator. This will be discussed in the next section. The parallels in all of the phases are essentially equal on average, in their performance in the machine. Therefore, they each “see” equal voltage and current, magnitudes and phase angles, when averaged over one alternating cycle. The stator bars in any particular phase group are arranged such that there are parallel paths, which overlap between top and bottom bars. The overlap is staggered between top and bottom bars. The top bars on one side of the stator bore are connected to the bottom bars on the other side of the bore in one direction while the bottom bars are connected in the other direction on the opposite side of the stator. This connection with the bars on the other side of the stator creates a “reach” or “pitch” of a certain number of slots. The pitch is therefore the number slots that the stator bars have to reach in the stator bore arc, separating the two bars to be connected. This is always less than 180 degrees. Once connected, the stator bars form a single coil or turn. The total width of the overlapping parallels is called the “breadth.” The combination of the pitch and breadth create a “winding or distribution factor.” The distribution factor is used to minimize the harmonic content of the generated voltage. In the case of a two parallel path winding, these may be connected in series or parallel outside the stator bore, at the termination end of the generator. The connection type will depend on a number of other design issues regarding current-carrying ability of the copper in the winding. In a two-parallel path, three-phase winding, alternating voltage is created by the action of the rotor field as it moves past these windings. Since there is a plus and minus, or north and south, to the rotating magnetic field, opposite polarity currents flow on each side of the stator bore in the distributed winding. The currents normally flowing in large turbo generators can be in the order of thousands of amperes. Due to the very high currents, the conductor bars in a turbo generator have a large cross-sectional area. In addition they are usually one single turn per bar, as opposed to motors or small generators that have multiple turn bars or coils. These stator or conductor bars are also very rigid and do not bend unless significant force is exerted on them.

ROTOR

The rotor winding is installed in the slots machined in the forging main body and is distributed symmetrically around the rotor between the poles. The winding itself is made up of many turns of copper to form the entire series connected winding. All of the turns associated with a single slot are generally called a coil. The coils are wound into the winding slots in the forging, concentrically in corresponding positions on opposite sides of a pole. The series connection essentially creates a single multi-turn coil overall, that develops the total ampere-turns of the rotor (which is the total current flowing in the rotor winding times the total number of turns). There are numerous copper-winding designs employed in generator rotors, but all rotor windings function basically in the same way. They are configured differently for different methods of heat removal during operation. In addition almost all large turbo generators have directly cooled copper windings by air or hydrogen cooling gas.

Rotor of a Turbo Generator

Cooling passages are provided within the conductors themselves to eliminate the temperature drop across the ground insulation and preserve the life of the

insulation material. In an “axially” cooled winding, the gas passes through axial passages in the conductors, being fed from both ends, and exhausted to the air gap at the axial center of the rotor. In other designs, “radial” passages in the stack of conductors are fed from sub slots machined along the length of the rotor at the bottom of each slot. In the “air gap pickup” method, the cooling gas is picked up from the air gap, and cooling is accomplished over a relatively short length of the rotor, and then discharged back to the air gap. The cooling of the end-regions of the winding varies from design to design, as much as that of the slot section. In smaller turbine generators the indirect cooling method is used (similar to indirectly cooled stator windings), where the heat is removed by conduction through the ground insulation to the rotor body. The winding is held in place in the slots by wedges, in a similar manner as the stator windings. The difference is that the rotor winding loading on the wedges is far greater due to centrifugal forces at speed. The wedges therefore are subjected to a tremendous static load from these forcesand bending stresses because of the rotation effects. The wedges in the rotor are not generally a tight fit in order to accommodate the axial thermal expansion of the rotor winding during operation. There are also many available designs and configurations for the end-winding construction andventilation methods. As in the rotor slots, the copper turns in the end-winding must be isolated from one another so that they do not touch and create shorts between turns. Therefore packing and blocking are used to keep the coils separated, and in their relative position as the rotor winding expands from thermal effects during operation. To restrain the end winding portion of the rotor winding during high-speed operation, retaining-rings are employed to keep the copper coils in place.

BEARINGS

All turbo generators require bearings to rotate freely with minimal friction and vibration. The main rotor body must be supported by a bearing at each end of the generator for this purpose. In some cases where the rotor shaft is very long at the excitation end of the machine to accommodate the slip/collector rings, a “steady” bearing is installed outboard of the slipcollector rings. This ensures that the excitation end of the rotor shaft does not create a wobble that transmits through the shaft and stimulates excessive vibration in the overall generator rotor or the turbo generator line. There are generally two common types of bearings employed in large generators, journal” and “tilting pad” bearings. Journal bearings are the most common. Both require lubricating and jacking oil systems, which will be discussed later in the book, under auxiliary systems. When installing the bearings, they must

be aligned in terms of height and angle to ensure that the rotor “sits” in the bearing correctly. Such things as shaft “catinery” must be considered and“pre-loading” or “shimming” of the bearings to account for the difference when the rotor is at standstill and at speed. Getting any of these things wrong in the assembly can cause the rotor to vibrate excessively and damage either the rotor shaft or the bearing itself. Generally, a “wipe” of the bearing running surface or “babbitt” results.

AUXILIARY SYSTEMS

All large generators require auxiliary systems to handle such things as lubricating oil for the rotor bearings, hydrogen cooling apparatus, hydrogen sealing oil, de-mineralized water for stator winding cooling, and excitation systems for field-current application. Not all generators require all these systems and the requirement depends on the size and nature of the machine. For instance, air cooled turbo generators do not require hydrogen for cooling and therefore no sealing oil as well. On the other hand, large generators with high outputs, generally above 400 MVA, have water-cooled stator windings, hydrogen for cooling the stator core and rotor, seal oil to contain the hydrogen cooling gas under high pressure, lubricating oil for the bearings, and of course, an excitation system for field current. There are five major auxiliary systems that may be used in a generator. They are given as follows:

1. Lubricating Oil System2. Hydrogen Cooling System3. Seal Oil System4. Stator Cooling Water System5. Excitation System

Each system has numerous variations to accommodate the hundreds of different generator configurations that may be found in operation. But regardless of the

generator design and which variation of a system is in use, they all individually have the same basic function as described before.

TRANSFORMER

A transformer is a static device consisting of a winding, or two or more coupled windings, with or without a magnetic core, for inducing mutual coupling between circuits. When an alternating current flows in a conductor, a magnetic field exists around the conductor. If another conductor is placed in the field created by the first conductor such that the flux lines link the second conductor, then a voltage is induced into the second conductor. The use of a magnetic field from one coil to induce a voltage into a second coil is the principle on which transformer theory andapplication is based.

A 220 kV Transformer at Power Plant

ANSI/IEEE defines a transformer as a static electrical device, involving no continuously moving parts, used in electric power systems to transfer power between circuits through the use of electromagnetic induction. The transformer is one of the most reliable pieces of electrical distribution equipment. It has nomoving parts, requires minimal maintenance, and is capable of withstanding overloads, surges, faults, and physical abuse that may damage or destroy other items in the circuit. Often, the electrical event that burns up a motor, opens a circuit breaker, or blows a fuse has a subtle effect on the transformer. Although the transformer may continue to operate as before, repeat occurrences of such damaging electrical events, or lack of even minimal maintenance can greatly accelerate the eventual failure of the transformer. The fact that a transformer continues to operate satisfactorily in spite of neglect and abuse is a testament to its durability. However, this durability is no excuse for not providing the proper care. Most of the effects of aging, faults, or abuse can be detected and corrected by acomprehensive maintenance, inspection, and testing program. Transformers are exclusively used in electric power systems to transfer power by electromagnetic induction between circuits at the same frequency, usually with changed valuesof voltage and current. There are numerous types of transformers used in various applications including audio, radio, instrument, and power. In Badarpur Thermal Power Station, we deal exclusively with power transformer applications involving

the transmission and distribution of electrical power. Power transformers are used extensively by traditional electric utility companies, power plants, and industrial plants. The term power transformer is used to refer to those transformers used between the generator and the distribution circuits, and these are usually rated at 220 kVA and above. Power systems typically consist of a large number of generation locations, distribution points, and interconnections within the system or with nearby systems, such as a neighbouring utility. The complexity of the system leads to a variety of transmission and distribution voltages. Power transformers must be used at each of these points where there is a transition between voltage levels. Power transformers are selected based on the application, with the emphasis toward custom design being more apparent the larger the unit. Power transformers are available for step-up operation, primarily used at the generator and referred to as generator step-up (GSU) transformers, and for step-down operation, mainly used to feed distribution circuits. Power transformers are available as single-phase or three-phase apparatus.

A Power Transformer at a Thermal Power Plant

CONSTRUCTION

A power transformer is a device that changes (transforms) an alternating voltage and current from one level to another. Power transformers are used to “step up” (transform) the voltages that are produced at generation to levels that are suitable for transmission (higher voltage, lower current). Conversely, a transformer is used to “step down” (transform) the higher transmission voltages to levels that are suitable for use at various facilities (lower voltage, higher current). Electric power can undergo numerous transformations between the source and the final end use point.

• Voltages must be stepped-up for transmission. Every conductor, no matter how large, will lose an appreciable amount of power (watts) to its resistance (R) when a

current (T) passes through it. This loss is expressed as a function of the applied current (P=I2R). Because this loss is dependent on the current, and since the power to be transmitted is a function of the applied volts (E) times the amps (P=IE), significant savings can be obtained by stepping the voltage up to a higher voltage level, with the corresponding reduction of the current value. Whether 100 amps is to be transmitted at 100 volts (P=IE, 100 amps X 100 volts = 10,000 watts) or 10 amps is to be transmitted at 1,000 volts (P=IE, 10 amps X 1,000 volts = 10,000 watts) the same 10,000 watts will be applied to the beginning of the transmission line.

• If the transmission distance is long enough to produce 0.1 ohm of resistance across the transmission cable, P=I2R, (100 amp)2 X 0.1 ohm = 1,000 watts will be lost across the transmission line at the 100 volt transmission level. The 1000 volts transmission level will create a loss of P=I2R, (10 amp)2 X 0.1 ohm = 10 watts. This is where transformers play an important role.

• Although power can be transmitted more efficiently at higher voltage levels, sometimes as high as 500 or 750 thousand volts (kV), the devices and networks at the point of utilization are rarely capable of handling voltages above 32,000 volts. Voltage must be “stepped down” to be utilized by the various devices available. By adjusting the voltages to the levels necessary for the various end use and distribution levels, electric power can be used both efficiently and safely.

• All power transformers have three basic parts, a primary winding, secondary winding, and a core. Even though little more than an air space is necessary to insulate an “ideal” transformer, when higher voltages and larger amounts of power are involved, the insulating material becomes an integral part of the transformer’s operation. Because of this, the insulation system is often considered the fourth basic part of the transformer. It is important to note that, although the windings and core deteriorate very little with age, the insulation can be subjected to severe stresses and chemical deterioration. The insulation deteriorates at a relatively rapid rate, and its condition ultimately determines the service life of the transformer.

Core

The core, which provides the magnetic path to channel the flux, consists of thin strips of high grade steel, called laminations, which are electrically separated by a thin coating of insulating material. The strips can be stacked or wound, with the windings either built integrally around the core or built separately and assembled

around the core sections. Core steel can be hot- or cold rolled, grain-oriented or non grain oriented, and even laser-scribed for additional performance. Thickness ranges from 0.23 mm to upwards of 0.36 mm. The core cross section can be circular or rectangular, with circular cores commonly referred to as cruciform construction. Rectangular cores are used for smaller ratings and as auxiliary transformers used within a power transformer. Rectangular cores use a single width of strip steel, while circular cores use a combination of different strip widths to approximate a circular cross-section. The type of steel and arrangement depends on the transformer rating as related to cost factors such as labour and performance.Just like other components in the transformer, the heat generated by the core must be adequately dissipated. While the steel and coating may be capable of withstanding higher temperatures, it will come in contact with insulating materials with limited temperature capabilities. In larger units, cooling ducts are used inside the core for additional convective surface area, and sections of laminations may be split to reduce localized losses. The core is held together by, but insulated from, mechanical structures and is grounded to a single point in order to dissipate electrostatic buildup. The core ground location is usually some readily accessible point inside the tank, but it can also be brought through a bushing on the tankwall or top for external access. This grounding point should be removable for testing purposes, such as checking for unintentional core grounds. Multiple core grounds, such as a case whereby the core is inadvertently making contact with otherwise grounded internal metallic mechanical structures, can provide a path for circulating currents induced by the main flux as well as a leakage flux, thus creating concentrations of losses that can result in localized heating. The maximum flux density of the core steel is normally designed as close to the knee of thesaturation curve as practical, accounting for required over excitations and tolerances that exist due to materials and manufacturing processes. For power transformers the flux density is typically between 1.3 T and 1.8 T, with the saturation point for magnetic steel being around 2.03 T to 2.05 T.There are two basic types of core construction used in power transformers: core form and shell form.

• In core-form construction, there is a single path for the magnetic circuit. For single-phase applications, the windings are typically divided on both core legs as shown. In three-phase applications, the windings of a particular phase are typically on the same core leg. Windings are constructed separate of the core and placed on their respective core legs during core assembly.

Schematic Diagram of Shell-form Construction• In shell-form construction, the core provides multiple paths for the magnetic circuit. The core is typically stacked directly around the windings, which are usually “pancake”-type windings, although some applications are such that the core and windings are assembled similar to core form. Due to advantages in short-circuit and transient-voltage performance, shell forms tend to be used more frequently in the largest transformers, where conditions can be more severe. Variations of three-phase shell-form construction include five and sevenlegged cores, depending on size and application.

Schematic Diagram of Shell-form Construction

Windings

The windings consist of the current-carrying conductors wound around the sections of the core, and these must be properly insulated, supported, and cooled to withstand operational and test conditions. Copper and aluminum are the primary materials used as conductors in power-transformer windings. While aluminum is lighter and generally less expensive than copper, a larger cross section of Al conductor must be used to carry a current with similar performance as copper. Copper has higher mechanical strength and is used almost exclusively in all but the smaller size ranges, where aluminum conductors may be perfectly acceptable. In cases where extreme forces are encountered, materials such as silver-bearing copper can be used for even greater strength. The conductors used in power transformers are typically stranded with a rectangular cross section, although some transformers at the lowest ratings may use sheet or foil conductors. Multiple strands can be wound in parallel and joined together at the ends of the winding, in which case it is necessary to transpose the strands at various points throughout the winding to prevent circulating currents around the loop(s) created by joining the strands at the ends. Individual strands may be subjected to differences in the flux

field due to their respective positions within the winding, which create differences in voltages between the strands and drive circulating currents through the conductor loops. Proper transposition of the strands cancels out these voltage differences and eliminates or greatly reduces the circulating currents. A variation of this technique, involving many rectangular conductor strands combined into a cable, is called continuously transposed cable (CTC).A view of Pancake Winding In core-form transformers, the windings are usually arranged concentrically around the core leg, which shows a winding being lowered over another winding already on the core leg of a three phase transformer. Shell-form transformers use a similar concentric arrangement or an interleaved arrangement. With an interleaved arrangement, individual coils are stacked, separated by insulating barriers and cooling ducts. The coils are typically connected with the inside of one coil connected to the inside of an adjacent coil and, similarly, the outside of one coil connected to the outside of an adjacent coil. Sets of coils are assembled into groups, which then form the primary or secondary winding. When considering concentric windings, it is generally understood that circular windings have inherently higher mechanical strength than rectangular windings, whereas rectangular coils can have lower associated material and labour costs. Rectangular windings permit a more efficient use of space, but their use is limited to small power transformers and the lower range of medium power transformers, where the internal forces are not extremely high. As the rating increases, the forces significantly increase, and there is need for added strength in the windings, so circular coils, or shell-form construction, is used. In some special cases, elliptically shaped windings are used. Concentric coils are typically wound over cylinders with spacers attached so as to form a duct between the conductors and the cylinder. As previously mentioned, the flow of liquid through the windings can be based solely on natural convection, or the flow can be somewhat controlled through the use of strategically placed barriers within the winding. This concept is sometimes referred to as guided liquid flow. A variety of different types of windings have been used in power transformers through the years. Coils can be wound in an upright, vertical orientation, as is necessary with larger, heavier coils;or they can be wound horizontally and placed upright upon completion. As mentioned previously, the type of winding depends on the transformer rating as well as the core construction. Several of the more common winding types are discussed further.

Taps-Turns Ratio Adjustment

The ability to adjust the turn’s ratio of a transformer is often desirable to compensate for variations in voltage that occur due to the regulation of the transformer and loading cycles. This task can be accomplished by several means. There is a significant difference between a transformer that is capable of changing the ratio while the unit is on-line (a load tap changing [LTC] transformer) and one that must be taken off-line, or de-energized, to perform a tap change. Most transformers are provided with a means of changing the number of turns in the high voltage circuit, whereby a part of the winding is tapped out of the circuit. In many transformers, this is done using one of the main windings and tapping out a section or sections. With larger units, a dedicated tap winding may be necessary to avoid the ampere-turn voids that occur along the length of the winding. Use and placement of tap windings vary with the application and among manufacturers. A manually operated switching mechanism, a DETC (deenergized tap changer), is normally provided for convenient access external to the transformer to change the tap position. When LTC capabilities are desired, additional windings and equipment are required, which significantly increase the size and cost of the transformer. This option is specified on about 60% of new medium and large power transformers. It should be recognized that there would be slight differences in this schematic based on the specific LTC being used. It is also possible for a transformer to have dual voltage ratings, as is popular in spare and mobile transformers. While there is no physical limit to the ratio between the dual ratings, even ratios (for example 24.94 X 12.47 kV or 138 X 69 kV) are easier for manufacturers to accommodate.

MAINTENANCE AND TESTING

Heat and contamination are the two greatest enemies to the transformer’s operation. Heat will break down the solid insulation and accelerate the chemical reactions that take place when the oil is contaminated. All transformers require a cooling method and it is important to ensure that the transformer has proper cooling. Proper cooling usually involves cleaning the cooling surfaces, maximizing ventilation, and monitoring loads to ensure the transformer is not producing excess heat. • Contamination is detrimental to the transformer, both inside and out. The importance of basic cleanliness and general housekeeping becomes evident when long term service life is considered. Dirt builds up and grease deposits severely limit the cooling abilities of radiators and tank surfaces. Terminal and insulation surfaces are especially susceptible to dirt and grease build up. Such buildup will usually affect test results. The transformer’s general condition should be noted

during any activity, and every effort should be made to maintain its integrity during all operations.

• The oil in the transformer should be kept as pure as possible. Dirt and moisture will start chemical reactions in the oil that lower both its electrical strength and its cooling capability. Contamination should be the primary concern any time the transformer must be opened. Most transformer oil is contaminated to some degree before it leaves the refinery. It is important to determine how contaminated the oil is and how fast it is degenerating. Determining the degree of contamination is accomplished by sampling and analyzing the oil on a regular basis.

• Although maintenance and work practices are designed to extend the transformer’s life, it is inevitable that the transformer will eventually deteriorate to the point that it fails or must be replaced. Transformer testing allows this aging process to be quantified and tracked, to help predict replacement intervals and avoid failures. Historical test data is valuable for determining damage to the transformer after a fault or failure has occurred elsewhere in the circuit. By comparing test data taken after the fault to previous test data, damage to the transformer can be determined.

SAFETY

Safety is of primary concern when working around a transformer. The substation transformer is usually the highest voltage item in a facility’s electrical distribution system. The higher voltages found at the transformer deserve the respect and complete attention of anyone working in the area. A 6.6 kV system will arc to ground over 1.5 to 2.5 in. However, to extinguish that same arc will require a separation of 15 in. Therefore, working around energized conductors is not recommended for anyone but the qualified professional. The best way to ensure safety when working around high voltage apparatus is to make absolutely certain that it is de-energized.

• Although inspections and sampling can usually be performed while the transformer is in service, all other service and testing functions will require that the transformer is deenergized and locked out. This means that a thorough understanding of the transformer’s circuit and the disconnecting methods should be reviewed before any work is performed.

• A properly installed transformer will usually have a means for disconnecting both the primary and the secondary sides; ensure that they are opened before any work is performed. Both disconnects should be opened because it is possible for generator or induced power to back feed into the secondary and step up into the primary. After verifying that the circuit is de-energized at the source, the area where the work is to be performed should be checked for voltage with a “hot stick” or some other voltage indicating device.

• It is also important to ensure that the circuit stays de-energized until the work is completed. This is especially important when the work area is not in plain view of the disconnect. Red or orange lock-out tags should be applied to all breakers and disconnects that will be opened for a service procedure. The tags should be highly visible, and as many people as possible should be made aware of their presence before the work begins.

• Some switches are equipped with physical locking devices (a hasp or latch). This is the best method for locking out a switch. The person performing the work should keep the key at all times, and tags should still be applied in case other keys exist.

• After verifying that all circuits are de-energized, grounds should be connected between all items that could have a different potential. This means that all conductors, hoses, ladders and other equipment should be grounded to the tank, and that the tank’s connection to ground should be verified before beginning any work on the transformer. Static charges can be created by many maintenance activities, including cleaning and filtering. The transformer’s inherent ability to step up voltages and currents can create lethal quantities of electricity.

• The inductive capabilities of the transformer should also be considered when working on a de-energized unit that is close to other conductors or devices that are energized. A deenergized transformer can be affected by these energized items, and dangerous currents or voltages can be induced in the adjacent windings.

• Most electrical measurements require the application of a potential, and these potentials can be stored, multiplied, and discharged at the wrong time if the proper precautions are not taken. Care should be taken during the tests to ensure that no one comes in contact with the transformer while it is being tested. Set up safety barriers, or appoint safety personnel to secure remote test areas.