PROJECT REPORT ONAND OVERVIEW OF POWER PLANT And TRANSFORMERS AKASH SRIVASTAVA 12BEE0019 PROJECT REPORT ON AND OVERVIEW OF POWER PLANT And TRANSFORMERS AKASH SRIVASTAVA 12BEE0019 PROJECT REPORT ON AND OVERVIEW OF POWER PLANT And TRANSFORMERS AKASH SRIVASTAVA 12BEE0019 ACKNOWLEDGEMENT: Our report will remain incomplete if we do not mention the efforts of those people who helped us in completing this project. Firstly we want to thank Mr. Binay Khalkho Manager-H.R. and Deepu Minz H.R. Dept TATA POWER Jamshedpur and the electrical department for letting us use the department data and carry out the necessary field work. We would like to express our gratitude to Mr.G.P.Shastry, Department Head, EMD, for his support, encouragement and all the technical help without which the completion of this project would have been impossible. We are also deeply indebted to Mr. Soumendra Mandal, EMD Project coordinator, whose technical input, stimulating suggestions and encouragement helped us in completion of report and provided us with practical on field knowledge of the subject. We also want to thank my co-trainees from various colleges who supported us in work. Their presence made our stay at TATA POWER a pleasant and learning experience in all aspects of life. CONTENTS Tata power Jojobera Thermal Power Plant Areas of operation • Coal Handling Plant • Boiler & Turbine Area • Transformer Yard • Electrical Switchyard • Electrostatic Precipitator Transformers Conclusion Bibliography Jojobera Thermal Power plant The Jojobera Thermal Power Plant, in Jamshedpur has an installed capacity of 547.5MW. It has a 5 generating units comprising of a single 67.5MW unit and four units of 120MW each. Units: Unit#1: 67.5MW (commissioned in 1996) Unit#2: 120MW (commissioned in 2000) Unit#3: 120MW (commissioned in 2001) Unit#4: 120MW (commissioned in 2005) Unit#5: 120MW (commissioned in 2011) In total there are total 9 number of transmission lines going to various substations in the city. Transmission Lines (each 132kv): Line – 1: Golmuri Line – 2: Golmuri Line – 3: Golmuri Line – 4: Main Ring Substation (MRSS – 4) Line – 5: Blower House (TATA STEEL) Line – 6: Bara, Sonari Line – 7: Main Ring Substation (MRSS – 2) Line – 8: Main Ring Substation (MRSS – 2) Line – 9: Tata Growth Shop Total number of bus couplers and bus sectionalizers used for transmission are: Bus sectionalizers: 4 Bus couplers: 3 Total bus sections: 6 The transmission line diagram of the plant is as follows: RAW MATERIAL (COAL) CHP BOILER AND TURBINE TRANSFORMER YARD SWITCHYARD ELECTROSTATIC PRECIPITATOR FLY ASH AS SIDE PRODUCT TO DIFFERENT PLACES BY TRANSMISSION LINES Areas of operation The main area of operation or the division of the thermal power plant is into the following areas: • Coal Handling Plant • Boiler & Turbine Areas • Transformer Yard • Electrostatic Precipitator • Switchyard COAL HANDLING PLANT Coal Handling Plant is the most essential part of any thermal plant. CHP is responsible for the procurement and the supply of the coal to the coal mill bunkers so that the required supply of pulverized coal is obtained for the generation of power. CHP in thermal power plant comprises the process from the unloading till filling bunkers of respective coal mills in different units. Function and Capacity • Procurement of coal • Unloading , sizing and storing of coal • Feeding of coal • The design capacity of CHP is 1176TPH and rated capacity is 100TPH • Middling coal from TISCO West Bokaro Collieries • IB Valley coal from Mahanandi Coalfields Ltd. Coals are crushed by primary crusher and secondary crusher. PROCESS FLOW CHART OF CHP Step 1 Railway Rakers Step 2 Wagon Tipplers Step 3 Hopper Step 4 Apron Feeder Step 5 Primary Feeder Step 6 Eccentric Disk Screen Step 7 Secondary Crusher Step 8 Conveyer Step 9 Bunker Specifications of Jojobera CHP plant: Capacity of CHP 1176 ton/hr Capacity of Wagon Tippler 105 ton Coal carrying capacity of one wagon 58-60 ton(approx.) Coal carrying capacity of one rake 3500 ton(approx.) Charging rate of side arm charger 20 wagon/hr Capacity of primary crusher 1176 ton/hr Capacity of secondary crusher 1176 ton/hr Total length of conveyer 3km(approx.) Input/ Output coal size primary crusher 600mm/150mm Input/ Output coal size secondary crusher 300mm/25mm Average coal consumption per day 3000 – 4000 ton Coal storage capacity of stockyard 1000 tons Major auxiliaries of CHP are: 1. Wagon Tipplers: The wagon tippler is driven by 110KV motor and the top lamping device is actuated by hydraulic cylinders. Cylinders are operated by a hydraulic power pack which gives necessary fluid flow and pressure by means of 30KV hydraulic motor to operate the hydraulic cylinder. All these motors are PLC controlled. Wagons arriving at CHP are placed by hydraulic operated side arm charger on wagon tippler which is driven by 415V ac, 125KV induction motor. TATA Honeywell PLC performs the entire of wagon tippling. Individual coal loaded wagons are placed on wagon tippler cradle. Wagon tippler raise command is given by operator and sequence of tippling operation starts in auto mode controlled by PLC. During tippling operation the along with loaded wagons rotates for 135 – 140 degrees and dumps the coal into the hooper. 2. Side Arm Chargers It is hydraulic equipment provided at wagon tippler to place coal wagons as per requirement avoiding the engagement of the locomotive during unloading operation. A batch of 20 – 22 coal loaded wagon can be handled at a time. A 450KW motor in side arm chargers is connected with the SAC hydraulic power pack which delivers necessary torque to the SAC. SAC in each operation gets connected to the rack of the loaded wagon by swing arm and brings the individual wagon to cradle for tripling. 3. Conveyer Belts: A conveyer belt (or belt conveyer) consists of two or more pulleys, with a continuous loop of material – the conveyer belt – that rotates about them. One or both of the pulleys are powered, moving the belt and the material on the belt forward. The powered pulley is called the drive pulley while the unpowered pulley is called the idler. 4. Primary Crusher: It is also known as rotator breaker. It is a horizontal drum having perforated screen plates inside the drum and is supporting rollers along with two guide rollers. Lifting shelves are fitted inside the drum which lifts up coal fed at one end during its rotation the coal is reduced in size and passes through the perforated screened plates having 150mm diameter holes. The uncrushed BOILER AND TURBINE AREA Boiler, also called the steam generator is the engineering device which generates steam at constant pressure. It is a closed vessel, generally made of steel in which vaporization of water takes place. Heat required for vaporization may be provided by the combustion of fuel in furnace, electricity, nuclear reactor, hot exhaust gases, solar radiations etc. Boiler mainly works on the principle of Rankine cycle. RANKINE CYCLE Rankine cycle is thermodynamic cycle derived from Carnot vapour power cycle for overcoming its limitations. Carnot cycle cannot be used in practice due to certain limitations. Rankine cycle has the following thermodynamic processes. 1 – 2: Isobaric heat addition 2 – 3: Adiabatic expansion 3 – 4: Isobaric heat release 4 – 1: Adiabatic pumping T – S, H – S and P – V representations are as shown below Practical arrangement in a simple steam power plant working on Rankine cycle is shown ahead. Thus in Rankine cycle, isothermal heat addition and heat rejection processes have been replaced by isobaric processes. Realization of ‘isobaric heat addition’ and ‘heat rejection’ in ‘boiler’ and ‘condenser’ respectively is in conformity with nature of operation of these devices. Isobaric heat addition can be had in boiler from sub cooled liquid to superheated steam without any limitations. Boiler mountings: • Water level indicator • Safety valve • High steam and low pressure safety valves • Fusible plugs • Pressure gauge • Stop valve • Feed check valve • Blowof cock • Manhole and Mudbox Boiler accessories: • Super heater • Economizer • Air preheater • Feed pump N.B :- Boiler mountings and accessories may vary for different types of boilers. Some of the boiler accessories and mountings are described below. • Super Heater: It’s purpose is to super heat steam and is a type of heat exchanger in which steam flows inside tubes and hot gases surround it. • Economizer: It is also a heat recovery device in which feed water is heated from heat available with exhaust gases. Thus hot feed water available from economizer lowers the fuel requirement in combustion economizer also helps in removal of dissolved gases by preheating of water and thus minimizes tendency of corrosion and pitting. • Feed pump: Feed pump is used for sending water into boiler at the pressure at which steam generation takes place. It is generally of three types i.e. centrifugal pump, reciprocating pump and injectors. • Safety valve: It’s function is to prevent the steam pressure from exceeding a limiting maximum pressure value. Safety valve should operate automatically by releasing excess steam and bring pressure down within safe limits. • Pressure gauge: It is used for pressure measurement. Pressure is continuously monitored so as to avoid occurrence of over shooting of boiler pressure, although safety devices to protect boiler against pressure rising beyond a limit is provided but pressure gauges are also used for monitoring pressure. • Stop valve: It regulates the flow of steam from the boiler. The is generally mounted on highest part of boiler shell and performs function of regulating the flow of steam from boiler. Stop valve generally has main body of cast steel; valve, valve seat and nut etc. are of brass. • Manhole and Mudbox: Manhole provides opening for cleaning, inspection and maintenance purpose. Mudbox is a collection chamber for collecting the mud. TURBINE AUXILLARIES: A steam turbine is a prime mover in which rotary motion is obtained by the gradual change of momentum of the steam. In a steam turbine following are the main parts: • The nozzle in which energy of high pressure is converted in to kinetic energy, so that steam issues from nozzle at high velocity. • The blade which change the direction of motion of steam issuing from the nozzle that force acts on the blade to change momentum and propel them. The auxiliaries are: Condensate extraction pump Hot well make up pump Grand stream condenser Surface condenser Extraction steam system LP Heater: A heater located between the condensate pump and either the boiler feed pump. HP Heater: A heater located downstream of the boiler feed pump. De – aerator: It is a device that is widely used for the removal of oxygen and other dissolved gases from the feed water to steam- generating boilers Operation: The steam cycle is working on a heat regeneration cycle. Feed water is supplied to the drum through the economizer outer limbs. Water in the boiler tubes absorbs heat from the furnaces. The mixtures of the water and steam is discharged into the boiler drums. The separated saturated steam is led to super heater where it is heated to about 810K. Superheated steam from the boiler is fed to the turbine via the turbine top valve, emergency stop and governing valve. Steam first enters the HP turbine, gets expanded here and then it is directed to the inlet of the LP turbine for further expansion. In the process the turbine starts rotating which in turn rotates the prime mover of the generator, thus producing electricity. Steam undergoing expansion in the allowed to flow through the condenser where the steam is condensed by cooling water supplied by C.W pumps. The condensate collected in the hot wells and pumped by vertical condensate extraction pumps to the de aerator through air ejector, gland steam condenser, drain cooler and LP heater. From the de aerator feed water storage tank, feed water is pumped to the economizer inlet header through two HP heater. During this process, the condensate steam is collected in different system. Heat is recovered at various points and is fed to the main feed water system. The ratings of UNIT #2 turbo generator of TATA Power Jojobera Power Plant is given below: 1. K.W : 120000 2. P.F : 0.8 3. KVA : 150000 TURBO GENERATOR A turbo generator is a combination of a turbine directly connected to an electric generator for the generation of electric power. Large steam powered turbo generators provide the majority of the world’s electricity and are also used by steam powered turbo electric ships. Smaller turbo – generators with gas turbine is often used as auxiliary power units. For base loads diesel generators are usually preferred, since they offer better fuel efficiency, but on the other hand diesel generator have a lower power density and hence require more space. The efficiency of larger gas turbine plants can be enhanced by using a combined cycle, where the hot exhaust gases are used to generate steam which drives another turbo generator. Generator produces alternating current: The generator is connected to the turbine drive shaft. It has a moving part – the rotor and a fixed part – the stator. The rotor’s outer surface is covered with electromagnets. The stator’s inner surface, or cylinder wall, is made up of copper windings. When the rotor turns inside the stator, the electrons in the copper windings ‘vibrate’. Their movement generates an electric currents, similar to the one created by Michael Faraday in his 1831 experiment on electromagnetic induction, but on a much larger scale. Turbine have a constant rotation speed: All the generators units in a power system must be synchronised. In other words it’s essential that they maintain an exact rotation speed. To ensure adequate power quality. Equipment that runs on electricity is designed to use alternating current of specific frequencies. This frequency depends on the generating unit’s rotation speed i.e. the no of times per second that rotor magnet travel past the stator winding. This frequency is expressed in cycles per second, or hertz(Hz), named after the German physicist Heinrich Hertz, to prove the existence of radio waves. Stator :- The stator consists of two party viz. The outer casing and inner frame supporting the core the winding outer casing is of welded construction welded tubes and ducts provide flow path for cooling. Inner stator consists of stator and core windings. Stator core stacked with insulated electrical sheet steel lamination clamping fingers ensures a uniform pressure and intensive cooling of stator core ends. Stator winding fractional pitched, two layers consisting of individual bars. To minimize the losses, the bars are separately brazed together and insulated from each other TRANSFORMER YARD The transformer yard of Jojobera thermal plant comprises of following transformers: TYPE OF TRANSFORMER QUANTITY 1. Generating transformer (GT) 5 2. Unit Auxiliary transformer (UAT) 5 3. Station transformer (ST) 3 4. Sub station transformer (SST) 8 Generating Transformer (GT)- 11KV/132KV The generating transformer is used as a step up transformer which produces 11KV and steps up this voltage to 132KV for transmission. Electricity produced at 11KV cannot be transmitted owing to large losses due to heavy current at the voltage level. A generating transformer is used to increase the voltage level to 132KV to meet the transmission requirements GTs are step up transformers with delta – connected LV windings are connected to the transmission lines. Specifications: GT#1 Make BHEL Rating 90MVA Voltage 11/132KV Cooling HV LV ONAF ONAN 30MVA 55MVA 90MVA 55MVA No Load Voltage HV LV ONAF ONAN 133V 139V 10.5V 10.5V Line Current HV LV ONAF ONAN 373.8A 228.4A 4943.7A 3024.5A Phase 3 Connection Y Frequency 50Hz Insulation level HV LV ONAF ONAN 660KV 275KV 75KV 25KV Weight (of oil) (total) 30500Kg 116000Kg GT#2, GT#3, and GT#4 Make BHEL Rating 90/120/150 MVA Voltage 11/132KV Cooling HV LV ONAF ONAN 30MVA 55MVA 90MVA 55MVA No Load Voltage HV LV ONAF ONAN 133V 139V 10.5V 10.5V Line Current HV LV ONAF ONAN 373.8A 228.4A 4943.7A 3024.5A Phase 3 Connection Y Frequency 50Hz Insulation level HV LV ONAF ONAN 275KV 650KV 28KV 75KV Weight (of oil) 30500Kg (total) 116000Kg Unit Auxiliary Transformer (UAT) 11KV/6.6KV UAT is a power transformer that supplies power to the auxiliary equipment of the power generating station during its normal operation. This transformer is connected directly to the generator output by a tap – off of the isolated phase bus duct and thus becomes the cheapest source of power to the generating station. It is generally a three – winding transformer where the primary winding is equal to the main generator voltage rating. The UAT’s are connected to the generators as step down transformers. Rated KVA of UAT is approximately 15% of the generator rating. Specifications: UAT#1 Make BHEL Rating 11/6.6KV No Load Voltage HV LV 11000V 6600V Line Current HV LV 656.1/787.3A 1045.9/1255.1A Phase 3 Vector group DdO Cooling ONAF/ONAN Impedance at normal temperature 8.76% Impedance at extreme temperature 8.05% Weight (of oil ) (total) 6265Kg 16500Kg Station Transformer (ST) – 132KV/33KV/6.6KV A critical function of any electricity generation and distribution system is stepping up or down of voltage at various points in the delivery network. The voltage is stepped up or down at substations using station transformers. The se may come in various sizes, depending upon the site transformers, but are normally connected to the network through overhead connections which results in exposed HV or LV bushings that necessitate a fence or other protective enclosure for personnel safety and security. At Tata Power Jojobera plant, there are three numbers of 3 winding 132/22/6.6KV station transformers (ST#1, ST#2, ST#3) to cater to station loads such as CHP,DM water plant, AIC plant, ESP unit etc. These also supply power to the customers as Tata Motors, Tata Cummins, and Lafarge Cement at 33KV. There are two numbers of feeders for each recipient, normally which are kept ON from the Jojobera plant end. Specifications: ST#1, ST#2, ST#3 Make BHEL Rating 75MVA Voltage 132/33/6.6KV Oil quantity 32250 Lts Cooling ONAF/ONAN Connection HV – Star, MV – Star, LV – Delta Vector group HV – MV: YNyno HV – LV: YNd1 Insulation level 1.) Impulse withstand voltage 2.) 1 min. Power frequency withstand voltage HV MV LV 550 75 60 230 28 20 No load losses at rated voltage and frequency 42 KW Load losses at principal tap and rated output 268 KW Auxiliary losses 5.2KW (Max.) Weight (of oil ) (total) 30500Kg 116000Kg TRANSFORMERS AT JOJOBERA PLANT AND THEIR USES: UST’s are located in power plant to step down the voltage level from 6.6KV to 415V. The rating for this transformer corresponds to the rating of the auxiliary load to be borne. Generally, these are stationed indoor and are dry type transformers. Power at Jojobera plant of Tata Power is evacuated at 132KV 33KV system supplies power to Lafarge cement plant, Tata Cummins and TELCO. For each of the aforesaid units, there are two feeders of 33KV each. To facilitate this is a 33 kV bus with two incomers and one bus couplers. Each incomer is capable of bearing the full load. There are six 6.6 kV buses. All the HT motors, 6.6kV/440V transformers are connected to the 6.6kV buses employ SF 6 breakers. If the unit trips then all the load gets transferred to the station transformer automatically. There are 14 415 V buses, all the 415V motors like ACW pumps, CCW pumps, valves, blowers, AC’s are connected to the 415V bus system. SWITCHYARD Switchyard is protected area where all the switchgears and its equipment are installed power transmission. Switchgear is used for switching, controlling and protecting device. Switchgear equipment is essentially concerned with switching and interrupting currents either under normal or abnormal operating conditions. Switchyard is hub for electrical power sources, transformers, remotely controlled arc snuffing breakers, metering devices etc. Switchyard forms the integral part of any power plant. These power plants have their main plant equipment integral controls as well as Plant Distributive Control System (DCS). While the entire plant is integrated at the DCS level, true unification is achieved by incorporating/integrating switchyard controls. Features: 1.) Monitoring of status of switchyard equipment like isolators, circuit breakers etc. 2.) Issue of close/open commands to isolators, breakers, etc. 3.) Monitoring of system parameters like voltage, current, frequency, MW, MVAR, energy. 4.) Controlling operations of protective relays. 5.) Presentation of information useful to operator in different forms. 6.) Generation of reports. 7.) Storage and revival of reports. 8.) Remote control and monitoring of different kinds of frequencies through fibre optics/PLCC communication like wave traps. Switchyard Equipments CURRENT TRANSFORMER (CT) Current transformer is used to measure current at reduced current as well as for the protection of the system. It is mainly a step down current to known ratio. The primary of this transformer consists of one or more turns of thick wire connected in series with the line. The secondary consists of large number of turns of fine wire which provides measuring instrument and relay a current which is a constant fraction of current in the line. It reduces heavy current flowing in the element of a power system to low values that are suitable for relay operation. Nickel iron is used for the construction of CT because it gives a good accuracy up to 5 times the rated current. Types of CT: a) Live Tank Current Transformer: In the live tank design, active part (i.e. core with secondary and primary duly insulated) is in the top tank. Top tank is live and secondary is insulated and taken to the bottom tank. b) Dead Tank Current Transformer: in dead tank design, active part is in bottom tank is in ground potential (i.e. dead) and the primary is insulated and taken to the top tank for primary termination. POTENTIAL TRANSFER (PT) Potential transformer is mainly a step down transformer which steps down the voltage to known ratio. It is used for protection as well as measurement of voltage in the system. The primary of this transformer consists of a large number of turns which provides measuring instruments and relays, a voltage which is known fraction of line voltage. Types of PT: a) Capacitive Voltage Transformer (CVT): CVT is used to measure high ac voltage. It consists of capacitive potential divider and step down transformer. Primary side of transformer is connected to the high voltage side together with capacitive and secondary side of transformer is having measuring instrument. Advantages of CVT are: Low impedance measuring device is used in CVT Cost of device reduced Loss is less b) Inductive Voltage Transformer (IVT): IVT are used for voltage metering and protection in high voltage network systems. They transform the high voltage into low voltage adequate to be processed in measuring and protection instruments secondary equipment, such as relays & recorders. ISOLATORS It is a disconnection switch which operates at no load condition both manually as well as remote. During faulty conditions circuit breaker operates first and then isolator is opened. After getting recovered from the fault, the isolator is closed first then the circuit breaker in order to avoid flashover. Isolators have provisions for a padlock so that inadvertent operation is not possible. In high-voltage or complex systems, these padlocks may be part of a trapped key-interlock systemto ensure proper sequence of operation. In some designs, the isolator switch has the additional ability to earth the isolated circuit thereby providing additional safety. Such an arrangement would apply to circuits which inter-connect power distribution systems where both ends of the circuit need to be isolated. So definition of isolator can be rewritten as Isolator is a manually operated mechanical switch which separates a part of the electrical power system The main difference in isolator and a circuit breaker is that an isolator is an off load device intended to be opened only after a current has been interrupted by some other control device. Safety regulations of the utility must prevent any attempt to open the disconnector while it supplies current. Classification of isolators Isolators are classified into three categories based on its breaking method. • Horizontal break: In these isolators the operating arm moves in a horizontal plane. It can be further classified into:- 1. Centre break 2. Double break • Vertical break • Pantograph CIRCUIT BREAKER The circuit breakers are the device, which is capable of making and breaking of an electrical circuit under normal and abnormal conditions. During normal operating condition the Circuit Breaker(CB) can be opened or closed by a station operator for the purpose of switching and maintenance. During the abnormal or faulty conditions the relays senses the fault and closes the trip circuit breaker. Thereafter the circuit breaker opens. The circuit breaker has two working positions, open & closed. These correspond to open circuit breaker contact and closed circuit breaker contact respectively. The operation of automatic opening and closing the contact is achieved by means of operating of the circuit breaker. As the relay contact closes the trip circuit is closed and the operating mechanism. In Tata Power, the 132kV breakers for GT#1,2,3 all ABB and arc SF 6 circuit breaker in which closing and tripping charged spring. Classification of Circuit Breaker a) Oil Circuit Breaker: It employs the basic technique of oil for arc extinction. Upon interruption of the electric circuit, an electric arc forms between the contacts of the circuit breaker. Because of the high temperature of the arc the oil is evaporated and oil vapours are partially decomposed liberating ethylene, methane and other gases. A gas bubble is formed in the arcing zone; the pressure in the bubble may be as high as several dozen mega N/m 2 . The arc is then extinguished, both because of its elongation upon parting of contacts and because of intensive cooling by the gases and oil vapour. b) Vacuum Circuit Breaker: Vacuum circuit breakers are circuit breakers which are used to protect medium and high voltage circuits from dangerous electrical situations. A vacuum circuit breaker is such kind of circuit-breaker where the arc-quenching takes place in vacuum. The technology is suitable for mainly medium voltage application. For higher voltage vacuum technology has been developed but not commercially viable. The operation of opening and closing of current carrying contacts and associated arc interruption take place in a vacuum chamber in the breaker which is called vacuum interrupter. The vacuum interrupter consists of a steel arc chamber in the centre symmetrically arranged ceramic insulators. The vacuum pressure inside a vacuum interrupter is normally maintained at 10 - 6 bar. c) Air Blast Circuit Breaker: It uses compressed air or gas as the interrupting or the breaking medium. Gases such as hydrogen, carbon-dioxide, nitrogen and Freon can be used as arc interrupting medium, but compressed air is the most suitable arc interrupting medium. The reason is nitrogen has the similar circuit breaking properties as that of compressed air medium. Carbon-dioxide has a drawback that is difficult to control owing to the freezing at valves and other restricted passages. Hydrogen has increased breaking capacity bit it is costlier. The air blast circuit breaker requires an auxiliary compressed air system which supplies air to the breaker air receiver. d) SF 6 Circuit Breaker: A circuit breaker in which the current carrying contacts operate in sulphur hexafluoride or SF 6 gas is known as an SF 6 circuit breaker. SF 6 has excellent insulating property. SF 6 has high electro-negativity. That means it has high affinity of absorbing free electron. Hence, for heavier and less mobile charged particles in SF 6 gas, it acquires very high dielectric strength. Not only the gas has a good dielectric strength but also it has the unique property of fast recombination after the source energizing the spark is removed. The gas has also very good heat transfer property. Due to its low gaseous viscosity (because of less molecular mobility) SF 6 gas can efficiently transfer heat by convection. So due to its high dielectric strength and high cooling effect SF 6 gas is approximately 100 times more effective arc quenching media than air. Due to these unique properties of this gas SF 6 circuit breaker is used in complete range of medium voltage and high voltage electrical power system. These circuit breakers are available for the voltage ranges from 33KV to 800KV and even more. LIGHTNING ARRESTER A lightning arrester is a device used electrical power systems and telecommunications systems to protect the insulation conductors of the system fromthe damaging effects of lightning. The typical lightning arrester has a high voltage terminal and a ground terminal. When a lightning surge (or switching surge, which is very similar) travels along the power line to the arrester, the current from the surge is diverted through the arrester to earth. If protection fails or is absent, lightning that strikes the electrical system introduces thousands of kilovolts that may damage the transmission lines, and can also cause severe damage to transformers and other electrical or electronic devices. Lightning produced extreme voltage spikes in incoming power lines can damage electrical home appliances. Here zinc oxide arresters are used. Zinc oxide arresters have energy absorption level. Types: 1. Rod gap arrester 2. Horn gap arrester 3. Multi gap arrester 4. Expulsion type lightning arrester 5. Valve type lightning arrester RELAYS A relay is an electrically operated switch. Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays. Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits as amplifiers: they repeated the signal coming in from one circuit and re-transmitted it on another circuit. Relays were used extensively in telephone exchanges and early computers to perform logical operations. A type of relay that can handle the high power required to directly control an electric motor or other loads is called a contactor. Solid-state relays control power circuits with no moving parts, instead using a semiconductor device to perform switching. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called "protective relays". BUS COUPLER The main function of bus coupler is to couple the busses. For each and every instrument a cable comes at the input terminal of bus coupler. Output terminal of this coupler goes to the control room. Thus each equipment can be controlled either manually or by control room. It comes into action if any fault occurs in the bus. Bus coupler configurations are available as non-terminated or internally terminated. If two or more non-terminated couplers are used on a bus, then the couplers at each end of the bus must be terminated externally with 78 ohmterminators on the unused bus connections of the end couplers. Alternately, internally single terminated couplers (with or without the non-functional bus connectors) can be supplied. BUS SECTIONALIZERS The sectionalizer is a self-contained, circuit-opening device used in conjunction with source-side protective devices, such as reclosers or circuit breakers, to automatically isolate faulted sections of electrical distribution systems. Power to operate the control circuitry and the mechanism is obtained from the line ELECTROSTATIC PRECIPITATOR (E.S.P) An electrostatic precipitator (ESP) is a highly efficient filtration device that removes fine particles, like dust and smoke, from a flowing gas using the force of an induced electrostatic charge minimally impeding the flow of gases through the unit. In contrast to wet scrubbers which apply energy directly to the flowing fluid medium, an ESP applies energy only to the particulate matter being collected and therefore is very efficient in its consumption of energy (in the form of electricity Exhaust gases contain large quantity dust particles which are emitted into the atmosphere. This poses threat to mankind as devastating health hazard. The dust issuing from exhaust of the plant can most effectively be prevented from entering into atmosphere by employing ESP. Here de dusting of exhaust gases from boiler is done. Exhaust gases contain large quantity dust particles which are emitted into the atmosphere. This poses threat to mankind as devastating health hazard. E.S.P advantages: • Ability to treat volume of gases at high temperature. • Ability to cope up with corrosive atmosphere. • Offers low resistance path to gas flow. • E.S.P uses intense electric forces to separate suspended particles from the flue gases. Processes involved: • Electric charging of suspended particles. • Collection of charged particles on collecting diodes. • Removal of particles from collecting diodes. • Five fields to keep emission within limits. • Electromagnetic control system is used. TRANSFORMERS A transformer is an electrical device that can change electrical energy of a given voltage into electrical energy at a different voltage level. It consists of two coils arranged in such a way that the magnetic field surrounding of one coil cuts through the other coil. When an alternating voltage is applied to (across) one coil induces an alternating voltage in the other coil. Transformers will not work with direct current, since no changing magnetic field is produced, and therefore no current can be induced. The factor which determines whether a transformer is a step up or step down type is the turn’s ratio. The turn’s ratio in the primary winding to the number of turns in the secondary winding. The TATA power Jojobera division has got the following types of transformers: Generating Transformer (GT) – 5 Station Transformer (ST) – 3 Substation Transformer (SST) – 8 Unit Auxiliary Transformer (UAT) – 5 Essential parts of a Transformer: LV Winding: The LV Winding or Low Voltage winding is that winding of a transformer which has “less number of turns” compared to the high voltage winding. As a result, the voltage generated in this winding i.e. LV winding, also the LV winding is generally ‘Delta’ connected. HV Winding: The HV winding or High Voltage winding has “more number of turns” as compared to the LV winding so voltage regulation is better. So the voltage required in this type of winding is more than the LV side. The HV winding is generally ‘Star’ connected. Transformer Core: The core makes up the bulk of the transformer and so selection of proper material plays an important role in the transformer’s overall function, working and efficiency. A range of different cores exist viz... steel laminated, solid, toroidal and air cores as well as variation of each within their respective categories. The core thus serves to greatly reduce the magnetizing current and confine the flux to a path which closely couples the windings. Generally all transformers are made of CRGO (Cold Rolled Grain Oriented Silicon Steel) The salient features of CRGO Transformer cores are: • Unmatched quality • High strength • Corrosion resistant • Long lasting • Reliable • Moisture resistant • High permeability Electrical bushings: It is an insulated device that allows safe passage of electrical energy through an earth field. If the strength of the electric field increases, it may be attracted to the earthed material and if sufficient voltage is present, leakage paths may develop within the insulation. If the energy of the leakage path overcomes the dielectric strength of the insulation, it may puncture the insulation and allow the electrical energy to conduct to the nearest earthed material causing burning and arcing. Thus, it should be so designed that it has the capability of enduring occasional and exceptional high voltage moments as well as the normal continual service withstand voltage. Transformer tank: Most power transformers are filled with transformer oil, a high dielectric fluid that both cools and insulates the windings. Transformer oil is a highly refined mineral oil that cools the windings and insulation bu circulating within the transformer tank. Radiator: Radiators are used to circulate the oil in a transformer by natural convection. Conservator: A conservator as the name suggests is a reservoir used in conserving or storing oil. If the transformer tank oil reduces due to a kind of overload or any other ambient conditions, the conservator will feed the oil into the main tank and if expansion occurs in the transformer oil, that oil will go to the conservator. Buchholz relay: In the field of electric power distribution and transmission, a “buchholz relay” is a safety device mounted on some oil filled transformers equipped with an external overhead oil reservoir/conservator. It is used as a protective device sensitive to the effects of dielectric failure inside the equipment. On a slow accumulation of gas, due to slight overload or any ambient condition, gas produced by decomposition of insulating oil accumulates in the top of the relay and forces the oil level downward. On load Tap Changer (OLTC): The OLTC is located in HV winding of a transformer. The tapping is taken from HV winding because.. • In HV winding, no. of turns is more, so voltage regulation is good. • The LV winding is nearer to the core, so that less insulation can be provided between core and winding. • In HV winding, current carrying capacity is less than LV winding. The function of OLTC is to give constant voltage to the secondary winding, rather maintain/regulate the output voltage of a transformer. Breather: When load on transformer increases or when the transformer is under full load, the insulating oil in the transformer gets heated up, expands and gets expelled out into the conservator, thus subsequently pushing the dry air out of the conservator tank through the silica gel breather. When the oil cools down, air from the atmosphere is drawn into the transformer. This is called “Breathing in of the transformer”. During the breathing process, the incoming air may contain moisture and dirt which should be removed in order to prevent any physical damage. Hence the air is made to pass through Silica gel breather, which will absorb the moisture in the air and ensure that only dry air enters into the transformer. Silica gel in the breather is blue when installed and they turn to pink colour after absorbing moisture, thus indicating replacement of the crystals. Thus silica gel breathers provide an economic and efficient means of controlling the level of moisture entering into the conservator tank during the breathing process. Neutral Grounding Resistor: The purpose of a Neutral Grounding Resistor (NGR) is to limit the ground fault current to a safe level so that all the electrical equipment in the power system is protected. The resistor should be the only current path between the neutral of the power transformers and ground. Cooling Fans: • Oil Natural Air Forced (ONAF): For oil filled transformers, the cooling ducts in the coils must be in sufficient number and size to allow dielectric fluid to flow through the coils to remove heat. This oil can move by simple convection (Oil Natural Air Natural) or it can be “forced cooled” by pumping fluid. Additionally, the tank surface must be large enough to transfer heat away from the fluid by the combined actions of conduction, convection or radiation. To maximise the cooling effect, cooling fans are added to expedite the heat removal through radiators. Thus ONAF uses natural circulation of the oil with the help of fans to increase the air flow and thus increase the rating of the transformer. • Oil Natural Air Natural (ONAN): This is the simplest transformer cooling system. The full form of ONAN is "Oil Natural Air Natural". Here natural convectional flow of hot oil is utilized for cooling. In convectional circulation of oil, the hot oil flows to the upper portion of the transformer tank and the vacant place is occupied by cold oil. This hot oil which comes to upper side, will dissipate heat in the atmosphere by natural conduction, convection & radiation in air and will become cold. In this way the oil in the transformer tank continually circulate when the transformer put into load. As the rate of dissipation of heat in air depends upon dissipating surface of the oil tank, it is essential to increase the effective surface area of the tank. So additional dissipating surface in the form of tubes or radiators connected to the transformer tank. • Oil Forced Air Forced (OFAF): OFAF means "Oil Forced Air Forced" cooling methods of transformer. In oil forced air natural cooling system of transformer, the heat dissipation is accelerated by using forced air on the dissipating surface but circulation of the hot oil in transformer tank is natural convectional flow. The heat dissipation rate can be still increased further if this oil circulation is accelerated by applying some force. In OFAF cooling system the oil is forced to circulate within the closed loop of transformer tank by means of oil pumps. Minimum Oil Gauge (MOG): MOG in transformer is used to show analogic indication of oil level inside the conservator tank and upon requirement; it also gives alarm signals when the oil level reduces below minimum/maximum level. Oil Temperature Indicator (OTI) & Winding Temperature Indicator (WTI): • Oil Temperature Indicator (OTI): The OTI measures the Top Oil Temperature. It is used for control and protection for all transformers. This device measures top oil temperature with the help of sensing bulb immersed in the pocket by using liquid expansion in the bulb through a capillary line to operating mechanism. A link and lever mechanism amplifies this movement to the disc carrying pointer and mercury switches. When volume of the liquid in operating mechanism changes, the bellow attached to end of capillary tube expands and contracts. This movement of bellow is transmitted to the pointer in temperature indicator of transformer through a lever linkage mechanism. • Winding Temperature Indicator (WTI): This device measures the LV and HV winding temperature. A winding temperature indicator or WTI is also used as protection of transformer. The basic operating principle of WTI is same as OTI. But only difference is that the sensing bulb pocket on transformer top cover is heated by a heater coil surrounded it. This heater coil is fed by secondary of current transformers associated with transformer winding. Hence the current through the heater coil is directly proportional to the electric current flowing through transformer winding. CONCLUSION This vocational training helped us in knowing what it takes to be in a corporate section. Working in an industry requires soft – skills, patience and the power of making adjustments. We got a good view of the working and use of different electrical machines and other equipments, understood the need of control rooms in every realm of the power plant and above all learned that “SAFETY IS IN YOUR OWN HAND”, without it we can suffer major consequences and it can even risk one’s job and life. Also, we could relate my theoretical knowledge with the practical field and got to know that both practical as well as theoretical knowledge has its own experience and they run in parallel tracks. Undergoing this training helped us to develop our inner self confidence and interactions with people in the plant premises thus giving me an interest in understanding basic engineering work. Through this training, we met people who shared their views, thoughts and outlooks required for working in an industry. Apart from the training, the culture Tata Power inculcates in each and every employee and even their trainees is really commendable.