Electrical furnace and Substation at Tata Steel

Study of Electrical Furnace in Heat treatment Shop and

Power System of West Plant Substation

- Nishant Ranjan

Acknowledgement

I am thankful to the entire TATA STEEL family for having extended full support and co-operation during the duration of my summer training to help me complete this project as well as gain valuable information as to how the Iron and Steel industry functions. This project report would not be complete without the help and cooperation of Mr. M C Jha (Foreman, Heat Tr. Shop). Every other member of IEM office had extended their hand of help throughout my course of summer training. I had a very good experience working with them and the office ambience was very much friendly. In particular, I would like to express my gratitude towards: Mr. Binay Kumar Agarwal (Manager, Spare Manufacturing Department) – My benevolent guide who was most helpful throughout my project.

Certification

This is to certify that Nishant Ranjan a student of 3rd year Electrical and Electronics Engineering from GITAM UNIVERSITY has undergone summer training at Spares Manufacturing Department of TATA STEEL under my guidance and has successfully completed the project titled “Study of Electrical Furnace in Heat Treatment Shop and Power System of West Plant Substation”. The duration of the training was from 3rd June to 28th June, 2014. He has shown a lot of initiative in learning during his training program and has enthusiastically completed all the work allotted. I wish all the best for his future endeavours.

Project Guide: Mr. Binay Kumar AgarwalManagerSpare Manufacturing DepartmentTATA Steel

Overview of TATA STEEL

Tata Iron and Steel Company was established by Dorabji Tata on August 26, 1907, as part of his father Jamsetji's Tata Group. Jamsetji Tata had started his quest for steel way back in 1882 but it was twenty-five years later, in December 1907 that the explorers found their way to Sakchi - at the confluence of the rivers Subarnarekha and Kharkai. On 27th February 1908 when the first stake was driven into the soil of Sakchi the dream had come alive. By 1939 it operated the largest steel plant in the British Empire. The company launched a major modernization and expansion program in 1951. Later, the program was upgraded to 2 MTPA project. In 1990, it started expansion plan and established its subsidiary Tata Inc. in New York. The company changed its name from TISCO to Tata Steel in 2005. The last decade has been marked by Tata Steel’s prominent role in the overall development of the country, even during phases of economic turbulence and its decisive foray into more and more global territory. Intense strategic thinking about future expansions, plans for organic growth and initiation of new projects are a few highlights in Tata Steel’s expanding and more penetrative roles in the larger perspective. The acquisition of NatSteel in 2004 was Tata Steel’s first overseas acquisition and the series of joint ventures and mergers that followed found a peak when the acquisition of Corus, happened in April 2007. But in every positive step that the Company has taken towards growth and expansion, involving diverse cultures and geographies, Tata Steel has never lost sight of its great heritage of social and community responsibility.


Tata Steel is headquartered in Mumbai, Maharashtra, India and has its marketing headquarters at the Tata Centre in Kolkata, West Bengal. It has a presence in around 50 countries with manufacturing operations in 26 countries including: India, Malaysia, Vietnam, Thailand, Dubai, Daggaron, Ivory Coast, Mozambique, South Africa, Australia, United Kingdom, The Netherlands, France and Canada. Tata Steel primarily serves customers in the automotive, construction, consumer goods, engineering, packaging, lifting and excavating, energy and power, aerospace, shipbuilding, rail and defence and security sectors. Tata Steel has set a target of achieving an annual production capacity of 100 million tons by 2015; it is planning for capacity expansion to be balanced roughly 50:50 between greenfield developments and acquisitions. Overseas acquisitions have already added an additional 21.4 million tonnes of capacity, including Corus (18.2 million tonnes), NatSteel (2 million tonnes) and Millennium Steel (1.2 million tonnes). Tata plans to add another 29 million tonnes of capacity through acquisitions. Major greenfield steel plant expansion projects planned by Tata Steel include: - A 6 million tonne per annum capacity plant in Kalinganagar, Odisha, India;


- An expansion of the capacity of its plant in Jharkhand, India from 6.8 to 10 million tonnes per annum; - A 5 million ton per annum capacity plant in Chhattisgarh, India (Tata Steel signed a memorandum of understanding with the Chhattisgarh government in 2005; the plant is facing strong protest from tribal people); - A 3 million ton per annum capacity plant in Iran; - A 2.4 million ton per annum capacity plant in Bangladesh; - A 10.5 million ton per annum capacity plant in Vietnam (feasibility studies are underway); - A 6 million ton per annum capacity plant in Haveri, Karnataka.

Departments of TATA Steel Plant, Jamshedpur: From raw material to the final product many units work upon the material. These units are -Blast furnace -LD1-LD2 -CRM -HSM -CO MILLS -SINTER PLANT 1,2,3 -MERCHANT MILL -SM & SPOS

Spares Manufacturing Department

The spares manufacturing department is divided into the following different sections: • Forge Shop• Heat Treatment Shop • Welding Shop • Fabrication Shop • Machine Shop • Segment Shop

Some of the machines available in the above mentioned shops are:

Forge Shop: Davy Press Steam Hammer Pneumatic Hammer Gear Turning machine Furnace


Machine Shop: • Skoda conventional horizontal boring machine- M/C #249, #253, #254, #257, #259 • Skoda CNC horizontal boring machine (CNC of Fagor and Kirloskar)- M/C #252, #255, #256,

#258, #260 • Plano miller (CNC of Siemens)- M/C #449 • Grinding machine (CNC of Siemens)- M/C #347• Conventional lathe machine -Without turret -With turret- M/C #172, #180 • CNC lathe machine (CNC of Fagor and Siemens)- M/C #89, #94, #97, #106 • Vertical boring machine (CNC of Fagor)- M/C #178

Fabrication Shop: • Sheet Bending machine • Sheet Straight machine • Sheet Shearing machine • Profile Gas Cutting machine (CNC of Barny Phantom, Tanaka, Kirloskar) • Penetrating machine


Welding Shop: • Gas Arc Welding machine • Submerged Arc Welding machine

Heat Treatment Shop: • Gas furnace (BOFCO #1, #2; NSR; WESMAN #1, #2) • Electrical furnace (BOFCO Tempering, NGC)

Heat Treatment Process

Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material. It is noteworthy that while the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding. Heat treatment techniques include:

• Hardening• Annealing• Normalizing• Tempering• Surface Hardening

Hardening

Case hardening or surface hardening is the process of hardening the surface of a metal object while allowing the metal deeper underneath to remain soft, thus forming a thin layer of harder metal (called the "case") at the surface. For steel or iron with low carbon content, which has poor to no hardenability of its own, the case hardening process involves infusing additional carbon into the case. Case hardening is usually done after the part has been formed into its final shape, but can also be done to increase the hardening element content of bars to be used in a pattern welding or similar process.The hardened metal is usually more brittle than softer metal, through-hardening (that is, hardening the metal uniformly throughout the piece) is not always a suitable choice for applications where the metal part is subject to certain kinds of stress. In such applications, case hardening can provide a part that will not fracture (because of the soft core that can absorb stresses without cracking) but also provides adequate wear resistance on the surface.

Annealing

Annealing consists of heating a metal to a specific temperature and then cooling at a rate that will produce a refined microstructure. The rate of cooling is generally slow. Annealing is most often used to soften a metal for cold working, to improve machinability, or to enhance properties like electrical conductivity.In ferrous alloys, annealing is usually accomplished by heating the metal beyond the upper critical temperature and then cooling very slowly, resulting in the formation of pearlite. In both pure metals and many alloys that can not be heat treated, annealing is used to remove the hardness caused by cold working. The metal is heated to a temperature where recrystallization can occur, thereby repairing the defects caused by plastic deformation. In these metals, the rate of cooling will usually have little effect

Normalizing

Normalizing is a technique used to provide uniformity in grain size and composition throughout an alloy. The term is often used for ferrous alloys that have been

austenitized and then cooled in open air. Normalizing not only produces pearlite, but also bainite sometimes martensite, which gives harder and stronger steel, but

with less ductility for the same composition than full annealing.

Tempering

Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to a much lower temperature than was used for hardening (which means below critical temperature). The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product.

Untempered martensitic steel, while very hard, is too brittle to be useful for most applications. A method for alleviating this problem is called tempering. Most applications require that quenched parts be tempered. Tempering consists of heating steel below the lower critical temperature, (often from 400 to 1105 ˚F or 205 to 595 ˚C, depending on the desired results), to impart some toughness. Higher tempering temperatures, (may be up to 1,300 ˚F or 700 ˚C, depending on the alloy and application), are sometimes used to impart further ductility, although some yield strength is lost.Tempering may also be performed on normalized steels. Other methods of tempering consist of quenching to a specific temperature, which is above the martensite start temperature, and then holding it there until pure bainite can form or internal stresses can be relieved. These include austempering and martempering.

Tempering

Tempering Colours

Steel that has been freshly ground or polished will form oxide layers when heated. At a very specific temperature, the iron oxide will form a layer with a very specific thickness, causing thin-film interference. This causes colours to appear on the surface of the steel. As temperature is increased, the iron oxide layer grows in thickness, changing the colour. These colours, called tempering colours, have been used for centuries to gauge the temperature of the metal. At around 350˚F (176˚C) the steel will start to take on a very light, yellowish hue. At 400˚F (204˚C), the steel will become a noticeable light-straw colour, and at 440˚F (226˚C), the colour will become dark-straw. At 500˚F (260˚C), steel will turn brown, while at 540˚F (282˚C) it will turn purple. At 590˚F (310˚C) the steel turns a very deep blue, but at 640˚F (337˚C) it becomes a rather light blue.The tempering colours can be used to judge the final properties of the tempered steel. Very hard tool steel is often tempered in the light to dark straw range, whereas spring steel is often tempered to the blue. However, the final hardness of the tempered steel will vary, depending on the composition of the steel. The oxide film will also increase in thickness over time. Therefore, steel that has been held at 400˚F for a very long time may turn brown or purple, even though the temperature never exceeded that needed to produce a light straw colour. Other factors affecting the final outcome are oil films on the surface and the type of heat source used.

Tempering

In Heat Treatment shop of Spares Manufacturing Department the materials which are to be sent in Machine shop for machining are given required hardness and after machining they are again brought to Heat Treatment shop for removal of excess hardness i.e. tempering.

Carburizing

Carburizing is a heat treatment process in which iron or steel absorbs carbon liberated when the metal is heated in the presence of a carbon bearing material, such as charcoal or carbon monoxide, with the intent of giving the metal required hardness. Some types of carburizing process are- liquid carburizing, gas carburizing, plasma carburizing, salt bath carburizing. In gas carburizing the surface carbon content as well as the case depth can be accurately controlled. It also gives uniform case depth. It is much cleaner and efficient method of carburizing. The main carburizing agent in this process is any carbonaceous gas such as methane, propane or alcohols. In this process it is necessary that the hydrocarbon gases should be diluted with a carrier gas to avoid heavy soot formation. In NGC Tempering machine of Heat Treatment shop the carrier gas is nitrogen

Furnace in Heat Treatment Shop

There is basically two types of furnace in Heat Treatment Shop:-a. Gas Chamber Furnaceb. Electrical Furnace

We have three types of Electrical Furnace • BOFCO Tempering Machine• NGC Tempering Machine• NGC Furnace

Gas Chamber Furnace

Short heating times due to the high power are a convincing argument. The chamber furnaces with powerful gas burners cover a wide variety of these processes. In the basic version the burners are manually ignited once at the start of the process. The automatic control system then takes over control of the temperature curve. At program end, the burners are automatically switched off. Depending on the process, the furnaces can be equipped with automatically controlled fan burners and safety technology for debinding. Especially in case of larger binder concentrations, gas furnaces have the advantage that the exhaust quantity can be significantly reduced as the binders are burnt off in the furnace, providing for downsizing of the exhaust cleaning.

Specifications:-- Tmax 1300 °C-Powerful, atmospheric burners for operation with coke gas and water vapour-Special positioning of the gas burners with flame guide top-down provides for good temperature uniformity-Fully automatic temperature control-Gas fittings with flame control and safety valve in accordance with DVGW (German Technical and Scientific Association for Gas and Water)-Multi-layer, reduction-proof insulation with light-weight refractory bricks and special back-up insulation result in low gas consumption-Self-supporting and rugged ceiling, bricks laid in arched construction or as fiber insulation

Gas Chamber Furnace

-Dual shell housing, side panels made of stainless steel (NB 300), for low outside temperatures-Solid, dual shell door-Exhaust hood with 150 mm (NB 300) and 00 mm (NB 400, NB 600) diameter connection-Over-temperature limiter with manual reset for thermal protection class 2 in accordance with EN 60519-2 as temperature limiter to protect the furnace and load

BOFCO Tempering Machine

• It is an Electrical furnace. Rating of the heater is 60kW. Heating element is Nichrome.• It is a chamber type furnace. • Dimension: 1550mm (depth) x 1000mm (width) x 450mm (height) • Temperature Range: 500°C to 1150°C with tolerance from ±5°C to ±10°C • Charging capacity: per batch is 1 ton. • Floor is made by ceramic bricks. • Charge Conveyance: Pushers, Rollers or Walking Beams• Processes: Hardening, Quench and Tempering, Normalizing, Annealing, Isothermal Annealing,

Spherodize Annealing, Solution Annealing, Stress Relieving, etc.• Material: Alloy Steels, Stainless Steels.

The charge is loaded into trays or baskets mounted on trays, and conveyed across the hearth using pushers, rollers or walking beams at a controlled rate. Conveyors and extractors are used to automate and integrate the heat treatment processes into other manufacturing processes at the plant.

An easy to use Control Panel provides the interface to setup the automation sequences and heat treatment parameters. The Control Panel and Process automation is implemented via PID Controller. Multi-point Digital Temperature Recorders are provided with an option to record various zone temperatures to a paper based system or to local storage that can subsequently


be interfaced to a PC for long term record keeping and analytics.

Hydraulic pushers are used for pushing the trays into the heating chamber & Electromechanical Extractor Mechanisms are used to unload the baskets or trays. Specially designed conveyors are used for transferring the baskets or trays from the Austenitising Furnace to the Quench elevator and subsequently to the Tempering Furnace and discharge station.


Design of Heating Coil:

The heating coils are mounted on both side walls. In each side wall 18 coils are there in 3 sets, each set having 6 coils. Previously 12 coils of each side walls and the remaining 6+6 coils from each side were connected in delta. But it was observed that there is a difference of 100°C in the temperature of the side walls. To overcome this 6 coils from one side and 5 from other side are taken and they are connected in series. Like this each branch of delta is made. Now there is only difference of about 10-20°C between the temperature of the sidewalls.

Theory:- When electric current flows through a conductor some loss occurs and this loss is almost inevitable, and more the resistance of the conductor, more the loss. This loss due to the electrical resistance of conductor is mainly responsible for the heating effect of electric current. As some electric power is converted into heat energy, this phenomenon can be described by Joule's law, which states that,

Where H is the generated heat in calories, i is the current that is flowing through the wire and it is measured in amperes, r is the resistance of the conductor in ohm(Ω) and t is the duration of current flowing in seconds. If we know the time of current flowing, the resistance of wire, and amount of electric current flow, we can easily find out the generated heat of the circuit. This heat


can be utilized in various ways.

We saw that the more the electrical resistance of the wire the more the generated heat in the circuit, but to know more accurately about the heating effect of current, we should know about it from the atomic level. As the flow of electric current is nothing but the flow of electrons there will always be resistance from the fixed atoms of the conductor. The fixed atoms of the wire resist the flow of electrons and as a result there are collisions and as the kinetic energy converts into heat energy we see that the wire is getting hot.


Heating Element: Nichrome

Patented in 1905, it is the oldest documented form of resistance heating alloy. A common alloy is 80% nickel and 20% chromium, by mass, but there are many others to accommodate various applications. It is silvery-grey in colour, is corrosion-resistant, and has a high melting point of about 1,400 °C (2,550 °F). Due to its resistance to oxidation and stability at high temperatures, it is widely used in electric heating elements, such as in appliances and tools. Typically, nichrome is wound in coils to a certain electrical resistance, and current is passed through it to produce heat.

The element will be coiled tube type supported with ceramic tubes, ceramics bobbins. These tubes will be suitably anchored with SS studs to the wall. The element terminal will be taken to the rear side of the furnace casing for suitable interconnection. The design of the heating element will be low watt density for longer life.

BOFCO Furnace Controller

PID Controller:

PID controller is used to control the temperature of the furnace. A PID controller calculates an "error" value as the difference between a measured process variable and a desired set point. The controller attempts to minimize the error by adjusting the process control inputs.

The PID controller calculation algorithm involves three separate constants parameters, and is accordingly sometimes called three-term control: the proportional, the integral and derivative values, denoted P, I and D. Simply put these values can be interpreted in terms of time: P depends on present error, I on accumulation of past errors, and D is a prediction of future errors, based on current rate of change. The weighted sum of these three actions is used to adjust the process via a control element such as the position of a control valve, a damper, or the power supplied to a heating element.


The output of the PID controller u(t) can be expressed in terms of the input e(t), as:

And the transfer function of the controller is given by:

The terms of the controller are defined as:Kp= Proportional gain, τd = Derivative time, and τi = Integral time


If the PID controller parameters (the gains of the proportional, integral and derivative terms) are chosen incorrectly, the controlled process input can be unstable, i.e., its output diverges, with or without oscillation, and is limited only by saturation or mechanical breakage. Instability is caused by excess gain, particularly in the presence of significant lag. So tuning a control loop is required for the adjustment of its control parameters (proportional band/gain, integral gain/reset, derivative gain/rate) to the optimum values for the desired control response.

In the PID controller time and temperature parameters are set for required for the tempering process. In certain time interval how much temperature will rise or it will remain constant, all these parameters are put in the controller. And the controller works as per requirement after getting the temperature feedback from the TC.


TC to PID controller connection diagram is shown below.

Drives of BOFCO Tempering Furnace

Incomer:Feeder no: 0Feeder Rating: 125 A SDFUType of Feeder: SDFU

Control Transformer:Feeder no: 1Feeder Rating: 415/220V, 50Hz, 500VaType of feeder: Control Supply

Zone -1 Heater:Feeder no: 2Feeder rating: 60KWType of feeder: Thyristor

Recirculation Fan:Feeder no: 3Feeder Rating: 3HPType of feeder: DOL


Connection diagram of BOFCO Furnace


AC Drive internal circuit of control panel

Thermocouple (TC)

Thermocouple is used to measure the temperature of the inside wall of the furnace i.e. the temperature at which the material is being tempered. One end of the thermocouple is inserted into the furnace. A thermocouple consists of two dissimilar conductors in contact, which produces a voltage when heated. The size of the voltage is dependent on the difference of temperature of the junction to other parts of the circuit. Although the voltage is within the range of millivolts. For measurement purpose we need to transfer this voltage, but as the amount of voltage is very less, most of it is lost during transmission. So this voltage is first converted to current by a transmitter. The output of the transmitter is in the range of 4-20 mA. The minimum current is not zero because even at lower temperature the voltage produced is not zero, so the current is also not zero. The temperature scale of the measuring device is calibrated in accordance with the current output of the transmitter

Thermocouple (TC)

Working PrincipleThe working principle of thermocouple is based on three effects, discovered by Seebeck, Peltier and Thomson. They are as follows:1) Seebeck effect: The Seebeck effect states that when two different or unlike metals are joined together at two junctions, an electromotive force (emf) is generated at the two junctions. The amount of emf generated is different for different combinations of the metals.2) Peltier effect: As per the Peltier effect, when two dissimilar metals are joined together to form two junctions, emf is generated within the circuit due to the different temperatures of the two junctions of the circuit.3) Thomson effect: As per the Thomson effect, when two unlike metals are joined together forming two junctions, the potential exists within the circuit due to temperature gradient along the entire length of the conductors within the circuit.

Thermocouple (TC)

How it WorksThe general circuit for the working of thermocouple is shown in the figure 1 above. It comprises of two dissimilar metals, A and B. These are joined together to form two junctions, p and q, which are maintained at the temperatures T1 and T2respectively. Remember that the thermocouple cannot be formed if there are not two junctions. Since the two junctions are maintained at different temperatures the Peltier emf is generated within the circuit and it is the function of the temperatures of two junctions.If the temperature of both the junctions is same, equal and opposite emf will be generated at both junctions and the net current flowing through the junction is zero. If the junctions are maintained at different temperatures, the emf’s will not become zero and there will be a net current flowing through the circuit. The total emf flowing through this circuit depends on the metals used within the circuit as well as the temperature of the two junctions. The total emf or the current flowing through the circuit can be measured easily by the suitable device.The device for measuring the current or emf is connected within the circuit of the thermocouple. It measures the amount of emf flowing through the circuit due to the two junctions of the two dissimilar metals maintained at different temperatures. In figure 2 the two junctions of the thermocouple and the device used for measurement of emf (potentiometer) are shown.Now, the temperature of the reference junctions is already known, while the temperature of measuring junction is unknown. The output obtained from the thermocouple circuit is calibrated directly against the unknown temperature. Thus the voltage or current output obtained from thermocouple circuit gives the value of unknown temperature directly.

YOKOGAWA display unit

The TC output is also connected to the YOKOGAWA display unit. There we can see the temperature of all the furnaces together.The function of the circuit is to provide a high accuracy multichannel thermocouple measurement solution. Achieving a precision thermocouple measurement requires a signal chain of precision components that amplifies the small thermocouple voltage, reduces noise, corrects nonlinearity, and provides accurate reference junction compensation (commonly referred to as cold junction compensation). This circuit addresses all these challenges for measuring thermocouple temperature with better than ±0.25°C accuracy.

YOKOGAWA display unit

Internal Circuitry of YOKOGAWA

DAQLOGGER Software

Daqlogger software which plots graphs of temperature vs. time. In the computer display we can observe heat treatment curve.The primary purpose of the DAQLOGGER software package is to acquire data at fixed intervals. Data acquisition is the function whose performance is considered most important. To improve the performance, we have designed the logger software so that it has the functions listed below1. Reading measurement data

at fixed intervals2. Writing measurement data

to data-sharing memory3. Converting measurement

data to file

Circulatory Fan

There is a circulating fan (3HP, 4 pole, 1440 rpm) with bare Impeller and hub, mounted on the top of the furnace.

The SS fabricated recirculation fan located at the roof level behind the top of baffle for circulating the hot air inside the working the chamber of the furnace. The fan shaft will extend through the roof lining, and be supported on Plummer Block bearings. The fan will be driven by V belt arrangement. The fan motor will be 3 HP. The fan shaft will also be of SS material

Quenching

There are basically three type of quenching:-1. Oil Quenching2. Water Quenching3. Plasma Quenching

*In Heat Treatment Shop we have Oil quenching and Water Quenching available

Oil Quenching:

Quench immersion is operated using an Elevator table. Quench operation can be completed within 22 seconds for the first tank and about 35 seconds for the second tank. The term quenching normally refers to the controlled cooling of steel components in a fluid to give specified propeties. the hardness and the other physical properties obtained depend up on the composition of the steel, the dimension of the component, the time and temparature of the heat treatment and the speed and duration of the quenching process.A number of quenching mediums such as molten salts, brine solutions and synthetic quenchants cab be used, but petroleum based quenching media find the widest application due to the following advantage.They are easier to control and give uniform hardness.• Suitable for large scale automation• These are non-corrosive and non –toxic.

Quenching

Here Metaquench oil is used. Metaquench grades have been specially formulated from highly refined petroleum oils with additives and have the following characteristics.• Good thermal properties• Good chemical and oxidation stability• High boiling points and low volatility• High flash and fire points

NGC Tempering Furnace

It has Electrical Furnace. NGC stands for New Gas Carburizing. The furnace of NGC is pit type. It has a lid which is opened by hydraulic system. The heating element of the coils of the furnace is nichrome. It has 3 coils each of 60 KW. The coils are fed from 440V, 50Hz 3 phase AC supply. The Heating mechanism of this NGC Tempering Furnace is similar to that of the BOFCO Tempering Furnace.


- The basic difference is the design of coils. NGC Tempering furnace has circular coils of diameter of about 2 inches forming a pair of 3 each. These coils are placed on the inner walls of the Furnace.

- The work pieces are pre-heated and then held for a period of time at an elevated temperature in the austenitic region of the specific alloy, typically between 820 and 940°C. During the thermal cycle the components are subject to an enriched carbon atmosphere such that nascent species of carbon can diffuse into the surface layers of the component. The rate of diffusion is dependent on the alloy and carbon potential of the atmosphere. Care must be taken to ensure that only sufficient carbon is available in the atmosphere at any one time to satisfy the take up rate of the alloy to accept the carbon atoms. In practice, this is defined in a carbon potential setpoint profile which runs concurrently with the temperature cycle. The setpoint may give a boost phase where the carbon potential would be typically set above 1.0% carbon but, as the cycle progresses and the effective case depth increases, the carbon setpoint will be reduced to complete the diffusion stage.

- It has two fans (3 phase induction motor). They are of 3HP and 5HP. One of them circulates the hot air inside the furnace uniformly and the other is for cooling purpose.

- There are 4 thermocouple of K-Type used here.


From the nameplate of the machines we can know the following things about it

• Maximum operating temperature is 700°C. • Capacity of the machine is 3 tons per batch. • Dimension of the machine is 3000mm (depth) x 1500mm (diameter). • Type of treatment done is tempering.

NFC Furnace 1

It has Electrical Furnace. NGC stands for New Gas Carburizing. The furnace of NGC is pit type. Here in this It has a lid which is opened by hydraulic system. The heating element of the coils of the furnace is nichrome. It has 5 coils each of 90+90 KW. The coils are fed from 440V, 50Hz 3 phase AC supply. The Heating mechanism of this NGC Furnace-1 is similar to that of the BOFCO Tempering Furnace.

NFC Furnace 1

- NGC Tempering furnace has coils in U shaped each forming a pair of five. There are five coils of each phase and they are short-circuited into one. These coils are placed on the inner walls of the Furnace.

- It has one fans (3 phase induction motor). They are of 3HP.It circulates the hot air inside the furnace uniformly.

- It has 3 Thermocouple of K- Type.

Specifications:

Dimension: 1500mm(depth) X 2000mm diameterCapacity: 2 Ton per batchMaximum Temp: 1100°C

Masibus

The operation of NGF Furnace 1 and NGC Tempering Furnace is controlled by Masibus.

Masibus:

This a microprocessor based scanner with built-in features. The basic scanner is having a capacity of 20 channels. The scanner can accept any industrial grade input like thermocouples, RTD, mV, mA or voltage or current input. The type of input is factory settable to any of the above types. The unit is either operated by 230V Ac or 110V Ac supply. The setting of operating power supply is done at factory only. The instrument is having two back plates. The first back plate is having connections for main power inputs and relay outputs. It is also having connection for parallel port and serial port

Power System of West Plant Substation

Overview of West Plant Substation:

• It is a 3.3kV substation. The substation is installed to supply power to different workshops and offices of Spares Manufacturing Department of TATA Steel.

• 3.3kV is supplied to the substation from Power House no. 1 and Segment Shop tie. • The 3.3kV bus is connected to three 1500kVA, 3300V/440V transformers and one 750kVA

rectifier transformer. The 3.3kV bus is also supplying power to Fabrication Shop Substation.

• The 440V bus is connected to the secondary side of the transformers. This bus is feeding different machines and cranes of the workshops.

• To feed the DC crane at 250V one 750kVA, 3300kV/440V transformer and a rectifier arrangement are used. The rectifier converts the 3 phase AC voltage to 250V DC.

• In both sides of the transformer (primary and secondary) circuit breakers are connected for safe operation. In the high voltage side vacuum circuit breakers of Siemens are installed. In the low voltage side air circuit breakers of Eswaran and Schneider and oil circuit breakers are connected.

West Plant Substation

West Plant Substation

The above diagram is the single line diagram of West Plant Substation. Power House 1 is feeding the 750kVA rectifier transformer and one of the distribution transformers. Segment shop tie is feeding the rest of the distribution transformers. These two high voltage buses are connected by a bus coupler which is a VCB. It is kept in OFF position; if one of the supplies fails the breaker is put to ON position. Similarly 440V bus #1 and #2 are connected by bus coupler, so that if one the transformer fails the other transformer supplies both the feeder.

The SLD of West Plant Substation is given aside

Transformer

West Plant substation has 4 transformers, 3 of them are for distribution purpose and one is for rectifying the alternating current to direct current. Distribution transformers are 3 phase, 1500kVA, 3.3kV/440V and the rectifier transformer is 3 phase, 750kVA, 3.3kV/440V. From the nameplate of the distribution transformer we know the following details. 1. Rating of the transformer is 1500kVA; 6300V, 3150V/440V.

2. Max voltage that can be applied to the high voltage side is 6930V. 3. The transformer is designed in such a way that we can get 440V in the secondary side though we apply different voltages in the primary side. We can apply 10 different voltages starting from 2835V up to 6930V and still get 440V in the secondary side. This is possible by connecting the high tension leads in different manners. 4. For the distribution purpose in West Plant substation (3300kV/440V) in high tension side leads 1 and 8; 7 and 14; 3 and 5; 10 and 12 are connected.

5. The vector group of transformer id Dy11. That means secondary line to neutral voltage leads primary line to virtual neutral voltage by 30°.

Transformer

Transformer6. Leads A2, B2, C2 in the high tension side and a2, b2, c2 in the low tension side are brought out of the transformer. 7. Operating frequency is 50Hz. 8. Per unit impedance of the transformer #2 is 4.71% and of transformer #3 is 5.25%. 9. Rectifier transformer- 750kVA, 50Hz former

Primary Seconday Tertiary

Rated

Volatage 6300V/3150V 268V 239V

Rated

Current 83.5A/177A 3000A 30A

Phase 3 6 3

Transformer

Advantage od using D-y transformer:

We get an advantage of having delta in the primary side. Third harmonic current can flow through the delta, as the current gets a loop to flow. The origin of the third harmonic current is sinusoidal magnetic flux. If flux is purely sinusoidal then current required to produce that flux cannot be purely sinusoidal. So we get a third harmonic component if we do the Fourier series expansion of the current. And the third harmonic components of all three phase R, Y, B are in same phase. So if both side of the transformer are star at the star point summation of the currents are not zero if the star point is not grounded. And the resulting effect is oscillation of neutral point. In we have delta in one side of the transformer third harmonic current can flow thorough the close loop.

Some Points:

• As the primary side is connected to the high voltage side and it is arranged in delta, the voltage induced per turn is higher if it was connected in star. So the insulation grade should be better and it is costly.

• Secondary side bus bars carries huge amount of current, so these bus bars are thick, whereas the primary bus bars are thin.

• Here in the west plant substation all the transformers are feeding different loads. No two transformers are operated in parallel to operate a single load. But as the vector groups of the transformers are same and their voltage ratings are also same they can be operated in parallel after checking the polarity.

Transformer

Maintenance:

• Transformer body temperature should be measured and noted down on regular basis.

• The radiator oil level should be checked. The oil level inside the indicator should not be less than one half.

• The colour of the silica gel in the breather is to be noticed. It is blue in normal condition. Its colour changes to pink when it absorbs moisture from the air. Then the silica gel is separated from the breather and then it is heated to vaporize the moisture.

• Transformer acidic neutrality (TAN) test and tanδ test (for seeing capacitance loss) are to be done in regular basis.

• The star point is grounded through neutral earthing pit and the transformer body is grounded through equipment earthing pit. The combined earthing resistance value should be measured on regular basis; its value should not be more than 1Ω. If it becomes more then saltwater is to be poured in the pit to reduce the resistance value. The information plate of the neutral earthing pit has red background and of equipment earthing pit has black background. On this plate designation of the earthing pit, date of test, earth pit value is to be written.

Rectifier

To feed the DC cranes of Spares Manufacturing Department the rectifier is needed. The output of the rectifier is 250V DC as required for the DC cranes. Rectifier has following parts.

• AC isolator panel (2 nos) 1. Rating: 500V AC, 1250A 2. Continuous operation3. pole • Thyristor converter (1 no)

1. 6 phase full controlled rectifier, single way 2. 750kW 3. Output: DC 250V, 3000A 4. Loading: 100% continuously, 125% for 2 hours, 200% for 10seconds 5. No of series path: 1 6. No of parallel path: 3 7. Fuse: 660V, 800A 8. Forced air cooled

Rectifier

• Reactor panel

1. Rating 250V, 3750A 2. Continuous duty cycle 3. Air cored 4. Inductance: 0.25mH 5. Provided with blower and forced air cooled

• Control cubicle

1. Constant potential through phase angle control of SCRs 2. Measuring instruments: AC ammeter and voltmeter, DC ammeter and voltmeter, power

factor meter. 3. Natural air cooled 4. DC isolator panel 5. 2 pole 6. 250V DC, 4000A 7. Continuous operation

Rectifier

• High speed DC circuit breaker (1 no)

1. Type: GEARAPID; SE-6000 2. Air circuit breaker 3. 4200A at 55°C 4. 750V DC, rated current 5000A

Operation of the rectifier assembly:

The high tension input three phase AC voltage is fed to a double wound step down transformer. The step-down secondary voltage is in double star configurations connected to hexa phase full controlled bridge which gives six pulse rectification.

The hexa phase AC output from the rectifier transformer is fed to the six phase full control thyristor bridge having AC and DC isolators connected at input and output respectively. DC air cored reactor at the output of the rectifier bridge limits the short circuit fault current level also provides filtering.

The hexa phase thyristor bridge consists of 3 high power silicon controlled rectifiers in parallel in each arm. Each SCR is protected by a fast acting fuse and dv/dt circuit. The SCR bridge is protected against input voltage transients by a suppression network comprising of rectifier bridges with resistors and capacitors.

Rectifier

3 phase full wave rectifier using thyristor, input voltage step down by a double wound transformer

Rectifier

Maintenance: • Regular lubrication of blower bearing and checking the shaft alignment are to be done.

• The power and control cubicle are to be cleaned regularly with vacuum cleaner to avoid dust collection on busbars, silicon devices, capacitors. While cleaning the cubicle must be electrically isolated and the capacitors must be discharged.

• The ventilation of the room where the rectifier assembly is placed has to be good. If the height of the room is less than 5 metres exhaust fan must be provided.

The sequence of the phase R, Y, B must be maintained in all interconnections. This is very important.

Circuit Breaker

Circuit breaker is a high power switching device which can make, break and carry current under normal as well as abnormal conditions. Circuit breakers are vital part of a power system for safe power supply. In West Plant Substation vacuum circuit breakers (VCB) is used in high tension side and air circuit breakers (ACB) and oil circuit breakers (OCB) are used in low tension side. As fault occurs, the fault impedance being low the current increases and the relay set actuated. The moving part of the relay moves because of the increase in the operating torque, the relay takes some more time to close the contact. Relay contacts close trip circuit of the circuit breaker closes and the trip coil is energised. The operating mechanism starts operating for the opening process. The CB contact separates. Arc is drawn between the circuit breaker contacts and it is extinguished through various mechanism in different types of circuit breakers. Modern circuit breakers have over load trip, short circuit trip, under voltage trip, shunt trip, earth fault trip indication and service, test, isolated position indication. As the OCBs installed in this substation are old they do not have these many indication. Along with it OCB has maintenance problem. So the old OCBs are being replaced by modern ACBs.

Circuit Breaker

Vacuum circuit breaker: 10 VCBs (new) are installed in West Plant Substation Made by Siemens • Rated voltage- 12kV, rated current- 1250A• Short circuit current- 31.5kA for 3 seconds • Impulse voltage- 75kV• Connected to the high tension side of the transformers • Pressure of the vacuum is in the range of 10-7 to 10-3 tor. 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. The vacuum as such is a dielectric medium and arc cannot persist in ideal vacuum. However the separation of current carrying contacts causes the vapour to be released from the contact giving rise to plasma. Thus, as the contact separate, the contact space is filled with vapour of positive ion liberated from contact material. The vapour density depends on the current of the arc. During decreasing mode of current wave the rate of release of vapour reduces and after the current is zero, the medium regains dielectric strength provided vapour density around the contact has substantially reduced. While interrupting the current of the order of few hundred amperes by separating flat contacts under high vacuum, the arc generally has several parallel paths, each arc path originating and sinking in hot spot of current.

Circuit Breaker

Thus the total current is divided in several arcs. The parallel arcs repel each other so that the arc tends to spread over the contact surface. Such an arc gets interrupted easily.

The advantages of using VCB are- • No emission of gases, pollution free.• Modest maintenance.• No explosion, silent operation • Long life But VCBs are more expensive and requires high technology.

Vacuum circuit breaker: 10 VCBs (new) are installed in West Plant Substation Made by Siemens

• Rated voltage- 12kV, rated current- 1250A• Short circuit current- 31.5kA for 3 seconds • Impulse voltage- 75kV • Connected to the high tension side of the transformers • Pressure of the vacuum is in the range of 10-7 to 10-3 tor.

Circuit Breaker

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. The vacuum as such is a dielectric medium and arc cannot persist in ideal vacuum. However the separation of current carrying contacts causes the vapour to be released from the contact giving rise to plasma.

Oil circuit breaker: 14 old OCBs are there in this substation connected to the low tension side of the transformer. As the current carrying contacts are separated under oil, the heat of the arc cause decomposition of the oil, the products of the decomposition being hydrogen gas and other gases like acetylene. The gases formed cause increase in pressure inside the arc-control device. The flow of gases is influenced by the design of arc-control device speed of contact travel, the energy liberated by the arc, etc. The high pressure gas in the arc-control device tries to escape through the side vents towards the top of the interrupter. While doing so, it tries to cool the arc and lengthen it into the vents. When current goes to zero, the arc diameter is very small and the gas flow is able to interrupt the arc. The interruption of the arc stops the generation of gas and the contact is filled with fresh dielectric oil.

Circuit Breaker

Disadvantages of using oil- • The decomposed products of dielectric oil are inflammable and explosive. If the oil circuit

breaker is unable to break the fault current, pressure in the tank may rise above safe limit and explosion may occur.

• The oil absorbs moisture readily. Dielectric strength reduces by carbonising which occur during arcing. The oil is needed to be replaced after a certain breaker operation. It needs regular maintenance.

• Oil is not a suitable medium for breakers which have to operate repeatedly.

Air circuit breaker: West Plant substation has 8 old Eswaran ACBs and 6 new Schneider ACBs. New ACBs have various trip indication mentioned above and service, test and isolated position indication.

Trip characteristics are often fully adjustable including configurable trip thresholds and delays. Usually electronically controlled, though some models are microprocessor controlled via an integral electronic trip unit. Every air circuit breaker is fitted with a chamber surrounding the contact. This chamber is called ‘arc chute’. The arc is driven into it. If inside of the arc chute is suitably shaped, and if the arc can be made conform to the shape, the arc chute wall will help to achieve cooling.

Circuit Breaker

The inner walls of the arc chute is shaped in such a way that the arc is not only forced into close proximity with it but also driven into a serpentine channel projected on the arc chute wall. Thus the arc path is lengthened. The main arc chute is divided into numbers of small compartments by using metallic separation plates. These metallic separation plates are actually the arc splitters and each of the small compartments behaves as individual mini arc chute. In this system the initial arc is split into a number of series arcs, each of which will have its own mini arc chute. So each of the split arcs has its own cooling and lengthening effect due to its won mini arc chute and hence the arc extinguishes.

Circuit Breaker

Maintenance:

• It is recommended to grease the breaker after some particular numbers of operation.

• In VCB contact resistance of all three poles are to be measured by CRM meter. According to TISCO standard its value should be less than 100µΩ.

• In VCB insulation resistance value between phase to phase and phase to neutral should be greater than (3.3+1)MΩ. For ACB IR value should be greater than 1MΩ.

• Appropiate parts are to be lubricated.

• Contact erosion is to be noticed and checked as per manual.

• Insulators, switching bars should be cleaned with damp cloth.

• OCBs need regular maintenance. Breakdown value of the oil is to be measured. It must not be less than. DGA(dissolved gas analysis) test are also be carried out. This called blood test of oil. This is done how much gas is dissolved in the oil. If it is less than the standard value then the OCB is prone to explosion as the gases formed by decomposion of oil are flammable.

Circuit Breaker

Safety precaution during handling High Tension Breaker

• The concerned IEM officer is to be informed before handling the high tension breaker.

• The voltage and current of the breaker which is going to be handled is to be checked. Also the current level should be zero before putting the breaker off.

• If power shut down job is given to external agency, necessary power clearance is to be taken for the same.

• The breaker is to be handled with personal protective equipment's (PPE) i.e. hand gloves, cool coat, power tester, ground rod, link road, fuse puller, barricading tape.

• The breaker is to be switched on/off with the consent of the concerned authority.

• Red tag is to be used for necessary information about the S/D taken for the breaker.

• The HT equipment is to be discharged with the help of discharge rod before starting the work.

• Before putting the breaker into service, insulation resistance value is to be taken through mugger.

Electrical furnace and Substation at Tata Steel

Engineering

Transcript of Electrical furnace and Substation at Tata Steel