“HINDUSTAN ZINC LIMITED”

An

Industrial Training Report

submitted

in partial fulfilment

for the award of the Degree of

Bachelor of Technology

in Department of Electrical & Electronics Engineering

Supervisor: Submitted By: Mr. Piyush Sharma Riya Purohit HOD EEE Roll No.12ECOEX042

Department of Electrical & Electronics Engineering

SS College of Engineering, Udaipur, Rajasthan

Rajasthan Technical University

SEPTEMBER, 2015-16

Candidate’s Declaration

I hereby declare that the work, which is being presented in the Industrial Training Seminar Report, entitled “Generator protection” in partial fulfillment for the award of Degree of “Bachelor of Technology” in Department of Electronics & Communication Engineering and submitted to the Department of Electronics & Communication Engineering, SS College of Engineering, Udaipur, Rajasthan Technical University is a record of my own investigations carried under the Guidance of Mr. Piyush Sharma Department of Electrical and Electronics Engineering , SS College of Engineering, Udaipur.

I have not submitted the matter presented in this report anywhere for the award of any other Degree. Supervisor: Submitted By: Mr. Piyush Sharma Riya Purohit HOD OF EEE Roll No.12ECOEX042

ACKNOWLEDGEMENT

It’s a great pleasure to present this report of summer training in HINDUSTAN ZINC LIMITED (DEBARI) in partial fulfillment of B-TECH Program under S.S. College of Engineering, Rajasthan Technical University, Kota. At the outset, I would like to express my immense gratitude to my training guide Mr. Kant. For guiding me right from the inception till the successful completion of the training.

I am falling short of words for expressing my feelings of gratitude towards him for extending his valuable guidance about technology, equipments and support for literature, critical reviews of project and the report and above all the moral support he had provided me with all stages of this training.

I would also like to thank my friends and all my group members for their help and cooperation throughout the training.

Riya Purohit

PREFACE

Practical training is a way to implement theoretical knowledge to practical use to become a successful engineer. It is necessary to have a sound practical knowledge because it is only way by which one can acquire proficiency & skill to work successfully different industries.

It is proven fact that bookish knowledge is not sufficient because things are not as ideal in practical field as they should be.

Hindustan Zinc Ltd. is one of the best examples to understand the production process & productivity in particular of Zinc.

This report is an attempt made to study the overall production system & related action of Zinc Smelter, Debari a unit a HZL. It is engaged in production of high grade zinc metal & other by products viz. Cd, sulphuric acid etc. since 1968 by adopting Hydro Metallurgical technology.

Riya Purohit

CONTENTS

Acknowledgement Preface 1. Company Profile 6 1.1 Vedanta 6 1.2 Hindustan Zinc Limited 6 1.3 Zinc Smelter Debari 7

2. Zinc 9 2.1 Introduction 9 2.2 Properties of Zinc 10 2.3 Zinc Smelting 10

3. Zinc Smelter Debari 12 3.1 General Process Overview 12 3.2 Raw Material Handling Section 12 3.3 Roasting Plant 13 3.4 Leaching Plant 15 3.5 Cell House 16 3.6 Melting & Casting 16

4. Project Work 18 4.1 Generator protection 18 4.2 Stator protection 19 4.3 Rotor protection

22

5. Conclusion

25

6. References 26

Chapter-1

Company Profile

1.1 Vedanta

Vedanta is an LSE-listed diversified FTSE 100 metals and mining company, and India’s largest non-ferrous metals and mining company based on revenues. Its business is principally located in India, one of the fastest growing large economies in the world.

In addition, they have additional assets and operations in Zambia and Australia. They are primarily engaged in copper, zinc, aluminium and iron businesses, and are also developing a commercial power generation business.

Founder of this recognition is Mr. Anil Agarwal, who is chairman of this group, a simple person without any special degree in management field but have a great experience in this field and a sharp sight of the future conditions and requirement.

1.2 Hindustan Zinc Limited

Hindustan Zinc Limited was incorporated from the erstwhile Metal Corporation of India on 10 January 1966 as a Public Sector Undertaking.

In April 2002, Sterlite Opportunities and Ventures Limited (SOVL) made an open offer for acquisition of shares of the company; consequent to the disinvestment of Government of India's (GOI) stake of 26% including management control to SOVL and acquired additional 20% of shares from public, pursuant to the SEBI Regulations 1997. In August 2003, SOVL acquired additional shares to the extent of 18.92% of the paid up capital from GOI in exercise of "call option" clause in the share holder's agreement between GOI and SOVL. With the above additional acquisition, SOVL's stake in the company has gone up to 64.92%. Thus GOI's stake in the company now stands at 29.54%.

Hindustan Zinc Ltd. operates smelters using

• Roast Leach Electro-Winning (RLE) • Hydrometallurgical (Debari, Vizag and Chanderiya Smelters) • ISP™ pyrometallurgical (Chanderiya Lead Zinc Smelter) and • Ausmelt™ (Chanderiya Lead Smelter) process routes.

Location 14 km from Udaipur, Rajasthan, India Hydrometallurgical Zinc Smelter Commissioned in 1968 Roast Leach Electrowining Technology with Conversion Process Gone through a series of debottlenecking 88,000 tonnes per annum of Zinc

Captive Power Generation 29 MW DG Captive Power Plant commissioned in 2003

Certifications BEST4 Certified Integrated Systems ISO 9001:2000, ISO 14001:2004, OHSAS 18001:1999, SA 8000:2001

Covered Area (Ha) 22.65

Total Plant Area (Ha) 126

1.3 Zinc Smelter, Debari-Udaipur

Location 14 km from Udaipur, Rajasthan, India. Hydrometallurgical Zinc Smelter Commissioned in 1968 Roast Leach Electrowining Technology with Conversion Process Gone through a series of debottlenecking 88,000 tonnes per annum of Zinc Captive Power Generation 29 MW DG Captive Power Plant commissioned in 2003 Certifications BEST4 Certified Integrated Systems ISO 9001:2000, ISO 14001:2004, OHSAS 18001:1999, SA 8000:2001 Covered Area (Ha) 22.65 Total Plant Area (Ha) 126

Products Range

a. High Grade Zinc (HG) (25 kgs) & Jumbo (600 kgs) b. Cadmium Pencils (150 gms) c. Sulphuric Acid + 98% concentration

Awards & Recognitions

a. International Safety Award: 2006 by British Safety Council, UK b. ROSPA Gold Award for prevention of accidents c. Operating Capacity (Per Year

Raw Material Supplies:-

a. Zawar Mines b. Agucha Mines c. Rajpura Dariba Mines

Product Buyers:-

a. Tata b. Bhel c. Steel Companies

TABLE 1.1:-Quantity of Materials

S. No MATERIAL QUANTITY 1 Zn 80,000MT

2 Acid 130,000MT

3 Cd 250MT

4 Zinc dust 360MT 5 Work force 876 Nos. 6 Managerial & Engineering Staff . 84 Nos. 7 Supervisory & Technical Staff 58 Nos 8 Labour 729 Nos. 9 (a) Skilled 154Nos. 10 (b) Semi-Skilled 555Nos.

Process Collaborators:-

a. Krebs Penorrova, France Leaching, Purification, Electrolysis b. Lurgi, GMBH, and Germany Roaster and gas clearing c. Auto Kumpu Finland RTP, Wartsila Plant d. I.S.C., ALLOY, U.K. Zinc dust plant, Allen Power Plant

Chapter-2

Zinc (Zn)

2.1 Introduction Zinc is a metallic chemical element with the symbol Zn and atomic number 30. In

nonscientific context it is sometimes called spelter. Commercially pure zinc is known as Special High Grade, often abbreviated SHG, and is 99.995% pure.

Zinc is found in the earth’s crust primarily as zinc sulfide (ZnS). Zinc (Zn) is a metallic element of hexagonal close-packed (hcp) crystal structure and a density of 7.13 grams per cubic centimeter. It has only moderate hardness and can be made ductile and easily worked at temperatures slightly above the ambient. In solid form it is grayish white, owing to the formation of an oxide film on its surface, but when freshly cast or cut it has a bright, silvery appearance. It’s most important use, as a protective coating for iron known as galvanizing, derives from two of its outstanding characteristics: it is highly resistant to corrosion, and, in contact with iron, it provides sacrificial protection by corroding in place of the iron.

Zinc ores typically may contain from 3 to 11 percent zinc, along with cadmium, copper, lead, silver, and iron. Beneficiation, or the concentration of the zinc in the recovered ore, is accomplished at or near the mine by crushing, grinding, and flotation process. Once concentrated, the zinc ore is transferred to smelters for the production of zinc or zinc oxide. The primary product of most zinc companies is slab zinc, which is produced in 5 grades: special high grade, high grade, intermediate, brass special and prime western. The primary smelters also produce sulfuric acid as a byproduct.

With its low melting point of 420° C (788° F), unalloyed zinc has poor engineering properties, but in alloyed form the metal is used extensively. The addition of up to 45 percent zinc to copper forms the series of brass alloys, while, with additions of aluminum, zinc forms commercially significant pressure die-casting and gravity-casting alloys. Primary uses for zinc include galvanizing of all forms of steel, as a constituent of brass, for electrical conductors, vulcanization of rubber and in primers and paints. Most of these applications are highly dependent upon zinc’s resistance to corrosion and its light weight characteristics. The annual production volume has remained constant since the 1980s. India is a leading exporter of zinc concentrates as well as the world’s largest importer of refined zinc.

2.2 Properties of Zinc (metallic) at 293K

TABLE 2.1:-Properties of Zinc Material Quantity 1. Melting Point

693K

2. Specific Latent Heat of Fusion 10 J/ Kg 3. Specific heat capacity 38510 J/ Kg 4. Linear expansivity

31/k

5. Thermal conductivity

111 W/m/k

6. Electric Sensitivity 5.9ohm-meter 7. Temp. Coefficient of resistance

40/k

8. Tensile Strength

150Mpa

9. Elongation

50%

10. Young’ modulus

110Gpa

11. Passion’s Ratio

0.25

2.3 Zinc Smelting

Zinc smelting is the process of recovering and refining zinc metal out of zinc-containing feed material such as zinc-containing concentrates or zinc oxides. This is the process of converting zinc concentrates (ores that contain zinc) into pure zinc.

The most common zinc concentrate processed is zinc sulfide, which is obtained by concentrating sphalerite using the froth flotation method. Secondary (recycled) zinc material, such as zinc oxide, is also processed with the zinc sulfide. Approximately 30% of all zinc produced is from recycled sources.

Globally, two main zinc-smelting processes are in use:

a. Pyrometallurgical process run at high temperatures to produce liquid zinc. b. Hydrometallurgical or electrolytic process using aqueous solution in combination with

electrolysis to produce a solid zinc deposit.

The vast majority of zinc smelting plants in the western world use the electrolytic process, also called the Roast-Leach-Electrowin (’RLE’) process, since it has various advantages over the pyrometallurgical process (overall more energy-efficient, higher recovery rates, easier to automate hence higher productivity, etc.).

In the most common hydrometallurgical process for zinc manufacturing, the ore is leached with sulfuric acid to extract the Zinc. These processes can operate at atmospheric pressure or as pressure leach circuits. Zinc is recovered from solution by electrowinning, a process similar to electrolytic refining. The process most commonly used for low-grade deposits is heap leaching. Imperial smelting is also used for zinc ores

Chapter-3

Zinc Smelter Debari

Zinc Smelter Debari have following main plants

3.1 General Process Overview

The electrolytic zinc smelting process can be divided into a number of generic sequential process steps, as presented in the general flow sheet set out below. In Summary, the Process Sequence is:

Step 1: Receipt of feed materials (concentrates and secondary feed materials such as zinc oxides) and storage; Step 2: Roasting: an oxidation stage removing sulphur from the sulphide feed materials, resulting in so-called calcine; Step 3: Leaching transforms the zinc contained in the calcine into a solution such as zinc sulphate, using diluted sulphuric acid; Step 4: Purification: removing impurities that could affect the quality of the electrolysis process (such as cadmium, copper, cobalt or nickel) from the leach solution; Step 5: Electrolysis or electro-winning: zinc metal extraction from the purified solution by means of electrolysis leaving a zinc metal deposit (zinc cathodes); Step 6: Melting and casting: melting of the zinc cathodes typically using electrical induction furnaces and casting the molten zinc into ingots.

Additional steps can be added to the process transforming the pure zinc (typically

99.995% pure zinc known as Special High Grade (’SHG’)) into various types of alloys or other marketable products.

3.2 Raw Material Handling Section(RMH)

Smelters use a mix of zinc-containing concentrates or secondary zinc material such as zinc oxides as feed to their roasting plant. Debari smelter is characterized by a relatively high input of secondary materials. Smelters located inland receive their feed by road, rail or canal depending on site-specific logistical factors and the type of feedstock (eg, secondary zinc oxides come in smaller volumes and are typically transported by road). Concentrate deliveries typically happen in large batches (eg, 5,000 to 10,000 tonnes).

Hindustan Zinc Smelter Debari is strategically located close to the Zawar mines that

serves as a global concentrate hub and provides for an extensive multi-modal logistical

infrastructure. It is 14 kms away from Udaipur well connected by rail, road and air. Most zinc smelters use several sources of concentrates. These different materials are blended to obtain an optimal mix of feedstock for the roasting process.

The zinc concentrate is delivered by trucks and is discharged into two

underground bins. Several belt conveyors transport the concentrate from the underground bins to the concentrate storage hall. A Pay loader feeds the materials into two hoppers. By means of discharging and transport belt conveyors including an over-belt magnetic separator, a vibro screen and a hammer mill, the materials are transported to the concentrate feed bin. Dross material from the cathode melting and casting process will be added to the feed material before the vibro screen. For moistening of the concentrate several spraying nozzles are foreseen in the concentrate storage hall, as well as on the conveying belt before the concentrate feed bin.

Blended feed from the concentrate feed bin is discharged onto a discharge belt conveyor, which in turn discharges onto a rotary table feeder. The roaster is fed then by two slinger belts.

3.3 ROASTING PLANT

In Roasting Plant, oxidation of zinc sulfide concentrates at high temperatures into an impure zinc oxide, called "Calcine". The chemical reactions taking place during the process are:

Approximately 90% of zinc in concentrates is oxidized to zinc oxide, but at the roasting temperatures around 10% of the zinc reacts with the iron impurities of the zinc sulfide concentrates to form zinc ferrite. A byproduct of roasting is sulfur dioxide, which is further processed into sulfuric acid. Zinc oxide obtained is then sent in leaching plant for further processing.

Firstly, zinc sulphide after ore concentration process is sent to furnace .There at 920-950 degree temperature zinc sulphide combustion takes place producing calcine. Further oxidation of SO2 maintains the temperature range, and then SO3 is sent to acid plant for production of sulphuric acid. HZL, Dariba has 2 roaster units R4 and R5. Zinc Sulphide Concentrate is introduced directly into the roaster and roasted in a turbulent layer, largely consisting of roasted material especially Zinc Oxide (ZnO). This layer has been heated to ignition temperature by thee preheating device. The desired reaction is maintained by an exothermic reaction of Sulfide Concentrate and air in the turbulent layer.

The surplus reaction heat is taken out of the roaster bed by cooling elements installed in the turbulent layer in the form of evaporator heating surfaces connected to the Waste Heat

Boiler. The waste heat is further utilized to generate electricity of about 9.6MW which is used in the plant.

Roaster part of plant also divided as follows:

1. Raw Material Handling (RMH)

2. Roaster

3. Waste Heat Recovery Boiler (WHRB)

4. Hot Gas Precipitator (HGP)

5. Gas Cleaning Plant (GCP

6. Sulphuric Acid Plant (SAP)

7. Acid Loading Plant

ACID PLANT

The utilization of sulphur containing gases after zinc concentrate roasting is carried out at the sulphuric acid plant resulting in marketable sulphuric acid.

For the treatment of Sulphur Dioxide (SO2) in the roasting off-gas, by passing through the Gas Drying Tower, the Sulphur Dioxide (SO2), cooled and saturated with water vapor, comes in direct contact with concentrated acid. Sulfuric Acid (H2SO4) of this concentration is very hygroscopic (absorbs waters) and the gas is practically free from water vapor after leaving the Drying Tower.

After the Drying Tower, the Sulphur Dioxide (SO2) has to be converted into Sulfur Trioxide (SO3) to allow the production of Sulphuric Acid (H2SO4) according to the following reactions:

SO2 + 1/2 O2 → SO3 SO3 + H20 → H2SO4

Gas coming out of hot gas precipitator has 7-8%of SO2 at 330 degree Celsius.SO2 gas is passed through scrubbing tower, which has sedimentation tank and SO2 stripper & wet gas precipitator

In presence of V2O5, oleum is formed which further gives H2SO4.

3.4 LEACHING PLANT

The calcine is first leached in a neutral or slightly acidic solution (of sulphuric acid) in order to leach the zinc out of the zinc oxide. The remaining calcine is then leached in strong sulfuric acid to leach the rest of the zinc out of the zinc oxide and zinc ferrite. The result of this process is a solid and a liquid; the liquid contains the zinc and is often called leach product. There is also iron in the leach product from the strong acid leach, which is removed in an intermediate step, in the form of jarosite. Jarosite is a waste therefore it is sent to Effluent treatment plant. There is still cadmium, copper, arsenic, antimony, cobalt, germanium and nickel in the leach product. Therefore it needs to be purified.

The basic leaching chemical formula that drives this process is:

ZnO +H2SO4→ZnSO4 + H2O

MeO + H2SO4→MeSO4+ H2O

Me→ Metals other than Zinc present in concentration

Leaching Area is distributed in following buildings:

1. Weak acid leaching building 2. Jarosite precipitation building 3. Purification building 4. Gypsum removal building

PURIFICATION

It uses zinc dust, Potassium Antimony Tartarate (PAT) and steam to remove copper, cadmium, cobalt, and nickel, which would interfere with the electrolysis process. After purification, concentrations of these impurities are limited to less than 0.02 milligram per liter Purification is usually conducted in large agitated tanks called Pachukas. The process takes place at temperatures ranging from 40 to 85 °C (104 to 185 °F). The zinc sulfate solution must be very pure for electrolysis to be at all efficient. Impurities can change the decomposition voltage enough to where the electrolysis cell produces largely hydrogen gas rather than zinc metal. That’s the reason zinc sulphide is passed through various thickeners and then hot filter beds where the zinc sulphate goes with the solution as it is soluble. This ZnSO4 solution has pH of 5 which is then sent to electrolysis process. Zinc calcine, leach solutions and cell house acid are mixed in 11 agitated tanks which are controlled to varied pH, from 2 to 5, by additions of cell house acid or calcine. Continuous pH monitoring is facilitated by submerged pH cells in controlled tanks.

After leaching, the acid leach slurry is distributed to four 24-m thickeners where the leach residues are separated from the clear zinc sulphate solution. The residues are filtered and washed before being pumped to the Lead Smelter for further processing to recover zinc and other metals. These recovered metals are recycled as a fume to the zinc circuit through the Oxide Leach Plant. Clear zinc sulphate solution flows continuously from the thickeners to the zinc dust purification circuit. Solution flow rate from this circuit is approximately 450 m/h. An increase in acid concentration from 3 to 4% resulted in a 5% increase in recovery. The higher the concentration of the acid the better the dissolution of the zinc, so zinc is recovered mostly.

3.5 CELL HOUSE

Zinc is extracted from the purified zinc sulfate solution by electro winning, which is a specialized form of electrolysis. The process works by passing an electric current through the solution in a series of cells. This causes the zinc to deposit on the cathodes (aluminum sheets) and oxygen to form at the anodes. Every 24 to 48 hours, each cell is shut down, the zinc-coated cathodes are removed and rinsed, and the zinc is mechanically stripped from the aluminum plates.

A portion of the electrical energy is converted into heat, which increases the temperature of the electrolyte. A portion of the electrolyte is continuously circulated through the cooling towers both to cool and concentrate the electrolyte through evaporation of water. The cooled and concentrated electrolyte is then recycled to the cells. This process accounts for approximately one-third of all the energy usage when smelting zinc.

Zinc contained in the purified Zinc Sulphate (ZnSO4) is recovered as metal in the Electrolysis Plant. Zinc Electro-Winning is a method of depositing Zinc Metal (Zn) on the surface of Aluminum Sheet (Al) in cell by passing electric current through the cell. The thickness of Zinc Plating depends on the time spent in the electrolysis cell, the amount of current, and the chemical composition of the cell.

The electrolysis cells are arranged in one electrical circuit of two rows of cells each. The circuit is serviced by two transformer rectifiers that are connected in parallel by Aluminum (Al) and Copper (Cu) bush bars. The cells are connected in series while the Anode/Cathode System in each cell is in parallel.

Principle electrolysis reaction:

ZnSO4 + electricity →Zn++ + SO4 --

Zn++ + 2e- →Zn

3.6 MELTING AND CASTING

Cathode melting will be carried out in two identical electric induction furnaces. The furnaces will have a guaranteed average melting rate of 22 tonnes per hour of zinc cathodes (maximum ~ 24 tonnes per hour). The melting rate is infinitely variable between 0% and 100% of the maximum melting rate and is controlled by the automatic control system to match the rate that molten metal is removed (pumped) from the furnace.

Each furnace is equipped with a still well equipped with one or more molten metal pumps. The pump delivers molten zinc to a launder system feeding the casting machine. Each furnace feeds a single casting line. In addition, provision is made to pump molten zinc from one of the furnaces to the zinc dust production plant.

In addition to cathode bundles, the furnace chutes are designed to receive metallic zinc from the dross separation plant and metallic zinc “skims” from the casting machines. This material is fed to any chute (normally one dedicated chute) from forklift transported to hoppers that have been raised to the charging floor by the freight elevator (lift). The required amount of nh4cl to enhance the melting of this material is manually added to each hopper prior to dumping in the charge chute.

When cathode zinc is melted, a layer of dross comprised mainly of zinc oxide entrained molten zinc droplets is produced. This dross must be removed from the furnace once in every 24 hours by manually skimming the dross from the surface of the bath in a process called drossing. This process consists of opening one of the doors on the side of the furnace, manually spreading a few kg of NH4Cl onto the dross layer, manually agitating the dross layer with a steel “rake” and finally using the “rake” to drag the dross through the open door of the furnace into a forklift tote bin. During the drossing process, the furnace is operated under conditions of increased ventilation to contain the fumes and dust that are generated by the agitation and dross removal processes. The totes of furnace dross are transported by lift truck to the dross cooling area, to await treatment in the dross separation plant.

Chapter 4

Project work 4.1 GENERATOR PROTECTION Introduction Generator protection and control are interdependent problems. A generator has to be protected not only from electrical faults (stator and rotor faults) and mechanical problems (e.g. Related to turbine, boilers etc), but it also has to be protected from adverse system interaction arising if generator going of out of step with the rest of system, loss of field winding etc. Under certain situations like internal faults, the generator has to be quickly isolated (shut down), while problems like loss of field problem requires an ‘alarm’ to alert the operator. Following is a descriptive list of internal faults and abnormal operating conditions. 1. Internal Faults

a. Phase and /or ground faults in the stator and associated protection zone b. Ground faults in the rotor (field winding)

2. Abnormal Operating Conditions. a. Loss of field. b. Overload. c. Overvoltage. d. Under and over frequency e. Unbalanced Operation e.g. single phasing. f. Loss motoring i.e. loss of prime mover. g. Loss of synchronization (out of step). h. Subsynchronous oscillation.

4.2 STATOR PROTECTION Differential protection for generators:

Differential protection is used for protection of the generator against phase to earth and phase to phase fault. Differential protection is based on the circulating current principle.

In this type of protection scheme currents at two ends of the protection system are compared. Under normal conditions, currents at two ends will be same. But when the fault occurs, current at one end will be different from the current at the end and this difference of current is made to flow through relay operating coils. The relays then closes its contacts and makes the circuit breaker to trip, thus isolate the faulty section. This type of protection is called the merz price circulating current system.

Limitations of this method: The earth fault is limited by the resistance of the neural

earthing. When the fault occurs near the neutral point, this causes a small current to flow through the operating coil and it is further reduced by the neutral resistance. Thus this current is not sufficient to trip the circuit breaker. By this protection scheme , one can protect only 80 to 85 percent of the stator winding. If the relays with low settings are used the it will not provide desire stability. This difficulty is overcome by using the modified differential protection.

Fif:4.1:-Percentage Differential Relay

Fig4.2:- characterstics of percentage differential

Stator inter turn fault protection :

Differential protection for stator does not provide protection against the inter-turn faults

on the same phase winding of the stator. The reason is that the current produced by the turn to turn fault flows in the local circuit

between the turns involved and thus it does not create any difference between the current entering and leaving the windings at its two ends where the CTs are mounted. The coils of the modern turbo generator are single- turn , so there is no need to provide inter –turn fault protection for the turbo generator. But the inter turn protection is necessary for the multi turn generator like hydro electric generator. Some times stator windings are duplicated to carry heavy current. In this case stator winding have two different paths.

In this type of protection primaries of the CTs are inserted in the parallel paths and

secondaries are inter connected. Under the normal condition current flowing through the two parallel paths of the stator winding will be same and no current flowing through the relay operating coil. Under the inter turn fault, current flowing through the two parallel path will be different and this difference in current flowing through the operating coil and thus causes the circuit breaker to trip and disconnect the faulty section. This type of protection is very sensitive.

Fig4.2:-Interturn Protection Relay Stator over heating protection:

Stator over heating is caused due to the overloads and failure in cooling system. It is very difficult to detect the over heating due to the short circuiting of the lamination before any serious damage is caused. Temperature rise depend upon I^2Rt and also on the cooling. Over current relays can not detect the winding temperature because electrical protection can not detect the failure of the cooling system.

So to protect the stator against over heating, embed resistance temperature detector or thermocouples are used in the slots below the stator coils. These detectors are located on the different places in the windings so that to detect the temperature throughout the stator.

Detectors which provide the indication of temperature change are arranged to operate the temperature relay to sound an alarm.

Fig4.3:-Over Heating Protection Relay

4.3 ROTOR PROTECTION PROTECTION OF THE GENERATOR DUE TO UNBALANCE LOADING:

Due to fault there is an imbalance in the three phase stator currents and due to these imbalance currents, double frequency currents are induced in the rotor core. This causes the over heating of the rotor and thus the rotor damage. Unbalanced stator currents also damage the stator.

Negative sequence filter provided with the over current relay is used for the protection

against unbalance loading. From the theory of the symmetrical components, we know that an unbalanced three phase currents contain the negative sequence component. This negative phase sequence current causes heating of the stator.

The negative heating follows the resistance law so it is proportional to the square of the

current. The heating time constant usually depend upon the cooling system used and is equal to I²t=k where I is the negative sequence current and t is the current duration in seconds and k is the constant usually lies between 3 and 20.

Its general practice to use negative current relays which matches with the above heating characteristics of the generator. In this type of protection three CTs are connected to three phases and the output from the secondary of the CTs is fed to the coil of over current relay through negative sequence filter.

Negative sequence circuit consists of the resistors and capacitors and these are connected in such way that negative sequencecurrents flows through the relay coil. The relay can be set to operate at any particularvalue of the unbalance currents or the negative sequence component current. Loss of Excitation Protection:-

Loss of field failure of or loss of excitation is same phenomena and same kind of protection is used. It is discussed here in the field failure topic.

Loss of field occurs due to tripping of the supply of the field current which occurs

because of the reasons.

(i) Loss of field to the main exciter. (ii) Accidental tripping of the field breaker. (iii) Short circuit in field circuits (iv) Poor brush contact in the exciter. Field Protection Phenomena: - When the field supply is tripped, it speed increased and it start behaving as induction generator so heavy currents are produced in the teeth and wedges of the rotor. Because of the drop in excitation voltage the generator output voltage drops slowly to compensate this voltage current start increasing then generator become under excited and start drawing reactive power 2 to 4 times the generator load.

Before losing excitation, the generator is delivering power to the system. But when loss

of field occur this large reactive load thrown on the system abruptly with loss of generator’s reactive power and it further causes voltage reduction and extensive instability. Protection against Loss of field:-

If the system has capability to tolerate the difference of reactive power then automatic protection is not required but if the system will be instable in this condition and has not capability to tolerate then automatic protection is required.

Under current Moving coil relay is connected across a shunt in series with field winding.

But in case of large generators which operate over a wide range of field excitation then this relay will not work properly because field failure due to the failure of the excitation is not detected by it because it is held in by the ac induced from the stator.

The most valid type of protection in this case is by using directional-distance type relay

operating by alternating current and voltage at the generator terminals. In offset-mho relay is used and its setting is like that when the excitation goes certain value then this relay start operating because machine start running asynchronously. Its characteristics are shown on R-X diagram.

When excitation is lost the generator impedance start a curve from the first quadrant to the fourth quadrant. This region is enclosed in the operating area of the relay so the relay will operate when the generator starts to slip poles and will trip the field breaker and disconnect the generator from the system. The generator may then return to service when the cause of failure is cleared.

CHAPTER-5 CONCLUSION It was just like a dream come true for me to pursue training in Hindustan Zinc ltd. It was really a learning experience for me to have a feel of different Industrial aspects. In this period I have Learnt those things, which I could not get from books i.e., the practical experience under the guidance of learned professionals. Special thanks for my college and Hindustan Zinc Ltd. Work .

References

• “Plant Operating Manuals” Zinc Smelter Debari, Udaipur • McCabe, Smith, Harriott “Unit Operations of Chemical Engineering” McGraw-Hill

International Editions. • http://www.hzlindia.com/index.aspx Official Website of Hindustan Zinc Limited • http://www.vedantaresources.com/default.aspx Offical Website of Vedanta Resources

HZLTraining Report