A Report on Transfrmer (Part-2)

Independent University Bangladesh

[TRANSFORMER-DESIGN & MANUFACTURING AT GEMCO LTD]

CHAPTER 1: Introduction 1.1 Internship An internship is a type of work experience for entry-level job-seekers. Internships involve working in your expected career field, either during a semester. Besides gaining valuable experience, we get exposed to the business environment and gain valuable references and network contacts. Career counselors, employers, and others have long touted the importance for students to work one or more internships during their university years. Internships are seen by students as potentially valuable tools to explore general career avenues as well as specific companies. Such arrangements can provide us with valuable work experience (both practical and for résumé enhancement) and an opportunity to line up a job before graduation. In addition to securing good work experience, students also may be able to gain academic credit learning by doing and being exposed to professionals working in the field provides valuable experience, a professional reference and often leads to a position upon graduation.

1.2 Origin of the Report The entitled report ― TRANSFORMER-DESIGN & MANUFACTURING AT GEMCO LTD is originated from the partial fulfillment of the Bachelor of Electrical & Electronics Engineering of Independent University Bangladesh. This is a report on the internship internship that I have completed from General Electric Manufacturing Company Ltd. I worked specifically in the Transformer Design & Testing Station. These particular departments‟ deals with the design, process, and quality check and control of the transformers of the GEM plant. During my internship I made a detailed study on the whole system of the transformer testing station and also the whole transformer manufacturing system. This documentation includes the details description of my study during my internship at General Electric Manufacturing Company Ltd.

1.3 Methodology For this particular study, the data was collected in three ways: from the practical field, from key personals and from different official documents.

1.4 Limitations The limitation I faced during my internship was: The major limitation faced was mainly time constraint, which prevented in-depth and detailed work related to the transformer manufacturing process. Some information which was confidential to the company was not revealed for the work. It is quite impossible to learn all the traits of the transformer manufacturing process within this short time. So there is some lacking of information regarding the topics of the report.

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CHAPTER 2: GEmco ltd 2.1 Company profile Figure 2.1: Logo & Motto of GEMCO ltd

Electricity is an index of civilized nation and a necessity on the day to day life of the people. National development of a country is judged by the quantum and amount of electricity it produces and consumes. The Government has taken effective plans to spread electrical network to all corners of the country with a view to developing infrastructure as well as building up and improving the rural economy. In order to achieve the above objectives the scheme of establishment of a heavy electrical complex in the name of General Electric Manufacturing Plant at North Patenga, Chittagong was formally and finally approved by the Government of Bangladesh in 1972. As an enterprise of Bangladesh Steel & Engineering Corporation, it was emerged as a public limited company in 1979 and renamed General Electric Manufacturing Company Limited popularly known as GEMCO. It is today the largest engineering and manufacturing enterprise in Bangladesh. GEMCO is now manufacturing and supplying a major portion of Electrical Distribution equipments required by the power sector of the country. In present open and free market scenario its products have been established as highly reliable and competitive with international manufacturers. GEMCO has already stamped its mark significantly through its products and services by maintaining highest degree of quality and technical standard. A vast reservoir of manufacturing skills and management - the GEMCO family comprises a formidable team of engineers, managers and trained technicians. Together they are able to offer high quality products and services with professional excellence. The enterprise is manufacturing and servicing quality Transformers and for national agencies like Bangladesh Power Development Board(BPDB), Rural Electrification Board(REB), Dhaka Electric Supply Authority (DESA), Dhaka Electric Supply Company(DESCO) etc.[enjoys 50% of the government quota] & also other private organizations.

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2.2 Products / Services: i) Transformers: Power Transformer, 3 Phase, 33/11KV, 1 MVA to 5 MVA Distribution Transformer, 3 Phase, 33/0.415 KV, 100 KVA to 250 KVA Distribution Transformer, 3 Phase, 11/0.415 KV, 50 KVA to 500 KVA Distribution Transformer, 1 Phase, 6.35/0.24 KV, 5 KVA to 50 KVA Distribution Transformer, 1 Phase, 11/0.24 KV, 15 KVA to 25 KVA other rated transformer transformer are manufactured manufactured as per requirements requirements of clients * Apart from these, other

ii) Other Electrical Items: 11 KV & 415 Volt, HT & LT Switch Gear. PFI Plant upto 250 KVAR, Drop out Fuse, 11 KV Disconnector, 11 KV, Lightning Arrestor, 11 KV iii) Repair & Miscell M iscellaneous aneous Works: Damaged Distribution & Power Transformers upto 20 MVA Manufacture & supply of Electrical Sub-station. Shearing & Bending of M.S. Plates upto 10 mm thickness Electroplating Facilities. Transformer Oil Centrifusing, Testing etc.

2.3 Installed & Attainable Capacity of the Unit: Installed Capacity: 6922 M. Tons (three shifts) per year Attainable Capacity: 1850 M. Tons or Equivalent 1875 Nos. of 200 KVA Transformers

(Single shift) per year.

2.4 Quality / Standard of the Products:

Manufactured and Tested as per International Standards IEC/ANSI/BS. ISO 9001-2000 Certified

2.5 location: North Patenga, Chittagong-4204, Bangladesh.

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2.6 Major Raw Material used by the Unit:

Name of Raw Materials

Specification of materials

Source Average Yearly Foreign/ Local Consumption

Silicon Steel Sheet

Grade M-5

Foreign

500 M.Tons

Copper Strips

Electrolytic Copper of 99.9% Foreign purity

80 M. Tons

Super Enameled Copper Wire

SWG -14 to 21 Copper of 99.9% purity

Local

100 M.Tons

M.S. Sheet

Prime Quality

Local

150 M.Tons

HT Insulator

Electrical Porcelain Insulator Local

5000 Pcs.

Transformer Oil

As per IEC-296 Class-1

Foreign

2500 Barrel

Insulated Paper & Board

Electrical Grade 100% Sulphate/Kraft Pulp

Foreign

20 M. Tons

2.7 Technical Data Capacity (KVA)

Losses (watt) No Load

Load at 75°C

Imp (%)

Regulation at unity power factor (%)

Efficiency at Unity Power factor (%)

at PF=1

Out line Dimension Approx (mm)

H

L

Oil (Kg)

Total Weight (kg)

24.0

104.0

W

6350/240V, SINGLE PHASE DISTRIBUTION TRANSFORMER

5

40

100

2.50

2.0

99.91

10

50

220

3.80

2.2

99.93

875

580

540

32.0

170.0

15

90

250

3.96

1.67

99.93

875

580

540

32.0

170.0

25

130

400

3.90

1.60

99.98

1000

690

540

56.0

320.0

11000/240V, SINGLE-PHASE TRANSFORMER

15

80

350

3.0

1.92

99.98

865

580

600

32.0

175.0

25

120

480

3.8

1.92

99.98

960

700

650

39.0

280

11000/415V, THREE-PHASE TRANSFORMER

50

190

1100

4.0

2.20

97.45

1300

1100 1100

640

148.0

480.0

100

245

1635

4.0

1.63

98.47

1325

1150

725

182.0

660.0

200

435

2820

4.0

1.52

98.38

1530

1200

900

244.0

985.0

250

520

3180

4.0

1.76

98.52

1530

1200

900

244.0

1100.0

300

650

4500

4.00

1.50

98.98

1610

1380

920

390.0

1450.0

500

1000

6200

6.36

1.24

98.988

1755

1510

1000

550.0

2100.0

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CHAPTER 3: Transformer 3.1 Defining Transformer A transformer makes use of Faraday's law and the ferromagnetic properties of an iron core to efficiently raise or lower AC voltages. It of course cannot increase power so that if the voltage is raised, the current is proportionally lowered and vice versa.

Figure 3.1:Worki 3.1:Working ng principl principlee of Transfor Transformer mer

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3.2Types of Transformers: Transformers are constructed so that their characteristics match the application for which they are intended. In power system, transformers are mainly of two type‟s power and distribution transformer. Apart from this, transformer types are also designated by the function the transformer serves in a circuit, such as an isolation transformer and many others.

Power transformer Installation: at substation. Connection: Δ- Δ or Y- Δ, 3 wire. Capacity: MVA. Voltage rating: usually stepped down to 11kv. Cooling system: ONAF, OFAF, ONWF, OFWF    



Distribution Distribut ion transformer At distribution and transmission. Δ-Y, 4wire. KVA. Usually stepped down from 11kv to 415v. Usually ONAN.    



3.3 About Distribution Transformers Most people don't realize that they see distribution transformers all the time. One of the most common applications is up on utility poles; they are the metal boxes and cylinders we see. Distribution transformers play an important role in reducing voltage so it can be used by customers in their homes and businesses. Distribution transformer reduces voltage to 120-240V single phase for residential customers, and 480-277 or 208Y-120 for commercial or light industry customers. Distribution transformers can vary in size, with the most common ranging in size from 5 KVA single phases to 2500 KVA. They come in single phase and three phases and can be pad mounted or pole mounted. mounted. The three phase pad mounted transformers are used with an underground primary circuit and three single-phase pole type transformers for overhead service. Common Application of Distribution Transformers Transmission lines Radar systems Photocopying machines Tool Machines Data processing equipment Telecommunications systems Test control and measuring systems Safety alarm and lighting plants        

Distribution Distribut ion transformers come in two types; dry-type and liquid. The Dry Type Distribution Transformers are usually smaller and do not generate much heat and can be located in a confined space at a customer's locat location. ion. The liquid types usually have oil which surrounds the transformer core and conductors to cool and electrically insulate the transformer. The Oil distribution transformer types are usually the larger and need more than air to keep them from overheating thus it acts as coolant & insulator.

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3.4 Oil Filled Distribution Transformer Defined

The majorities of high voltage transformers are bathed in transformer oil and are classified as oil filled transformers. The oil used used in an oil filled transformer is a highly refined mineral oil that is stable at high temperatures Oil filled transformers can operate in an overload condition of up to 30%, which makes them the most tolerant and efficient of any transformer design. The primary reason to use oil in transformers is as a coolant agent and provides part of the electrical insulation between internal live parts. For the larger oil filled transformers , the tank may have radiators through which the oil circulates by natural convection so it improves cooling. These large oil filled transformers may also have fans, oil pumps and oil to water heat exchangers. Oil filled transformers also undergo a prolonged drying process using electrical self-heating, or the application of a vacuum, or both to ensure that the transformer is completely free of water vapor before the cooling oil is intro introduced. duced. This prevents electrical breakdown under load. Basic function of the oil is to remove heat from the core and coil and it it is also a lso part of the insulation system. The transformer oil, here at GEMCO, is imported from foreign countries, especially from India. Sometime the oil is imported as crude oil and sometime as refined oil. Now a day„s almost refined oil is imported. Though it is refined there also different kinds of impurities exists in the Transformer Oil. They are: i) Mechanical impurities ii) Chemical impurities iii) Solid impurities etc. To separate these impurities from the Transformer Oil, different process are approached. They are: i) Oil Centrifuging ii) Oil Degassing and iii) Oil Filtering Before purification the oils are collected from the drums by motor pump or gear pump and stored in the reservoir. In GEM plant, the capacity of each oil reservoir is 12 barrels. Due to the presence of purification technology the same transformer oil is recycled and reused.

Figure 3.2: Transformer oil barrel

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Transformer Oil specifications from GEMCO: Mineral oil. Clear and transparent. Free from suspended matter/sediments. Density (at 21 ºC) =0 .895 gm/cc. Density (at 29 ºC) =0 .8110 gm/cc. Specific gravity = 0.85 Flash point (Min) = 140 ºC. Pour point (Max) = -30 ºC. Viscosity = 240 CST (at -10 ºC), 30 CST (at 20 ºC) and 09 CST (at 50 ºC). Acidity (Max) = 0.05mg KOH/gm. Burning point = 160 ºC and above. Dielectric strength (at 2.5 mm electrode gap) =50 kV (RMS) Water and Mechanical impurities should not present.

            

3.5 Transformer Construction Type: There are two general types of transformers 1. Core type transformer 2. Shell type transformer These two differ by the manner in which the windings are wound around the magnetic core. The advantages of each type are:

core-type (or three limbs) is the most commonly used method of construction, the smaller core means less weight and expense. Core-type provides better insulation between the primary and secondary because of its inherent two windings. shell-type (or five limbs) is used for larger transformers because they can be made with a reduced height. Shell-type provides a higher degree of mechanical protection to the winding because it is surrounded by core. In shell-type core is the most expensive part, while in core-type conductor is the costliest factor. From a manufacturing standpoint, core-type and shell-type designs are very similar. The magnetic core is a stack of thin silicon-steel laminations about 0.35 mm thick for 50 Hz transformer. In order to reduce the eddy current losses, these laminations are insulated from one another by thin layers of varnish. In order to reduce the core losses, transformers have their magnetic core made from cold-rolled grain-oriented sheet steel (C.R.G.O). This material, when magnetized in the rolling direction, has low core loss and high permeability. Apart from this, transformer could be further classified depending on the way the core is cut and stacked such as at GEMCO the core cutting might be straight or angular.

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Core Type Transformer:

Figure 3.3:(a) Core-type Transformer

In the core-type, the windings surround a considerable part of steel core as shown in fig (a). The core type transformers require more conductor material and less iron when compared to shelltype. The vertical portions of the core are usually called limbs or legs and the top and bottom portions are called the yoke. For single phase transformers, core-type has two legged core. In order to reduce leakage flux, half of the L.V. winding is placed over one leg and other half over other leg. For H.V. winding also, half of the winding is placed over one leg and the other half over the other leg. L.V. winding is placed adjacent to the steel core and H.V. winding outside, in order to minimize the amount of insulation required. Shell Type Transformer: In the core-type, the steel core surrounds a considerable part of the windings as shown in fig (b). Shell-type transformer has three legged core. The L.V. and H.V. windings are wound on the central limb. In order to reduce leakage flux, the windings are interleaved or sandwiched. The shell type transformers require more iron and less conductor material when compared to core-type. There are two types of windings employed for transformers. 1. Concentric coils. 2. Interleaved coils. The concentric coils are used for core-type transformers and interleaved coils for shell-type.

Figure 3.3:(b) Shell type transformer

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3.6 Transformer Outlook Figure 3.4: Typical Transformer parts labeling labeling

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3.7 Primary Transformer Accessories:

(a)The conservator tank is an expansion vessel party filled with oil and connected to transformer tank. Its function is to ensure that the transformer tank is completely filled with oil every time. The system allows the transformer tank to remain full despite contraction or expansion of the fluid due to temperature change. Figure 3.5: (a) Conservator tank

(b)Silica gel is used in conservator tank to absorb moisture and remove dust/foreign impurities from air sucked in. Whenever transformer is loaded or unloaded, oil temperature inside the tank rises or falls, air volume inside the tank changes resulting in pushing out or sucking in the air. During this breathing process air is made to pass over the silica gel crystals which absorb moisture, dust/foreign impurities get trapped in oil cup. Oil cup are made of transparent yet unbreakable polycarbonate plastic.

Figure 3.5: (b) Silica gel

(c)Radiators are used in a transformer to cool the transformer oil through natural air or forced air flowing in these radiator fins. As the transformer oil temperature goes down due to cooling it goes to the transformer tank from bottom ,cool the windings and gets heated, and then returns to the radiator for next cooling .This cycle repeats as the oil flow is also natural due difference in temperature of oil on bottom and top.

Figure 3.5: (c) Radiator

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(d)The bushing is a hollow insulator, allowing a conductor to pass along its centre and connect at both ends to other equipment. Bushings are often made of wet-process fired porcelain, and may be coated with a semi-conducting glaze to assist in equalizing the electrical stress along the length of the bushing. The inside of the bushing may contain paper insulation and the bushing is often filled with oil to provide additional insulation. Bushings for mediumvoltage and low-voltage apparatus may be made of resins reinforced with paper. The use of polymer bushings for high voltage applications is becoming more common.

Figure 3.5: (d) Bushing

(e)Surge arresters are devices that help prevent damage to apparatus due to high voltages. The arrester provides a low-impedance path to ground for the current from a lightning strike or transient voltage and then restores to a normal operating conditions. ROD GAP ARRESTER It is a very simple type of diverter and consists of two 1.5 cm rods, which are bent at right angles with a gap in between as shown in Fig. It could be labeled as the protective device for transformer. Figure 3.5: (e) Surge arrester

(f)The rating plate carries all the important specification of the transformer. The common specifications are, KVA rating, voltage rating, connection type, vector group, tapping, cooling type, and ambient temperature. It also bears the manufacturing date and model. Apart from this, there are other specifications which are usually in the manual. Figure 3.5: (f) Rating plate

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(g)A transformer tap is a connection point along a transformer winding that allows a certain number of turns to be selected. This

means, a transformer with a variable turn‟s ratio

is produced, enabling voltage regulation of the output. The tap selection is made via a tap changer mechanism.

Figure 2.5: (g) Tapping

(h)Transformer tank envelops the active part, filled with completely with oil. The body has no contact with the active part. The paint at the inner side is oil resistive and the outer paint is anti-corrosion. Even if it is just a container but requires much consideration during design.

Figure 3.5: (h) Transformer Tank

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3.8 Working Transformer parts:

Figure 3.6: A typical transformer winding over the core(transformer leg)

Theoretically the primary and secondary winding are located on the opposite legs of the core but in actual constructions these are always interleaved to reduce leakage flux. The core is a combination of laminated sheets in order to limit loss due to eddy current. In order to avoid reluctance at the joint where the laminations are butted against each other, the alternate layers are stacked differently to eliminate these joints. Insulating cylinders are used to separate the core and coils, and the coils themselves. For proper oil circulation oil duct is used between coil and layers. Apart from this, the winding have insulation strip at the ends to hold the windings tightly and insulating it from the core.

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Figure 3.7: Transformer core-type(transformer core-type(transformer leg)

(a) Angular cut

(b) Straight cut

Angular Reduce reluctance against the magnetic flux by reducing the gaps between sheets (only four joints at four corners). Hence lessen the iron loss and betters the performance.



Performance





Cost





Preference



Straight Provides more reluctance to magnetic flux due to more number of gaps (eight joints i.e. double the angular cut). Hence performance is well below angular cutting. Cost is more as huge amount of Cost is well below the core is wasted due to the cutting off angular cut. at the corner to make it angular Preferred where efficiency is vital. Mostly used due to the less cost. 





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CHAPTER 4: Manufacturing & Design 4.1 The Technology for Manufacturing Oil-immersed Distribution Transformers:

GEMCO is an order based manufacturing company. When the company gets an order, it first goes to the production planning and management division (PPM). Here at PPM, it ‟s primary functions is to: 1. 2. 3. 4. 5. 6. 7.

Select production target, collect raw material, provide job order and material control slip, Monitoring, Prevent tender‟s document, Estimate, To draw sketch of the parts of required product and if necessary change the old drawing.

GEMCO‟s corporate thinking is driven by the commitment to quality. The complete in hou se manufacturing process helps to ensure optimum quality of the transformer. It have the complete testing facility to test and check all the major raw-materials used in manufacturing transformers. CRGO laminations are cut to achieve minimum magnetic flux distortion. To ensure the best quality of the coils of transformers, best quality winding wires and strips are collected through international tender. Fabrication section ensures the robustness, quality and fine finish of the tank and radiators of transformers. The manufacturing process of Transformers covers the following departments/functions: 1. Transformer Tank Fabrication 2. Surface Finishing and Painting 3. Coil Winding 4. Core Assembly 5. Core – Coil Assembly 6. Active Part (Assembled Core &Coils) Drying 7. Tank-up 8. Testing

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Figure 4.1: Process flow diagram of GEMCO ltd

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4.11: COIL WINDING: Coil winding of two types: 1. High Voltage Coils: HV coils are components of finished transformer. They are made on HV winding machines.

Figure 4.2: HT winding 

    

A hollow cylindrical former of predetermined diameter and length is being used as base over which coil is wound. Generally Super Enameled Copper Wire is used as basic raw material. The Coils are made in number of layers. The starting and finishing leads of each coil are terminated on one side of the coil. The leads are properly sleeved and locked at number of points. Oil channel is provided between mid-layers for proper cooling of HT coil.

2. Low Voltage Coils:

Figure 4.3: LT winding 



LV coils are also of the components of o f transformer. The procedure pr ocedure of making low voltage coil is generally same as that of HV-coils, described earlier. The shape of the basic raw-material is rectangular widely known as copper strip.

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4.12: CORE ASSEMBLY:

Fig4.4:core assembling     

The basic raw-material is COLD ROLLED GRAIN ORIENTED (CRGO) Silicon Electrical Steel Sheet. It is in the form of thin sheets & cut to size as per design. Generally three different shapes of core laminations are used in one assembly. These laminations are assembled in such a manner that there is no air gap between the joints of two consecutive sheets. The entire assembly is done on a frame commonly known as core channel (Yoke beam). These frames being used as clamping unit/units of the core assembly.

4.13: CORE-COIL ASSEMBLY:

Fig 4.5: Core-coil assembling  

        

The components produced in the coil winding and core assembly stage is then taken into core-coil assembly stage. The core assembly is vertically placed with the foot plate touching the ground. The top yoke of the core is removed. The limbs of the core are tightly wrapped with cotton tape and then varnished. Cylinder made out of insulating press board is wrapped on all the three limbs. Low voltage coil is placed on the insulated core limbs. Insulating block of specified thickness and number are placed both at the top and bottom of the low voltage coil. High voltage coils are placed over the low voltage coils. Gap between each section of high voltage coils and low voltage coils including top & bottom clearances is maintained with the help of oil ducts, as per design/drawings. The top yoke is re-assembled. Top core frame including core bolts and tie rods are fixed in position. Primary and secondary windings are connected as per requirements. Phase barrier between HV phases at the top are placed as per requirements. Connections to the tapping switch (if required) are made. Finally complete active part(Core-Coil Assembly) is placed into vacuum drying chamber. GEMCO ltd

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4.14: FINAL ASSEMBLY (TANK-UP): The final assembly activities are used to carry out after getting Tanks, Covers, all other related parts and dried up Active Parts. The procedure is as follows:           

The active part is taken out from the drying chamber and the “Megger Test” is carried

out. Only if the Megger value is as per the specification, the active part may be taken for final assembly. The tanks supplied by the fabrication section are brought to the final assembly section duly painted. Fitting like HV H V insulators, insulat ors, LV bushings, phase plate, conservator with o il level indicator are fitted on the top cover of the tank. Nuts-bolts, long pin, short pin, tap changer fittings, angle bolts etc of dried up Active Part are properly retightened. Active part is then placed inside the tank and properly locked by side clamp. Operating knob of tap changer switch is fitted, wherever required. Top cover is then fitted to seal the tank. Connections of primary and secondary to the terminal bushings are made before the Top Cover bolted with Tank. New, filtered, centrifuged transformer oil is filled in the tank up to the mark indicated on the oil level indicator. The completed transformer then placed for routine test after the oil is cooled.

4.15: TRANSFORMER TANK:  There is a state-of-art-in-house facility for manufacturing transformer tanks.  All tanks are made of high quality steel and can withstand vacuum as specified by the international standards and the customer. All welding joints are tested, ensuring 100% leak proof and mechanical strength. 4.16: PAINTING: The entire procedure of painting is done under two stages:

1. Surface Preparation/Cleaning of tanks, Covers, Conservators etc.:  Cleaning of tank is done normally by chipping/grinding. The outside surface of the tank with radiator fitted, Conservator, Top Cover and related  parts are shot blasted to achieve a very fine cleaned surface. 2. Painting of tanks: After surface preparation and cleaning the tanks, a coat of oil resistant paint is applied on  the internal surface of the tank.  The outside surface is painted with a coat of Stoving Enamel Red Oxide primer, termed as prime coat and subsequently with one coat of Stoving Enamel Final (grey), termed as final coat. 3. Drying:  The painted tanks, top cover, conservator with related parts are placed into the drying chamber where they are kept about 40-45minutes at 120°C. GEMCO ltd

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4.17: FINISHING:

Figure 4.6: Transformer after f inishin inishing g              

Fittings and accessories are checked. Transformers are filled with oil up to the required level marking. Loose accessories like earthing terminals, bimetallic connectors, dial type thermometers, name plate, rating plate etc. are also checked for proper fitting. Lugs/hooks for lifting and towing complete unit. Facilities for lifting core and coil assembly. Base designed for platform mounting on poles. First filling of oil shall comply with BS: 148/1972 or latest revision of IEC-60296 standard. Each H.T. bushing shall have bolted type bi-metallic connector suitable for accommodating ACSR/AAC having diameter range from 9 to 14.5 mm. Each L.T. bushing shall have bolted type bi-metallic connector suitable for 2 2 accommodating Copper/ACSR/ AAC of area range 2x95mm to 2x120mm . HT & LT bushings are installed on the top cover of the transformer. Dial type thermometer for oil temperature mounted on the top cover of the tank. Earthing terminal shall be at the bottom corner of the tank. Nameplate with transformer rating and winding diagram made of stainless steel has engraved letters filled with black enamel. The tank, conservator and radiator shall be painted with 2 (two) coats of grey finishing paint on an anti corrosive suitable prime.

4.18: QUALITY: 

The Quality Assurance Plan is guided by the urge to provide uncompromising quality to the valued customers to ensure lasting satisfaction. Each transformer undergoes a prescribed range of the tests as per IEC 60076 standards.

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4.19: TESTING:

Figure 4.7: Transformer at testing station

Each and every transformer undergoes routine tests as per IEC 60076 requirements. In accordance with IEC the following tests are carried out on finished Transformer as per sequence: Di-electric strength test of transformer oil.   Measurement of Insulation Resistance.  High Voltage Test. Induced Over Voltage Test.   Ratio Test.  Vector Group Test.  Measurement of No-Load Loss and Excitation Current.  Measurement of Load-Loss and Impedance Voltage.  Measurement of Winding Resistance.  Temperature Rise Test.

4.2: New Oil-immersed Transformer Warranty:

For a period of TWELVE MONTHS from the date of delivery, the equipment is guaranteed to operate in accordance with its nameplate when operated under normal load, usage, conditions and with proper care, installation and supervision. There is no liability for damage resulting from improper installation or operation of the equipment, voltage surges, negligence of others, accidents, natural forces, and operations beyond rated capacities, or misuse. Any mechanical or electrical parts proving defective under such normal operation and installation will be repaired or replaced. The seller shall not be liable for special or consequential damages or for delay in performance of this warranty. Seller shall not be responsible for repairs

or replacement made by others without the seller‟s consent.

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4.3: Design of a 150KVA Transformer 11/.415KV, 3 phase, 50Hz vector group Dyn-11.

Raw materials: 

LT Coil dimension, Thickness=5.6mm, width=11.2mm, insulation=.45mm



HT coil type- SWG17, dia=1.42/1.53



CRGO Silicon Electrical Steel Sheet with thickness 0.3mm &flux density Bm≤1.7Wb/m²

Common design formulae: 

The circumscribing diameter of the core= Cross-sectional area of core/0.69364



Reactance per unit for cylindrical coil, Xpu=

2Πfμ0*lmt(AT)(a+ Thickness of LT+HT) LT+HT )

Ez*hc Where AT=ampere turn, Ez=Voltage/turn, hc= height of coil.

3

Calculation: LT side(Y-connection),

Let turns T2=50, Then, Ez(voltage/turn)= 415/√3 = 4.792 50 Let Bm=1.58Wb/m², then cross-section area of the core, A=Ez/(4.44*f*Bm) =4.792/ (4.44*50*1.580) =13661.763mm² Chosen a six-step core, A=0.69364*d² , Therefore, core dia d=√(13661.763/.69364)=140.34≈140mm

Now, I2=150/(√3*.415)=208.680 & cross-sectional area of LT, a2=5.6*11.2*.985=61.7792mm², where .985 is the correction factor (cf) and its value is arbitrary depending on the designer.

Hence the current density, δ2=I2/a2=208.680/61.7792=3.37amp/mm². Area of LT with insulation= (5.6+.45)(11.2+.45)=6.05*11.65 The LT coil is to be wounded in two layers therefore the turns per layer is 50/2=25 which is taken to be 26 for convenience. During winding LT coil is bounded by equalizer ring of width 22mm at top & button. Hence height of LT coil= 26*11.65*(cf=1.095)+22*2=311+44=355mm. Actual height of LT, Lc2=26*11.65*1.095= 311mm

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The LT coil is separated from core with a circular circular insulation card with thickness of 5mm Hence inside dia of LT=d+5*2=140+10=150mm. Oil duct between two layers of LT for oil circulation=3mm Thickness of LT winding= (thickness of LT coil+ insulation)*2 +oil duct = (5.6+.45)*+3 =17mm Outside dia of LT=150+17*2=184mm Mean dia = (inner dia +outer dia)/2= (184+150)/2=167mm

Circumference of LT, Lmt2= Π*167=524.645mm Resistivity of LT, R2= (ρ*Lmt2*T2)/a2=(0.021x10ˉ³*524.646)/61.7792=0.0089169Ω Cu loss in LT = 3*(I2)²*R2= 3*208.680²*0.0089169≈1200watt Weight of copper in LT= 3*Lmt2*a2*T2*density

= 3*524.646*61.7792*50*8.9μ ≈ 45 Kg

HT side (Δ-connection), I1=150/(3*11)= 4.545Amp T1= 50*11/.2396= 2294 For tapping position-1, T1=150/(3*11.275)2352 Turns per tapping=2352-2294= 58 Tapping position I II III IV V

Turns 2352 2294 2236 2178 2120

Cross- sectional area of HT, a1= Π*d²/4= 1.58367 & current density δ1= I1/a1= 4.5454/1.58367= 2.87Amp/mm² HT coil height=LT coil height=355mm Width of HT side insulation at both top and button= 10mm Hence total height of HT coil= 355-10*2=335mm Actual height of HT, Lc1= 216*1.52*1.02=334.8mm Turns/layer= height of the coil/(dia of the HT wire*cf)=335/(1.53*1.02)= 216 Total length of HT coil= 216*1.52*1.02= 334.8mm

Number of layers= 2352/216= 10.88≈ 11 layers

Outside dia of LT= 184mm Oil duct between LT & HT =4mm HT insulating cylinder thickness= 3mm Hence inside dia of HT= 184=(4+3)*2= 198mm GEMCO ltd

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Thickness of oil duct between HT layers =3mm Thickness of insulating papers between layers=0.12 Thickness of HT winding= layers*dia +oil duct+layers*thickness of insulating paper*2 = 11*1.53+3+11*0.12*2

= 22.47≈25mm for convenience.

Outside dia of HT winding= 198+25*2=248mm Mean dia = (248+198)/2=223mm

Circumference of the HT Lmt1= Π*223= 700.575mm Resistivity of HT, R1= (ρ*Lmt1*T1)/a1= (0.021x10ˉ³*700.575*2294)/1.58367 = 21.31Ω Loss in HT= 3*I1*R1= 1320watt≈1300watt. Total loss= loss in HT+ LT= 1300+12002500watt Weight of copper in HT= 3*Lmt1*a1*T1*density

= 3*700.575*1.5836*2353*8.9μ = 69.67≈72kg

Ampere-turn in LT, AT=I2*T2=50*208.686 Lmt= Lmt1+Lmt2 = 700.575+524.646 700.575+524. 646 2 2

= 612.61mm≈0.61261M

Lc= Lc1+Lc2 = 334.8+355= 322.9432= 0.3229432M 2 2 Recalling the previous values, Voltage per turn in LT, Ez=4.792 The summation of oil duct & insulating cylinder between HT & LT, a=4+3=7mm=.007M Thickness of HT=22mm= 0.022M Thickness of LT=17mm=.017M Oil duct between HT layer, a`=3mm= 0.03M The reactance per unit for this particular design is given by the derived formula, Xpu= 2Πfμ0*lmt(AT)(a+ Thickness of LT+HT +a`) Ez*Lc 3 4 ^-7

= 2Π*50*4Π*10 *0.6126*50*208.686(0.007 + 0.022+0.017 + 0.003) 4.792*0.3229432 3 4

=0.02075)≈0.0338357 = 1.630636*(cf =0.02075)≈0.0338357

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Total equivalent resistance in primary, Rp=R1+R2*(N1/N2)²=21.31+0.0089169(2294/50)²= 40.07984 & Rpu= Rp/Rbase= 40.07984*4.5454/11000= 0.01656172

Hence, total impedance Zpu= √(Rpu+Xpu)=0.03955 Therefore the calculated values are: LT coil weight=44.5kg HT coil weight=72kg LT winding height=335mm HT winding height=335mm Total copper Loss=2560watt Inside dia of LT= 150mm Outside dia of LT=184mm Inside dia of HT=198mm Outside dia HT=248mm Impedance voltage=3.95% Core loss=380watt           

Core design: Window Height = winding height + thickness of wooden insulation between coil & core =355mm+10*2=375mm Centre to centre distance between legs= HT outside dia+10mm = 248 +10=260mm For 6-step core section, width of lamination pieces used: 0.32% 0.46% 0.63% 0.77%

0.88%

0.95%

The % is of dia of core.

1. 0.95*140=133≈130mm 2. 0.88*140=123.2≈120mm 3. 0.77*140=107.8≈110mm 4. 0.63*140=88.2≈90mm 5. 0.46*140=64.4≈65mm 6. 0.32*140=44.8≈40mm

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Thickness of steps is given by: 1. √(140² -130²)=51.96mm 2. √(140² -120²)-51.96=20.4mm 3. √(140² -110²)-51.96-20.4=14.4mm 4. √(140² -90²)-51.96-20.4-14.4=21.6mm 5. √(140² -65²)-51.96-20.4-14.4-21.6=15.6mm 6. √(140² -40²)-51.96-20.4-14.4-21.6-15.6=10.8mm

Steps 1 2 3 4 5 6

Size 130 120 110 90 65 40

Thickness 51.6 20.4 14.4 21.6 15.6 10.8

quantity 51.6/03=172 20.4/0.3=68 14.4/0.3=48 21.6/0.3=72 15.6/0.3=52 10.8/0.3=36

Actual A= (130*172+120*68+110*48+90*72+ (130*172+120*68+110*48+90*72+65*52+40*36)*0.3*(cf=0.96)=13 65*52+40*36)*0.3*(cf=0.96)=13564.8mm² 564.8mm² & Bm=4.792/(222*0.0135648)=1.59 Width of upper is chosen to be 120mm

Length of sheet from steps 1-6=615-120=495mm

120mm

Length of upper sheets, 1’:step-1,length=650-130*2=390mm

130 mm 615mm

375mm

2’:step-2,length=390+(130-120)=400mm 3’:step-3,length=400+(120-110)=410mm 4’:step-4,length=410+(110-90)=430mm 5’:step-5,length=430+(90-65)=455mm 6’:step-6,length=455+(65-40)=480mm

Length of lower sheets steps 1-6=260mm

260mm

260mm

width of step 8)1-4=120mm & 9)5-6=100mm

650mm

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Data obtained: Dimension pf sheets

Position 1 2 3 4 5 6 1’ 2’ 3’ 4’ 5’ 6’

A b

Length

495 495 495 495 495 495 390 400 410 430 455 480 260 260

Width

130 120 110 90 65 40 120 120 120 120 100 100 120 100

Quantity(pcs) 516 204 144 216 156 108 172 68 48 72 52 36 720 2688

The above design gives a brief assumption about the transformer electrical design but in practice further complex issues is to be dealt with. Apart from this, there is the mechanical design, which deals with the design of the transformer tank which also incorporates complicated tasks. Summarizing the design: 1. Specification of the transformer to be designed 2. Chose Ez, Bm, 3. Calculate A and D for core 4. Find conductor size of HT & LT winding w inding and calculate current density 5. Choose layout of windings-- nos of turns per layer , nos of layers, distance between coils 6. Calculate R, X, Z. 7. Calculation of performance 8. Designing the tank

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CHAPTER 5: Transformer Testing 5.1 Testing Station: Transformers are very reliable devices for power transmission and distribution system and can provide service for a long time if maintained and serviced regularly. They are also the single most costly item in any substation. To protect against failures that can result in costly repairs and long periods of downtime. The best insurance against transformer failure is to ensure that they are properly installed and maintained. The complete line of testing equipment makes testing easy and accurate. After the completion transformers are sent to the testing station for routine testing. In the Testing Station transformers are tested through the IEC (International Electromechanical Commission) standards. If the transformers are passed the testing standard of the IEC, then the transformers are send to the delivery section. Otherwise faulted transformers are sent to repair.

There is various kind of testing techniques, not necessarily every transformer manufacturing company must follow the same techniques. But there are some techniques which are common and essential techniques that everyone must follow. Among the testing, there are some which are considered less important and could be ignored; some are given important over other and ignored, and sometime one testing can yield the result of other testing. However, in GEMCO, they testing section is supplied with a evolution sheet that has nine compulsory testing, of which eight are test in the testing section and the remaining one is the oil breakdown voltage test which is carried out at the lab. The eight compulsory testing at testing section are: 1. Winding insulation testing 2. Power frequency withstand test 3. Ratio test 4. Induced over voltage test 5. Vector group test 6. No-load test 7. Load – loss loss test 8. DC resistance measurement test Depending on time and work-load, some of the testing is ignored such as vector group test because the ratio test confirms the same result. Some of the tests are repeated on both the active part and after final assembly, such as the test no1 & 8. Also, apart from the above other tests are carried out on need basis such as the impulse test. Beside transformer, test is also carried out on the insulation i.e. bushing and the test is known as Lightning Arrestor Test.

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5.11 Winding insulation testing: It is a test of the insulation properties. "Megger" is used to check the insulation of active part of the transformer, and to check for any moisture and short between body and active part also between HT and LT (as shown in table). In this test 1000v/2000MΩ Megger is used. If the active part is not properly dried, the meggermeter will show very low MΩ. Hence before this test, the active part of the transformer is heated at 100º C temperature for 72 to 96 hours. Heating process is done in an Electric drying chamber or Vacuum drying chamber. However in rainy season the result will never be accurate so it is discourage to carry the test.

Table 5.1: Megger test Measuring points

Resistance (MΩ)

15 sec.

30 sec.

Ratio

Body to HT Body to LT HT to LT Method: For instance in case of body to HT, the LT side is shorted and grounded whereas the HT side is shorted and connected to one probe of the megger with other probe connected to body. A

MΩ

N

B a

C b

HT

c

LT Body Fig 5.1: Connection for Megger test gnd Two readings are taken, one at 15 sec and other at 60 sec. The ratio of the resistance values at 60sec to 15sec is called dielectric absorption ratio (DAR) which must be greater than 1.3 according to IEC. The above is carried out similarly for LT-body and HT-LT with HT earthed and body earthed respectively.

5.12 Power frequency test: This test is also done to check the insulation strength of active part. At first the HT sides and LT sides are shorted through a wire or copper bar. Then this test is done by supplying 28 KV at HT side for 1 minute with LT side and body grounded. Again almost same process is carried by supplying 2.5 KV at LT for 1 minute with HT and body grounded. If there is any insulation fault, the supplied voltage will be totally grounded and the voltmeter will not show any voltage. The whole process is shown by this simple diagram: 230 V

Step up

36 KV

Regulate

2.5 KV

Supply

28 KV

Supply

LT

HT

Fig 5.2: Power frequency test

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5.13 Induce test: This test is done to check the turn-to-turn, layer-to- layer and coil to coil insulation of the active part is properly done or not. In this test, the double rated voltage of 830 V is supplied to the three phases (a, b, c) of LT sides at a frequency of 200 Hz for 30 seconds. The frequency is increased to four times the power frequency so as to prevent the current from doubling and burning the conductors. [Emf/turn =444.4NfΦ, but Φ is proportional to I. Then emf/turn=444.4NfI=kfI. If emf is doubled then both f & I could be double Hence f is increased to 4 times to prevent I from doubling] If there is any insulation fault, the ammeters connected with the three phases (a, b, c) of LT sides, will show a current. 5.14 Ratio test: In this test, 70 V is supplied to the each phases (a, b, c) of LT sides and the corresponding HT value for each tapping position is stepped down by a PT and measured. The voltages are taken from all the three phases (A, B, C) of HT sides by voltmeter. The voltages in the five tapes (I, II, III, IV, and V) of the tap changer are observed and the ratio of HT to LT is calculated for the desired values given in the table below.

Table 5.2: HT-LT Voltage ratios Tap No.

HT-LT Voltage Ratio

I II III IV V

27.16 26.5 25.8 25.8 24.5

The purpose of the test is to check the active part for proper tapping with a tolerance of +5%. Basically the type of fault that is observed in this test is the inter-change between positions of the tapping. In this case the active part in checked for for proper tapping position but if the error still persist then this is could be due to failure in layer to layer insulation or improper winding.

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5.15 Vector Group test: This test is performed to check the vector positions as well as the delta (Δ) connection in HT side and star (Y)connection in LT side of the transformer are properly connected or not. This test is done by supplying 400 V at the HT sides of the transformer. In this test, „A‟ phase of HT and „a‟ phase of LT is shorted. If V Cb > others i.e. VAB, VBc, VBb then the results confirms vector group: Dyn-11. Here, Dyn-11means Delta connected HV winding, star connected LV winding with neutral brought out; LV is leading HV with 30°. Vector group means it define the primary & secondary side connection type of the transformer says D for the delta connections & Y for the star connections then the 11 stands for the phase displacements between the primary side & secondary side. In the sense in a clock while 11„o‟clocks shows the angle between the small needle & big needle will be 30 degree, as the phase displacement will be 30 degree. Vector group of a transformer describes the relation between primary voltage vectors and the secondary voltage vectors.

DYN11 means: D - Primary DELTA connection Y - Secondary STAR connection N - Neutral Point available in the secondary star connection 11 - 30 degrees lag (11o clock position for better understanding) The coil connection for HT in case of DYN_11 is given below

Fig 5.3: Connection for DYN-11

And the after shorting the „A‟ and „a‟ phase the resultant vector diagram stand as given below. A

a c

C

b

B Fig 5.4: Vector group triangle

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5.16 No load loss test: The purpose of this test is to calculate the core loss. At no load the primary input current has to supply the eddy current and hysteresis loss in the core and a small amount of copper loss in conductor. This current is used to calculate the core loss. In the test, for convenience the rated voltage is supplied at the LT side with the HT side open. Hence the transformer is at no-load and the current in primary is the loss current. This current is measured with the ammeter for each phase and the wattmeter reading W1 & W2 is noted.

Fig 5.5: Double wattmeter connection for Y

The loss is then calculated as shown below: W1 – W2 = W3 × wmf Here, wmf = wattmeter multiplying factor ratio =Voltmeter scale*ammeter scale × Ct ratio × Pt ratio Total wattmeter division 5.17 Full Load test: Load loss is also widely known as Copper loss. This test is performed by connecting the supply to HT side and the LT side is shorted. When the LT is shorted it acts as a load with very low low impedance and draws a high current. The supply at HT is then varied until the rated current at HT is achieved and the corresponding voltage at HT for this current is called the impedance voltage which is measured with the voltmeter and is well below the rated voltage. For this rated current the corresponding wattmeter reading of W1 & W2 are noted and the load loss is calculated as in no-load calculation. 

% impedance voltage = impedance voltage/rated voltage * 100 The % impedance voltage must lie within the range of 3.6≤% i.v. ≤ 4.4. If the value is lower than 3.6 it means low impedance and conductors might burn And if the value is more than 4.4 it means high impedance hence higher loss.

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5.18 DC Resistance test: This test is done to check the different connections of the transformer are properly connected or not. This test is done by supplying phase to phase DC voltage and DC current at the HT sides of a transformer. Again DC voltage and DC current supplied phase to phase at the LT sides of a transformer. The DC current is adjusted by barriers. At last Ohm„s law is applied to calculate resistive value i.e. ohmic value. If the current is not flowing, there is an open connection across the respective phases. 5.19 Lightning Arrestor Test: In GEM plant, Lightning arrestor test is also done by testing section. After assembling the lightning arrestors are sent to the lightning arrestor test section. This test is done to check the resistivity property of the lightning arrestors. In this test, 80K V DC voltage supplied to the top side of the lightning arrestor. And bottom side of the lightning arrestor is grounded. Then the leakage current is measured by an ammeter. The ammeter is connected by a CT (current transformer). If the leakage current is lower than 3 ampere, the lightning arrestor is passed the quality check and it is accepted. Otherwise the lightning arrestor is rejected. A lightning arrester tester includes two tubular poles respectively carrying probe terminals, one probe terminal being connected in series through a high-voltage rectifier and high resistance circuit disposed within the pole to one terminal of a DC microammeter mounted on the outside of the one pole, the other probe terminal being connected through a high resistance circuit housed within the second pole to the other meter terminal. A diode is connected in parallel with the meter to protect the meter from high leakage currents. In use, the probe terminals are respectively connected to a high-voltage power transmission or distribution line and one terminal of an arrester, the other terminal of which is grounded or, alternatively, to ground and to one terminal of an arrester, the other terminal of which is connected to a highvoltage power transmission or distribution line. The meter displays DC leakage current through the arrester.

Fig 5.6: Lightning arrestor

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5.2 A GEMCO-Transformer Inspection Report

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In the above Inspection report, it includes all the specification of the transformer and the testing results. After completion it is checked by the in charge and supplied to the consumer. Explanation of some attributes in the report:  



Type of cooling: ONAN, it stands for OIL natural Air natural Standard: IEC-76/BS-171, i.e. the transformer is manufactured in accordance to the international electric commission standard no 76 and Bangladesh standard no 171. Type: Tm-200/11-76T1/97; where 200 are the KVA rating, 11 is the voltage rating, 76 is the model and 97 is the manufacturing year. These information are the specification and the other information are the exact testing result that are described in the report.

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CHAPTER 6: CPL- Central plant laboratory 6.1 Purpose of CPL: It is one of the important sections of GEM plant. It„s also known as quality control section. Quality Control means, the activities which are performed to keep the quality of any product within acceptable or tolerable limit of its specification, drawing (if any) and documents are called Quality Control. Products Quality is determined by the characteristics it possesses, which again depends on: a) Specification b) Price/Cost c) Use d) Choice e) Environment To keep the quality within an expected performance limit, products must have to go through some indirect process, such as- Testing, Inspecting, Checking etc. which ultimately will control its quality. 6.11 Wire Resistivity test: Measuring resistance is an important part of maintaining high voltage copper wires and copper strips. This test is done by measuring the resistance of the copper wire or strip. Resistance is measured by micro-ohmmeter. In this test, a copper wire or strip of 1000mm length is used as sample. Then two electrodes of the micro-ohmmeter are connected to the end of two copper wire terminals. Then resistance is measured by microohmmeter. After that, resistivity of the copper wire or strip is measured by this formula:

ρ =RA =RA L

where, ρ = Resistivity, R = Resistance, A = cross -sectional area and L = length

PURPOSE OF TESTING 1. Calculations of the I2R component of conductor losses. 2. Calculation of winding temperature at the end of a temperature test cycle. 3. As a base for assessing possible damage in the field.

Fig 6.1: Wire resistivity test measuring device.

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6.12 Oil breakdown voltage test: The dielectric test measures the ability of the oil to withstand electric stress without failure. The higher the value, the lower the presence of contaminants such as water, dirt, or other conductive particles. Oil breakdown voltage test is also known as oil insulation test. Insulation oil in power transformers serves as a heat transfer medium and as an insulating liquid. Two reasons motivated for the investigation of its insulation properties: Firstly power transformers are often operated under aged conditions. Thus the moisture content in oil increases, aging products become dissolved and particles are dispersed. Secondly, transformers are operated under novel environmental conditions, were low or high pressures exist. A safe service necessitates the thorough investigation of these influences.

The breakdown voltage in mineral insulation oils was investigated under the influences of moisture, acidity, pressure and particles using electrodes at 2.5mm gap according to IEC156/95. In GEM plant oil breakdown voltage test is done by an old non-automatic machine. At first 100 mg of purified transformer oil poured into the ceramic test cup. There are two electrodes are inserted from the two sides in the test cup. The electrodes are setup at 2.5mm gap. After pouring oil into the test cup, the cup leaves idle for fifteen minutes, to blow up the bubbles made from pouring oil into the cup. Then the test cup is placed into the breakdown voltage testing machine. The machine is powered from 220V line. There is a step-up transformer into the machine which increases power to some kilovolts. Then the machine switched on and the voltage applied between the electrodes increases slowly. The voltage increasing must be done slowly and uniformly, otherwise machine will show wrong result. At some KV, the insulation capacity of the oil will breakdown and current will pass by sparking or arching between the electrodes through the oil. This voltage is the breakdown voltage of the transformer oil.

Figure 6.2: Oil break-down voltage test machine

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6.13 Flash point and ignition point test: Flash point: The flash point of oil is a measure of its inflammability and means ability of a substance to burn in air. It is the temperature at which sufficient vapor is produced to explode with air if brought in contact with a flame. The flash point gives an indication of the volatility of oil; the higher value, the more suitable it is as a lubricant. In GEM plant, flash and ignition points are tested by the Open-Cup method. Here flash point test is done by an old non-automatic machine. At first 100 mg of purified transformer oil poured into the machine. There is a thermometer and stir strip inserted form the top cover of the machine. The thermometer and stir strip are set at some distance upper from the bottom surface of the machine. After pouring oil into the machine, the machine is powered from an autotransformer by 220V line. There is an electric heating coil inside the machine by which the oil is heated. Then the autotransformer switched on and the voltage is regulated to increase or decrease the heating temperature of the oil. The temperature of the oil increasing must be done slowly and uniformly, otherwise temperature of the oil will cross the flash point temperature. After reaching the temperature at 30ºC, a flame inserted from the top cover and observed that, weather the oil vapor explode with the flame or not. If a blue flame is seen arching at the surface of the oil, then this temperature is the flashpoint of the transformer oil Ignition point: It is the lower temperature at which the oil continues to burn after a flame has been applied to its surface. Ignition point is also known as t he fire point po int of oil. Fire Point is the minimum sample temperature at which vapor is produced at a rate sufficient to sustain combustion for at least 5 seconds. Fire point is tested by Cleveland Closed Cup. In GEM plant, after founding the flash point of the transformer oil, the top cover of the flash point testing machine is removed. Then slowly increase the temperature of the flash point testing machine by autotransformer. Then a flame has been applied to the surface of the oil and observed that, weather the oil continues to burn after the flame has been applied to its surface or not. If a blue flame is seen continuously burning at the surface of the oil, then this temperature is the ignition point of the transformer oil. 6.14 Viscosity test: Viscosity is co-efficient of resistance to flow with in a liquid. As the higher viscosity, the liquid is the thicker and less mobile are its particles and vice versa. If the oil is thicker, it cannot properly dissipate the heat generated by the coil and core. So the lower the viscosity, the better oil for the transformer. Viscosity at 40°C or 100°C measures a lubricant's resistance to flow (fluid thickness) at temperature and is considered an oil's most important physical property. Depending on lube grade, viscosity is tested at 40 and/or 100° Celsius. Apart from the above test, various other tests do take place at CPL, such paint is also tested for

purity and viscosity and also before every raw material enters GEMCO it is tested for proper quantity, specification and dimension such as bushings, surge arrester etc. GEMCO ltd

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CHAPTER 7: Conclusion 7.1 Concluded with Gratitude I would once again like to thank Almighty Allah for giving me the ability to successfully complete the internship program and the internship report. Internships provide us opportunities to gain experience in relevant field which "bridges the gap" between the academic environment and a practical work environment. Besides, this internship program made me to realize the value of working together as a team and as a new experience in working environment, which challenges us every minute. Power sector is the most important and developing sector in Bangladesh. And distribution transformer is one of the key elements of the distribution system. Without distribution transformer we cannot imagine power distribution system. So, the manufacturing of the distribution transformer is very important for our distribution system and it also very important to compensate the losses of the distribution system. General Electric Manufacturing Company Ltd is now the market leader in the field of Transformer manufacturing in Bangladesh. The main reason working behind this success is their manufacturing process and materials. During my stay in GEMCO Ltd I studied the transformer manufacturing process, specifically the transformer testing station. The study was very useful for me. I experienced a lot of things which I had only heard or read about. As three months passed away, my internship just concluded with effect from today. I am glad to have undergone this internship because it taught me various important things and also got me to explore my own interests. The work I made during my internship gave me a lot of knowledge that could help me in flourishing my future carrier.

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REFERENCES



http: ttp:// //en en.w .wiikipe kipedi dia. a.org org// www.scribd.com/doc/





A study on Transformer



Transformer testing and design

http:/ http://ww /www.b w.bsec sec.gov .gov.bd .bd/h /html tml/en /ent_gel t_gelectri ectric.ph c.phpp

BIBLIOGRAPHY



Elec Electri trical cal power power sys system tem desi design, gn, byM. V. Deshpande.



Powe ower sy system tem, by byV. K. Mehta.

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A Report on Transfrmer (Part-2)

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