Insulation System in Turbo Generators (2)

INSULATION SYSTEM IN TURBO GENERATORS A Mini Project Work Submitted in partial fulfillment of the Requirements for the award of degree of

BACHELOR OF TECHNOLOGY In

ELECTRICAL & ELECTRONICS ENGINEERING By

D.VAMSHI RAJ (08J51A0214) S.ANURAG (08J51A0241) A.ARUN KUMAR (08J51A0201) S.SUMAN (08J51A0240) Under The Guidence Of

Mr.J.P.Balaji (HOD) Dept. of Electrical and Electronic Engineering

Mannan Institute of Science and Technology Aloor(V), Chevella(M), Ranga Reddy(Dist.).

2011 ACKNOWLEDGEMENT We take this opportunity to record our gratitude to all those who helped us in successful completion of the project. We take immense pleasure in thanking Mr. BABU RAO garu , Principal of our college and Mr. J.P. BALAJI garu, Head of the department for having permitted us to carry out this project.

We wish to express our deep sense of gratitude to our Internal Guide MR.M.N.V. SURYA PRASAD and MR.S.JITHENDER REDDY for her able guidance and useful suggestions, which helped us in completing the project work in time. We immensely be grateful to Sri. B. PRASADA RAO garu, CEO (chief executive officer) of BHARAT HEAVY ELECTRICALS LIMITED for giving us permission for undergoing study project training in their company. We wish to thank MR. S.K.SAHOO garu, HRD-MANAGER, BHEL for allotting us this project.

Finally, we wish to express our profound thanks to all the employees, in charges and workmen without whose support, completion of this project would have been impossible.

ABSTRACT

BHEL is the largest engineering and manufacturing enterprise in India in the energy related infrastructure sector today which manufactures turbo generators (2-pole and 4-pole) ranging up to 150 MHz. The manufacturing process of turbo generator is mainly divided into stator section and rotor section where stator frame, stator core, stator windings, end covers are received from the stator building section and rotor with rotor windings and rotor retaining rings is received from rotor section and assembled at the assembly section. Each and every process is carried out in a sequential process. Turbo generators are designed with the Closed circuit air cooling with water or air coolers mounted in the pit. The layout of the manufacturing plant is such that it is well streamlined to enable smooth material flow from the raw material stages to finished goods. The raw material that are produced for manufacture are used only after thorough material testing in the testing lab and with strict quality checks at various stages of productions. Latest technologies like vacuum press impregnated micalastic high voltage insulation, polyester fleece tape impregnation for outer corona protection are implemented to produce high quality insulation for turbines, outstanding performance and long lasting lifetime.

INDEX ABOUT THE COMPANY:

BHARAT HEAVY ELECTRICALS LIMITED. INTRODUCTION

Introduction History of Turbo Generators Principle of operation Synchronous generators classification based on the medium used for generation Components of Turbo Generator Stator Rotor STATOR

Stator Stator frame Stator core The purpose of stator core Preparation of laminations Compounding operation Blanking & notching operation The different operation in manufacturing of laminations Deburring operation

ROTOR

Rotor Rotor shaft Rotor winding Construction Conductor material Insulation Rotor slot wedges Rotor Retaining rings INSULATION SYSTEM

BHEL insulation system for Turbo Generators BITUMEN system & life extension Various insulation system & practices VACCUM PRESSURE IMPREGNATION

VPI system Introduction to VPI system Features and benefits VPI of resin poor insulated jobs Process of VPI 1. General 2. Preheating 3. Impregnation 4. Post curing 5. Electrical testing CONCLUTION & FUTURE SCOPE

ABOUT THE COMPANY BHARATH HEAVY ELECTRICALS LIMITED

ABOUT THE COMPANY: BHARAT HEAVY ELECTRICALS LIMITED  Bharat Heavy Electrical Limited (BHEL) is today the largest engineering Enterprise of India with an excellent track record of performance.  Its first plant was set up at Bhopal in 1956 under technical collaboration with M/s. AEI, UK followed by three more major plants at Haridwar, Hyderabad and Tiruchirapalli with Russian and Czechoslovak assistance.  These plants have been at the core of BHEL’s efforts to grow and Diversify and become India’s leading engineering company.  The company now has 14 manufacturing divisions, 8 service centers and 4 power sector regional centers, besides project sites spread all over India and abroad and also regional operations divisions in various state capitals in India for providing quick service to customers.BHEL manufactures over 180 products and meets the needs of core sectors like power, industry, transmission, transportation (including railways), defense, telecommunications, oil business, etc.  Products of BHEL make have established an enviable reputation for high quality and reliability. BHEL has installed equipment for over 62,000 MW of power generation for Utilities, Captive and Industrial users.  Supplied 2,00,000 MVA transformer capacity and sustained equipment operating in Transmission & Distribution network up to 400kV – AC & DC, Supplied over 25,000 Motors with Drive Control System Power projects.  Petrochemicals, Refineries, Steel, Aluminum, Fertilizer, Cement plants etc., supplied Traction electric and AC/DC Locos to power over 12,000 Km Railway network. Supplied over one million Valves to Power Plants and other Industries.  This is due to the emphasis placed all along on designing, engineering and manufacturing to international standards by acquiring and assimilating some of the best technologies in the world from leading companies in USA, Europe and Japan, together with technologies from its-own R & D centers.  BHEL has acquired ISO 9000 certification for its operations and has also adopted the concepts of Total Quality Management (TQM).  BHEL presently has manufactured Turbo-Generators of ratings up to 560 MW and is in the process of going up to 660 MW.  It has also the capability to take up the manufacture of ratings unto 1000 MW suitable for thermal power generation; gas based and combined cycle power generation as-well-as for 13 diverse industrial applications like Paper, Sugar, Cement, Petrochemical, Fertilizers, Rayon Industries, etc.  The Turbo generator is a product of high-class workmanship and quality. Adherence to stringent quality-checks at each stage has helped BHEL to secure prestigious global orders in the recent past from Malaysia, Malta, Cyprus, Oman, Iraq, Bangladesh, Sri Lanka and Saudi Arabia. The successful completion of the various export projects in a record time is a testimony of BHEL’s performance.  Bharat Heavy Electrical Limited (BHEL) is, today, a name to reckon with in the industrial world. It is the largest engineering and manufacturing enterprises of its kind in India and is one of the leading international companies in the power field.

 BHEL offers over 180 products and provides systems and services to meet the needs of

core sections like: power, transmission, industry, transportation, oil & gas, nonconventional energy sources and telecommunication.  A wide-spread network of 14 manufacturing divisions, 8 service centers and 4 regional offices besides a large number of project sites spread all over India and abroad, enables BHEL to be close to its customers and cater to their specialized needs with total solutions-efficiently and economically.  An ISO 9000 certification has given the company international recognition for its commitment towards quality.  With an export presence in more than 50 countries BHEL is truly India’s industrial ambassador to the world. BHEL Hyderabad unit’s manufacture includes the following.  Gas turbines  Steam turbines  Compressors  Turbo generators  Heat Exchangers

 Pumps  Pulverizers

 Switch Gears  Oil rigs

BHEL Hyderabad is the only one in Asia that has the latest type of insulation system called the Vacuum Pressure Impregnation System.

INTRODUCTION: TURBO GENERATOR A turbo generator is a turbine directly connected to electrical generator for the generation of electric power. An electrical generator is a machine which converts mechanical energy to electrical energy.

HISTORY OF TURBO GENERATORS: Generators are based on the theory of electromagnetic induction, which was discovered by Michael Faraday in 1831, a British Scientist. Faraday discovered that if an electric conductor, like a copper wire, is moved through a magnetic field, electrical current will flow(be induced) in the conductor. So the mechanical energy of the moving wire is converted into the electric energy of the current that flows in the wire.

PRINCIPLE OF OPERATION: Turbo generator or A.C. generators or alternators operates on the fundamental Principles of FARADAYS LAWS OF ELECTROMAGNETIC INDUCTION. In them the standard construction consists of armature winding mounted on stationary element called stator and field windings on rotating element called rotor. The stator consists of a cast-iron frame, which supports the armature core, having slots on its inner periphery for housing the armature conductors. The rotor is like a flywheel having alternating north and south poles fixed to its outer rim. The magnetic poles are excited with the help of an exciter mounted on the shaft of alternator itself. Because the field magnets are rotating the current is supplied through two slip rings. As magnetic poles are alternately N and S, they induce an e.m.f and hence current in armature conductors. The frequency of e.m.f depends upon the no. of N and S poles moving past a conductor in 1 second and whose direction is given by Fleming ’s right hand rule.

SYNCHRONOUS GENERATORS CLASSIFICATION BASED ON THE MEDIUM USED FOR GENERATION:  Turbo generators in Thermal, nuclear, Gas station • High speed – 3000 rpm • Min poles – 2 poles • Horizontal construction • Cylindrical rotor  Hydro generators in hydel plants • low speed – 1000 to 500 rpm • more poles – 6 or more • vertical construction • salient rotor

COMPONENTS OF TURBO GENERATOR: STATOR  Stator Frame  Stator Core  Stator Windings  End Covers

ROTOR  Rotor Shaft  Rotor Windings  Rotor Retaining Rings

The following auxiliaries are required for operation:  Bearings  Cooling System  Oil Supply System  Excitation System

STATOR

S TATOR STATOR FRAME: The stator frame is of welded steel single piece construction. It supports the laminated core and winding. It has radial and axial ribs having adequate strength and rigidity to minimize core vibrations and suitably designed to ensure efficient cooling. Guide bars are welded or bolted inside the stator frame over which the core is assembled. Footings are provided to support the stator foundation.

Fig: stator frame S TATOR CORE:-

The stator core is stacked from the insulated electrical sheet steel lamination and mounted in supporting rings over the insulated dovetail guide bars. In order to minimize e current losses core is made of thin laminations. Each lamination layer is made of individua sections. The ventilation ducts are imposed so as to distribute the gas accurately over the c and in particularly to give adequate support to the teeth. The main features of core are – 1. To provide mechanical support. 2. To carry efficiently electric, magnetic flux. 3. To ensure the perfect link between the core and rotor.

THE PURPOSE OF STATOR CORE  To support the stator winding  To carry the electromagnetic flux generated by rotor winding. So selection of material for building up of core plays a vital role. The losses in the core are of two types. Hysterysis Loss: Due to the residual magnetism in the core material. Hysterysis loss is given by Wh α βmax 1.6 Eddy Current Loss: Due to the e.m.f induced in the core of the stator. Eddy current loss is given by We α βmax2 f2 t2 In order to reduce the hysterysis loss, silicon alloyed steel, which has low hysterysis constant is used for manufacture of core. The composition of silicon steel is Steel - 95.8% Silicon – 4.0% Impurities - 0.2% From the formula it is seen that eddy current loss depends on the thickness of the laminations. Hence to reduce the eddy current loss core is made up of thin laminations which are insulated from each other. The thickness of the laminations is about 0.5mm. The silicon steel sheets are of COLD ROLLED NON-GRAIN ORIENTED (CRANGO) type as it provides the distribution of flux throughout the laminated sheet.

PREPARATION OF LAMINATIONS: For high rating machines each laminations is build of 6 sectors (stampings), each of 60 cut according to the specifications. Press tools are used in the manufacture of laminations. Press tools are mainly of two types.  Compounding tools.  Blanking and slot notching tools. LAMINATIONS ARE MANUFACTURED IN TWO DIFFERENT WAYS

COMPOUNDING OPERATION: In this method the stamping with all the core bolt holes, guiding slots and winding slots is manufactured in single operation known as Compounding operation and the press tool used is known as Compounding tool. Compounding tools are used for the machines rated above 40 MW.

BLANKING AND NOTCHING OPERATIONS: In case of smaller machines the stampings are manufactured in two operations. In the first operation the core blot holes and guiding slots are only made. This operation is known as Blanking and the tools used are known as Blanking tools. In the second operation the winding slots are punched using another tool known as Notching tool and the operation is called Notching.

THE DIFFERENT OPERATIONS IN MANUFACTURE OF LAMINATIOS: Deburring operation: In this operation the burrs in the sheet due to punching are deburred. There are chances of short circuit within the laminations if the burrs are not removed. The permissible is about 5 micrometer. For deburring punched sheets are passed under rollers to remove the sharp burs of edges.

Varnishing: Then depending on the temperature withstand ability of the machine the laminations are coated by varnish which acts as insulation. The lamination sheets are passed through conveyor, which has an arrangement to sprinkle the varnish is obtained. The sheets are dried by a series of heaters at a temperature of around 260-350 C. Two coatings of varnish are provided in the above manner till 12-18mm thickness of coat is obtained. The prepared laminations are subjected to following tests:  Xylol test To measure the chemical resistance.  Mandrel test - When wound around mandrel there should not be any cracks.  Hardness test - Minimum 7H pencil hardness.  IR value test - For 20 layers of laminations insulation.

ASSEMBLY OF CORE: The stator laminations are assembled as separate cage without stator frame. The entire core length is made in the form of packets separated by radial ducts to provide ventilating passages for the uniform cooling of the core. The thickness of each lamination is 0.5mm and the thickness of lamination separating the packets is about 1mm. The lamination separating each packet has strips of nonmagnetic material that are welded to provide radial ducts. The segments are staggered from layer to layer so that a core of high mechanical strength and uniform permeability to magnetic flux is obtained. Stacking mandrels and bolts are inserted into the windings slot bores during stacking provide smooth slot walls.

Fig: assembly of core To obtain the maximum compression and eliminate under setting during operation, the laminations are hydraulically compressed and heated during the stacking procedure when certain heights of stacks are reached. The complete stack is kept under pressure and located in frame by means of clamping bolts and pressure plates. The clamping bolts running through the core are made of nonmagnetic steel and are insulated from the core and pressure plates to prevent them from short circuiting the laminations and allowing the flow of eddy currents. The pressure is transmitted from the clamping plates to the core by clamping fingers. The clamping fingers extend up to the ends of the teeth thus, ensuring a firm compression in the area teeth.

Fig: assembled core The core building or assembling method depends on the insulation system used. 1. For resin rich insulation system the laminations are stacked in the frame itself. 2. For resin poor insulation system (VPI) cage core of open core design is employed.

STATOR WINDINGS: Stator winding is the one which induces emf and supplies the load. Stator winding is placed in the slots of stator core. Due to the advantages of generation and utilization of 3 phase power we use three phase windings for generation. So number of slots must be a multiple of 3 (or 6 if two parallel circuits are required).

Generally two layer lap winding, chorded to about 5/6 pitch which practically eliminates 5th and 7th harmonics from the flux wage or open circuit induced emf wave is used. The stator coil is made up of number of strips instead of single solid piece to reduce the skin effect. Copper material is used to make the coils. This is because Copper has high electrical conductivity with excellent mechanical properties. ii. Immunity from oxidation and corrosion. iii. It is highly malleable and ductile metal. i.

There are two types of coils manufactured in BHEL, Hyderabad.  Diamond pulled multiturn coil (full coiled):  Roebel bar (half coiled). Generally diamond pulled multiturn coils are used for low capacity machine. In this coils are pulled in a particular shape similar as diamond that’s why they are called so. In large capacity machines we use ROEBEL bars. These coils were constructed after considering the skin effect losses. In the straight slot portion, the conductors or strips are transposed by 360 degrees. The transposition is done to ensure that all the strips occupy equal length under similar conditions of the flux. High purity (99%) copper conductors/strips are used to make the coils. This results in high strength properties at higher temperatures so that deformations due to the thermal stresses are eliminated. The high voltage insulation is provided according to the resin poor mica base of thermosetting epoxy system. Several half overlapped continuous layers of resin poor mica tape are applied over the bars. The thickness of the tape depends on the machine voltage.

Fig: stator windings

3.6.1 Slot Discharges: Slot discharges occur if there are gaps within the slot between the surface of the insulation and that of the core. This may cause ionization of he air in the gap, due to breakdown of the air at the instances of voltage distribution between the copper conductor and the iron. Within the slots, the outer surface of the conductor insulation is at earth potential, in the overhanging it will approach more nearly to the potential of the enclosed copper. Surface discharge will take place if the potential gradient at the transition from slot to overhang is excessive, and it is usually necessary to introduce voltage grading by means of a semiconducting (graphite) surface layer, extending a short distance outward from the slot ends.

3.6.2 MANUFACTURE OF STATOR COILS: Various operations carried out during manufacture of stator coil are 1. Set the straightening and cutting machine using guide pilot. 2. Cut the conductor strips as per the requirement. 3. Set the press for “Roebel Transposition”. 4. Assemble strips with respect to template and transpose. 5.Assemble both halves of coil sides to from 1.One Roebel half bar 2. Insert insulation of halves between quarter bars matching the straight part zone as per drawing.       

Fig: stator coils  6.Cure half coil on hydraulic press. This process is known as Baking.

 (a) Remove insulation at the ends of the strips.

 (b) Test for inter-strip and inter-halves shorts.  7.Set the universal former as per standards. Check the setting of universal former for  (a)Length of straight part also mark diagonals/former walls inside for cross check.  (b)Check for marking made by template.  8.(a) Place the bar on former.  (b) Form the overhang bends as per standards.  © Remove clamps and inserts overhand insulation to both roebel halves with an application of araldite mixture.  (d) The bar is allowed to cure by giving supply to heating clamps.  9.Remove heating clamps and take out the bar halves from former.  (b)Round off sharp edges of straight part and dress up overhang halves insulation of both halves with out damage to copper strip insulation and to copper stacks.

3.6.3 PROCESS OF TAPING: 1.Tape the bar with Resin poor fine mica paper tape on straight part of bar taking copper foil outside the tape. 2.Tape with one layer of conductive polyester fleece tape. a)Provide main insulation b)OCP protection tape 3.Tape the straight part of bar with conductive polyester fleece tape with starting and ending shall be on straight part of bar. 4.Tape with mica splitting tape with accelerator taking Ocp layer into and leaving. 5.Tape the straight part of bar with polyester Conductive fleece tape. 6.Provide End Corona protection taping. 7.Provide overhang with protective tape (Polyester glass tape) 8.Test for inter-strip shorts.

Fig: mica taping machine.

3.7 STATOR END COVERS: The stator end covers are attached to end flanges of stator frame and also rest on the foundation plate. The end covers are made up of non-magnetic material (Aluminium castings) to reduce stray load and eddy current losses.

3.7.1 PHASE CONNECTORS: The phase connectors consist of flat copper sections, which results in low specific current loading. The phase connectors are wrapped with resin rich mica tape. After curing the connectors are attached to the pressure plate with clamps and bolts.

3.7.2 RESISTANCE TEMPERATURE DETECTORS : The temperature measurements on the generator are made with RTDs. They are placed at various sections of the core and winding. When making measurements with RTDs the resistance element is exposed to the temperature to be measured. The RTD works on the principle of the change in electrical resistance of a conductor due to temperature. R= Ro (1+ α T) ;Where Ro = reference resistance at room temperature; R= temperature coefficient of resistance ; T = temperature difference in C.

ROTOR

ROTOR: Rotor is the rotating part of alternator. It is used to support field winding placed in slots on the rotor. FOR 2-POLE GENERATOR: Solid rotors are manufactured from forged alloy steel with suitable alloying elements to achieve very high mechanical and superior magnetic properties. This type of rotor can withstand even upto speed of 3000 rpm. Rectangular or trapezoidal rotor slots are accurately machined to close tolerances on slot milling machine.For indirectly cooled generator rotors, ventilation slots are machined in the teeth. FOR 4-POLE GENERATOR: For directly cooled rotors, sub slots are provided for cooling Generator rotors of 1500 RPM are of round laminated construction. In this case rotor is made up of two parts (1) core, (2) lamination. The outer diameter of core and the inner diameter of laminations are equal. So for inserting the core inside the laminations the laminations are first red heated at medium temperature for 15 hours in BELL FURNACE. After that the core is shrunk fitted inside the laminations. Thus punched and varnished laminations of high tensile steel are mounted over machined shaft and are firmly clamped by end clamping plates.

Fig: rotor body.

ROTOR SHAFT :

Rotor shaft is a single piece solid forming manufactured from a vacuum casting. It is forged from a vacuum cast steel ingot. Slots for insertion or the field winding are milled into rotor body. The longitudinal slots are distributed over the circumference such that two solid poles are obtained. To ensure that only a high quality product is obtained, strength tests, material analysis and ultrasonic tests are performed during the manufacture of rotor. The high mechanical stresses resulting from the centrifugal forces and short circuit torque’s call for a high quality heat treated steel. After completion, the rotor is balanced in various planes at different speeds and then subjected to an over speed test at 120% of the rated speed for two minutes.

Fig. rotor shaft. Approximately 60% of rotor body circumference has longitudinal slots which hold the field winding. Slot pitch is selected so that the two solid poles are displaced by 180 degrees. The rotor wedges act as damper winding within the range of winding slots. The rotor teeth at the ends of rotor body are provided with axial and radial holes enabling the cooling air to be discharged into the air gap after intensive cooling of end windings.

ROTOR WINDINGS :The rotor windings consist of several coils inserted into the slots and series connected such that two coil groups form one pole. Each coil consists of several series connected turns, each of which consists of two half turns connected by brazing in the end section. Thickness of each strip can be made upto 10.5 mm but here in BHEL we make only upto 5.3 mm. The rotor bearing is made of silver bearing copper ensuring an increased thermal stability. For ventilation purpose the slots are provided on the coil and on inter strip insulation layer both. The individual turns of coils are insulated against each other by interlayer insulation. L-shaped strips of laminated epoxy glass fiber fabric with nomex filter are used for slot insulation.

Fig: rotor winding. The slot wedges are made of high electrical conductivity material and thus act as damper windings. At their ends the slot wedges are short circuited through the rotor body. The inter space between the overhang is called slot through.

CONSTRUCTION The field winding consists of several series connected coils inserted into the longitudinal slots of rotor body. The coils are wound so that two poles are obtained. The solid conductors have a rectangular cross section and are provided with axial slots for radial discharge or cooling air. All conductors have identical copper and cooling duct cross section. The individual bars are bent to obtain half turns. After insertion into the rotor slots, these turns are brazed to obtain full turns. The series connected turns of one slot constitute one coil. The individual coils of rotor are connected in a way that north and south poles are obtained.

CONDUCTOR MATERIAL: The conductors are made of copper with a silver content of approximately 0.1%. As compared to electrolytic copper, silver alloyed copper features high strength properties at high temperatures so that coil deformations due to thermal stresses are eliminated.

INSULATION The insulation between the individual turns is made of layer of glass fiber laminate. The coils are insulated from the rotor body with L-shaped strips of glass fiber laminate with nomex interlines. To obtain the required leakage paths between the coil and the rotor body thick top strips of glass fiber laminate are inserted below top wedges. The top strips are provided with axial slots of the same cross section and spacing as used on the rotor winding. Insulation b/w overhang is done by blocks made of HGL.

To protect the winding against the effects of centrifugal forces, the winding is secured in the slots with wedges. The slot wedges are made of copper alloy featuring high strength and good electrical conductivity. They are also used as damper winding bars. The slot wedges extend beyond the shrink seats of retaining rings. The wedge and retaining rings act on the damper winding in the event of abnormal operations. The rings act as short circuit rings in the damper windings.

END WINDING BRACING The spaces between the individual coils in the end winding are filled with insulated members that prevent coil movement. Two insulation plates held by HGL high glass laminate plates separate the different cooling zones in the overhangs on either sides.

ROTOR RETAINING RINGS The centrifugal forces of the rotor end winding are contained by single piece rotor retaining rings. Retaining rings are made of non-magnetic high strength steel in order to reduce stray losses. Each retaining ring with its shrink fitted. Insert ring is shrunk on to the rotor body in an overhang position. The retaining ring is secured in the axial position by snap rings.

Fig: rotor retaining rings The rotor retaining rings withstand the centrifugal forces due to end windings. One end of each ring is shrunk fitted on the rotor body while the other end overhangs the end windings without contact on the rotor shaft. This ensures an unobstructed shaft deflection at the end winding.

ROTOR FANS

The cooling air in generator is circulated by two axial flow fans located on the rotor shaft one at each end. To augment the cooling of the rotor winding, the pressure established by the fan works in conjunction with the air expelled from the discharge parts along the rotor. The blades of the fan have threaded roots for being screwed into the rotor shaft. The blades are drop forged from an aluminium alloy. Threaded root fastenings permit angle to be changed. Each blade is secured at its root with a threaded pin.

Fig: rotor fan

BEARINGS VENTILATION AND COOLING

INSULATION SYSTEMS BHEL INSULATION SYSTEM FOR TURBO GENERATORS: BHEL had Bitumen insulation system for low & medium rating TGS and switched over to resin rich Thermo setting type as a step towards increasing reliability and upgrading technology. Micalastic system has been adopted for high rating machinery.

BITUMEN SYSTEM & LIFE EXTENSION:  The experience with Bitumen system has been generally satisfactory & practically negligible service failure has been reported on these sets.  Mechanical damage most commonly associated with this system ie., tape separation, due to thermal expansion of the winding during normal or abnormal temperature eyeing is not met any of sets.  Though outage due to insulation failures has been considerably low, yet these machinery would need to be attended to have life extension above their estimated life of 25 years.  Major inspection of the machine condition is by checking the healthiness of windings & life of bar insulation.  Rehabilitation, if needed, requires restoration of varnish, removal of bitumen & cleaning, tightening of fasteners/supports, modification of busbars, use of new wedges & other winding components. 

The replacements are required because of vibration / external damage etc.

VARIOUS INSULATION SYSTEMS & PRACTICES: Large & medium range motors are provided with following insulation system. 1.Resin flux Insulation System : This system is used on earlier designs & where duplicate or spare motors to suit the customer requirements are required. In the coming years this system may become absolute. 2.Resin Rich micalastic Insulation System : The system provides use of Resin rich polyester backed epoxy micafolium on straight portion & resin rich polyester backed epoxy mica paper tape on overhang with a final layer of polyester shrink tape. The system is highly productive during coil manufacture and housing. 3.Resin poor Micalastic Insulation System: Resin poor micalastic system is adopted for large range Ac Induction and synchronous machines. Theses are designated to meet specific customer requirement hence for unique in nature to each other. The main insulation consists of resin poor epoxy mica paper tape all over the oil periphery with varying number of layers on straight and overhang portions.

The brief comparison of Resin poor over Resin rich is as follows:

RESIN POOR

RESIN RICH

1.Epoxy resin content is about 8%.

1.Epoxy resin content is about 40%.

2.This method follows Thermo Setting Process.

2. This method also follows Thermo Setting Process.

3.There is a need for addition of resin from outside.

3. Further addition of resin is not required from outside.

4.Time required for this cycle is less. 5.Repairing is very difficult.

4.Its a very long process and time consuming. 5.Repairing is easy. 6.Over all cost is more.

6.Over all cost is less compared to resin rich.

VACUUM PRESSURE IMPREGNATION INTRODUCTION TO VPI SYSTEM: DR. MEYER brought the VPI system with the collaboration of WESTING HOUSE in the year 1956. Vacuum Pressure Impregnation has been used for many years as a basic process for thorough filling of all interstices in insulated components, especially high voltage stator coils and bars.  VPI is a process, which is a step above the conventional vacuum system. VPI includes pressure in addition to vacuum, thus assuring good penetration of the varnish in the coil.  The result is improved mechanical strength and electrical properties. With the improved penetration, a void free coil is achieved as well as giving greater mechanical strength.  With the superior varnish distribution, the temperature gradient is also reduced .  In order to minimise the overall cost of the machine & to reduce the time cycle of the insulation system vacuum pressure Impregnated System is used.  The stator coils are taped with porous resin poor mica tapes before inserting in the slots of cage stator, subsequently wounded stator is subjected to VPI process, in which first the stator is vacuum dried and then impregnated in resin bath under pressure of Nitrogen gas.

Fig: VPI system

Features and Benefits: • State-of-the-art process for completely penetrating air pockets in winding insulation.

• Increases voltage breakdown level. (Even under water!) • Proven submergence duty system • Improved heat transfer- windings are cooler, efficiency is improved. • Improves resistance to moisture and chemicals. • Increases mechanical resistance to winding surges.

Vacuum Pressure Impregnation of resin poor insulated jobs:

Preheating Vacuum to be maintained

Variant-01 Variant-02

Variant-03

60 5 C for 605C for 3hrs 12hrs 0.4mbar 0.2mbar/0.4mbar

603C for 12hrs

40min

(both together shall not exceed 50hrs including rising time) 0.2mbar for 9hrs Stopping vacuum pumps 0.4mbar for 17hrs for 10min shall check 17hrs vacuum drop. The vacuum drop shall not exceed by 0.06mbar for 10min 80min 80min

3bar

4bar

4bar

3hrs

3hrs

3hrs

Vacuum heating time 3hrs

Increase in pressure Maximum pressure Pressure holding Post curing