1 MANUFACTURING AND DESIGN OF INSULATION SYSTEM FOR AIR COOLED TURBO GENERATOR BY V.P.I PROCESS A PROJECT REPORT SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF BACHELOR DEGREE IN ELECTRICAL ENGINEERING SUBMITTED BY G.VENKATESH BABU (04A21A0258) M.K.CHAITANYA SARMA (04A21A0216) M.V.SATYA TEJA (04A21A0254) L.PRANEETH CHAITANYA (03A21A0226) UNDER THE ESTEEMED GUIDANCE OF R.K.MANOHAR Sr DGM Quality Control(E.M) BHEL, Ramachandra puram T.Ravi. M.E.., Asst prof. Swarnandhra College Narsapuram
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MANUFACTURING AND DESIGN OF INSULATION SYSTEM FOR
AIR COOLED TURBO GENERATOR BY V.P.I PROCESS
A PROJECT REPORT SUBMITTED IN PARTIALFULFILLMENT OF THE REQUIREMENTS
FOR THE AWARD OF
BACHELOR DEGREEIN
ELECTRICAL ENGINEERING
SUBMITTED BY
G.VENKATESH BABU(04A21A0258)
M.K.CHAITANYA SARMA(04A21A0216)
M.V.SATYA TEJA(04A21A0254)
L.PRANEETH CHAITANYA(03A21A0226)
UNDER THE ESTEEMED GUIDANCE OF
R.K.MANOHARSr DGM
Quality Control(E.M)BHEL, Ramachandra puram
T.Ravi. M.E..,Asst prof.
Swarnandhra CollegeNarsapuram
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CERTIFICATE
This is to certify that the project entitled “MANUFACTURING AND DESIGN OF INSULATION SYSTEM FOR AIR COOLED TURBO GENERATOR BY V.P.I PROCESS” Submitted byG.VENKATESH BABU (04A21A0258)M.KRISHNA CHAITANYA SARMA (04A21A0216) M.V.SATYA TEJA (04A21A0254) L.PRANEETH CHAITANYA (03A21A0226) In partial fulfillment of “BACHELORS DEGREE IN ELECTRICAL AND ELECTRONICS ENGINEERING” for the academic Year 2007-2008 of IV-Year from SWARNANDHRA COLLEGE OF ENGINEERING AND TECHNOLOGY, affiliated to JNT UNIVERSITY, WEST GODAVARI DIST., A.P, INDIA.A record of bonafide work carried by them under my guidance in “BHEL, RAMACHANDRAPURAM, HYDERABAD-32”.
SIGNATURE OF PROJECT GUIDE SHRI R.K.MANOHARDGM,B.Tech(Elect),(SQC&OR)Electrical Machines,(Quality Control), BHEL,Ramachandrapuram.
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1. ABSTRACT
In developing countries like India, power generation is a major break through to meet
the present demands of the nation. Power generation of several types are on forefront, the dominant
component of power generation is TURBO-GENERATOR which produces large capacity, the word
“TURBO” stands for turbine drive. Generally the turbines used to drive these turbo-generators are of
reaction type.
In large-scale industries manufacturing generators, insulation design plays a vital role.
Insulation is known to be the heart of the generator. If insulation fails, generator fails which leads to
the loss of crores of rupees. The latest technology for insulation in the world and adopted by BHEL,
(Hyderabad) unit is “VACUUM PRESSURE IMPREGNATION “which is of resin poor
thermosetting type. This type is preferred as it is highly reliable and possesses good mechanical,
thermal properties and di-electric strength. As the quantity of resin used is less, hence the over all cost
of insulation is reduced.
In our project we have made a detailed study of the VPI system of insulation. This
system is employed by BHEL first in the country and second in the world next to Germany.
Project Associates:
G.Venkatesh Babu (04A21A0258)
M.K.Chaitanya Sarma (04A21A0216)
M.V.Satya Teja (04A21A0254)
L.Praneeth Chaitanya (03A21A0226)
Project Guide External: Project Guide Internal:
Mr. R.K.Manohar., Mr. T.Ravi. M.E..,
Sr.D.G.M, Assistant prof.- EEE dept.
Quality Control (E.M), Swarnandhra College of
B.H.E.L. R.C.Puram. Engg and technology
APPROVED BY
HOD OF EEE
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TABLE OF CONTENTS1. ABSTRACT 3
1.1 ACKNOWLEDGEMENTS 10
1.2. PROFILE OF BHEL 11
1.3. PREFACE 13
2. INTRODUCTION 14
2.1 DRAWBACKS OF EARLY VPI PROCESS 14
2.2 ADVANTAGE OF PRESENT RESIN POOR VPI PROCESS 15
3. INTRODUCTION TO VARIOUS PARTS OF A GENERATOR 17
3.1 STATOR 17
3.2 ROTOR 18
3.3 FIELD CONNECTIONS AND MULTI CONTACTS 19
3.4 EXCITATION SYSTEM 19
3.5 PERMANENT MAGNET GENERATOR AND AVR 20
3.6 VARIOUS LOSSES IN A GENERATOR 23
4. MANUFACTURE OF GENERATOR 26
VARIOUS STAGES IN MANUFACTURE OF GENERATOR 26
4.1 STATOR MANUFACTURING PROCESS 26
4.1.1 STATOR CORE CONSTRUCTION 27
4.1.2 PREPARATION OF STATOR LAMINATIONS 27
4.1.3 RECEPTION OF SILICON STEEL ROLLS 27
4.1.4 SHEARING 27
4.1.5 BLANKING AND NOTCHING 27
4.1.6 COMPOUND NOTCHING 27
4.1.7 INDIVIDUAL NOTCHING 28
4.1.8 DEBURRING 28
4.1.9 VARNISHING 28
5. STATOR CORE ASSEMBLY 29
5.1 TRAIL PACKET ASSEMBLY 29
5.2 NORMAL CORE ASSEMBLY 29
5.2.1 STEPPED PACKET
ASSEMBLY 29
5.2.2 NORMAL PACKET
ASSEMBLY 29
5.2.2.1 IN PROCESS
PRESSING 30
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5.2.2.2 FITTING OF
CLAMPING BOLTS 30
6. STATOR WINDING 31
6.1 CONDUCTOR MATERIAL USED IN COIL MANUFACTURING 31
6.2 TYPES OF CONDUCTOR COILS 31
7. ELECTRICAL INSULATION 33
7.1 STATOR WINDING INSULATION SYSTEM FEATURES 35
7.1.1 STRAND INSULATION 35
7.1.2 TURN INSULATION 39
7.1.3 GROUND WALL INSULATION 40
7.1.4 SLOT DISCHARGES 41
7.2 INSULATING MATERIALS 41
7.2.1 CLASSIFICATION OF INSULATING MATERIALS 42
7.2.2 INSULATING MATERIALS FOR ELECTRICAL MACHINES 42
7.3 ELECTRICAL PROPERTIES OF INSULATION AND FEW DEFINITIONS 44
7.3.1 INSULATION RESISTANCE 44
7.3.2 DIELECTRIC STRENGTH 44
7.3.3 POWER FACTOR 44
7.3.4 DIELECTRIC CONSTANT 44
7.3.5 DIELECTRIC LOSS 44
8 RESIN IMPREGNATION 45
8.1 INSULATION MATERIALS FOR LAMINATIONS 46
8.2 VARNISH 47
8.3 RESIN POOR SYSTEM
8.4 RESIN RICH SYSTEM 48
9. MANUFACTURE OF STATOR COILS 48
9.1 FOR RESIN POOR PROCESS 48
9.1.1 RECEPTION OF COPPER CONDUCTORS 50
9.1.2 TRANSPOSITION 50
9.1.3 PUTTY OPERATION 50
9.1.4 STACK
CONSOLIDATION 51
9.1.5 BENDING 51
9.1.6 FINAL TAPING 51
9.2 FOR RESIN RICH PROCESS 52
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9.2.1 PUTTY WORK
52
9.2.2 FINAL TAPING 52
9.2.3 FINAL BAKING
53
10. AN OVERVIEW 54
10.1 ADVANTAGES OF RESIN POOR SYSTEM 54
10.2 DISADVANTAGES OF RESIN POOR SYSTEM 54
10.3 ADVANTAGES OF RESIN RICH SYSTEM 54
10.4 DISADVANTAGES OF RESIN RICH SYSTEM 54
11. ASSEMBLY OF STATOR 54
11.1 RECEPTION OF STATOR CORE 55
11.2 WINDING HOLDERS ASSEMBLY 55
11.3 STIFFENER ASSEMBLY 55
11.4 EYE FORMATION 55
11.5 CONNECTING RINGS ASSEMBLY 56
11.6 PHASE CONNECTORS 56
12. THE VPI PROCESS 56
12.1 INTRODUCTION TO VPI PROCESS 56
12.2 HISTORY 57
12.3 VPI PROCESS FOR RESIN POOR INSULATED JOBS 59
12.3.1 GENERAL 59
12.3.2 PREHEATING
59
12.3.3 VACUUM CYCLE
60
12.3.4 IMPREGNATION
61
12.3.5 POST CURING
62
12.3.6 ELECTRICAL TESTING 63
12.4 GLOBAL PROCESSING 63
12.5 RESIN MANAGEMENT 63
12.6 SPECIFIC INSTRUCTIONS 63
12.7 PRECAUTIONS 64
12.8 FEATURES AND BENEFITS 64
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13. FACILITIES AVAILABLE IN VPI PLANT BHEL 66
13.1 DATA COLLECTION OF SAMPLES 68
13.1.1 INDO-BHARAT –II ROTOR 68
13.1.2 INDO BHARAT –II STATOR 70
13.1.3 HIGH VOLTAGE LEVELS OF STATOR/ROTOR WINDINGS FOR MULTI-
TURN MACHINES 74
13.1.4 TESTING RESULTS OF INDO BHARAT-II ROTOR 75
13.1.5 TESTING RESULTS OF INDO BHARAT-II STATOR 75
14. COMPARISON BETWEEN RESIN POOR AND RESIN RICH SYSTEMS 77
14.1 DRAWBACKS 78
14.2 SUGGESTIONS 78
14.3 JUSTIFICATION 78
15. PRESENT INSULATION SYSTEMS USED IN THE WORLD 83
15.1 WESTINGHOUSE ELECTRIC CO: THERMALASTIC™ 84
15.2 GENERAL ELECTRIC CO: MICAPALS I AND II™, EPOXY MICA MAT™,
Various stages in generator manufacturing:In our project we have a detail study of only stator, rotor and the insulation system
used for it. The parts excitation system, cooling system and bearings are external to the generator and
are treated as a completed one and are out of scope of our record. Now, generator manufacturing can
be broadly divided into three main parts:
1. Stator manufacture
2. Rotor manufacture
3. Exciter manufacture
The various stages involved in the generator manufacture and their sub processes are shown in
the flow diagram given below. This facilitates manufacture erection and transport of the stator.
B.3: flow diagram showing various stages involved in generator manufacture.
Now these sub processes are explained in detail below. Let us start with Stator.
4.1 STATOR MANUFACTURE PROCESS:This stator manufacturing is a combination of two individual sub processes, namely
Stator core construction and
Coil construction and their assembly.
4.1.1 STATOR CORE CONSTRUCTION:
The stator core isn’t a solid iron type and is the assembly of strips of laminations. As the
reasons are explained in the Section 3.1 in the introduction to stator.4.1.2 PREPARATION OF STATOR LAMINATIONS
As explained above, stator laminations are the important parts of the stator core and they
should be manufactured as per the design requirements and involves the following sub processes.
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4.1.3 RECEPTION OF SILICON STEEL ROLLS:
The silicon steel rolls received are checked for their physical, chemical, mechanical and
magnetic properties as per the specifications mentioned above.
In order to reduce the Hysterisis loss, silicon alloyed steel, which has low Hysterisis
constant is used for the manufacture of core. The composition of silicon steel is
Steel - 95.8 %
Silicon - 4.0 %
Impurities- 0.2 %
From the formula for eddy current loss 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 0.5
mm. The silicon steel sheets used are of COLD ROLLED NON-GRAIN ORIENTED
(CRANGO) type as it provides the distribution of flux throughout the laminated sheet.
4.1.4 SHEARING: The cold rolled non grained oriented (CRNGO) steel sheets are cut to their outer
periphery to the required shapes by feeding the sheet into shearing press. For high rating
machines each lamination is build of 6 sectors (stampings), each of 60 cut according to the
specifications.
4.1.5 BLANKING AND NOTCHING:
Press tools are used in making the core bolt holes and other notches for the laminations.
Press tools are mainly of two types.
i. Compound notching tools.
ii. Individual notching tools.
4.1.6 COMPOUND OPERATION:
In this method the stamping with all the core bolt holes, guiding slots and winding
slots is manufactured in single operation known as Compound operation and the press tool
used is known as Compounding tool. Compounding tools are used for the machines rated
above 40 MW. Nearly 500 tons crank press is used for this purpose.
4.1.7 INDIVIDUAL OPERATIONS:
In case of smaller machines the stampings are manufactured in two operations. In the
first operation the core bolt 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.
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4.1.8 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.
B4: Figure showing the shape of laminations after the completion of notching and deburring
operations.
4.1.9 VARNISHING:
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, and a coat of varnish is obtained.
The sheets are dried by a series of heaters at a temperature of around 260 – 350 oC. Two
coatings of varnish are provided in the above manner till 12-18 micrometer thickness of coat
is obtained. Here instead of pure varnish a mixture of Tin and Varnish is used such that the
mixture takes 44sec to empty a DIN4 CUP.
The prepared laminations are subjected to following tests.
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i) Xylol test - To measure the chemical resistance.
ii) Mandrel test - When wound around mandrel there should not be any cracks.
iii) Hardness test - Minimum 7H pencil hardness.
iv) IR value test - For 20 layers of laminations insulation resistance should not be less than
1M.
5. STATOR CORE ASSEMBLY:
5.1 TRAIL PACKET ASSEMBLY:Clamping plate is placed over the assembly pit; stumbling blocks are placed between the
clamping plates and the assembly pit. Clamping plate is made parallel to the ground by checking with
the spirit level. One packet comprising of 0.5 mm thickness silicon steel laminations is assembled
over the clamping plates by using mandrels and assembly pit .after assembling one packet thickness
of silicon laminations, inner diameter of the core is checked as per the drawing also the slot freeness
is checked with inspection drift .There should not be any projections inside or outside the slot. If all
the conditions are satisfied the normal core assembly is carried out by dismantling the trial packets.
5.2 NORMAL CORE ASSEMBLY
5.2.1 Stepped packed assembly: Stepped packets are assembled from the clamping plate isolating each packet with ventilation
laminations up to 4 to 5 packets of thickness 10cms for an air cooled turbo generator of 120MW.
5.2.2 Normal packet assembly: Normal packet assembly is carried out using 0.5 mm silicon steel laminations up to
required thickness of 30mm by using mandrills and inspection drift after normal packet assembly
completion, one layer of HGL laminations are placed and one layer of ventilation lamination are
placed over them. Then again normal packet assembly is carried as above. The thickness of each
lamination is 0.5 mm and the thickness of lamination separating the packets is about 1 mm. 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.
5.2.2.1 In process pressings 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.
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The packets are assembled as above up to 800mm as above and 1 st pressing is carried
using hydraulic jacks up to 150kg/cm2 and the pressing is carried out for every 800mm and a pre final
pressing is done before the core length almost reach the actual core. Now the core is tested for the
design specifications and the compensation is done by adding or removing the packets.
5.2.2.2 Fitting of clamping bolts: The complete stack is kept under pressure and locked in the 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 the 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 of
the teeth. The stepped arrangement of the laminations at the core ends provides an efficient support
to tooth portion and in addition contributes to the reduction of stray load losses and local heating in
that area due to end leakage flux.
The clamping fingers are also made of non-magnetic steel to avoid eddy-current losses. After
compression and clamping of core the rectangular core key bars are inserted into the slots provided in
the back of the core and welded to the pressure plates. All key bars, except one, are insulated from
the core to provide the grounding of the core.
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WINDINGThe next important consideration is winding. The stator winding and rotor winding consist of several
components, each with their own function. Furthermore, different types of machines have different
components. Stator windings are discussed separately below.
6. STATOR WINDINGThere are three main components in a stator, they are
a. copper conductors (although aluminum is sometimes used).
b. The stator core.
c. Insulation.
6.1 CONDUCTING MATERIAL USED IN COIL MANUFACTURING:
Copper material is used to make the coils. This is because
i) Copper has high electrical conductivity with excellent mechanical properties
ii) Immunity from oxidation and corrosion
iii) It is highly malleable and ductile metal.
6.2 TYPES OF CONDUCTOR COILS:
Basically there are three types of stator winding structures employed over the range from 1 KW to
1000 MW.
1. Random wound stators.
2. Form-wound stators using multi turn coils.
3. Form-wound stators using Roebel bars.
Out of these, two types of coils are manufactured and used in BHEL, Hyderabad.
1) Diamond pulled multi-turn coil (full coiled).
2) Roebel bar (half-coil).
In general, random-wound stators are typically used for machines less than several hundred
KW. Form-wound coil windings are used in most large motors and many generators rated up to 50 to
100 MVA. Roebel bar windings are used for large generators. Although each type of construction is
described below, some machine manufacturers have made hybrids that do not fit easily into any of the
above categories; these are not discussed in the project.
Generally in large capacity machines ROEBEL bars are used. 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. The transposition provides for a mutual neutralization of the voltages induced
in the individual strips due to the slot cross field and ensures that no or only small circulating currents
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exists in the bar interior. Transposition also reduced eddy current losses and helps in obtaining
uniform e.m.f. More about transposition is discussed later in the section with diagrammatic quote.
The copper is a conduit for the stator winding current. In a generator, the stator output current
is induced to flow in the copper conductors as a reaction to the rotating magnetic field from the rotor.
In a motor, a current is introduced into the stator, creating a rotating magnetic field that forces the
rotor to move. The copper conductors must have a cross section large enough to carry all the current
required without overheating.
B5 schematic diagram for a three –phase Y-connected stator winding with two parallel conductors per phase
The diagram shows that each phase has one or more parallel paths for current flow. Multiple
parallels are often necessary since a copper cross section large enough to carry the entire phase
current may result in an uneconomic stator slot size. Each parallel consists of a number of coils
connected in series. For most motors and small generators, each coil consists of a number of turns of
copper conductors formed into a loop. The rationale for selecting the number of parallels, the number
of coils in series, and the number of turns per coil in any particular machine is beyond the scope of
our project.
The stator core in a generator concentrates the magnetic field from the rotor on the copper
conductors in the coils. The stator core consists of thin sheets of magnetic steel (referred to as
laminations). The magnetic steel acts as a low-reluctance (low magnetic impedance) path for the
magnetic fields from the rotor to the stator, or vice versa for a motor. The steel core also prevents
most of the stator winding magnetic field from escaping the ends of the stator core, which would
cause currents to flow in adjacent conductive material.
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7. ELECTRICAL INSULATIONThe final major component of a stator winding is the electrical insulation. Unlike copper
conductors and magnetic steel, which are active components in making a motor or generator function
the insulation is passive. That is, it does not help to produce a magnetic field or guide its path.
Generator and motor designers would like nothing better than to eliminate the electrical insulation,
since the insulation increases machine size and cost, and reduces efficiency, without helping to create
any torque or current. Insulation is “overhead,” with a primary purpose of preventing short circuits
between the conductors or to ground. However, without the insulation, copper conductors would
come in contact with one another or with the grounded stator core, causing the current to flow in
undesired paths and preventing the proper operation of the machine. In addition, indirectly cooled
machines require the insulation to be a thermal conductor, so that the copper conductors do not
overheat.
The insulation system must also hold the copper conductors tightly in place to prevent
movement. The stator winding insulation system contains organic materials as a primary constituent.
In general, organic materials soften at a much lower
B6 Photographs of end windings and slots of random wound stator (Courtesy TECO Westing house)
temperature and have a much lower mechanical strength than copper or steel. Thus, the life of a stator
winding is limited most often by the electrical insulation rather than by the conductors or the steel
core. Furthermore, stator winding maintenance and testing almost always refers to testing and
maintenance of the electrical insulation.
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B7 Photograph of a form wound stator winding (courtesy TECO Westing house)
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.
B8 A single form wound coil being inserted into two slots
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7.1 STATOR WINDING INSULATION SYSTEM FEATURES
The stator winding insulation system contains several different components and features which
together ensure that electrical shorts do not occur, that the heat from the conductor I2R losses are
transmitted to a heat sink, and that the conductors do not vibrate in spite of the magnetic forces.
The basic stator insulation system components are the:
1. Strand (or sub conductor) insulation
2. Turn insulation
3. Ground wall (or ground or earth) insulation
Figures B9 , B10 and B11 show cross sections of random-wound and form-wound coils in a stator
slot, and identify the above components. Note that the form-wound stator has two coils per slot; this
is typical.
Figure B14 is a photograph of the cross section of a multi-turn coil. In addition to the main insulation
components, the insulation system sometimes has high-voltage stress-relief coatings and end-winding
support components.
The following sections describe the purpose of each of these components. The mechanical, thermal,
electrical, and environmental stresses that the components are subjected to are also described.
7.1.1 Strand Insulation
In random-wound stators, the strand insulation can function as the turn insulation, although extra
sleeving is sometimes applied to boost the turn insulation strength in key areas. Many form-wound
machines employ separate strand and turn insulation. The following mainly addresses the strand
insulation in form-wound coils and bars. Strand insulation in random wound machines will be
discussed as turn insulation. Section 1.4.8 discusses strand insulation in its role as transposition
insulation.
There are both electrical and mechanical reasons for stranding a conductor in a form wound coil or
bar.
From a mechanical point of view, a conductor that is big enough to carry the current needed in the
coil or bar for a large machine will have a relatively large cross-sectional area. That is, a large
conductor cross section is needed to achieve the desired ampacity. Such a large conductor is difficult
to bend and form into the required coil/bar shape. A conductor formed from smaller strands (also
called sub-conductors) is easier to bend into the required shape than one large conductor.
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B9 C.S of a random stator winding
slot
From an electrical point of
view, there are reasons to make
strands and insulate them from
one another. It is well known
from electromagnetic theory
that if a copper conductor has a
large enough cross-sectional
area, the current will tend to flow on the periphery of the conductor. This is known as the skin effect.
The skin effect gives rise to a skin depth through which most of the current flows. The skin depth of
copper is 8.5 mm at 60 Hz. If the conductor has a cross section such that the thickness is greater than
8.5 mm, there is a tendency for the current not to flow through the center of the conductor, which
implies that the current is not making use of all the available cross section. This is reflected as an
effective AC resistance that is higher than the DC resistance. The higher AC resistance gives rise to a
larger I2R loss than if the same cross section had been made from strands that are insulated from one
another to prevent the skin effect from occurring. That is, by making the required cross section from
strands that are insulated from one another, all the copper cross section is used for current flow, the
skin effect is negated, and the losses are reduced.
In addition, Eddy current losses occur in solid conductors of too large a cross section. In the slots, the
main magnetic field is primarily radial, that is, perpendicular to the axial direction. There is also a
small circumferential (slot leakage) flux that can induce eddy currents to flow. In the end-winding, an
axial magnetic field is caused by the abrupt end of the rotor and stator core. This axial magnetic field
can be substantial in synchronous machines that are under-excited.
By Ampere’s Law, or the ‘right hand rule’, this axial magnetic field will tend to cause a current to
circulate within the cross section of the conductor (Figure 1.11). The larger the cross sectional area,
the greater the magnetic flux that can be encircled by a path on the periphery of the conductor, and
the larger the induced current. The result is a greater I2R loss from this circulating current. By
reducing the size of the conductors, there is a reduction in stray magnetic field losses, improving
efficiency.
The electrical reasons for stranding require the strands to be insulated from one another. The voltage
across the strands is less than a few tens of volts; therefore, the strand insulation can be very thin. The
strand insulation is subject to damage during the coil manufacturing process, so it must have good
mechanical properties. Since the strand insulation is immediately adjacent to the copper conductors
that are carrying the main stator current, which produces the I2R loss, the strand insulation is exposed
38
to the highest temperatures in the stator. Therefore, the strand insulation must have good thermal
properties. Section 7.1.1 describes in detail the strand insulation materials that are in use. Although
manufacturers ensure that strand shorts are not present in a new coil, they may occur during service
due to thermal or mechanical aging. A few strand shorts in form-wound coils/bars will not cause
winding failure, but will increase the stator winding losses and cause local temperature increases due
to circulating currents.
B10 C.S of a form wound multi-turn slots containing form wound multi-turn coils.
7.1.2 Turn Insulation
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The purpose of the turn insulation in both random- and form-wound stators is to prevent shorts
between the turns in a coil. If a turn short occurs, the shorted turn will appear as the secondary
winding of an autotransformer. If, for example, the winding has 100 turns between the phase terminal
and neutral (the “primary winding”), and if a dead short appears across one turn (the “secondary”),
then 100 times normal current will flow in the shorted turn. This follows from the transformer law:
npIp = nsIs (1.1)
Where n refers to the number of turns in the primary or secondary, and I is the current in the
primary or secondary. Consequently, a huge circulating current will flow in the faulted turn, rapidly
overheating it. Usually, this high current will be followed quickly by a ground fault due to melted
copper burning through any ground-wall insulation. Clearly, effective turn insulation is needed for
long stator winding life.
B11 C.S of a form wound multi-turn slots directly cooled roebel bars
The power frequency voltage across the turn insulation in a random-wound machine can range
up to the rated phase-to-phase voltage of the stator because, by definition,
the turns are randomly placed in the slot and thus may be adjacent to a
phase-end turn in another phase, although many motor manufacturers may
insert extra insulating barriers between coils in the same slot but in
different phases and between coils in different phases in the end-windings.
Since random winding is rarely used on machines rated more than 600 V
(phase-to-phase), the turn insulation can be fairly thin. However, if a
motor is subject to high-voltage pulses, especially from modern inverter-
fed drives (IFDs), inter-turn voltage stresses that far exceed the normal
maximum of 600 Vac can result. These high-voltage pulses give rise to
failure mechanisms, as discussed in Section 8.7.
The power frequency voltage across adjacent turns in a form-wound multi-turn coil is well defined.
Essentially, one can take the number of turns between the phase terminal and the neutral and divide it
into the phase–ground voltage to get the voltage across each turn. For example, if a motor is rated
4160 Vrms (phase–phase), the phase–ground voltage is 2400V.
This will result in about 24 Vrms across each turn, if there are 100 turns between the phase end and
neutral. This occurs because coil manufacturers take considerable trouble to ensure that the
inductance of each coil is the same, and that the inductance of each turn within a coil is the same.
Since the inductive impedance (XL) in ohms is:
XL = 2_fL (1.2)
Where f is the frequency of the AC voltage and L is the coil or turn inductance, the turns appear as
impedances in a voltage divider, where the coil series impedances are equal. In general, the voltage
40
across each turn will be between about 10 Vac (small form-wound motors) to 250 Vac (for large
generator multi turn coils).
The turn insulation in form-wound coils can be exposed to very high transient voltages associated
with motor starts, IFD operation, or lightning strikes. Such transient voltages may age or puncture the
turn insulation. This will be discussed in later sections. As described below, the turn insulation
around the periphery of the copper conductors is also exposed to the rated AC phase–ground stress, as
well as the turn–turn AC voltage and the phase coil-to-coil voltage.
Before about 1970, the strand and the turn insulation were separate components in multi turn coils.
Since that time, many stator manufacturers have combined the strand and turn insulation. Figure 1.12
shows the strand insulation is upgraded (usually with more thickness) to serve as both the strand and
the turn insulation. This eliminates a manufacturing step (i.e., the turn taping process) and increases
the fraction of the slot cross section that can be filled with copper. However, some machine owners
have found that in-service failures occur sooner in stators without a separate turn insulation
component.
Both form-wound coils and random-wound stators are also exposed to mechanical and thermal
stresses. The highest mechanical stresses tend to occur in the coil forming process, which requires the
insulation-covered turns to be bent through large angles, which can stretch and crack the insulation.
Steady-state, magnetically induced mechanical vibration forces (at twice the power frequency) act on
the turns during normal machine operation. In addition, very large transient magnetic forces act on
the turns during motor starting or out-of-phase synchronization in generators. These are discussed in
detail in Chapter 8. The result is the turn insulation requires good mechanical strength.
The thermal stresses on the turn insulation are essentially the same as those described above for the
strand insulation. The turn insulation is adjacent to the copper conductors, which are hot from the I2R
losses in the winding. The higher the melting or decomposition temperature of the turn insulation, the
greater the design current that can flow through the stator.
In a Roebel bar winding, no turn insulation is used and there is only strand insulation. Thus, as will be
discussed in later sections, some failure mechanisms that can occur with multi turn coils will not
occur with Roebel bar stators.
7.1.3 Ground wall InsulationGround wall insulation is the component that separates the copper conductors from the
grounded stator core. Ground wall insulation failure usually triggers a ground fault relay, taking the
motor or generator off-line.* Thus the stator ground wall insulation is critical to the proper operation
of a motor or generator. For a long service life, the ground wall must meet the rigors of the electrical,
thermal, and mechanical stresses that it is subject to.
41
B12 C.S of multi-turn coil, where the turn insulation
and strand insulation are same
7.1.4 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 the 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 semi-conducting (graphite) surface
layer, extending a short distance outward from the slot ends.
So insulation of these stator bars is an inevitable task. It is worth now to know about
insulation. As we told before insulation is the heart of the generator now let us move to the most
interesting and important topic insulation.
7.2 INSULATING MATERIALS:Insulating materials or insulators are extremely diverse in origin and properties. They are
essentially non-metallic, are organic or inorganic, uniform or heterogeneous in composition, natural
or synthetic. Many of them are of natural origin as, for example, paper, cloth, paraffin wax and
natural resins. Wide use is made of many inorganic insulating materials such as glass, ceramics and
mica. Many of the insulating materials are man-made products and manufactured in the form of
resins, insulating films etc., in recent years wide use is made of new materials whose composition and
organic substances. These are the synthetic Organo-silicon compounds, generally termed as silicones.
Properties of a good Insulating Material: The basic function of insulation is to provide insulation live wire to live wire or to the earth. A
good insulating material needs the following physical and electrical properties.
1. It should be good conductor to heat and bad conductor to electricity.
2. It should withstand the designed mechanical stress.
3. It should have good chemical and thermal resistively and environmental resistively.
4. High dielectric strength sustained at elevated temperatures.
42
5. High resistivity or specific resistance
6. Low dielectric Hysterisis.
7. Good thermal conductivity.
8. High degree of thermal stability i.e. it should not deteriorate at high temperatures.
9. Low dissipation factor.
10. Should be resistant to oils and liquid, gas flames, acids and alkalis.
11. Should be resistant to thermal and chemical deterioration.
43
7.2.2 CLASSIFICATION OF INSULATING MATERIAL:The insulating material can be classified in the following two ways.
I. Classification according to substance and materials.II. Classification according to temperature.
a. Classification according to substance and materials:1. Solids (Inorganic and organic)
EX: Mica, wood slate, glass, porcelain, rubber, cotton, silks, rayon, ethylene, paper and cellulose
materials etc.
1. Liquids (oils and varnishes)
EX: linseed oil, refined hydrocarbon minerals oils sprits and synthetic varnishes etc.
2. Gases
EX: Dry air, carbon dioxide, nitrogen etc.
A.1 CLASSIFICATION ACCORDING TO TEMPERATURE:
Class Permissible temperature
Materials
Y 90 Cotton, silk, paper, cellulose, wood etc neither impregnated nor immersed in oil. These are unsuitable for electrical machine and apparatus as they deteriorate rapidly and are extremely hygroscopic.
E 120 Synthetic material of cellulose base B 130 Mica, asbestos, glass fibre with suitable bonding substance F 155 Material of class B with binding material of higher thermal
stability. H 180 Glass fibre and asbestos material and built up mica with
silicon resins. C Above
180Mica, porcelain, quartz, glass (without any bonding agent) with silicon resins of higher thermal stability.
A.1 Classification according to temperature
44
7.2.2 INSULATING MATERIALS FOR ELECTRICAL MACHINES:
In resin rich system of insulation Mica paper will give a good dielectric strength and Glass fiber tape
will give a good mechanical strength and Epoxy resin can withstand up to 155 degree Centigrade so it
gives a good thermal properties. Resin rich and Resin poor insulating materials are characterized by
the contact of the Epoxy Resin. In Resin rich system the content of Epoxy Resin tape is 40% so it is
named as RESIN RICH SYSTEM, and in Resin poor system the content of Resin tape is 8%. By VIP
impregnation process, the required amount is added to then conductor bars after assembling the core
and placing the winding in the core. In resin rich system before placing of coils in the stator slots the
rich tape will be wrapped over the bars. Nevertheless, this system has the following disadvantages:
1. This system is very time consuming and very long procedure.
2. Total cost of the system is more.
In order to minimize the over all cost of the machine and to reduce the time cycle of the system, the
VACUUM PRESSURE IMPREGNATION SYSTEM is being widely used. This process is very
simple, less time consuming and lower cost.
BHEL, HYDERABAD is equipped with the state of the art technology of VACUUM
PRESSURE IMPREGNATION.
The core or coil building and assembling method depends on the insulation system used. The
difference in core building is
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.
The manufacturing of coils also differs for both as explained above for core.
1. For resin poor process
2. For resin rich process
50
9. MANUFACTURE OF STATOR COILSManufacturing of stator coils depends on the type of the insulation process used for the stator.
I.e. the process is different for resin rich and resin poor process although few of the sub processes are
same for both.
9.1 FOR RESIN POOR PROCESS:In this process 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.
9.1.1 Reception of copper conductors:
The copper conductors rolls are
received is checked for physical and
mechanical properties. First piece is
checked for specifications such as
length and if found satisfactory, mass
cutting to desired length is carried out
by feeding into the cutting mills. B13 Side View showing one way of transposing insulated strands in stator bar.
9.1.2 Transposition: Conductors are adjusted one over another for a given template and the bundles are
transposed by 360 degrees by setting the press for “Roebel Transposition”. Now they are bundled and
consolidated by tying with cutter tape at various places.
Similarly all the bundles are processed. Thus each stator bundle has a transposed coils in each phase
such that the flux distribution is equal and hence the induced e.m.f.
9.1.3 Putty operation:
All the transposed bars are shifted to putty operation. Here a single bar is taken for putty
operation by filling up the uneven surfaces on the width face by filling with NOMAX. I.e., NOMAX
sheets are inserted in the crossovers on the width face to the both ends. Form mica net is placed over
the width face of the bar on both sides & wrapped with PTFE (poly tetra flamo ethane) tape.
9.1.4 Stack consolidation:
Now 2 to 3 bars are inserted into hydraulic presser and they are pressed horizontally and
vertically to a pressure up to 150kg/cm2. At the same time the bars are subjected o heating from 140
to 160 degrees for duration of 2-3 hrs. Then the bars are unloaded and clamped perfectly. Now inter
half and inter strip testing is carried out and the dimensions are checked using a gauge.
51
9.1.5 Bending:
Each of the samples is placed over the universal former & the universal former is aligned to
the specifications. The bar is bent on both the sides i.e. on turbine side (TS) and exciter side
(ES).the 1st bend and the 2nd bend is carried out and continued by over hang formation. Now the
3rd bend is carried by inserting nomax sheet from the end of straight part to the end of 3 rd bend and
the bars are clamped tightly. Now the clamps are heated to 60 degrees for 30mins. Inter half and
inter strip tests follows.
9.1.6 Final taping:
The taping may be machine or manual taping and the taping is done according to the
type of insulation used. In case of resin poor system, resin poor tape is wrapped by 9*1/2 over lap
in the straight portion up to overhang and 6*1/2 over lap layers in the intermittent layers. The
intermittent layers are follows….
1st intermittent layer is ICP (internal corona protection) tape. This is wrapped by
butting only in straight portion.
2nd is split mica tape. One layer of split mica is wrapped by butting & using conductive
tape at the bottom so that split mica is not overlapped.
Next layer is O.C.P (outer corona protection). OCP tape is wrapped final in straight
portion by but joint up to end of
straight portion on both the sides.
Next intermittent layer is
ECP (end corona protection). ECP tape is
wrapped from the end of straight portion
up to over hang over a length
of 90-110mm. Now the bars are wrapped
B.14 Cross-section of a multi
turn coil, where three turns and three
strands per turn
finally with hyper seal tape from straight portion to the end of 3rd bend in overlapping layers for
protecting the layers from anti fingering. The IH & IS tests follows and the bars are discharged to the
stator winding.
9.2 FOR RESIN RICH SYSTEM:
52
The coil manufacture is same as in case of resin poor but differ in a few stages. The
Conductor cutting and material used is same as resin poor system. Transposition is done same as that
of resin poor system. Stacking of coils is done. In this case high resin glass cloth is used for
preventing inter half shorts. There is a difference in putty work.
9.2.3 Putty work:
Nomex is used in between transposition pieces. 775 varnish is applied over the straight
portion of bar and mica putty is applied on the width faces of the bars.
Mica Putty mixture is a composition of SIB 775 Varnish, mica powder and china clay in the ratio of
100:50:25.
Straight part baking is done for 1hour at a temperature of 160C and a pressure of 150kg/sq.cm.Then
bending and forming is done. Half taping with resin rich tape is done for over hangs and reshaping is
done. To ensure no short circuits half testing of coils is done.
9.2.4 Final taping:
Initial taping and final tapings is done with resin rich tape (Semica Therm Tape) to about 13-
14 layers. The main insulation layers are 12*1/2 overlap in the straight portion and 9 layers in the
overhang.
B15 Layout of a mould used in baking of stator by Resin rich process
9.2.5 Final baking:
53
Final baking is done for 3hrs at a temperature of 160C in cone furnace. The bar is fed into the
baking mould.
The bar is heated for 1 hr at 90 degree to get gelling state.
The temperature of the mould is increased to 110 degrees in 30 mins and simultaneously the
moulds are tightened. Now in this process 155 of the resin is oozed out only 25% will be
remain. Now the bar is unloaded and checked for final dimensions, sharp corners, depressions,
charring, hollow sounds etc.,
Gauge suiting is done. I.e. the dimensions are made to compromising with the design.
Conductive/graphite coating (643) is applied on the straight portion and semi-conductive
coating 642 from end of straight portion to 3rd bend to pre transition coating on both sides.1st
coating for 90mm, 2nd coating for 100mm and 3rd coating for 120mm on both sides.
The bar is allowed for drying and epoxy red gel is applied from the end of straight portion to
the 3rd bend on both sides and allow for drying.
High voltage testing is done at 4 times that of rated voltage and tan testing, inter strip, inter
half testing are done. Tan values must be less than 2%.
54
10. AN OVERVIEW
10.1 ADVANTAGES OF RESIN POOR SYSTEM OF INSULATION:
It has better dielectric strength
Heat transfer coefficient is much better
Maintenance free and core and frame are independent
It gives better capacitance resulting in less dielectric losses due to which the insulation life will be
more
The cost will be less and it is latest technology
Reduction in time cycle and consumption for MW also less and it gives high quality
10.2 DISADVANTAGES OF RESIN POOR SYSTEM OF INSULATION:
If any short circuit is noticed, the repairing process is difficult and need of excess resin from
outside.
Dependability for basic insulating material on foreign supply
10.3 ADVANTAGES OF RESIN RICH SYSTEM OF INSULATION:
Better quality and reliability is obtained
In case of any fault (phase - ground/ phase – phase short) carrying the repair process is very easy.
Addition of excess resin will be avoided because of using resin rich mica tape
10.4 DISADVANTAGES OF RESIN RICH SYSTEM OF INSULATION:
It is a very long procedure
Due to fully manual oriented process, the cost is more
It is possible to process stator bars only.
Even though the advantages and disadvantages of both the process are explained above, resin
poor process is the best of all, as the resin content used is almost only 35% compared to resin poor
process and also show good insulation properties justified in the later sections.
55
11. ASSEMBLY OF STATOR
The completed core and the copper bars are brought to the assembly shop for
assembly.
11.1 RECEPTION OF STATOR CORE:
Stator core after the core assembly is checked for the availability of foreign matter, so coil
projections are checked in each slot. HGL gauge is passed in each and every slot to detect bottom
core projections.
11.2 WINDING HOLDER’S ASSEMBLY:
Assemble all the winding holders on both sides by adapting to the required design size.
Check all the wedge holders by a template and they are assembled as per the design requirement.
Tighten all the bolts relevant to winding holders and lock them by tag welding. Assemble HGL rings
on both the sides by centring with respect to core. Subject each individual for pressing in pressing
fixture at a pressure of 60 kg/cm2 for 30 minutes. Inter half test is conducted for each individual bar
before assembling into the stator.
Now stator bar assembling is carried out by centring to the core and check for proper
seating of bottom bars with T-gauge and checked for third bend matching, over hang seating etc..,
rein force the overhang portion of stator bars by inserting glass mat in between the bars and tying
them with neoprene glass sleeve. This process is carried out for all respective bottom bars .now the
pitch matching is checked on both sides both the generator and the exciter side.
Now high voltage testing is carried out on the stator.
11.3 STIFFENERS ASSEMBLY:
Stiffeners are assembled on both sides and then checked for physical feasibility of top bar
by laying into the respective slot. Check for uniform gap in the over hang and top bar matching to the
bottom bar pitch on both sides. Assemble all the top bars by inserting inner layer inserts and also
assemble relevant RTD’s (Resistance Temperature detectors) where ever they are required as per the
design.
After completion of top bars, reinforce overhangs by inserting Glass-mat and tying with
Neoprene glass sleeve and also check for the third bend matching on both the sides. Then the core is
subjected to high voltage DC test and inter half short circuit tests.
11.4 EYE FORMATION:
Join bottom conductors and top conductors forming an eye, by brazing the conductors with
silver foil. Segregate eyes into two halves on both sides and test for inter half shorts. Insert Nomax
into two halves and close them.
56
Brazing makes the electrical connection between the top and bottom bars. One top bars strand
each is brazed to one strand of associated bottom bar so that beginning of the strand is connected with
out any electrical contact with the remaining strand. This connection offers the advantage of
minimizing three circulating currents.
11.5 CONNECTING RINGS ASSEMBLY:
The connecting rings are assembled on exciter side as per the drawing and connect all the
connectors to the phase groovers by joining and brazing with silver foil. Clean each individual phase
groove, insert nomax sheet and tape with semica folium. Subject the whole stator for HVDC test.
Terminate the three RTD’s in the straight portion and the 3-RTD’s in the over hang portion on both
turbine and exciter side except one for earthing requirement.
11.6 PHASE CONNECTORS:
The phase connectors consist of flat copper sections, the cross section of which results in a low
specific current loading. The connections to the stator winding are of riveted and soldered tape and
like wise wrapped with dry mica/glass fabric tapes. The phase connectors are firmly mounted on the
winding support using clamping pieces and glass fabric tapes.
Thus we have a completed stator here. Now this stator is sent for VPI process because in there is a
chance of damage to the insulation due to the following reasons
During the stator assembly, the bars are beaten with rubber hammers to fit into the slots
Also there is a chance of void spaces in between the stator conductors and the core due to the
use of solid insulating materials, which lead to slot discharges.
So in order to fill these voids and to gain good insulating properties the stator is VPI processed. Let
us start with an introduction to the process and the early materials used for this process and the
advancement of this process to our present resin poor VPI process.
57
12. THE VPI PROCESS
12.1 INTRODUCTION TO VACUUM PRESSURE IMPREGNATION SYSTEM (VPI)
12.2 HISTORY
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. Prior to development of Thermosetting resins, a widely used insulation system for 6.6kv and
higher voltages was a Vacuum Pressure Impregnation system based on Bitumen Bonded Mica
Flake Tape is used as main ground insulation. After applying the insulation coils or bars were placed
in an autoclave, vacuum dried and then impregnated with a high melting point bitumen compound.
To allow thorough impregnation, a low viscosity was essential. This was achieved by heating the
bitumen to about 180C at which temperature it was sufficiently liquid to pass through the layers of
tape and fill the interstices around the conductor stack. To assist penetration, the pressure in the
autoclave was raised to 5 or 6 atmospheres. After appropriate curing and calibration, the coils or bars
were wound and connected up in the normal manner. These systems performed satisfactorily in
service provided they were used in their thermal limitations. In the late 1930’s and early 1940’s,
however, many large units, principally turbine generators, failed due to inherently weak thermoplastic
nature of bitumen compound.
Failures were due to two types of problems:
a. Tape separation
b. Excessive relaxation of the main ground insulation.
Much development work was carried out to try to produce new insulation systems, which
didn’t exhibit these weaknesses. The first major new system to overcome these difficulties was
basically a fundamental improvement to the classic Vacuum Pressure Impregnation process. Coils
and bars were insulated with dry mica flake tapes, lightly bonded with synthetic resin and backed by
a thin layer of fibrous material. After taping, the bars or coils were vacuum dried and pressure
impregnated in polyester resin. Subsequently, the resin was converted by chemical action from a
liquid to a solid compound by curing at an appropriate temperature, e.g. 150 C. this so called
thermosetting process enable coils and bars to be made which didn’t relax subsequently when
operating at full service temperature. By building in some permanently flexible tapings at the evolutes
of diamond shaped coils, it was practicable to wind them without difficulty. Thereafter, normal slot
packing, wedging, connecting up and bracing procedures were carried out. Many manufacturers for
producing their large coils and bars have used various versions of this Vacuum Pressure Impregnation
58
procedure for almost 30 years. The main differences between systems have been in the types of
micaceous tapes used for main ground insulation and the composition of the impregnated resins.
Although the first system available was styrenated polyester, many developments have taken place
during the last two decades.
Today, there are several different types of epoxy, epoxy-polyester and polyester resin in
common use. Choice of resin system and associated micaceous tape is a complex problem for the
machine manufacturer.
Although the classic Vacuum Pressure Impregnation technique has improved to a significant
extent, it is a modification to the basic process, which has brought about the greatest change in the
design and manufacture of medium-sized a.c. industrial machines. This is the global impregnation
process. Using this system, significant increases in reliability, reduction in manufacturing costs and
improved output can be achieved. Manufacture of coils follows the normal process except that the
ground insulation consists of low-bond micaceous tape. High-voltage coils have corona shields and
stress grading applied in the same way as for resin-rich coils, except that the materials must be
compatible with the Vacuum Pressure Impregnation process. Individual coils are inter turn and high-
potential-tested at voltages below those normally used for resin-rich coils because, at the un-
impregnated stage, the intrinsic electric strength is less than that which will be attained after
processing. Coils are wound into slots lined with firm but flexible sheet material. Care has to be taken
to ensure that the main ground insulation, which is relatively fragile, is not damaged. After inter-turn
testing of individual coils, the series joints are made and coils connected up into phase groups. All
insulation used in low-bond material, which will soak up resin during the impregnation process. End-
winding bracing is carried out with dry, or lightly treated, glass-and/or polyester-based tapes, cords
and ropes. On completion, the wound stator is placed in the Vacuum Pressure Impregnation tank,
vacuum-dried and pressure-impregnated with solvent less synthetic resin. Finally, the completed unit
is stoved to thermo set all the resin in the coils and the associated bracing system.
After curing, stator windings are high-potential-tested to the same standard. Loss-tangent
measurements at voltage intervals up to line voltage are normally made on all stators for over 1kv. A
major difference between resin-rich and vacuum pressure impregnation lies in the importance of this
final loss-tangent test; it is an essential quality-control check to conform how well the impregnation
has been carried out. To interpret the results, the manufacturer needs to have a precise understanding
of the effect of the stress-grading system applied to the coils. Stress grading causes an increase in the
loss-tangent values. To calculate the real values of the ground insulation loss-tangent, it is necessary
to supply from the readings the effect of the stress grading. For grading materials based on the
materials such as silicon carbide loaded tape or varnish, this additional loss depends, to a large extent
upon the stator core length and machine voltage.
59
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 and therefore, there is a lower hot spot rise compared to the
average rise.
In order to minimize 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.
The chemical composition of our resin type and its advantages are explained in the later
sections. Now let us discuss the various stages involved in VPI process for resin poor insulated jobs.
VPI process is done in the VPI camber. For higher capacity stators of steam turbine or gas
turbine generator stators, horizontal chamber is used where as vertical chamber is used for smaller
capacity systems such as Permanent Magnet Generator (PMG), coil insulation of small pumps and
armature of motors etc..,
12.3 VACUUM PRESSURE IMPREGNATION OF RESIN POOR INSULATED JOBS:
VPI process for a stator involves the following stages.
2. Preheating
3. Lifting and shifting
4. Vacuum cycle
5. Vacuum drop test
6. Heating the resin
7. Resin admission.
8. Resin settling
9. Pressure cycle
10.Aeration.
11.Post curing cycle
12.Cleaning
60
12.3.1 General instructions before VPI process:The jobs that are entering tank for Vacuum Pressurised Impregnation shall not have any oil
based coatings. Any such, rust preventive/ corrosion preventive viz., red oxide etc., shall be
eliminated into the tank. Jobs shall be protected with polyethylene sheet for preventing dust or dirt on
jobs, till it is taken up for impregnation. Resin in the storage tank shall be stored at 10 to 12 C and
measured for its viscosity, viscosity rise. Proper functioning of the impregnation plant and curing
oven are to be checked by production and cleared for taking up of job for impregnation.
12.3.2 Pre heating: The foremost stage of VPI, the completed stator is placed in the impregnation vessel and kept
in an oven for a period of 12 hours at a temperature of 60 deg. Six thermocouples are inserted at the
back of the core to measure the temperature. The temperature should not exceed to 85 deg .Smaller
stator can be inserted directly into the impregnation chamber. The job is to be loaded in the curing
oven and heated. The temperature is to be monitored by the RTD elements placed on the job and the
readings are logged by production. The time of entry into the oven, time of taking out and the
temperature maintained are to be noted. Depending on convenience of production the jobs can be
preheated in impregnation tank by placing them in tubs.
The impregnation tubs used for impregnation of jobs are to be heated in the impregnated tank itself,
when the jobs are preheated in the curing oven
12.3.3 Insertion of tub with job into the impregnation tank:The wound stator is lifted and shifted into the tub. By the time, the preheating of job is completed, it
is to be planned in such a way that the heating of tub and tank heating matches with the job. This is
applicable when the job is heated in the curing oven separately. The preheated job is to be transferred
into the tub by crane handling the job safely and carefully with out damage to the green hot insulation
the tub is then pushed in the 140 tank furnace or also called as vacuum tank, after which the lid is
closed and the tank furnace was heated to 60 +/- 3 deg The warm tub with job is inserted into
impregnation tank by sliding on railing, in case of horizontal tank. The thermometer elements are to
be placed at different places on the job. The connection for inlet resin is to be made for collection of
resin into tub. After ensuring all these lid of the impregnation tank is closed. In case of vertical tank
61
the job along with tub is slinged and inserted carefully into impregnation tank without damage to
insulation
12.3.4 Vacuum cycle:
The pre heated job will be placed in the impregnation chamber by a hydraulic mechanism.
The vessels are kept clean and the resin available in the vessel is wiped out. Methylene and traces of
resin should not be allowed on the inner side of the tank. Now the vacuum pumps are all switched on
and a vacuum pressure of about 0.2 mb is maintained for about 17 HRS, after which the wound stator
is subject to vacuum drop test.
12.3.5 Vacuum drop test: This drop test is important phase, all the vacuum pumps are switched off for about 10 mins,
and the vacuum drop is measured and it is checked whether it exceeds 0.06mb, if it exceeds 0.06mb
then it is subject to repetition of vacuum cycle for another 6 to 8 hrs, else it is sent to the next cycle
12.3.6 Drying the job in vacuum The job is to be dried under vacuum. Drain out the condensed moisture/ water at the exhausts
of vacuum pumps for efficient and fast vacuum creation. Also check for oil replacement at pumps in
case of delay in achieving desired vacuum.
12.3.7 Heating the resin in the storage tank The completion of operations of drying and the heating of the resin in the storage tank are to
be synchronised. The heating of resin in the tank and pipeline is to be maintained as at preheating
temperature .i.e. the temperature is maintained at 60+/- 3 deg ,including pipeline
12.3.8 Admission of resin into impregnation tank The resin is allowed into the impregnation tank tub if required from various storage tanks
one after the other, such that the difference in pressure fills the tank, up to a level of 100mm above
the job generally, after which the resin admission is stopped. After 10mins of resin settling the tank is
to be pressurised by nitrogen. While admitting resin from storage tanks pressurise to minimum so that
nitrogen will not affect resin to spill over in tank.
12.3.9 Resin settling:The resin is allowed to settle for
about 4mins in such a way that bubble
formation ceases
12.3.10 Impregnation Pressurising/gelling After the resin has settled the job
is subject to pressure cycle of 4 kg/ cm2
62
of dry nitrogen into the vacuum tank after obtaining 4 kg/cm2, this is subject for 2 hrs. in this stage
the resin is impregnated into the micro pores of the stator and is very firmly embedded into the
crevices of the stator ,so thus acting as a tough layer of insulation for the stator ,being indestructible
in the long run. B16 vertical VPI tanks for
smaller jobs
12.3.11 Withdrawal of resin
from impregnation tank to
storage tank
The resin that is pressurised as
per pressure cycle is drawn into the
tank by the opening of relevant
valves will allow the resin to come
back to the storage tank. The job also
shall be allowed for dripping of
residue of resin for about 10min.
After dripping, withdrawal of resin in various storage tanks is to be carried out. This is necessary
because resin is a very costly material.
12.3.12 Taking out the tub with job from impregnation tankThe lid is then opened after taking precautions of wearing mask and gloves for the operating
personnel as a protection from fumes. The job is withdrawn from impregnation tank by sliding on
railing for horizontal and slinging on to crane for vertical impregnation tanks.
12.3.13 Post curing:The job is post heated. The
time and temperature to which the
job has to be scheduled is varies
according to the type of job and is
given in the table. The time at which
the heating is started, achieved and
maintained is to be logged. The
63
wound stator is subject to 140 +/- 5 deg. After obtaining 140 deg the stator is subject for 32 hrs. The
stator is then made to rotate at 1 rpm up to 120 deg. It is then allowed for cooling without opening
the doors till the temperature reaches 80 deg, after attaining the temperature of 80 deg, the doors are
opened and wound stator is sprayed with epoxy red gel on the overhangs and is allowed for drying.
12.3.14 Cleaning: Entire wound stator is cleaned for resin drips, after which its subjected to HV and tan delta tests
12.4ELECTRICAL TESTING:
All jobs that are impregnated till above process are to be tested for electrical tests. After
ensuring that all the temperature/vacuum conditions stipulated for drying, impregnation and curing
operations have been properly followed, the job is to be released for this operation.
12.5GLOBAL PROCESSING:
Processing details depends very much on the machine type, on customer’s defined parameters and
type of mica tapes.
Generally the VPI system is used in impregnation vessels up to 30T where the rotor/stator is
impregnated at elevated temperatures. Machine parts usually are preheated (also under vacuum) in
order to remove moisture and to reduce viscosity during impregnation.
12.6RESIN MANAGEMENT:
After impregnation the VPI bath is pumped into storage tanks and cooled down to 5-10C and
should be stored in dry conditions in order to obtain a long bath life. Actual bath life depends on
additional parameters, e.g., impregnation temperature and duration of impregnation, impurities in the
bath, wash-out of catalyst from mica tapes into the un- accelerated resin system (B), replenishment
rate, moisture exposure etc,. The viscosity of the bath should be checked periodically in order to
maintain a suitable viscosity for impregnation.
Impregnated, yet uncured machine parts in unconditioned atmosphere may pickup moisture.
Therefore curing directly after impregnation or storage in moisture controlled area is recommended.
Generally machine parts are rotated when removed B17 Showing the resin tank in
which resin is stored.
from the bath and during the first part of
curing in order to avoid drip off.
Evaporation of hardener during the
vacuum cycle leads to a change in the
resin/hardener ratio in the bath and
has to be compensated. Therefore
replenishment is mixing ratios of 100-
120pbw of hardener HY 1102 per
100pbw MY 790-1 are generally
64
used. Replenishment mixing ratios depend on actual processing parameters and conditions and have
to be evaluated at the customer site.
Due to excellent latency of the system (A) MY 790-1/HY 1102/DY 9577/DY073 the replenishment
volume to maintain a constant viscosity is comparatively small, even if impregnation is performed at
40-50C.
On single coils and roebel-bars the mica insulation is normally covered with a tight glass tape to
prevent drainage of the impregnation resin.
12.7SPECIFIC INSTRUCTIONS:
Depending on the insulation materials and the accelerating agent in use, a ramped curing
schedule is recommended.
In systems with high reactivity, where the accelerator can be include in the mica-tape, a fast
gelation can be obtain with a temperature-shock, and draining can so be reduced or avoided.
Standard curing with the standard accelerated mixture (system A) is:11 at 90C plus 18 h at
140C
12.8PRECAUTION:
To determine whether cross linking has been carried to completion and the final properties are
optimal, it is necessary to carry out relevant measurements on the actual object or to measure
the glass transition temperature. Different gelling and cure cycles in the manufacturing
process could lead to a different cross linking and glass transition temperature respectively.
12.9 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.
An overview of entire VPI process with the time taken for each process according to the type of the
job used is given below in a tabular form
VACUUM PRESSURE IMPREGNATION OF RESIN POOR INSULATED JOBS:
Variant Description
01 Brushless exciter armature, PMG stators and Laminated rotors 02 Stator wound with diamond pulled coils.
65
03 Stator with half coils
Variant-01 Variant-02 Variant-03Any other
information
Preheating 605C for 3hrs 605C for 12hrs
603C for 12hrs
Vacuum to be maintained
0.4mbar 0.2mbar/0.4mbar
<0.2mbar(both together
shall not exceed 50hrs including
rising time)
Vacuum heating time 3hrs
0.2mbar for 9hrs 0.4mbar for 17hrs
Stopping vacuum pumps for 10min shall
check 17hrs vacuum drop. The vacuum
drop shall not exceed by
0.06mbar for 10min
Increase in pressure
40min 80min 80min
Maximum pressure 3bar 4bar 4bar
Pressure holding 3hrs 3hrs 3hrs
Post curingAt1405C for
14hrsAt1405C for 32hrs
At1405C for 32hrs
A.6 Table showing temperature and time to be maintained for different type of jobs in VPI
66
13. FACILITIES AVAILABLE IN VPI PLANT IN BHEL
The major facilities available in VPI plant are:
Steam furnace for preheating
Size of chamber: 2 * 2 * 6.5 M
Maximum temperature: 160C
Electrical power consumption: 75KW
Work place: 1425
Work centre: 3215
Stream inlet: 200-250C
Impregnated tubs for keeping jobs
For vertical impregnation: As per respective tech. Document.
For horizontal impregnation: As per respective tech. Document.
Specifications of plant:
Impregnation medium
(a) Epoxy resin (class F solvent free) and hardener mix in 1:1 ratio as per TG34967
(b) Epoxy resin (class F solvent free) and hardener mix in 1:1 ratio as per TG34931
Horizontal impregnation chamber
Diameter: 4000mm
Cylindrical length: 9000 mm
Operating over pressure: 6 bar
Operating vacuum: 0.15 mbar
Operating temperature: 90C
Loading weight of
impregnation object: Maximum of 120 tonnes
Maximum leakage rate: less than 1mbar/lit/sec.
Moving load: 140 tonnes.
Static load: 170 tonnes
Pressure medium for impregnation
Pressure medium: Dry nitrogen
Operating pressure: 6 bar.
Nitrogen storage capacity: 52cubic meter at 25 bar.
Resin storage capacity
Total storage: 5*9000L+1*3000L
Operating parameters of each tank
Operating vacuum: 0.5mbar
Operating over pressure: 0.5bar
67
Operating temperature: 80C
Resin filters(stainless steel washable)
Filter fineness: 150microns
Output (maximum): 1000lits/min
Vacuum system
Root pumps: 2No.s, 5.5KW each
Suction capacity: 2000cubic meter/hr
Vacuum pumps(4No.s, 7.5KW each)
Suction capacity: 250 cubic meter/hr
This system is provided with separator filter with activated carbon filters, to protect the vacuum
pumps from resin and hardener vapours.
Refrigeration system
The resin inside the tanks has to be stored at 102C. this can be stored for indefinite period with
a brine chilling/refrigeration system.
The brine storage capacity: 1*25000L+1*26000L
Composition of brine: 40%Mono Ethylene glycol and 60%water
Heating and cooling system
The heating of resin in the storage tanks and the impregnation chamber is by circulating the
heated brine through the heat exchangers, to heat by saturated steam. The hot brine is cooled to
about 40C by circulating water through coolers and then the brine is chilled to -10C and stored
in the tanks.
Post heating of job
(a) Explosion proof steam drier and electrical heating superposed.
Size: 7*4.5*4.5M
Maximum weight of job: 80 tonnes
Maximum temperature: 150C
(b) Indirectly heated hot air circulating oven (gas fired)
Size: 9*4.5*4.5M
Maximum weight of job: 170 T/120T with facility for rotation.
Maximum temperature: 150C
68
13.1 DATA COLLECTION OF SAMPLES
During the project two jobs have been impregnated in VPI Plant, the data has been collected and
recorded in the project report.
13.1.1 INDO-BHARAT-II ROTORPREHEATING:
Indo Bharat II rotor is loaded for preheating in steam furnace on 30-5-2007 at 18:00hrs.
Date and timeRTD-I(C) RTD-II(C) Furnace air
temperatureRemarks
30.5.2007 19:00 32.0 30.0 45.6
Rotor temperature is reached to 603C at 2:00hrs on 31.5.2007 and it is maintained for 4 hrs i.e., up to 6:00 on 31.5.2007
1.6.2007 3:30 116.2 113.1 34.8 Resin admission completed at 14:25 hrs on 31.5.2007
69
1.6.2007 5:30 129.6 125.7 33.8 Pressurization started at 14:45 hrs on 31.5.2007
1.6.2007 7:30 137.6 133.2 33.2 Pressurization completed at 15:30hrs on 31.5.2007
1.6.2007 9:30 145.7 140.2 36.5 Pressurization hod up completed at 18:30hrs on 31.5.2007
1.6.2007 11:30 145.7 141.6 38 Resin withdrawal to storage tanks is from 18:30 to 18:45hrs on 31.5.2007
1.6.2007 13:30 144.7 143.4 42.8 Rotor loaded in gas furnace at 19:15hrs on 31.5.2007
1.6.2007 15:30 144.1 143.0 43.8 Rotor temperature is reached to 131.6 to 145.7C at 8:30hrs on 1.6.2007 and it is maintained for 14hrs i.e., up to 22:30hrs on 1.6.2007
1.6.2007 17:30 144.0 144.5 42.81.6.2007 19:30 143.0 143.0 41.0 Furnace is switched at
22:30hrs on 1.6.2007 and circulation fans are kept running till the job temperature is reached to 70C to 75C.
Indo-Bharat-II stator is loaded for preheating in steam furnace on 7-5-2007 at 23:30hrs.
Date and Time RTD-I(C) RTD-II(C) Furnace air
temperature(C)Remarks
7.5.2007 23:30 36.3 36.1 Stator temperature is
reached to 60.5C to
62.9C(603C) at
7:30hrs on 8.5.2007
and it is maintained
for 12hrs i.e., up to
19:30hrs on
8.5.2007
8.5.2007 1:30 43.6 42.9
8.5.2007 3:30 52.0 51.74
8.5.2007 5:30 55.9 56.0
8.5.2007 7:30 60.5 62.9 Stator is loaded in
vac(140) tank at
21:00hrs on
8.5.2007
8.5.2007 9:30 61.3 62.9
8.5.2007 11:30 60.3 62.4
8.5.2007 13:30 60.3 62.6 Vac. Pump is started
at 2:30hrs on
9.5.2007
8.5.2007 15:30 62.5 62.9
8.5.2007 17:30 62.9 62.66
8.5.2007 19:30 62.4 62.1
A.10 Preheating of Indo Bharat II stator
72
Vacuum cycle:
Date and TimeVacuum in
graph (mbar)
Vacuum in
meter (mbar)
Job
temperature
(C)
Resin cycle
8.5.2007 22:00 -- -- 54.37 Resin tanks 025,102 are
heated for impregnation
8.5.2007 0:00 -- -- 54.89 Viscosity of resin at 60C
is 33CP
9.5.2007 2:00 -- -- 59.02 Viscosity after aging is
36.10CP
9.5.2007 3:30 0.65 0.65 61.6 9.5.2007 and 10.5.2007
9.5.2007 5:30 0.41 0.40 63.59 Resin admission started at
19:45hrs
9.5.2007 7:30 0.28 0.29 64.2 Resin admission
completed at 19:55hrs
9.5.2007 9:30 0.22 0.22 63.2 Pressurisation started at
20:00hrs
9.5.2007 11:30 0.19 0.19 62.3 Pressurisation of
4kg/sq.cm reached at
21:20hrs
9.5.2007 13:30 0.18 0.18 62.1 Pressurisation hold up for
3hrs is at 0:20hrs
9.5.2007 15:30 0.17 0.17 62.0 Resin withdrawn to
storage tanks is from
0:30hrs –1:00hrs
9.5.2007 17:30 0.14 0.14 61.8 Stator loaded in hot air
furnace from 1:00hrs –
1:30hrs on 10.5.2007
9.5.2007 19:30 0.14 0.14 61.3
A.11 Vacuum cycle of Indo Bharat II stator
73
POST CURING:
Date and
Time
ESOH
15T
TSOH
06B
ESW
02
TSW
13Core
Room
temperatureRemarks
10.5.2007
1:30hrs70.0 76.4 62.4 62.5 63.4 33.1
10.5.2007
4:30hrs126.7 131.4 94.7 102.3 98.8 31.7
10.5.2007
7:30hrs144.3 154.1 125.4 134.5 126.1 31.6
10.5.2007
10:30hrs147.7 154.9 139.9 145.1 140.6 34.8
10.5.2007
13:30hrs137.6 144.4 139.3 141.6 140.7 38.0
10.5.2007
16:30hrs136.9 144.2 140.0 140.9 140.6 38.4
10.5.2007
19:30hrs140.2 143.6 140.1 140.7 140.2 37.2
10.5.2007
22:30hrs144.4 151.3 143.7 145.1 144.1 35.9
Job temp. is reached to
1405C i.e., from 136.2C to
145.6C at 9:30hrs on
10.5.2007 and it is maintained
for 32hrs i.e. up to 17:30hrs on
11.5.2007.
11.5.2007
1:30hrs143.1 146.7 145.2 145.1 145.2 33.8
11.5.2007
4:30hrs144.3 151.0 143.6 144.0 144.7 31.1
11.5.2007
7:30hrs135.7 142.1 144.3 145.1 145.0 31.3
11.5.2007
10:30hrs135.0 135.7 135.1 135.0 135.8 34.8
11.5.2007
13:30hrs135.6 141.4 135.4 135.6 135.9 38.3
11.5.2007
17:30hrs148.0 149.2 142.8 142.2 142.1 39.8
Furnace is switched off at
17:30hrs on 11.5.2007 and
circulation fans kept running
till the job temperature is
reached from 70C- 75C
A.12 Post curing of Indo Bharat II stator
74
13.1.3 High voltage levels of stator/rotor windings for multi turn machines:
S.No. Description HV level HV in kv remarks
Stator winding
1. After laying and wedging of
coils
18.9/1’ RTD,IT test
2. After OH spacers and
forming eyes
18.03/3’ RTD,IT test
3. Before impregnation
17.5/1’ R, RTD test
4. After impregnation
26.0/1’ R, RTD, Tan, leakage
reactance test5. Customer
acceptance25.0/1’ Rotor winding
Rotor winding
1. After laying first coil
UT+1400 2.9 Pole drops
2. After laying second coil
UT+1250 2.75 Pole drops
3. After laying third coil
UT+1100 2.6 Pole drops
4. After laying fourth coil
UT+950 2.45 Pole drops
5. After laying fifth coil
UT+800 2.3 Pole drops
6. After laying sixth coil
UT+650 -- Pole drops
7. After all connections
-- -- R, Pole drops
8. After tech. rings assembly
-- 2.15
9. After bandage -- 2.0 R, Pole drops10. After
impregnation-- 1.9 R, Pole drops
11. After excitation cable assembly
-- 1.8 R, Pole drops
12. After balancing UT+200 1.7 R,Z with 50Hz
A.13 High voltage levels of stator/rotor windings for multi turn machines
75
13.1.4 TESTING RESULTS OF INDO-BHARAT-II ROTOR
Customer Name: INDO-BHARAT-II ROTOR
M/c rating: 10.8MW, 12kv, 1500rpm.Test: Z, R and H.V test.
Stage: After impregnation.
Ambient temperature: 35C
Ohmic resistance: 0.264 (rotor temperature was more)
Voltage( volts) Current(amps)215.0 0.5
367.5 1.0
523.0 1.5
High voltage test: IR value before H.V. test at 15”/60” -- 200/300 M H.V. applied at 1.9kv /1’ – withstood IR value after H.V. test at 15”/60” -- 200/300 M
13.1.5TESTING RESULTS OF INDO-BHARAT-II STATOR:
Customer Name: INDO-BHARAT-II STATOR
M/c rating:10.0MW, 12kv, 0.8pf, 650A, 1500rpm.
Test: H.V test.
Stage: after impregnation.
Ambient temperature: 36C
A PHASE: IR value at 2.5kv IR value before H.V. test -- 1000/2000 M H.V. applied at 26-25kv /1’ – withstood IR value after H.V. test -- 1000/2000M
B PHASE: IR value at 2.5kv IR value before H.V. test -- 1000/2000 M H.V. applied at 26-25kv /1’ – withstood
76
IR value after H.V. test -- 1000/2000M
C PHASE: IR value at 2.5kv IR value before H.V. test -- 1000/2000 M H.V. applied at 26-25kv /1’ – withstood IR value after H.V. test -- 1000/2000M
A.17 tabular form showing open circuit voltages and currents obtained in the testing
82
14.3.4. Graphs
Fig. 2Comparative functional testing using the stator slot model confirms the progress achieved in extending the service life of 13.8-kV insulation of varying compositions.
KV
TA
N δ
83
RESISTANCE MEASUREMENT:Instrument: Micro ohm meter
Resistance at 25c (m) Resistance at 20c (m)
Rotor 264 258.92
R*75 = (( 235+75)/(235+20)) x R20= (310/255) x (0.2587)=0.3147