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OPERATIONAL FAULTS IN SYNCHRONOUS GENERATORS

Major Project Report(Part-I)

Submitted in Partial Fulfillment of the Requirements for Semester-III of

Master of Engineering

In

Electrical Engineering

(Electrical Power System)

By

Suryadeep D. Zala

090070707006

GUJARAT TECHNOLOGICAL UNIVERSITY


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Certificate

This is to certify that the Major Project Report (Part-I) entitled OPERATIONALFAULTS OF SYNCHRONOUS GENERATORS submitted by Mr.Suryadeep D. zala

(090070707006), towards the partial fulfillment of the requirements for Semester-III ofMasterof Engineering (Electrical Engineering) in the field of (Electrical PowerSystem) of Gujarat Technological University is the record of work carried out by himunder our supervision and guidance. The work submitted has reached a level requiredfor being accepted for examination. The results embodied in this major project, to thebest of my knowledge, have not been submitted to any other University or Institution foraward of any degree or diploma. Date:

Prof. R.P.Mehta Prof.B.A.Oza

(Project Guide) (PG Co-ordinator)Department of Electrical Engineering Department of Electrical Engineering

Birla Vishvakarma Mahavidyalaya Birla Vishvakarma Mahavidyalaya

Vallabh Vidyanagar Vallabh Vidyanagar

Head-EE

Department of Electrical Engineering

Birla Vishvakarma Mahavidyalaya

Vallabh Vidyanagar


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Acknowledgements

With immense pleasure, I would like to present this report on the dissertation work related toOPEARATIONAL FAULTS OF SYNCHRONOUS GENERATORS. I am very thankful to all those whohelped me for the successful completion of the first phase of the dissertation and for providing valuableguidance throughout the project work.

I would first of all like to offer thanks to Prof.R.P.Mehta, Guide, whose keen interest and excellentknowledge base helped me to finalize the topic of the dissertation work. He has shown keen interest inthis dissertation work right from beginning and has been a great motivating factor in outlining the flow ofmy work. I would also like to thank to Mr.N.B.Shah & Mr.S.H.Kahar(Ex.Engineers,Gsecl) who hascreated interest for finding ways to improve continuity of generators in me. Also I would like to extend my

thanks to Prof. B.A.Oza PG Co-ordinator, BVM, Vallabh Vidyanagar.

My sincere thanks and gratitude to Dr.B.R.Parekh, Head, Electrical Engineering Department, BirlaVishvakarma Mahavidyalaya for his continual kind words of encouragement and motivation throughoutthe Dissertation work. I am thankful to Birla Vishvakarma Mahavidyalaya ,Vallabh Vidyanagar forproviding all kind of required resources. I would like to thank The Almighty, my family, for supporting andencouraging me in all possible ways. I would also like to thank all my friends who have directly orindirectly helped in making this dissertation work successful.

- Suryadeep D. Zala090070707006


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Contents

Certificate

Acknowledgements

1 Scope of thesis work 5

2 Work plan for various stages including Dissertation Phase-1 and Phase-2 6

3 Introduction 8

4 Faults in synchronous generators 12

5 Literature Review 40

6 Conclusion & Future Work 50

7 References 51


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Scope of thesis work

Today's societies are critically dependent on the reliable supply of electricity. The

reliable operation of large synchronous generators in power stations is therefore

of paramount importance. Thus, power utilities are most keenly concerned in

taking measures to avoid catastrophic failures and to minimize the impact of

generator outages due to faults. Each generator constitutes one of the most

expensive single pieces of equipment in the power station. Although they are

mostly reliable and generally require minimal maintenance, faults can and do

occur with varying degrees of consequences ranging from minor outages to

catastrophic failures. When faults occur, cause-effect relationships need to be

identified systematically. This is necessary for having a better understanding of

the fault formation mechanisms so that preventive measures can be taken,

although sometimes the immediate pressures for returning a generator to servicemay impede a thorough investigation. The purpose of this thesis is to report on

practical problems in the operation of large synchronous generators and offer

advice on remedy & try to improve the performance.


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Work plan for various stages

Including Dissertation Phase-1

and phase-2

Work plan for dissertation Phase-1

Sr. no. Month Work

1 August-2010 (1)searching for the topic and requested Prof.R.P.Mehta to guide for the

dissertation work

(2)Confirmation from Prof. R.P.Mehta for the guidance

2 September-2010 Collection of relevant materials including books & IEEE papers

3 October-2010 (1)Study of Generator operation, factors affecting its performance &

Occurrence of Faults

(2)Literature Review

4 November-2010 (1)Problem identification and deciding the methodology for the work

(2)First Presentation in front of guide


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CHAPTER:1

INTRODUCTION

Over the last half century much advancement has been made in the field of large

synchronous generators. The need for larger unit sizes has driven the

development of better and more innovative designs. Manufacturing processes

have steadily improved.

The advent of new insulation materials has contributed to increased

reliability in operation at lower cost. The so-called Class F fiber-glass epoxy

insulation system, now recognized worldwide as the best modern insulation

system for commercial high voltage Generators, has all but replaced the hot dip

varnish insulation systems known as Class B insulation. Sophisticated designoptimization, clever use of modern materials and more efficient cooling systems

have been some of the key achievements in the field of design and manufacturing

of large synchronous generators. For example, the emergence of new technology

in internal water-cooled conductors within hydrogen cooled generators in the

1960s was, for the power generation industry, a quantum leap in building higher

capacity generators to answer the quest of the rapid growth in power generation

demands with improved generation efficiency.

Toward the end of the 20th century, privatization reforms swept through the

electricity industry in many developed countries. It is now widely observed that,

especially during the transition period, there have been shortfalls of capital

investment in upgrading and replacing aged equipment in the electricity supply

industry in all areas of generation, Transmission and Distribution.


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Major investment decisions have been heavily influenced by economic

drivers, which predominantly focus on short term interests, causing a serious

decline in support for load growth and difficulties in maintaining a satisfactory

Level of reliability for electricity supply. This is reflected in the reduction of

spinning reserve in the national electricity power systems. Due to the economic

realities of the electricity market, there is also a tendency that many old

generators built in the 1950's and 1960's are still kept in service or even recalled

from retirement. This has caused major concerns for power generation

companies for whom the reliability of operation of large synchronous generators

is of critical importance. Hence, innovative measures need to be taken so as to

avoid the risk of catastrophic failures, minimize the impact of generator outages

and ultimately making the best use of existing generating assets.

As participants in the modern electricity market, generating units, inevitably,

are required to be more flexible and responsive. No different from turbines and

power station auxiliaries, large synchronous generators have been operated

under increasingly tough conditions with significantly greater numbers of start-up

and shut-down, faster loading rates and a wider range of output fluctuation.

Cyclic operation in peak load or Load following modes is more suitable to fast

response gas turbine driven and hydro-electric generating units, whereas the

traditional large base load machines are designed for large volume, steady output

generation. Changing the operational mode of older machines from base load to

peak load or load following poses significant risks to large Generators, make harsh

problems related to ageing and, ultimately, reducing their Remanent (remaining

after the magnetizing field has been removed) service life.

As a matter of fact, a number of problems have emerged over time in

generators, especially in those manufactured during the transition period of the

1960s and 1970s. These problems have varied from fractures of stator endwinding conductors, causing hydrogen to leak into the stator coolant, to fretting

damage of the high voltage insulation system, causing partial discharge and, in

some severe cases, electrical short circuits. Such problems are observed

particularly in machines of the type known as waterbox generator, discussed in


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further detail in further chapters, where relevant constructional features are

described along with a number of internal faults.

The faults described in detail cover stator core localized overheating, fractured

stator conductors, rotor retaining ring cracking, rotor coil deterioration and inter-turn short circuits. One highlight of these problems is the failures of stator end

windings, which originate in one way or another, from mechanical deterioration

of the end-winding support structures. Observations on symptoms are given

which include evidence of insulation fretting dust found in stator conductor

interspace and on support structures. Fractures in conductors have been

attributed to copper work hardening due to uncontrolled movement of

conductors. Effects such as hydrogen gas leaking into the stator coolant water,

posing an increased risk for failure.

Also a range of test and investigation techniques commonly employed to

detect faults in large synchronous generators discussed in it. An inspection and

test plan is proposed, providing broad guidelines for monitoring the service

condition of generators.

The plan includes routine inspection, condition monitoring strategies and

appropriate test techniques. Early detection of failure in machines is absolutely

critical in avoiding major damage. Condition Monitoring (CM) is one of the most

effective means of failure prevention by early fault detection in modern asset

management practice.

In order to optimize the maintenance strategies, the traditional time-based

maintenance routines have been replaced with condition-based activities.

Although capital investment is required to setup appropriate CM systems, such

early fault detection can prevent major failures, downtime and repay their

investment many times over. Offline diagnostics, on the other hand, play animportant role in 'health spot checks' for synchronous generators. They can be

carried out routinely or as required and are great complement to the online

systems.


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A range of offline inspections and test techniques is suggested for detection of

specific faults of stator and rotor components. In general, it is observed that visual

inspection by an experienced inspector, who is familiar with a particular type of

generator, does not require sophisticated equipment and can be by far the most

beneficial in discovering many faults and the early beginning of faults.

Most problems within large generators can be traced back to two main original

sources: machine design and operating regime. The former may explain why

fracturing of some components is more common in some topologies than in

others, whereas the latter may have to do with the temptation in meeting the

raised generation requirements due to market demands. The thesis presents

examples of both types, with detailed accounts being given in each case.

Particular attention is paid to condition monitoring, both online and off-line, as abasis for developing preventive maintenance strategies to achieve reliable

operation with extended service life.


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CHAPTER : 2

FAULTS IN SYNCHRONOUS GENERATORS

Large synchronous generators are critically important to the reliability ofpower systems. Any fault, no matter how insignificant it may be perceived to be,

is a potential thread to its operation. Unchecked faults may ultimately lead to

catastrophic failure with dire consequences.

The majority of internal faults of large synchronous generators are in one way

or the other, related to mechanical related origins, of which the mechanical wear

and tear including structural deterioration are amongst the most common causes.

Electrical components such as the stator conductors are designed with generoussafety margins and rarely develop faults from within themselves, provided that

they are operated under the specified operational conditions. Some faults

originate from inherent weaknesses in generator design or in manufacturing

techniques and are often difficult to be remedied. On the other hand, operating

conditions constitute the other significant contributor to the development of

faults in large synchronous generators. Particularly, excesses in operation beyond

the original design capabilities can cause major damage or exacerbate existing

problems within the generators.


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2.1 FAULTS OF THE STATOR CORE

2.1.1 Constructional Features of Stator Core

Stator core of a large generator is made up of a large number of stamped thin

segmental magnetic steel laminations. Depending on the design and size of the

generator, there could be from 4 to 12 segments of laminations per layer. The

lamination material usually has high magnetic permeability and low electrical

losses. Each segment of lamination is coated with a thin layer of insulating varnish

so that when the core laminations are compressed together, each lamination is

electrically insulated from one another minimizing eddy current losses when the

core is excited.

Key bars of trapezoidal cross section are anchored axially along the internal

bore of stator frame to form a robust support structure for the core laminations.

Each lamination segment has dovetail shaped slots stamped on its outer

periphery and parallel radial slots stamped on its inner periphery to

accommodate the conductors. When the core is built up, each lamination

segment is sequentially located and locked into the appropriate key bars at the

dovetail slots. This allows the lamination segments to be accurately located,

aligned and supported at the key bars. The lamination stacks are then tightlycompressed at stages under pressure to form a rigid core, providing a strong

foundation for stator windings to be housed at the same time completing the

stator magnetic circuits.


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Figure 1. Typical arrangement in the back of stator core of a 210 MW

synchronous generator.

Longitudinally, the core is divided into sections. Each section is sandwiched

between two rings of thicker core plates and I-section spacers. Once built with the

adjacent sections, they form a network of radial cooling ducts. These ducts

provide essential cooling for the deeper parts of core laminations and passage for

control of hydrogen flow. In most generator cores, these cooling ducts are built at

short intervals of few centimeters along the entire length of the core while othersare built in a portion of the core only, depending on the design of cooling gas flow

control. Core lamination intervals are called core packet.

At both ends of the core, where magnetic flux density is the greatest due to the

high concentration of magnetic fringing flux, additional cooling provisions are


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problems are difficult and expensive to rectify. In large generators, stator cores

are inevitably of complex design and, if not constructed correctly, major problems

can and do occur after a few years in service. Some problems can escalate to

damaging stator windings. It is also important to follow the correct procedures

when the stator core is constructed, in addition to having the correct design and

materials. Each core lamination must be adequately insulated, pressed tightly

together and remain tight for the life of the generator. Loose core laminations can

lead to major core damage and catastrophic failures.Figure 3 illustrates such a

core fault. Following the discovery of a core fault during a routine inspection, a

thorough investigation was carried out on a 210 MW generator that had been in

service for 15 years. This particular generator suffered a core relaxation problem.

The condition deteriorates over time due to cyclic thermal and electromagnetic

forces acting on the laminations. In this case, the core was built with a thin layer

of compressed paper base material at approximately 100 mm intervals repeated

for the entire core length. Over time, the paper layers had been slowly

compressed further in service causing the relaxation. As the retaining pressure in

axial direction reduces, core 15 lamination vibration increases allowing the thicker

protective end plates on both sides of the lamination packets to rattle against the

'I' section gas duct spacer and eventually braking away. At the time of the repair,

many similar areas were found to have looseness and electrical arcing similar tothat shown in Figure 3.

Evidently, the broken away pieces of the core lamination material, bouncing

around in the air gap when the rotor is in motion cause mechanical impact

damage to both the stator and rotor surfaces. Lamination short circuits were

found in many places on the stator core. The stator and rotor windings were

contaminated with steel debris and carbon from the areas of coreburning.


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2.1.3 Stator Core End Plates Hot Spots

For optimum performance, generator stator core designs incorporate many

special features. Of these, core efficiency is the main design feature measured by

core losses. Other features such as cooling, rigidity, durability against thermal

cycling, and ease of construction are also very important. Magnetic field

distributions within the generator during normal and abnormal operating

conditions are also carefully considered in generator core designs. Improper

design or construction can lead to irreparable damage. Dreaded problems such as

core end plate burning are amongst those consequences. Some older stator cores

built in the 1960's and 1970's have been known to suffer a chronic problem of

local overheating at the end regions, labelled core end plate burning. After the

problem was experienced in several generators, it was found that the core end

plates of these generators were made of magnetic materials which are unsuitable

and highly susceptible to induced eddy currents.

When generators operate in leading power factor mode the air gap flux

density increases excessively. This leads to a significant increase in the fringingflux density in the end winding region, inducing eddy currents in the core end

plates, resulting in local overheating and burning. As the burning activities occur

on and behind the core end plate, usually in inaccessible locations, it is difficult to

detect. Figure 4 illustrates a typical magnetic field distribution in the end winding

region.


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Figure 4. Typical magnetic field distribution in the end winding region of a

synchronous generator

Once the core burning starts, it forms a so called 'hot spot" and gradually

grows. The heat generated from the hot spots can cause damage to the

surrounding lamination insulation, encouraging further burning. In at least three

severe cases, the hot spots were found to have spread through the conductor

insulation causing stator winding-to-earth faults. In other cases, the damage was

found to have extended to the generator air gap and stator core teeth where the

magnetic flux density is high, causing rapid core deterioration.

2.1.4 Back-of-Core Burning Fault

Back-of-core burning fault, as its name suggests, occurs in the back of the core

between the stator core laminations and their support key bars. The burning is a

result of electrical arcing from the current transfer between core laminations and

the key bars through poor electrical contacts. The key bars have an important


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function to provide accurate alignment and mechanical support for individual

laminations through a dovetail arrangement, as described previously in Section

2.1.1. Generally, there are about 20 to 30 key bars in a stator core of a 210 MW

generator.

There are two main types of stator core commonly known as (1) insulated

core and (2) non-insulated core. As their names suggest, the insulated cores are

electrically insulated from the stator frame whereas the non-insulated cores have

their laminations directly built on the dovetail shape key bars, which are

electrically earthed to the stator frame.

The key bars are electrically short circuited at the ends to form a squirrel

cage, designed to carry the induced currents caused by the stator leakage flux in

the back of the core. The induced currents flow between the network of key bars,

core laminations and core end plates and are highly complex in orientation,

fluctuating in magnitude depending on the operating condition and the degree of

magnetic saturation in the stator core. Especially during high leading power factor

generation, the saturation of magnetic field

can cause the induced currents in the back of the core to intensify near the core

ends. Back-of-core burning faults have been found on the top, sides and bottom

of turbo-generator stator cores.

Even though the back-of-core burning fault does not threaten an immediate

outage to the affected generator, it poses a major concern. As the electrical arcing

activity occurs, it produces metal globules at the fault location as a result of high

temperature burning of core lamination and key bar metals. Eventually, the

globules dislodge themselves from their 'birth place' and find their way into the

generator air gap causing serious contamination to the generator internal

components with thousands of small broken metal globules. This ramifies tosecondary problems such as degrading the high voltage winding insulation,

creating magnetic termites on stator conductors, promoting interturn short

circuits on rotor windings, causing interlaminar short circuits to stator core and

other mechanical impact damage to rotor components such as slot wedges and

rotor retaining rings.


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2.2FAULTS OF THE STATOR WINDINGS

Stator winding faults can have devastating consequences in terms of generating

plant availability. Although they can occur in any types of generator, some are

more prone to developing stator winding faults due to inherent design

weaknesses. One such type is known as the waterbox generator, which made its

debut halfway through the 20th

centuiy. Figure 7 depicts the end view of a typical

waterbox generator. Although reasonably reliable when new, its inherent

weaknesses became evident after about 10 years of operation. The design proved

to be maintenance intensive and hazardous due to occurrences of hydrogen

leaking into the stator coolant.

2.2.1 Constructional Features of Stator Windings

2.2.1.1 Conductors

There are two layers of coil side within each stator slot. Each coil side consists

of a bundle of insulated prismatic hollow copper strands, referred to as sub-

conductors. The sub-conductors are individually insulated from each other to

minimize internal eddy currents, and are transposed along the stator core to

minimize internal circulating currents in between the strands, referred to as

Roebel transposition, further improving machine efficiency. These hollow sub-

conductors are designed to carry the stator currents as well as to reticulate the

cooling water. At each end of the half coils, the strands are pneumatically and

electrically connected together and soldered to a hollow termination nozzle

making it a gas tight joint. This nozzle is mounted onto the back of waterbox. Each

termination nozzle takes cooling water from the waterbox for conductor cooling

at the same time carries electrical current. Each pair of corresponding termination


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nozzles is connected together via copper links to complete the electrical circuits

of the windings.

Due to the physical arrangement of the waterboxes, there is a larger gap in

between the adjacent waterboxes; the space in between the conductors ofadjacent phase groups is much greater than that of others in the same phase

group. Since the stator core slots are made with equal spacing, there are subtle

differences in coil pitch lengths and shape variations of conductors in the same

phase group at the end winding regions

This unique feature makes stock-holding of spare conductors difficult and

expensive. Most waterbox generators utilize so called, semi-flexible coils; that is

the middle section of the coil in the core region is more solid. In contrast, the

overhang sections are more flexible. Following the insulation wrapping process,

the middle straight section of the coils in the core section is pressed in formers to

exact dimensions and heated to cure the resin enriched insulation simultaneously.

The overhang end sections are, however, not pressed but insulated differently to

the core section in such a way that they are more flexible making coil installation

process easier. The insulation materials and coil construction methodologies have

varied with time and technologies available. The early generators utilized resin

rich mica-bituminous hot dip varnish insulation whereas the more recentgenerators have been equipped with resin rich fiber-glass epoxy insulation

system.

2.2.1.2End-Winding Support System

In the case of waterbox generators, a unique end winding support

system is utilized in order to restrain the stator conductors in the correct position

against electromagnetic forces. The stator end winding support system at each

end of the generator consists of one support comb to restrain the conductor ends

and support the resin waterboxes, nonmetallic support brackets to provide back

bone support for the end windings and to connect the support comb to the stator

frame structure at core end, a frustum cone on the involute side and a network of


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nylon tie rods. Each end-winding support comb consists of six matching segments

fabricated from resin-bonded fabric laminate material. It is mounted on the end

of the support brackets in parallel with stator core laminations to form a locating

platform for the ends of the

stator conductors. The stator conductors are further secured in position with a

resin-bonded fabric laminate ring fitted to the inner periphery of the support

comb by a series of radial nylon studs through the comb width. For additional

stiffness, the comb is secured to the stator core compression plate via a set of

long brass studs in parallel with the support brackets.

[End winding support system looking from behind the end winding involute

showing radial support brackets.]

2.2.1.3 Waterbox

A waterbox assembly consists of a cast resin epoxy base with conductor mounting

holes and securing studs, a cover and a carbon free rubber gasket . Waterboxes

are installed at both ends of the stator winding for the purpose of reticulating the


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cooling water into the hollow stator conductors. They also serve as an additional

barrier of electrical insulation between the high voltage conductors and the

nearby outer casing at earth potential. Once installed correctly with all

conductors in position, the waterbox assembly becomes gastight to an operating

hydrogen pressure of about 300 kPa. Stator conductors of the same phase group

are mounted through the back of a waterbox in two rows corresponding to the

inner and outer layers of the stator windings. Electrical connections between the

inner conductors and outer conductors are made with copper clamps called

'conductor links' forming stator windings.

Stator coolant water is pumped into each waterbox through hollow

terminal leads at one end of the generator. The cooling water is then forced

through the conductors to the corresponding waterbox at the opposite endbefore returning to the external cooling, filtering and pumping system.

[A typical end winding arrangement in a 210 MW generator, showing

waterbox base, coil links, coil end nozzles and spacing between conductor

ends.]


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2.2.2Sub-ConductorCracking Problem

In normal operation, the generator is filled with hydrogen gas at 300 kPapressure and the stator coolant system is charged with demineralised water at

200 kPa. The purpose for the differential in the pressure is to prevent water

leakage into the stator insulation system. Should any small leaks occur in the

stator coolant system due to faults such as cracks in conductors or leak gaskets,

hydrogen gas escapes into the stator coolant water and is collected in a gas

catchment chamber. An alarm is triggered and hydrogen gas is vented safely into

the atmosphere. Usually, hydrogen leaks are small and the loss of hydrogen

pressure is insignificant. However, if the cracks were allowed to grow, excessivehydrogen gas in stator coolant water could seriously reduce cooling capacity of

the sub-conductors leading to local overheating, eventually, deformation of the

subconductors and severe damage to the stator conductor. Even though most

stator designs incorporate thermal protection features with thermocouples

embedded in the core and various stator slots, overheating of this nature tends to

be localising around the affected conductor and have minute impact on the

average thermal distribution of the stator. If the fault location is away from a

thermocouple, its temperature is less likely to be registered. In the extreme cases,

the stator windings could suffer catastrophic failure.Such a gas leakage into the

stator coolant water is commonly called 'gas-in-statorcoolant leak'.

2.2.3End Winding Looseness Problem

Investigation of the end winding structure of many generators have found

that in most cases where cracked subconductors are found there is evidence of

excessive interconductor movement within the end-winding and relative to their

support structures. As the conductors vibrate in service, they rub against one

another and their support system, thus generating the so-called insulation fretting

dust, as exemplified in Figure. The fretting dust is relatively harmless to the


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generator and is a useful indicator for visual inspection of looseness. However,

excessive amount of insulation fretting dust is a cause for alarm as it is indicative

of a significant loss of insulation material; a sign of dangerous degradation of

conductor insulation system, thus increased risk of discharge and electrical faults.

Visual inspection is arguably the most practical and effective method for

detecting insulation fretting and deterioration of end winding insulation related

to conductor vibration. The relaxation of the conductors and their support

systems invariably leads to the loosening of the support comb and the involute

frustum support cone, which then vibrates in sympathy with the conductors,

causing insulation fretting, as illustrated in Figure 18. This accelerates the rate of

deterioration of the end winding support system.

[End winding relaxation causing abrasion wear on conductor insulation

indicated by insulation fretting dust.]


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[Stretched and unlocked nylon tie rods further promoting end winding

relaxation.]

2.2.4.1 A Case Study of stator winding fault

2.3ROTOR FAULTS

2.3.1 Constructional Features of Synchronous Generator Rotor

Only high speed turbo cylindrical generator rotor is discussed in this

thesis. Large turbo generator rotor is manufactured from one-piece alloy steel


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forging with winding slots and ventilation slots machined in the forging. The rotor

shafts are usually hollow and supported at both ends by white metal journal

bearings. Most high speed turbo generators of significant size would normally be

hydrogen cooled and their rotor shafts provide a good sealing collar for the

hydrogen seal to perform satisfactorily. Some rotor designs utilise sliprings as

means of connecting the rotor windings to the main excitation DC source, either

from a static excitation system or a DC main exciter. Other designs utilise a

brushless excitation system involving an AC main exciter and rotating rectifiers

eliminating the need for rotor sliprings. In both cases, the physical connections

from the rotor winding to its external DC supply are made through axial leads

placed in the centre bore of the shaft at the exciter end called 'up-shaft leads'.

Radial connections,also called 'radial pins' complete the electrical connection

between the up-shaft leads and the rotor windings. Design considerations of the

rotor insulation system are not only to satisfy the electrical requirements for the

windings but more importantly, to withstand the enormous centrifugal forces

acting on the windings

The rotor end windings are robustly built with strong insulation packing

blocks in between each coil groups in both radial and axial directions to prevent

coil distortion that could result from mechanical forces under steady state and

transient conditions. Each end winding is held in position by a retaining ring and a

balance ring.The rotor end windings are insulated from their retaining rings with

a strong insulation wrapping that allows sufficient electrical insulation property

but also withstands the mechanical pressure between the end winding coils and

the retaining rings. Furthermore, it is also critical that the design of insulation

system, rotor winding coils and their retaining systems have careful

considerations for thermal expansion for the winding coils to avoid distortions of

the copper coils.


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2.3.1.1 Rotor End Winding Retaining Rings

As the name suggests, retaining rings are of cylindrical shape that

manufactured from extremely high yield strength steel alloy, designed to

withstand enormous centrifugal forces from the weight of rotor copper end

windings. The retaining rings' main function is to retain the rotor end windings.

The design details of retaining rings can vary between manufacturers. However,

they commonly have one end of their cylinder interference fitted to the rotor

body and the other to a so called balance ring. On installation, the end rings are

heated to a designed temperature usually about 250C. As the thermal expansion

overcomes the interferential fit, the retaining ring is slid over the insulated end

winding onto the rotor shrink landing while being hot. It is extremely important

that the interference fit it designed and manufactured correctly so that the end

rings stay tight in position to at least 120% of the rotor rated speed. To further

secure the retaining rings in position and to prevent the retaining rings to move

away from the shrink landing, a locking mechanism is built into the nose section in

the form of a bayonet, step, clip ring or screw thread as shown in figure.

[Common types of retaining ring designs.]


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2.3.1.2Rotor Windings

In the case of a high speed, 3000 rpm, large turbo generators, the rotor has a two

pole winding made up from serial connected double helical bare copper coils

forming North South magnetic poles. Each turn of the rotor winding is

insulated from each other with thin insulation strips, called 'inter-turn insulation'

forming rotor coil stacks. The coil stacks are then insulated from the rotor body

with strong fibre-glass insulation troughs call 'slot liners'. The coils are retained in

the slots by strong metallic slot wedges that are keyed in the rotor slots by their

inverted-T shape and the retaining ring at each end. Cooling circuits are formed in

the windings by using a network of hollow conductors and carefully designed

radial cooling ducts made up by cut-out slots or perforated holes along each coil

turn.

In most rotors, the cooling gas is forced through the coils in both axial and

radial directions. Carefully designed ventilation features are incorporated in the

coil stack in each rotor slot and in the slot wedges to maximize the gas flow

through the rotor. The bulk of exhaust gas from the rotor flows into the air-gap

and then through stator core cooling ducts to further remove heat from the

stator. As the rotor windings normally carry thousands of Amperes, a properly

designed cooling system for rotor coils is

extremely important.

2.3.1.3Rotor Radial Connectors

Rotor radial connectors constitute another one of the critical components of

the generator rotor. The connectors provide electrical connections from rotor

windings to a DC supply via sliprings or rotating rectifiers. The main components

of the connectors are made of high conductivity hard drawn copper, fitted to and

electrically insulated from the rotor shaft. As the connectors rotate with the rotor,


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their design must have robust mechanical construction and must have a locking

mechanism to ensure that their components are secured in position. hi hydrogen

cooled generators, they are also required to be gas tight to prevent gas leakage to

the atmosphere. A typical arrangement of a radial connector assembly.

2.3.2 RotorRetaining Ring Faults

2.3.2.1 Stress Corrosion Cracking

In addition to the centrifugal forces produced by the rotor end windings, the

retaining rings' own mass and the shrink fit forces contribute a

significant radial loading on the rings. Iii fact, studies have found that only

approximately 25% of the force is contributed by the end windings' mass and 75%

from the retaining ring itself. These forces stress the retaining rings to over 50%

yield strength at the normal operating speed. In-service failure of rotor retaining

rings is catastrophic. Many studies and failure investigations on generator rotor

retaining rings have been undertaken in the70

's and80

's by large powergeneration utilities and technical organisations such as EPRL They all come to the

same conclusion, namely that the intergranular stress corrosion cracking of

nonmagnetic materials such as 1 8Mn4Cr steel is responsible for most failures of

rotor retaining rings.Statistically, while the failure rate of rotor retaining rings is

less than 0.01 percent, the consequence of this type of failure is enormous. In

most cases where a retaining ring contained cracks greater than the critical crack

size, its failure leads to a total destruction of the generator.

The majority of retaining ring replacements is due to crack discovery below

critical crack size by routine Non Destructive Testing (NDT), risk management or

preventative asset management initiatives. For example, according to the

experience of one OEM, about 80% of non-magnetic 1 8Mn4Cr retaining rings

inspected had indications of stress corrosion defects either requires replacement


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or remedial work, of which, approximately 5 percents requires immediate

replacement because of critical size defects, and the rest was refurbished and

returned to service for later replacement. To date there is still a considerably

large number of 1 8Mn4Cr retaining rings remaining in service with strict control

of generator internal environment

and regular NDT programs.

2.3.2.2 Electrical Arcing on Rotor Retaining Rings

This fault, while considered minor and often overlooked, is quite common

especially in older generator rotors. Electrical arcing damage is often found at the

interface between the retaining rings and rotor body or balance rings or rotor slot

wedges. The arcing spots are a result of electrical arcing across the components

driven by negative phase sequence currents generated on the rotor surface

during generator's abnormal operations such as motoring, pole slipping or lost of

excitation. Such arcing spots could alter the material properties locally, especially

hardness. In the worst case, an arcing spot could lead to initiation of cracks and

eventually cause catastrophic failure to the retaining ring.


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2.3.3Copper Dusting Problem in Rotor Windings

The radial cooled rotor design utilises channels underneath each winding slot,called sub slot, to deliver the cooling gas to the centre of the rotor from both

ends. The cooling gas is forced under the rotor end windings to pressurise the sub

slots, then bleeding radially outward through perforations along the rotor coils as

depicted in Figure. The cooling gas continues its travel past the air gap and

exhausts through stator core cooling ducts before transfer its heat to Hydrogen

coolers typically placed horizontally behind the stator core or vertically at each

end of the stator. In order to maximize heat exchange efficiency between the

copper coils and the cooling gas, each coil turn in the slot section of the rotorwinding is made up of two layers, so called half coil, of bare copper strips

uninsulated from each other. The half turns are brazed to the curved sections in

the end windings to complete the coil. Each copper strip has elongated slots cut-

out for its entire

straight length. The slots are slightly offset from the centre in opposite direction

so that when stacked up together they form a zigzag path for cooling gas to flow

through as shown in Figure. In radial cooled rotors, a significant proportion ofcopper dust is generated in between the pairs of half turns. The rest is generated

by the rubbing action at the interfaces between the copper coils and the fiber

glass insulation surrounding them such as slot liner, top insulation pads and inter-

turn insulation.


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[Cross sectional view of rotor winding (a) radial cooling, (b) axial cooling.]

Copper dusting is generated both when generator is on-line and off-line with

a different dynamic mechanism. When the generator rotor is rotating at full

speed, all rotor coils are pushed outward against the rotor slot retaining wedges

by enormous centrifugal forces. This causes the rotor coils and their insulationsystem to lock up together.

2.3.3.1 Consequences of Rotor Copper Dusting

The generation of copper dusting can have a devastating impact on the

operation and reliability of generators. The copper dust is highly conductive andcan cause short circuits between rotor turns, commonly called rotor shorted turn,

or to ground, commonly called rotor earth fault. In relative terms, the rotor

shorted turn problem is relatively less serious. The voltage between two adjacent

coils is in the order of only a few volts. However, the inter-turn insulation is very

thin: once a sufficient amount of copper dust is deposited across two adjacent


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coils, the rotor current would partially by-pass the rest of the winding causing

imbalance in the magnetic field, resulting in an increase in shaft vibration. Sudden

step changes in shaft vibration with the fundamental component at the exact

generating frequency is usually the tell tail of a rotor winding inter-turn short

circuit fault. The rotor earth fault, on the other hand, is much more serious and

usually leads to extensive secondary damage to the rotor. Due to the massive

inductance of the rotor windings, an earth fault could produce devastating arcs

leading to burning damage to rotor components surrounding the fault.

Fortunately, earth faults in rotor windings, directly caused by copper dusting

alone, are relatively uncommon. They normally occur in cohort with other

problems within the rotor or its protection systems. In addition to continuous

online vibration monitoring systems, most of large turbo generators have a rotor

earth fault protection system that continuously monitors the rotor impedance.

2.3.4Rotor Coil Overheating

Distortion damage of rotor coils is commonly found amongst large turbo

generator rotors. In comparison to salient pole rotors, large high speeddistributed winding rotors have a relatively complicated system of rotor coils,

insulation, separator blocks, retaining rings and so on. In addition, all components

in the rotor windings are designed to move in harmony with each other, within

certain design limits, to cope with the inevitable thermal expansion and

contraction of rotor coils. The ventilation systems in these rotors are complex and

play a very important role in their operation. Deterioration or misalignment of

insulation components in the rotor windings often cause blockages to the

ventilation system and coil overheating.


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2.3.7 RotorUp-Shaft Lead Fault

Up-shaft lead assembly provides electrical connection from the outboard

radial connectors to the inboard radial connectors of large hydrogen cooled

synchronous generator rotors. The up-shaft lead assembly is inserted in the rotor

shaft bore and usually contains gas tight seals to prevent hydrogen leaks. The up-

shaft lead assembly functions as an electrical conduit conducting DC current to

energize the rotor windings. A typical up-shaft lead assembly comprises two

heavy 'D' cross sectional copper bars, an insulation separator between the D-

sections and an insulating tube that doubles up as a shell for the whole assembly.

The outer end of the leads is connected to the outboard radial connectorassembly and the inner end is connected to the inboard radial connectors.

The insulation tube is made from paper resin impregnated bakelite or fibre

glass based insulation materials in modem rotors. Given that the rated

[ A typical crack in up-shaft lead insulation tube in a 210 MW generator.]


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rotor voltage is usually around 400 volts, even with a crack the insulation tube

should provide adequate electrical insulation grading. However, the concern is

that when the crack is contaminated with moisture or conductive dust, it can

develop a significant risk of electrical earth fault or across the windings within the

rotor shaft bore. Such faults, whilst relatively rare, can cause considerable

damage to the rotor.


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Part-I IntroductionToday's societies are critically dependent on the reliable supply of electricity. The reliable

operation of large synchronous generators in power stations is therefore of paramount

importance. Thus, power utilities are most keenly concerned in taking measures to avoid

catastrophic failures and to minimize the impact of generator outages due to faults. Each

generator constitutes one of the most expensive single pieces of equipment in the power

station. Although they are mostly reliable and generally require minimal maintenance, faults

can and do occur with varying degrees of consequences ranging from minor outages to severe

failures.

When faults occur, cause-effect relationships need to be identified systematically. This is

necessary for having a better understanding of the fault formation mechanisms so that

preventive measures can be taken, although sometimes the immediate pressures for returning

a generator to service may impede a thorough investigation.

Over the last half century much advancement has been made in the field of large Synchronous

generators. The need for larger unit sizes has driven the development of better and more

innovative designs. Manufacturing processes have steadily improved. The advent of new

insulation materials has contributed to increased reliability in operation at lower cost.

Sophisticated design optimization, clever use of modern materials and more efficient cooling

systems have been some of the key achievements in the field of design and manufacturing of

large synchronous generators

Different ways to improve the continuity of supply of generator are shown below different

researchers are working on this era by using different methods by publishing different paper

over past years.

Part-2 Different Techniques

I have presented the different techniques to improve the continuity of generator. I mention

the considerations which would be taken care of research papers.

Chapter:3 Literature review on Operational Faults in Large Synchronous Generators

Suryadeep D. Zala

M.E. (EPS)


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(a) Recent problems experienced with motor and generator windings IEEE Paper No. PCIC-

2009-6 by G.C. Stone, M. Sasic, D. Dunn, I. Culbert

The problem of building more efficient machines which has less cost & time consuming as

well as with higher continuity.

Some of the methods employed to accomplish this include

(1) Reducing the conductor cross section

(2) Reducing the insulation thickness

(3) Reducing the amount of steel core material

(4) Developing manufacturing methods that result in less time to manufacture.

Each of these methods tends to increase the operating temperature of the windings or put

additional voltage stress on the electrical insulation. The author indicates different problems

associated with the machine parts such as rotor & stator while applying one of the methods

shown above to achieve more efficiency.

Some suggestions are also given by him to avoid premature failure of stator & rotor of machine

such as to reduce partial discharge level by proper dimensioning of it. Also to take failure into

consideration while designing different machines. the parameters are selected as which makes

system more stable

Problems such as coil abrasion in the slot, electric stress relief coating deterioration and partialdischarges in the end winding have led to failures in as short as 5 years of operation. This

anecdotal information is supported by the fact that PD for some manufacturers is higher for

recently made machines than for similar machines made in excess of 10 years ago.

To avoid premature failures, users of modern air-cooled machines should ensure they have a

good purchase specification and ensure the manufacturer has an appropriate QA program in

place.

Different problems occur due to the reduction of material as well as time. The faults due to

partial discharge and some of the other faults can be avoided by means of proper study of theeffects of one component on other while change in design quality assurance programs as well

as condition monitoring helps in reducing faults proper maintenance would proper

maintenance would also help in improving continuity of the machine and there by increase in

efficiency


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(b)Effect of manufacturer winding age and insulation type on stator winding Partial discharge

levels by G. C. Stone and V. Warren From DEIS, September/October 2004, Vol. 20, No. 5

Partial discharges (PD) are a symptom and sometimes a cause of many types of motor and

generator stator winding insulation system deterioration mechanisms in utility generators.

Online PD measurement has been able to determine whether the electrical insulation is

deteriorating because of loose coils in the slots, resulting in insulation abrasion; thermal

deterioration or load cycling, which leads to insulation delamination and electrical tracking

caused by partly conductive contamination of the end windings

At the end of 2003, over 60,000 test results had been accumulated over 10 yr, and simple

statistical analysis has been applied to the database to extract Information that can help test

users to interpret PD results better. In addition, experience has accrued on what operating and

environmental parameters may affect the PD readings as well as the deterioration in the stator

insulation condition.

Procedures used to indicate and analyze insulation condition

(1) Data Presentation (2) Database to the End of2003 (3) Database Analysis Method

With thousands of machines monitored for as long as 25 yr with the same method, on-line PD

testing has become a recognized, proven tool to help maintenance engineers identify which

stator windings need off-line testing, inspections, and/or repairs. Within the statistical accuracy

possible with several thousands of independent results, it seems that critical PD levels only

depend on operating voltage, hydrogen pressure, manufacturer, and the specific type of PDsensor and instrumentation used.

The paper illustrates the important parameters which effect the partial discharge levels of

winding. The study shows that this method helps in selection of parameters in new design

development as well as in findings the areas which needed the maintenance and reduces the

time & money

(c) An Accurate Approach to Earth Fault Detection for Generator Stator Using

Fuzzy Logic byH. Khorashadi Zadeh

In this paper the use of fuzzy logic for stator earth fault detection is presented. A generator

model is simulated using EMTDC. Earth faults are simulated between 0.1% to 100% distancepoints from the generator neutral. The combination of both EMTDC simulation and neural

network presented in this paper is to introduce a new, complementary method that will


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perform better in instances where the interpretation of traditional methods is somewhat

dubious.

A lot of attention has been focused on generators single-phase to ground fault which is one of

the main causes for shutting down a generator. The causes may be due to insulation

degradation in the windings as well as environmental influence such as moisture or oil in

combination with dirt settles on the coil surfaces outside the stator slots. This often leads to

electrical tracking discharges in the end winding which eventually punctures the ground wall.

Stators winding fault must be avoided since the amount of time wasted and the cost for

repairing a generator is enormous.

Hence, it is necessary to prevent such occurrences by incorporation reliable protection and

monitoring schemes. The differential relay does not cover the entire stator winding of the

generator. In addition some of the faults may well occur before the sensitivities of the

protective relay operate, while some types of deterioration can progress rapidly with littleadvance warning of the failure; therefore it is critical to get frequent updates of the condition

of components.

Protection relaying is just as much a candidate for the application of pattern recognition. The

majority of power system protection techniques are involved in defining the system state

through identifying the pattern of the associated voltage and current waveforms measured at

the relay location. This means that the development of adaptive protection can be essentially

treated as a problem-pattern Recognition/classification.

Artificial Intelligences (AIs) are powerful in pattern recognition and classification. They possessexcellent features such as generalization capability, noise immunity, robustness and fault

tolerance. Consequently, the decision made by an AI-based relay will not be seriously affected

by variations in system parameters. AI-based techniques have been used in power system

protection and encouraging results are obtained

A fuzzy logic scheme to identifications of generator stator windings fault is presented.

Simulation studies are performed and the relays performance with different internal fault is

investigated. The obtained results show that the proposed fuzzy logic-based fault detector

represents a proper action.

It can operate with proper sensitivity thus the use of fuzzy scheme can make it possible to

extend reliability and sensitivity fault detectors for generators.


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The paper illustrates about the earth fault detection as well as the use of fuzzy logic for the

relaying operation of the simulated model of small power system. It is being advantageous as

operation does not affect other parameters of system. the only problem is to implement a fuzzy

logic system requires proper definition as well as proper design of simulated model. Also it is

being complex, time consuming and costly. it can be used for small systems.

(d)Effects of Types of Faults on Generator Vibration Signatures by R. Oliquino, Jr., S. Islam,SMIEEEand H. Eren,

Generators are frequently subjected to high currents and voltages caused by electrical

disturbances in the power system. Faults in particular subject the generator to stresses beyond

its design limits and cause high temperature increase, amplify and distort air gap torques, and

create unbalanced flux densities. Even more stressful as a consequence of faults are sudden

loss of load, fault clearance and reclosing. Mechanically, the abnormal forces that are

generated excite the rotor and as a result, amplify the shafts normal mode of oscillation. Theobjective of this paper is to present the results on the experiment conducted regarding the

effects of selected types of electrical faults on generators vibration signatures. It aims to

examine the generated vibration frequencies, changes in rotor shaft orbits and increase in

vibration magnitudes as a result of faults. In this paper, the results on the experiment

conducted regarding the effects of selected types of electrical faults on generator vibration

signatures are presented. It aims to examine the generated vibration frequencies, changes in

rotor shaft orbits and increase in vibration magnitudes due to three-phase fault (3PF), line-to-

line fault (LLF), line-to ground fault (LGF) and double-line-to-ground fault (LLGF). In the

experiment, the above conditions were applied to the terminals of a loaded 5KVA three phase

synchronous generator. The vibration responses for each condition were examined, analyzed

and compared.

It is evident from the plots that the vibration frequencies are random, which means that the

generator motion is not periodic. Random motion is associated with severe mechanical or

structural looseness.

The effect of3-phase fault (3PF), line-to-line fault (LLF), line-to-ground fault (LLGF) and line-to-

ground fault (LGF) on the 5KVA model generator vibration signature is evidently seen

particularly on the time and XY plots. LGF appears to have caused the strongest magnitude gainfollowed by LLF. However, 3PF has the highest average gain. This reveals that although LGF and

LLF could produce the strongest vibration magnitudes, a 3PF will most likely generate strong

vibration responses.

LGF also exhibited the greatest effect on the generator shaft deflection by a wide margin

against 3PF, which came in second. Additionally, LGF fault has the highest average shaft


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(4) Sub conductor cracking problem

In normal operation, the generator is filled with hydrogen gas at 300 kPa pressure and the

stator coolant system is charged with demineralised water at 200 kPa. The purpose for the

differential in the pressure is to prevent water leakage into the stator insulation system.

Any small leaks occur in the stator coolant system due to faults such as cracks in conductors or

leak gaskets, hydrogen gas escapes into the stator coolant water and is collected in a gas

catchments chamber. An alarm is triggered and hydrogen gas is vented safely into the

atmosphere. Usually, hydrogen leaks are small and the loss of hydrogen pressure is

insignificant. However, if the cracks were allowed to grow, excessive hydrogen gas in stator

coolant water could seriously reduce cooling capacity of the sub-conductors leading to local

overheating, eventually, deformation of the sub conductors and severe damage to the stator

conductor.

(5) End Winding Looseness Problem

Investigation of the end winding structure of many generators have found that in most cases

where cracked sub conductors are found there is evidence of excessive interconductor

Movement within the end-winding and relative to their support structures. As the conductors

vibrate in service, they rub against one another and their support system, thus generating the

so-called insulation fretting dust.

Excessive amount of insulation fretting dust is a cause for alarm as it is indicative of a significant

loss of insulation material; a sign of dangerous degradation of conductor insulation system,

thus increased risk of discharge and electrical faults.

(6)Conductor Damage by Foreign Objects Magnetic Termites

A small metallic object accidentally left in the generator during an overhaul or a fragment of

lamination broken away from the core can cause major failures.

(B)Different Faults occur of Rotor

(1) Rotor Retaining Ring Faults

(a) Stress Corrosion Cracking

When in service, rotor retaining rings endure extremely high levels of stresses . In addition to

the centrifugal forces produced by the rotor End windings, the retaining rings' own mass and

the shrink fit forces contribute a Significant radial loading on the rings. Excessive stress may

damage the rings.


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(3) Stator Core Ring Flux Test

(4) Stator Core Lamination Tightness Test

(5) Stator Slot Wedge Tightness Test

(6) End Winding Vibration Testing

(a) On-line Condition Monitoring

(b) Off-line Evaluation

(7) Stator Windings Pneumatic Tests

(8) Stator Core End Plate Hot Spot Inspection

(9) Generator Rotor Retaining Ring Inspection

(a) Traditional Dye-Penetrant Inspection

(b) In-Situ Inspection

(10) Rotor RSO Test

(11) Rotor Coil Volt-Drop Test

(12)Thermo graphic Examination

(13) Radiographic Examination

Part 4 conclusion

In this paper, the different faults occur in synchronous generator as well as their detecting

methods are presented.

Different methods are selected according to the surrounding environment & their compatibility

towards desired condition.

Database method gives the detailed study of effects on different parameters while in operating

conditions. It is very useful where detailed study of machines needed but it has a limitation that

it is very lengthy process as well as data that are used from the database may not fulfill the real

time requirement. in real time data may differ according to the surrounding environment

Vibration signatures are helpful to indicate the effect of rotor vibration because of different

faults. It is a model base analysis & limited to provide efficiency above certain range.


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The use of fuzzy logic & artificial intelligence gives the accurate results but it is difficult in design

as well as implementation. Also the programming of the network is very difficult & costly.

The methods for detecting & diagnostic methods are described in the paper. The mixed

approach of different methods are used according the accuracy required in given time period.

References

[1] O. Gl and T. V. Tran, 'Stator End Winding Problems in Large Synchronous Generators and Some

Remedies', ICEM - International Conference on Electrical Machines, Bruges - Belgium, August 2002.

[2] G. C. Stone, Edward A. Boulter, Ian Culbert and Hussein Dhirani, 'Electrical Insulation for Rotating

Machines Design, Evaluation, Aging, Testing, and Repair'. IEEE Press series on Power Engineering,

Institute of Electrical and Electronics Engineers, 2004.

[3] ESIPC - Electricity Supply Industry Planning Council, 'The South Australian Annual Planning Report -

2005', Executive Summary, June 2005.

[4] EWBANK PREECE AUSTRALIA, 'Preliminary Study into Two-shifting Operation at Torrens Island Power

Station', Consultancy report for SA Generation Corporation, March 1997.

[5] 5. Denison ETSA Torrens Island Power Station, 'Ageing of Machines with Respect to Load or Field

Current Cycling', CIGRE Working Group 11-01 Questionnaire 90-3, reply letter to the CIGRE Paimel 11

convernor, December 1992.

[6

] J. E. Timperley and J. R. Michalec, 'Estimating The Remaining Service Life of Asphalt-Mica StatorInsulation', IEEE Transactions on Energy Conversion, Vol. 9, No. 4, pp 686-694, December 1994.

[7] D. J. Petty, 'Recent Experiences in the Refurbishment of Large Generators', Cigre APi 1 Conference

1996 - Sydney, No. 15, September 1996.

[8] M. Liese, J. Ber, R. Gem and W. Schier, 'Life Extension Methods and Experiences with Turbine

Generator Rehabilitation and Uprating', Cigr APi 1 1990 Paris Session, No. 11-104, August 1990.

[9] T. R. Blackburn, R. C. Sheehy and B. T. Phung, 'Deterioration of Stator Bar Insulation: use of Phase-

Resolved Partial Discharge Monitoring During Accelerated Ageing Tests', Cigre APi 1 Conference 1996 -

Sydney, No.8

, September 1996

.


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Chapter:4

Conclusion and future work

Conclusion: From above it is Proved that different types of faults are occur due to the fatigue of

machine component. The avoidance of these faults are necessary to maintain continuity of

operation also to increase performance. These can be avoid by focusing on maintenance

procedures & condition monitoring. The advantages are come up with reduction in

maintenance cost which can be used for further development also better continuity will give

rise in revenue

Future Work:After referring various work done in this area the different faults & their detecting,diagnosis techniques are studied and try to find remedial solutions & improve the performance of

machine


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Chapter : 5

References

[1] O. Gl and T. V. Tran, 'Stator End Winding Problems in Large Synchronous Generators andSome Remedies', ICEM - International Conference on Electrical Machines, Bruges - Belgium,

August 2002.

[2] G. C. Stone, Edward A. Boulter, Ian Culbert and Hussein Dhirani, 'Electrical Insulation for

Rotating Machines Design, Evaluation, Aging, Testing, and Repair'. IEEE Press series on

Power Engineering, Institute of Electrical and Electronics Engineers, 2004.

[3] ESIPC - Electricity Supply Industry Planning Council, 'The South Australian Annual

Planning Report - 2005', Executive Summary, June 2005.

[4] EWBANK PREECE AUSTRALIA, 'Preliminary Study into Two-shifting Operation at

Torrens Island Power Station',Consultancy report for SA Generation Corporation, March 1997.[5] 5. Denison ETSA Torrens Island Power Station, 'Ageing of Machines with Respect to

Load or Field Current Cycling', CIGRE Working Group 11-01 Questionair 90-3, reply letter to

the CIGRE Paimel 11 convernor, December 1992.

[6] J. E. Timperley and J. R. Michalec, 'Estimating The Remaining Service Life of Asphalt-Mica

Stator Insulation', IEEE Transactions on Energy Conversion, Vol. 9, No. 4, pp 686-694,

December 1994.

[7] D. J. Petty, 'Recent Experiences in The Refurbishment of Large Generators', Cigre APi 1

Conference 1996 - Sydney, No. 15, September 1996.

[8] M. Liese, J. Ber, R. Gem and W. Schier, 'Life Extension Methods and Experiences with

Turbine Generator Rehabilitation and Uprating', Cigr APi 11990 Paris Session, No. 11-104,

August 1990.

[9] T. R. Blackburn, R. C. Sheehy and B. T. Phung, 'Deterioration of Stator Bar Insulation: use

of Phase-Resolved Partial Discharge Monitoring During Accelerated Ageing Tests', Cigre APi 1

Conference 1996 - Sydney, No. 8, September 1996.

[10] T. Tran Optima Energy Torrens Island Power Station, 'TIPS 'B4' Generator Stator

Complete Rewind Option Torrens Island Power Station', Departmential Letter, ETSA,December 1997.


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[11] A. Nuttall - The Electricity Trust of South Australia, 'Metallurgical examination of

Torrens Island Power Station Unit B2 Generator Stator Subconductors', MetallurgicalInvestigtion Report, report number MET\94\041, December 1994.

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