Ives Ti Gating Failed Transformer

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INTERNATIONAL JOURNAL Of ACADEMIC RESEARCH Vol. 2. No. 3. May 2010

INVESTIGATING FAILED TRANSFORMER:CASE STUDY OF A 100KVA, 11/0.415Kv BURNT-OUT DISTRIBUTION

TRANSFORMER, BENIN CITY, NIGERIA

S.O. Igbinovia1, N. Igbinovia2

1

Electrical/Electronic Engineering Dept. Faculty of Engineering, University of Benin2Procurement Section, Power Holding Company of Nigeria,

Benin Electricity Distribution Company, Benin City (NIGERIA) [email protected], [email protected]

ABSTRACT

A burnt–out 100KVA, 11/0.415Kv distribution transformer with 26.51 voltage and current transformation ratiosrespectively have been investigated in this work.

The methodology is based on the concept of copy creativity and the development of technology within. Routineand type tests were adopted to find out the probable causes of its failure and extent of damage. The tests results couldnot be adjudged to definitive type of fault, hence we had to dismantle the transformer, unblande the yoke and onthorough investigation, the first two segments of the four layers coils group on the yellow-phase limb had inter-turn short

circuit and completely burnt. The re-wound segments had taps at the 134 and 200 turn’s positions from the top. The re-assembled transformer was subjected to pre-commissioning test with the secondary circuit indicating sharp flash-over onthe blue-phase an indication of short-circuit when 2.5 Kv was injected, while the primary circuit withstood the 4.5 Kvinjected into it. This outcome prevented subjecting the rehabilitated transformer to further final tests.

The reasons attributed to the cause of damage to the high-voltage blue–phase windings segment could bemanufacturer error due to damping of the blue–phase coils with an iron channel thus leading to gradual windinginsulation failure resulting from the transformer oil overheating. The flash experienced from the secondary circuit whenpressure tested indicated ether inexperience in re-assembling, connecting the joints and tap change contacts. Thecalculated transformer equivalent circuit parameters from the no-load and on-load tests gave constant no-loss of 20watts; enable the performance characteristics of the devices.

Key words: Unblade yoke, reblade yoke, coils, insulation, oil overheating, transformer laminations, segments,performance characteristics

1. INTRODUCTION

Electric energy generating stations are sited near their primary huge resources to reduce production cost andtransmission lines losses. The generating units in most cases for technical reasons are built for not higher than 15 to 25kilo volts and their associated power can be transmitted over a few kilometres [ Faulkenberry L.M and Coffer W, 1996,Olle E.I, 1925]. Since the energy generated cannot be totally consumed by the industries and its environ, the large bulksof power for full utilisation has to be transported to localities which lie hundreds and thousands of kilometres away frompower sources where high demand for it exist to large industrial centres and districts. To technically, safely andeconomically evacuates quality power over several thousand kilometres, pylons, conductors, insulators, switchgears,reactive power compensators, power factor connection equipment, switching devices, protection schemes andtransformers are some of the associated equipment required in the circuits to achieve high tension transmission lines(like 66Kv, 132Kv, 330Kv, 450Kv, 500Kv, 750Kv, etc.) depending on the countries electricity regulatory act. In power systems, transformers built in few volts – amperes to several thousand kilovolt-amperes are static electromagneticdevices that transform voltage from higher level to lower level and vice versa. (i.e. step–up the voltage at the generatingend and step-down the voltage at the consumer’s end). In these wise transformers are very important links between

generation stations and the end users of the generated energy which cannot be conserved [Glover J.D and Sarma M.S,2001] as shown in figure1, must be given due engineering, installation, maintenance and repairs in case of failures toguarantee adequate and reliable power supply. Since this is the backbone of technological revolution and developmentalsustainability, this work was started with the following key desire;

The technological development of a country depends to some extent on the development from within Practical realisations of technical concepts and in so doing if some relevant skills, crafts and practices of

engineering and technology we be acquired Economic use of natural available resources/materials for making or improving parts or the whole device

(research and development) Practical awareness of engineering designs and related problems and how these problems are overcome The need for on-the-job training as necessary repair techniques and experiences are being gained and

developed Experimental effort to find out what technologies (apart from usual transformers space theory) are involved in

the manufactures, construction and design factors

Therefore all over the world, the dissatisfaction in the mind of consumers of electric energy from power utilitiesinsist ant power disruption and breakdown of power systems key components like generators, transformers, switchgears,relays, etc. mostly in developing countries which lacks the technological know-how and thus very far from self-sustenance, value engineering and the principle of break-even and cost benefit analysis must be the watch dog of



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nations utilities like Nigeria National Electric Power Authority (NEPA) now Power Holding Company of Nigeria (PHCN)that now have the mandate not to practice monopolised vertically structured electricity body. From value analysis the unitcost and selling price in naira of a given device (like transformers, generators, switch-gears etc.) can be expressed as [Kuale P.A and Chuku A.U, 1980];

Where = Total number of items manufactured for a given period

= All conceivable cost items (labour, management, energy, research and development, materials, charges on

capital investment, interest etc.)

= Sellers profit

= Market inflation factor

= No. of locations (in zones) into which the country is divided for estimating the broken-down items

If such reparable devices breakdown per year in an ith location, the total breakdown of the same type in a

given country becomes;

To effectively be able to repair some of or all these failed devices economically mean the development of some of the skills and practices involved in the technology of manufacturing of these devices/equipment /apparatus, otherwise themean time to restoration will be so discouraging and frustrating. In other to avoid these colossal losses utility boardsspecify capitalisation formulae along with enquiries as [Singh B, 1982];

Capitalised costs in naira = quoted cost of transformer in naira + (no- load loss in watts) x 13.2 + (load loss inwatts) x 4.6 ………………………………………….…… (4)

Equation 4 represents the operating cost over a certain period of time, and in this way offers by variousmanufactures are compared. Factors compound rate of interest, depreciation, load factor etc affects capitalised cost andbest selected values for the losses are used to give minimum capitalised cost. The cost optimisation of these devices

depends on the economic selection of the core area, current density, other design features and technology developed.Therefore in general the distribution transformer cost can be classified as (1) the cost of investment, (2) the cost of lost of energy due to losses in the transformer and (3) the cost of demand cost (i.e. the cost of lost capacity) due to the losses inthe transformer [Gonen T, 1986].

2. PRINCIPLE OF OPERATION OF TRANSFORMER

Faraday’s law of electromagnetic induction (1831) manifest itself in the operation of this static electromagneticdevice using the following relationships; [ Faulkenberry L.M and Coffer W, 1996, Glover J.D and Sarma M.S, 2001, SinghB, 1982, Deshpande M.V, 1994, Deshpande M.V, 1986, Say M.G, 1958, Symonds A, 1980, Mehta D.P and ThumannA, Matsch L.W, 1977, Gebert K.L and Edwards K.R, 1974, Theraja B.L and Theraja A.K, 2008];

n N

N

V

V

2

1

2

1...... ............................................................................................................(5)

Weber t SinV N

t CosV N

,2

221

1

1

1

................................................(6)

weber V N

m ,2

1

1 ),44.4/(103 f r S weber …………………………….……(7)

From equation 7, the voltage per turn (VPT) an important design parameter is given as;

maxmaxmax

2

2

1

144.4

2

2

2 B fA

fAB

N

V

N

V E it

, volts...................................(8)

= k S …………………………………………………………………………………………..(9)

Where,

V1 = voltage induced in the primary winding, voltsV2 = voltage induced in the secondary winding, voltsN1 = number of turns in the primary circuitN2 = number of turns in the secondary circuit





n = transformation ratio

2

t Sin = sinusoidal wave form of the sinusoidal excited electromagnetic device

= sinusoid ally vary flux of amplitude and frequency f , Weber’s

= angular frequency rad/sec

r = ratio of IN m / , constant for a type of transformer of a given type, service and method of

construction

K =constant ( 31044.4 fr ), which depends on type, material and labour

cost, factory organisation, etcS = KVA rating of transformer f = supply frequency, HZ

max = amplitude of sinusoidal flux density, which depends on the type of magnetic

material and the losses ratio

, Telsa

A = net cross–sectional area of the iron coreThe same assumptions are used in analysing ideal and practical transformers in which the core material is

characterised by infinite magnetic permeability and zero core reluctance ( 0 )), and infinite resistivity, a finite valueof flux requires an magneto motive force (mmf) of zero such that whether under no-load and under loading conditions,the following relationships holds for the mutually coupled circuit; (Gonen T, 1986, Say M.G, 1958, Matsch L.W, 1977,Theraja B.L and Theraja A.K, 2008 )

02211 N I N I ......................................................................................................... (10)

n N

N

I

I

L

1

1

21 ................................................................................................................(11)

While the obtainable power from the device is given by; ( Symonds A, 1980, Mehta D.P and Thumann A,Theraja B.L and Theraja A.K, 2008, Draper A, 1971,)

P= = Cos I V L L3 , watts........................................... ……………. ..(12)

Where:

= Ampere – turn balance= current flowing in the primary circuit, amps.

(=-I L )= current flowing in the secondary circuit, amps.

L L I V , = supply line voltage and current respectively

Cos = the load power factor

= the no-load phase angle

This power is not limited in ideal transformer but in the practical (i.e real) transformer, the actual losses whichoccur although relatively small set a definite limit to its power capability [ Deshpand M.V, 1994, Say M.G, 1976]

3. DESIGN EQUATIONS

One of the outlined objectives of this study was to be able to design for any of the failed designed features andreturn the failed system back to its initial functional state. Thus from the very small rating, IVA to very large rating,

500MVA the basic design equations applied are [ Deshpand M.V, 1994, Deshpand M.V, 1986]:

max

3

322.2

10

JB fF

S A A

C

wi ………………………………....................................……………(13)

max

3

122.2

10

JB fF

S A A

C

wi ……………………………………………………………………….(14)

The empirical formula for F c is;

KV VA

F C ln2.010

ln08.01.0

…………………………………………………(15)

While the ratio of magnetic loading to electric loading is;

11 I N

f n ………………………………………………………………………………………..(16)

Where,





yoke laminations in a circle of predetermined diameter shown in figure 2. The cross members (yokes) served to completethe magnetic circuit. These top yokes shown in figure3 were unbladed to remove the burnt coil and rebladed after thecoils were rewound and assembled. The studied transformer uses mineral oil as the cooling medium for transferring thethermal power generated (losses) in the core and windings to ambient air, while in power transformers the losses aredissipated to running water by thermosyphon actions and pumps. It was also found that major and minor types of insulations were used in the transformer manufacture. Suitable ducts were kept between the core and coil as well asbetween coils. This particular transformer has tapping range of 20% in 16 steps of 1.25%. The coarse/fine tappingarrangement has a modified version with a linear switch arrangement for four coarse sections each equivalent to four fine

sections. The disadvantage of this arrangement is that high surge voltage can occur on the tap charger at transition.Other types of tapping winding arrangement used in transformers are linear – tapping winding arrangement and reverse – tapping winding arrangement.

5. DETERMINATION OF THE CAUSE OF FAULT AND REPAIR

The failed transformer was subjected to routine defects determination, results obtained is as presented inTables1, while Table2 presents other measurements taken.

A

B

HIGHH VOLTAGE COIL

IRON-CORE

INSULATING BOARD

INSULATION BETWEEN HV COIL AND LV COIL

OIL

....

........

.

.. . . .... ...

...

..

..

.. .

... . ...... ......

.... .... . .

........ ...

..

. . ..... ... ......

..

..

.

.

.

.

....

.. .. .. .. ...

Fig 2. Shows core-section of a 3- transformer with circuiting strip AB

Fig 3. Shows the top view of a 100KVa, 3-oburnt-out (11/0.415KV) distribution transformer

SECONDARY END BOX SECONDARY TERMINAL LEADS

STALK OF LAMINATINGS

TANK

PRIMARY END BOX

PRIMARY TERMINAL LEAD

TAP CHANGER CHORD TAP - ADJUSTER





Table1. Insulation resistance test

High – Voltage windings test High – Voltage windings and body of transformer (tank)

Description Reading Description Reading

Red – Yellow 0.75 ohms Red phase – earth 1000 ohmsYellow – Blue 0.75 ohms Yellow phase – earth 1000 ohmsRed – Blue 0.75 ohms Blue phase – earth 1000 ohms

Low Voltage Windings to EarthContinuity Test

Low Voltage to High Voltage Side

Description Reading Description ReadingsRed phase to earth ohms Red phase – red phase Mohms

Yellow phase to earth ohms Yellow phase – yellow phase Mohms

Blue phase to earth ohms Blue phase to blue phase Mohms

Red phase – lamination core ohms

Yellow phase – core lamination ohms

Blue phase – core lamination ohms

Between phases – core ohms

Core -core 0.0 ohmsRemarks Readings suggest that

transformer is okay

Low voltage side after unblading Continuity test of the high voltage side to low voltage sideafter unblading

Description Reading Description ReadingRed phase – yellow phase 1.0 ohms Red phase – red phase ohms

Yellow phase – blue phase 1.0 ohms Yellow phase – yellow phase ohms

Red phase – blue phase 1.0 ohms Blue phase – blue phase ohms

All Phases to earth ohms All phases to earth ohms

Remarks Readings suggest thattransformer is okay

Table 2. Other measurements taken after unblading

S/N Description Measurement

1 Diameter of coil 1.295mm (0.048”, SWG 18)

2 Diameter of lamination 1.00mm

3 Lamination triangular groove length 11cm4 Length of lamination 58.00cm

5 Length to centre of groove 29cm6 From end of lamination to the diagonal 10cm7 Number of coil – turns in each segments 748 (actual ±10)8 Tapping positions in the 3

rdsegment 134 & 200 turns position from the top

9 Wood packing insulation found in between coils segments withcringed paper

10 Break-down dielectric strength voltage of the transformer oil Average of (22, 32 and 30Kv) gave 28Kv, which is okaysince its above 25Kv for old samples

11 Temperature of the transformer oil that have been off – load for over a year

31OC

12 Wooden former of 180mm diameter and 40mm height was usedin rewinding the transformer

From the tests results, there were no clear indications of the extent of damage. Determination of the defectstherefore involved my dismantling, unblading and manual unwinding of burnt coils in the transformer. We found out thattwo segments of the blue-phase high voltage assembly were completely burnt. After thorough examination, weconcluded that the failure could be due: to incorrect assembly of the high voltage winding coils which probably came incontact with the laminations thus causing inter–turn short-circuit resulting in over heating of the cooling oil causing itnatural ageing and wear of the winding insulation, periodic overloading of the transformer and dynamic stresses resultingfrom through short–circuit currents and the burnt contact surfaces of the tap–charger attributed to excessive–arcing fromfrequent switching currents. Considering the efforts it took to localise the faults, the faults in the transformer would havebeen identified by the colour of the gas in the electromagnetic relay if the transformer had one.

The wound dried windings were impregnated while still hot at temperature of 75OC by immersing them for aboutthirty minutes in a bath filled with glyptic varnish before they were baked in an oven at a temperature about 100 OC for 24hours to increase the winding electric strength, make the winding monolithic and increase in the mechanical strengthafter which they were stored at room temperature of ±5OC. Spot welding equipment was used in the soldering process tothe compensators and ends of the windings. After the coupling, the immersed transformer core and coil properly dried ina heat chamber to expire the moisture contained in the winding and core insulation having been left opened for quite

some time was then subjected to pre –commissioning test.





6. EXPERIMENTAL TEST RESULTS AND ANALYSIS

Transformers either in new or repaired state is usually subjected to routine, special and type tests to confirm if thedevice will meet up with the required specifications in conformity with designer and IS 2026 (part 1) – 1977 “ specificationfor power transformer” and users desire. These tests include [Singh B, 1982, Gonen T, 1986, Deshpande M.V, 1994,Say M.G, 1958, Symonds A, 1980, Mehta D.P and Thumann A, Umeojiaka A.O, 2009, Stigant S.A, Lacey H.C, andFranklin A.C, 1973]:

1. Core insulation tests to prove that the magnetic circuit is positive

2. Ratio tests: ±0.5% of the declared ratio or ±10% of the percentage impedance whichever is thesmaller is considered3. Phasing/Additive and subtractive polarity(Flick)/Vector group verification tests4. Insulation/Earth resistance tests5. Resistance of windings test (continuity test), not applicable to zig-zag transformer windings6. Surge – voltage withstand test; (a)Full- wave test, (b)Chopped – wave test (type test)7. Separate – source – voltage withstand tests8. Induced – over – voltage withstand and internal discharge tests.9. No load loss and no – load magnetising current test

10. Noise test11. Load loss and leakage impedance voltage test12. Zero – sequence impedance test (special test)13. Top oil temperature rise test (type test) or Heat run test14. Capacity confirmation test

It is important to note that a transformer Basic Insulation Level (BIL) between 5 and 30 times the insulation class

is the peak transient voltage level that the transformer can withstand for a specified time. While the insulation class of atransformer is the maximum root mean square working voltage of the transformer [Faulkenberry L.M and Coffer, 1996].To buttress this fact the high voltage test of the repaired transformer is presented in Table3.

Table 3. High Voltage Flash Test

Voltage applied Description Remark

4.5 KV Between core and boltsBetween high voltage phases

No break-down of insulationNo break-down of insulation

2.5KV Between low voltage phases Break-down occurred due to short-circuit

The sudden abrupt short–circuit fault was not apparent at the time we ascertained the causes of the transformer break-down. It was suspected this could have resulted due:

i, to inexperience in handling the essential components after the burnt H. V. layers rewound in NEPA Ijora

Electrical Central workshop for a week and assemblyii none use of adequate testing instrument/equipment, special repair/ assembly tools andiii since the other parts were left in the workshop and the layers I brought back from Lagos must have

accumulated dust and moisture, not dried and transformer oil not filtered due to unequipped laboratory, and hurriedlyassembled to carry out the final tests.

From the no-load and on-load tests carried out and the equivalent transformers parameters calculated thesimplified equivalent circuit as shown in figure 4 can be used in evaluating the performance characteristics of the devicehaving constant no-loss of 20 watts at different loading conditions.

Fig. 4. Simple equivalent circuit of the practical transformer

Where,V 1, V2 = the primary and secondary voltages respectivelyI P = current drawn from the source of supplyI O = no- load currentI C = copper loss current component, amps, in phase with V 1





I m = magnetising current, amps, lags 900

behind V 1 1

P I = active current component, amps

I L = load current, ampsUsing Figure 4 the following relationships can be determined [Glover J.D and Sarma M.S, 2001, Singh B, 1982,

Deshpande M.V, 1986, Matsch L.W, 1977, Draper A, 1971];

The device percentage impedance, %Z = 22

%% eqeq X R , ohms………… .(18)

Where the equivalent circuit resistance R = 2

2

1 R N

N R

…, ohms………………(19)

While the equivalent circuit reactance X eq = X1 +

2

N

N X 2 …, ohms……………….(20)

The voltage regulation of this device at a specified time and power factor, the primary voltage remaining constantis;

=

s

Leq Leq L

s

s

E

Sin X Cos R I X

E

V E

1002

……………………………….(21)

While the efficiency of the practical transformer can be determined using the relationship;

Efficiency () = 1 - LossesOutput

Losses

…………………………… ……………(22)

The total losses include electric circuit loss, magnetic loss and dielectric loss. The load at which the efficiency ishighest can be found from [Singh B, 1982, Gonen T, 1986];

%Load = 1002

1

X copperloss

ironloss

…………………………………………………………(23)

Where;

R 1 , R 2 = primary and secondary resistance respectively

X 1 , X 2 = primary and secondary reactance respectively

E s = secondary no-load voltage

The losses in a transformer are roughly proportional to its weight, that is, to (length)3

, and the surface area

through which the dissipation of the losses takes place depends on (length)2

, it then follows that the larger thetransformer the more difficult is the problem of heat dissipation unless material is to be used in uneconomic proportion].The practical transformer depending on the operating conditions, the calculated copper temperature rise must not

exceed 250 c0starting from an initial temperature value of 90 c0

for water cooling and 105 c0for air cooling. The

temperature rise is calculated from [Say M.G, 1958];

C at T

K

T

at T at d

0

11

1

2

620

2

2

………………………………………………...…..(24)

Where,

t = time in sec.T 1 = 1

+234.5 C 0

1 = initial temperature in C 0

K d = eddy-loss ratio at 75 C 0

a = 0.0025 x loss in W. per kg. at 1 , or in terms of current density in A. per mm2, a = 1.9 2

T1 x105

.

With all these parameter values affecting the efficiency value the all day efficiency of a practical transformer isgiven by the relationship[Deshpand M.V, 1994];

All the efficiency =hrsrinkwhintransforme Inputofthe

hrserinkwhinetransformOutputofth

24

24……………………(25)

6.1 Precautions

To operate a transformer close to the normal life span of about 30years depends on the operational condition andthe maintenance procedures adopted. Thus service life of the oil in a practical transformer is halved if its temperature is





10OC above normal [Singh B, 1982, Gonen T, Deshpande M.V, Deshpande M.V, 1986, Say M.G, 1958]. Therefore if theoil is to have a longer service life, transformers should be operated so that top-oil temperature does not exceed 85OC(i.e. should not exceed 50OC above an ambient temperature of +35OC). Installation of The Internal Fault Dectector (IFD)which functions incorporates the pressure-relief device(PRD} and provides a visual indication on whether the transformer has faulted internally from IFD Corp.(Vancouver, Canada)[ Desrosiers D, 2006] to improve worker productivity, enhancecustomer service and increase overall safety associated with transformer failures as found on pole-mounted transformersshould be incorporated in distribution transformers to improve electric energy consumption and remove the decisionwhether to re-fuse a transformer and put it back in service.

7. FACTORS THAT CAN IMPAIR TRANSFORMER PERFORMANCE

Careless handling of lamination (blows, mechanical hymy, scored insulation) during unblading andreblading

Cooling arrangement (i.e. areas of maximum temperature in the core windings, the so – called hotspots, always below specified maximum values)

Over- heating of the transformer core and windings can damage the core (burning of the core) andaccurate ageing of the laminations and thus reduction of the life cycle of the transformer

Lowered flash point caused by cracking (decomposition) of oil due to local overheating due to severalinternal faults (like breakdown of the internal insulation between the core steel laminations, poor contact in the trap changer switch, surge voltages which can develop particularly across the tappingrange, windings or terminals under the cover, partial turn – to – turn short-circuit in winding whichprovides a short – circuit for a heavy current) that can result in very serious overheating.

Mechanical impurities Increased viscosity of the oil Reduced electric strength(breakdown voltage)of the oil (28KV when tested for the burnt transformer) Increased acidity of the oil as a result of oil deterioration Moisture continent increase Inability to carryout the required maintenances Unavailability of testing equipments/tools

8. EXPERIENCES GAINED

Experiences acquired in this work include: Dismantling and blading techniques acquired Re-winding skill and repair techniques acquired Designing for any of the faulty components to achieve new system was acquired

The best maintenance procedures to be adopted for practical transformers was also acquired

9. CONCLUSION

The investigation of the burnt 100KVA, 11/0.415kV distribution with 26.51 voltage and current transformationratios has been successful in this work. The burnt two segments of the high voltage blue-phase having 748 turnsrewound with standard wire gauge (SWG) 18 subjected to routine and final tests tested okay but when the re-assembledtransformer was pressure tested on the low voltage side flashed at 2.5KV indicating short circuit of the inter-turnswindings a fault that did not present itself before we took the burnt segment to NEPA rewinding section at Ijora Lagos for rewinding. The investigation was very interesting and challenging because of the enormous skills, techniques andexperiences required in carrying out perfect repairs and refurbishment of power equipments like transformers a veryimportant link in power transmission and utilization. Though we were able to determine the machine equivalent circuitparameters that enabled the performance characteristics to be analysed, putting the re-assembled transformer on loadbecame unrealistic because of the flash-over experienced. If tertiary institutions laboratories are equipped with modernpower equipments/tools, carrying out investigations of this nature will go a long way in our nation self reliance andtechnological break through.

REFERENCES

1. Faulkenberry L. M & Coffer W, “Electrical Power Distribution and Transmission,” Prentice–Hall, Inc.Simon & Schuster /A Viacom.Company,Upper Saddle River, New Jersey 07458, USA. ISBN 0-13-249947-9. PP94 – 124, 129 -138. 1996.

2. Olle E.I, “Basic Electric Power Engineering” Reading Mass: Addison-Wesley Pub. Co. PP114-127. 19253. Glover J.D & Sarma M.S, “Power System Analysis and Design” Third Edition, Brooks/ Cole Thomson

Learning Inc., USA, ISBN 0-534-95367 -0. PP71-117. 20014. Kuale P.A & Chuk A.U. “Paper Presented on Repairs and Tests of an 11/.415kv Transformer in

NASA/SAN Engineering Section proceedings. 19805. Singh B, “Electrical Machine Design” New Delhi: Vikas, XVi, 404P: ill, ISBN 0-7069 -1806-1, PP246-278,

19826. Gonen T, “Electrical Power Distribution System Enginering” McGRAW – Hill, Inc., U.S.A. ISBN 0-07-

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7. Deshpande M.V, “Electrical Machines”, First Edition, Wheeler Publishing, A Division of A.H. Wheeler &Co. Ltd. Allahabad 211001, ISBN 81-85814-23-6, PP 2-11, 14-15, 22-41, 56-76, 78-119. 1994

8. Deshpande. M.V, “Elements of Electrical Power Station Design”, Third Edition, Wheeler Publishing, ADivision of A.H. Wheeler & Co. Ltd. Allahabad, New Delhi. ISBN 81-85614-33-4, PP399-401. 1986

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12. Matsch L.W, “Electromagnetic and Electromechanical Machines”, A Dun-Doncalley Publisher, New York,USA. ISBN 0-7002 -250 -3, PP90 -140. 1977

13. Gebert K.L & Edwards K.R., “Transformers Principles and Applications” Second Edition, AmericanTechnical Society Chicago 60637, USA. ISBN: 0-8269 -1602-3, PP22-274, 314 -330. 1978

14. Theraja B.L.& Theraja A.K, “A Textbook of Electrical Technology”, 24th Revised Edition, Published by S.Chand & Company Ltd., 7361, Ram Nagar, New Delhi – 110055, PP1115 – 1186, 1210- 1221, 1236 -1238. 2008

15. Drapper A, “Electrical Machines”. The English Language Book Society and Longman, New York, USA,Second Edition” ISBN: 0582-30509-8, 1971

16. Umeojiaka A.O, “Types of tests carried out on power and distribution transformers substation” Paper presented in a one day workshop at Benin Zone Power Holding Company of Nigeria, Benin City. July2009.

17. Stigant S.A. & Franklin A.C, “J & P Transformer Book”, Tenth edition, Published by Newness-Butterworths, The Butterworth & Co. (Publishers) Ltd., ISBN: 0 408 00094 5, PP653-667. 1973

18. Desrosiers D, “Has the Transformer Failed”. Hydro-Quebec, Oct1, Penton Media, Inc. 2006.

Ives Ti Gating Failed Transformer

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