Quality Analysis in Production and Operation of ...

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Production Engineering Thesis

2020-03-16

Quality Analysis in Production and

Operation of Transformers: the Case of

Tatek Transformer Factory

Girmay, Hailemariam

http://hdl.handle.net/123456789/10428

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BAHIR DAR UNIVERSITY

BAHIR DAR INSTITUTE OF TECHNOLOGY

SCHOOL OF RESEARCH AND GRADUATE STUDIES

FACULTY OF MECHANICAL AND INDUSTRIAL ENGINEERING

Thesis on: - Quality Analysis in Production and Operation of

Transformers: the Case of Tatek Transformer Factory

By:- Hailemariam Girmay

A thesis Submitted to Bahir Dar Institute of Technology

Presented in partial Fulfillment of the Requirements for the Degree of Masters of Science

in Production Engineering and Management in the (Industrial Engineering Stream)

Advisor. Dr. Ing. Ephrem Gidey

Bahir Dar, Ethiopia

Dec-2018

ii

Quality analysis in Production and Operation of Transformers: in the case of Tatek transformer factory

Declaration

I hereby declare that the work which is being presented in this thesis entitled

―Quality Analysis in Production and Operation of transformers: in the case of Tatek-

Transformer factory.‖ is original work of mine, has not been presented for a degree in

any other university and all the resources or materials used for this thesis have been duly

acknowledged.

Hailemariam Girmay Date

This is to certify that the above declaration made by the candidate is correct to the best of

my knowledge.

Dr.Ing. Ephrem Gidey Date (Advisor)


ACKNOWLEDGEMENT

First and foremost, thanks to my friends and colleagues of Ethiopian Electric Utility

(EEU) for giving me the strength, ability and patience to start and complete this

research study.

Next, I would like to thank my advisor Dr. Ephrem Gidey for his follow up,

forwarding crucial documents and comment during process guidance and

insightful comments through this thesis and throughout my time here.

I would also wish to thank my friend Kassa, in charge of Energy portfolio and logistic

head in EEU for his encouragement, supporting me by providing software and

relevant documents with respect to this research and throughout the course of the

study.

Special thanks goes again to EEU staffs for providing me with the necessary

resources and materials. Thanks to Mr. Tesfaye Alemayehu and Mengstu Aseres,

both form Quality Assurance and Quality control team, Abadi Gebrehiwot from

Energy Management, Demeke Tsehay and Kasaye heads of wire business

(Distribution) EAAR and SAAR respectively. I would also like to thank Mrs.

Sirgut Mitiku, in charge of EHS, Health and Quality department executive

secretary for preparing and arranging and editing my document in every stage

ofthis thesis process.

Thanks are also due to my family and friends who have always been by my side to

support and encourage me and for bearing with me.

Hailemariam Girmay

Dec-2018, Addis Ababa

i


Contents

TABLE OF CONTENTS

Page ACKNOWLEDGEMENT i

Table of Content ii

List of Figures iv

List of Tables v

ACRONYMS vi

Abstract vii

CHAPTER ONE ............................................................................................................................. 1

1.INTRODUCTION ....................................................................................................................... 1

1.1 BACKGROUND OF THE STUDY................................................................................................. 2

1.2. STATEMENT OF THE PROBLEM ............................................................................................... 3

1.3. OBJECTIVES OF THE STUDY.................................................................................................... 3

1.3.1 General objective ............................................................................................................ 3

1.3.2 Specific Objectives ......................................................................................................... 3

1.4 SCOPE AND LIMITATIONS OF THE STUDY ................................................................................ 4

1.5 SIGNIFICANCE OF THE STUDY .................................................................................................. 4

2.0 LITERATURE REVIEW ......................................................................................................... 6

2.1.2. LOAD CHECKING................................................................................................................. 8

2.1.3. Protection system Failure................................................................................................... 9

2.1.4 SHORT CIRCUIT EXPOSURE ............................................................................................... 10

2.1.5. Un balanced loading and earth fault protection 11

2.1.6 Installation components and work man ship quality 12

2.1.7 Preventive maintenance schedule and consistency replacement of aged

transformers 12

2.1.8 RISK MITIGATION PLAN AND PREVENTION MECHANISM .........................................

2.1.9 Job quality Controlling procedure

............ 13

14

2.3 Testing in Distribution Transformers 16

2.3.1 Manufacturing quality 17

2.3.2 Quality Production Certification 17

2.3.3 Routine Test 17

2.3.3.1 Separate Source Test

19

2.3.3 2. Induced over voltage test 20

2.3.3 3. Measurement of the no load loss and no load current 20

2.3.3. 4. Measurement of the winding resistance 20

ii

2.3.3.5 Measurement of the load loss and impedance voltage

20

2.3.3. 6. Measurement of voltage ratio and vector grouping 20

2.3.3.7. Measurement of the insulation resistance 20

3. RESEARCH METHODOLOGY.............................................................................................. 25

3.1 Research Design 23

3. 2 Data Collection 28

3.3 Research analysis 29


4. DATA COLLECTION AND ANALYSIS ............................................................................... 30

4.1 DATA COLLECTION ............................................................................................................. 30

4.2. Pareto Analysis for the supplied transformers 42

4.3. Primary Data form site 55

4.3.2. EEU's central region 55

4.3.3. EEU's North Addis Ababa 56

4.3.4. EEU's West Addis Ababa Region 58

4.3.5. Site investigation is east Addis Ababa region of the utility 60

4.4. Summary findings on site 62

4.5. Operation proceduie 63

CHAPTER FIVE

5. CONCLUSION AND RECOMMENDATION ................................................................... 64

5.1 Conclusion .............................................................................................................................. 64

5.2 Recommendations:……………………………………………………………………….… 65

REFERENCE

Annex

iii


List of Figures

Fig 2.1 index age of transformers--------------------------------------------------------------------------16

Fig: 2.2. Literature of Cause and effect diagram-------------------------------------------------------23

Fig: 2.3. Standard graph of Pareto------------------------------------------------------------------------24

Fig 4.2: Insulation resistance test in MV and LV ------------------------------------------------------31

Fig 4.3 Test between MV and Ground-------------------------------------------------------------------32

Fig 4.4: Test between LV and Ground-------------------------------------------------------------------32

Fig 4.5. Winding resistance test---------------------------------------------------------------------------33

Fig 4.6: Voltage ratio test---------------------------------------------------------------------------------34

Fig 4.7: No load loss and no load current test----------------------------------------------------------36

Fig 4.8: Load test impedance voltage--------------------------------------------------------------------37

Fig 4.9: Separated source power frequency test-------------------------------------------------------38

Fig 4.10: Induced over voltage with stand test---------------------------------------------------------38

Figure:-4.11 SPSS result of pareto chart of failed transformers KVA------------------------------43

Figure. 4.12 Progress of failure transformer based on year -----------------------------------------43

Fig 4.13 Total supplied and failed transformers ------------------------------------------------------45

Fig 4.14 Annual Failure progress -----------------------------------------------------------------------46

Fig 4.15 Failure rate states based on kVAs-------------------------------------------------------------47

Fig4:16 SPSS result of Failure reasons pareto chart-------------------------------------------------49

Fig4:17 Failure reasons in Quantity --------------------------------------------------------------------50

Fig 4:18 Percentage of as per the types and kinds of reasons ---------------------------------------51

Fig 4:19 Cause and Effect Diagram for Transformer failing ---------------------------------------53

Fig 4: 20 Age of METEC-EPEI's transformers -------------------------------------------------------54

Fig 4.21፡- Transformer without any protection--------------------------------------------------------57

Fig4.22 ፡- Transformer with pillar (Proper installations)---------------------------------------------58

Fig 4;23፡- Cable without sheath and cause for transformer failed and power interruption-------60

Fig 5: 1 Recommendation replacing HRC fuse with Pole mount circuit breaker -----------------67

iv


List of Tables

Table: 2.1 The standard capacity value. ------------------------------------------------------------------10

Table 2.2 cable current carrying capacities-------------------------------------------------------------11

Table: 2.3. Example of Pareto data sheet ---------------------------------------------------------------23

Table:- 4.1 Routine test samples from 13/02/2010 E.C. to 23/05/2010 E.C ------------------------30

Table 4.1 Supplied Transformers Data collection-------------------------------------------------------41

Table:- 4.2 Pareto table for Addressing issues based KVA on priority------------------------------42

Table 4.3 Secondary Data of supplied VS failed transformers of 3 Historical years collected

from EEU's Procurement Department and Transformer workshop of EEU.-------------44

Table 4.4 Comparison of Annual failure rate of transformers-----------------------------------------45

Table. 4.5 Type of Transformers Failure status -------------------------------------------------------46

Table 4:6 Failed transformer failures based on kind of the fault.-------------------------------------48

Table:-4.7 Pareto table for causes of failure reasons---------------------------------------------------48

Table 4:8 Failure reasons of Distribution transformers in percentage ------------------------------50

Table 4:9 Age of the failed Transformers ---------------------------------------------------------------54

v


ACRONYMS

EEU Ethiopian Electric Utility

EEP Ethiopian Electric Power

EPEI Ethiopian power engineering industry

HV High Voltage

MV Medium Voltage

LV Low voltage

IEC International electro-mechanical Commission

SPSS Statically Package for Social Science

LPF Lighting proof fuse

RCA Root cause analysis

IEEE Institute of Electrical Electronics Engineering

ANSI American National Standard institution

JIS Japanese industrial Standard

BS British Standard

HRC High Rupturing Capacity

EAAR Eastern Addis Ababa Region

SAAR Southern Addis Ababa Region

PMCB Pole mounted Circuit Breaker

PLW Procurement Logistic & warehouse

DVDF Double Voltage Double Frequency

vi


Abstract

Analysis Quality in Production and Operation is a systematic standard based management

approach which targets customer satisfaction by applying product inspection on process of

production before use and on the operation during utilizing the specific product. The main

purpose of this research is to analyze Quality transformers in production and Operations in using

Transformers manufactured in Ethiopia by Tatek Transformer factory. The research covers only

transformer type of distribution oil immersed. Protection methods are considered and also there

is detailed analysis about distribution transformers in the testing, inspection and operational

failure reasons.

The Quality analysis in production and operation of distribution transformer analysis resulted

Primary data for testing and inspection within the factory, secondary data collected from the

EEU's maintenance center and site observation within the distribution network. The scientific

approach to analysis these collected data are IEC standard parameters, Pareto and cause and

effect analysis.

The study addresses 20, 940, 22 transformers are deemed in sampling for testing within the

premises of the factory the transformers, secondary data collected from EEU’s maintenance

and site observations respectively. The voltage level and the capacity in kVAs deemed for

this study are 15 and 33KVs and form 25-1250kVAs. I have received the report of 940 from

the four regions of Addis Ababa, and this secondary data reveals that, among the reasons

reported the issue of overload, lightening and unbalanced loading and earthlings fault were from

the worst to the least in terms of reason.

Over all, this study finalized and concludes that the failure rate is 9.6%, this show very high and

beyond the expectation to be. Failure rate 0.05% is the accepted rate. The manufacturer share for

damaging of transformer is 20.11% this is assured this much of transformers are found damage

before energized to the existing distribution network. This is also can happened because of

missing design test, there is no operational manual on how to loading and. The other 79.89%

share for the failing of transformer is on the Utility side; this is also because of not using

protection devices, unbalanced load, over current, improper usage of ratings and phase balance.

vii


CHAPTER ONE

1. INTRODUCTION

In today’s world it has become increasingly important for companies to be able to compete

on a global competitive market through producing quality and reliable products. Companies

are no longer just competing for business within their own local markets, but with

companies that reach far across the globe. [9] For companies to be able to compete on this

level they must strive to produce their products more quality and meet the expected

standards effectively and more efficiently than ever before and then other companies

engaged in the same business [10]. International electro-mechanical Commission (IEC)

Standards has been becoming increasingly popular for the competitions of standard

products [11. The same quality of using the good product is required by utility companies.

Most utility companies including EEU have to adopted best practices in installation,

operation and maintenance that have to be taken to harvest the best out of the good

transformers they purchased.[11]

Therefore, as there has to be a dynamic value adding quality production process in

manufacturers to settle their being ever market leading manufacturing companies, there

should be a best figure in using and caring for their assets and being reliable service

provider for the customers, who spend much of its expenses for expansion and

modernization than being busy in a daily replacement of failed transformers in addition to

securing its financial capability to be best place for its employees, too.

This study in particular is concerned to investigate if the frequency of failing transformers

in Addis Ababa is related to the poor quality transformers manufacturing of distribution

power transformer in Ethiopia by Ethiopian power engineering industry (EPEI) by

evaluating their product against standard quality(IEC standard 600076-1, 3, 5, 8&10.), by

conducting tests and inspections within the factory.[11]

Of course, there are believed to be more operational factors which this study is

going to reveal their level of impact in affecting their best performance desired and then

transformer is analyzed accordingly.

BY HAILEMARIAM GIRMAY BI T-BDU,2018 Page 1


1.1 Background of the Study

All across the globe, countries and corporations are facing pressure from world leaders to

produce and manage power with greater quality of transformers. In today’s world it has

become increasingly important for companies to be able to compete on a global competitive

market considering the quality product. Companies are no longer just competing for

business within their own local markets, but with companies that reach far across the globe.

[Sarah Kahn 2014] states that in the electrical equipment industry, ―globalization has

increased in this industry due largely to the following: increased international trade; foreign

takeovers of companies and joint ventures; growing global demand for industry products,

particularly in the Asia-Pacific region; the off shoring of manufacturing operations to low-

wage cost countries by industry firms; and the outsourcing of production to third parties in

low-wage cost countries.

There are several reasons studying the level of good transformer production in terms of

quality assurance during manufacturing and also insuring the good utilization working

practice in securing all type of protection and programmed servicing that the transformer

deserves during its service time.

Primary reason is, the rate of transformer failure per year in Ethiopia is large and the major

recent year supplier is EPEI and hence the root cause should be investigated to limit the rate

by taking appropriate data collected from the total supplied and number of transformers

failed during service providing within three consecutive years.

As it is clear known from the history of industrial revolution, the supply of huge amount of

energy at a very reliable and quality service is mandatory to support industrial

transformation. As part of this, making study on quality on the most electrical equipment of

which is distribution transformers is crucial.

Finally, it is clear that the cost of replacement shall be kept minimum as the country is in a

situation of vast electrification which needs a financial capability to do it by itself. This is

also what has to be addressed to minimize the replacement finance, time and human

devotion cost together with all other resource it consumes.



1.2 Statement of the problem

The major customers of the transformers of EEU and its technical staffs usually report that

transformers supplied by EPEI were not tested properly, do not comply with standards,

some parts are not fixed properly during loading and uploading, there is no output while

energizing and one phase or two-phase line is/ are missing. Besides, some private

customers are buying transformers from overseas due to the fact that the local transformers

are not of good quality. The transformer maintenance team of EEU has reported 142

transformers were damaged from 2015 to 2017 (2008-2009E.C).To sum up, EEU stated

that the one of the main reasons for power interruption is failure of transformers. On the

other hand, the manufacturer stated that "transformers are being manufactured following

the IEC standards; the problem is not because of supply of poor quality transformers but

due to EEU’s failure in installing standard protective devices, not properly erecting and not

managing the load properly". The problem already exists, power supply is continuously

being interrupted and transformer are failing and in some cases damaged.

Therefore, this study intends investigate the quality of transformers starting from the

manufacturing process up to the operation on site in such away to indentify the root cause

of severe transformers failures and to develop and forward scientific improvement

approaches.

1.3. Objectives of the study

1.3.1 General objective

The main objective of this study is to analyze quality in production based on the IEC

standards and operation failure reasons for the transformers supplied by EPEI. so as to

come up with alternative scientific transformer operational performance improvement

approaches.

1.3.2 Specific Objectives

1. To analyze transformer production process based on the IEC standards.

2. To examine which type of quality problem belong to the utility company and which

one belong to the manufacturer or EPEI.



3. To analyze the root cause of transformer failure

4. To develop transformer quality improvement scheme

1.4 Scope and Limitations of the study

This study exclusively focuses on Production and operation failure reasons on the

distribution transformers manufactured locally in Ethiopia by Ethiopian power Industry

Engineering (EPEI). Primary sample is taken from the factory for testing within the

factory from five production batch based on the contract signed between EEU and

EEPI. The research also covered 940 transformers which failed within three

consecutive years and were reported to EEU's maintenance department. Additionally,

some data was taken from site observation within Addis Ababa region to ensure that

the data from EEU's maintenance department reflects the realities on operation site.

The scope of this study for operation standard is mainly focused on transformers which

are installed by EEU's and are pole mounted distribution transformers and rated from

25 to 1250kVA of both 15and 33kV lines. In addition, the limitation of this study was

getting similar study done before.

1.5 Significance of the study

The share of in this theoretical knowledge of the study is such that the quality problems of

locally produced and operated transformers would be new contributions to the body of

knowledge in the area of quality of transformers. In addition, the findings would be an

important factors for the improvement of currently applied operational standards.

The findings of this research is to identify the present problems and its causes so that

provide remedy solutions and reported to the manufacturers for adhere to the international

best practices in the field. This thesis also got supportive to conduct further research to

meet the standards and operation serving on transformers. EEU and EPEI would also

benefit from this research when the findings and recommendation are put in to action. EEU

would be benefited from saving frequent interruptions, high cost for maintaining

transformers, rework to reenergize the transformer, customer and higher public

representative resentment. Customers and stakeholders also benefited from this research

power would not interrupted frequently, damaging materials or house hold used through

power supply, researchers, government, lecturers and students would also benefited from



for further study, calling and encourage of local and foreign investors, performing their

daily task without wastage of time and being power interruption would be a reason

respectively



CHAPTER TWO

2.0 LITERATURE REVIEW

2.1 Overview to Transformer

Transformer is an electricity distribution network element which is used in transmission,

distribution systems and it is used to raise the voltage level of the generated power in to a

transmittable voltage at generation stations and reduce it in to a user friendly voltage levels

at substations and distribution lines, so that the generated energy can be used for different

purposes ranging from domestic up to medium and large industries. Based on the reason

they are fixed anywhere in the network they are classified in to two as step up and step

down transformers. [14]

The step down transformers is used for electric power distribution and sub transmission

extensions to reduce the voltage form High voltage in to distribution level and sub

transmission voltage levels. And named as Generation-transmission station transformers

and Substation -distribution transformer and distribution customer transformers. There are

several types of transformer used in the distribution system such as single phase

transformer, three phase transformer, pole mounted transformer, pad mounted transformer,

and underground transformer.[1, 12, 14]

Even though they all work based on a common principle of electromagnetic induction

joining their primary and secondary winding through a common ferromagnetic metal core,

the size of Distribution transformers are generally small compared to sub transmission and

big substation transformers. This is due to power demanded to be supplied from these

transformers and the insulation against their relatively lower voltage and the heat rejection

capacity of the oil needed to keep the insulation safe live longer, is smaller than the sub

transmission and substation transformers.[13]

Distribution transformers are commonly called by their power supply capacity and level of

their medium voltage, accordingly EEU uses distribution transformer named as 25kVA-

1250AkVA, in regard to voltage ratings.



2.2 Standard of Distribution Transformer in Operation

To minimize the risk of failure of transformers during operation utility companies develop

and follows a document known as code of practice and operation and maintenance manual

derived from other countries experience and manufacturers caution and product name

plates. And EEU has this type of document which I have used as a reference and short

listed issues which are relevant to this thesis

It is a document that includes sets of rules and regulations during performing transformer

related works like transporting loading and unloading installing and commissioning as well

as schedules and frequencies of regular preventive maintenances on the transformers so that

the transformer can deliver the desired service for the customer.

For this thesis we only focus on the operational transformers these includes

1. Lists of types ( lightening, overload, unbalance and dropout fuses) and rating of

protection devices for each KVA size of transformers in 15 and 33 KV voltage

labels

2. Installation components and work man ship quality.

3. Loading quality( load checking)

4. Preventive maintenance schedule and consistency replacement of aged transformers

5. Commissioning and job quality approval.

2.1.0. Failure reasons on distribution network

Lightening arrestor are used to protect the transformers against damage resulted from

lightening stroke. And the standard lightening protection system installation is stated in the

manual to include

Three lightening arrestors, bare copper wire with diameter of 35 sq mm and two earth rods

made of two options copper 2m or galvanized iron 3m length .

The connection has to be using factory accessories supplied with the lightening arrestor for

connecting the three arrestors with the line and the earth wire at the pole top and welding

with the earth rod and the dawn comer of the earth wire.[18]



2.1.1 Overload and over current protection

There are two points where stated at the operation manual that should be provided over load

and over current protections these are Dropout fuse at the MV side and HRC fuses at the

LV side of the transformer terminals. Dropout fuses are intended to prevent upcoming

current trying to pass through the transformer and HRC uses are intended to prevent excess

current demand to be drawn out of the transformers.

Loading history this is the historical data recorded daily, hourly, monthly and annually by

station operators or any automation. Data sources includes the date ,time of hour of

occurrence ,the type of situation of interest( Load, Overload, short circuit, earth unbalance,

surge etc) and the magnitude or value of the issue. [7]

Beyond the above instantaneous values daily, monthly and annual values and frequency of

occurrence and much aggregate information is there so that using this recording of loading

history will to measure the time of hit by high load, lightening stroke, growth rate of the

load demand and hence determine the action that should be taken like upgrading,

rehabilitation, or select the type of detail test that should be done to a particular transformer

than suggesting all type of tests while investigating a claim of failure to transformers.

Since this is much practically an daily experiences is done in substation and transmission

system operation departments including EEP but it is expected to be done twice a year in

distribution transformers based on the inspection schedule. [1]

2.1.2. Load checking

This is removing unplanned work order issuing and unconsciously loading the transformer

over the rated standard and it also is a means of simple manually performing of loading

history data of case 5 above.

It has a number of a advantages including carefully adding new demands so that the

transformers are not overloaded, The rate of request is used to put an estimated date of up

grading or putting additional transformers nearby as rescue measure and risk mitigation

plan [1,18]. Dropout fuse and HRC have specific value defined as the capacity or rating of

the protection which varies with respect to the capacity of each transformers. They limit the

maximum current that should pass through the transformers while the transformer remains

safe.



2.1.3. Protection system Failure

The main function of the protection system is to protect the transformer from faults by first

detecting the fault and then resolving it as fast as possible.

So the failure of protection system means allowing the danger system to pass through the

transformers and looking forward to see the burn of transformers and blown out. If it

cannot fix the fault, it isolates it so that it may not damage the transformer. Protection

systems include the Buchholz protection, pressure relief valve circuitry, surge protection

and Sudden Pressure Relays, LV HRC fuse and circuit breakers.

The Surge protector protects the transformer from over voltage by allowing specific

magnitude of voltage to go to transformer and for the rest alternate route is found.This

means they are like diodes reverse biased ,they only will act as a shorted to ground line

provision (an easy way for current to pass) than passing through relatively higher

impedance terminal of the transformer connection.

Failure in surge protection causes high voltage to pass to the windings which becomes

damaged because of its effects as listed above. [5].

Moisture, heat and corrosion are the main reasons of the failure of surge protection as it

causes overheating and short circuit in it.

There are different protection systems and installation architectures that can be adopted

from world class standards in utility installation and protection system manufacturers like

ABB circuit breakers for over load and over current protection corresponding to the

designed levels of protection of transformers. Anything beyond this value is going to be

isolated from the transformer, Siemens HRC FUSES, BREAKERS and Schneider

electronics arrestors and Dropout Fuses are the most common protection devices

manufacturers [6]

To investigate health index and failure cause and modes of failure of transformers the

following issues are very important parameters together with the above [4]



2.1.4 Short circuit exposure

This is the level of exposure of the transformer to the stated conditions of 1 and 2 above

and also the dawn stream network installation qualities to seem are going to create short

circuit sooner or later [11].

This means that any weak points at the transformer out let cable, and Distribution box

connections and opened to the rain and dust also poor LV and MV network are supposed to

be exposure to short circuit. The exposure will raise its risk to highest level when they are

not equipped with protection system.

KVA

MV (D.O fuse

element in

ampere.)

LV (HRC fuse total

sum in ampere.)

50KVA 10 60

100KVA 15 120

200KVA 20 240

315KVA 35 380

630KVA 55 750

Table 2.1 The standard capacity value is specified as follows

Above 630KVA transforms protection against over load and over current is made using

MV and LV switchgear of 75A for 800kVA MV side by 1000A LV side while up to 110A

for 1250KVA MV side and 1500A LV side

The installation of these protection devices as stated on the manual shall consider the

following qualities.

Use appropriate connectors and cable lugs, avoid lose connection, use appropriate cable

sizes (diameters as per the designed transformers capacity) and cable types.

The appropriateness of the cables and connector accessories are specified as follows.



cable current carrying capacities

Table 2.2 cable current carrying capacities

2.1.5 . Un balanced loading and earth fault protection

The quality of connecting customer loads is determined by the equality of current passing

through each of the three windings and zero current through the neutral earthed wire. This

is done by evenly distributing customer connections throughout the routine operation of

EEU, any un balanced load will result in circulating current within the transformer if it is

not provided with neutral wire connected to the ground it will cause damage on one of the

windings and hence appropriate neutral grounding must be provided on LV side of the star

connected distribution transformer. The standard neutral grounding is stated as follows.[18]

The loading quality of DT states about the maximum amount of 80% of load that a given

transformer must be loaded and the maximum unbalanced loading that should be taken as

tolerable if higher correction action should be taken or the transformer will sooner or later

get damaged. [18]



The Operational manual regulation says 85% of the transformer rating shall correspond to

the load of the customer, during design stage and found unsafe if 100% and above so that

transformer capacity upgrading and/or additional transformer erecting or load shifting to the

nearby transformers are suggested as remedial actions. And 10% unbalance between the

phase currents is the maximum unbalance current and suggests load balancing tasks to be

launched if higher than 10% .[18]

It must have 2 earth rods buried 5mt apart and one neutral line from the transformer neutral

terminal is drawn and connected to the 2 earth rods before 5mt. the connection between the

neutral wire and transformer must be using factory supplied connector and the earth wire

and rod shall be welding. And no lose connection shall be observed.

2.1.6 Installation components and work man ship quality

The installation accessories and work man ship qualities are stated as

Use standard connectors and joints and creeping hand tools and dynamometer to fix

them

Avoid lose connections and missed connections

Avoid under sized wiring and overrated protection devices

Avoid uncovered terminals at the LV distribution boxes and clean any metal scraps

on the work bench. .[18]

2.1.7 Preventive maintenance schedule and consistency replacement of aged

transformers

It provides a time bounded consistent technical inspection followed by preventive

maintenance as per the investigations and suggestions during inspection

EEU has a checklist annexed with the operational manual which includes the following

important preventive measure insights. These are

Cooling oil level

Oil leakage, dust and other external maters accumulation on surface of the

transformers

LV distribution box physical condition remarks

HRC fuse rating recording column for each out going LV lines



Line and Phase voltages measurement recording at three different loading hours

(normal, peak, off peak hours)

Phase and neutral current measurement recording at corresponding voltage

measurement recording hours

Pole support strength and uprightness remarks

Potential short circuit wiring and clearance remarks

Neutral earth and lightening protection system continuity remarks

Availability and connection of dropout fuse and arrestors remarks

Lists of recommendations on correction actions that should be approved and

commenced officially. [18]

2.1.8 Risk mitigation plan and prevention mechanism

Since transformer is a valuable asset of Utility companies it must be provided necessary

operational risk mitigation plans which should be routinely assessed and any indication of

possible occurrence of failure reason must be cleared. These activities includes, many

utility companies adopted the following specific strategies to address the predominant

causes and consequences of failure: [5]

preventive maintenance; contingency planning;standardisationprescriptive technical

specifications, quality control measures, failure point awareness and environmental.

They mean a regular at least twice a year before summer and just after summer

transformers have to be paid a visit and seen for any problem defect or need of service it

requires and must be followed by a corrective action, doing this will significantly reduce

the number of transformers failed per year.

And also during installation and weather fit and range of operations expected to meet

should be there conditioned to the local area of application, and so should be standard range

of protection devices equivalent to the standardized technical specification un like to

buying same type for a country like us with all type of world weather existence should

require higher expertise in designing specific prescription of transformers. In addition to

this quality control measures ,failure point awareness shall always be recorded and known

to further studies and understand the trend of failure and propose the solution in designing a

change in risk mitigation plan,[5,12]



To optimize the cost effectiveness of the preventive maintenance the task could be arranged

based on the priority number generated from the site inspection data and calculated as in

equation: PN = Severity * Occurrence * Detection [1]

The transformer core is made up of laminated sheets of Ferro magnetic material used to

provide a controlled path to guide the magnetic flux generated in the primary voltage side

of the transformer to the secondary coil so that most of the flux generated is passed through

the secondary side, too. This is meant there is minimum loss of energy during voltage

transformation, that’s why we call the capacity of the transformer ideally constant. The

core is generally not a solid bar of steel, rather a construction of many thin laminated steel

sheets or layers. This construction is used to help eliminate and reduce heating loss and

heating effects. By providing a kind of obstruction intentionally created on the eddy current

Path so that the cause of the heat is limited. It is a mechanism of voltage regulation. This

allows for variable turn ratios to be selected in discrete steps so that the voltage level of the

user is kept standard. Transformers with this mechanism obtain this variable turn ratio by

connecting to a number of access points known as taps along either the primary or

secondary winding. [12,13]

2.1.9 . Job quality Controlling procedure

This is all about sort of work flows and sequences as well as approval and corrective or

rejection and authorizing work progress based on the approved design.

EEU has a procedure that specifies sequences of procedures and delegations of power for

each department related to transformer installation and new customer connection on

existing transformers and stated as follows. .[18]

A) New transformers installation

1. New transformer installation shall pass through planning and design department to

decide the KVA size of the transformer, wiring sizes and sets of protection devices

and all lists of materials the job requires based on the Customers request

information.

2. Following the approval of the work authorization the customer is made to pay the

cost of material lists

3. The issue is then forwarded to the technician teams to commence the installation



4. After the team declares completion of the transformer installation the overall work

is evaluated in reference with the design and planning design before energizing.

This stage includes correction or rework if need be.

5. Following the approval of energizing he transformer is left for service.[18]

B) Customer connection request from existing transformer

EEU customer service policy and procedure regulates the following sequences of tasks

before authorizing the connection.\

1. The technician is sent to the customers’ address of service request and transformer

to conduct the following inspection and material cost estimation.

1.1.Transformer KVA size

1.2.HRC fuse size

1.3.LV cable and conductor size

1.4.The voltage and current measurement recording at different loading hours of

each out going LV feeders.

1.5.The distance of the customer service request from the transformer and distance

of meter point from the existing LV line that passes by his address.

2. The data is then received by office engineers and verify the existing transformer can

accommodate the new request immediately or after some network augmentation.

3. Lists of materials for the service and for network improvement are listed and made

the customer aware of to pay

4. After the customer pays the issue is handed over to working teams to commence the

connection.

5. The overall installation work is checked for safety and energized and declared the

job is completed.

The above controlling procedures are supposed to be strictly followed by respective offices

and task forces so that the transformers are provided with all protection devices at deserved

installation quality as well as avoid unconsciously introducing new demands to existing

transformers and potential risks of transformer damage. .[18]



2.2.0. Age of transformers

Transformers are going to be called aged when they are found to be 20 or more under

service network for distribution transformers and 40-50 year for power transformers.

The first 2 are much related to the occurrence of the nature failure source and the last

except 8 are operational works that has to be done before the occurrence of failure

indications. Even though 3-8 are important in determining thehealth index oftransformers

as stated by [1] early age transformers show a relatively small in risk index but the

dispertion of the result as shown below indicates bad shaped and conclude that age alone

should not be taken as a measure of health index. [11]

In regard to transformer age , MR. Brian Sparling, Jacques Aubin supported their study in

titled by ―Determination of Health Index for Aging Transformers in View of Substation

Asset Optimization‖ which was published GE engineering presented to techcon in 2010 by

stating ―some interesting data has been published on its relationship to Heath Index. As in

the figure below, presented by, Hydro-Quebec, at Cigre 2008. It characterizes the

classification of health condition versus age for 2300 transformers 49 kV to 765kV. The

health condition is expressed here as a risk index leading to new a transformer in excellent

condition scoring near a zero value and aged units in bad shape score up to 45 on a scale 0-

64. Obviously, there is an upward trend, as transformer conditions tend to degrade with

time. However, there is a large dispersion of results indicating that age alone cannot be used

to assess 9 transformer conditions and should not be assigned an important weight in Health

Index determination.‖

Fig 2.1 index age of transformers



Which means that the risk level of early age transformers is lower than the aged ones;

especially there is a sudden raise in risk index to a bad level after 20 years. But the

distribution of the population is so dispersed that it shall be considered as not a critical

measure of being a reason of raising factor for failure rate [1,11,12]

2.3 Testing in Distribution Transformers

2.3.1 Manufacturing quality

The quality of transformer as any product depends on the raw material quality, raw

material handling, production process and product handling qualities.

In view of the above parameters there are internationa standards that has to be tested and

passed as an indication of following good production process quality. The quality of the

product is blieved to be an indication of fulfilment of the above critical issues. Had it not

been the case it is impossible to get an acceptaable product specification test results. To

insure this there are many tesing s that has to be done and its result has to be seen, they

include the following [10]

2.3.2 Quality Production Certification

The common transformer standard used as the reference in Ethiopia is based on

International Electro technical Commission (IEC). However, there are also other standard

that being used like American National Standard Institutes (ANSI) or Japanese Industry

Standard (JIS) and British Standard (BS). Fit for use in Ethiopia network system. The test

can be categorized as routine test, type test and special test.[10] Ethiopia and other utility

always required Short Circuit Testing Liaison (STL) members to witness and to verify the

test during customer witness testing process.[7]

2.3.3 Routine Test

Routine test is required to be performed on every transformer produced by the manufacturer

mainly within the manufacturer’s premises. Test under routine test are namely ratio test,

winding resistance, insulation resistance, separate source voltage withstand test, induced

over potential, no load loss and load loss, short circuit impedance test. Manufacturers are

required to supply the transformer within the required losses and impedances values at all

time. [7,10]



2.3.4 Type Test

Design test or the common named as‖ type test ― is a test which required to be done only

once for every new design made by the manufacturer by third authorized body. It is

intended to check the design characteristics. The test can be said much more destructive if

compared to the routine test. The tests are temperature rise test and impulse voltage test.

Temperature rise test is a test in which a transformer is put under full load and the

temperature of the windings and the oil are monitored as per design values. This test need

long hours and mostly take about 12 to 24 hours to perform. Impulse test is a dielectric test

in which it has to be tested to insure the transformer capability and to withstand high

voltages transient especially during lightning, switching and during fault. The transformer

needs to reduce the voltages within specific duration to the IEC standard. All of these tests

cannot be performed in Malaysia (Ethiopia)using facilities from the manufacturers high

voltage laboratory.[10,7]

2.3.5 Special Test

Special test is a test specially required by utility to be performed on a new design

transformer, like type test category. This test is conducted in the presence of the purchaser

or its representative as specified in the tender. Test fall under this category are short circuit

test, noise level test and zero sequence impedance test. Short circuit test which is a

destructive test cannot be performed in Ethiopia since the testing facility is not available.

Testing is done to test the transformer ability to withstand both thermal and dynamic effects

during short circuit. Thermal ability is demonstrated by calculation while dynamic ability is

proven through actual physical testing. Most of the time the test is performed with STL

members like CESI of Italy, KEMA of Holland, ASTA of Australia, etc. For transformer

certification and acceptance with utility in Ethiopia it is a must for the manufacturers to

pass the physical short circuit test. There are also other special test required like partial

discharge test and harmonic test but these tests only limited to special distribution

transformer used for example in oil and gas industry, factory, etc [7, 10,13]. Most of the

time EEU use IEC standards for transformers and same time IEC and Bs for other items.

Tenders given out by utility or private sectors will mostly refer to IEC standard or else

stated otherwise.

The routine test held consists of several types of tests;



2.3.3.1. Separate source test

2.3.3.2. Induced over voltage test

2.3.3.3. Measurement of the no load loss and no load current

2.3.3.4. Measurement of the winding resistance


2.3.3 6. Measurement of voltage ratio and vector grouping

2.3.3.7. Measurement of the insulation resistance

2.3.3.8 Separate Source Test

This test is important in testing the insulation and clearance of the HV/LV coil to

core/earth and HV coil to LV coil by using a single-phase test transformer. The test value

for HV is 15Kv33kV while for LV is 400kV: The values used to test are usually higher to

check the ability to withstand. This test is carried out for 60 sec and no short circuit

between HV to LV and other parts [10]. The calibration is performed on test circuit. The

factor Ku is calculated. The wiring is connected as per test circuit. Voltage is applied at 1.8

of rated voltage for 30 seconds. Then it would be reduced to 1.3 of rated voltage for 3

minutes. The discharge value is recorded in the report during 3 minutes period. Charge is

recorded at 90,150 and 210 seconds. The value obtained must be below 10pC.

Testing of any electrical equipment indicates the extent to which the equipment is able to

comply with a customer’s requirements. In this paper testing of distribution transformer is

considered. Manufacturers test thousands of distribution transformers at worldwide

locations each week. The primary incentive is to make sure the transformers meet

manufacturing specifications .Tests are part of a manufacturer’s internal quality assurance

program .A manufacture’s own criteria have to be fulfilled in addition to requirements

specified by customers and applicable standards, Besides, equipment of testing machineries

should calibrated within six to one year period and design test should also performed within

five years base time.. [10,13]



2.3.3 2. Induced over voltage test

This is to test the insulation between turns of the winding (HV and LV). Three phases line

fed to the LV terminals of the transformer. Twice of rated voltage is fed at frequency of 125

Hz (433Vx2=866V –feed on LV side). Duration of this test is 48 seconds and no short

circuit between turns [10,13]. Here for Ethiopia context (400Vx2=800V –feed on LV side)

2.3.3 3. Measurement of the no load loss and no load current

The transformer is energized with rated voltage, normally feed from the LV side and the no

load loss is directly from megger Power analyzer meter. Through this test the result must be

guaranteed value. Tolerance for No Load Loss (Po) is +15% [10].

By using the DC power supply (current is controlled), the phase to phase is measured on

HV and LV side. The resistance value is determined by calculating V/I formula.

Approximately 10% of the rated current is applied on HV side and 15amp current on LV

side [7.10].

2.3.3. 4. Measurement of the winding resistance

By using the DC power supply (current is controlled), the phase to phase is measured on

HV and LV side. The resistance value is determined by calculating V/I formula.

Approximately 10% of the rated current is applied on HV side and 15amp current on LV

side [10,13].


The LV side is short circuited; feed on HV side with 50% of the3 rated current. Losses (PL)

and impedance (ez) which determined at ambient temperature shall be corrected to

temperature 120°C by calculation. The tolerance for Load Loss (PL) is +15% and

Impedance voltage (ez) is +/- 10% [10].

2.3.3. 6. Measurement of voltage ratio and vector grouping

All connection is connected to ratio measuring device. Ratio value must not exceed the

guaranteed ratio value. The tolerance of the ratio is +/- 0.5% on the principal tap [10].

2.3.3.7. Measurement of the insulation resistance

Meggar tester is used at 2.5kV DC. The value obtained must be above 1kΩ/V [10].



In addition to the above, a case study was made by C. Ndungu, J.Nderu, L. Ngoo, P. Hinga

in Kenya to verify root causes of higher failure rate of Distribution transformers and

verified the following based on the local observation they made in 2017 [1] They recall

from [2] that Standard acceptable annual failure rate of transformer is 1-2% and referred

from [ 3, 4, 5 and 6] that life span of transformers larger than 100KVA is 35 while less than

100 KVA are 25 year and stated ―It is important to note that, average life expectancy of the

transformers is directly proportional to the average life of insulating materials. On steady

state power supply, harmonics and variations in frequency are the main factors that

accelerated aging of insulation materials and hence premature failure of distribution

transformers‖ and the phenomenon and their effect are Operational miss uses like poor

protection system, replacing the damaged transformer without being sure of clearing the

root cause and not performing preventive maintenances like tree trimming, sagging and

phase to phase clearance [19]

In my thesis I come up with the failure rate of transformers is 9.6% which similarly is going

to be considered high failure rate as compared to 1-2 % standard and average age of

transformers failed is less than 3 years excluding those which fail before giving any service

which is far beyond the standard life expectancy 35 for >=100KVA and 25 years for

<100KVA transformers. But I have also investigated that there is a20% initial condition

failure that EEU has to address with the manufacturer. Sever operational miss use were also

observed in Addis Ababa regions. The unbalanced loading, overloading of transformers,

totally missing of any of protection devises, and almost abandoned regular inspection and

preventive maintenances were among worsening situations towards aggravating failure rate

I observed during my survey. And hence I can say that the survival of transformers only

depends on the probability of fault occurrences. Also found similar to Kenya that EEU

transformers always get replaced after damage without insuring the root cause investigation

and clearing is done. Especially poor protection system and poor installation workmanship

resulted from negligence of cru management were among my observation that has to be

reviewed by EEU.

International journal of engineering science and research technology also has released a

publication work done by Tarini Dewangan , Miss Pragya Patel from Dr. C. V. Raman

institute of science and technology department of electrical engineering in May, 2017 on

premature distribution transformer failure prevention[25] And come up with the results of



their assessment as Electrical disturbances, Overloading 29.43%, Lightings Strike 17.32%,

Loose Electrical Connections, High Resistance 7.3%, Maintenance Issues, Oil

Contamination 5.91%, Moisture Ingress 4.03%, Line Surge, Other Issues 3.25% and further

recommend the failure results can be significantly reduced by doing corrective measures

against the following lists of major contributors of raising the failure rate of

transformers[25]

1) Poor earthing system, 2. Absent of lightening Arresters, 3. overloading of a transformer,

4 Lightning Surges, 5 Line Surges/External Short Circuit 6.Thief on apparatus, 7.

Consumers wrong connection 8. Poor Workmanship-Manufacturer. 9. Deterioration of

Insulation. 10. Moisture.11. Inadequate Maintenance,12 Sabotage, Malicious Mischief and

13. Loose Connection

I have used the above recommendations to be included in my check list to verify that EEU

is using them as transformer failure risk mitigation plans during my site survey and found

EEU has only included them in his code of practice and operation and maintenance

procedure but none of them were practiced as a result transformer failure is found as Due to

lightening strike 25.21%, Due to overload and over current 25.64%, due to unbalanced

loading and earth fault 29.04%, Initial condition failure 20.11% and hence in addition to the

recommendations stated in[25] I shall recommend Utility companies must develop a

procedure and continuously perform factory testing followed by on delivering tests as well

as pre commissioning tests. Last but not least EEU has to refresh its distribution team on

its’ own operational and maintenance manual that I believe has included most of cares that

should be given to transformers throughout the life time of transformers.

2.4. Cause and effect chart.

Is a tool for describing Quality of products can be Best described in production industries to

improve their products quality and production process improvements using assessment on

one or all of the following major parameters ,which have a relation with the product and

process. These are Man power, Material, Management, Machinery, Method and

Measurement. [26]

And I have used this resource I.e. cause and effect chart from the resources on the

references by optimizing the parameters that fits my investigation parameters.



Manpower

Material Machine

Effect

Method Measurement

Management

2.5. Pareto chart

Fig: 2.2. Literature of Cause and effect diagram

Pareto analysis have been used as a means of describing root causes of poor product quality

and production process by prioritizing issues in to 80 to 20 weighted percentage

contribution of effects. And as a result I have used the tool from the resource described as a

reference

Mr. Aschner have presented an example to demonstrate the purpose of pareto in priority

indexing and benefit of pareto analysis in optimization of addressing the effects of variables

Table: 2.3. Example of Pareto data sheet


Dent Scratch Hole Others Crack Stain Gap

Defects 104 42 20 14 10 6 4

Qu

an

tity


The result of the above table came as in the following bar graph He described the result

as,‖The quality will be improved as longer as the bar chart if we work on the defects stated

as Dent‖

120

100

80

60

40

20

0

Fig: 2.3. Standard graph of Pareto

I have used the method by optimizing the prepared my own parameters table and

values of my investigation results to show the priority of addressing the causes of failure of

transformer and also which transformer KVA sizes accounts for the major raise of failure

rate of transformers .



CHAPTER THREE

3. RESEARCH METHODOLOGY

3.1 Research Design

The study of this thesis focuses on analyzing Quality in production and operation,

inspection and testing of transformers within the Tatek transformers factory. It also

includes analyzing the secondary data from EEU's maintenance department damaged

transformers within two consecutive years. The analysis uses graphs presentations and

samples of pcs of transformer taken from site visit on the installed and already serving

transformers. As per the EEU and EPEI contract agreement signed on Feb, 2014 5%

samples from are batch of production exclusively set aside for testing and inspection

within the company to be analyzed with respect to the parameters of IEC 60076

standards. Investigations on transformers being observed on site and checked all

standard installation and accessories installed. Analysis done based on the required

observation and standards. For the transformers failed on operation, 940pcs reported

as failed during serving and analyzed using excel and different graphs software.

The excel and different graphs in this work begins with identifying modes of failures,

age, over load, unbalanced phases, during special events like holidays and

protective devices. Revealing underlying causes of the failure modes and

proposing appropriate remedial measures. In this respect, the excel and different

graphs study forms the background to focus on recurring failures or components

that have most significance for the reliability of transformers.

The work presented in this study is done based on IEC tasting and inspection parameters,

pareto and effect and cause and effect analysis. The Pareto anal ysis is belong to

anal ysis identify the most severe transformers, recurring and hard to detect

failures according to EEU maintenance staff opinion. In this case the staff opinion

serves to emphasize in finding remedial measures and keep the focus of the work on

failures that have strong link with the reliability of power transformers instead of

acting upon each failure regardless of their relevance. The testing and inspection

have been executed as per the IEC 60076 standards are describe based on routine

test within the factory and design test by third authorized high equipment testing center.

Routine test: test to be carried out on each job within the factory premises



1. Measuring of the winding Resistance

Procedures: - Use Digital multi meter to measure winding resistance,

Then, connect the two terminal ends of the meter in between the two terminal ends of R and

S, R and T, and S and T phases of the primary side windings. See the diagram of Fig 4

attached here with. The expected magnitude of winding resistance varies and depends on

the design, cross-sectional area of the winding wire, number of turns of the winding,

resistivity of the winding wire, and so on even though transformers do have the same

voltage level and KVA. Note that this happens not only for the primary windings but also

for the secondary windings. Besides, consider that the higher the KVA the lower resistance

value and vise versa. It is because

Resistance (R) = Resistivity*length

Cross-Sectional Area

From the mathematical expression, since the lower KVA transformers’ have lower cross

sectional area it implies to have the higher resistance value. Therefore the expected

measuring resistance value to be is higher than that of the transformer with the higher rated

KVA.

Similarly, repeat this procedure to measure the secondary side winding resistance. The

expected measuring resistance value to be is in milliohm.

2. Measuring of insulation Resistance

Procedures: -use the Megger of 5KV rating measuring instrument.

To get measure between MV and LV connect the two output terminals ends of the megger

to MV and LV terminals respectively. It is strongly recommended the measured value to be

nearly infinity to get the most best insulation level which assures long service life of the

transformer during operation. The supply voltage must be 5KV. See the diagram of Fig 4.1

attached here with.

3. Separate source voltage withstand test

Procedure: - Shunt the primary side and connect to the supply voltage of 38KV or 70KV

for one minute while the secondary side must be shunt with its neutral and be connected

with the ground (earthling). The expected result from this test is to withstand this over

voltage. Shunt the secondary side and connect to the supply voltage of 3KV or 8KV for one



minute while the primary side must be shunt and connected with the ground (earthling).

The expected result from this test is to withstand this over voltage.

4. Induced over voltage withstand (DVDF)

Procedure: -Supply the voltage for one minute to the secondary side windings of the

transformer at its double rate voltage level and double rate frequency setting the primary

side windings open. The expected result from this test is to with stand this over voltage.

5. Measurement of voltage

Procedures: - Supply the rated voltage to the primary side of the transformer when

secondary side windings are at open condition. Then, the displayed voltage ratio readings

from the meter in the laboratory must be the same as the calculated above.

6. Measurement of no load loss and Current

Procedures: - supply the rated voltage to the primary side winding setting the secondary

side open. Then, induced voltage is created on the secondary side windings so that due to

magnetic current created in the iron core which results No-Load loss is displayed on the

meter which is associated to its no-load current. The displayed value of No-load loss must

be precisely the same as the calculated value.

7. Measurement of load loss and Impedance (efficiency & Regulation)

Procedures:- supply the current source to the primary side winding until the supplied

current reaches the rated current setting the secondary side short circuit Then, due to this

current load loss is created and displayed on the meter which is associated to its impedance

voltage.

Type test: This type of test is carried out during design test up to destructive and is given

by authorized and third body. For transformer the following Design test (Type test)

executed.

1. All type tests,

2. Lighting Impulse test, and

3. Temperature test

Special test: This test measures the ability of the transformers to withstand the mechanical

and thermal stress caused by the external short circuit.



1. Additional Impulse test

2. Measurement of Zero phase sequence impedance test

3. Measurement of acoustic noise level

4. Measurement of Harmonic of the no load current, and

5. Magnetic balance test. sense sampling

3. 2 Data Collection

The sampling data is considered as the whole sampling damaged and reported to EEU's

maintenance center. Primary and secondary data were collected from Ethiopian

power engineering transformer factory located at Tatek and from Ethiopian Electric

utility Addis Ababa regions and transformer workshop maintenance of the utility.

Primary data was collected from the factory and EEU’s Addis Ababa’s wire

business heads during site visit and Secondary data was collected from EEU’s

maintenance department. Secondary data collection began by selecting 940 transformers

which were given for maintenance from four of the Addis Ababa regions and use the

details of their history card as secondary data source and the information in the history

card were filtered to include the following points for our analysis

1. Duration of transformer failed

2. Evaluating the service life of the failed transformer

3. Cause for failure

4. Over loading

5. Unbalanced connection and Protection

The detailed secondary data is attached as annex and the table below shows the

summarized and rearranged form which is made ready for software analysis and added

special event during the date of failure like, holiday, peak hour, and rainy seasons. Data is

collected Five percent from each patch following the contract agreement between

EEU and EEPI taken for the quality analysis through testing, data collection on the

site for failure on the history of failed transformers within 2015-2018 (2008-2010 E.C)

collected for the purpose evaluating the hypothesis and recommendation purpose of

the study.



3.3 Research analysis

The study data was analyzed based on the data collected from the factory, EEU's

maintenance center and site observations. Accordingly, the factory data is analyzed

based on the IEC standards of transformer testing and inspection whereas the data from

the maintenance centers of EEU is analyzed using the Pareto and cause and effect analysis.

For the data collected from the EEU's maintenance center it is analyzed by applying the

Pareto analysis and for the data collected from the site it analyzed based on the cause and

effect analysis.



CHAPTER 4

4. Data Collection and Analysis

4.1 Data collection from the factory:- The following table is primary data taken

during testing as per the IEC 60076 standards, these data are routine test or factory test

performed within Tatek Transformer factory. The methods, procedures and expected values

are mentioned under the table

NO

ITEM Description METEC

(Serial No.)

KVA

Customer Product

Year

Date (E.C)

1 New .prd(15kv/0.4 2502936 25 EEU 2010E.C 13/2/2010

2 New .prd(15kv/0.4 20002724 200 EEU 2010E.C 13/2/2010

3 New .prd(33kv/0.4 2502538 25 EEU 2010E.C 13/2/2010

4 New .prd(33kv/0.4 2502596 25 EEU 2010E.C 13/2/2010

5 New .prd(33kv/0.4 2502597 25 EEU 2010E.C 13/2/2010

6 New .prd(33kv/0.4 20001913 200 EEU 2010E.C 13/2/2010

7 New .prd(15kv/0.4 5001822 50 EEU 2010E.C 4/3/2010

8 New .prd(15kv/0.4 31503247 315 EEU 2010E.C 4/3/2010

9 New .prd(33kv/0.4 2502718 25 EEU 2010E.C 4/3/2010

10 New .prd(33kv/0.4 2502721 25 EEU 2010E.C 4/3/2010

11 New .prd(33kv/0.4 10001778 100 EEU 2010E.C 4/3/2010

12 New .prd(33kv/0.4 10001783 100 EEU 2010E.C 14/3/2010

13 New .prd(33kv/0.4 80000227 800 EEU 2010E.C 14/3/2010

14 New .prd(15kv/0.4 63001111 630 EEU 2010E.C 9/4/2010

15 New prd. (33kv/0.4 80000243 800 EEU 2010E.C 26/04/2010

16 New prd. (33kv/0.4 10001850 100 EEU 2010E.C 4/5/2010

17 New prd. (33kv/0.4 80000251 800 EEU 2010E.C 4/5/2010

18 New prd. (33kv/0.4 20001952 200 EEU 2010E.C 16/05/2010

19 New prd. (33kv/0.4 10001854 100 EEU 2010E.C 23/05/2010

20 New prd. (33kv/0.4 20001970 200 EEU 2010E.C 23/05/2010

Table:- 4.1 Routine test samples from 13/02/2010 E.C. to 23/05/2010 E.C.



Testing type and Methodology of Distribution transformers based on the IEC

standard

1. Insulation Resistance Test: -the above data are tested based on the following the

IEC standards; this activity is performed to know the insulation level in between

MV to Ground, LV to Ground, and LV to MV. The magnitude of the measuring

value is in Giga Ohm. When the magnitude of the insulation approaches to infinity

it indicates that the transformer insulation level is very efficient and reliable so that

the durability during service giving is very high. The measuring instrument to

measure insulation resistance is known as MEGGER.

Fig 4.2: Insulation resistance test in MV and LV

Expected Measuring Value = Infinity

To measure between MV and ground connect the two output terminals ends of the megger

to MV terminals and body of the transformer respectively. It is strongly r000ecommended

the measured value to be nearly infinity to get the most best insulation level which assures

long service life of the transformer during operation. The supply voltage better to be 5KV.

See the diagram of Fig 4.2 attached here with.



Fig 4.3 Test between MV and Ground


To measure between LV and ground connect the two output terminals ends of the megger

to LV terminals and body of the transformer respectively. It is strongly recommended the

measured value to be nearly infinity to get the mostbest insulation level which assures long

service life of the transformer during operation. The supply voltage better to be 2.5KV -

5KV. See the diagram of Fig 4.3 attached here with.

Fig 4.4: Test between LV and Ground


2. Winding Resistance test: - this test helps to identify whether the windings of each

phase for both MV and LV are equal. It means those MV windings of each phases

(R, S, T) are exactly equal with each other. Besides, those LV windings of each

phases are also equal with each other.

It can be expressed mathematically as:


Therefore,

Tap Position Voltage Level Voltage Ratio Calculated Result

5 15,750 15,750/220 71.59

4 15,375 15,375/220 69.89

3 15,000 15,000/220 68.18

2 14,625 14,625/220 66.48

1 14,250 14,250/220 64.77


RT + XLT , Rሰ+ XLሰ , RR+ XLR for MV side

Rተ+ XLተ , Rሰ+ XLሰ , Rr + XLr for LV side

Fig 4.5. Winding resistance test

Expected Measuring Value = Lower KVA, Higher Resistance

Higher KVA, Lower Resistance

3. Voltage Ratio Test: -it is the turn ratio of the medium voltage at different tap position

with the RMS voltage of 220vwhich is induced voltage of phase to neutral from the

secondary side.

For instance, for 15KV distribution transformer the voltage ratio mathematically expressed

as the following.

VR = Voltage level of primary side

220

Note that Transformer has +5% tolerance from its rated voltage level.



Hence, when 15KV transformer is being tested in the Laboratory the displayed result of

voltage ratio must be the same as the calculated value as shown above.

Fig 4.6: Voltage ratio test

Expected Measuring Value = 68.0105 - 75.1695

66.3955 - 73.3845

64.771 – 71.589

63.156 – 69.804

61.5315 – 68.0085

4. Vector Group Verification test: - Three phase transformer windings can be connected

several ways. For example, for 0 degree phase shift Dy0, Dd0, Yy0, for 30 degree phase

shift Yz1, Dy1, for 150 degree phase shift Yd5, Dy5, Yz5, and so on. The determination of

vector group of transformers is very crucial before connecting two or more transformers in

parallel. So, Vector Group Verification test: is an approach to identify the phase shift

(angle) difference between the primary windings and secondary windings introduced due to

that particular configuration of transformer winding connection.

If two transformers of different vector groups are connected in parallel then phase

difference exist between the secondary side windings of the transformer and large

circulating current flows between the two transformers which is very detrimental. The

Procedure and diagram is the same as Fig 4.6



Expected Measuring Value = Depends on the connection type of MV &

LV windings.

5. No-Load loss and No-Load Current Test: - it helps to know the power dissipated or

consumed by the secondary side windings at open circuit condition when the primary side

windings are energized or supplied the rated voltage. The power consumed is from the iron

core where magnetic current is created and is known as Iron loss (I2R).

It is recommended that the iron loss to be consumed by the Iron core is not to exceed 1.5%

of the rated load current.

Therefore, when transformer is checked or tested in the laboratory the expected No-load

current must not exceed as described above.

This explanation can be expressed mathematically as shown below.

For example, for 100KVA transformer with voltage rating of 15KV,

Let the displayed reading of the

No-Load loss from the Wattmeter be: 281.51 watts

No-Load current from the Ammeter be: 1.02 Amps

Therefore, when calculating the no-load current in terms of percentage, it shows:

1.02A*100% = 0.71%

144.34A

Then the result shows that this transformer comply or satisfied the recommended value

which is <1.5%.



Fig 4.7: No load loss and no load current test

Expected Measuring Value for different ratings:

–100kVA: Po: 380W, – 160kVA: Po: 480W,

– 250kVA: Po: 750W, – 315kVA: Po: 1050W,

– 400kVA: Po: 1320W, – 500kVA: Po: 1630W, Pt(+70°C) : 5960W

Where:

– Po: the Losses without load

6. Load loss and Impedance Voltage Test: -it is also useful to know the power dissipated

or consumed by the primary side windings during being energized or supplied the rated

current at short circuit condition of the secondary side windings. The power consumed is

from the primary winding which is known as copper loss.

For example, for 100KVA transformer with voltage rating of 15KV,

Supply the current source to the primary windings until it reaches its rated current which is

3.84Amps.

Then, the displayed reading of the

Load loss from the primary wattmeter is 1282.76 watts

Displayed voltage from the voltmeter is 601.35Volts



Therefore, when calculating the impedance Voltage in terms of percentage it shows:

Impedance Voltage = .601.35*100% = 4.01%

15,000V

Then the result shows that this transformer complies or satisfied the recommended value.

Fig 4.8: Load test impedance voltage

Expected Measuring Value for different ratings:

–100kVA: Pt(+70°C) : 1800W – 160kVA: Pt(+70°C) : 2550W

– 250kVA: Pt(+70°C) : 3120W – 315kVA: Pt(+70°C) : 4050W

– 400kVA: Pt(+70°C) : 5000W – 500kVA: Pt(+70°C) : 5960W

Where: – Pt(+70°C) : the losses with ful load at ‖ +70°C ―

7. Separate Source Power Frequency withstand test: -The main purpose of this test is to

assure the insulation level of each windings of both the primary and secondary sides to be

ok when both the primary and secondary side windings inject the high voltage of 38KV and

3KV or 70KV and 8KV for one minute for both primary and secondary respectively.

Therefore, if both primary and secondary side windings with stand the given voltage the

transformer is said to be in a reliable condition of insulation so that can give service in

operation.



Fig 4.9: Separated source power frequency test

Expected Measuring Value: With stand the given voltage with in a given period of time;

that is one minute.

8. Induced Over voltage withstand test: - The secondary side of the transformer will be

supplied 800V which is double voltage of the rated voltage at 100HZ frequency which is

double from its rated frequency keeping the primary side open. That is this test is known as

Double Voltage Double Frequency Test.(DVDF).

Fig 4.10: Induced over voltage with stand test

Expected measured value for different ratings: With stand the given voltage and frequency

within a given period time; that is 1 minute.



Summary of tests and inspections performed within the factory.

For confirming the specifications and performances of a transformer it has to go through

numbers of testing procedures. Some tests are done at manufacturer premises before

delivering the transformer. Mainly two types of transformer testing are done at

manufacturer premises- type test of transformer and routine test of transformer as per IEC

standard mentioned above

In EPEI Transformer factory case all testes of routine test performed as per the IEC 60076

standards recommendation. But, the following type testesnever performed and tests type or

prototype tests have high impact for failure in operation.

1. Dielectric tests of transformer, 2 Vector Group Verification

3. Vector Group Verification, 4 Temperature Rise test,

4. Impulse test of a transformer, 5. Tests on on-load tap-changer, and

6. Vacuum tests on tank and radiators.

The Dielectric test of transformer is generally performed in two different steps, likewise,

separate source voltage withstand test and induced voltage withstand test of transformer,

which we have discussed one by one below. This dielectric test is intended to check the

ability of main insulation to earth and between winding. If the insulation of the active parts

of the transformer with the earth is weak the transformer fails to operate.

The Vector Group of transformer is an essential property for successful parallel operation

of transformers. Hence every transformer must undergo through vector group test of

transformer at factory site for ensuring the customer specified vector group of transformer.

The phase sequence or the order in which the phases reach their maximum positive

voltages, must be identical for two paralleled transformers. Otherwise, during the cycle,

each pair of phases will be short circuited. Therefore, it is very difficult to use two

transformers in parallel without knowing the vector group.

Temperature rise test of Transformer is included in type test of transformer. In this test

we check whether the temperature rising limit of transformer winding and oil as per

specification or not. Here the test is allowed to be continued until the top oil temperature



rise does not vary more than 1oC per hour for four consecutive hours. The least reading is

taken as final temperature rise of the oil. This test is very useful mainly to determine and

know the transformer heat effect at peak load. It means, during operation at peak load there

will be heat developed in the windings, oil, and the tank. Therefore, if heat developed

without limit the transformer tends to be malfunction. Thereby, it is strongly recommended

the developed heat has to be at steady state condition.

Impulse test of a transformer: - Lighting is a common phenomenon in transmission lines

because of their tall height. This lightning stroke on the line conductor causes impulse

voltage. The terminal equipment of transmission line such as Power Transformer then

experiences this lightning impulse voltages. Again during all kind of online switching

operation in the system, there will be switching impulses occur in the network. The

magnitude of the switching impulses is about 3.5 times the system voltage.

Insulation is one of the most important constituents of a transformer. Any weakness in the

insulation may cause failure of transformer. To ensure the effectiveness of the insulation

system of a transformer, it must confirms the dielectric test. But the power frequency

withstand test alone cannot be adequate to demonstrate the dielectric strength of a

transformer. That is why impulse test of transformer performed on it. Both lightning

impulse test and switching impulse test are included in this category of testing. Hence,

this test is very helpful to determine how the insulation of the transformer withstand the

incoming high voltage at instant time. If it withstands the transformer is at good condition

for operation; if not it fails.

On-load tap changer testing: - The tap changer allow ratio to be increased or decreased by

fractions of a percent. Any of the ratio changes involve a mechanical movement of a

contact from one position to another. It is this contact that needs to be checked by way of its

resistance. The contact may go bad for a number of reasons.

1. Misaligned when manufactured causing insufficient surface contact. Full load

current overheats contact surface causing it to burn.

2. Current passing through contact exceeds full load rating.

3. Tap changing operation not "Make before break" creating internal arcing of contact

surface.

Tap changers are divided into two types; On-load (OLTC) and Off-load/de-energized

(DECT). The OLTC allows selection of ratio change while the transformer is in service.



This means that the ratio of a transformer can be changed while power (current) is still

passing through it. Thereby, to protect the transformer not to be burnt due to those bad

reasons listed above it is recommended to perform on-load tap changer testing.

Vacuum tests on tank and radiators: - This test is mainly useful for the transformers

which are oil immersed type. When manufacturing the tank and its radiator if the welding

process is poor the oil of the transformer will be leaked so that it causes the minimum level

of oil in the tank, this in turn causes also the burning of the active parts of the transformer

which is the malfunction of the transformer. To protect this phenomena Vacuum tests on

tank and radiators has to be performed. All in all, these testing are very crucial and which

are determining the quality of transformer and insure the performance of transformers in

operation. Besides, insuring these test through other testing company is witnessing

transformer products being reliable and leads to compute international bid. EPEI's

transformer factory located in Tatek doesn't execute these test which is during the design or

Prototype of new products. Finally, these tests should tested for official confirmation being

the supplying locally manufacturing are reliable and Met the IEC standard. Meanwhile, not

testing during design test by other body leads for failure reasons in operation since they are

able to withstand operational effects, same phase are missing before energized due to poor

handling and the manufacturer didn't have operational manual for how to how to transport,

installed, maintained and out operate.



4.2 Pareto analysis for the supplied transformers

SNo

Rating of

Transformer

2015

2016

2017

Total

Quantity

1 25KVA 368 1153 797 2318

2 50KVA 312 509 278 1,099

3 100KVA 412 412 742 1,566

4 200KVA 314 559 598 1,471

5 315 KVA 723 820 952 2,495

6 400KVA 38 59 30 127

7 500KVA 25 45 40 110

8 630KVA 42 118 184 344

9 800KVA 26 40 46 112

10 1250KVA 73 6 70 149

Total 2333 3721 3737 9791

Table 4.1 Supplied Transformers Data collection

Rating of

Transformer

Count

Percentage

Cumulative

Percentage

25KVA 68.00 7.23 7.23

50KVA 135.00 14.36 21.60

100KVA 260.00 27.66 49.26

200KVA

215.00

22.87

72.13

315 KVA 217.00 23.09 95.21

400KVA 4.00 0.43 95.64

500KVA 1.00 0.11 95.74

630KVA 36.00 3.83 99.57

800KVA

2.00

0.21

99.79

1250KVA 2.00 0.21 100.00

Table:- 4.2 Pareto table for Addressing issues based KVA on priority



Figure:-4.11 SPSS result of pareto chart of failed transformers KVA

The above pareto chart indicates that transformer sizes ranging from 25KVA up to

315KVA accounts for the 80% of the failed transformers and addressing issues related to

these sizes will bring about a significant percentage reduction on transformer failure.

The above table 4.1 indicates the delivering of transformers from the METEC significantly

increased from 2015 to 2016 and kept as more as it was in 2016 even in 2017. This is

depicted as in the following graph.

4000

3000

2000

1000

0

Qty 2015GC 2016GC 2017GC

Qty

Figure. 4.12 Progress of failure transformer based on year



Secondary Data of supplied VS failed transformers of 3 Historical years collected from

EEU's Procurement Department and Transformer workshop of EEU.

2015Gc 2016Gc 2017Gc Total Rating of

Transformer

Failure

rate

Supplied Failed Supplied Failed Supplied Failed Supplied Failed

25KVA 368 1 1153 12 797 55 2318 68 2.933563

50KVA 312 12 509 49 278 74 1099 135 12.28389

100KVA 412 22 412 98 742 140 1566 260 16.60281

200KVA 314 23 559 71 598 121 1471 215 14.61591

315 KVA 723 16 820 67 952 134 2495 217 8.697395

400KVA 38 0 59 0 30 4 127 4 3.149606

500KVA 25 0 45 0 40 1 110 1 0.909091

630KVA 42 4 118 14 184 18 344 36 10.46512

800KVA 26 0 40 2 46 0 112 2 1.785714

1250KVA 73 0 6 2 70 0 149 2 1.342282

Total 2333 78 3721 315 3737 547 9791 940

Table 4.3 Secondary Data of supplied VS failed transformers of 3 Historical years

collected from EEU's Procurement Department and Transformer workshop of EEU.

Pareto Analysis: The most frequently failed types of transformers are the 100,200, and

315kVAs. This, shows 74% percentage of failure is occurring by the 30% types of

transformers. From the above table we have observed that the numbers of transformers

failed increased from year to year since 2015. And also they were 315, 100,and 200 and

50KVA transformers which were most frequently failing transformers respectively. And

these are the transformers sizes which appear in public service except 25kva which is used



in ethio telecom towers. To have a clear insight how the failure of transformers and supply

are figured let us look at the following graph.

Total Supplied Vs Failed transformers

2500

2000

1500

1000

500

0

Purchased Failled

Fig 4.13 Total supplied and failed transformers

As it is indicted within the graph above, the most supplied type of the transformer is the

315kVA and the next one is also 25kVAs. The most failed is also 100kVA and 315kVA

respectively. It shows 100 and 315kVAs are most demanded transformers which mostly

requested and connected to the private and public customer loads. Besides, these types of

transformers are found being missed operational distribution standards and some of them

also connected over its capacity expected to be.

Comparison of Annual failure rate of transformers

Research covered years annual failure rate

2015G.C 3.34

2016G.C 8.46

2017G.C 14.63

Table 4.4 Comparison of Annual failure rate of transformers



The table above and the graph bellow show that there is no progress in reducing the failure

rate of transformers from year to year, Ruther it increased by 5.12% from 2015 to 2016 and

by 6.17% from 2016 to 2017. And the issue still remains a problem that EEU should

concern.

Annual failure Progress

20

15

10

annual failure rate

5

0

2015G.C 2016G.C 2017G.C

Fig 4.14 Annual Failure progress

The Supply demand Of EEU is increasing from year to year at almost same rate after 2016

and doesn’t seem to reduce for several years even up to this year, But the per unit failure

rate of failure of transformers should have to reduce by performing a selective intervention

on either the quality control document revision or /and operational misuse.

To made some selective criteria let us analyze the data found in the above table further up

to which type of transformers do have a per unit supply rate of failure. I.e. The ratio of

failure to purchase during these three years

Type of Transformers Failure status

KVA Failure rate

25KVA 2.933563417

50KVA 12.28389445

100KVA 16.60280971

200KVA 14.61590755

315 KVA 8.69739479

400KVA 3.149606299

500KVA 0.909090909

630KVA 10.46511628

800KVA 1.785714286

1250KVA 1.342281879

Table 4.5 Type of Transformers Failure status


%fa

ilure

rat

e o

ver

the

3 y

era

s p

eri

od


18

16

14

12

10

8

6

4 Failure rate

2

0

KVA

Fig 4.15 Failure rate states based on kVAs

From the above figure we can observe that the 100KVA transformers are the highest

percentage of failure in reference to the amount of 100KVA transformers supplied similarly

200,50,630,315 are the next sequentially having failure rate of next to 100KVA. This

means 16+ out of the 100 100KVA transformers fail within three years, and 14+ out of

100,200KVA transformers fail within three years 12+ out of 100 50KVA transformers fail

within three years etc. these rates are very high as compared to the ever increasing cost and

demand of transformers for new expansion

Causes of failed transformers reported analysis

I had a secondary data of the transformers failed within these three years and they include

in their history card when they return them to the work shop. And the data is filtered and

organized as in the following table. unlike the fault by the manufacturers the secondary data

collected from the Maintenance center of EEU summarized as the fault happened are due

to:-

1. Get installing without having protective device s like lightning arrestors, dropout fuses

and HRC fuses.

2. Unbalancing and grounding earthing and

3. Overloading

4. Failed to connect before connected to the existing network



Failed transformer failures based on kind of the fault

Total failure reason for 33 and 15KV

KVA

Lightening

Unbalanced

Earth

Overload

Failed before

energized

Total

25KVA 13 15 25 15 68

50KVA 36 35 40 24 135

100KVA 70 66 73 51 260

200KVA 73 46 54 42 215

315 KVA 43 56 70 48 217

400KVA 0 3 1 0 4

500KVA 0 0 1 0 1

630KVA 2 16 9 9 36

800KVA 0 2 0 0 2

1250KVA 0 2 0 0 2

Total 237 241 273 189 940

Table 4:6 Failed transformer failures based on kind of the fault.

Reason

Count

Percentage

Cumulative

percentage

Lightening 237 25.21 25.21

Unbalanced earth 241 25.64 50.85

Overload 273 29.04 79.89

Initial condition 189 20.11 100.00

Table:-4.7 Pareto table for causes of failure reasons



Fig4:16 SPSS result of Failure reasons pareto chart

From the above pareto chart we can see that Lightening, unbalanced earth and overloading

of the transformers accounts for 80% of failure reasons reported, and hence if we prepare

and work on mitigation plans on these issues we can achieve a significant improvement on

failures. To address the cause and effects related to our site survey and observations we

have put as in the following cause and effect chart.

The above table 4.5 shows there are 940 transformers various KVA sizes of METEC

origins failed within these three years and as depicted in the graph below most frequently

failed transformer types were 100KVA, 315KVA, 200KVA, and 50KVA transformers.

And the most reason reported as being the reason of failure is overload and unbalanced

loading followed by Lightening and initial condition problems. This means that

transformers capacity upgrade and additional transformers installation are required as a

relief of existing transformers. Furthermore the load balancing and customer network

reconfiguration are needed and also lightening protection system has to be reviewed and

correction action has to be made before every summer season.

But the failure during commissioning which is thought to have been factory error are also

seen to be significantly more in number. The relative impact shall have a significant



difference between other three reasons and factory error; however, they are viewed as

equally significant issues as in the next topic.

1000 900 800 700 600 500 400 300 200

0

Lightening

Unbalanced Earth

Overload

Failed before energized

Total

Fig4:17 Failure reasons in Quantity

Percentage of failure reasons as a measures of prioritizing issues

To further develop the relative strength of the issues of the four reasons reported let us see

the following summary and graphs

Failure reasons of Distribution transformers within the percentage

KVA

Lightening

Unbalanced

Earth

Overload

Failed

before

energized

25KVA 19.11765 22.05882 36.76471 22.05882

50KVA 26.66667 25.92593 29.62963 17.77778

100KVA 26.92308 25.38462 28.07692 19.61538

200KVA 33.95349 21.39535 25.11628 19.53488

315 KVA 19.81567 25.80645 32.25806 22.11982

400KVA 0 75 25 0

500KVA 0 0 100 0

630KVA 5.555556 44.44444 25 25

800KVA 0 100 0 0

1250KVA 0 100 0 0

Total average 25.21277 25.6383 29.04255 20.10638

Table 4:8 Failure reasons of Distribution transformers in percentage



From the table above and the graph bellow we can see that Overload accounts for 29+ % of

the failure of transformers and unbalanced loading 25.36+ % followed by 25.21% of

lightening and 20.10 % of initial condition failure.

Percentage failure of each transfomer KVA by each reason

100%

80%

60%

40%

20%

0%

Lightening Unbalanced Earth Overload Failed before energized

Fig 4:-18 Percentage of as per the types and kinds of reasons

As we can see the failure of unbalanced loading and lightening are only 5% higher than the

factory error, which means the quality of the product is seriously questionable that EEU has

to review. As we can see the failure of unbalanced loading and lightening are only 5%

higher than the factory error, which means the quality of the product is seriously

questionable that EEU has to review.

The lightening impact is seen to be significant on 200KVA and 100KVA transformers

followed by 315KVA, 50KVA and 25KVA. In this case we can also see that big size

transformers are seen to be less affected for some reason that we are going to verify it on

site investigation information.

The unbalanced load is seen to affect more of the 100KVA transformer followed by

315KVA, 200KVA and 50KVA. And we can see there is a much far less impact of

unbalanced load on big size transformers also.

When we observe the overload impact it is seen to affect most of the 315KVA transformers

followed by 100KVA, 200KVA and 50KVA transformers, similarly the big size

transformers are much less affected by overload.



The failure of transformers during commissioning ,which means failure due to initial

conditions of the transformers were also severely observed on the 100,200,315 and 50KVA

transformers respectively followed by 25 and 630KVA transformers. This amount of

transformer failure due to initial condition is significant that must be closely investigated

especially for a company which is suffering from material shortage and financial

challenges.

Finally in 15KV transformers we can summarize that the overall failure reasons reported

have much impact on those transformers which are less than 400KVA size and greater than

50KVA.


Po

or P

rocess Ex

Pro

cedu

re

Is n

ot fo

llow

ed


Cause Effect

Man Power Material Machine

Control

Supervised

Negligence

Protection device missed

Overrated

Miss use

Direct connection

Improper modification

& Re Use

Missed Testing

Procedure

Preventive

Maintenance

Construction

Improper stock

laming

Failed

transformers

O.P Manepment

Managemen

Sample quality

controle controle

PLW based on

only 5% sample

Poor imitation

quality Load checking Commissioning

Method Measurement

Fig 4.19 Cause and Effect Diagram for transformer failing


A

vera

ge s

erv

ice

ye

ar


Age of the failed transformers

As per the literature review transformers may fail due to aging but only after 20+ years of

service. and the guarantee period is 5+ years; But in case of METEC transformers they are

all failing almost before 3 years of service to reveal this let us see the following

KVA

Average

25KVA 1.50

50KVA 1.98

100KVA 1.76

200KVA 2.43

315KVA 1.26

400KVA 2.00

500KVA 0.50

630KVA 1.32

800KVA 0.50

1250KVA 1.00

Table 4:9- Age of the failed Transformers

Excluding the failure of transformers on initial condition the earliest age failure is within

0.5 years and the oldest service year is 2.43 years this generally means transformers from

METEC are under age and within warranty period that EEU could have claimed however

their warranty period agreement is stated as one year which clearly is in need of revision.

3.00

2.50

2.00 1.50

1.00

0.50

0.00

Average

Transformer KVA

Fig 4: 20 Age of METEC-EPEI's transformers



For this thesis we have make use of two data sources as secondary data source which

includes number of received and purchased transformers from METEC categorized by

voltage levels and KVA size from the year 2015 up to 2017 as in the following table

Table 4.1 number 15KV and 33KV transformers inspected and purchased from METEC

since 2015 up to 2017.

4.3 Primary collected from site observation on the EEU distribution

network

I have made site observations on four of Addis Ababa regions of EEU and focusing

on the following issues

Are the transformer stations equipped enough to protect the transformers

Is the installation quality good enough to enable the protection devices’ operation

Do the regions make due care to the transformers during operation

Do the construction team conduct a site test prior to energizing the transformers

4.3.1 The protection devices and their installation status

The survey is made based on the check list I have stated at the literature review and I

come up with the following results

4.3.2 EEU’s Central Region

Debrezeit No. 1 Kebele 12:- 315KVA transformer is installed without lightning

arrestor and drop out fuse on the primary side; however, there is fuse box on the secondary

side with three HRC fuses rated 400A.

Debrezeit No. 1 Kebele 12 mwmhiranSefer:- 50KVA transformer is installed without

lightning arrestor and drop out fuse on the primary side, besides, there is no fuse connected

on the secondary side which is out going line to the customer.

Debrezeit No. 1 Kebele 12 Addis Sefer:- 315 kVA transformer is installed without

lightning arrestor and drop out fuse on the primary side, besides, there is no fuse connected

on the secondary side which is out going line to the customer and another outgoing has only

two phases with HRC fuses 200A and 300A rating.



Debrezeit No. 1 Bale Godana- 315 kVA transformer is installed without lightning arrestor

and drop out fuse on the primary side, besides, there is no fuse connected on the secondary

side which is out going line to the customer and another outgoing has HRC fuses with

300A, 300A, and 300A rating.

Debrezeit No. 1 end of No.60 Bus station:-315 kVA transformer is installed without

lightning arrestor and drop out fuse on the primary side, besides, it has three independent

out goings in which two of them in each box with 300A, 300A, 300A HRC fuse rating and

the other one has only two fuses with rating of each 200A and one phase is connected

directly.

Debrezeit No. 2 near bishoftu cliff፡- 315 kVA transformer is installed without lightning

arrestor and drop out fuse on the primary side, besides, it has three independent out goings

in which two of them are connected directly and the other one has two fuses with rating of

each 200A and one phase is with 400A rating.

Debrezeit No. 2 tajima bridge bajaj station:- 315 kVA transformer is installed without

lightning arrestor and drop out fuse on the primary side, besides, it has three independent

out goings which are connected directly and always causes for the transformer to be

burned.

4.3.3 EEU's North Addis Ababa Region

According to the information given from regional wire business head the place where

frequent power interruption occurs is Holleta District. It is because the main reasons are:

1. The substation is saturated with its peak load.This case had been informed with

official letter written on the date 02/13/2007 with reference No.054/HSC/08 to

regional head. Even though it is informed the region do not discontinue to connect

the new line for the new customers.

2. Besides, in Holleta almost all transformers are connected without any protection so

that this case also informed to regional wire business head with official letter

written on the date 10/08/2008 but no action has taken place till now instead the

activity to connect the new line to the new customers still is ongoing.

During surveillance of transformers within the Holleta district the Quality office

observed the followings:



Mehal Ketema 44:- Near Birhan Bank 630 kVA transformer is installed without lightning

arrestor and drop out fuse on the primary side, besides, the outgoing of the secondary side

is connected directly.

Fig 4.21፡- Transformer without any protection

1. AddisuGebeya 315 kVA transformer is installed without lightning arrestor and drop

out fuse on the primary side, besides, the outgoing of the secondary side is

connected directly.

2. In front of Gottera School: - 200 kVA transformer is installed without lightning

arrestor and drop out fuse on the primary side, besides, the outgoing of the

secondary side is connected directly.

3. DandiBoru Flower Farm: - 100kVA transformer is installed without lightning

arrestor and drop out fuse on the primary side, besides, it has burnt three times.

4. Kaf Rose Flower Farm: - 315 kVA transformers is installed without lightning

arrestor and drop out fuse on the primary side, besides, it has burnt two times.

5. Yewel Stone Crusher፡ 100kVA transformer is installed without lightning arrestor

and drop out fuse on the primary side, besides, it has burnt once.

6. Holleta Farm: - 630 kVA transformers is installed without lightning arrestor and

drop out fuse on the primary side, besides, it has burnt once.



4.3.4 EEU's West Addis Ababa Region

When comparing with the rest of the region this region relatively uses protection devices

for the installed transformers; besides, the outgoing of the secondary side is installed with

pillar and its outgoing cable is Copper Cable.

Fig4.22 ፡- Transformer with pillar (Proper installations)

In this region at different sites, researcher has observed the followings; the surveyed sites

are those who are in frequent power interruption:

1. FM Surrounding:- 630kVA transformer is installed without lightning arrestor but has

drop out fuse on the primary side, besides, it has four out goings with two boxes each

with two by 250A and one by 350A rated foses..

2. SIlte Sefer:-630 kVA transformer is installed with lightning arrestor and drop out fuse on

the primary side, besides, it has four out goings with four boxes each with



(400A,300A,315A), (400A,355A,315A), (400A,300A,350A), (300A,300A,3A) fuse

ratings.

3. Zenebe work ZelekeTejBete:- 315 kVA transformer is installed with lightning arrestor but

without drop out fuse on the primary side, besides, it has three out goings with three

boxes each with (315A,315A,350A), (315A,315A,350A), (350A,350A,350A), fuse

ratings.

4. Mekanisa Amigo Caffe Near safe children፡-100 kVA transformer is installed without

lightning arrestor and drop out fuse on the primary side, besides, it has one out going

with one box each with (250A, 250A, 300A) fuse ratings.

5. On the way to Jemo at Mekanisa፡- the MV line which is installed to supply water pump,

when energized the pin insulator is bursting so that the region is in frightened to

reconnect the line. Therefore, still the line is not reconnected.

6. Yemane (Wedi) Condominium: - the insulation sheath of the cable which comes to the

fuse box is worn out so that it is very dangerous and hazardous. ፡



Fig 4;23፡- Cable without sheath and cause for transformer failed and power

interruption

4.3.5 Site investigation in East Addis Ababa Region of the utility

During the face to face discussion with technical distribution line staffs of the region named

Ato Biru and Ato Tesfahun the following constraints can be noticed:

Wooden poles are not impregnated very well.

In distribution line where chain insulator has to be used, pin insulator are using

instead at dead end and T-off position.

Using jumper is not in proper situation.

cable lag which is made of brass is not compatible with secondary side stud which

is made of copper. This incompatibility causes wear out the studs.

Using Fuses and Fuse boxes which are not comply the quality standards.

When using ABC cables termination is not performed as per standard and extra

length of ABC cables will be coiled in th line which causes stress. On the top of

this, frequently interrupted line with different sites are deemed.

1. The Transformer with rating of 315KVA at Gerji surrounding:

Installed without lightning and drop out fuse.

Has two out goings with two fuse boxes each with (315A, 350A, 300A) and

(350A, 300A, 300A) fuse ratings respectively.

The two outgoing lines are connected with ABC cables instead of using

Copper cables.

Silicagel of the transformer do not change properly during deterioration.



The transformer is erected very near to the rasident’s fence within about 20

cm gap.

2. The Transformer with rating of 315KVA at Gerji condominium surrounding:

Installed with lightning and drop out fuse.

Has three out goings with three fuse boxes each with (300A, 350A, 250A)

(250A, 300A, 300A) and (400A, 315A, 350A) fuse ratings respectively.

The three outgoing lines are connected with ABC cables instead of using

Copper cables.

Silica gel of the transformer do not change properly during deterioration.

3. The Transformer with rating of 315KVA at Gerji wetader area and surrounding:

Installed with lightning Arrestor but without drop out fuse.


(315A, 300A, 300A) (250A, 250A, 250A) fuse ratings respectively.

The three outgoing lines and primary side lines are connected with ABC

cables instead of using Copper cables.

Silica gel of the transformer do not change properly during deterioration.

The transformer is erected very near to the resident’s fence within about 20

cm gap.

4. The Transformer with rating of 315KVA at Gerji sun shine condominium

surrounding:

Installed with lightning Arrestor and drop out fuse.


(300A, 300A, 250A) and (400A 350A 315A) fuse ratings respectively.

The two outgoing lines are connected with ABC cables instead of using

Copper cables.

During celebration of Ethiopian New Year 2010 it is obviously observed since the day

before the eve power interruption has occurred and transformers are burnt at different sites.

Here are the followings:

1. On the date 04/03/2009 at Legetafo CCD real state compound 315 KVA

transformer with serial No. 77388 burnt because of over load and chaned by the

other transformer with serial No. 03217 but again burnt this transformer.



2. On the date 04/03/2009 at Kottebe Mesalemia surrounding 315 KVA transformers

with serial No. 28855 burnt because of over load.

3. On the date 05/03/2009 E.C Ayat Quter 1 condominium Block 24 surrounding 200

KVA transformers with serial No. 01344 burnt because of over load.

4. On the date 01/01/2010 Goroselasie 315 KVA transformer with serial No. 01094

burnt because of over load.

5. On the date 02/01/2010 Legetafo Mamitie Real State surrounding 200 KVA

transformer with serial No.032072 burnt because of over load.

Section switch is used partially to disconnect the line from the net work. However,

in EEU case most of section switches are shunted so that do not give the required service.

To show the evidence, the place at Gerji surrounding the two section switches are shunted

because the contact point of the section switch is copper where as the conductor which has

to be connected with the section would be aluminum, this improper connection results

incompatible with each other.

In this region the places where difficulties of frequent power interruption occur always at

Bole Lemi Industry Park. The line of this site is installed with double line using AAC

cables. These double lines rest on a single pin insulators using cross arm. It is because

during the time of construction of the lines there was scarce resource of cross arm.

Therefore, till the interview conducted no action has taken place to get rid of this problem

From the above site observations we can summarize the investigation as follows

Even though it is not mature enough to generalize that site conditions I surveyed is major

issue of raising failure rate of transformers between these years due to the my small sample

size and the coincidence year I observed and the historical data I used, however it indicates

the survival of the transformers I observed depends only on the withstanding tolerance of

the transformers and on the frequency of occurrences of various faults as well as magnitude

of the fault current.

4.4 Summary of Findings on site

I found there are some transformers connected without protection, poor installation,

unbalanced load and overloads. The standard protection devices for distribution

transformers are:- HRC Fuses rated according the load and capacity of transformers,



dropout fuse for over current protection, Lightening arrestors for protection against

lightening stroke and grounding system for unbalanced current relief during excess

unbalanced current circulation occurrences and providing an information for protection

devices to act before the transformer gets damaged. Besides,

There are transformer protection devices installed but they are much more than the

standard operating capacity of the transformer so that they bypass the over load and

over current demand of the load,

There are also transformers which have non uniform size of protection devise and also

partially availability of devices. This also causes a partial or total damage to

transformers due to the bypassed fault on one of the phase windings and also on one

side of the LV network depending on the direction of fault.

There are bad installations which may adhere the desired functionalities of

protective devise among them are

Manual splicing and Using ABC cables instead of standard connectors and

underground cables

Improper earthling system installation and wiring connections

Leaving the HRC fuse terminals open to weather human access, so that it

may cause improper functioning and bypass the undesired current flow.

4.5 Operation procedure

The EEU operation manual states that the transformers installation shall pass a number of

sequences of tasks that has to be performed before energizing.

One of them is the transformer must be tested without load at the completed transformer

station before energizing referred as ―pre commissioning step.‖ in the reference document

However for different reasons ,especially the non availability of proper testing equipment’s

like MEGGER and Earth test Meters‖ site testing is not carried out this time. There for

even though it is clear that transformer damaged when energized without load side

installation being connected suggests initial condition failure, it is hard for EEU to claim

the supplier and hence take the risk towards itself.



CHAPTER FIVE

5. Conclusion and Recommendation

5.1 Conclusions

Even though the factory testing conducted shows there is a confirming mechanism of

quality product before it is delivered to customer there are still missing very crucial tests

which should performed by third authenticated body during the design phase(Proto type/

test type) there are also high contribution in failing in operation and looked at its results so

the result of the statistical significance analysis of the failed transformers and the site

observation of the network and transformer preventive maintenance practice it shows I have

enough evidence to conclude the following generalizations. The failure rate is 9.6% this

shows it out of expected manufacturing quality error.

Almost all failed transformers are get failed early age. The expected average service life of

transformers are 40 year based on the literature review sates .Besides, the 20.11% failure of

transformers failed at initial condition before connected to the existing distribution network.

This suggests that those transformers supplied by METEC between these years missed are

missing standard quality of production. The missing quality aspects are because of not

conducted designed test, recommending only 5% for routine test for sampling and there is

no operational manual which suppose how to loading, uploading, transporting, packaging,

and operating and maintain.

The failure percentage of failure rate are increasing from year to year and EEU has

to act up on any measures that should be taken to improve on the failure of

transformers

The other 79.89% failure share is because of failures on the operation missing of

operational standards during installations. Based on the secondary data collected from the

maintenance center and site observation, transformers on operations are found get

damaging because of missing protective devices like lighting arrestor and dropout out

fuses, unbalanced load connection and overload connection 25.21%,25.64% and 29.04%

respectively.

The site observations tells leads as, EEU should provide appropriate protection devices ,

should also work based on the EEU's O&M manual.

From the above conclusions I would like to pass the following recommendations and

operational model to minimize their effect and reduce their risk level.



5.2 Recommendations:

The missed designed testing items must be full filled to verify if the manufacture is

producing quality and standard transformers following the recommended IEC 60076

The missed testing schedules at the factory and additional quality control revision has

to be done on new basis by conducting a mutual study of their products

Agreement has to be reached to include random checking of production process to

verify the consistency of quality production process agreed.

EEU must follow its own operation procedure and start claiming the supplier if it

verifies poor transformers before energizing and burning it.

EEU shall schedule and sustain transformer stations inspection and preventive

maintenances to minimize the loss of transformers at early age trough suggested

inspection investigations correction action and insure reliable customer services.

EPEI should provide operational manual to EEU which supports how to installed,

operate, transport and maintenance.

EEU should test the transformer by megger before energized in the network and should

also check the customer load and standard protection proved or not.

The protection system of transformers must be insured as any unhealthy conditions

could occur any time, and it is also the company’s valuable asset. EEU shall avoid

creating the issues and eradicate the issues by performing a regular inspection of the

transformer stations as well as provide the required standard protection at least to the

above Known cause of fault occurrences and keep them serve for long period of time.

As a matter of fact EEU has its own standard protection and installation procedure of

transformer installation the problem is not following that standard rating and

installation as well as knowingly or un knowingly adding additional load to the

transformers. This is because a customer has also a technical obligation to keep the

safety of the transformers to prevent them from uncontrolled wrong power signal.

Which by default will destroy the transformers?

The problem I found during site observation is related to the un availability of some

protection resources will force them to install without protection. So I recommend the

work shall stay paned until appropriate materials are delivered. And by doing so it will

lead the technicians accept as if they are doing legally the right thing to install them



wrongly. If it is not resolved earlier it will lead to being a bad operational practice

adopted by all its technicians.

The operational issues

These include the regular visiting of the transformer stations and looking any of the

preventive maintenance recommended and conducting the preventive maintenance it

deserves. EEU has in principle adopted many years ago but left over these days. It must

have started to neglect it the operational procedure someday in the past and it becomes

totally forgotten as being operational excellence to provide a service at a desired pre

specified design and construction standard and new connection service. Since we have seen

that not all transformers have got failed at equal period, as long as we made some

correction action at least as per the standard of EEU itself twice a year we could have

rescue those which fail in less than 10 years. And also check the load on the transformers

before new request is additionally connecting to it. By doing this we can minimize the raise

of the overload failure reason and failure of transformers due to overload. This means an

operational quality control model must be there to sustain the good operational and

maintenance best practices. As a result I propose the following Model for operational

control system.

EEU must see itself by conducting a separate study on how much is it loosing due to its

own failure to meet the operational standards so that it can make awareness appraisal prior

to taking administrative and legal measures over the sources of failure.

EEU should use PMCB in place of HRC fuse for monitoring the load management.

Applying this, EEU would be benefited from installing improper fuse rate,

frequently interruption, damaging transformers, repeatedly buying fuses. Protection

is vital part of electrical power systems. The secondary of low voltage distribution

transformers of EEU are protected using fuse. However, pole mounted circuit

breaker (PMCB) [5]; unlike the conventional protection, would help network

operators to have a better protection mechanism of their transformers against short

circuit and overloading while enhancing reliability, saving investment cost and

transformer sizing forecast.



Fig 5: 1 Recommendation replacing HRC fuse with Pole mount circuit breaker

The PMCB presented here is designed and manufactured by Schneider Electric. It is based

on the thermal image technology. Its tripping unit is composed of 3 thermistors which heat

proportionally to the windings of the transformer (joule effect). The system is continuously

watching the critical transformer heating point of 120 complying by the IEC 60076-7

above which the transformer is in danger. Hence, unlike the conventional protection; fuse,

it does not act depending on the amount of current only. It rather checks if the thermal

effect of it. If the current flowing through is not in an amount and duration that crosses the

thermal limit of the transformer, it does not interrupt the operation i.e. advanced reliability.

Furthermore, it has got a 24 hours clock that indicates the period accumulation the load gets

above 85% of the transformer’s nominal load. Each time the load exceeds 85%, the clock

hand turns and registers all these periods. When the sum of the periods reaches 24 hours,

the PMCB will trip or signal a red flag depending on your PMCB managing choice.

Investing in such a unit will help in curbing several costs. The multiple costs; but not

limited to, that one can get rid of are:

Replacement of conventional protection (Material + Labor + Logistics)

Storage Costs (fuses)

This unit is in use in several countries such as Kenya. A pilot case is running in Ethiopia as

well.



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