Power distribution, operation and maintenance of comilla palli bidyut samity 1

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Practicum Report On

Power Distribution, Operation and

Maintenance of Comilla Palli Bidyut

Samity-1

Prepared By:

Kawsar Ahmed

ID: 12105297

Prepared For

Engr. Md. Abul Bashar

Associate Professor and Coordinator

Department of Electrical and Electronics Engineering

IUBATInternational University of Business Agriculture and Technology

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Letter of Transmittal

April 15, 2016

Engr. Md. Abul Bashar

Associate Professor and Coordinator

Department of Electrical and Electronics Engineering

IUBAT- International University of Business Agriculture and Technology

4, Embankment Drive Road, Sector 10, Uttara Model Town, Dhaka -1230, Bangladesh.

Sub: Letter of Transmittal of the Practicum Report

Sir,

I am submitting my practicum report on, Power Distribution, Operation and Maintenance of Comilla Palli Bidyut

Samity-1 to you. Thank you for giving a chance to work with the Power Distribution, Operation and

Maintenance of Comilla Palli Bidyut Samity-1 as part of educational and other facilities. It was superlative

opportunity for me to work on this topic to actualize my theoretical knowledge in the practical area and to have an

enormous experience on the maintenance. With my little knowledge of the subject I tried to make it as interesting

and as accurate as possible.

I would be very kind of you, if you please take the trouble of going through the report and evaluate my performance

regarding this report.

Yours sincerely

----------------------- Kawsar Ahmed

ID # 12105297

Program: BSEEE

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Training Concern LetterTraining Concern Letter

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Training confirmation letter Student Declaration

I am Kawsar Ahmed ID# 12105297 student of the Department of Electrical and Electronics

Engineering under the College of Engineering and Technology (CEAT) would like to declare that the

project paper on Power Distribution, Operation and Maintenance is prepared for the fulfillment of

Practicum, as the partial requirements of academic purpose from my own concept and idea, after

completion of three months practicum in of Comilla Palli Bidyut Samity-1.

We also confirm that the report is only prepared to fulfill the academic requirement and not for any other

purposes.

Faithfully Yours,

.

Kawsar Ahmed

ID # 12105297

Program: BSEEE

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Acknowledgement

At first I thank to Almighty Allah who gave me energy to complete this task. Then, I would like to express my

gratitude and respect to some generous persons for their immense help and enormous cooperation.

First of all I would like to pay my gratitude to Honorable Vice Chancellor Prof. Dr. M. Alimullah

Miyan for giving me chance to prepare my research about this splendid topic.

I am very much grateful to Engr Md. Abul Bashar Associate Professor and Coordinator of Electrical and Electronics Engineering Department, (IUBAT) International University of Business Agriculture and Technology. Also

I would like to thanks my practicum supervisor Abu Bakr Siddique faculty of EEE.

After that I would like to express my special gratitude Md Yousuf Ali, Deputy General Manager

(Technical) in Comilla Palli Bidyut Samity-1. Who helped me a lot by his brilliant guidelines to make this

report, without which it was not possible to conduct it.

I would also like to thank all the faculty members of IUBAT and for their critical advice and guidance.

I have given my best effort to prepare and represent the report. I hope my endeavor will be beneficial to

the audience and urge for exonerate eye for any mistake or error found in the report. Finally, we also feel

it is an important thing to acknowledge and thanks to our faculties and all the employees of Comilla Palli

Bidyut Samity-1 who helped me a lot to provide a valuable forum for the exchange of ideas and

information.

Kawsar Ahmed

ID # 12105297

Program:BSEEE

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Executive Summary

As technology is advancing the consumptions of power is steadily rising. There are three steps for proper

electrification these are:

1. Electric Power generation.

2. Electric Power Transmission.

3. Electric Power Distribution.

This three are equally important for proper electrification, without any one of this three the electricity

system will be incomplete. Power Grid Company ltd is the BPDB (Bangladesh Power Development

Board) wound company which is the only authorized company for the Electric Power Transmission sector

in Bangladesh. Rural Electrification Board (REB) has many sub- stations all over the country which are

connected through the distribution line, these stations are called sub-station. This project paper provides

the synopsis assessment of all the systems existing 33/11KV Comilla Palli Bidyut Samity-1, Chandina.

Rural Electrification Board (REB) has the vast electricity distribution network all over the country & the

electric power plants are connected with the transmission line to assure the continuity of electric power.

The electric power plants produce power & feed in to the transmission line. All power plants are

connected parallel with the transmission and distribution line. Comilla Palli Bidyut Samity-1, Chandina.

Has AIS (Air Insulated Switchgear) switchyard.

Every sub-station is controlled by some experienced manpower, including one in charge, 2/3 engineers,

4/5 technical staffs. All kinds of maintenance work of the sub-station done by them in addition, sub-

station operation work done by the engineers. Every year annual maintenance work is done in every sub-

station in according to the official schedule. This thesis report is prepared in according to the operation

and maintenance procedure of 33/11KV Sub-Station, including emergency maintenance work.

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TABLE OF CONTENTS

Topic Page No

Preparatory Part Title Page-------------------------------------------------------------------------------------------------------I

Letter of Transmittal------------------------------------------------------------------------------------------II

Training Concern Letter------------------------------------------------------------------------------------III

Training confirmation letter--------------------------------------------------------------------------------IV

Students Declaration ----------------------------------------------------------------------------------------V

Acknowledgement- -----------------------------------------------------------------------------------------VI

Executive Summary ---------------------------------------------------------------------------------------VII

Table of Content -------------------------------------------------------------------------------------------VIII

List of Tables ------------------------------------------------------------------------------------------------7I

List of Figures -----------------------------------------------------------------------------------------------7I

Chapter-1

Introductory part

1.1 Origin of the report --------------------------------------------------------------------------------------1

1.2 Objective Of the study ----------------------------------------------------------------------------------1

1.2.1 Broad Objective------------------------------------------------------------------------------------1

1.2.2 Specific Objective----------------------------------------------------------------------------------1

1.3 Scope-------------------------------------------------------------------------------------------------------2

1.4 Background -----------------------------------------------------------------------------------------------2

1.5 Methodology ----------------------------------------------------------------------------------------------2

1.6 Limitations ------------------------------------------------------------------------------------------------2

Chapter-2

Organization Part

2.1 Introduction------------------------------------------------------------------------------------------------3

2.2 Company profile------------------------------------------------------------------------------------------3

2.3 Comilla Palli Bidyut Samity-1--------------------------------------------------------------------------4

2.4 Organizational chart of Comilla Palli Bidyut Samity-1---------------------------------------------5

2.5 Consumer Information of Comilla PBS-1-------------------------------------------------------------6

2.5.1 Tariff-wise Consumer No--------------------------------------------------------------------6

2.5.2 Disconnected Consumer No-----------------------------------------------------------------6

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Chapter-3

Transformer

3.1 Introduction of Transformer-----------------------------------------------------------------------------7

3.2 Classification of Transformer---------------------------------------------------------------------------7

3.3 Parts of the transformer----------------------------------------------------------------------------------8

3.3.1 Transformer Tank----------------------------------------------------------------------------------9

3.3.2 Conservator tank-----------------------------------------------------------------------------------9

3.3.3 Transformer Core---------------------------------------------------------------------------------10

3.3.4 Transformer Coil or Winding-------------------------------------------------------------------10

3.3.5 Silica Gel------------------------------------------------------------------------------------------10

3.3.6 Buckholz Relay-----------------------------------------------------------------------------------11

3.3.7 Radiator--------------------------------------------------------------------------------------------11

3.3.8 Bushing--------------------------------------------------------------------------------------------11

3.3.9 Transformer Tap----------------------------------------------------------------------------------12

3.3.10 Insulation and Impregnation-------------------------------------------------------------------12

3.3.11 Insulation material------------------------------------------------------------------------------12

3.3.11.1 Transformer Oil-------------------------------------------------------------------------------12

3.3.11.2 Electrical grade paper------------------------------------------------------------------------13

3.3.11.3 Press board-------------------------------------------------------------------------------------13

3.3.11.4 Parma wood------------------------------------------------------------------------------------13

3.3.11.5 Conductor material---------------------------------------------------------------------------14

3.3.11.6 Insulating tapes-------------------------------------------------------------------------------14

3.3.11.7 Bakelite----------------------------------------------------------------------------------------14

3.3.11.8 Gasket------------------------------------------------------------------------------------------14

3.4 Transformation Ratio ----------------------------------------------------------------------------------15

3.5 Voltage regulation of transformer--------------------------------------------------------------------15

3.6 Efficiency of transformer -----------------------------------------------------------------------------15

Chapter-4

Power Distribution

4.1 Introduction----------------------------------------------------------------------------------------------16

4.2 Power Distribution--------------------------------------------------------------------------------------17

4.3 Substations-----------------------------------------------------------------------------------------------17

4.3.1 Classification of sub-station-------------------------------------------------------------------------17

4.3.1.1 Indoor Sub-station ---------------------------------------------------------------------------------18

4.3.1.2 Transformer Sub-stations -------------------------------------------------------------------------18

4.3.2 Requirements of Installing a Substation-----------------------------------------------------------18

4.3.3 Equipment of the Substation------------------------------------------------------------------------19

4.3.3.1 Bus-bar ---------------------------------------------------------------------------------------------19

4.3.3.1.1 Single bus-bar system ---------------------------------------------------------------------------19

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4.3.3.1.2 Duplicate bus-bar system -----------------------------------------------------------------------20

4.3.3.1.3 Materials of bus-bars ----------------------------------------------------------------------------20

4.3.3.2 Insulators--------------------------------------------------------------------------------------------20

4.3.3.3 Isolating Switches----------------------------------------------------------------------------------21

4.3.3.4 Circuit Breaker or Auto Circuit Recloser-------------------------------------------------------21

4.3.3.5 Power Transformers--------------------------------------------------------------------------------21

4.3.3.6 Instrument Transformation-----------------------------------------------------------------------22

4.3.3.6.1 Current Transformer (C.T.) --------------------------------------------------------------------22

4.3.3.6.2 Voltage Transformer (P.T.) --------------------------------------------------------------------22

4.3.3.7 Fuse---------------------------------------------------------------------------------------------------22

4.3.3.7.1 Fuse characteristic -------------------------------------------------------------------------------23

4.3.3.7.2 Properties of reliable fuse -----------------------------------------------------------------------23

4.3.3.8 Optimum component protection -----------------------------------------------------------------23

4.3.3.9 Earthling switch ------------------------------------------------------------------------------------23

4.3.3.10 Neutral grounding (Earthing) -------------------------------------------------------------------23

4.3.3.11 Types of grounding -------------------------------------------------------------------------------24

4.3.3.12 Power factor ---------------------------------------------------------------------------------------24

4.3.3.12.1 Methods of power factor improvement ------------------------------------------------------24

4.3.3.12.1.1 Static capacitor -------------------------------------------------------------------------------24

4.3.3.12.1.2 Synchronous condenser ---------------------------------------------------------------------25

4.3.3.12.1.3 Phase advancers ------------------------------------------------------------------------------25

4.3.3.12.1.4 Calculating power factor --------------------------------------------------------------------26

4.3.3.12.1.5 Power factor correction ---------------------------------------------------------------------26

4.3.3.12.1.5.1 Power factor correction has the following advantages -------------------------------27

4.3.3.12.1.5.2 The advantages of power factors improvement ---------------------------------------27

4.3.3.12.1.6 Cost benefits of power factors improvement ---------------------------------------------27

4.4 Substation of Comilla Palli Bidyut Samity-1--------------------------------------------------------28

4.4.1 Circuit Breaker----------------------------------------------------------------------------------------29

4.4.2 Lightning Arrester------------------------------------------------------------------------------------29

4.4.3 CT and PT---------------------------------------------------------------------------------------------29

4.4.4 Fuse-----------------------------------------------------------------------------------------------------29

4.4.5 Metering------------------------------------------------------------------------------------------------29

4.4.6 Isolator--------------------------------------------------------------------------------------------------29

4.4.7 Power Transformer-----------------------------------------------------------------------------------20

4.4.8 Voltage Regulator-------------------------------------------------------------------------------------32

4.4.9 Feeder---------------------------------------------------------------------------------------------------33

4.4.10 Distribution-------------------------------------------------------------------------------------------33

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Chapter 5

Maintenance

5.1 Substation Maintenance--------------------------------------------------------------------------------34

5.2 Line Maintenance---------------------------------------------------------------------------------------34

5.3 Transformer Maintenance------------------------------------------------------------------------------34

5.3.1 Single phase transformer-----------------------------------------------------------------------------35

5.3.2 Power transformer------------------------------------------------------------------------------------39

Chapter-6

Transformer Test

6.1 Testing ---------------------------------------------------------------------------------------------------40

6.1.1 Megger test (Insulation resistance test) ------------------------------------------------------------40

6.1.2 Winding resistance test ------------------------------------------------------------------------------41

6.1.3 Ratio Test ----------------------------------------------------------------------------------------------42

6.1.4 No-load loss test --------------------------------------------------------------------------------------44

6.1.5 Full load loss test -------------------------------------------------------------------------------------46

6.1.6 Impedance test ----------------------------------------------------------------------------------------47

Chapter-7 Supplementary Part

Recommendation --------------------------------------------------------------------------------------------48

Conclusion ---------------------------------------------------------------------------------------------------48

Appendix -----------------------------------------------------------------------------------------------------49

References ----------------------------------------------------------------------------------------------------51

Topics Pages No

List of Table

Table No: 2.1 main features of rural electrification in Bangladesh-------------------------------------4

Table No: 2.2 Tariff-wise Consumer No-------------------------------------------------------------------6

Table No: 2.3 Disconnected Consumer No----------------------------------------------------------------6

List of Figure

Figure3.1: Transformer -------------------------------------------------------------------------------------8

Figure3.3.1: Transformer Tank------------------------------------------------------------------------------9

Figure3.3.2: Conservator tank-------------------------------------------------------------------------------9

Figure3.3.3: Transformer Core-----------------------------------------------------------------------------10

Figure3.3.4: Transformer winding-------------------------------------------------------------------------10

Figure3.3.5: Silica Gel--------------------------------------------------------------------------------------10

Figure3.3.6: Buchholz relay--------------------------------------------------------------------------------11

Figure3.37: Radiator-----------------------------------------------------------------------------------------11

Figure3.3.8: Transformer Bushing------------------------------------------------------------------------11

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Figure3.3.9: Transformer Tap------------------------------------------------------------------------------12

Figure3.3.10: Transformer oil Tank-----------------------------------------------------------------------13

Figure3.3.11: Insulating tapes------------------------------------------------------------------------------14

Figure3.3.12: Sealing / Gasket-----------------------------------------------------------------------------14

Figure4.1: Insulator------------------------------------------------------------------------------------------20

Figure4.2: Auto Circuit Recloser--------------------------------------------------------------------------21

Figure 4.3: Power factor improvement by static capacitor---------------------------------------------25

Figure4.4: Comilla PBS-1 Single Line Diagram--------------------------------------------------------28

Figure4.5: Meter in Comilla PBS-1 Substation---------------------------------------------------------29

Figure4.6: Power Transformer in Comilla PBS-1------------------------------------------------------30

Figure4.7: Power Transformer name plate---------------------------------------------------------------31

Figure4.8: Voltage regulator-------------------------------------------------------------------------------32

Figure4.9: Feeder control panel and display-------------------------------------------------------------33

Figure5.1: Transformer primary coil wire----------------------------------------------------------------35

Figure5.2: Coil binding-------------------------------------------------------------------------------------36

Figure5.3: complete coil binding--------------------------------------------------------------------------36

Figure5.4: core binding of transformer-------------------------------------------------------------------37

Figure5.5: after coil and core binding in the transformer----------------------------------------------37

Figure5.6: After repair complete single phase transformer--------------------------------------------38

Figure5.7: power transformer without tank--------------------------------------------------------------39

Figure5.8: power transformer coil binding in Savar central workshop-------------------------------39

Figure 6.1.1: Megger meter---------------------------------------------------------------------------------41

Figure6.1.2: Multimeter display show value-------------------------------------------------------------41

Fig 6.1.3: Multi-meter connection & output result for Winding resistance test---------------------41

Fig 6.1.4: Winding resistance test result form for 200KVA transformer----------------------------42

Figure6.1.4: HT terminal connected with 3 phase supply----------------------------------------------42

Figure 6.1.5: 3 phase voltage supply regulator----------------------------------------------------------43

Figure6.1.6: Clamp Meter----------------------------------------------------------------------------------43

Figure6.1.7: No current is taking HT side----------------------------------------------------------------43

Fig 6.1.8: Turn ratio test result form for 200KVA Transformer--------------------------------------44

Figure 6.1.9: Measuring the no load current by clamp meter------------------------------------------45

Figure 6.1.10: No load loss test result form for 200KVA Transformer------------------------------45

Fig 6.1.11: LT Terminal is shorted with wire------------------------------------------------------------46

Fig 6.1.12: HT terminal is connected with 3 phase supply --------------------------------------------46

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Chapter-1

Introduction

1.1 Origin of the report:

This report on topic name is Power Distribution, Operation and Maintenance of Comilla

Palli Bidyut Samity-1 is prepared by Kawsar Ahmed for the Bachelor of Science in Electrical

& Electronics Engineering program at the department of Electrical & Electronics Engineering,

under IUBAT-International University of Business Agriculture and Technology as an integral

part of the internship. He has done this practicum report based on power distribution, operation

and maintenance at Comilla Palli Bidyut Samity-1 under the instruction of Md Yousuf Ali,

Deputy General Manager (Technical) in Comilla Palli Bidyut Samity-1.

1.2 Objective Of the study:

1.2.1 Broad Objectives:

The main objectives are to extrovert my theoretical knowledge to the practical field with

adequate conceptualization and understanding how to operate a substation and maintenance of

the rural distribution network to meet the increasing demand of electricity, to minimize the load

shedding in the rural areas under Rural Electrification (RE) program and thus achieving

reliability of distribution system for greater satisfaction of consumers.

1.2.2 Specific Objectives:-

Achieve the knowledge of regarding my topics. Such as

Observation of rural distribution network

About the system of REB

About the system of Palli Bidyut Samity

About substation

About transformer

Maintenance system of REB

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1.3 Scope:-

For preparing this report, I have studied vastly about the process of Electricity transmission and

distribution system, Transmission line, Substation, Transformer, Conductors and maintenance.

This will increase my electrical engineering knowledge about operation and maintenance as well

as help me learn more about more in Future. This report will, without any doubt, be very helpful

for my future life.

1.4 Background of the Study:-

I have studied about 4 years at IUBAT International University of Business Agriculture and

Technology Department of Electrical and Electronics Engineering. I have learned theoretical

about my subject. So I have applied my knowledge in the practical life.

1.5 Methodology:-

I did visited Savar grid substation and their own distribution station to observe the distribution

system, also visited Savar central workshop for maintenance. For protection purpose REB uses

mostly vacuum circuit breaker and for higher safety SF6 circuit breaker on their substations they

also use auto circuit recloser. I also learn about bus bar arrangement, staking design and

supervision and other electrical equipment. I also observe the departments of REB, it has five

individual departments. They are Nipor Section, Engineering Section, Member Service, Finance

Section and Consultant Service. I observed the revenue collection of the month September which

was done by the Finance Department. During this time I also learn about load shedding and how

they maintain the schedule of load shedding. This observation helps me to complete my

internship and gives me a practical knowledge on electricity generation and distribution.

1.6 Limitations:-

During practicum in Comilla Palli Bidyut Samity-1, a huge amount of information has been

accumulated from the company. The employees were very much cooperative. They helped me a

lot, thus it was very much easy to understand most of the technical terms. But as it is a

commercial organization, sometimes the employees were busy with their official work.

Therefore at all time it was not convenient to communicate with them because of their hard

work. Yet they tried to help me as much as possible by them.

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Chapter-02

Organizational Overview

2.1 Introduction:

Internship is the part of the Bachelor degree in Electrical and Electronic department of IUBAT-

International University of Business Agriculture and Technology. I get the theoretical and

practical knowledge from Internship which is important for my career. I had completed my

degree major area in Power system, Power Station, Switchgear. For this reason I have done my

internship in Comilla Palli Bidyut Samity-1 under Bangladesh Rural Electrification Board which

is the leading electricity distribution company in rural and industrial area in Bangladesh. Comilla

Palli Bidyut Samity-1 has ensured me, the best support for developing the practical knowledge

with the help of the theoretical knowledge. In my internship I have learned how power plant

generate power, what is the distribution system. The objective of this internship report is it will

be the important source for the people, who want to do thesis in power sector or power

distribution system.

2.2 Company profile:

The Rural Electrification Board of Bangladesh has been providing service to rural member

consumers for over 38 years. Continued support from the Government of Bangladesh, the donor

community, consulting partners, and member consumers will help this program continue to

expand, providing the gift of electricity to millions more Bangladeshi households, businesses,

and industries.

Rural Electrification Board Act, 2013 has been established instead of Rural Electrification Board

Ordinance, 1977 (Ordinance No. LI of 1977) and the name of Board is Bangladesh Rural

Electrification Board which was responsible for electrifying rural Bangladesh. Since its

inception, the purpose of the program has been to use electricity as a means of creating

opportunities for improving agricultural production and enhancing socio-economic development

in rural areas, whereby there would be improvements in the standard of living and quality of life

for the rural people.

Today there are 77 operating rural electric cooperatives called Palli Bidyuit Samity (PBS), which

bring service to approximately 1,38,37,163 new connection being made and more than 2,91,534

km of line has been constructed.

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The followings are the main features of rural electrification in Bangladesh as on January, 2016.

Name Number Number of PBSs organized 77 Number of PBSs operating commercially 77 Number of district under the program 61 Number of Up-Zillas under the program 453 Number of villages electrified 56,302 Total distribution line constructed 2,94,897 Km Total distribution line energized 2,80,686 Km Total 33/11 KV sub-stations constructed and commissioned 721 (548 Constructed

by BREB, 85 Constructed by Private, 88 taken over from PDB/DPDC/OTHERS)

Installed Capacity of Sub-stations 7598 MVA Total number of consumers 1,41,98,117 Total number of irrigation pumps connected 2,37,442 System Loss 12.52% (cumulative), 09.88% (Jan'2016)

Table No: 2.1

REB sets forth the following major objectives in implementing the rural electrification program:

Ensure peoples participation in policy formulation in a democratic way.

Provide reliable and sustainable electricity to the rural people at affordable price.

Improve economic condition of the rural people by using electricity in agriculture,

cottage and agro based industry.

Improve living condition of rural peoples.

Bring about entire rural Bangladesh under RE program or an area coverage basis.

2.3 Comilla Palli Bidyut Samity-1:

Since its inception in 1981, Comilla Palli Bidyut Samity-1 is playing a vital role in Agricultural,

Industrial and Socio-Economic Development of Comilla District. The Rural Electrification

Program conducted by Comilla Palli Bidyut Samity-1 has acted a leap-forward in the

development of socio-economic structure of rural areas in Comilla District as well as entire

Bangladesh. It has significant and sustained impact on agricultural growth, industrialization and

business & commercial activities in the rural areas. It is a consumer owned entity organized on

the basic principles of Co-operative for distribution of electric power to its members and operates

on No Loss - No Profit basis for the mutual benefits of all its Members.

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2.4 Organizational chart of Comilla Palli Bidyut Samity

of Comilla Palli Bidyut Samity-1:

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2.5 Consumer Information of Comilla PBS-1:

2.5.1 Tariff-wise Consumer No (As on December 2014):

Tariff Consumer No.

Percentage (%)

Cunsumer Connected in Current Month

Billed Consumer

Domestic 56500 72.73 206 52362 Commercial 6778 8.72 7 3146 Charitable Institute 1542 1.98 3 1468 Irrigation 6778 8.72 7 3146 General Power 6778 8.72 7 3146 Large Power 2 0 0 2 Street Light 6 0 0 3

Table No: 2.2

2.5.2 Disconnected Consumer No (As on December 2014):

Tariff Consumer No.

Percentage (%)

Arrear Amount (Tk) DNP List No.

Domestic 4131 47.12 1381 1200 Commercial 400 42.43 927 400 Charitable Institute 150 1.71 79 51 Irrigation 400 42.43 927 400 General Power 400 42.43 927 400 Large Power 0 0 0 0 Street Light 3 50 17 0

Table No: 2.3

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Chapter-3

Transformer

3.1 Introduction of Transformer

A Transformer is a device that transfers electrical energy from one circuit to another by

electromagnetic induction (transformer action). The electrical energy is always transferred

without a change in frequency, but may involve changes in magnitudes of voltage and current.

Because a transformer works on the principle of electromagnetic induction, it must be used with

an input source voltage that varies in amplitude. There are many types of power that fit this

description; for ease of explanation and understanding, transformer action will be explained

using an ac voltage as the input source.

3.2 Classification of Transformer

Transformers are constructed so that their characteristics match the application for which they

are intended. The differences in construction may involve the size of the windings or the

relationship between the primary and secondary windings. Transformer types are also selected

by the function the transformer serves in a circuit, such as an isolation transformer. Transformers

can be constructed so that they are designed to perform a specific function. A basic

understanding of the various types of transformers is necessary to understand the role

transformers play in todays nuclear facilities. Trade Engineering are manufacturing different

type of transformer but most of them power and distribution transformer giving as below:

a. 2000KVA ~ 28000KVA 33/11KV Power Transformer

b. 5KVA ~ 3000KVA 11/0.415KV Distribution Transformer

c. 50KVA ~ 750 KVA 33/0.415KV Auxiliary Transformers

The applications of each of the following types of transformers:

1. According to Procedure:

i. Step up transformer

ii. Step down transformer

2. According to construction of Core:

i. Core type transformer

ii. Shell type transformer

3. According to Application of Transformer:

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i. Power transformer

ii. Distribution transformer

iii. Auto Transformer

iv. Instrument transformer

Instrument transformer is two types:

a. Current transformer (C.T)

b. Potential transformer (P.T)

4. According to Frequency:

i. Audio frequency transformer

ii. Radio frequency transformer

5. According to Number of phase:

i. Single phase transformer

ii. Poly phase transformer

3.3 Parts of the transformer

1. Transformer tank

2. Conservator

3. Transformer Core

4. Coil or winding

5. Bushing

6. Radiator

7. Breather

8. Silica Gel

9. Transformer oil

10. Oil level indicator

11. Temperature gauge

12. Thermo-meter

13. Buchholz relay

14. Drain coke

15. Earth point

Figure3.1: Transformer

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3.3.1 Transformer Tank

The transformer tank shall be fabricated from steel and shall be of robust construction. Care

should be taken at the manufacturing stage so as not to have leaks during transportation or when

the transformer is continuously operated at rated power.

Transformer tank envelop the active part, filled with

completely with oil. The body has no contact with the

active part. The paint at the inner side is oil resistive and

the outer paint is anti-corrosion. Even it is just a container

but require much consideration during design. With the

exception of radiator elements, all external joints shall be

seam welded. There shall be only one vertical seam weld

for the fin radiator and the other three vertical corner

edges of the transformer shall be formed by bending.

Corner ribs shall be avoided for the fin radiator. The

bearing surface of the tank to which bushings are

clamped shall be substantially flat. All matching faces of

joints shall be made oil tight and finished with a smooth

surface to ensure that the casketing materials make a satisfactory joint. Flanges and covers of

tanks shall be of sufficient thickness to prevent any depression occurring, which would retain

water around the bolts. The horizontal edges of the cover plate shall be bent over the tank flange

to facilitate water dripping out of the tank. The bent collar width shall be about 10mm to 15mm.

All the nut and bolts used shall be hot dip galvanized and spaced at sufficiently close intervals to

avoid buckling of either flange or covers and shall provide reasonably uniform compression of

the gasket. Each transformer shall be provided with a minimum of two closed lifting lugs. The

minimum diameter of the hole or width of the slot shall be 25 mm. The two lifting lugs shall be

located such that there would be a minimum of 50 mm between the lifting chain and the nearest

part of the bushings. All transformers shall be suitable for outdoor mounting on pole or plinth

platforms and shall have four mounting lugs with 12 mm diameter holes suitable for bolting the

transformer to the platform. Bolt hole spacing shall be as specified by the purchaser to suit

mounting requirements.

3.3.2 Conservator tank

Conservator tank is an expansion vessel party filled with

oil and connected to transformer tank. Its function is to

ensure that the transformer tank is completely filled with

oil every time. The system allows the transformer tank to

remain full despite contraction or expansion of the fluid

due to temperature change.

Figure3.3.1: Transformer Tank

Figure3.3.2: Conservator tank

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3.3.3 Transformer Core

In an electrical power transformer, there are

primary, secondary and may be tertiary

windings. The performance of a transformer

mainly depends upon the flux linkages

between these windings. For efficient flux

linking between these windings, one low

reluctance magnetic path common to all

windings should be provided in the

transformer. This low reluctance magnetic

path in transformer is known as core of

transformer. Transformer normally use laminated silicon steel core for reduce

hysteresis loss and eddy current loss.

Figure3.3.3: Transformer Core

3.3.4 Transformer Coil or Winding

Transformer coil or winding two types one

is primary winding another is secondary

winding or H.T winding and L.T winding.

H.T winding wire is thin than L.T winding.

Transformer winding wire is laminated by

super enamel insulation

Figure3.3.4: Transformer winding

3.3.5 Silica Gel

Silica gel is used in conservator tank to

absorb moisture and remove dust/foreign

impurities from air sucked in. Whenever

transformer is loaded or unloaded, oil

temperature inside the tank rises or falls,

air volume inside the tank changes

resulting in pushing out or sucking in the

air.

Figure3.3.5: Silica Gel

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3.3.6 Buckholz Relay

Buchholz relay is a type of oil and gas

actuated protection relay universally used on

all oil immersed transformers having rating

more than 500 KVA. Buchholz relay is not

provided in relays having rating below 500

KVA from the point of view of economic

considerations.

Figure3.3.6: Buchholz relay

3.3.7 Radiator

Radiators are used in a transformer to cool

the transformer oil through natural air or

forced air flowing in these radiator fins. As

the transformer oil temperature goes down

due to cooling it goes to the transformer tank

from bottom, cool the winding and gets

heated, and then return to the radiator for next

cooling.

3.3.8 Bushing

The bushing is a hollow insulator, allowing a

conductor to pass along its center and connect

at both ends to other equipment. All bushings

shall be of porcelain clad, of the highest

quality. Cree page distance of the bushing

insulator shall be 290mm for 12kV

transformer and 720 mm for 36kV

transformers. The inside of the bushing may

contain paper insulation and the bushing is

often filled with oil to provide additional

insulation.

Figure3.3.8: Transformer Bushing

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3.3.9 Transformer Tap

A transformer tapes connection points along

a transformer winding that allow a certain

number of turns to be selected. This means, a

transformer with a variable turn ratio is

produced, enabling voltage regulation of the

output. The tap selection is made via a tap

changer mechanism.

Figure3.3.9: Transformer Tap

3.3.10 Insulation and Impregnation

The quality of transformer and especially the dielectric strength response of its insulation depend

on the treatment of its active part. TSTL transformers the treatment consists in an alternation of

hot air heating and prolonged vacuum so as to remove all moisture from the insulating materials.

Pre compressed press board spacers used in the active part provide a rigid insulation structure

with low partial discharge levels. A clean, dust free environment ensures the highest standards in

quality. By using automated oil filling and processing system and by virtue of leak proof joints in

the transformers, there is no oil spillage. The oil used for impregnation complies with BS-171

and every consignment received is tested before being pumped into storage tanks.

3.3.11 Insulation material

Apart from main raw materials like ergo, ms, insulating materials are also employed for building

up a transformer. The following are some of the insulating materials used in transformer.

1. Transformer oil

2. Electrical grade paper

3. Press board

4. Parma wood

5. Conductor material

6. Insulating tapes

7. Bakelite

3.3.11.1 Transformer Oil

Insulating oil forms a very significant part in the transformer insulation system and has the

important functions of acting as Electrical insulation as well as a coolant dissipates heat

P a g e | 2 4

losses. The basic raw material for the

production of transformer oil is crude

petroleum. Proceed transformer oil

will be received by us to use in the

transformer after filtering.

Breakdown voltage strength is the

basic Parameter of the transformer

oil. It serves to indicate the presence

of containing agents like moisture

carbon particles etc. breakdown

voltage should not be less than 50 KV

after filtration. Figure3.3.10: Transformer oil Tank

3.3.11.2 Electrical grade paper

Paper is made from cotton, straw and wooden pulp. Paper is generally used as inner layer

insulation in windings, covering on conductors, paper tubes for terminal load insulation etc.

3.3.11.3 Press board

Pressboard is widely used insulating material for making a variety of components used in

electrical and mechanical design of a transformer like paper. Pressboard is also made from wood

pulp cotton, jute, ham etc.

Generally pressboards are of three grades

i. Grade 3 pre compressed

ii. Grade 3 normal

iii. Grade K

Pressboard can be converted into any required shaped as Runners, spacers, support blocks,

cleats, phase barriers, cylinders etc.

3.3.11.4 Parma wood

Parma wood is used in the areas which require higher mechanical and lower electrical strength.

Parma wood is used for making a variety of insulation components like coil clamping ring, cleat,

support, core and yoke clamp, bolt and nut etc. Haldu and Teak wood seasoned planks are also

used as a filter material between core limbs and enveloping coils and as yoke step filters.

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3.3.11.5 Conductor material

The following types of conductors are used for making different types of coils.

a) Double paper covered rectangular strip.

b) Double paper covered round wire.

c) Super enameled rectangular strip

d) Super enameled round wire.

3.3.11.6 Insulating tapes

This is used in winding coils to maintain the

phase after the former is removed. This is used

on core limbs to bind all the laminations before

inserting LV winding.

Figure3.3.11: Insulating tapes

3.3.11.7 Bakelite

This is used for Ducts, Tap Link Boards, cleats, common rings etc.

3.3.11.8 Sealing / Gasket

The transformers shall be of the hermetically sealed type and provided with a satisfactory lid

sealing gaskets. The gasket shall of the good

quality to maintain the sealing effect through

its life span and shall prevent seeping of oil

due to ageing and extreme operating

temperature. Gaskets provided with the

transformers shall be suitable for making oil

tight joints, and there would be no

deleterious effects on either gaskets or oil

when the gaskets are continuously in contact

with hot oil. No gaskets shall be used in

which the material of the gasket is mounted

on a textile backing. Exterior gaskets shall be

of rubberized cork material, weatherproof

and shall not be affected by strong sunlight. Figure3.3.12: Sealing / Gasket

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3.4 Transformation Ratio

Voltage ratio

Ep/EP = 4.44Npm 10/4.44Nsm 10

Or

Ep/Ep = Np/Ns

Current ratio

Ep Ip = Es Is Or Is/Is = Ep/Es

Turns ratio

=

= a

(a=Transformation Ratio)

3.5 Voltage regulation of transformer

The difference between the output voltage at no load and the output voltage at rated load, divided

by the output voltage at rated load, is called the voltage regulation of the transformer.

Mathematically,

=

3.6 Efficiency of transformer

The output of a transformer is slightly less than the input because of the losses in the windings

and the iron core. The ratio of the two, expressed as a percentage in the efficiency of the

transformer. The output and input are expressed in watts or kilo-watts.

=

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Chapter-4

Power Distribution

4.1 Introduction

This Chapter will focus on the distribution side. Substation is used for distribution purpose. To

establish a substation, there are some important points which have to be maintained. Different

types of substations are used to meet the different requirements. Normally step-up substation is

used at end of the generator side. Circuit breaker is protective element used in substation to avoid

the abnormal situation. Arrester and earthing is also used to avoid the overcharging situation.

Current transformer, potential transformer and bus bar arrangement are the common feature of

distribution substation. Typically, transformer is used to ensure the electricity supply in both end

of the substation and the household.

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4.2 Power Distribution

Electricity distribution is the final stage in the delivery of electricity to end users. A distribution

system's network carries electricity from the transmission system and delivers it to consumers.

Typically, the network would include medium-voltage (less than 50 kV) power lines, electrical

substations and pole-mounted transformers, low-voltage (less than 1 kV) distribution wiring and

sometimes electricity meters.

The same phenomenon is applicable for REB. At first power come to the Grid Station (400 KV

or 230KV)from National Grid, then power transmitted to the substation (400/230 KV) or

230/132KV then 123/33KV. From substation power is transmitted to the bus bar (33/11 KV) and

then to the consumer through transformer.

4.3 Substations

The assembly of apparatus used to change same characteristics of electric power supply e.g.

voltage, ac to dc; frequency etc is called a substation.

4.3.1 Classification of sub-station

There are several ways of classifying sub-stations. However, the two most important ways of

classy- flying they are according to (1) service requirement and (2) constructional features.

According to service requirement: A sub-station may be called upon to change voltage level or

improve power factor or convert a.c. power into d.c. power etc. According to the service

requirement, sub-stations may be classified into:

1. Transformer sub-station

2. Switching sub-stations

3. Power factor correction sub-stations

4. Frequency changer sub-stations

5. Converting sub-stations

6. Industrial sub-stations

According to constructional features: A sub-station has many components (e.g. circuit

breakers, switches, fuses, instruments etc.) which must be housed properly to ensure continuous

and reliable service. According to constructional features, the sub-stations are classified as:

i. Indoor sub-station

ii. Outdoor sub-station

iii. Underground sub-station

iv. Pole-mounted sub-stations

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4.3.1.1 Indoor Sub-station

For voltage up to 11kv the equipment of the substation is installed indoor because of economic

consideration .However when the atmosphere is contain with impurities these substation can be

erected for voltage up to 440 v

4.3.1.2 Transformer Sub-stations

The majority of the sub-stations in the power system are concerned with the changing of voltage

level of electric supply. These are known as transformer sub-stations because transformer is the

main component employed to change the voltage level. Depending upon the purpose served,

transformer sub-stations may be classified into

1) Step-up sub-station

2) Grid sub-station

3) Secondary sub-station

4) Distribution sub-station

Step-up Substation: Step-up substation is being situated at end of the generator side.

Grid Substation: From the step-up substation, electric power at 400KV, 220KV or 132KV

etc. is transmitted by 3-phase. 3 wire overhead system to the outskirts of the city. Here electric

power is received by the primary grid substation which reduces the voltage level to 66KVor 33

KV or any another type for secondary transmission. Generally grid substation is of outdoor type.

Secondary Substation: From the grid substation, electric power is transmitted at 66 KV or 33

KV or another type by 3-phase, 3-wire system to various secondary substations located at the

strategic points in the city. At a secondary substation the voltage is further stepped down to 11

KV. The 11 KV lines run along the important road sides of the city. It may be noted that big

consumers are generally supplied power at 11 KV for further handling with their own

substations. The secondary type substations are also called outdoor type substations.

Distribution Substation: The electric power from 11 KV lines is delivered to distribution

substations. These substations are located near the consumers localities and to 400V or 230V. 3

phase or 4 wire for supplying to the consumers. The voltage between any two phases is 400V

and the voltage between any phase and neutral is 230V.

4.3.2 Requirements of Installing a Substation:

i. It should be located at a proper site. As far as possible, it should be located at the centre

of gravity of load.

ii. It should provide safe and reliable arrangement. For safety, consideration must be given

to the maintenance of regulation clearances, facilities for carrying out repairs and

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maintenance, abnormal occurrences such as possibility of explosion or fire etc. For

reliability, consideration must be given for good design and construction, the provision of

suitable protective gear etc.

iii. It should be easily operated and maintained

iv. It should involve minimum capital cost.

4.3.3 Equipment of the Substation:

The equipments required for a substation which depends on the type of the substation and service

requirements. However a transformer has the following equipments. These equipments are

described in the given below:

4.3.3.1 Bus-bar

When a number of generator or feeders operating at the same voltage have to be directly

connected electrically, bus-bar are used as the common electrical component. Bus-bars are

copper rods or thin walled tubes and operated at constant voltage. Thus electrical bus bar is the

collector of electrical energy from one location.

The selection of any bus bar system depends upon the following-

1. Amount of flexibility required in operation

2. Immunity from total shut-down

3. Initial cost of the installation

4. Load handled by the bus-bar

Classification of bus-bar

1. Single bus bar system.

2. Sectionalized bus bar.

3. Duplicate bus bar.

4. Ring bus bar.

5. One and half breaker arrangement

4.3.3.1.1 Single bus-bar system

As the name suggests, it consists of a single bus-bar and all the incoming and outgoing lines are

connected to it. The chief advantages of this type of arrangement are low initial cost, less

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Figure4.1: Insulator

maintenance and simple operation. However, the principal disadvantage of single bus-bar system

is that if repair is to be done on the bus-bar or a fault occurs on the bus,

4.3.3.1.2 Duplicate bus-bar system

This system consists of two bus-bars, a main bus-bar and a spare bus-bar. Each bus-bar has

the capacity to take up the entire sub-station load. The incoming and outgoing lines can be

connected to either bus-bar with the help of a bus-bar coupler which consists of a circuit breaker

and isolators. Ordinarily, the incoming and outgoing lines remain connected to the main bus-bar.

However, in case of repair of main bus-bar or fault occurring on it, the continuity of supply to the

circuit can be maintained by transferring it to the spare bus-bar. For voltages exceed- in 33kV,

duplicate bus-bar system is frequently used.

4.3.3.1.3 Materials of bus-bars

Copper and aluminum are used for bus bars. Copper being scare and costly, aluminum is being

increasingly used for bus bars. The material used for bus bars should have low resistivity, higher

softening temperature, good mechanical properties and low cost. Now aluminum is being

increasingly used for various switchgear applications. While using aluminum for bus-bars, the

difficulties arise due to following aspects:

1. Higher resistivity hence associated problems of temperature rise.

2. Lower tensile strength than copper

3. Lower thermal conductivity than copper.

4. Higher coefficient of linear expansion than copper.

5. Higher joint resistance and associated problems about jointing

6. Special welding techniques are necessary.

4.3.3.2 Insulators

We know that the insulator serves two purposes one

is they support the conductors (or bus-bars) and

confine the current to the conductors. In my

internship I have seen porcelain insulator which is the

most commonly used material for the manufacture of

insulators is porcelain. There are several types of

insulators (e.g. pin type, suspension type, post

insulator etc.). And their use in the substation will

depend upon the service requirement. For example,

post insulator is used for bus-bars. A post insulator

consists of a porcelain body, cast iron cap and flanged

cast iron base. The hole in the cap is threaded so that

bus bar can be directly bolted to the cap.

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4.3.3.3 Isolating Switches

In substation, it is often desired to disconnect a part of the system for general maintenance and

repairs. This is accomplishing by an isolating switch or isolator. An isolator is essentially a knife

switch which is designed to open a circuit under no load. In other words, isolator switches are

operated only when the lines in which they are connected carry no current.

4.3.3.4 Circuit Breaker or Auto Circuit Recloser

A circuit breaker is equipment which can open or close a circuit under normal operation as well

as fault condition. In Comilla PBS-

1 substation, all the circuit breaker

is auto circuit recloser which

contains SF6 gas. In closed

position of the breaker, the contacts

remain surrounded by SF6 gas and

the pressure of the gas is 2.8

kg/cm. When the breaker operates

the pressure of the gas is goes to

14kg/cm. We know that the high

pressure flow of SF6 can absorbs

the free electron easily. During my

internship I know that the pressure

of the SF6 circuit breaker are Figure4.2: Auto Circuit Recloser

2.8 kg/cm in normal condition of the circuit breaker and the abnormal condition this pressure

rise to the 14kg/cm. If the pressure goes down then the circuit breaker do not able to do proper

work and it may be risky for the whole system and also it is dangerous to system protection. This

kind of circuit breaker is costly because of the high cost of the SF6 gas. But it is environment

friendly because it cannot deposit the high amount of carbon. And it also low maintenances cost

and the light foundation requirement. Over all in this grid substation has ensure that the

protection is valid and it is user friendly.

4.3.3.5 Power Transformers

A power transformer is used in a substation to step- up or step down the voltage. It is important

part for a substation. I have also mentioned that power transformer is required for any type of

substation. Except at the power station, all the subsequent sub- station use step- down

transformers to gradually reduce the voltage of electric supply and finally deliver it at utilization

voltage. The modern practice is to use 3-phase transformers in substation, although 3 single

phase bank of transformers can also be used. The use of 3-phase transformer permits two

advantages. Firstly, only one 3 phase load tap changing mechanism can be used. Secondly, its

installation is much simpler that the three single phase transformers. I know from my internship

P a g e | 3 3

the power transformer is gradually installed upon lengths of rails fixed on concrete slabs having

foundation 1 to 1.5 m deep. For rating up to 10 MVA, naturally cooled, and also forced air

immersed transformers are used. For higher ratings, the transformers are generally normal and

forced air cooled.

4.3.3.6 Instrument Transformation

In the grid substation, the main lines in sub-station operates at high voltage and carry current of

thousand of amperes which we have seen. The measuring instruments and protective devices are

designed for low voltage (generally 110V) and currents (about 5A). Therefore, they will not

work satisfactorily if mounted directly on the power lines. This difficulty is overcome by

installing instrument transformers on the power lines. The function of these instruments of these

instrument transformers is to transfer voltage or currents in the power lines to values which are

convenient for the operation of measuring instruments and relays. There are two types of

instrument transformation viz.

1. Current Transformer (C.T)

2. Potential Transformer (P.T)

4.3.3.6.1 Current Transformer (C.T.)

A current transformer in essentially a step-up transformer which steps down the current to a

known ratio. The primary of this transformer consists of one or more turns of fine wire and

provides for the measuring instruments and relays a current which is a constant fraction of the

current in the line. Suppose in the Comilla PBS-1 substation, a current transformer rated at 100:5

A is connected in the line to measure current. If the current in the line is 100 A, then current in

the secondary will be 5A. Similarly, if current in the line is 50A, then secondary of C.T. will

have a current of 2.5 A. Thus the C.T. under consideration will step down the line current by a

factor of 20.

4.3.3.6.2 Voltage Transformer (P.T.)

It is essentially a step down the voltage to a known ratio. The primary of this transformer

consists of a large number of turns of fine wire connected across the line. The secondary winding

consists of a few turns and provides for measuring instruments and relays a voltage which is a

known fraction of the line voltage. Suppose in the Comilla PBS-1 substation, a potential

transformer is rated at 132KV/33KV is connected to a power line. If line voltage is 132KV, then

voltage across the secondary will be 33KV.

4.3.3.7 Fuse

A fuse is a short pitch of metal, inserted in the circuit, which melts when excessive current flows

thought it i.e. Fuse is a simplest current interrupting devices for protection from excessive

P a g e | 3 4

current. It is used for overload and or short circuit protection in medium voltage (into 650) and

low voltage (up to 400v) installations

4.3.3.7.1 Fuse characteristic

1. Low melting point.

2. High conductivity.

3. Least deterioration due to oxidation.

4. It carries the normal current without overheating.

4.3.3.7.2 Properties of reliable fuse

It must be remembered that fuse wire is always connected to the line wire. Fuse must be of

correct value. For example, in a 15A circuit, the fuse wire used is thick and of 15A rating.

4.3.3.8 Optimum component protection

Fuses reduce short circuit (fault) currents that flow to a low value by "current limitation". There

is no need for complex short circuit calculations and no concerns about costly future upgrades

due to system expansion with increased fault currents. Their compact size offers low cost over-

current protection for the highest short circuit levels.

Safety: Fuses do not produce gas, flames, arcs or other materials when clearing any value of

over-current up to the highest level of short circuit. In addition, the speed of operation on high

short circuit currents limits significantly the flash hazard at the fault location.

Reliability: No moving parts to wear out or become contaminated by dust, oil or corrosion and

no nuisance tripping. If a fault occurs, the fuse immediately operates in its predetermined manner

or co-ordinates with other circuit components. The cause of the fault is then ascertained,

corrected and a new fuse fitted. Fuse replacement ensures protection is restored to its original

state of integrity. It should be stressed that the time taken for the replacement is very small in

relation to the fault correction.

Simple co-ordination: Standardized fuse characteristics and a high degree of current limitation

ensure that there is simple and effective co-ordination between fuses and with other devices.

4.3.3.9 Earthling switch

Earthing switch is connected between the line conductor and earth. Normally it is open. When

the line is disconnected, the earthing switch is closed so as to discharge the voltage trapped on

the line. Though the line is disconnected, there is some voltage on the line to which the

capacitance between line and earth is charged. This significant in high voltage system. Before

starting the maintenance work these voltages are discharged to earth by closing the earthing

switch.

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4.3.3.10 Neutral grounding (Earthing)

The three phase 50Hz AC power systems with neutral grounding at every voltage level are used

for generation, transmission, distribution and utilization. The neutral points (star points) of star

connected 3 phase winding of power transformers, generators, motors, earthing transformers are

connected to low resistance ground. Such a connection is called Neutral grounding

4.3.3.11 Types of grounding

1. Underground system: It is used no more. The neutral is not connected to earth also called

insulated neutral system. In Standard Group this system is used.

2. Solid Grounding: The neutral is directly connected to ground without any intentional

impedance between neutral and ground. The coefficient of earthing is less than 80% for such

systems.

3. Reactance Grounding: Reactance is connected between neutral and ground.

4. Resonant Grounding: an adjustable reactor of correctly selected value to compensate the

capacitive earth currents is connected between neutral and earth. The coil is called Peterson coil

or Arc suppression coil or earth fault neutralizer.

4.3.3.12 Power factor

The cosine of angle between voltage and current in and a c circuit is known as power factor.

4.3.3.12.1 Methods of power factor improvement

Normally, the power factor of the whole load on a large generating station is in the region of 0.8

to 0.9. However, sometimes it is lower in such cases It is generally desirable to take special steps

to improve the power factor. This can be achieved by the following equipments

a) Static capacitor

b) Synchronous condenser

c) Phase advancers

4.3.3.12.1.1 Static capacitor

The power factor can be improved by connecting capacitor in parallel with the equipment

operating at lagging power factor. The capacitor draws a leading current and partly or completely

neutralizes the lagging reactive component of load current. This raises the power factor of the

load. For three phase loads, the capacitor can be connected in delta or star as shown in fig. Static

capacitors are invariably used for power factor improvement in factories.

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4.3.3.12.1.2 Synchronous condenser

A synchronous motor takes a leading current when over excited and therefore, behaves as a

capacitor. And over excited synchronous motor running on no load is called synchronous

condenser. When such a machine is connected in parallel with the supply, it takes a leading

current which partly neutralizes the lagging reactive component of the load. Thus the power

factor is improved.

4.3.3.12.1.3 Phase advancers

Phase advancers are used to improve the power factor of induction motors. The low power factor

of an induction motor is due to the fact that its stator winding draws exciting current which lags

behind the supply voltage by 90. If the exciting ampere turns can be provided from some other

a.c. source, then the stator winding will be relieved of exciting current and the power factor of

the motor can be improved. This job is accomplished by the phase advancer which is simply an

a.c. exciter. The phase advancer is mounted on the same shaped as the main motor and is

connected in the rotor circuit of the motor. It provides exciting ampere turn to the rotor circuit at

slip frequency. By providing more ampere turns then required, the induction motor can be make

to operate on leading power factor like an over excited synchronous motor.

The electrical energy is almost exclusively generated, transmitted and distributed in the form of

alternating current. Therefore the question of power factor immediately comes into picture. Most

of the loads are inductive in nature and hence have low lagging power factor. The low power

factor is highly undesirable as it causes an increase in current, resulting in additional losses of

active power in all the elements of power system from power station generator down to the

utilization devices. In order to ensure most favorable conditions for a supply system from

engineering and economical standpoint, it is important to have power factor as close to unity as

possible.

4.3.3.12.1.4 Calculating power factor

As was mentioned before, the angle of this power triangle graphically indicates the ratio

between the amount of dissipated (or consumed) power and the amount of absorbed/returned

power. It also happens to be the same angle as that of the circuit's impedance in polar form.

When expressed as a fraction, this ratio between true power and apparent power is called the

power factor for this circuit. Because true power and apparent power form the adjacent and

hypotenuse sides of a right triangle, respectively, the power factor ratio is also equal to the cosine

of that phase angle. Using values from the last example circuit:

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Power Factor =True Power

Apparent Power

Power Factor =119.365W

169.26VA

Power Factor = 0.705

45.152 = 0.705

4.3.3.12.1.5 Power factor correction

Power factor correction is the method to reduce the lagging power factor in inductive loads by

fixing a high value capacitor across the phase and neutral close to the load. When the Voltage

and Current are in phase with each other in an AC circuit, the energy from the source is fully

converted into another form to drive the load and in this case power factor is in unity. When the

power factor drops, the system becomes less efficient. As a rule a drop from unity to 0.9 in the

power factor increases the current requirement to 15% or more.

A power factor of 0.7% increases the power requirement to around 40%. This is much severe in

the case of inductive loads such as Motors, Refrigerators, Inverters etc. In these inductive loads,

current lags the voltage leading to lagging power factor. But opposite condition occurs if

current leads the voltage. This is called leading power factor. Power factor correction is the

method to reduce the lagging power factor in inductive loads by fixing a high value capacitor

across the phase and neutral close to the load. These capacitors have leading power factor so that

it will neutralize the lagging power factor of the load.

4.3.3.12.1.5.1 Power factor correction has the following advantages

a) Load becomes more efficient.

b) It prevents the wastage of energy due to heat generation.

c) Maintains voltage stability.

4.3.3.12.1.5.2 The advantages of power factors improvement

a) Reactive component of the network is reduced and so also the total current in the system from

the source to end.

b) Power losses are reduced in the system because of reduction in current.

c) Voltage level at the load end is increased.

d) KVA loading on the source generators as also on the transformers and lines up to the

capacitors reduces giving capacity relief. A high power factor can help in utilizing the full

capacity of your electrical system.

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4.4 Substation of Comilla Palli Bidyut Samity-1

Figure4.4: Comilla PBS-1 Single Line Diagram

P a g e | 3 9

In figure 4.4 Comilla PBS-1 single line diagram firstly Comilla PBS gets power from Power

Grid Company of Bangladesh Limited (PGCB) and Summit Power Limited. PGCB and Summit

supply 33KV in the auto circuit recloser then substation modify it and distribute it 11KV.

4.4.1 Circuit Breaker

In Comilla PBS-1 use auto circuit recloser (ACR) instead of SF6 circuit breaker. The rating of

this ACR is 360 ampere and 33KV

In this substation use two ACR one in the beginning and other in the ending point of substation.

Both of them use for protection of the substation equipments though they use it as like switch.

4.4.2 Lightning Arrester

Lightning arrester use for protect from over voltage generally from thunder. Here lightning

arrester rating is 36KV.

4.4.3 CT and PT

Current Transformer (CT) and Potential Transformer (PT) use for measuring current and voltage

in high voltage and current. If we want to calculate higher voltage and current directly we will

need bigger measuring instrument which will be higher cost. For this reason we use CT and PT.

Here Current Transformer (CT) ratio is 600:5

Potential Transformer (PT) ratio is 6350:240

4.4.4 Fuse

Fuse generally use for protect from over current. In this substation use drop down fuse or air

break fuse. The rating of this fuse is 220Ampere in primary side and secondary side rating is

110Ampere.

4.4.5 Metering

By the meter we can measure present current,

voltage, power factor and maximum current,

voltage etc. There was analog meter now they

are using digital meter.

4.4.6 Isolator

Isolator is like a switch. It use after the circuit

breaker for ensure line are totally off mode.

Mainly it use while substation or line need

maintenance. Isolator has a manual system for

operation.

Figure4.5: Meter in Comilla PBS-1

Substation

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4.4.7 Power Transformer

Power Transformer is the main part of a substation. In Comilla PBS-1 use 10MVA power

transformer.

Figure4.6: Power Transformer in Comilla PBS-1

In Comilla PBS-1 they use three power transformer every of them 10MVA power. Transformer

can vary up to 5% voltage.

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Figure4.7: Power Transformer name plate

Transformer nameplates general requirements:

Following are the minimum information and Data which to be shown on a transformer nameplate

Name of manufacturer; Serial number; year of manufacture; Number of phases; KVA or MVA

rating; Frequency; Voltage ratings; Tap voltages; Connection diagram; Cooling class; Rated

temperature in C; Polarity (for Single Phase Transformers); Phasor or vector diagram (For

Polyphase or Three Phase Transformers); % impedance; Approximate mass or weight of the

transformer; Type of insulating liquid; Conductor material of each winding; Oil volume (of each

transformer Container/Compartment); Instruction for Installation and Operation

P a g e | 4 2

4.4.8 Voltage Regulator

Voltage regulator is a device which can voltage up or down in a limitation range according to

consumer demand. Actually voltage regulator is an auto transformer. In Comilla PBS-1 those

transformer we use they can voltage up or down up to 10%.

Figure4.8: Voltage regulator

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4.4.9 Feeder

Distribution feeder circuits are the connections between the output terminals of a distribution

substation and the input terminals of primary circuits. The distribution feeder circuit conductors

leave the substation from a circuit breaker. In Comilla PBS-1 have 9 feeders. Every feeder have

own controlling system and also there has a display which shows maximum current, voltage,

power factor and present current , voltage, power factor, the times its operation. It shows up to

one month data.

Figure4.9: Feeder control panel and display

4.4.10 Distribution

Distribution line is connected form the feeder. For different area needs different feeder. In

distribution system normally 11KV line up to 80% and other are 400 volt ore 230 volt in REB. In

our country maximum consumer need 400volt and 230 volt for this reason we use a transformer

11/0.4 KV or three single phase transformer bank for three phase. In REB they use only single

phase transformer because of lost cost and easy maintenance.

P a g e | 4 4

Chapter 5

Maintenance

Generally Comilla PBS-1 has three type of maintenance those are:

1) Substation maintenance

2) Line maintenance

3) Transformer maintenance

5.1 Substation Maintenance

Every time substation needs maintenance if any problem arises. Without problem substation

needs check all equipment of its quality and condition at least one time in a year.

Note: For maintenance purpose every time substation has to be shut down and line has to ground.

First of all we have to check circuit breaker whether it has any problem or will be any problem.

In the time of power transformer checking it have to be 1000M of insulation test in megger.

Grounding test must in 0.4M

5.2 Line Maintenance

There are many types of line maintenance:

Some of maintenance has to be done immediately which mean whenever problem creates it had

to be solved within four hours. Like over head line tear. For this type of problem immediately

line has to be shut down.

Some of maintenance can be done within 1-2 days like transformer problem.

Some of maintenance can be done within a week like pole or tower got slanted to ground also

trees came to the line.

5.3 Transformer Maintenance

In REB have two type of transformer maintenance.

One is single phase transformer and another is power transformer.

Thy repair single phase transformer in there PBS workshops. And power transformer repair in

central workshop at Savar in Dhaka. I visited both place and saw maintenance.

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5.3.1 Single phase transformer

If any transformer create problem it bring in the workshop and the worker see it what type of

problem. Workers open the transformer and take transformer oil into a separate tank. Then

separate every part from the transformer and mark every part by serial a number. If any

transformer core damage badly which is not possible to repair those type of transformer rejected

totally by workshop worker.

If any transformer coils damage any other problem thy repair it.

Now I am describing how the worker repairs a single phase transformer.

After marking transformer every part when they get time to repair it they take it.

The main part of a transformer is coil binding. In Comilla PBS-1 they bind coil manually by the

worker.

There have two type of coil one is primary coil and another is secondary coil. Secondary coil

strong and hard but primary coil is thin and weak. That is why secondary coil normally do not

damage. Primary coil damage normally and need to repair it. Here in figure5.1 I am showing a

high voltage or primary coil wire.

Figure5.1: Transformer primary coil wire.

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In this figure5.2 I am showing that the worker binding transformer coil.

Figure5.2: Coil binding

In this figure5.3 transformer coil after binding coil.

Figure5.3: complete coil binding

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Figure5.4: core binding of transformer

In figure5.4 worker binding core in coil. After binding coil they bind core in transformer then

they keep coil in heat chamber for absorbing moisture from the coil.

In figure5.5 transformer coil into the tank

Figure5.5: after coil and core binding in

the transformer.

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After all complete they give oil in the tank and connect with bushing.

Figure5.6: After repair complete single phase transformer

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5.3.2 Power transformer

Single phase transformer and power transformer maintenance procedure are same.

Figure5.7: power transformer without tank

Figure5.8: power transformer coil binding in Savar central workshop.

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Chapter-6

Transformer Test

6.1 Testing

A number of tests are required to physically determine the electrical characteristics of power and

distribution transformers. Many of those tests are indicated below:

1. Megger test (Insulation resistance test)

2. Winding resistance test

3. Ratio Test

4. No-load loss test

5. Full load loss & Impedance test

6. Power frequency high voltage test

7. Dielectric strength of oil

8. Polarity test

In REB workshop has done 6 tests for a transformer those are:

1. Megger test (Insulation resistance test)

2. Winding resistance test

3. Ratio Test

4. No-load loss test

5. Full load loss test

6. Impedance test

6.1.1 Megger test (Insulation resistance test)

The megger test is a measure of the insulation resistance. Remove solid connections of windings

to ground. The following tests are made using a 1000V megger meter:

Measure from the high-voltage winding to the low-voltage winding and ground. (Low-voltage

winding is grounded.) H-LG

Measure from the low-voltage winding to the high-voltage winding and ground. (High-voltage

winding is grounded.) L-HG Table No: 6.1

Measure from the high- and low-voltage windings to ground. HL-G SL NO

HT+ Earth LT + Earth HT+ LT Remarks

01 1000+ 1000+ 1000+ OK

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We use a megger meter to measure the insulation

resistance. It range is 0 to 1000+.Megger meter has

two terminal .At the starting of megger testing , we

connect two terminal to the body ( short) to set

megger pointer at 0 position.

Then we connect body to HV side.

1000. Then we connect one terminal to HT side

other terminal to LT side. Megger shows 1000. If

any fault occurs then in megger meter

not show 1000. 1000 means that insulation resistance

property is Ok of transformer. Then we record

megger readings on table6.1.

6.1.2 Winding resistance test

For measuring winding resistance test only a

Multimeter is need. In winding resistance test

LT side connection should be open. No voltage

source is required. Winding resistance only

measured in HT side.

At first two terminal of multi

connected to the HT A & B terminal. Then turn

on the multimeter & set it pointer in Ohm for

measuring resistance. Wait for few second.

Result will show in multimeter display.

After measuring resistance in HT A

we measure HT B & C terminal winding resistance. T

output result is shown in multimeter in fig

Fig 6.1.3: Multi-meter connection & output result for Winding resistance test

We use a megger meter to measure the insulation

to 1000+.Megger meter has

two terminal .At the starting of megger testing , we

connect two terminal to the body ( short) to set

Then we connect body to HV side. In megger shows

1000. Then we connect one terminal to HT side &

other terminal to LT side. Megger shows 1000. If

ult occurs then in megger meter, pointer will

1000. 1000 means that insulation resistance

property is Ok of transformer. Then we record

Figure 6.1.1:

Winding resistance test

For measuring winding resistance test only a

Multimeter is need. In winding resistance test

LT side connection should be open. No voltage

source is required. Winding resistance only

t two terminal of multi-meter is

& B terminal. Then turn

ultimeter & set it pointer in Ohm for

ew second. Figure6.1.2: Multimeter display show value

display.

After measuring resistance in HT A & B terminal, then we measure HT A & C terminal. And last

C terminal winding resistance. The process is shown in fig

is shown in multimeter in figure6.1.4.Then the result is recorded in result sheet.

meter connection & output result for Winding resistance test

.1.1: Megger meter

Figure6.1.2: Multimeter display show value

C terminal. And last

he process is shown in figure 6.1.3. The

ecorded in result sheet.

meter connection & output result for Winding resistance test

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After measuring the winding resistance HT A & B, B & C , C & A terminal in 200KVA

transformer .

Fig 6.1.4: Winding resistance test result form for 200KVA transformer

6.1.3 Ratio Test

T