Golis University

Faculty of engineering
Department of Electrical power Engineering

Entiteled: TRANSFORMER PROTECTION

Prepared By:
1. Mohamed Jama Dahir
2. Abdalla Faisal Osman
3. Abdiqani Muse Hassan
4. Ibrahim Mahdi Essa

Supervisor:
Eng Mohamed AbdiAziz Mohamed

AUGUST 2021

I
 GOLLIS UNIVERSITY

FACULTY OF ENGINE EERING

DEPARTMENT OF ELECTRICAL ENGINEERIG

Final year project submitted in fulfillment of the requirements

for the Degree of the BSc (Honor) in Electrical power
Engineering

TRANSFORMER PROTECTION

Prepared By:
Mohamed Jama Dahir

Abdalla Faisal Osman

Abdiqani Muse Hassan

Ibrahim Mahdi Essa

SUPERVISOR:

A Research Project Submitted In Partial Fulfillment of the

Requirements for Degree of Bachelor of

Electrical Engineering

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 Transformer Protection

CERTIFICATE OF ORIGINALITY

Gollis University as partial fulfillment of the requirement of the Bachelor of

Science information technology has not been submitted as an exercise for a

degree of any other universities. I also clarify that the work described here is

entirely my own except for excerpts and summaries whose sources are

appropriately cited in the references.

This report may be made available within university library for the purpose of

consultation.

DATE

Abdalla Faisal Osman [16118]

Abdikani Mouse Hassan [ ]

Mohammad jama Dahir [ ]

Ibrahim Mahdi Osamn [ ]

Signature: ------------------------

iii
 Transformer Protection

APPROVAL SHEET

This thesis entitled:

Transformer protection

Prepered by:

Abdalla Faisal Osman [16118]

Abdikani Mouse Hassan [ ]

Mohammad jam Dahir [ ]

Ibrahim Mahdi Osamn [ ]

In requirement for the degree of bachelor of electrical engineering, Gollis

University has been accepted

SUPER VISOR…………………

SIGNATURE……………………

DATE…………………….

DECLARATION OF THESIS

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 Transformer Protection

Transformer protection
Title of thesis

We, (MOHAMED JAMA DAHIR, ABDALLE FAISAL OSMAN,

ABDIQANI MUSE HASSAN,IBRAHIM MAHDI ESSA)

Hereby declare that the thesis is based on my original work except for quotations and
citations which have been duly acknowledged. I also declare that it has not been previously or
concurrently submitted for any other degree at (GU) or other institutions.

Witnessed by

________________________________ __________________________

Signature of Author Signature of Supervisor

Permanent address: Name of Supervisor

ENG:MOHAMED ABDIAZIZ

Date : _____________________ Date : __________________

Dedication

v
 Transformer Protection

We dedicated in this research thesis to our dear families specially our beloved
parents for their
Endurance, support, and encouragement throughout our study period.
Abdalla Faisal Osman
I would like to dedicate this thesis to my family specially my beloved parents
Daddy. Faisal
Osman Jama and Mom: Sahra abdi Abdilahi who had raised me to be the
person I am
Today. Also I like to dedicate my beloved friend Suhayb Osman Ahmed who
support me
Financially, morally and encouraged me during my study.

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 Transformer Protection

ACKNOWLEDGEMENT

First of all, Thanks to Allah who allows for completing this research
thesis book?

We would like to thank our supervisor Eng.Mohammed Abdi asis,

For providing tremendously supportive environment for our
research, through his always o hand during this research, guided
feedback, thoughts and ideas , and whenever we needed help and
moral support.

We would like to thank our classmates and teachers for always been
with us during observation and collection of questions.

We also would like to thank Eng khaddar jaamac, head of

department of electrical engineering. Gollis University for his
assistance towards the success full completion of this book.

Last but not least, we wish to thank our dear families, colleagues
and friends who have not mentioned here for their full financial and
technical support during our study. Especially to our much loved
fathers, mothers and sisters and brothers who gave us financial,
moral and spiritual aid

Abstract

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 Transformer Protection

Transformers are the most superior unit in power system, it consists of

several parts such as Windings, core and oil tank, each one of these parts

must be protected from

Abnormal condition and faults. Several of protection functions and principles

must be discussed and understand for strong Transformer protection schemes.

Transformer protection schemes were carried out by collecting Transformer and

protection data, for analyzing these schemes and their simulation using ETAP

program and protection relays.

Different scenarios of abnormal conditions and faults were created by using

ETAP Simulation to study and analyses. Same thing is said to protection relay,

different current and voltage values were injected to simulate the abnormal

conditions in order to display the relay response time. Transformer protection

system were studied and simulated.

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 Transformer Protection

TABLE OF CONTENTS

Table of Contents
Declaration................................................................................................................................vi
Dedication................................................................................................................................vii
ACKNOWLEDGEMENT.....................................................................................................viii
List of acronyms........................................................................................................................x
List of tables..............................................................................................................................xi
LIST OF FIGURES................................................................................................................xii
Abstract...................................................................................................................................xiv
Chapter one................................................................................................................................2
1.0 Introduction.........................................................................................................................2
1.1 Background of the study.....................................................................................................2
1.1.0: History of the transformer.............................................................................................3
1.2. Problem statement..............................................................................................................3
1.3. Purpose of the study...........................................................................................................5
1.4. Objective of the study........................................................................................................6
1.5. Limitation of the study.......................................................................................................6
1.6. Scopes..................................................................................................................................7
1.7. The significance of the study.............................................................................................7
1.8. Research question...............................................................................................................8
Chapter 2 Literature review.....................................................................................................8
2.1 introductions........................................................................................................................8
2.2 types of transformer............................................................................................................9
2.2.0. Step-Down Transformer...............................................................................................10
2.2.1. Step-Up Transformer....................................................................................................13
2.3. Operation of Transformers.............................................................................................14
2.4. Cooling System of transformer.......................................................................................15
2.4.0 Transformer Cooling Methods..............................................................................17
2 ) Oil immersed transformer................................................................................................18
2.5. Maintenance of Transformer..........................................................................................22
2.5.0. Maintenance Actions...............................................................................................23
2.6 Transformer protection....................................................................................................24
1) Buchholz (gas) relay............................................................................................................26

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2) Pressure Relay....................................................................................................................30
3 ) DMCR protection relay.....................................................................................................32
2.7 Protection components......................................................................................................37
CHAPTER 3 METHADOLOGY..........................................................................................42
3.1 introductions......................................................................................................................42
3.2 Differential Protection Scheme in a Power Transformer...........................................42
Principle of Differential Protection......................................................................42
3.3 Differential Protection of a Transformer........................................................................44
3.4 Connection for Differential Protection for Transformer..............................................45
3.5 Working of Differential Protection System....................................................................45
3.6 Problem Associated with Differential Protection System..............................................46
3.7 Transformer Protection for Different Types of Transformers.....................................46
Table 3.1. The protection system used for a power transformer according to their
category....................................................................................................................................47
3.8 Common Types of Transformer Protection....................................................................47
3.8.1. Overheating Protection in Transformers....................................................................48
3.8.2. Over current Protection in Transformer....................................................................50
3.8.3. Differential Protection of Transformer.......................................................................50
3.8.4. Transformer Differential Protection Working:.........................................................51
3.8.5. Restricted Earth Fault Protection...............................................................................52
Buchholz (Gas Detection) Relay...........................................................................53
3.8.6. Over-fluxing Protection................................................................................................56
CHAPTER 4 RESULTS.........................................................................................................58
4.1 system profile.....................................................................................................................58
Power Transformer Protection Systems................................................................58
Table 4.1. Failures can be detected with corresponding protection..................................61
Single phase over current, ground fault and tank ground-fault protection......................62
Table 4.2. Different transformer protection systems...........................................................62
Differential Protection of Power Transformer.....................................................62
Transformer Differential Protection Principle.....................................................63
4.2 systems Design...................................................................................................................65
Table 4.3 transformer description.........................................................................................66
4.3 problems arising in differential protection in power system (transformer)..........68
CHAPTER 5 CONCLUSION AND RECOMMINDATION..............................................69
5.1 Conclusion..........................................................................................................................69

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 Transformer Protection

5.2 Recommendation...............................................................................................................69
REFERENCES........................................................................................................................71

List of acronyms
VT /PT Voltage Transformer / Potential Transformer

A/D Analog / Digital

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 Transformer Protection

DFT Discrete Fourier Transform

DSP Digital Signal Processing

AF Air Forced

AN Air Natural

ONAN Oil Natural Air Natural

ONAF Oil Natural Air Forced

OFAF Oil Forced Air Forced

AFWF Air Forced Water Forced

HV High Voltage

LV Low Voltage

MVA Mega Volt Ampere

KVA Kilo Volt Ampere

EMF Electromotive Force

CT Current Transformer

DCMR Detection Measurement and Control Relay

AC Alternating Current

DC Direct Current

IP Primary Current

IS Secondary Current

VP Primary Voltage

VS secondary voltage

NP primary turning

NS secondary turning

IDMT inverse definite minimum time

I /O Input / output

List of tables

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 Transformer Protection

TABLE 3.1. THE PROTECTION SYSTEM USED FOR A POWER TRANSFORMER

ACCORDING TO THEIR CATEGORY-------------------------------------------------------------------
TABLE 4.1. FAILURES CAN BE DETECTED WITH CORRESPONDING PROTECTION.----------------
TABLE 4.2. DIFFERENT TRANSFORMER PROTECTION SYSTEMS-------------------------------------
TABLE 4.3 TRANSFORMER DESCRIPTION-----------------------------------------------------------------

LIST OF FIGURES

FIGURE 1.0 POWER TRANSFORMER--------------------------------------------------------------------------

1.1.1: HISTORY OF THE TRANSFORMER--------------------------------------------------------------------
FIGURE 2.1: POWER TRANSFORMER.-----------------------------------------------------------------------
FIGURE 2.2: A STEP-DOWN TRANSFORMER.-------------------------------------------------------------
FIGURE 2.3: A STEP-DOWN TRANSFORMER.-------------------------------------------------------------
FIGURE 2.4: STEP-UP TRANSFORMER----------------------------------------------------------------------
FIGURE 2.5: STEP-UP TRANSFORMER----------------------------------------------------------------------
FIGURE 2.6: OPERATION OF TRANSFORMER-------------------------------------------------------------
FIGURE 2.7: COOLING SYSTEM OF TRANSFORMER----------------------------------------------------
FIGURE 2.8: FORCED AIR COOLED TRANSFORMER.----------------------------------------------------
FIGURE 2.9: OIL NATURAL AIR NATURAL COOLING OF TRANSFORMER-------------------------
FIGURE 2.10: OIL NATURAL AIR FORCED COOLING OF TRANSFORMER----------------------------
FIGURE 2.11: OIL FORCED AIR FORCED COOLING METHOD.-----------------------------------------
FIGURE 2.12: OIL FORCED WATER FORCED COOLING OF TRANSFORMER-----------------------
FIGURE 2.13: MAINTENANCE OF TRANSFORMER-------------------------------------------------------
FIGURE 2.13: TRANSFORMER PROTECTION .-----------------------------------------------------------
FIGURE 2.14: THE BUCHHOLZ GAS RELAY.-------------------------------------------------------------
FIGURE 2.15: THE BUCHHOLZ GAS RELAY.-------------------------------------------------------------
FIGURE 2.16: FUNCTION OF THE BUCHHOLZ RELAY:-------------------------------------------------
FIGURE 2.17: THE EXAMPLE OF OIL LOSS DUE TO LEAKAGE.----------------------------------------
FIGURE 2.18: FUNCTION OF THE BUCHHOLZ RELAY:-------------------------------------------------
FIGURE 2.19: PRESSURE RELAY WITH ITS COMPONENT.----------------------------------------------
FIGURE 2.20: PRESSURE RELIEF DEVICE.----------------------------------------------------------------
FIGURE 2.21: PRESSURE RELIEF DEVICE.----------------------------------------------------------------
FIGURE 2.22: DMCR PROTECTION RELAY.-------------------------------------------------------------
FIGURE 2.23: THE NORMAL CONDITION OF THE DMCR RELAY.-----------------------------------
FIGURE 2.24: THE PRESSURE CAPTOR UNIT. III.--------------------------------------------------------
FIGURE 2.25: THE ADJUSTABLE PRIMARY AND SECONDARY THERMOSTATS.---------------------
FIGURE 3.1: DIFFERENTIAL PROTECTION----------------------------------------------------------------
FIGURE 3.2: CONNECTION OF DIFFERENTIAL PROTECTION------------------------------------------
FIGURE 3.3: OVER HEAT PROTECTION-------------------------------------------------------------------
FIGURE 3.4: TEMPERATURE THERMOMETER------------------------------------------------------------
FIGURE 3.5: DIFFERENTIAL PROTECTION OF TRANSFORMER----------------------------------------
FIGURE 3.6: DIFFERENTIAL PROTECTION WORKING--------------------------------------------------
FIGURE 3.7: BUCHHOLZ RELAY---------------------------------------------------------------------------
FIGURE 3.8: GAS BUCHHLOZ GAS DETECTION RELAY-------------------------------------------------
FIGURE 3.9: GAS BUCHHLOZ GAS DETECTION RELAY-------------------------------------------------
FIGURE 3.10: THE BUCHHOLZ GAS RELAY.-------------------------------------------------------------

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 Transformer Protection

FIGURE 4.1: THE TRANSFORMER SUBSTATION----------------------------------------------------------

FIGURE 4.2. TRANSFORMER DIFFERENTIAL PROTECTION DIAGRAM-------------------------------
FIGURE 4.3. DIFFERENTIAL . TRANSFORMER PTOTECTIOM ETAP------------------------------------
FIGURE 4..4. DIFFERENTIAL . TRANSFORMER PTOTECTIOM ETAP-----------------------------------

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 Transformer Protection

Chapter one

1.0 Introduction
This chapter discusses the background to the study related to transformer

protection, the statement of the problem, purpose of the study, limitation of

the study.

1.1 Background of the study

The history of electrical power technology throughout the world is one

of steady and, in recent years, rapid progress, which has made it possible to

design and construct economic and reliable power systems capable of

satisfying the continuing growth in the demand for Electrical energy. In this,

power system protection and control play significant part, and Progress in

design and development in these fields has necessarily had to keep pace with

Advances in the design of primary plant, such as generators, transformers,

switchgears, Overhead lines and underground cables, indeed, progress in the

fields of protection and Control is a vital prerequisite for the efficient operation

and continuing development of Power supply systems as whole.

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 Transformer Protection

Figure 1.0 power: transformer

1.1.0: History of the transformer

A transformer refers to a device converts electrical energy from one circuit

then onto the next through inductively coupled circuits. A transformer is

accustomed to bringing voltage up or down in an AC electrical circuit. A

transformer can be utilized to change over AC power to DC power with help of

rectifiers.

There are transformers everywhere in each house; they are inside the dark

plastic case, which

You connect to the socket to energize your mobile phone or different gadgets.

These sorts are frequently called "divider warts." They can be substantial, as

in national utility systems, or it can be little implanted inside hardware.

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 Transformer Protection

The property of induction was found in the 1830's yet it was not until 1886

that William Stanley, working for Westinghouse manufactured the first solid

business transformer. His work was based on some simple plans by the Gang

Company in Hungary (ZBD Transformer 1878), and Lucien Gaillard and John

Dixon Gibbs in England. Nikola Tesla did not concoct the transformer as a few

questionable sources

Have asserted. The Europeans specified above did the first work in the field.

George Westinghouse, Albert Schmid, Oliver Shallenberger and Stanley made

the

1.2. Problem statement

Power transformers play a significant role in the power grid. Power

transformers provide the majority of electric power, power transformers

widely used in wind power generation. Losing power transformer can greatly

impact the stability and the reliability of the power system.

Thus, developing a secure and reliable method to protect power

transformers can bring huge benefits to the grid. As one of the most

expensive and important power apparatus in the power grid, power

transformers should be protected securely and dependably.

Generally, the faults in oil immersed transformers can be divided in to

two types of faults, namely incipient faults and major faults .The incipient

fault are like: short turn and windings, short between phase to phase, open

circuit.

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 Transformer Protection

Abnormal operating condition protection means the protection of the power

transformer

when it is experiencing abnormal operating condition, e.g. abnormal frequenc

y, over excitation, under excitation, out-of-step, etc.

Legacy protective functions are being used to provide fault protection, such as

differential protection, negativesequence protection, instantaneous/time overc

urrent protection, and distance protection However, the above mentioned lega

cy protective functions can only cover stator ground fault up to 85-90% of the

winding. Legacy protective functions, such as differential protection, negative-

sequence protection, instantaneous/time overcurrent protection, are designed

to detect internal fault of induction generators. For the power transformer

protection usually there are more than ten protective functions used to protec

t the device and they need to coordinate with each other. Present protective

technology has evolved from conventional electromechanical relays to

numerical relays. Compared with electromechanical relays, numerical relays

are microprocessor based digital devices, which analyze power system

voltages, currents and other quantities for the purpose of detection of faults

in the power system. The characteristics and behaviors of several protective

functions can be programmed together into one numerical relay. This is

definitely a big step for the development of protective relays.

For the power transformer protection, usually there are more than ten

protective functions used to protect the device and they need to coordinate

with each other. This coordination is very complicated and it may bring

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human or setting errors when facing faults. Furthermore, the protective

functions that are being used in the numerical relays now still mimic the

concepts of electromechanical relays.

They are basically direct comparisons between the pre-defined settings

and real- time measurements after simple calculation. Since the advanced

microprocessors are being utilized now, they have the capability to perform

high-speed complicated calculation. This is a huge potential that has not been

fully explored yet so that a new revolution in protective relays could be

started to seek the full potential of numerical relays.

1.3. Purpose of the study

Transformers are one of the most critical and expensive components of any

distribution system. It is an enclosed static device usually drenched in oil, and

hence faults occurring to it are limited. But the effect of a rare fault can be

very dangerous for the transformer, and the long lead time for repair and

replacement of transformers makes things even worse. Hence power

transformers protection becomes very crucial.

Faults occurring on a transformer are mainly divided into two types, which

are, external faults and internal faults, to avoid any danger to the

transformer, an external fault is cleared by a complex relay system within the

shortest possible time. The internal faults are mainly based on sensors and

measurement systems. We will talk about those processes further in the

article. Before we get there, it is important to understand that there are

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 Transformer Protection

many types of transformers and in this study, we will discussing mainly about

power transformer that is used in distribution systems.

1.4. Objective of the study

The objectives of the project are to study protection of the power

system elements, Power station specially the transformers and analyze

different Methods to protect the Transformer from potential faults.

1.5. Limitation of the study

This study has the following limitations including

1. More transportation cost and in adequate resources

2. In adequate time and unwillingness of respondents

3. Limited time which will effect collection of the data due to long distance

to collect the questionnaire

1.6. Scopes
The subject scope of this study focused transformer protection in 26

June area.

The area which is the focus of this study, 26 June located central of

Hargeisa Somaliland.

1.7. The significance of the study

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 Transformer Protection

This study is significance to many areas and direct use full such as

1. The study can be used as secondary data by academic institutions who

want detailed information about this study.

2. The study will be helpful students who are interesting and involving

this research

3. The study will be useful for government institutions like ministry of

energy and minerals and other institutions such as international

organizations.

4. The first beneficiaries of this study will be energy suppliers or power

companies in Somaliland

1.8. Research question

1. What is transformer protection?

2. What is the possible solution for this problem?

3. How we can protect the transformer?

Chapter 2: Literature review

Figure 2.1: power Transformer.

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2.1 introductions

Transformer is a device that transfers electrical energy from one circuit

to another by magnetic coupling without requiring relative motion

between its parts. It usually comprises two or more coupled windings,

and, in most cases, a core to concentrate magnetic flux. An alternating

voltage applied to one winding creates a time-varying magnetic flux in

the core, which induces a

Voltage in the other windings. Varying the relative number of turns

between primary and secondary windings determines the

Ratio of the input and output voltages, thus transforming the voltage

by stepping it up or down between circuits.

2.2 types of transformer

There are several different kinds of transformers; they can be

categorized in different ways according to their purpose, use and

construction. We will deal with the transformer which is used in

transmission and distribution power networks. There are two types of

transformer used in transmission and distribution power system network.

Namely, step up and step down transformer. Generally, they are used for

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 Transformer Protection

stepping up and stepping down the voltage level in transmission and

distribution of electrical power network.

A transformer comprises of a soft iron coil with two coils twisted

around it, which are not joined to each other. These coils can either be

twisted on isolated appendages of the iron center or be organized on top of

one another. The coil that supplies the alternating voltage is known as the

primary coil or primary winding. At the point when an alternating potential

difference is supplied, the subsequent alternating current in the primary coil

creates a changing attractive field around it. This changing field induces an

alternating voltage in the secondary coil. The extent of the induce voltage

coming about because of the impelled current in the secondary coil relies

upon the number of turns in the secondary coil. Transformers are one of

the most efficiency devices. If a transformer is 100% effective, then the

power in the primary coil must be equivalent to the power in the secondary

coil.

We know that: p =v *I

If the transformer is 100% effective, then:

Power in primary coil = Power in secondary coil

VP x IP = V S x IS

Where,

Vp = Voltage on the primary

windings. Vs = Voltage on the

secondary windings

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 Transformer Protection

Ip = Current in the primary

windings.

Is= Current in the secondary windings.

2.2.0. Step-Down Transformer

A step-down transformer has fewer turns on the secondary coil than

on the primary coil. The induced voltage in the secondary coil is less than

the connected voltage over the primary coil or as it were; the voltage is

stepped-down. If a transformer steps up the potential difference, then the

current is stepped down by the same proportion. If the transformer steps

down the voltage, then the same proportion steps up the current. The

higher voltage beginning from the step-up transformer at the power plant

is diminished by the utilization of a step-down transformer situated in a

substation numerous miles away at the other side of the transmission line.

The utilization of these substations of step-down transformer successfully

brings down the voltage and in the meantime raises the current at the

other side of the line.

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 Transformer Protection

Figure 2.2: A Step-Down Transformer.

Figure 2.3: A Step-Down Transformer.

The relationship between the voltage and the number of turns in each

winding is given by:

VP NP

VS NS

It can be written as:

VP × Ns = VS × Np

To obtain the secondary voltage then it can be written as:

VS = VP X NS

NP

The relationship between the current and the number of turns in

each winding is given by:

IP = N S

IS NP

It can be written as:

IP x Np= Is x Ns

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 Transformer Protection

To obtain the secondary current then it can be written as:

IS = IPXN P

NS

Where,

Vp = potential difference (voltage) input on the primary coil

Vs = potential difference (voltage) output on the secondary coil

Np = number of turns of wire on the primary coil

Ns = number of turns of wire on the secondary coil

Ip = Current in the primary coil

Is= Current in the secondary coil.

2.2.1. Step-Up Transformer

A step-up transformer is the direct inverse of a step-down

transformer. There are more turns on the secondary winding than in the

primary winding in the step-up transformer. In this manner, the voltage

induced into the secondary transformer is higher than the one supplied

over the primary winding. Due to the standard of conservation of power,

the transformer changes from low voltage and high current to high voltage

and low current. In other word, the voltage has been stepped up. The

purpose to use a step up transformer is that, when a current flow in the

transmission lines some energy is lost as heat. The higher current the more

energy is lost. To avoid these heat losses, we make the current lower and

therefore we step up the voltage. Step up transformers are used at the

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 Transformer Protection

power station to produce the very high voltage needed to reduce the power

losses on the transmission power network system. However, very high

voltages are really dangerous for use in the home. Therefore, step down

transformers are used at the distribution network to reduce the voltage to a

safe level.

Figure 2.4: Step-Up transformer

Figure 2.5: Step-Up transformer

2.3. Operation of Transformers

The transformer operates based on the concept that energy

can be effectively exchanged by magnetic induction starting with one

winding then onto the next alternating to wind by a varying magnetic field

created current. An electrical voltage is induced when there is a relative

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 Transformer Protection

movement between a wire and a magnetic field. Exchanging current, (AC)

gives the movement required by altering course, which makes a crumpling

and growing magnetic field. The two windings are connected together with

a magnetic circuit that must be regular to both windings. The connection

associating the two windings in the magnetic circuit is the iron core on

which both windings are wound. Iron is a greatly decent conductor for

magnetic fields. The core is not a solid bar of steel, yet is developed of

numerous layers of slight steel called laminations. One of the windings is

assigned as the primary and the other winding as the secondary. Since the

primary and secondary are twisted on the same iron core, when the

primary winding is energized by an AC source, an exchanging magnetic

field called flux is set up in the transformer core. The flux made by the

connected voltage on the primary winding induces a voltage on the

secondary winding. The primary winding acquires the energy, which is

known as the input. The secondary winding releases the energy and is

known as the output.

Figure

Figure 2.6: operation of transformer

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 Transformer Protection

The primary and secondary windings comprise of aluminum or

copper channels twisted in curls around an iron core and the quantity of

turns in every loop will decide the voltage transformation of the

transformer. Every turn of wire in the primary winding has an equivalent

offer of the primary voltage. The same is instigated in every turn of the

secondary. In this manner, any distinction in the quantity of turns in the

secondary when contrasted with the primary will deliver a voltage change.

In many transformers, the high voltage winding is twisted specifically over

the low voltage winding to make effective coupling of the two windings.

The voltage transformation is an element of the turns proportion. It might

be alluring to change the proportion to get evaluated yield voltage when

the approaching voltage is somewhat not the same as the normal voltage.

2.4. Cooling System of transformer

Since an ideal transformer is not achieved yet so, practical

transformers have different types of losses such as copper loss, hysteresis

loss and eddy current loss. These losses in the transformers are

approximately 1% of its full load rating. Most of these losses are converted

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 Transformer Protection

into heat. Increasing this heat will rise the temperature of the transformer

continually which may cause damages in paper insulation and liquid

insulation medium of the transformer. Therefore, it is essential to use an

external cooling system to control the temperature within certain limits to

ensure long life of the transformer.

Figure 2.7: cooling system of transformer

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 Transformer Protection

2.4.0Transformer Cooling Methods

There are different cooling methods used in electrical power

transformer depending upon their size and ratings. The bigger transformer

size we have the more sophisticated cooling method it needs.

Transformer cooling methods can be divided in two types:

1) Dry type transformers.

This method also can be divided in two types:

a) Air natural (AN)

Air natural or self-air cooled transformer is generally used for small

ratings transformers up to 3 MVA. Basically, this method uses the natural

air flow surrounding the transformer as cooling medium. Thus, heat will

dissipate in air automatically.

b) Air forced (AF)

Air natural method is inadequate to use it for transformers rated

higher than 3MVA. Therefore, fans or blowers are required to force the air

towards the core and windings so, hot air is gained cooled due to the

outside natural conventional air.

However, the air forced should be filtered in order to prevent the

accumulation of dust particles in ventilation ducts. This method is useful for

transformers rated up to 15MVA. Figure (2.8) shows an example of forced

air cooled transformer.

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 Transformer Protection

Figure 2.8: forced air cooled transformer.

2) Oil immersed transformer

This is the most common cooling method for electrical power

transformers. Generally, the transformer core and windings are immersed

in the oil which has a good electrical insulating property and high thermal

conductivity.

This method can be divided into four types regarding to their size

and ratings.

a) Oil Natural Air Natural (ONAN).

This method of transformer cooling is widely used for oil immersed

transformers rated up to 30MVA. The heat which is generated in the

winding and core will transfer to the oil in the tank. Furthermore, we know

that hot oil flows upward and the cold oil comes down according to the

principle of convection. Therefore, the heated oil flows in the upward

direction and then will goes via the tubes or radiators. While these tubes or

radiators filled with cold oil so, the hot oil which comes from the upper

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 Transformer Protection

portion of the radiators will push the cold oil down to the oil tank, and the

heated oil will dissipate in the radiators due to the natural air flow around

the transformer. In this manner, the oil in the transformer tank will keeps

circulating because of natural convection and it will dissipate the heat in

atmosphere due to natural conduction. Figure (2.9) shows an example of

oil natural air natural.

Figure 2.9: Oil Natural Air Natural Cooling of Transformer

b) Oil Natural Air Forced (ONAF).

Oil natural air natural is not enough to cool the transformer rated

higher than 30 MVA. Thus, the dissipating surface can be improved by

applying forced air such as fans. Forced air gives faster heat dissipation

than natural air. In this manner, fans are mounted near to the radiator

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 Transformer Protection

surface and it can be provided with an automatic speed arrangement, to

control the fans by turn it on or to give the fans higher speed to keep the

temperature at permissible limit. Generally, oil natural air forced method is

useful for transformers rated up to 60MVA. Figure (2.10) shows an example

of oil natural air forced cooling of transformer.

Figure 2.10: Oil natural air forced cooling of transformer

c) Oil Forced Air Forced (OFAF).

For transformers rated higher than 60MVA, oil natural air forced is

not adequate to dissipate the heat properly. Thus, oil circulation needs to

be improving in somehow. Therefore, the oil circulation can be forced

through the radiators with the help of a pump, and then compressed air is

forced to flow on the radiators with the help of fans. Furthermore, the

radiators or heat exchangers can be designed to be mounted separately

from the transformer tank and connected via pipes at top and bottom of

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the transformer tank. Generally, oil forced air forced cooling method is

provided for high rating transformers at substations or power stations.

Figure (2.11) shows an example of oil forced air forced cooling method.

Figure 2.11: Oil forced air forced cooling method.

d) Oil Forced Water Forced (OFWF).

We know that the ambient temperature of the atmosphere air is

much higher than the water in same weather condition. Thus, water can be

used for better heat exchanger medium than air. Furthermore, oil forced

water forced cooling system designed to be separately from the

transformer tank, the hot oil forced to flow via the top pipe with the help of

pump to the heat exchanger, while the cold water forced to flow in the heat

exchanger with help of pump too. Note that, oil not mixed with cold water

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in the heat exchanger. Moreover, the hot oil dissipates the heat in the cold

water which is taken away to cool again in separate coolers. While the

cooled oil which comes out from the heat exchanger are back again to the

transformer through the bottom pipe. Generally, oil forced water forced

cooling system used in very large transformers rated higher than 500MVA.

An example of OFWF shown in figure (2.12).

Figure 4.6: Oil Forced Water Forced Cooling of

Figure 2.12: Oil Forced Water Forced Cooling of Transformer

2.5. Maintenance of Transformer

The unwavering quality and execution of a transformer is

significant to power dissemination. Any breakdown of such gear involves

loss of income for both suppliers and in addition customers. Accordingly, it

is of highest significance to keep them operational all through their

presence. Support is the best way to keep them fit for obligation. Minor

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 Transformer Protection

debasement in transformers and circuit breakers ought not to be

disregarded because in the end it turns into the powerless point in the

hardware.

Recognizing these shortcomings ahead of time causes from various

perspectives to minimize upkeep cost, forestall unexpected blackouts and

give smooth force dissemination on the system. Nonstop checking of this

hardware should be completed forever with a specific end goal to

distinguish any corruption and disappointments utilizing legitimate

instruments taking into account producers information. Utilizing the

information gathered, the condition appraisal of every gadget can be

developing. While the working standards of transformers have continued as

before for about a century, the difficulties of keeping up and testing

transformers have developed alongside transformer outline and

development. Present day transformers are intended to closer resistances

than at any other time. Appropriate testing is fundamental for assessing the

state of a transformer.

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 Transformer Protection

Figure 2.13: maintenance of Transformer

2.5.0. Maintenance Actions

1. The oil level in oil top under silica gel breather must check in one-month

interim. In the event that it is discovered the transformer, oil inside the

glass comes below the predefined level, oil to be beat up according to

indicated level.

2. Breathing openings in silica gel breather ought to additionally be

checked month to month and legitimately clean if require, for

appropriate breathing activity.

3. In the event that the transformer has oil, filled bushing the oil level of

transformer oil inside the bushing must be independently checked in the

oil gage appended to those bushing. This activity additionally to be done

month-to-month premise.

2.6 Transformer protection

Electrical power transformers are one of the main components in

power system. Starting from the very high power transformer rated higher

than 500MVA which is usually located at the power stations, or the medium

size of transformers which can be founded at the distribution station. Faults

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in all sizes of transformers are common and costly. It is so dangerous if it's

not protected properly. The whole transformer might be damaged and may

lead the power station to be shut down in case of high power transformers

used at power stations. However, distribution transformers may leave

thousands of houses without electricity. Therefore, electrical power

transformers must be protected in very high level to avoid any unwanted

consequences. Furthermore, transformer protection reduces the risk of

catastrophic failure to simplify eventual repair and to minimize the time of

disconnection for faults within the transformer. An extended operation of

the transformer under abnormal condition will diminish the life of the

transformer. This means we should provide adequate protection for quicker

isolation of the transformer under such conditions.

However, there are many different types of transformers depending

upon their size, use and location on the power system, each of them may

have different type of protection too. Therefore, it's quite difficult to cover

all the transformers protections. So, we will discuss further one type of high

power transformer that is oil immersed transformers. There are three

common protection devices used in oil immersed transformer Buchholz gas

relay, pressure relay and DMCR relay.

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Figure 2.13: Transformer protection.

Figure 2.14: The Buchholz gas relay.

Buchholz (gas) relay.

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The Buchholz relay is an electromechanical faults detector used in oil

immersed transformers. The Buchholz gas relay is placed in the connecting

piping between the transformer tank and the oil conservator as it shown in

figure (2.14.). Often the Buchholz gas relay is installed in a bypass pipe to

make it possible to take it out of service. For reliable operation, the

conservator pipe must be slant slightly.

The faults in oil immersed transformers can be divided in two types of

faults, namely incipient faults and major faults.

The incipient faults are like:

• Current through defective supporting and insulating structures

• Defective windings

• Core fault

The major faults are like:

• Short between turn and windings

• Short between phases to phase.

• Open circuit.

Figure 2.14: The Buchholz gas relay.

Figure 2.15: The Buchholz gas relay.

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The Buchholz gas relay provides two functions, an alarm signal when

incipient faults detected in the transformer. Also it initiating disconnection

of the transformer when a major fault has been detected in the

transformer.

How the Buchholz gas relay identify the faults!

The Buchholz gas relay can identify the transformer faults regarding

to the gas in the oil. When incipient faults happen in the transformer, it will

heat up the oil slowly and produces ionize gases. These gases will go up

towards the conservator, accumulates in the Buchholz relay until it’s exceed

permissible limit then it will give an alarm signal. However, a major fault is

identified by the Buchholz gas relay by the fact that, when a major faults

happen in a transformer it will produce rapidly a large volume of gases.

This large volume of gases causes a steep buildup of pressure and

displaces oil. This rapid oil flow from the transformer tank towards the

conservator. Thus, the Buchholz gas relay will sense this rapid flow in

somehow and will disconnect the transformer.

Function of the Buchholz relay:

Figure (2.16) below shows an example of the Buchholz (gas) relay

which consist of a double float, each one carries a mercury switch.

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Figure 2.16: Function of the Buchholz relay:

During the normal operation, the Buchholz relay is completely filled

with oil and the tow floats at the top of their position due to buoyancy and

both of the mercury switches are open as it shown in the blue color in the

figure (2.16). However, when an incipient fault occurs in the transformer,

the gas in the oil moves upwards and will accumulates in the Buchholz

relay and affecting the oil level. This will cause the upper float to move and

close the circuit and an alarm signal is tripped, while the lower float will not

have effected and the gas will flow to the conservator.

Figure 2.17: the example of oil loss due to leakage.

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Figure (2.18) shows the situation when a major fault occurs in the

transformer, the oil flow will reach the damper in the relay, if the damper

has been moves in flow direction, the lower float will move and close the

circuit and will cause the transformer to be disconnected. Furthermore, the

Buchholz relay can detect the very low oil level too in case of leakage. If

the oil level falls, the upper float will move down. Thus, an alarm circuit is

tripped. If the oil level decreasing continually until the conservator, piping

and the Buchholz relay become empties.

Figure 2.18: Function of the Buchholz relay:

Thus, the lower float will move too and actuates a switch contact so,

the transformer is disconnected. Figure (2.17) above shows the example of

oil loss due to leakage.

2.7 Protection components

A collection of protection devices (relays, fuses, etc.). Excluded are

devices such as CT’s, CB’s, Contactors, etc.

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2.7.0. Fuses

Probably the oldest, simplest, cheapest, and most-often used type of

protection device is the fuse. The operation of a fuse is very straightforward:

The thermal energy of the excessive current causes the fuse-element to melt

and the current path is interrupted.

Technological developments have made fuses more predictable, faster,

and safer (not to explode) Fuses are very inexpensive and they can operate

totally independently, that is, they do not need a relay with instrument

transformers to tell them when to blow.

This makes them especially suitable in applications like remote ring

main units, etc. (1)

2.7.1. Relays

The most versatile and sophisticated type of protection available today,

is undoubtedly the relay/circuit-breaker combination. The relay receives

information regarding the network mainly from the instrument transformers

(voltage and current transformers), detects an abnormal condition by

comparing this information to pre-set values, and gives a tripping command to

the circuit-breaker when such an abnormal condition has been detected. The

relay may also be operated by an external tripping signal, either from other

instruments, from a SCADA master, or by human intervention. Relays may be

classified according to the technology used:

2.7.1.0. ELECTROMECHANICAL RELAYS

These relays were the earliest forms of relay used for the protection of

power systems, and they date back nearly 100 years. They work on the

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principle of a mechanical force causing operation of a relay contact in

response to a stimulus. The mechanical force is generated through current

flow in one or more windings on a magnetic core or cores, hence the term

electromechanical relay. The principle advantage of such relays is that they

provide galvanic isolation between the inputs and outputs in a simple, cheap

and reliable form –therefore for simple on/off switching functions where the

output contacts have to carry substantial currents, they are still used.

Electromechanical relays can be classified into several different types as

follows:

a) Attracted armature

b) Moving coil

c) Induction

D) thermal

e) Motor operated

f) Mechanical

2.7.1.1. STATIC RELAYS

Introduction of static relays began in the early 1960’s. Their design is

based on the use of analogue electronic devices instead of coils and magnets

to create the relay characteristic.

Early versions used discrete devices such as transistors and diodes in

conjunction with resistors, capacitors, inductors, etc., but advances in

electronics enabled the use of linear and digital integrated circuits in later

versions for signal processing and implementation of logic functions. While

basic circuits may be common to a number of relays, the packaging was still

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essentially restricted to a single protection function per case, while complex

functions required several cases of hardware suitably interconnected. User

programming was restricted to the basic functions of adjustment of relay

characteristic curves. They therefore can be viewed in simple terms as an

analogue electronic replacement for electromechanical relays, with some

additional flexibility in settings and some saving in space requirements. In

some cases, relay burden is reduced, making for reduced CT/VT output

requirements.

2.7.1.2. Digital Relays

Digital protection relays introduced a step change in technology.

Microprocessors and microcontrollers replaced analogue circuits used in static

relays to implement relay functions. Early examples began to be introduced

into service around 1980, and, with improvements in processing capacity, can

still be regarded as current technology for many relay applications. However,

such technology will be completely superseded within the next five years by

numerical relays. Compared to static relays, digital relays introduce A/D

conversion of all measured analogue quantities and use a microprocessor to

implement the protection algorithm. The microprocessor may use some kind

of counting technique, or use the Discrete Fourier Transform (DFT) to

implement the algorithm. However, the typical microprocessors used have

limited processing capacity and memory compared to that provided in

numerical relays. The functionality tends therefore to be limited and restricted

largely to the protection function itself. Additional functionality compared to

that provided by an electromechanical or static relay is usually available,

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typically taking the form of a wider range of settings, and greater accuracy. A

communications link to a remote computer may also be provided.

2.7.1.3. Numerical Relays

The distinction between digital and numerical relay rests on points of

fie technical detail, and is rarely found in areas other than Protection. They

can be viewed as natural developments of digital relays as a result of

advances in technology. Typically, they use a specialized digital signal

processor (DSP) as the computational hardware, together with the associated

software tools. The input analogue signals are converted into a digital

representation and processed according to the appropriate mathematical

algorithm. Processing is carried out using a specialized microprocessor that is

optimized for signal processing applications, known as a digital signal

processor or DSP for short. Digital processing of signals in real time requires a

very high power microprocessor. In addition, the continuing reduction in the

cost of microprocessors and related digital devices (memory, I/O, etc.)

naturally leads to an approach where a single item of hardware is used to

provide a range of functions („one-box solution‟ approach). By using multiple

microprocessors to provide the necessary computational performance, a large

number of functions previously implemented in separate items of hardware

can now be included within a single item.

2.7. 2. Instrument transformers (CT\VT)

Relays need information from the power network in order to detect an

abnormal condition. This information is obtained via voltage and current

transformers (collectively called instrument transformers), as the normal

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system voltages and currents are too high for the relays to handle directly,

and the instrument transformers protect the relay from system ‘spikes’ to a

certain extent.[1]

CHAPTER 3 METHADOLOGY

3.1 introductions

Generally Differential protection is provided in the electrical power transformer

rated more than 5MVA.

The Differential Protection of Transformer has many advantages over other

schemes of protection.

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1. The faults occur in the transformer inside the insulating oil can

be detected by Buchholz relay. But if any fault occurs in the

transformer but not in oil then it cannot be detected by Buchholz

relay. Any flash over at the bushings are not adequately covered

by Buchholz relay. Differential relays can detect such type of

faults. Moreover Buchholz relay is provided in transformer for

detecting any internal fault in the transformer but Differential

Protection scheme detects the same in faster way.

2. The differential relays normally response to those faults which

occur inside the differential protection zone of transformer.

3.2 Differential Protection Scheme in a Power Transformer

Principle of Differential Protection

Principle of Differential Protection scheme is one simple conceptual technique.

The differential relay actually compares between primary current and

secondary current of power transformer, if any unbalance found in between

primary and secondary currents the relay will actuate and inter trip both the

primary and secondary circuit breaker of the transformer.

Suppose you have one transformer which has primary rated current and

secondary current is. If you install CT of ratio I/1A at the primary side and

similarly, CT of ratio is/1A at the secondary side of the transformer. The

secondaries of these both CTs are connected together in such a manner that

secondary currents of both CTs will oppose each other.

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In other words, the secondaries of both CTs should be connected to the same

current coil of a differential relay in such an opposite manner that there will

be no resultant current in that coil in a normal working condition of the

transformer. But if any major fault occurs inside the transformer due to which

the normal ratio of the transformer disturbed then the secondary current of

both transformers will not remain the same and one resultant current will flow

through the current coil of the differential relay, which will actuate the relay

and inter trip both the primary and secondary circuit breakers. To correct

phase shift of current because of star-delta connection of transformer winding

in the case of three-phase transformer, the current transformersecondaries

should be connected in delta and star as shown here.

Figure 3.1: differential protection

At maximum through fault current, the spill output produced by the small

percentage unbalance may be substantial. Therefore, differential protection of

transformer should be provided with a proportional bias of an amount which

exceeds in effect the maximum ratio deviation.

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...

3.3 Differential Protection of a Transformer

The transformer is one of the major equipment in power system. It is a

static device, totally enclosed and usually oil immersed, and therefore the

fault occurs on them are usually rare. But the effect of even a rare fault may

be very serious for a power transformer. Hence the protection of power

transformer against possible fault is very important.

The fault occurs on the transformer is mainly divided into two type

external faults and internal fault. External fault is cleared by the relay system

outside the transformer within the shortest possible time in order to avoid any

danger to the transformer due to these faults. The protection for internal fault

in such type of transformer is to be provided by using differential protection

system.

Differential protection schemes are mainly used for protection against

phase-to-phase fault and phase to earth faults. The differential protection

used for power transformers is based on Mars-Prize circulating current

principle. Such types of protection are generally used for transformers of

rating exceeding 2 MVA.

3.4 Connection for Differential Protection for Transformer

The power transformer is star connected on one side and delta

connected on the other side. The CTs on the star connected side are delta-

connected and those on delta-connected side are star-connected. The neutral

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of the current transformer star connection and power transformer star

connections are grounded.

The restraining coil is connected between the secondary winding of the

current transformers. Restraining coils controls the sensitive activity occurs on

the system. The operating coil is placed between the tapping point of the

restraining coil and the star point of the current transformer secondary

windings.

Figure 3.2: connection of differential protection

3.5 Working of Differential Protection System

Normally, the operating coil carries no current as the current are

balanced on both the side of the power transformers. When the internal fault

occurs in the power transformer windings the balanced is disturbed and the

operating coils of the differential relay carry current corresponding to the

difference of the current among the two sides of the transformers. Thus, the

relay trips the main circuit breakers on both sides of the power transformers.

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3.6 Problem Associated with Differential Protection System

When the transformer is energizing the transient inrush of magnetizing

current is flows in the transformer. This current is as large as 10 times full

load current and its decay respectively. This magnetizing current is flows in

the primary winding of the power transformers due to which it causes a

difference in current transformer output and it makes the differential

protection of the transformer to operate falsely.

To overcome this problem the kick fuse is placed across the relay coil.

These fuses are of the time-limit type with an inverse characteristic and do

not operate in short duration of the switch in the surge. When the fault occurs

the fuses blow out and the fault current flows through the relay coils and

operates the protection system. This problem can also be overcome by using

a relay with an inverse and definite minimum type characteristic instead of an

instantaneous type

3.7 Transformer Protection for Different Types of

Transformers
The protection system used for a power transformer depends on the

transformer's categories. A table below shows that,

Transformer Rating – KVA

Category
1
3 Phase
Phase

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5 -
I 15 – 500
500

501 –
II 501 – 5000
1667

1668
III 5001 - 30,000
- 10,000

>
IV >30,000
10,000

Table 3.1. The protection system used for a power transformer

according to their category

 Transformers within the range of 500 KVA fall under (Category I &

II), so those are protected using fuses, but to protect transformers

up to 1000 kVA (distribution transformers for 11kV and 33kV)

Medium Voltage circuit breakers are usually used.

 For transformers 10 MVA and above, which falls under (Category III

& IV), differential relays had to be used to protect them.

Additionally, mechanical relays such as Buchholtz relays, and sudden pressure

relays are widely applied for transformer protection. In addition to these

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relays, thermal overload protection is often implemented to extend a

transformer’s lifetime rather than for detecting faults.

3.8 Common Types of Transformer Protection

1. Overheating protection

2. Over current protection

3. Differential Protection of Transformer

4. Earth Fault Protection (Restricted)

5. Buchholz (Gas Detection) Relay

6. Over-fluxing protection

3.8.1. Overheating Protection in Transformers

Transformers overheat due to the overloads and short circuit conditions. The

allowable overload and the corresponding duration are dependent on the type

of transformer and class of insulation used for the transformer.

Higher loads can be maintained for a very short amount of time if it is for a

very long, it can damage the insulation due to temperature rise above an

assumed maximum temperature. The temperature in the oil-cooled

transformer is considered maximum when its 95*C, beyond which the life

expectancy of the transformer decreases and it has detrimental effects in the

insulation of the wire. That is why overheating protection becomes essential.

Large transformers have oil or winding temperature detection devices, which

measure oil or winding temperature, typically there are two ways of

measurement, one is referred to hot-spot measurement and second is

referred to as top-oil measurement, the below image shows a typical

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 Transformer Protection

thermometer with a temperature control box from reinhausen used to

measure the temperature of a liquid insulated conservative type of

transformer.

Figure 3.3: over heat protection

The box has a dial gauge which indicates the temperature of the

transformer (which is the black needle) and the red needle indicates the alarm

set point. If the black needle surpasses the red needle, the device will activate

an alarm.

If we look down, we can see four arrows through which we can configure the

device to act as an alarm or trip or they can be used to start or stop pumps or

cooling fans.

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 Transformer Protection

Figure 3.4: temperature thermometer

As you can see in the picture, the thermometer is mounted on the top of the

transformer tank above the core and the winding; it's so done because the

highest temperature is going to be at the center of the tank because of the

core and the windings. This temperature is known as the top oil

temperature. This temperature gives us an estimate of the Hot-spot

Temperature of the transformer core. Present-day fibber optic cables are used

within the low voltage winding to accurately measure the temperature of the

transformer. That is how overheating protection is implemented.

3.8.2. Over current Protection in Transformer

The over current protection system is one of the earliest developed protection

systems out there, the graded over current system was developed to guard

against over current conditions. Power distributors utilize this method to

detect faults with the help of the IDMT relays. That is, the relays having:

1. Inverse characteristic, and

2. Minimum time of operation.

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The capabilities of the IDMT relay is restricted. These sorts of relays have to

be set 150% to 200% of the max rated current; otherwise, the relays will

operate for emergency overload conditions. Therefore, these relays provide

minor protection for faults inside the transformer tank.

3.8.3. Differential Protection of Transformer

The Percentage Biased Current Differential Protection is used to protect power

transformers and it is one of the most common transformer protection

schemes that provide the best overall protection. These types of protection

are used for transformers of rating exceeding 2 MVA.

The transformer is star connected on one side and delta connected the other

side. The CTs on the star side are delta-connected and those on the delta-

connected side are star-connected. The neutral of both the transformers are

grounded.

The transformer has two coils, one is the operating coil and the other is

the restraining coil. As the name implies, the restraining-coil is used to

produce the restraining force, and the operating-coil is used to produce the

operating force. The restraining-coil is connected with the secondary winding

of the current transformers, and the operating coil is connected in between

the equipotential point of the CT.

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Figure 3.5: differential protection of transformer

3.8.4. Transformer Differential Protection Working:

Normally, the operating coil carries no current as the current is matched on

both sides of the power transformers, when an internal fault occurs in the

windings, the balance is altered and the operating coils of the differential

relay start producing differential current among the two sides of the

transformer. Thus, the relay trips the circuit breakers and protects the main

transformer.

3.8.5. Restricted Earth Fault Protection

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 Transformer Protection

Figure 3.6: differential protection working

A very high fault current can flow when a fault occurs at the transformer

bushing. In that case, the fault needs to be cleared as soon as possible. The

reach of a particular protection device should be only limited to the zone of

the transformer, which means if any ground fault occurs in a different

location, the relay allocated for that zone should get triggered, and other

relays should stay the same. So, that is why the relay is

named restricted earth fault protection relay.

In the above picture, the Protection Equipment is on the protected side of the

transformer. Let's assume this is the primary side, and let's also assume there

is a ground fault on the secondary side of the transformer. Now, if there is a

fault on the ground side, because of the ground fault, a Zero Sequence

Component will be there, and that will circulate only on the secondary side.

And it will not be reflected in the primary side of the transformer.

This relay has three phases, if a fault occurs, they will have three

components, the positive sequence components, the negative sequence

components, and the zero sequence components. Because the positive

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sequins components are displaced by 120*, so at any instant, the sum of all

the currents will flow through the protection relay. So, the sum of their

currents will be equal to zero, as they are displaced by 120*. Similar is the

case for the negative sequence components.

Now let us assume a fault condition occurs. That fault will be detected by the

CTs as it has a zero-sequence component and the current starts flowing

through the protection relay, when that happens, the relay will trip and

protect the transformer.

Buchholz (Gas Detection) Relay

Figure 3.7: buchholz relay

The above picture shows a Buchholz relay. The Buchholz relay is fitted in

between the main transformer unit and the conservator tank when a fault

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occurs within the transformer; it detects the resolved gas with the help of a

float switch.

If you look closely, you can see an arrow, gas flows out from the main tank to

the conservator tank, normally there should not be any gas in the transformer

itself. Most of the gas is referred to as dissolved gas and nine different types

of gasses can be produced depending on the fault condition. There are two

valves at the top of this relay, these valves are used to reduce the gas build-

up, and it's also used to take out a gas sample.

When a fault condition occurs, we have sparks between the windings, or in

between windings and the core. These small electrical discharges in the

windings will heat the insulating oil, and the oil will break down, thus it

produces gases, the severity of the breakdown, detects which glasses are

created.

A large energy discharge will have a production of acetylene, and as you may

know, acetylene takes a lot of energy to be produced. And you should always

remember that any type of fault will produce gases, by analyzing the amount

of gas, we can find the severity of the fault.

How Buchholz (Gas Detection) Relay Works?

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 Transformer Protection

Figure 3.8: gas buchhloz gas detection relay

As you can see from the image, we have two floats: an upper float and a

lower float, also we have a baffle plate that is pushing down the lower float.

When a large electrical fault occurs, it produces a lot of gas than the gas flows

through the pipe, which shifts the baffle plate and that forces the lower

floated down, now we have a combination, the upper float is up and the lower

float is down and the baffle plate has tilted. This combination indicates that a

massive fault has occurred. Which shuts down the transformer and it also

generates an alarm. The image below shows exactly that,

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 Transformer Protection

Figure 3.9: gas buchhloz gas detection relay

But this is not the only scenario where this relay can be useful, imagine a

situation where inside the transformer there is a minor arcing that is

happening, these arks are producing a small amount of gas, this gas produces

a pressure inside the relay and the upper float gets down displacing the oil

inside it, now the relay generates an alarm in this situation, the upper float is

down, the lower float is unchanged and the baffle plate is unchanged if this

configuration is detected, we can be sure that we have a slow accumulation of

gas. The image below shows exactly that, now we know we have a fault, and

we will bleed out some of the gas using the valve above the relay and analyze

the gas to find out the exact reason for this gas build-up.

This relay can also detect conditions where the insulating oil level falls due to

leaks in the transformer chassis, in that condition, the upper float drops, the

lower float drops, and the baffle plate stays in the same position. In this

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condition, we get a different alarm. The below image shows the working. With

these three methods, the Buchholz relay detects faults.

Figure 3.10: The Buchholz gas relay.

3.8.6. Over-fluxing Protection

A transformer is designed to operate at a fixed flux level exceed that flux level

and the core gets saturated, the saturation of the core causes heating in the

core that quickly follows through the other parts of the transformer that leads

to overheating of components, thus over flux protection becomes necessary,

as it protects the transformer core. Over-flux situations can occur because of

overvoltage or a reduction in system frequency.

To protect the transformer from over-fluxing, the over-fluxing relay is used.

The over-fluxing relay measures the ratio of Voltage / Frequency to calculate

the flux density in the core. A rapid increase in the voltage due to transients

in the power system can cause over fluxing but transients die down fast,

therefore, the instantaneous tripping of the transformer is undesirable.

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 Transformer Protection

The flux density is directly proportional to the ratio of voltage to frequency

(V/f) and the instrument should detect the ration if the value of this ratio

becomes greater than unity, this is done by a microcontroller-based relay

which measures the voltage and the frequency in real-time, then it calculates

the rate and compares it with the pre-calculated values. The relay is

programmed for an inverse definite minimum time (IDMT characteristics). But

the setting can be done manually if that is a requirement. In this way, the

purpose will be served without compromising the over-flux protections. Now,

we see how important it is to prevent the tripping of the transformer from

over-fluxing.

Hope you enjoyed the article and learned something useful. If you have any

questions, leave them in the comment section or use our forums for other

technical queries.

https://circuitdigest.com/article/all-about-transformer-protection-and-

transformer-protection-circuits

CHAPTER 4 RESULTS

4.1 system profile

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 Transformer Protection

A power transformer’s main task is to transform an electrical power from one

voltage level to the other voltage level. A power transformer is the most

important part of the electrical system as well as well the most expensive

part. The function of all other electrical equipment (e.g. circuit breakers,

instrument transformers, etc.) is to protect the power transformer.

Considering the importance of the transformer and its high cost compared to

other equipment, it is reasonable to install high-quality systems for protection

against external failures from the network or internal power transformer

failures.

Power Transformer Protection Systems

The external failures which appear somewhere in the network (overvoltage,

short circuit, overload, atmospheric discharge, etc) can cause the troubles for

the transformer (which is part of that network). E.g. short circuits in the

network can cause the significant heating of the transformer bus bars and

windings.

The copper losses I2R are increased with the square of the current and

dissipated as heat. Also, failures can appear inside the power transformer,

such as windings short circuit, inter-turns short circuit, and a short circuit

between phases, faults in the core, transformer tank, and breakthroughs on

the transformer bushing. When it comes to the failure location, the power

transformer protection systems can be divided into external and internal

protections.

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The main task of the protection system is to separate the transformer from

the energy supplying as soon as possible, thus preventing unintended

consequences and major transformer damages.

The protection system is designed to be able to signalize if irregularities

occurred in the electrical system which could lead to the transformer failure.

After a preset relay blocking time (operation time delay), the protection

system sends the signal to the circuit breaker which will turn off the

transformer from the system before the failure affects them.

The power transformer substation with the protected transformer, circuit

breaker and measurement current transformers is illustrated in Figure 1. The

different transformer protection systems according to the operation criteria

are listed in Table 1.

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 Transformer Protection

Figure 4.1: The transformer substation

The protection
The operation criteria The failure location
system

Current differences Differential Internal/external

criteria protection protection

Over current
High current criteria External protection
protection

Gas evaluation criteria Buchholz relay Internal protection

Thermal overload
High-temperature criteria Internal protection
protection

Zero-sequence current Ground fault

External protection
criteria protection

Distance
Line impedance criteria External protection
protection

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 Transformer Protection

Table 4.1. Failures can be detected with corresponding protection.

The different protection systems can detect the different faulty conditions in

the transformer. The

The transformer
The protection system
faulty conditions

Transformer

overloading or Thermal overload protection

overheating

The external short

Over current and distance protection
circuit in the network

The transformer
Differential, over current and Buchholz relay
internal short circuit

The transformer

internal single phase Single phase over current, ground fault and tank groun

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 Transformer Protection

The transformer
The protection system
faulty conditions

short circuit or ground-

fault protection

Table 4.2. Different transformer protection systems

Differential Protection of Power Transformer

The transformer differential protection (ΔI) is a reliable and safe protection as

well as the most important and most commonly used transformer protection.

It is used for

protecting the power transformer with nominal power above 8 MVA (it is

usually not used in case of a transformer with lower nominal power up to 4

MVA).

The ΔI covers almost all short circuits inside the transformer such as short

circuit: between phases, inter-turns, between phase and ground. If the

transformer neutral is directly grounded, this protection also covers insulation

breakthrough through all windings. If the transformer neutral is isolated the

ΔI covers only faults between two phases but not single phase failures.

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 Transformer Protection

Transformer Differential Protection Principle

The differential protection (ΔI) principle is based on comparing the output

and input transformer currents as illustrated in Figure 2.

In the normal network condition, power transformer operates with the

nominal current. The current transformers (CT) are selected with

corresponding turns ratio that the currents in CT secondary sides are equal. In

this case, there is no current flow through ΔI (ΔI=0) because CTs secondary

currents have equal amplitude and phase displacement value. The ΔI will not

operate.

In faulty condition, the transformer current value will be much higher than

the nominal current which will cause ΔI>0. In this case, the protection will

operate and take transformer off from the service.

Figure 4.2. Transformer differential protection diagram

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 Transformer Protection

Theoretically, this protection system seems very simple. But in reality, the

protection operating criteria is not as simple. The ΔI challenges are listed

below:

 The primary and secondary transformer currents are usually

different. The current transformer should be properly selected so

that the differential current in normal condition is ΔI=0.

 Different transformer vector groups have different current phase

displacement on the primary and secondary side.

 The CTs on both transformer sides should have approximately

the same saturation characteristics regarding the knee point and

saturation curve.

 The tap changer operation (transformer voltage regulation) can

cause the ΔI current through protection circuit because of the

changing transformer turns ratio.

 When the transformer is first energized, it causes the current in

only one transformer side and disturbs the ΔI balance.

 CT saturation and the DC current component phenomenon

cause the current differences.

 The external ground fault in electrical system from the low

voltage transformer side can cause the zero-sequent current

component which can operate the ΔI.

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 Transformer Protection

Nowadays, analog and digital differential protection can be found in the

electrical system. The analog system uses the old-fashioned mechanical

solutions, while the new digital technology solves the issues by using

software.

New electrical systems are designed according to the digital protection

systems. The digital systems resolve the using of the interconnection

transformers, the higher threshold ΔI value, chokes (inductors) and

capacitors.

4.2 systems Design

The procedure in designing transformer protection is to evaluate and

analyze appliances to be used on the protection system.

This design contains several components or elements which are :

power grid sized 100 MVA , bus1 size 12.47kv , bus 12.47 , circuit breaker 1

size 100A , circuit breaker 2 size , bus 4.16 kV , bus 2 4.16kv , transformer 5

MVA , over current relay , current transformer 1 , current transformer 2 , and

4MVA lumped load The connection of this elements will make complete small

system the purpose of this small system is to protect the transformer against

overloads , over voltages, under voltages ,over currents , over frequency ,

over temperature , over pressure and many others.

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 Transformer Protection

Parameters Description

Type Step down transformer

Primary voltage 12.47kv

Secondary voltage 4.16

Cooling system ONAN

Position Outdoor

Protection type Differential

Capacity 5 MVA

Circuit breaker 15 KA

Table 4.3 transformer description

Figure 4.3. Differential. Transformer protection etap

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 Transformer Protection

Figure 4...4. Differential. Transformer protection etap

4.3 Problems arising in differential protection in

power system (transformer).

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 Transformer Protection

1. Difference in lengths of pilot wires on either sides of the relay. This is

overcome by connecting adjustable resistors to pilot wires to get

equipotential points on the pilot wires.

2. Difference in CT ratio error difference at high values of short circuit

currents

That makes the relay to operate even for external or through faults.

This is overcome by introducing bias coil. 3. Tap changing alters the ratio of

voltage and currents between HV and LV sides

And the relay will sense this and act. Bias coil will solve this. 4.

Magnetizing inrush current appears wherever a transformer is energized on its

primary Side producing harmonics. No current will be seen by the secondary.

CT’s as there is no Load in the circuit. This difference in current will actuate

the differential relay. A harmonic Restraining unit is added to the relay which

will block it when the transformer is energized.

CHAPTER 5 CONCLUSION AND RECOMMINDATION

5.1 Conclusion

Transformers are the most important and expensive unites in power

system grid. Thus, Transformer protection must be taken in the consideration.

A real time implementation of multifunctional digital relaying scheme in ETAP

for scaled Transformer is presented in this project. The 3-phase power circuit

simulation and prototype Implementation of various protection schemes

applicable to transformer has been carried out.

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 Transformer Protection

This project presents a real time implementation of multifunctional

digital relaying scheme In ETAP for scaled transformer. The 3-phase power

circuit simulation and prototype Implementation of various protection

schemes applicable to transformer has been carried out The developed model

is capable of eliminating various type of normal and abnormal faulty Condition

of transformer. Faults protection scheme is designed and implemented to

detect all

Types of faults. Various type of transformer protection was

implemented successfully by ETAP programmer With satisfactory result.

5.2 Recommendation

The lack of main and reliability back-up protection schemes in the

event of abnormalities And faults, the lack of comprehensive monitoring, the

occurrences of abnormalities and Faults without protecting the supply units

from them is the main reason for instability of Power supply. To make a

comprehensive and effective protection for the transformer, Transformer

protection design scheme must also take into account some additional

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 Transformer Protection

considerations to increase the performance of the protection scheme, such as

Scheduling Maintenance for transformer units to avoid frequent outage of the

electrical supply, in the Event of very sever fault the relay must trip the Circuit

Breakers in very short time like What happened in the transient study of over

frequency by using ETAB implementation? Also Study and simulation for

generation unit (both generator and its step-up transformer) could be done

using protection relays of ETAP software besides online relay testing.

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 Transformer Protection

REFERENCES

1 https://ieeexplore.ieee.org/document/4562642

2 https://circuitdigest.com/article/all-about-generator-protection-

circuits

3 http://ecetutorials.com/transformer/transfomer-cooling-methods

4 http://hubpages.com/technology/Transformer-Cooling (04/2016)

5 http://electrical-engineering-portal.com/4-power-transformer-

protection-devicesexplained-in-details (04/2016)

6 http://cuiet.info/modelQnpaper/EN%2014%20107(2).pdf (03/2016)

7 http://electrical-engineering-portal.com/dmcr-protection-relay-for-

oil-transformer

(04/2016)

8 http://electrical-engineering-portal.com/protecting-oil-type-

transformer-withbuchholz-relay (05/2016)

9 http://www.electricaleasy.com/2014/06/cooling-methods-of-

transformer.html

(06/2014)

10 https://circuitdigest.com/article/all-about-transformer-protection-and

transformer protection-circuits

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 Transformer Protection

Mohamed, A., & Sultan, H. (2016). Electrical power transformer.

Karlskrona, KK: Blekingle institute of technology press.

Sun . L. (2017). Protection of synchronous generator. Georgia, GG:

Georgia institute of technology press.

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