MAKERERE UNIVERSITY

COLLEGE OF ENGINEERING, DESIGN, ART AND TECHNOLOGY

SCHOOL OF ENGINEERING

DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING

THIRD YEAR INDUSTRIAL TRAINING REPORT

AT

ABB UGANDA LIMITED

BY

GASTER MUSANJE

O8/U/453

THIS REPORT IS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE AWARD OF THE DEGREE OF

BACHELOR OF SCIENCE IN ELECTRICAL ENGINEERING

JUNE –AUGUST 2011

DECLARATIONI GASTER MUSANJE, do declare that this work is original, from my industrial training at ABB Uganda Limited and it has never been presented in any institution of higher learning

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for the award of any degree unless otherwise where the respective authors has been acknowledged.

GASTER MUSANJE (08/U/453)

Signature…………………………………... Date…………………………………..

This work has been submitted with the approval of the following supervisors:

COLLEGE SUPERVISOR

ASSOC. PROF DR.E LUGUJJO

DEPARTMENT OF ELECTRICAL ENGINEERING

MAKERERE UNIVERSITY

Signature………………………… Date…………………………….

MR.EMMANUEL LAGU

OPERATIONS DEPARTMENT

ABB UGANDA LIMITED

Signature…………………………. Date……………………………..

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DEDICATION To Ms. Robinah Mbabazi, Ivan Ndaula, Rogers Golooba, Dennis Walugembe, Irene Zansanze, Lydia Ndagire, Pherry Nassolo and Alice Lwantale.

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ACKNOWLEDGEMENTS I would like to acknowledge my supervisors Assoc. Prof E. Lugujjo, Mr. Emmanuel Lagu, Mr. Edward Semakula and Mr. Nylus Rupiny for their contribution and guidance in this work and their support during the course of the training.

In a special way, I would like to thank Ms. Norah Kipwola the Managing Director of ABB Uganda Limited, through whose hands I was offered the opportunity to train with this company.

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PREFACEThis is a third year industrial training report. The training was carried out at ABB Uganda Limited as from: 13th June 2011 to 15th August 2011.

This report consists of five chapters.

Chapter one is a general introduction that basically contains; the objectives of Industrial Training, the merits of Industrial Training and the ABB company profile.

Chapter two covers the theoretical concepts of Electric power generation, the excitation system, transformers and the electrical substation.

Chapter three focuses on the practical work done that includes the installation of the excitation system at Nalubaale Hydro Power Plant, autotransformer cleaning at Civil Aviation Authority and the configuration of the REF542 relays at the UMEME Kitante Road Substation.

Chapter four contains the challenges faced during the industrial training, the achievements registered, the conclusion and possible recommendations

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ACRONYMSABB Asea Brown Boveri

AC Alternating Current

ACSR Aluminum Conductor Steel Reinforced

AN Air Natural

AVR Automatic Voltage Regulator

CAA Civil Aviation Authority

CT Current Transformer

CVT Current Voltage Transformer

CB Circuit Breaker

DC Direct Current

EMF Electromotive Force

HV High Voltage

Hz Hertz

KVA Kilo Volt Amperes

LV Low Voltage

MVA Mega Volt Amperes

OVP Over Voltage Protection

ONAN Oil Natural Air Natural

ONAF Oil Natural Air Forced

OFAF Oil Forced Air Forced

OFAN Oil Forced Air Natural

OFWF Oil Forced Water Forced

rpm Revolutions per minute

SCR Silicon Controlled Rectifier

SES Static Excitation System

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S/S Sub Station

VT Voltage Transformer

TABLE OF CONTENTSDECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENTS iv

PREFACE v

ACRONYMS vi

LIST OF FIGURES ix

LIST OF TABLES xi

CHAPTER ONE: INTRODUCTION 1

1.1 Industrial training 1

1.1.1 Objectives of Industrial training 1

1.1.2 Merits of Industrial training 1

1.2 Background to the company 2

1.2.1 Organisation structure of ABB 2

1.2.2 Products, services and expertise 3

1.2.3 The ABB look 4

CHAPTER TWO: ELECTRIC POWER GENERATION, EXCITATION SYSTEM, TRANSFORMERS AND ELECTRICAL SUBSTATIONS 4

2.1 Electric power generation 4

2.1.1 The basic generator 5

2.2 The excitation system 9

2.2.1 Static excitation system 10

2.2.2 Merits of the excitation system 17

2.2.3 Power supply18

2.2.4 Protection 19

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2.3 Transformers 20

2.3.1 Construction of a transformer 21

2.6.2 Principle of operation of a transformer 26

2.6.3 Types of transformers26

2.6.4 Transformer name plate data 27

2.6.5 Transformer losses 30

2.6.6 Transformer tests 31

2.6.7 Autotransformer 33

2.6.8 Current transformer 34

2.6.9 Voltage transformer 35

2.7 Electrical substation 35

2.7.1 Classification of a substation 36

2.7.2 Components of a substation 39

2.7.3 Substation equipment 40

CHAPTER THREE: PRACTICAL WORK DONE 49

3.1 Nalubaale power station49

3.1.1 Tools used 49

3.1.2 Sequence of operation 49

3.2 Civil Aviation Authority51

3.2.1 Tools used 51

3.2.3 Sequence of operation 51

3.3 UMEME kitante road substation 52

3.3.1 Tools used 52

3.3.2 Sequence of operation 53

CHAPTER FOUR: CHALLENGES, ACHIEVEMENTS, CONCLUSION AND RECOMMENDATIONS 57

4.1 Challenge faced during Industrial training 57

4.2 Achievements 57

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4.3 Conclusion 57

4.4 Recommendations 57

REFERENCES 59

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LIST OF FIGURESFigure 1: Organizational Chart of ABB....................................................................................3

Figure 2: The ABB logo.............................................................................................................4

Figure 3: The major parts of an AC generator..........................................................................6

Figure 4: Field Excitation..........................................................................................................6

Figure 5: Development of the Sine wave...................................................................................7

Figure 6: Three phases with electrical degree separation........................................................7

Figure 7: Output voltage pattern...............................................................................................8

Figure 8: The synchronous generator units at Nalubaale power station..................................9

Figure 9: Output stator terminal for one phase.......................................................................10

Figure 10: Static exciter panel.................................................................................................11

Figure 11: 11kV cables from the stator terminal to the excitation transformer......................12

Figure 12: Excitation transformer with 415V output cables...................................................12

Figure 13: The thyristor bridge or power converter..............................................................13

Figure 14: The field discharge resistor...................................................................................14

Figure 15: The control unit chamber of the exciter panel.......................................................15

Figure 16: The Transformer in the exciter panel.....................................................................19

Figure 17: Basics of Excitation systems..................................................................................20

Figure 18: The power station showing the role of the Excitation system................................20

Figure 19: Transformer construction......................................................................................22

Figure 20: Autotransformer tank at CAA................................................................................23

Figure 21: Autotransformer bushings at CAA.........................................................................23

Figure 22: Surge Arrestor........................................................................................................24

Figure 23: Pressure relief valve..............................................................................................24

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Figure 24: Breather on Autotransformer at CAA....................................................................25

Figure 25: On- load Tap Changer Control Unit (Automatic and Manual) at CAA................25

Figure 26: Step up and Step down transformers.....................................................................27

Figure 27: Two step down power transformers (33/11kV) at the UMEME Kitante Road Substation.................................................................................................................................27

Figure 28: Excitation transformer at Nalubaale Power Station.............................................28

Figure 29: The power triangle................................................................................................29

Figure 30: Wiring set up for open circuit test..........................................................................32

Figure 31: Wiring set up for short circuit test.........................................................................32

Figure 32: Autotransformer.....................................................................................................33

Figure 33: Inside the autotransformer at CAA showing the input cables...............................34

Figure 34: Two 11kV incoming UMEME lines to CAA...........................................................34

Figure 35: Current transformers.............................................................................................35

Figure 36: Voltage transformer...............................................................................................35

Figure 37: Inside the 33/11kV UMEME Kitante Road Substation showing 11kV Distribution relays........................................................................................................................................36

Figure 38: Kawanda Substation (220/132kV) is an outdoor substation..................................37

Figure 39: Kitante Road Substation (33/11kV) is an indoor substation..................................38

Figure 40: 330kV Gas Insulated Underground Substation.....................................................38

Figure 41: Existing Air Insulated Substation (72kV-1200kA, 2 X 15MVA)............................39

Figure 42: After replacement with a Gas Insulated Substation...............................................39

Figure 43: AC distribution board at the Kitante Road Substation..........................................40

Figure 44: Elementary tripping circuit for a circuit breaker..................................................41

Figure 45: A cross section of one module of an air blast circuit breaker...............................42

Figure 46: A 630A air blast circuit breaker............................................................................42

Figure 47: Cross section of a Vacuum circuit breaker at CAA...............................................43

Figure 48: 40.5kV SF6 Circuit Breaker...................................................................................43

Figure 49: An Oil circuit breaker............................................................................................44

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Figure 50: Cross section of an oil circuit breaker...................................................................44

Figure 51: 400 to 12000A Disconnector and earthing switch.................................................45

Figure 52: Disc Insulator.........................................................................................................47

Figure 53: Solid core insulator...............................................................................................47

Figure 54: Hollow porcelain insulator....................................................................................47

Figure 55: Layout of a substation showing its components....................................................48

Figure 56: Some of the tools used during the installation work..............................................49

Figure 57: The exciter panel being removed from its wooden crate.......................................49

Figure 58: The exciter panel being lifted by the overhead crane............................................50

Figure 59: The 240V DC cables coming from the exciter panel to the rotor of the synchronous generator.............................................................................................................50

Figure 60: Toolbox and the Megger used at CAA...................................................................51

Figure 61: Carrying out the continuity test.............................................................................51

Figure 62: Carrying out the insulation test.............................................................................52

Figure 63: Cleaning the autotransformer...............................................................................52

Figure 64: Laptop, USB optical probe and Digital camera....................................................53

Figure 65: ABB REF542 Conf start window...........................................................................53

Figure 66: Choosing "Load from REF542plus"......................................................................54

Figure 67: REF542plus Communication port properties dialog box.....................................55

Figure 68: IEC 60870-5-103 properties dialog box................................................................56

Figure 69: The "Save As" dialog box.......................................................................................56

LIST OF TABLESTable 1: Excitation transformer nameplate data.....................................................................28

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CHAPTER ONE: INTRODUCTION

1.1 Industrial training Makerere University’s, College of Engineering, Design Art and Technology programs require all the students during their second and third year recess term to move out for industrial training with an intention of merging the theoretical aspects with practical field experiences. As a result industrial training is a core requirement for the award of a degree in any program offered in this faculty.

1.1.1 Objectives of Industrial trainingThe main objectives of industrial training include;

1) Industrial training has a major objective of enabling students acquire the practical skills, general knowledge and experience in the field commonly known as hands on.

2) Industrial training enables engineering students to understand the major roles an engineer is supposed to play in the engineering fraternity.

3) During this training period, trainees improve their public relations skills which are a fundamental requirement in any business sector.

4) It also helps to appreciate the ethical basics of engineering practice in the engineering world.

5) Lastly, industrial training is one of the course requirements for the award of a degree in Bachelor of Science in Electrical Engineering.

1.1.2 Merits of Industrial traininga) To the student

The student gets hands on the experience of the theoretical work done in class. It provides a suitable atmosphere for the student to operate with different people

under different conditions and rules. To be able to have an experience of the real engineering work in the field It’s an opportunity to make more friends as one associates with more people. It keeps the student academically occupied during holidays other than having to sit at

home Enables one to use and see most of the practical equipment that may not be available

in the university laboratory. The student obtains a better chance of acquiring a job in his or her training job as

he/she becomes more knowledgeable about the company b) To the company

It provides an opportunity for the company to freely train students from whom they can recruit employees in case they have vacancies

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It enables the company employees to refresh their memories as they explain to the employees the operation of most of the equipment.

It increases the amount of man power available for the various field operations in the company

It is another way of making the company known to the people as trainees talk about it to their friends and relatives.

Enables the company to fulfil the obligation of giving back to the community

1.2 Background to the companyThe history of ABB companies’ dates back to the late 19th century and this is a long and illustrious record of innovation and technological leadership in many industries.

ABB Uganda limited is part of the ABB group of companies, a multi-national company with headquarters based in Zurich, Switzerland. ABB are basically of two companies that merged together in 1988. One of the companies was ASEA whose headquarters was in Sweden and the other was BROWN-BOVERI with headquarters in Switzerland.

ABB is a leader in power and automation technologies that enable utility and industry customers to improve performance while lowering environmental impact. This is achieved through provision of solutions for securing energy efficient transmission and distribution of electricity, and for increasing productivity in industrial, commercial and utility operations. This group of companies operates in around 100 countries and employs around 112000 people.

ABB Uganda Limited was registered on 7th March, 1996 and it is incorporated in Nairobi, Kenya, where the East African regional headquarters are located. The Eastern African region comprises of Uganda, Kenya, Tanzania, Ethiopia, Eritrea, Somalia, Rwanda, Burundi and Djibouti.

1.2.1 Organisation structure of ABB ABB has a well developed organization, which ensures control, reporting and interrelationship among the employees. The General Manager ensures that responsibilities and authorities are defined and communicated to the employees in the organization and job descriptions of personnel who matter for quality of products and services are given for clarity of roles, responsibility, authority and inter-relationship.

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Figure 1: Organizational Chart of ABB

1.2.2 Products, services and expertiseIndustrial equipment;

a) Power factor correction equipmentb) Automation and drives systemsc) Industrial AC/DC motorsd) Low voltage switch geare) Distribution switch gearf) Distribution boardsg) Electrical cablesh) Electrical installation materialsi) Process instrumentation, analyzers, water and oil metersj) UPS systems (400VA-400kVA)k) Power stabilizersl) Generators: 7.5KVA to 2500KVA

Power Transmission and Distribution Products;a) High voltage cables and conductorsb) Distribution and power transformersc) Medium and high voltage switch geard) Control and protection equipment for power supply networkse) Filter and reactive power compensation systemf) Transmission and distribution power lines

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Managing Director

Sales Engineer

General Manager

Sales and Marketing Operations Manager

Senior Sales

Financial Manager

Project ManagerAccounts Assistant

Caretaker

Sales Assistant

Operations Assistant

Operations Assistant

g) Transmission and distribution Automation, SCADA systems

Services provided;a) Turn key projectsb) These include installation, Testing and commissioning of electrical products. c) Preventive maintenance and service of electrical and electrochemical systems, d) Overhauls of electrical and electrochemical systemse) Assistance in assessing customer needs, requirements and after sale services.

1.2.3 The ABB lookAn evolution that strengthens the ABB identity while keeping its customers in focus. ABBs’ slogan is “Power and Productivity for a Better world”.

Figure 2: The ABB logo

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CHAPTER TWO: ELECTRIC POWER GENERATION, EXCITATION SYSTEM, TRANSFORMERS AND

ELECTRICAL SUBSTATIONS

2.1 Electric power generationThe relationship between magnetism and electrical current was discovered and documented by Oerstad in 1819. He found that if an electric current was caused to flow through a conductor a magnetic field was produced around that conductor.

In 1831, Michael Faraday discovered that if a conductor is moved through a magnetic field, an electrical voltage is induced in the conductor. The magnitude of this generated voltage is directly proportional to the strength of the magnetic field and the rate at which the conductor crosses the magnetic field. The induced voltage has a polarity that will oppose the change causing the induction – Lenz’s law.

This natural phenomenon is known as Generator Action and is described today by Faraday’s Law of Electro Magnetic Induction which is mathematically stated as:

Where: e= induced emf, N=Number of turns of the coil and = rate of change

of the flux.

The negative sign indicates that the induced emf sets up in a direction so as to oppose the change causing it. All rotary generators built today use the basic principles of Generator Action.

2.1.1 The basic generatorThe basic generator will be studied below.

The elementary ac generator

The elementary ac generator consists of a conductor (or loop of wire) in a magnetic field (usually produced by an electromagnet). The two ends of the loop are connected to slip rings and they are in contact with two brushes. When the loop rotates it cuts magnetic lines of force, first in one direction and then the other. In the first half turn of rotation, a positive current is produced and in the second half of rotation produces a negative current. This completes one cycle of ac generation.

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Figure 3: The major parts of an AC generator

Figure 4: Field Excitation

Development of the sine wave

At the instant the loop is in the vertical position, the loop sides are moving parallel to the field and do not cut magnetic lines of force. In this instant, there is no voltage induced in the loop. As the loop rotates, sides will cut the magnetic lines of force inducing voltage in the loop. When the loop is in the horizontal position, maximum voltage is induced. The rotation of the coil through 360 degrees results in an ac sine wave output

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Figure 5: Development of the Sine wave

Three phase voltage

Three phase voltage is developed using the same principles as the development of single phase voltage. Three (3) coils are required positioned as shown below.

Figure 6: Three phases with electrical degree separation

A rotating magnetic field induces voltage in the coils which when aggregated produce the familiar three phase voltage pattern.

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Figure 7: Output voltage pattern

Types of generators

Essentially, there are two basic types of generators:

o Asynchronous (Induction) generators

o Synchronous generators

a. Induction generator

The induction generator is nothing more than an induction motor driven above its synchronous speed by an amount not exceeding the full load slip the unit would have as a motor. Assuming a full load slip of 3%, a motor with a synchronous speed of 1200 rpm would have a full load speed of 1164 rpm. This unit could also be driven by an external prime mover at 1236 rpm for use as an induction generator. The induction generator requires one additional item before it can produce power – it requires a source of leading VAR’s for excitation. The VAR’s may be supplied by capacitors (this requires complex control) or from the utility grid.

Induction generators are inexpensive and simple machines, however, they offer little control over their output. The induction generator requires no separate DC excitation, regulator controls, frequency control or governor.

b. Synchronous generator

Synchronous generators are used because they offer precise control of voltage, frequency, VARs and WATTs. This control is achieved through the use of voltage regulators and governors. A synchronous machine consists of a stationary armature winding (stator) with many wires connected in series or parallel to obtain the desired terminal voltage. The armature winding is placed into a slotted laminated steel core. A synchronous machine also consists of a revolving DC field - the rotor. A mutual flux developed across the air gap between the rotor and stator causes the interaction necessary to produce an EMF. As the magnetic flux developed by the DC field poles crosses the air gap of the stator windings, a sinusoidal voltage is developed at the generator output terminals. This process is called electromagnetic induction. The magnitude of the AC voltage generated is controlled by the amount of DC exciting current supplied to the field.

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If “FIXED” excitation were applied, the voltage magnitude would be controlled by the speed of the rotor (E=4.44fnBA), however, this would necessitate a changing frequency! Since the frequency component of the power system is to be held constant, solid state voltage regulators or static exciters are commonly used to control the field current and thereby accurately control generator terminal voltage. The frequency of the voltage developed by the generator depends on the speed of the rotor and the number of field poles. For a 60 Hz system, Frequency = speed (rpm)*pole pairs/60.

Figure 8: The synchronous generator units at Nalubaale power station

Generator parts and functions

Generator Frame: Provides the structural strength and rigidity for the generator and serves as a housing to guide cooling air flow

i. Inner End Shield: Is a baffle used to form a path for cooled air

ii. Generator Fan: Provides continuous circulation of cooling air

iii. Rotating Field: A magnetic field which induces AC voltage in the stator windings

iv. Collector Rings: Provide a connection and path for DC power into the rotating field windings

v. Main Coupling: Is the connection to the drive shaft of the Generator

vi. Stator Core: Houses the stationary windings and forms a magnetic path necessary for induced voltages

vii. Air Gap: Is the radial clearance between the rotating field and the stator core

viii. Stator Coil End Turns: Formed when coils leave one slot in a stator core and are returned to a different slot

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ix. Terminal Leads: Serve to conduct the three phase voltage and current flow from the generator stator to the external system

Figure 9: Output stator terminal for one phase

2.2 The excitation systemIn power plants, the excitation system has a powerful impact on generator dynamic performance and availability, it ensures quality of generator voltage and reactive power, i.e. quality of power plant delivered energy to consumers.

The main function of the excitation system is to:

i) Provide variable DC current with short-time overload capability

ii) Control terminal voltage with suitable accuracy

iii) Ensure stable operation with network and/or other machines

iv) Contribute to transient stability subsequent to a fault

v) Communicate with the power plant control system and to keep machine within permissible operating range.

2.2.1 Static excitation systemThis involves feeding rotor directly from thyristor bridges via brushes. Static excitation system plays a very important role in modern interconnected power system operation due to its fast acting, good response in voltage & reactive power control and satisfactory steady state stability condition. For machines of 500 MW & above and fire hazards areas, Brushless Excitation System is preferred due to larger requirement of current & plant safety respectively.

In order to maintain system stability in interconnected system networks, it is necessary to have fast acting excitation system for large synchronous machines which means the field current must be adjusted extremely fast to the changing operational conditions. Besides

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maintaining the field current and steady state stability the excitation system is required to extend the stability limits. It is because of these reasons the static excitation system is preferred to conventional excitation systems.

In this system, the AC power is tapped off from the generator terminal stepped down and rectified by fully controlled thyristor Bridges and then fed to the generator field thereby controlling the generator voltage output. A high control speed is achieved by using an internal free control and power electronic system. Any deviation in the generator terminal voltage is sensed by an error detector and causes the voltage regulator to advance or retard the firing angle of the thyristors thereby controlling the field excitation of the alternator.

This equipment controls the generator terminal voltage, and hence the reactive load flow by adjusting the excitation current. The rotating exciter is dispensed with the Transformer & silicon controlled rectifiers (SCRS) are used which directly feed the field of the Alternator.

Figure 10: Static exciter panel

Components of the static excitation system

Static Excitation Equipment Consist of;

1) Excitation (Rectifier) Transformer

The excitation power is taken from generator stator output terminals and fed through the excitation (rectifier) transformer which steps down to the 11kV from the stator to 415V which is the required voltage for the SCR Bridge and then fed through the field breaker to the generator field. The rectifier transformer used in the SES should have high reliability as failure of this will cause shutdown of the power station.

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Figure 11: 11kV cables from the stator terminal to the excitation transformer

Dry type cast coil transformer is suitable for static excitation applications. The transformer is selected such that it supplies rated excitation current at rated voltage continuously and is capable of supplying ceiling current at the ceiling excitation for a short period of ten seconds. Rectifier transformers directly connected to the generator terminals and feeding power to the field of the machine via thyristor converters, plays an important role in an excitation system and in turn power generation Reliability of this transformer has to be ensured in all respects. The selection of the secondary voltage of excitation transformer depends upon the field forcing voltage. The primary voltage is the same as that of generator terminal voltage.

Current rating is dependent on the maximum continuous current in the field winding. Generally the power rating of the Excitation transformer used in Static Excitation System is around 1 % of the rating of generator in MVA. These transformers are of class "F" insulation and indoor type. The cooling method of the excitation transformer is Air Natural (AN).

Figure 12: Excitation transformer with 415V output cables

2) SCR Output stage

The SCR output stage consists of a suitable number of bridges connected in parallel. Each thyristor bridge comprises of six thyristors, working as a six pulse fully controlled bridge. Current carrying capacity of each bridge depends on the rating of individual thyristor.

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11kV

415V output

Thyristors are designed such that their junction temperature rise is well within its specified rating. By changing the firing angle of the thyristors, variable output is obtained. Each bridge is controlled by one final pulse stage and is cooled by a fan. These bridges are equipped with protection devices and failure of one bridge causes alarm. If there is a failure of one or more thyristor bridges then the excitation current will be limited to a predetermined value lesser than the normal current. However, failure of the third, bridge results in tripping and rapid de-excitation of the generator. The above is applicable for four bridges thyristor with (n-1) principle operation.

Figure 13: The thyristor bridge or power converter

3) Excitation Start Up and Field Discharge Equipment

For the initial build-up of the generator voltage, field-flashing equipment is required. The rating of this equipment depends on the no-load excitation requirement and field time constant of the generator. From the reliability point of view, provisions for both the AC & DC field flashing are provided. The field breaker is selected such that it carries the full load excitation current continuously and also it breaks the maximum field current when the three phase short circuit occurs at the generator terminals.

The field discharge resistor is normally of non-linear type for medium and large capacity machines i.e. voltage dependent resistor. To protect the field winding of the generator against over voltages, an over voltage protection along with a current limiting resistor is used to limit the over voltage across the field winding. The OVP operates on the insulation break over Principle. The voltage level at which OVP should operate is selected based on insulation level of field winding of the generator.

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4) Field Discharge

During load condition whenever the Field breaker, opens suddenly there will be a surge voltage in the rotor which will damage the rotor winding insulation. To avoid this, the rotor winding is connected to the earth through a field discharge Resistor thereby by passing the surge voltage to earth. Field discharge greatly helps to limit the damages. 'Non-linear field discharge resistance is used which helps in faster field suppression/discharge.

Figure 14: The field discharge resistor

5) Regulator & Operational Control Circuits (Control Electronics)

Regulator is the heart of the system. This regulates the generator voltage by controlling the firing pulses to the thyristors.

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Figure 15: The control unit chamber of the exciter panel

6) Error detector & amplifier

The Generator terminal voltage is stepped down by a three phase PT and fed to the AVR. The a.c. input thus obtained is rectified, filtered and compared against a highly stabilized reference value and the difference is amplified in different stages of amplification. The AVR is designed with highly stable elements so that variation in ambient temperature does not cause any drift or change in the output level. Three CTs sensing the output current of the generator feed proportional current across variable resistors in the AVR. The voltage thus obtained across the resistors, can be added vectorially either for compounding or for transformer drop compensation. The percentage of compensation can be adjusted as the resistors are of variable type.

7) Grid - control unit

The output of the AVR is fed to a grid control unit, it gets its synchronous AC. reference through a filter circuit and generates six double, pulses spaced 600 electrical degrees apart whose position depends on the output of the AVR i.e. the pulse position varies continuously as a function of the control voltage. Two relays are provided, by energizing which, the pulses can be either blocked completely or shifted to inverter mode of operation

8) Pulse - amplifier

The pulse output of the ""Grid control unit "' is amplified further at intermediate stage amplification. This is also known as pulse intermediate stage. The unit has a DC power supply, which operates from a three phase 38OV supply and delivers +15V,1 – l5V,+5V, and a coarse stabilized voltage VL. A built in relay is provided which can be used for blocking the 6 pulse channels. In a two channel system (like Auto and Manual), the changeover is effected by energising/ de-energizing the relay.

9) Pulse final stage

This unit receives input pulses from the pulse amplifier and transmits them through pulse transformers to the gates of the thyristors. A built in power supply provides the required DC supply to the final pulse and amplifier. Each Thyristor bridge has its own final pulse stage. Therefore, even if a thyristor bridge fails with its final pulse stage, the remaining thyristors bridges can continue to cater to full load requirement of the machine and thereby ensure (n-1) operation.

10) Manual control channel

A separate manual control channel is provided where the controlling DC signals in taken from a stabilized DC voltage through a motor operated potentiometer. The DC signal is fed to a separate grid control unit whose output pulses after being amplified at an intermediate stage can be fed to the final pulse stage. When one channel is working, generating the required pulses, the other remains blocked. Therefore a changeover from "Auto" to "Manual'

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control or vice versa is effected by blocking or releasing the pulses of the corresponding intermediate stage. A pulse supervision unit detects spurious pulses or loss of pulses at the pulses bus bar and transfers control from Automatic Channel to manual channel.

11) Follow-up unit

To ensure a smooth changeover from 'Auto"' to Manual" control, it is necessary that the position of the pulses on both channels be identical. A pulse comparison unit detects any difference in the position of the pulses and with the help of a follow-up unit actuates the motor operated potentiometer on the "'Manual"' Channel to turn in a direction so as to eliminate the difference. However, while transferring control from "Manual"' to "Auto" mode any difference in the two control levels can be visually checked on a balance meter and adjusted to obtain null before change over.

12) Limit controllers

When a generator is running in parallel with the power network, it is essential to maintain it in synchronism without exceeding the rating of the machine and also without the protection system tripping. Only automatic Regulator cannot ensure this. It is necessary to influence the voltage regulator by suitable means to limit the over excitation and under excitation. This not only improves the security of the parallel operation but also makes operation of the system easier. However limiters do not replace the protection system but only prevent the protection system from tripping unnecessarily under extreme transient conditions. The AVR also has a built-in frequency dependent circuit so that the machine is monitored to run at the rated frequency and to ensure that the regulated voltage is proportional to frequency. With the help of a potentiometer provided in the AVR, the circuit can be made to respond proportionally to voltage above a certain frequency and proportional to a voltage below a certain frequency. The range of adjustment of this cut off frequency lies between 40 and 60 Hz. The static excitation system is equipped with three limiters which act in conjunction with the AVR. These limiters are as under;

13) Rotor current limiter

The unit basically comprises an actual value converter, a limiter with adjustable PID characteristics, a reference value, dv/dt sensor and a signalisation unit. The field current is measured on the AC input side of the thyristor converter and is converted into proportional DC voltages. The signal is compared with an adjustable reference value, amplified, and with necessary time lapse fed to the voltage regulator input. Rotor current limiter avoids thermal overloading of the rotor winding and is provided to protect the generator rotor against excessively long duration over loads. The ceiling excitation is limited to a predetermined limit and is allowed to flow for a time which is dependent upon the rate of rise of field current before being limited to the thermal limit value.

14) Rotor angle limiter

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This unit limits the angle between the voltage of the network centre and the rotor voltage or it limits the angle between the generator voltage and the rotor voltage. It comprises an actual value converter, a limiting amplifier with adjustable PID characteristics and a reference value unit. The limiting regulator operates as soon as the DC value exceeds the reference value. For its operation the Unit is given separate power supply from a DC power pack. It generates a DC signal proportional to the load or rotor angle from the stator current and voltage by means of a simple analogue circuit. The device takes over as soon as the set limit angle is exceeded. By increasing the excitation and ignoring opposite control signals the unit is prevented from failing out of step.

15) Stator current limiter

This unit functions in conjunction with an integrator unit which provides the necessary dead time and the gradient, which can be adjusted by potentiometers. The regulator consists essentially of a measuring converter, two comparators, two PID regulators and a DC power pack. A discriminator in the circuit differentiates between inductive and capacitive current. The positive and negative signals processed by two separate amplifiers are brought to the output stage and only that output which has to take care of the limitation is made effective. Stator current limiter avoids thermal over loading of the stator windings. Stator current limiter is provided to protect the generator against long duration of large stator currents. For excessive inductive current it acts over the AVR after a certain time lag and decreases the excitation current to limit the inductive current to the limit value. But for excessive capacitive current it acts on the AVR without time delay to increase the Excitation and thereby reduce the capacitive loading. This is necessary as there is a risk for the machine falling out of step during under excited mode of operation.

16) Slip stabilizing units

The slip stabilizing unit is used for the suppression of rotor oscillations of the alternator through the additional influence of excitation. The slip as well as acceleration signals needed for the stabilization are derived from active power delivered by the alternator. Both the signals, which are correspondingly amplified and summed up, influence the excitation of the synchronous machine through AVR in a manner as to suppress the Rotor oscillations. (http://www.pdffactory.com/static excitation system)

2.2.2 Merits of the excitation systemi) Performance

1) Faster Response in voltage & Reactive Power Control

2) Higher Accuracies of Voltage control

3) Faster discharge of field energy with inversion of thyristor bridge output

4) Provision of limiters & stabilizing equipment help to improve dynamic & transient stability

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5) Largely independent of variation in the excitation supply voltage & frequency

ii) Operational Maintenance

1) Redundancies in different Circuits increase reliability and availability

2) Adequate monitoring facilities aid fault finding & reduce down time

3) Absence of rotating parts enables less maintenance

4) Lower SCR value for larger generators (Reduces weight/compact in size & cost)

iii) General

1) Up rating of the machine can be done by adding additional power circuits/adding more redundancies

2) Location of the equipment can be planned independent of the machine thereby increasing the flexibility in the plant layout

3) Retrofitting of Static Excitation Equipment for old slow acting exciter

4) Length of the machine shorter when compare to other excitation equipment

2.2.3 Power supply The voltage regulating equipment needs an AC supply 38OV 3 Phase for its power supply units which is derived from the secondary side of the rectifier transformer through an auxiliary transformer. This voltage is reduced to different levels required for the power packs by means of multi-winding transformers.

A separate transformer supplies the synchronous voltage 3x38OV for the filter circuit of each channel and the voltage relay. During testing and pre-commissioning activities when generator voltage is not available, the station auxiliary supply 3 Phase 415V can be temporarily connected through an auxiliary step down transformer for testing purpose with the help of a regulator test/service switch.

The supply for the thyristor Bridge fan is taken from an independent transformer which gets its input supply from the secondary of the excitation transformer. The control & protection relays need 48V & 24VDC which are delivered from the station battery by means of the DC/DC converters, which are internally protected against overload.

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Figure 16: The Transformer in the exciter panel

2.2.4 ProtectionThe following modes of protection are provided in the Static Excitation Equipment;

1) Rectifier transformer over current instantaneous and delayed.

2) Rectifier transformer over Temperature

3) Rotor Over-Voltage

4) Rotor earth fault.

5) Fuse failure monitoring circuit for thyristors

6) Loss of control voltage (48V & 24V)

7) dv/dt protection of SCR by snubber net works

8) Cooling System failure for thyristors

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Figure 17: Basics of Excitation systems

Figure 18: The power station showing the role of the Excitation system

2.3 TransformersTransformers have been used since the inception of alternating-current generation, a century ago. While operating principles of transformers remain the same, the challenges of maintaining and testing transformers have evolved along with transformer design and construction.

A transformer is an energy coupling device that takes electrical energy at one voltage (from its source) converts it into another voltage. The new voltage may be higher (for the case of a step up transformer) or lower (for the case of a step down transformer), or it may be desired to have a specific value. (For the case of an Autotransformer)

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Figure 18: Basic transformer

2.3.1 Construction of a transformerA transformer primarily comprises of three major parts ie:

i. Core

The core is typically made from very thin steel laminations, each coated with insulation. By insulating between individual laminations, losses are reduced. The steel core provides a low resistance path for magnetic flux. Both high and low voltage windings are insulated from the core and from each other, and leads are brought out through insulating bushings. A three-phase transformer typically has a core with three legs and has both high- voltage and low-voltage windings around each leg. Special paper and wood are used for insulation and internal structural support.

ii. Coils(windings)

They are made of paper insulated copper or aluminium wire (magnet wire) which is wound around a laminated iron core. The primary winding is wound over the secondary winding but the two coils are well insulated from each other and the core to prevent shorts and grounds.

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Figure 19: Transformer construction

iii. Enclosure(structural parts)

For a transformer to be useful, its internal components (coils and windings) must be

connected to the physical/external components which are then connected by wires to the

desired load. These external components include:

a) Transformer tank

The transformer tank encloses all the internal components of the transformer which are immersed in oil.

Transformer oil performs two functions

It is used as an insulating medium between the HV and LV coils. This provides high resistance between the two windings which prevents short circuits within the transformer.

It cools the coils. As current passes through the copper coils, heat is generated which is drawn away by the oil.

The transformer tank has fins connected on to the sides. These fins also enhance transformer cooling by increasing the surface area which allows air to circulate naturally over the transformer.

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Figure 20: Autotransformer tank at CAA

b) Bushings

Bushings are insulators with small holes running vertically through them. Studs that are fixed to either the HV coil or the LV coil are fixed in the holes so that the coils have no direct contact with the transformer tank which is normally earthed. If the bushing is contaminated with deposits of dust and salt, it may lead to flashover troubles. This can occur especially in heavy polluted areas or coastal areas.

Figure 21: Autotransformer bushings at CAA

c) Surge Arrestors

Surge arrestors are either polymeric or porcelain insulators connected between the HV winding of the transformer and the earth to protect the transformer by arresting surges and take them to earth. Suitable sizes of surge arrestors are used for corresponding voltages.

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Bushings

Figure 22: Surge Arrestor

d) Pressure relief valve

Pressure Relief Valve is a safety element that a transformer employs to prevent heavy damages to the tank in case of sudden rise of the internal pressure. This Valve has been designed in order to remove the excess pressure in a very short time. As soon as the pressure in the tank rises above the predetermined safe limit, the valve operates and allows the pressure to drip instantaneously thereby avoiding damage to transformer body.

Figure 23: Pressure relief valve

e) Breather

The oil quantity in the oil cup must be kept around the oil level line on the cup. If the oil is low, moisture in the air is not absorbed or if in excess the silica gel in the breather is wetted and the moisture absorbing performance of the silica gel deteriorates.

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Figure 24: Breather on Autotransformer at CAA

In other words, failure of the breather promotes deterioration of the insulating oil and affects the life

of the transformer.

a) Tap changer

A transformer tap is a connection point along a transformer winding that allows the number of turns to be selected. This means, a transformer with a variable turn’s ratio is produced, enabling voltage regulation of the secondary side. Selection of the tap in use is made via a tap changer mechanism. Tap points are usually made on the high voltage or low current side of the winding in order to minimise the current handling requirements of the contacts.Power transformers have automatic tap changers while distribution transformers make use of off -load tap changers. On load tap changers can monitor the load and change the voltage tap but with off load tap changing, information about the voltage variations is obtained from customers who are connected to that transformer. This can be achieved through complaints such as dim light, customers connected to the same transformer may not all receive power.

Figure 25: On- load Tap Changer Control Unit (Automatic and Manual) at CAA

b) Earth terminal

The earth terminal on the transformer tank provides the earthing point on the transformer. The transformer must be earthed to protect it from faults that may be internal or external. The harking horns and surge arrestors arrest lightening and surges respectively and ground them through the earth terminal. The earth terminal is connected to the ground through a steel wire.

c) Painting

Painting of the transformer tank is intended to protect the metal surface from corrosion and improve the outward appearance. In the course of years of use, the film deteriorates due to

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sunlight, temperature changes, humidity and chemical atmosphere. When it is left in those conditions, not only the appearance becomes poor, but also the life of the transformer tank or other parts may be shortened

2.6.2 Principle of operation of a transformerTransformer function is based on the principle that electrical energy is transferred efficiently by magnetic induction from one circuit to another. When one winding of a transformer is energized from an AC source, an alternating magnetic field is established in the transformer core. Alternating magnetic lines of force called “flux,” circulate through the core. With a second winding around the same core, a voltage is induced by the alternating flux lines. A current passing through a conductor will induce a magnetic field around the conductor.

The primary winding is connected to a 60 hertz alternating current voltage source. Magnetic flux is induced into the core which is coupled through to the secondary. A voltage is induced into the winding, and a current is produced through the load. The magnitude of the voltage in the secondary is determined by the formula below that involves the turn’s ratio/voltage ratio of the transformer.

Turns Ratio (K) =

Secondary voltage is derived from the above equation as follows:

Significance of turns ratio

If the primary has eight turns, and the secondary has four turns. The turns ratio is 2:1, thus, the voltage of the primary winding of the transformer is twice that of its secondary winding. For the same input voltage, an increase in the number of turns will decrease the flux density and vice versa.

2.6.3 Types of transformersTransformers are broadly categorized as step up or step down transformers

Step up transformer

When the number of turns or voltage on the secondary of a transformer is greater than that of the primary, it is known as a step-up transformer.

Step down transformer

When the number of turns or voltage on the secondary is less than that of the primary, it is known as a step-down transformer.

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Figure 26: Step up and Step down transformers

Figure 27: Two step down power transformers (33/11kV) at the UMEME Kitante Road Substation

2.6.4 Transformer name plate dataThe name plate that is normally tagged on the transformer tank gives relevant information/data about the transformer. Below is the nameplate data of an excitation transformer at Nalubaale Power Station:

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May & Christe GmBH

Continuous, THREE PHASE,CLASS F,POWER TX

COOLING AN RATED

VOLTAGE

H.V 11000 V

FREQUENCY 50 Hz L.V 415V

VOLT

VARIATION

CFR RATED

CURRENT

H.V 21.5A

KVA 410 L.V

AMBIENT 45ºC YEAR 1990

WEIGHT 2200kg %

IMPEDANCE

4.1

VECTOR

SYMBOL

Dd0

SERIAL NO: 20435

Table 1: Excitation transformer nameplate data

Figure 28: Excitation transformer at Nalubaale Power Station

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Cooling

Increasing the cooling rate of a transformer increases its capacity. Cooling methods must not only maintain a sufficiently low average temperature but must prevent an excessive temperature rise in any portion of the transformer. Ducts are arranged to provide free circulation of oil through the core and coils, warmer and lighter oil rises to the top of the tank, cooler and heavier oil settles to the bottom. Several methods have been developed for removing heat that is transmitted to the transformer oil from the core and windings and these include:

ONAN

OFAF

AN

ONAF

OFAN

OFWF

A given transformer can have a combination of cooling parts to permit a change in the type of cooling

e.g. ONAN/ONAF.

Frequency

Frequency refers to the number of complete cycles of alternating voltage and current made per second.

Power rating

The kVA rating of a three phase transformer provides the amount of its apparent power. Apparent

power consists of both the active power (P/kW) and reactive power (Q/kVAR). The Power triangle showing the relationship between reactive, active and apparent power is shown below.

Figure 29: The power triangle

oil

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This indicates the amount of oil required to fill up the transformer tank during normal operation of the transformer. It also specifies that oil is used as the coolant as well as insulating medium.

Rated voltage

This shows the rated line voltages of both the HV and LV sides. This means that with no losses in the transformer, the transformer can successfully step down 11000 volts on the HV side to 415 volts on the LV side.

Rated current

This indicates the corresponding rated currents for both the HV and LV windings. The LV current is usually called the Load current of the transformer because it is connected to load. This information is also useful in determining the fuses on both the HV and LV sides. If all transformer losses are neglected, the formula shows that the higher the voltage, the lower the current and vice versa.

Where:

N1 = number of turns connected to input voltage

N2 = number of turns connected to output voltage

U1 = input (primary) voltage

U2 = output (secondary) voltage

I1 = input (primary) current

I2 = output (secondary) current

Ambient temperature

The maximum winding temperatures reached at any one time should not exceed 45ºC respectively.

Vector symbol

This nomenclature provides important information about the way in which three phase windings are connected and any phase displacement that occurs

ImpedanceThe specification is used to derive the testing voltage/impedance voltage for ratio and short circuit tests. It is normally expressed as a percentage of Primary voltage. In this case the impedance voltage would be 4.1% of 11kV.

2.6.5 Transformer lossesTransformer losses are attributable to several causes and may be differentiated between those originating in the windings, sometimes termed copper loss, and those arising from the magnetic circuit, sometimes termed iron loss. The losses vary with load current, and may furthermore be expressed as "no-load" or "full-load" loss, respectively. Winding resistance dominates load losses, whereas hysteresis and eddy currents losses contribute to over 99% of the no-load loss. The no-load

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loss can be significant, meaning that even an idle transformer constitutes a drain on an electrical supply, and lending impetus to development of low-loss transformers.

Copper losses (full load loss)

Current flowing through the windings causes resistive heating of the conductors.

Copper loss, in watts, can be found using the formula;

Copper loss =

Where IP    =    primary current IS    =    secondary currentRP =    primary winding resistance RS =    secondary winding resistance

Core/Iron (no load) losses

There are two kinds of losses associated with the core:

i. Hysteresis losses

Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core. For a given core material, the loss is proportional to the frequency, volume of the lamination and is a function of the peak flux density to which it is subjected

ii. Eddy current losses.

Ferromagnetic materials are good conductors, and a solid core made from such a material also constitutes a single short-circuited turn throughout its entire length. Eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is a complex function of the square of supply frequency and Inverse Square of the material thickness, it is therefore important to use thin laminations of high-grade electrical steel/iron.

2.6.6 Transformer testsSeveral tests are performed on the transformer to ensure safety of the person working on the transformer as well as to determine acceptable limits within which it can operate safely with maximum output. Tests are carried out by following certain procedures.The different tests that must be carried out include:

i. Open circuit testii. Short circuit test

iii. Voltage ratio test iv. Dielectric testv. Pressure test

vi. Continuity testvii. Insulation resistance / Megger test

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All the above tests are carried out in the distribution transformer workshop apart from the open circuit test.

Open circuit /No load test

The purpose of this test is to determine the no load loss or core loss. Since no current flows through the secondary winding, copper losses are taken to be negligible.The wiring set up is as shown below:

Figure 30: Wiring set up for open circuit test

Open circuit test procedures are as follows:

With the secondary open, the primary voltage is increased from zero to rated voltage, where the rated voltage is on the name plate stamp. A digital multimeter is used as an ammeter to measure the open circuit current. A wattmeter is used to measure the open circuit power. The power measured is equivalent to the core losses.

Short circuit (full load) test

The short circuit test is aimed at determining the copper losses in both the HV and LV windings.

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Figure 31: Wiring set up for short circuit test

Short circuit test procedures are normally as follows:

With the secondary terminals shorted, the primary voltage is increased from zero until the rated current is reached in the primary. At this point the primary voltage is measured. It is usually much less than rated voltage. Again, the power and current are measured.If the deviation between the test values and rated values is less than 20%, then the transformer is recommended for use.

Voltage ratio test

The aim of this test is to determine whether the turn’s ratio obtained form the test corresponds with that on the name plate.

Insulation test

The insulation test can be carried out by either using the insulation Megger or by performing a pressure test on the transformer. This test provides adequate information about the insulating medium that is used in the transformer.

Megger (Insulation Resistance) test

The insulation test indicates the condition of the insulation, that is, the degree of dryness of paper insulation, presence of any foreign containment in oil and also any serious defects in the transformer. Insulation resistance measurements will vary widely from one transformer to another.

Continuity Test

The continuity test guarantees that there is electrical continuity of all the phases of the HV and LV windings. This can be determined by using a Megger. The HV-HV and LV-LV values were 0Ω, which showed that the HV and the LV windings were continuous.

Dielectric strength test

This test is performed to determine the insulation (oil) breakdown voltage. The breakdown voltage drops significantly in the presence of moisture and impurities in the oil. When measuring the insulation breakdown voltage, sufficient care must be taken in sampling and testing.

2.6.7 AutotransformerIt is possible to obtain transformer action by means of a single coil, provided that there is a “tap connection” somewhere along the winding. Transformers having only one winding are called autotransformers, shown below.

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Figure 32: Autotransformer

Figure 33: Inside the autotransformer at CAA showing the input cables

Figure 34: Two 11kV incoming UMEME lines to CAA

An autotransformer has the usual magnetic core but only one winding, which is common to both the primary and secondary circuits. The primary is always the portion of the winding connected to the AC power source. This transformer may be used to step voltage up or down. If the primary is the total winding and is connected to a supply, and the secondary

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Input cables

Shared Line by CAA and UMEME Entebbe Customers

Dedicated Line strictly for CAA

circuit is connected across only a portion of the winding (as shown), the secondary voltage is “stepped-down.” If only a portion of the winding is the primary and is connected to the supply voltage and the secondary includes all the winding, then the voltage will be “stepped-up” in proportion to the ratio of the total turns to the number of connected turns in the primary winding. (ALSTOM, 1987)

2.6.8 Current transformer

A current transformer is designed to provide a current in its secondary which is accurately proportional to the current flowing in its primary. Current transformers (CTs) are commonly used in metering and protective relaying. They facilitate the safe measurement of large currents, often in the presence of high voltages. The current transformer safely isolates measurement and control circuitry from the high voltages typically present on the circuit being measured. Current transformers have their primary windings connected in series with the power circuit, and so also in series with the system impedance.

Figure 35: Current transformers

2.6.9 Voltage transformer

Used for metering and protection in high-voltage circuits. They are designed to present negligible load to the supply being measured and to have a precise voltage ratio to accurately step down high voltages so that metering and protective relay equipment can be operated at a lower potential. They provide the input voltage to the relays which is usually 110V.

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Figure 36: Voltage transformer

2.7 Electrical substationThe present day electrical power system is AC i.e. electric power is generated, transmitted and distributed in the form of Alternating current. The electric power is produced at the power station, which is normally located at favourable places, generally away from the consumers. It is delivered to the consumer through a large network of transmission and distribution. It may be desirable and necessary to change some characteristic (e.g. Voltage, ac to dc, frequency, p.f. etc.) of electric supply. This is accomplished by suitable apparatus called sub-station for example, generation voltage (11kv or 6.6kv) at the power station is stepped up to high voltage (Say 220kv to 132kv) for transmission of electric power. Similarly near the consumer’s localities, the voltage may have to be stepped down to utilization level. This job is again accomplished by suitable apparatus called sub-station.

Figure 37: Inside the 33/11kV UMEME Kitante Road Substation showing 11kV Distribution relays

DefinitionAn electrical substation is a subsidiary station of an electricity generation, transmission and distribution system where voltage is transformed from high to low or the reverse using power transformers. On the other hand, a switch is a junction connecting the transmission and distribution system to the power plant.The difference between a substation and switch yard is that, the function of a substation is to step up or down the voltages as per requirement. It receives electrical power via incoming transmission lines and delivers electrical power via the outgoing transmission lines while the switch yard is to deliver the generated power to the nearest grid.

2.7.1 Classification of a substationThere are several ways of classifying sub-station. However the two most important way of classifying them, these include;

According to service requirement

According to service requirement sub-station may be classified into.

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i. Transformer sub-station Those sub-stations which change the voltage levels of electrical power supply are called TIF s/s.

ii. Switching sub-station This sub-station simply performs the switching operation of power line.

iii. Power factor correction S/SThose sub-stations which improve the p.f. of the system are called p.f. correction s/s. these are generally located at receiving end s/s.

iv. Frequency changer S/S Those sub-stations, which change the supply frequency, are known as frequency changer s/s. Such s/s may be required for industrial utilization.

v. Converting sub-station These are sub-station which change AC power into DC power are called converting s/s. Ignition is used to convert AC to DC power for traction, electroplating, electrical welding etc.

vi. Industrial sub-station Those sub-stations, which supply power to individual industrial concerns, are known as industrial sub-station.

According to constructional features

According to constructional features, the sub-station are classified as;

i. Outdoor Sub-Station For voltage beyond 66KV, equipment is invariably installed outdoor. It is because for such Voltage the clearances between conductor and the space required for switches, C.B. and other equipment becomes so great that it is not economical to install the equipment indoor.

Figure 38: Kawanda Substation (220/132kV) is an outdoor substation

ii. Indoor Sub-station

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For voltage up to 11KV, the equipment of the s/s is installed indoor because of economic consideration. However, when the atmosphere is contaminated with impurities, these sub-stations can be erected for voltage up to 66KV.

Figure 39: Kitante Road Substation (33/11kV) is an indoor substation

iii. Underground sub-station In thickly populated areas, the space available for equipment and building is limited and the cost of the land is high. Under such situations, the sub-station is created underground.However the following should be carefully considered:

The design of underground s/s requires more careful consideration; The size of the s/s should be as minimum as possible. There should be reasonable access for both equipment & personal. There should be provision for emergency lighting and protection against fire. There should be good ventilation.

For an underground substation, its highly preferred to construct a Gas Insulated Substation (GIS) because of its inherent safety and compact structure as compared to an Air Insulated Substation (AIS).

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Figure 40: 330kV Gas Insulated Underground Substation

The figures below show the difference in the physical space occupied by an Air Insulated Substation and a Gas Insulated Substation.

Figure 41: Existing Air Insulated Substation (72kV-1200kA, 2 X 15MVA)

Figure 42: After replacement with a Gas Insulated Substation

iv. Pole-mounted sub-station This is an outdoor sub-station with equipment installed overhead on H.pole or 4-pole structure. It is the cheapest from of s/s for voltage not exceeding 11KV (or 33KV in some cases). Electric power is almost distributed in localities through such sub-station.

The lighting arresters are installed on the H.T. side to protect the sub-station from lighting strokes. The T/F step down voltage to 400V, 3phase, 4 wire supply. The voltage between any two lines is 400 V & between line & neutral is 230V. The oil circuit breaker installed on the L.T. side automatically isolates the mounted sub-station.

2.7.2 Components of a substationSubstation comprises three main components:

a) Primary System Primary system comprises all equipment which is in service at the nominal voltage system.

b) Secondary System

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The secondary system comprises all equipment which are used for the control, protection, and measurement and monitoring of primary equipment.

c) Auxiliary Supply System Auxiliary supply system comprises all equipment such as AC supplies and DC supplies that enable protection, control, measurement and monitoring equipment to operate.

i. DC Supply and Distribution Systems These are provided to ensure that a secure supply is available at all times to power protection systems, control equipment and initiate tripping of circuit breakers and comprise:

ii. Batteries Either lead acid or nickel-alkali types at ratings from tens of ampere hours to hundreds of ampere hours at voltages of 30V, 50V, 125V and 250V depending on the application.

iii. Battery Charger Usually constant voltage, current limited types with boost charge facility to supply standing loads and maintain the battery fully charged whilst the auxiliary AC supply is available.

iv. Distribution BoardAs the name implies, provides a system of distribution, isolation and protection for DC supplies to all equipment within the substation.

v. LVAC SuppliesAn auxiliary AC supply and distribution system which supports the operation of the substation by providing power for cooling fan motors, tap change motors, circuit breaker mechanism charging systems and Disconnector drives in addition to the normal heating, lighting and domestic loads.

Figure 43: AC distribution board at the Kitante Road Substation

2.7.3 Substation equipmentA substation consists of the following equipment:

Circuit Breakers

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A circuit breaker is a mechanical switching device capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying for a specified time and breaking currents under specified abnormal circuit conditions such as those of short circuit. As systems have increased in size and complexity, the circuit breaker has been called upon to have better short circuit interrupting performance, to operate faster and to tolerate higher and higher system voltages. Initially as fault currents increased circuit breakers became more and more complex to achieve the required performance, particularly when 400kV systems with fault currents of up to 63kA were designed. A circuit breaker can be operated manually (local) or remotely under normal conditions and automatically in case of a fault condition. Remote control is made possible through relays.

Action of a circuit breakerA circuit breaker consists of moving and fixed contacts called electrodes. Under normal conditions, these contacts remain closed and will not open automatically unless the system develops a fault. The contacts can be opened manually or by remote control whenever desired. When a fault occurs on any part of the system, the trip coils of the circuit breaker get energized and moving contacts are pulled apart by some mechanism, thus opening the circuit.The triggering action that causes a circuit breaker to open is usually produced by means of an overload relay that can detect abnormal line conditions. For example, the relay coil in the figure below is connected to the secondary of a current transformer. The primary carries the line current of the phase that has to be protected. If the line current exceeds the preset limit, the secondary current will cause relay contacts C1, C2 to close. As soon as they close, the tripping coil is energized by an auxiliary dc source. This causes the three main line contacts to open, thus interrupting the circuit.

Figure 44: Elementary tripping circuit for a circuit breaker

Types of circuit breakersa) Air blast circuit breakers

These circuit breakers interrupt the circuit by blowing compressed air at a supersonic speed across the opening contacts. Compressed air is stored in reservoirs at a pressure of about 3Mpa and is replenished by a compressor located in a substation. The most powerful circuit

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breakers can typically short circuit currents of 40 KA at a line voltage of 765 KV in a matter of three to six cycles on 60Hz line. The noise accompanying the air blast is so loud that noise suppression methods must be used when the circuit breakers are near residential areas.

Figure 45: A cross section of one module of an air blast circuit breaker

Figure 46: A 630A air blast circuit breaker

b) Vacuum circuit breakerIn such circuit breakers, vacuum is used as the quenching medium. Since vacuum offers a high insulating strength, it has superior quenching properties than any other medium. When contacts of the circuit breaker are opened, the interruption occurs and the first current zero with dielectric strength between the contacts builds at a rate of 1000th of times higher than obtained with other circuit breakers. Thus a vacuum arc is different from the general class of low and high pressure arc.

In the vacuum arc, the neutral atoms, ions and electrons do not come from the medium in which the arc is drawn but they are obtained from the electrodes themselves by evaporating the surface material. Because of the large mean free path for the electrons, the dielectric strength of the vacuum is 1000 times more than when gas is used as the interrupting medium.

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These circuit breakers are hermetically sealed; consequently, they are silent and never become polluted. Their interruption capacity is limited to about 30KV. For higher voltages, several circuit breakers are connected in series. Vacuum circuit breakers are normally used in underground distribution systems.

Figure 47: Cross section of a Vacuum circuit breaker at CAA

c) SF6 Circuit BreakerHere, totally enclosed circuit breakers insulated with SF6 gas are used.

Action

During interruption, electrical discharges and arcs will decompose SF6 gas. On cooling, a large part of the decomposed gas recombines hence quenching the arc. Reactions may however, also occur with design material (e.g. with vaporizing arcing contact material). This results in the formation of gaseous sulphur fluoride and solid metallic fluoride powder. In the presence of water or moisture there is the development of hydrogen-fluoride and sulphur dioxide. Some of these decomposition products are conspicuous through their unpleasant piercing odour.

Figure 48: 40.5kV SF6 Circuit Breaker

d) Oil circuit breakersOil circuit breakers are composed of a steel tank with insulating oil. In One version, three porcelain bushings channel the 3 phase line currents to a set of fixed contacts. Three movable contacts actuated simultaneously by an insulated rod open and close the circuit. When the circuit breaker is closed, the line current for each phase penetrates the tank by way of a

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porcelain bushing, flows through the first fixed contact, the movable contact, the second fixed contact, and then on out by a second bushing.

If an overload occurs, the tripping coil releases a powerful spring that pulls on the insulated rod, causing the contacts to open. As soon as the contacts separate, a violent arc is created, which volatilizes the surrounding oil. The pressure of the hot gases creates turbulence around the contacts. This causes cool oil to swirl around the arc, thus extinguishing it. In modern high power breakers, the arc is confined to an explosion chamber so that the pressure of the hot gases produces a powerful jet of oil. The jet is made to flow across the path of the arc, to extinguish it.

Figure 49: An Oil circuit breaker

Figure 50: Cross section of an oil circuit breaker

Disconnectors and Earth Switches

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Disconnectors (Isolators) are devices which are generally operated off-load to provide isolation of main plant items for maintenance or to isolate faulted equipment from other live equipment.

Open terminal Disconnectors are available in several forms for different applications. At the lower voltages, single break types are usually of the ‘rocker’ type or single end rotating post types being predominant. At higher voltages, rotating centre post, double end rotating post, vertical break and pantograph type Disconnectors are more common. Disconnectors are usually interlocked with the associated circuit breaker to prevent any attempt being made to interrupt load current.

Disconnectors are not designed to break fault current although some designs will make fault current. Most Disconnectors are available with either a manual drive mechanism or motor operated drive mechanism and the appropriate drive method must be selected for a particular Disconnector in a particular substation e.g. in a remotely controlled unmanned double bus bar substation the bus bar selector Disconnectors would be motor operated to allow ‘on load’ bus bar changes without a site visit being required.

Disconnector mechanisms incorporate a set of auxiliary switches for remote indication of Disconnector position, electrical interlocking and current transformer switching for busbar protection. Earthing switches are usually associated and interlocked with Disconnectors and mounted on the same base frame. They are driven by a separate, but similar, mechanism to that used for the Disconnectors.

This arrangement avoids the need for separate post insulators for the earth switch and often simplifies interlocking. Normally earth switches are designed to be applied to dead and isolated circuits and do not have a fault making capability, however special designs are available with fault making capability if required.

One practical point worth noting is that line or cable circuit earth switches are normally interlocked with the local line Disconnector but reliance is placed on operating procedures to ensure that the circuit is isolated at the remote end before the earth is applied.

Figure 51: 400 to 12000A Disconnector and earthing switch

Instrument Transformers

a) Current Transformers

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The majority of current transformers used in substations are bar (i.e. single turn) primary type but their method of installation varies considerably. In metal clad switchgear they are usually mounted around the insulated connections between circuit breaker fixed connectors and the cable box terminals, whereas in open terminal substations they may be mounted around the bushings of transformers or dead tank circuit breakers. Alternatively where live tank switchgear live is used, the current transformers are mounted in a form known as the post type current transformer where the secondary windings are fitted into a housing insulated from earth by a hollow support insulator. The secondary windings and leads are insulated from the housing and the secondary leads, also heavily insulated, are brought down to a terminal box at the base of the support insulator.

b) Voltage Transformers The choice is basically between ‘wound’ voltage transformers and ‘capacitor’ voltage transformers. Generally where high accuracy metering standard outputs are required the wound voltage transformer is used and where protection and instrumentation outputs only are required a capacitor voltage transformer is often more cost effective at voltages above 145kV. A further advantage of capacitor voltage transformers is that they can be used to provide coupling facilities for power line carrier systems used for protection, signaling, telemetry or telecommunications.

Bus Bars

A bus bar is used to interconnect the loads and sources of electrical power. It connects the incoming and outgoing transmission lines. It also connects generator and main transformer in a power plant.Bus bars are either flexible or rigid. Flexible bus bars are made of ACSR conductors and are supported by disc insulator strings on both sides with gantries. Rigid bus bars are made up of aluminium tubes and are supported on post insulators.

The design of bus bars is done on the basis of the following parameters rated current, rated voltage, rated frequency and the rated insulation level.

Insulators

All the current carrying parts in the substation are supported on insulators. The insulators provide mechanical support to the conductors and are subject to normal operating voltage and transient over voltage. The insulators should not fail due to mechanical load or over voltages. The insulators should have sufficient mechanical strength to withstand the maximum wind loading, ice loading, dead load and other kind of loads. The insulator should not flash over under any conditions of humidity, dirt and salt contaminates.

Materials used for insulators include; Porcelain Glass Types of insulatorsa) Disc insulator

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This is used for transmission lines and in substations for supporting flexible ACSR conductors. The required number of disc insulator units are assembled together to form a string. Therefore these insulators are also called string insulators.The number of units in the string is decided by rated voltage and insulation levels. For higher mechanical loads, double/triple/quadruple strings are used.

The disc insulators are used for two applications; Suspension spring Tension spring

Figure 52: Disc Insulator

b) Solid core insulatorThese have a higher mechanical strength and are used for supporting isolators, bus bars, circuit breakers etc.

Figure 53: Solid core insulator

c) Hollow porcelain These are used for chambers of circuit breakers, CTs, VTs, bushings, surge arrestors. For oil filled and nitrogen filled porcelain housings, internal surfaces are provided with anti-tracking glazed surface like external glazing. For SF6 filled housing, special epoxy coating may be considered for internal surfaces.

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Figure 54: Hollow porcelain insulator

The figure below shows the layout of a typical substation

Figure 55: Layout of a substation showing its components

Where: A: Primary power lines' sideB: Secondary power lines' side 1. Primary power lines2. Ground wire3. Overhead lines4. Transformer for measurement of electric voltage (VT)5. Disconnect switch6. Circuit breaker7. Current transformer (CT)8. Lightning arrester9. Main transformer10. Control building11. Security fence12. Secondary power lines.

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CHAPTER THREE: PRACTICAL WORK DONEThis chapter covers the entire field work that was done during the industrial training period.

3.1 Nalubaale power stationAt the Nalubaale Power Station, Installation of the UNITROL 6080 Exciter panel at Unit 8 and Unit 3 synchronous generators was done

3.1.1 Tools usedClaw hammer, claw bar, ladder, tape measure, chisel, metal grinder, overhead crane, racket handle, strapper belts, set of combinational spanners, lock nuts, bolts(10mm), drilling machine, cable reel, bi-metal hole saw, circular filler, pipe bridge, grip pliers, side cutter, gripping tool, torque spanner, masking tape and screw drivers.

Figure 56: Some of the tools used during the installation work

3.1.2 Sequence of operationi. Opening of the crate containing the UNITROL 6080 exciter panel.

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Figure 57: The exciter panel being removed from its wooden crate

ii. Measurements were made to ascertain the accurate space that the exciter panel will occupy since the old exciter panel was smaller in size.

iii. Using an overhead crane, the exciter panel was lifted and placed accurately in the demarcated space and the front drilled into the ground.

Figure 58: The exciter panel being lifted by the overhead crane

iv. The gland plates were removed and suitable sizes of holes drilled through for the passage of the control, phase and the DC cables.

v. Bus bar extensions were obtained and bolted firmly onto the existing bus bars.

vi. The earth rod was connected to the earth terminal of the exciter.

vii. All the control cables from the excitation transformer were terminated as per the cabling schedule and five extra protection relays added in the exciter.

Figure 59: The 240V DC cables coming from the exciter panel to the rotor of the synchronous generator

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3.2 Civil aviation authorityThe practical work done involved:

Carrying out Continuity and Insulation tests.

Cleaning of the Autotransformer

3.2.1 Tools usedThe following tools were used.

Megger(1000V)

13 and 14mm combinational spanner

13mm racket spanner

Ladder

Figure 60: Toolbox and the Megger used at CAA

3.2.3 Sequence of operation For the continuity test, the Megger leads (two leads) were connected between

any of the phases while for the insulation test one Megger lead was connected

to the phase and the other to the body of the autotransformer.

Figure 61: Carrying out the continuity test

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Figure 62: Carrying out the insulation test

Cleaning of the autotransformer involved removing the metallic casing and

using a towel soaked in filla (paraffin) to clean the bushings and Perspex

partitions.

Figure 63: Cleaning the autotransformer

3.3 UMEME kitante road substationThe practical work done involved Configuring the REF542 on the 11kV and 33kV relays by changing the baud rate from 9600bit/s to 192000bit/s and the ASDU from 3.1 to 9.

3.3.1 Tools usedThe following tools were used.

Laptop and a USB Optical probe.

Digital Camera.

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Figure 64: Laptop, USB optical probe and Digital camera

3.3.2 Sequence of operation For the configuration of the REF542, the following procedures were followed:

-The USB Optical probe is connected to the Optical communication port on

the relay.

-The program ABB REF542 Conf is opened on the laptop as shown below.

Figure 65: ABB REF542 Conf start window

- On the menu bar select Connect and choose Load from REF542plus.

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Figure 66: Choosing "Load from REF542plus"

- A dialog box opens and click on Properties.

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Figure 67: REF542plus Communication port properties dialog box

- Then change the Baud rate and the ASDU and select Download To Device from the REF542plus Communication port properties dialog box.

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Figure 68: IEC 60870-5-103 properties dialog box

- On the menu bar select File and Save As to save the downloaded configurations.

Figure 69: The "Save As" dialog box

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CHAPTER FOUR: CHALLENGES, ACHIEVEMENTS, CONCLUSION AND RECOMMENDATIONS

4.1 Challenge faced during Industrial trainingThe major challenge faced was Inadequate project activities. At times there was shortage of field projects hence certain days had no practical work activities.

4.2 AchievementsTraining at ABB Uganda gave me an opportunity to practically and theoretically experience the diverse electrical engineering field. Throughout the training I was able to understand the various concepts in the electrical engineering world for example the excitation system, the autotransformer, current and voltage transformers, configurations of the REF542 to mention but a few. At ABB I was stationed at the operations department where i had the opportunity of carrying out field activities in different parts of the country thereby training me to be a dynamic employee.

While at ABB, I learnt that while certain problems can be solved at the site others do require the intervention of the ABB engineering team at the factory where the equipment was manufactured. For example during the RTU configurations at the UMEME Kitante Road Substation, there were e-mail exchanges with ABB German to find the final solution to the communication issues at the site.

Above all, I learnt the significance of maintaining a good formal and informal relationship in a company and how they can promote favourable human relations and teamwork. This is enables smooth coordination among the workers and hence, smooth work flow.

4.3 ConclusionNo doubt the experience as an internee in the Operations Department of a Multinational electrical engineering company like ABB will serve as a step stone to better my carrier in Engineering field. The practical experiences gathered will be helpful in my future assignments as a student and as well as an Engineer. All the Engineering aspects being considered should be balanced with the appropriate safety requirements to avoid injury and loss of life during operations.

4.4 Recommendations I would recommend that students are given a guideline of the areas that they are

expected to encounter during their industrial training period so that the company supervisor can allocate tasks effectively.

ABB operations department should come up with a tentative training schedule for its interns that can be adjusted periodically in case of any abrupt projects that were earlier not included in the schedule.

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The College should set up an Industrial training office specifically aimed at bridging the gap between students and organisations. The office should also give guidance to students regarding academic and behavioural conduct during the training period.

Finally, ABB should continue with their policy of offering industrial training so that the upcoming professionals, who are still at undergraduate level, get to know what goes in the field and what is required of them as they pursue their profession.

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REFERENCES 1. Gec Alstom T&D Protection and Control Ltd, Protective Relays Application Guide, 1987.

2. U.S. Department of the Interior ,Bureau of Reclamation, Technical Service Centre, Infrastructure Services Division .Hydroelectric Research and Technical Services Group Denver, Colorado, Transformers: Basics, Maintenance, and Diagnostics, April 2005, Pages 1-20.

3. Transformers, http://wikipidea.org/transformers, 5th August 2011.4. Static Excitation System, http://www.pdffactory.com/static excitation system, 8th

August 2011, Pages 6-14.5. Notes taken down during my training at ABB Uganda Ltd.

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