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					Chapter 2                                                                     Training Experience


CHAPTER 2

                                                                          Training experience

2.1    Ceylon Electricity Board (CEB)

2.1.1. System control centre

2.1.1.1 Introduction

System control centre can be considered as the heart of electrical power system of the country
where frequency control, voltage maintenance, economic dispatch of generation and maintenance
work in transmission network are carried out. The demand or rate of energy consumption of the
whole country is a function of time which changes from time to time and generation has to be
dispatched accordingly, to meet with the system demand while maintaining the system frequency
within the appropriate range. This is done by monitoring and managing the operations of all
power stations and grid substations of the national grid.


The System Control Center consists of three sub divisions, Operational Planning Division,
System Operations and Operational Audit division.

2.1.1.2 Operational Planning Division

Operation planning section is responsible for weekly dispatch planning, interruption planning in
transmission lines, balancing hydro and thermal generation and machine outage scheduling.
Load flow studies, and load analysis too carried out by this section. Interruptions have to be
scheduled in such a manner where system load can be handled by remaining transmission
network effectively without outages and stability problems. System control centre simply cannot
dispatch hydro power stations according to energy requirements. Out of two hydro complexes in
srilanka, mahaweli complex is not merely power generation project but is implemented as a
multipurpose project which includes irrigation as well. Authorized personals representing CEB,
mahaweli authority and Department of irrigation meets weekly and prepares water management
plan for the week. The priority of water utilization used in planning is as per below.


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Akila Fernado, Dept. of Electrical Engineering                               University of Moratuwa
Chapter 2                                                                     Training Experience



1. Water service and drainage
2. Environment
3. Irrigation requirements
4. Power Generation

2.1.1.3 System operations

This section is functioning under a Chief engineer and handles day to day dispatch planning,
dispatch of power plants, and system operations. The control room is equipped with a mimic
diagram which indicates the present status of the transmission network including generation
stations, transmission lines, grid substations, pond and reservoir levels as well as the system
frequency. System control engineer has to decide on dispatching generations based on water
levels of the reservoirs and ponds, voltage levels of different bus bars, Economies of dispatching
thermal power stations Operation policy which is another important aspect is as follows.

1. Safety of people
2. Protection of equipment
3. Availability of supply
4. Quality of supply
5. Economics of supply

The operation policy of system control centre includes certain criteria regarding frequency and
voltage level that has to be maintained all time by system control centre.
       Frequency       50Hz ± 1%                 Voltage     220KV ± 5%
                                                             132KV ± 10%
                                                             33kv ± 2%
Whenever the frequency struggles to maintain its value within the specified range, immediate
actions have to be made in order to prevent system being disturbed which would ultimately result
in a total black out. Blackout is a name used to signify a total system failure which results due to
a sudden frequency drop down. Therefore maintaining the system frequency is very much
important as anything else. Frequency controlling is done by victoria, kotmale, samanalawewa
(Automatically adjusted) or new laxapana (Manually operated) power stations. And its duty of

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Akila Fernado, Dept. of Electrical Engineering                               University of Moratuwa
Chapter 2                                                                    Training Experience

system control engineer to assign frequency control to either of these stations. The water level of
reservoirs and current active power output of the station has to be considered when assigning
frequency control.
Load curve in srilanka rapidly varies with time which is not a good indication from the country‟s
perspective. In the load curve two identifiable peaks can be seen namely Morning peak and
Night peak. The system demand at night peak is much higher than at other peaks. Since the
Night peak is more considerably higher comparatively, and the system demand cannot be
obtained from existing power generation owned by CEB so it has to purchase power from
Individual Power Purchasers (IPP) in order to match the system demand with the generation

This shows that the industrial load is not strong enough to dominate the load curve and in fact it
is dominated by the domestic usage. For the safety of the system, 5% of the total present demand
is kept standby in order to connect if needed at some point. This is called the “spinning reserve”
of the system.

2.1.1.4 Communication

Good communication system is vital for proper functioning of the whole power system. Signals
on pond and reservoir levels, real time generation data, alarms, feeder currents and bus bar
voltages all has to be effectively communicated to system control centre in order to carryout
operations in an effective manner. There are SCADA (supervisory control and data acquisition),
PLC (power line carrier), and usually telephony system used for communication. PLC system,
communication is carried out using power line as medium. There are wave traps at every grid
substation and switch yard which filters data and makes it available to system control.

2.1.1.5 Training Experience

Our short stay at System control centre was really helpful and valuable period of time in which
we managed to get an overall picture about the whole system. This helped us a lot when we
stepped in to the remaining places as we have already been briefed about the operations of the
each and every unit of the network and how they links with each other. We were also briefed on
how the load cure is varying during a normal day, on a special day etc. Concisely, the scope of
system control is considered to be from the synchronous breakers at generation stations up to
132KV feeders at distribution substations.
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Akila Fernado, Dept. of Electrical Engineering                              University of Moratuwa
Chapter 2                                                                    Training Experience


2.1.2. Generation and Transmission planning divisions (Head office)

2.1.2.1 Introduction

Generation and Transmission planning divisions are two of the most functioning units positioned
at the CEB head office with the intention of establishing long term goals based on the demand
forecasts while considering economical aspects as well. The term planning doesn‟t explain the
effort carrying out by them since it‟s not just planning involved. They have to make sure that
whatever the plan they‟re carrying forward is the most effective and the economically optimum
option available out of all possible outcomes.

2.1.2.2 Generation planning division

Generation planning branch deals with long term generation planning and economical aspects of
electrical power industry. Generation planning is essential to fulfill the statutory duty of CEB
because maintaining an efficient and reliable supply of electricity is a responsibility of CEB.
Primary objectives of generation planning process are


Evaluating the feasibility of new generations to the system in terms of plant and system
characteristics
Investigating and envisaging future operations of hydro and thermal generations to determine
the most economical operating policy for reservoirs and thermal plants.
Simulating the power system to determine the economically optimum mix of generation plants
to meet future demand at acceptable reliability levels for upcoming
15years.
To investigate the robustness of economically optimum plan by checking its sensitivity to
changes of key input parameters.(demand forecast, discount rate, fuel price etc..

2.1.2.2.1 Demand forecast

Long term national demand forecast is prepared for a life span of 20 years based on econometric
estimates of future power and energy demand. In these econometric models, transactional figures
of the past will be analyzed against several independent variables such as previous year‟s
demand, Gross Domestic Product (GDP), GDP per capita, Population, Average, Electricity Price
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Akila Fernado, Dept. of Electrical Engineering                              University of Moratuwa
Chapter 2                                                                       Training Experience

and number of consumer accounts using regression techniques. Domestic, industrial, general
purpose and other (religious and street lighting) are the four categories of various consumer
sectors and their demand forecasts have to be prepared separately. In addition load factors also
forecasted (based on trend analysis) and using those data, peak demand forecast is derived

2.1.2.2.2 Long term Generation Expansion Plan

Making and updating the long term generation expansion plan is another important task carried
out by generation planning branch. Generation projects usually involve very large investments
and therefore economic lifespan of the projects and impact on other resources like refineries,
ports too has to be taken into account. This carries details of retiring and committed generations,
and sensitivity adjustments. Evaluating the status of present system and screening available
generation technologies comes after that. The Long Term Generation Expansion Plan is
implemented using software called WASP (Wien Automated System Planning Package)

                                         Retirement/
                                        Commissioning
                                                           Power staions
                   Economic cost Data
                                                           Characteristics



                                                                     System Epansion
      Demand Forecast                                               Constants/ Reliability
                                                                           index




 Existing Plants
                                        WASP                                   Candidate
                                                                              Power plants




          Figure 2.1 – simple illustration of how data is being inserted in to the software




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Akila Fernado, Dept. of Electrical Engineering                                University of Moratuwa
Chapter 2                                                                  Training Experience

2.1.2.3   Transmission planning division




                                 NATIONAL POWER AND
                                   ENERGY DEMAND
                                      FORECAST




                                                                        DISTRIBUTION
                                     GRID WISE DEMAND                DEVELOPMENT PLANS
                                         FORECAST




    GENERATION                  CAPACITY ENHANCEMENT AND
   EPANSION PLAN                 TRANSMISSION CONNECTION
                                        PROPOSALS


                                         POWER FLOW
                                           STUDIES


                                        SYSTEM STUDIES
                                                                               NO


                                         SATISFACTORY



                        LONG TERM TRANSMISSION EXPANSION PLAN




            Figure 2.2 – Schematic representation of the transmission planning process



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Akila Fernado, Dept. of Electrical Engineering                            University of Moratuwa
Chapter 2                                                                    Training Experience

Transmission planning branch is much related to generation planning branch. Transmission
network too has to be upgraded as generation expansion plans are being implemented. The
transmission system should be upgraded to match with increasing system demand and generation
expansions. So transmission planning branch basically deals with that process.

The process of transmission planning mainly includes

     Making Grid wise load forecast and proposals for capacity enhancement.
     System studies
           Load flow studies
           Stability studies
           Short circuit studies
Main objectives of the long term transmission development studies are as follows,

-   Figure out the areas that need to developed to ensure reliability and stability of the existing
    transmission system.
-   Estimating financial plans for the development of transmission system.

2.1.2.4 Training experience

Training at Generation planning branch was an experience different from other places since we
had to mainly deal with economical and other factors related to planning. Throughout the stay we
went through previous generation plans prepared by department and learned about generation
planning process from engineers. It was a week that revealed some concealed but highly
important areas of electrical power industry to us as trainees.


We were able to study the basics of transmission planning and long term transmission
development plan during our stay at transmission planning branch. We studies previous
transmission plans and learned about planning procedures from electrical engineers. The training
was pretty interesting which enlightened us about the importance of planning process in
maintaining system stability and quality of service up to standards.




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Akila Fernado, Dept. of Electrical Engineering                              University of Moratuwa
Chapter 2                                                                      Training Experience


2.1.3. Rathmalana and Pannipitiya grid substations

2.1.3.1. Introduction

For transmission operations and maintenance, we were assigned to Rathmalana and Pannipitiya
grid substations by Chief engineer of the transmission operations and maintenance branch at
Kent road. We spent rest of the two weeks at grid substations getting acquainted with equipments
and maintenance work.
PannipitiyA GSS, which is one of the five main grid substations in colombo region is fed by two
(132KV and 220 KV) lines from kolonnawa and biyagama respectively.




                                            Biyagama
                                             (220 kV)

                                                                    Mathugama
                                                                       ma
                        Kolonnawa          Pannipitiya GSS
                         (132 kV)
                                                                     Horana




                                    Dehiwala -UG             Rathmalana
                                      (132 kV)




                        Figure 2.3 power flow diagram of pannipitiya GSS


Rathmalna GSS which does not involve in power transmission (it is an end substation and only
distributes power to consumers) is fed by two 132KV lines from pannipitiya which is then
stepped down to 33kV and connected to 7 feeders for distribution purposes.




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Akila Fernado, Dept. of Electrical Engineering                                University of Moratuwa
Chapter 2                                                                     Training Experience

2.1.3.2. Components of grid substation

During our stay we were able to study about components of a Grid substation and purpose of
each and every device. Given below are the main components of a grid substation.

· Transformers
· Isolators
· Circuit breakers
· Bus bars
· Current Transformers
· Potential Transformers
· Feeders
· Surge arrestors


2.1.3.2.1 Circuit breakers


Circuit breakers are located in each side of the transformers before isolators as well as at each
                             feeder. The main purpose of circuit breakers is to safely disconnect
                             a current carrying circuit. Breakers can be categorized by using
                             operating voltages, current handling capability and arc quenching
                             mechanism. Usually all 220KV and 132KV breakers use SF6 as
                             quenching medium while some 33KV breakers use oil as well.
                             Operating mechanism is either charged spring or pneumatics.
                             Circuit breakers can be operated remotely or manually. SF6 is
                             preferred due to its high dielectric strength which makes it possible
                             to quench the huge electric arc produced.
     Figure 2.4 circuit
          breaker
2.1.3.2.2 Isolator

Isolator is a simple switch like contactor that visibly isolates the particular circuit that has been
disconnected by Circuit breaker. Operation of isolator without operating the circuit breaker
would result in disastrous consequences as a huge arc will be produces at isolator. The most

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Akila Fernado, Dept. of Electrical Engineering                               University of Moratuwa
Chapter 2                                                                   Training Experience

important purpose of isolator is visibly isolating the required circuit. Isolators can either be
operated manually or remotely from control room.

2.1.3.2.3 Surge arrestor

Used for lightning protection in substations. This device is capable of diverting lightning surges
to earth and protect transformers and other equipment.

2.1.3.2.4 Current Transformers

Current transformer is used to measure high currents. It‟s basically a transformer which makes it
possible to measure high currents. Current transformers are also used for protection relays.
Measuring high currents is not possible without a CT. There are individual CT‟s for each feeder
and supply line for measurement purposes.

2.1.3.2.5 Potential Transformers

Potential transformers which also called as Voltage transformers are used to measure high
voltages. PT‟s are used at every bus bar to measure voltages. Usually capacitor type voltage
transformers are used in substations because electromagnetic version is relatively expensive.

2.1.3.3 Maintenance work

Routine maintenance of grid substation is usually done by maintenance teams from pannipitiya.
Maintenance work usually includes

    Cleaning and washing insulators to remove surface contamination and restore insulation
       levels
    Checking isolator proper functioning including remote operation
    Attending to faults
    Checking insulation levels using Megger

Maintenance work should be carried out with the permission of system control centre since
interruptions can be required for maintenance work




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Akila Fernado, Dept. of Electrical Engineering                             University of Moratuwa
Chapter 2                                                                  Training Experience

2.1.3.4 Gas insulation substation (GIS)

Gas insulation subsidiary (GIS) is a
compact set of equipments including the
bas bars, Isolators, circuit breakers,
current transformers, voltage transformers
which uses a lesser amount of indoor
space. SF6 is used as the quenching
medium. Less area, high sensitivity, less
maintenance and tidiness are some of the
advantages of the GIS system.                    Figure 2.5 220 KV GIS at Kelanithissa

Disadvantages of GIS system

    Uncertainty due to unseen isolator openings.
    If single part is broken then the whole system has to be replaced.
    High initial cost

2.1.3.5 Protection mechanisms

There are different protection types used for different sections of the substation. All the
protection schemes are powered by DC system and battery banks

    33KV feeders - only over current and earth fault relays are used for 33KV feeders.
       Electronic relays are used for all protection circuits and not latest numerical relays.
       Settings for each relay can be done at substation for each line and feeder. Other than
       relays, there are auto reclosers for each 33KV feeder and parameters like reclosing time
       can be changed in control room.


    Line protection – main protection device for line protection (132KV) is Distance relay.
       And there are additional auto reclosers which has synchrocheck feature in addition to
       normal reclosing function. There‟s a backup over current relay for 132KV line.




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Akila Fernado, Dept. of Electrical Engineering                            University of Moratuwa
Chapter 2                                                                     Training Experience

    Transformer – transformer has differential protection,
       Over    current   protection,   Restricted    earth   fault
       protection for HV and LV, and standby earth fault
       protection (senses small fault currents that exist for
       long times). Other than these, there are mechanical
       protections like high temperature, Buchholz relay
       protection as well.


                                                                       Figure 2.6 Buchholz relay

2.1.3.6 Protection schemes

2.1.3.6.1 Over current and Earth fault protection

The fault current depends on the position of the fault and therefore relays are set to operate at
given values so that the relay nearest to the fault will operate and trip the Circuit breaker. Unlike
over current protection earth fault current can be very small and therefore set points of relays
have to be fixed accordingly. Earth fault current causes a zero sequence current which is not
present in a balance system.

2.1.3.6.2 Differential Protection

Differential protection is used as primary protection in system and is supposed to operate in the
shortest time. Differential protection measures currents from two CT‟s at two ends of the line
which is to be protected and communicated to relays located at each end by either fiber optic or
PLC communication. The relay will trip the circuit breaker if according to the difference
between two currents.

2.1.3.6.3 Distance Protection

Distance protection schemes used the concept of impedance to locate the fault location and trips
the circuit breaker accordingly. This measures Voltage and current as complex quantities at each
place and calculates impedance which is used as a measure of distance to the fault location.
There is a concept of zones of protection in distance protection and corresponding breakers will
tripped based in zones to which the fault belongs to.

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Akila Fernado, Dept. of Electrical Engineering                               University of Moratuwa
Chapter 2                                                                      Training Experience

2.1.3.7 Transmission transformers

Coolant oil also acts as an insulator to prevent sparking between the active parts of the
                                     transformer. Silica gel is there to remove moisture in air
                                                     Moisture
                                     Silica (Blue)                      pink

                                     With the use of silica gel inner parts of the transformer will
                                     be protected from getting corroded. When air is mixed with
                                     oil inside the conservator, sludge will be deposited at the
                                     bottom of the transformer.




       Figure 2.7 Silica gel

Two main cooling methods in a transformer

ONAN             - Oil Normal Air Normal
ONAF             - Oil Normal Air Forced



2.1.3.8 Training experience

The stay at Pannipitiya and Rathmalana Grid substations was a very interesting period of our
training program, where we were able to study maintenance procedures and get familiar with
components of a substation. There was a maintenance crew who was at work for two days during
our stay. We were able to witness Megger test on insulators and other maintenance work in
substation. Later we visited the battery banks and studied protection mechanisms and control
room operations with the help of Electrical superintendents. We were also exposed to the basics
of power system protection and their operations.




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Akila Fernado, Dept. of Electrical Engineering                              University of Moratuwa
Chapter 2                                                                    Training Experience


2.1.4 Samanalawewa hydro power plant

Training at Samanalawewa hydro power station was another memorable period of time we came
across during our industrial training. The power station has installed capacity of 120MW from
two vertical Francis turbines each capable of delivering 60MW at an average head of 320meters.
                                                 Samanalawewa is one of few places where
                                                 frequency control is done due to low response
                                                 time compared to thermal power stations. Other
                                                 than that in case of system blackout line
                                                 energizing can be done from samanalawewa
                                                 switchyard. So by all means samanalawewa
                                                 power station is a critical node of the national
                                                 grid.

    Figure 2.8 Samanalawewa dam

2.1.4.1Introduction

Samanalawewa project was started in 1987 and was connected to the national power system in
year 1992.plant supplies 405GWh of energy at a plant factor of 43%. A voltage of 10.5 is being
generated by the plant and is stepped up to 132kV for the purpose of transmission. Average head
difference is 320mbetween the reservoir and the power station and is fed from the reservoir to
the power station through a 5.2km long and 4.5m diameter concrete tunnel. Damn is made of a
clay core supported by rock filled shoulders with a height of 107.5m and a length of 530m.
walawe ganga is the main contributor and belihuloya also involves in supplying water. Damn
contains three relief gates to control the water when needed at a rate of 1200m3s-1. Water leak at
the right bank of the reservoir is still an unsolved problem. Samanalawewa plant was awarded
with ISO certificate in year 2003 and ISO 14001 environmental management certificate in year
2006 for the quality and the standards maintaining in their operations.

2.1.4.2 Turbine and Governor

Water coming through the penstoke is divided in to two paths in order to run two generators.
MIV (Main inlet valve) is the main inlet which controls the water flow to the turbines. In order to
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Akila Fernado, Dept. of Electrical Engineering                              University of Moratuwa
Chapter 2                                                                  Training Experience

prevent the damages to the turbine, a bypass valve is used to equalize the pressure at the two
ends of the MIV. Bypass valve opens automatically before opening the MIV.




                 MIV (Main inlet
                     valve)
                                          Bypass valve


                              Figure 2.9 operation of bypass valve

Governor which is an electro hydro type machine and the MIV get compressed air through the
air accumulator. Two standby compressors are there to pressure the air, if the pressure goes
down suddenly.

Governor       Oil tank level                        540-707mm

               Oil pressure                          45-54 kgcm2

               Oil temperature                       30-50 °C

               Dead time (time taken to respond) - less than 0.25s

               Speed droop                         - 0-10%

The speed of the turbine is controlled by the governor which controls wicket gates using
hydraulic pistons. Main inlet valves (MIV) are usually either opened or closed and are not
partially opened or closed so flow rate and therefore active power input of the generator is
completely controlled by guide veins and wicket gates. PID controller is used in governor control
system and speed sensing is done by PT and CT.. There are two modes in governor namely,

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Akila Fernado, Dept. of Electrical Engineering                            University of Moratuwa
Chapter 2                                                                     Training Experience

frequency control mode and manual mode. In manual mode the given active power output is
constantly maintained irrespective of the voltage. In this case, frequency control is done by
Victoria kothmale or Laxapana. Usually hydro power stations are preferred for frequency control
because of low response time. The Reactive power is balanced by maintaining voltage using
AVR which controls the excitation using thyristors.

2.1.4.3. Bearings used

This whole turbine generator assembly is a massive arrangement and holding it stationary while
the rotor is moving, becomes a big problem. There are three hydraulic bearings namely, upper
guide, lower guide and thrust bearings used for this purpose. Guide bearings maintain the
horizontal position of the assembly while thrust bearing is responsible for bearing all the weight
of this assembly. High pressure oil is injected on to thrust bearings (in-between thrust bearings
and rotor) to lift the rotor to rotate. High pressure oil is supplied until it reaches 400rpm and
thereafter rotor could lift it self without the support of the HP oil layer due to its high speed. LP
oil is injected to the thrust bearings. SSG (Sped signal generator) is used to sense the rotating
speed which is normally 500rpm.

2.1.4.4. Starting the Generator

Dc excitation is done via a transformer using the generated voltage. But before the machine starts
a battery bank (126V is needed for excitation) is used to supply the excitation current and
automatically gets removed from the system once the generator starts producing power.


  Cooling     Gen        Gen        Gen      MIV       Gover      FCB       Gen      Load       Load
              HP         LP        Opera               nor        Open      CB       MVAR       MW
  water
  pump                              tion               valves



                                             Starting sequence


                                           Closing sequence


                          Figure 2.10 starting & Closing sequence of a generator


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Akila Fernado, Dept. of Electrical Engineering                               University of Moratuwa
Chapter 2                                                                    Training Experience

A Co2 bank is also installed for the protection purposes. Whenever a fire develops inside the
generator, Co2 will be sprayed on to it. Power transformer protection is activated by a pressurized
water filled detector system which operates to trip the deluge valve when a fire occurs.

2.1.4.5 Training experience

The training experience we had in Samanalawewa power station was indeed a great one which I
would like to rate as a rare opportunity. We were able to visit the dam site, power station, and
switchyard where we were able to learn briefly about all components and subsystems mentioned
above with the help of operational engineer, control and instrumentation engineer and electrical
engineer all who were very helpful during our stay. We visited the power house numerous times
and studied about components and overall arrangement of the establishment. Unfortunately there
were no routine maintenance or repair work in the power station during our stay but we were
lucky to get a fair knowledge of all systems of a hydro power station which is indeed a precious
opportunity.


2.1.5 Kelanithissa thermal power plant & combined cycle power plant

2.1.5.1. Introduction

Kelanithissa thermal power plant, which is one of the main three thermal plants comprises of
seven generators. (Six machines with 20MW each and another machine with 115MW). Two
steam generators which were used during the early stage has been removed from generation and
currently only six machines are at running conditions including the 115MW generator. Though
the cost per a unit power generated (approximately 42ra in a small generator) is comparatively
high than that of hydro plants, kelanitissa is used to meet with the following requirements.

      For the restoring process during a blackout.
      To supply the system VAR requirement.
      To be operated during a peak demand.
      To be operated during a plant outage.

Introduction of a combined cycle plant (addition of the steam turbine unit) was mainly due to the
high energy loss of the gas turbine unit. Steam turbine was planned to generate energy by
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Akila Fernado, Dept. of Electrical Engineering                              University of Moratuwa
Chapter 2                                                                   Training Experience

producing steam using the exhaust heat of the gas turbine. Combined cycle plant supplies a total
power of 165MW (105MW from gas and 60 MW from steam) to the national power
requirement.

2.1.5.2 Process (gas turbine)




Generator        Load         Gas turbine          Joe         Accessory             Diesel
               gear box          unit            clutch        gear box              engine



            Figure 2.11 schematic representation of the thermal power production
            process

Load gear box is used to synchronize the turbine speed with the generator speed. Small
generators are operated using a diesel engine at the start and once it reaches the speed
1000rpmfire ignitions will start. When the generator reaches 3000rpm diesel engine disengages
from the system. Importance of having a joe clutch is to disengage the shaft from the diesel
engine after reaching 1500rpm.

Oil taken from kolonnawa is sent to the oil treatment plant to remove moisture, carbon and
sludge and will be stored in tank 5&6. Oil treatment plant comprises of six extractors of which at
least two on operation.

Air is compressed by the compressor which is made of 18 stages of blades. Almost 80% of
compressed air produced is used for cooling purposes of the turbine and the generator. About 5%
is sent to the atomizing air unit which is responsible for producing atomized air. This atomized
air is circulated until it reaches up to the desired pressure. There are 10 combustion chambers in
a gas turbine and once the atomized air reaches the desired pressure level it will be sent to the
combustion chamber. All these chambers are connected to each other and chamber 1&2 are
equipped with spark plugs, which are being supplied with 1500AC. A mixture of diesel and

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Akila Fernado, Dept. of Electrical Engineering                             University of Moratuwa
Chapter 2                                                                       Training Experience

atomized air which comes to the chamber as a spray (mist) is burnt inside the chamber. Then
with the help of the compressed air it will be taken to the turbine unit. Excess diesel of all the
chambers will be collected by the drain in the fifth chamber.

2.1.5.3 Process (combined cycle plant)

Diesel and naptha are used as fuels for the plant. Since naptha is very explosive, it is not being
used at the start and stop. Diesel is used for the purpose and once the machine starts producing
power it would be converted in to naptha. Gas turbine consumes naptha at a rate of 11liters/ sec
and it is sent to the machine along with purified air which is injected at a rate of 410 kgs -1.

Gas turbine comprises of 14 chambers which are connected to each other. Due to the
unavailability of compressed air at the beginning, an external source is needed to run the turbine.
Power output is controlled by varying the fuel supply.

2.1.5.4 Synchronous motor mode

Small generators are commonly used as synchronous motors to supply the VAR requirement of
the Colombo city (which has a vast amount of industrial loads). This is done by increasing the
excitation and making the voltage higher than the system. VAR flows from high voltage side to
the low voltage side.

2.1.5.5 Training experience
Our short stay at Kelanithissa power station was really helpful and valuable period of time in
which we could learn many new things about diesel engines, generators and other subsystems of
a thermal power station. All personals including chief engineer, Control room officers, Shift
engineers and other technical staff, were extremely helpful which caused our stay to be a
productive one despite short period of time. We also visited the battery bank and DC system of
the power station which is essential for communication and other control equipments. We were
able to observe and identify details of the components of different subsystems attached such as
cooling system, boilers and other subsystems located on the engine floor.




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Akila Fernado, Dept. of Electrical Engineering                                 University of Moratuwa
Chapter 2                                                                      Training Experience


2.2 Lanka Electricity Company (Pvt) Ltd

2.2.1. Branch office (Moratuwa)

We have undergone training at LECO Moratuwa branch office during our industrial training
period. There are multiple Customer service centers (previously referred to as depots) under the
supervision of one branch office. Other than supervision of Customer service centers (depots),
branch office deals with the legal and administrative aspects of the business. Inspection for new
connections or constructions of new substations should be approved by branch office and are
implemented under the guidance of branch office. Other than that, route survey for new
connections, LV design, and preparation of estimates are also done at branch office.

2.2.1.1 New connection procedure

1. Application should be handed over to branch office
2. Site inspection and preparation of estimate will be done at the branch office within 14 days.
3. The estimates should be sent for approval depending on the size of the project.
4. Estimate is given to the customer for payments to be made.
5. Construction work is assigned to the contractors and issue of materials.
6. The work should be inspected by LECO
7. Commissioning

2.2.1.2 Route survey

Route survey is required for relatively large projects where some principles are used to select one
or more possible routes over which the line should pass. Selecting the shortest possible path,
routing in the direction of possible future roads, and avoiding disrupting environment is usually
preferred. Route survey is done for projects like new housing schemes, installation of new
transformers, and expansion of feeders by adding new line sections. Factors like span of poles,
angle for respective poles, services at each pole, branches of trees and new road development
projects has to be considered when doing route survey.




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Akila Fernado, Dept. of Electrical Engineering                                University of Moratuwa
Chapter 2                                                                    Training Experience

2.2.1.3 Electricity tariff and Accounting

There is an accounting section in branch office which deals with legal and financial aspects of
energy business. The tariff is imposed on LECO by No. 1572/25 gazette which gives explicit
details of electricity tariff for each consumer sector. Billing is done transformer wise and there
are walk paths assigned to Revenue officers who visits customer premises and issue the bill.
There are revenue officers for each branch and electricity bills are issued from 20th of the
particular month up to 10th of following month. 30% of the billing amount is charged from
consumers exceeding 90 units, for fuel adjustment. There is a 14day credit limit allowed for
consumers before dispatching red notices of disconnection. After 10 days from the issue of the
notice of disconnection LECO has the authority to disconnect power for the particular customer.
(But generally, this system is applying for the bulk customers only). There are two main ways of
disconnecting. Pole disconnection and meter disconnection. If disconnected, customer will have
to pay 800/= for meter disconnection and 1238/= for pole disconnection to get the connection
back. Reconnection fee should only be made at the branch office. Billing system is implemented
using information system and pronto software is used in LECO.


2.2.1.4 Administration

There‟s a branch administration officer for each branch and administration division that is
responsible for human resource management and taking care of administrative work of the
branch. General office staff is responsible for administration officer including drivers and
clerical staff. Taking leaves, overtime, remuneration etc... are handled by administration division


2.2.1.5 Training experience

We had an interesting training experience at Moratuwa Branch office under the guidance of
branch engineer and electrical engineer both who were really helpful and caused to create a
friendly environment. We went through LECO construction manuals and standard cost manuals
and learned about branch operations from Electrical Engineer. The electrical engineer gave us a
demonstration of route survey and LV design during our stay.



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Akila Fernado, Dept. of Electrical Engineering                              University of Moratuwa
Chapter 2                                                                     Training Experience


2.2.2 Customer Service Centre

I have undergone training at LECO Moratuwa north Customer Service Centre(depot) during my
industrial training period which gave me the opportunity to work with technical staff and people
who carry out repair work and maintenance. There are multiple Customer service Centres under
one branch office in LECO. Customer service centers are mainly responsible for construction
and maintenance of MV and LV distribution systems. The main functions of Customer service
centre are providing new connections, fault clearance of power line construction and
maintenance of substations, and attending reported faults and maintenances in their respective
regions. Training at customer service centre was a pleasant experience where we could get
familiar with substation (transformer) arrangement and procedure of giving new connections.
Including different devices and components used in distribution network. The depot works over
the clock in order to attend faults and keep distribution network safe and efficient. It‟s in this
place where transformers, DDLO‟s (Drop down lift off), insulators, power cables, Load break
switches and Auto reclosers are really used for construction work and fault clearance.


2.2.2.1 Consumer substation (Transformers)
Each depot or Customer service centre has a set of transformer under their control of which
maintenance work has to be carried out by them. Consumer substations are usually fed by 11KV
high tension distribution lines which get transformed to 400V low tension distribution lines. LV
side is star connected 3 phase arrangement where start point is grounded at substation. There are
many occasions where bulk consumers are given supply by an entire transformer. Dielectric
strength of transformer oil has to be checked periodically to ensure the safe and reliable
operation.

There are different categories of transformers based on mounting methods.

    Single pole mounted transformers – for transformers with capacity less than 160KVA.
       This is the most common type of transformer we can see everywhere beside roads etc..
    Double pole mounted transformers – usually for transformers with rated capacity more
       than 160KVA. Some bulk consumers too are given double pole mounted transformers
       depending on their energy consumption.

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Akila Fernado, Dept. of Electrical Engineering                               University of Moratuwa
Chapter 2                                                                    Training Experience

    Plinth mounted transformers – largest type of distribution transformers which has highest
       rated capacity. The transformer is plinth mounted because of its size which makes it
       unstable in double pole mounted arrangement.


Given below is a diagram which depicts an arrangement of distribution transformer with its
auxiliary components.




               Figure 2.12 connection of a distribution transformer

The diagram shows all necessary components associated with Consumer substation, which
includes DDLO, Surge arrestors and grounding scheme. We can see that lightning earth is
connected with equipment earth and is grounded together while the neutral point of the LV start
connection is grounded separately. This is to avoid lightning surges to go through neutral point.

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Akila Fernado, Dept. of Electrical Engineering                              University of Moratuwa
Chapter 2                                                                     Training Experience

2.2.2.2 DDLO – Drop down Lift Off fuse

This is basically a fuse used in every consumer substation (transformer). DDLO drops the
conductor when fuse blows off due to high currents at primary side of the transformer. This can
be visually inspected. Other than that DDLO can be manually operated on load, to cut off power.
DDLO can be operated from ground level by using an insulated rod.

2.2.2.3 Load Break Switch – LBS

Load break switch is much similar to a circuit breaker used in transmission network. These are
on load switches that can break a current carrying circuit using arc quenching mechanisms. LBS
are used in distribution network to sectionalize the network which makes it possible to carry out
system interruptions with minimum number of consumers being affected. LBS can be operated
from ground using a rod fixed at pole where LBS is connected.

2.2.2.4 Auto Recloser

Auto reclosers are used to clear out temporary faults of transient nature which exist only for a
short period of time. Sudden short circuit or fault currents due to wind, rain or swinging of a
branch of a tree can cause temporary faults in power lines which naturally clear off. The auto
recloser is used to reclose the circuit in such scenarios. When the circuit is disconnected due to
fault detected, auto recloser closes and connects the circuit after a programmed time period of
few minutes or seconds to see whether the fault still exist, the circuit will be permanently
disconnected if the fault is not cleared after the number of recloses has elapsed.

2.2.2.5 Training experience

We had an exciting training experience at Customer service centre where we were able to get
familiar with many types of equipment we were only heard about until then. The staff was
friendly and easy to work with. I studied material handling and store procedures with the help of
stores assistant, and went to high voltage interruption and for few other domestic faults, with
technical staff of the depot.




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Akila Fernado, Dept. of Electrical Engineering                               University of Moratuwa
Chapter 2                                                                   Training Experience

We also visited a customer (ALT Freshtex Garment Finishing Colombo (Pvt) Ltd) who
requested for a tariff change from General Purpose to Industrial, with few technicians and
prepared a report after observing the plant and its current KW status.

                        Item                            Quantity         Total kW
 Air Compressor                                                      1              55     √
 Exhaust Blower                                                      1              45     √
 Exhaust Blower                                                      1              10     ×
 Vertical Kebab Machine                                             11              44     √
 Horizontal Kebab Machine                                            6              15     ×
 Sample Washing Machine                                              1              15     √
 Sample Dryer Machine                                                1              10     √
 Sample Kebab                                                        2               5     ×
 Lights                                                            150              20     √
 Air Conditioner                                                     3               6     ×
 Oven                                                                1               3     √
 Sample Oven                                                         1              15     √
 Ventilation Exhaust Fan                                            10              10     √
 Grinding Machine                                                    9               7     √
 Sample Hydro Extractor                                              1               2     √
 Sample Spray Booth                                                  1               6     ×


                      Table 2.2 KW status report of ALT Freshtex Garment


Consumer had not mentioned about water pumps and some of the kitchen accessories. Apart
from them, some of the items were not found in the Production Area (marked with „×‟).
Therefore, Depot officers decided that kW value was not sufficient for a Tariff change.




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Akila Fernado, Dept. of Electrical Engineering                             University of Moratuwa
Chapter 2                                                                     Training Experience


2.2.3 Engineering division

Due to the New Year holidays we had only couple of days to spend at the LECO engineering
division. But through hard work and proper time management we made sure that we are not
droppings anything which was on offer.

Procurement Process, Making Specifications, Maintaining Global Positioning System
(GPS)/Graphical Information System (GIS) of LECO network and load forecasting/load flow
analysis are the main functions carried out by LECO Engineering division.

2.2.3.1 Procurement Process

This is basically a purchasing procedure of any good or service to a company. In LECO,
Purchasing process is done in a particular method. Stock controller, who is responsible for
maintaining stocks, inspects the stock level frequently and if it is below the required amount a
requisition note which includes a description of the good, required quantity and current stock
level etc will be prepared by himself and submits it to the System Development Manager (SDM)
for his approval.
If the calculated cost is less than 10 million, check whether there are registered suppliers
available. If there are any they would call for quotations first and if there aren‟t any quotations
would be called from all the suppliers. Quotation opening is done in a transparent way and once
they are received, they will be evaluated with respect to technical and financial aspects. After the
final decisions are made, purchase order is granted and the letter of award will be sent. After
receiving the goods to the ware house, an inspection will be carried out by an engineer. If
calculated cost is over 10million then bidding document should be prepared and then follows a
similar process as mentioned above.


2.2.3.2 Specifications

Specification is something important in the procurement process and it is done in order to obtain
the required standards of what they purchase. The Specification includes general conditions,
scope, standards, test reports of certain components and it gives the basic idea to the supplier as
what the requirement they are looking for.

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Akila Fernado, Dept. of Electrical Engineering                              University of Moratuwa
Chapter 2                                                                   Training Experience


2.2.3.3 Graphical Information System (GIS)/ Global Positioning System (GPS)

Graphical Information System (GIS) is a computer system which captures, stores, manages and
presents data related to the assets in the network which owns by LECO. It displays data related to
positions on the earth surface. GPS is a Satellite Navigation System which is funded by and
controlled by the U. S. Department of Defense (DOD) while there are many thousands of civil
users of GPS world-wide and here LECO uses this as a subsystem of GIS.

2.2.3.4 Load forecasting

Load forecasting too is an important function carried out by the Engineering division.
Forecasting predicts the number of consumer load patterns and distribution planning. A large
variety of mathematical methods have been developed for load forecasting. Load forecasting at
LECO is done annually for a period of five years. Forecast is based on budgeted information of
the previous year. Growth of each specific category such as domestic, industrial or commercial is
taken to calculation.

2.2.3.5 Load Flow analysis

Load flow analysis is carried out to guarantee the most advantageous control of the existing
distribution network system. This is also required to enhance the distribution system in order to
meet up with the consumer demands. In other words, load flow analysis is carried out to
investigate whether the present distribution system can match the requirements of the future.
Load flow is carried out branch wise and is done for five more years from the date of study.

2.2.3.6 Training experience

We were able to learn the basics of procurement Process, GIS (GPS), load forecasting, load flow
analysis with the help of Electrical engineers of the branch during our 2days training at the
engineering division. Though we lacked field work and chance to deal with equipments as in
other sections we could get a valuable introduction to various sections, during our short stay at
the Engineering division.




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Akila Fernado, Dept. of Electrical Engineering                             University of Moratuwa
Chapter 2                                                                   Training Experience


2.2.4 System Operations

During the last week at LECO, we have undergone training at Distribution control centre and
Ekala training centre. The distribution control mainly deals with overall system controlling and
performance evaluation. Distribution control centre is much similar to System control centre of
CEB but less sophisticated since this only deals with distribution network. DCC also has mimic
diagram which represents LECO distribution network. System interruptions are scheduled and
have to be carried out under the supervision of Distribution control centre. It‟s the distribution
control centre that gives explicit instructions to each team of technicians during system
interruption. Effective coordination of teams is essential for successful completion of any
interruption. Lack of proper coordination can give disastrous consequences especially in High
Voltage maintenance. Other than this, performance evaluation and reporting is also done in
Distribution control centre.

2.2.4.1 Performance measurement indices

There are some performance measurement indexes used in Distribution control centre for the
purpose of evaluating system performance.
SAIDI – System Average Interruption Duration Index
The average total duration of interruptions of supply that a consumer experiences in the period is
represented by this index.




SAIFI – System Average Interruption Frequency Index
This is the average number of interruptions of supply that a consumer experiences in the period.




CAIDI – Consumer Average Interruption Duration Index


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Akila Fernado, Dept. of Electrical Engineering                             University of Moratuwa
Chapter 2                                                                     Training Experience

This is the average duration of an interruption of supply for consumers who experience an
interruption of supply in the period.




It can be seen that CAIDI = SAIDI/SAIFI.

2.2.4.2. Interruption scheduling

    Request letter should be sent to the head office by the branch office
    If the requested day is not a special one, they would give the approval
    A paper advertisement will be published

2.2.4.3 Reporting


Preparation of outage report, Energy management report and service call report too comes under
the tasks of Distribution control centre. Outage report contains consumer hours lost due to
failures and planned interruptions and considers above mentioned performance measurement
indices to evaluate the quality of supply. This also has details of the number of failures, number
of planned interruptions, no of feeder failures etc branch wise. It helps to evaluate the
performance of each and every branch in order to maximize the profit. Service call report is a
monthly report that contains details of number of service calls, average speed of calls, type of
faults and speed of restoration.

2.2.4.4 Meter testing

LECO has its training center and the service center located at Jaela. Meter testing, transformer
repairing are done at the service center, Meter testing is done with the help of a special test
equipment called “test bunch” which has the capability of testing 10 single phase or three phase
meters at a time.




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Akila Fernado, Dept. of Electrical Engineering                               University of Moratuwa
Chapter 2                                                                        Training Experience




                                                                          0.5 power
                                    Unity power factor
                                                                          factor lag
                  5% Ib             100% Ib                Im              100% Ib




                  +/- 3.5            +/- 2.5             +/- 2.5               +/- 3


         Ib      -Base current (indicated on the name plate of the meter)

         Im      -Maximum current

                                  Figure 2.13 sample test result sheet

Tuning is done with the use of no-load and full load adjustments. For the benefit of LECO, error
is likely to be kept positive since it is harder to achieve zero error mark.

Positive error - disk is rotating faster than usual

Negative error - disk is rotating slower than usual

If the o.5 power factor error is greater than 3% mark, it can be adjusted using the creep test. A
sample test was carried out to simulate the procedure of testing a bulk meter.

2.2.4.5 Training experience

Training experience we had in system operations division of LECO was very interesting and
useful one which gave us an overall idea of distribution system and its operations. We went
through previous outage and service call plans and learned about operational procedures from
staff and control engineer. Two days we spent at Ekala training center was also a new learning
episode in our training calendar. We were shown how the kWh meters are being tested and also
showed us how a transformer is being repaired. We were also briefed about how the oil test is
done.
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Akila Fernado, Dept. of Electrical Engineering                                  University of Moratuwa
Chapter 2                                                                     Training Experience

2.3 LTL Transformers (PVT) Limited

I spent the latter half of my industrial training at LTL Transformers which is a part and parcel of
the group LTL Holdings. LTL Holdings, formally known as Lanka Transformers Group of
Companies is a dynamic Engineering Enterprise engaged in Manufacturing, Construction &
Service industries spanning across Electrical, Mechanical and Civil engineering fields.

2.3.1 Main Objectives

      Improvement of efficiency by reducing on load and no load losses.
      Emphasis on lowering costs by economizing the use of materials and planning
       manufacturing processes that would optimize labor inputs.
      The complying designs are satisfactory with respect to dielectric strength, mechanical
       endurance, dynamic and thermal withstand of winding in event of short circuits.
      Use of materials economically in order to achieve lower cost, lower weight, and reduce
       size and better operation performance.
      Specified performance characteristics such as temperature rise, noise level, impedance
       etc.
      Strong mechanical design, final finish and appropriate accessories.
      Ability to withstand high impulsive voltage surges.
      Overload capacity.
      Elimination or reduction of stray losses, hot spots, electrical gradients, etc. by electrical
       and magnetic field analysis.

2.3.2 Production process

The plant is divided into several departments, which carry out different processes synchronously,
that will lead to a successfully finished Transformer. Each department comprises of supervisors,
skilled workers with a specified set of functions. These Departments are: Insulation department,
Winding department, Core cutting department, Core building department, Assembly stage 1,
Assembly stage 2, Fabrication department, finishing department, Testing department, Dispatch
department and Switchgear Department.


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Akila Fernado, Dept. of Electrical Engineering                               University of Moratuwa
Chapter 2                                                                         Training Experience

Other than producing transformers, Transformer plant provides services in repairing
transformers, manufacturing and testing switchgear etc.

2.3.2.1 Design, Research & Development

LTL Transformers are designed to fully meet with all the latest national and international
standards, including IEC, BS, and ANSI and often even exceeding the standards applicable to
concerned markets.

                                                 With a commitment to Continuous Improvement,
                                                 the Research and Development wing of LTL
                                                 Transformers (Pvt) Ltd which consist of several
                                                 design engineers have been painstakingly and
                                                 continuously working to develop new products, and
                                                 also to expand the quality of what they supply,
                                                 using modern technology and better materials for
                                                 the benefit of the customers. This would not only be
                                                 economical in terms of cost and energy, but would
                                                 also give an all time superior product, providing
     Figure 2.14 Transformer Design
                                                 total satisfaction to the end user.

Special software programs developed in technical collaboration with ABB Norway and the in-
depth expertise of the design engineers and draught person ensure the accuracy and reliability of
all designs with respect to optimization of various factors.

2.3.2.2 The Core Design

The core is made from high-quality; grain-oriented silicon steel and sheets are cut to length by
latest GEORG cutting line. The stacked core provides a superior flux path by utilizing a step-lap
that joins the core legs to the top and bottom yokes. The effective support for the core together
with a step-lap joint ensures optimum performance in relation to existing currents, sound levels
and iron losses.



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Akila Fernado, Dept. of Electrical Engineering                                   University of Moratuwa
Chapter 2                                                                     Training Experience

The core is done in such a way that it efficiently fills the opening in the coil, leaving a minimum
of unused space. The short yoke between the legs of the core reduces the external flow between
the active leg core materials, resulting in an increase in efficiency. The stacked core is more
uniformly and firmly supported with metal or laminated wood clamps to prevent movement of
the core structure so as to improve sound level characteristics and mechanical strength to
withstand short circuit forces.




                                  Yoke




                                                        Center Limb

            End Limb

                                         Figure 2.15 Parts of the core

2.3.2.2.1 Main Characteristics

      The 45°cut of the core sheets and step lap design guarantees optimum flow of magnetic
       flux in the core joint area resulting reduction in no load loss and no load current

      Low sound levels are achieved using the step-lap stacking pattern

      Various types of magnetic steel (Laser treated / Amorphous etc) are chosen to match the
       desired loss level

      The simple supporting structures and clamping devices of the core contribute to the
       compactness of the design.
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Akila Fernado, Dept. of Electrical Engineering                               University of Moratuwa
Chapter 2                                                                      Training Experience

As shown in the figure there is an overlap between two sets of lamination layers. It minimizes
the effect of the unavoidable air gap which occurs at joints and reduces the flux leakage at the
joint. This also gives mechanical strength to the core. If nuts and bolts are used to strengthen the
core, it will lead to stray losses. Normally it is use three lamination sheets at a time. Reducing the
number of lamination sheets here reduces the flux leakage and then the core loss.

2.3.2.2.2 Core material

In order to reduce the core loss of the transformer, core is constructed using Cold Rolled Grain
Oriented Silicon Steel sheets which are insulated by Aluminium Phosphate. The addition of
Silicon reduces the hysteresis loss by increasing the resistivity. Permeability should be a high
value to reduce the magnetic current.

Properties of core steel:

       Density: 7.65 Kg/dm3
       Loss at 50 Hz, 1.7T:0. 98w/Kg


     Common grades of with brand names of core steel

                    Type                                      Thickness (mm)
ZH90                                             0.23
MOH                                              027
M3                                               0.23
M4                                               0.27
M5                                               0.30
M6                                               0.35

                                Table 2.1 thickness of core
2.3.2.2.3 Eddy current loss                steel

The time varying magnetic flux induces currents in the paths perpendicular to the direction of the
flux flow. These currents produce losses in the core plates which account under iron loss. This is
known as Eddy current loss.

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Akila Fernado, Dept. of Electrical Engineering                                University of Moratuwa
Chapter 2                                                                       Training Experience




Eddy current is proportional to the thickness and conductivity of the material.

2.3.2.2.4 Hysteresis loss

Each time the direction of the magnetization is reversed, some useful energy is wasted in
overcoming internal friction. This is known as hysteresis loss and it also produces heat in the
core. Hysteresis loss is proportional to the frequency and depending on the area of the Hysteresis
loop, in turn is a characteristic of the material and a function of the peak flux density.

2.3.2.2.5 Copper loss

Heat produced by electrical currents in the conductors of transformer windings.

2.3.2.2.6 Core stacking

                                         Such issues have made the core structure an important
                                        aspect in the designing process. Transformer cores are
                                        mainly divided in to two sections. Core type and shell
                                        type. In modern core plates a few percentage of silicon is
                                        alloyed to reduce the conductivity and hence the eddy
                                        current loss. Laminations of lower thickness are used to
                                        take advantage of the lower eddy losses. Thickness range
                                        used here at LTL is between 0.23mm-0.3mm. Typical
                                        thickness value is 0.27mm.

    Figure 2.16    while stacking

Lower the thickness higher the eddy current losses in a transformer would be. In the case of a
transformer, eddy current loss is normally greater than the Hysteresis loss and both these losses
deposit under the iron loss component. Due to the high temperature CO2 and SO2 could be
generated in transformer oil.
After stacking is done, Chemifix is applied around the core to protect core from corrosion and
the bottom yoke is clamped with wooden clamps. The wooden clamps are made out of a special
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Akila Fernado, Dept. of Electrical Engineering                                 University of Moratuwa
Chapter 2                                                                       Training Experience

wood called “Balau” which has the special ability to withstand against temperature rises. Before
using them, they are kept immersed in an oil bath to increase temperature withstand capability.
Wooden clamps are tightened together using lengthy steel bolts. Further they are tightened with
metal strips and binding clips.

2.3.2.3 Insulations of the Transformers

Since transformer is dealing with huge currents and voltages, level of insulation is a must to suit
with international standards. Voltages in side a transformer varies from point to point with large
ups and downs. It should be noticed that most of the transformer problems occur due to
insulation failures and thus it is significantly contributing towards the performance, operation
and life time in the long run. Deterioration of the Insulation is mostly depending on the
temperature and the moisture content, when the transformer is under operation.

2.3.2.3.1 Press Board

Pressboard is only a thick insulation paper matt made using
number of paper layers at the wet stage of manufacturing. Two
categories of Pressboards are in attendance,

      That builds up, purely from paper layers in the wet stage
       without any bonding agent.
      That builds up, usually to a greater thickness by bonding
       individual boards using suitable adhesive.                          Figure 2.17      Press Board

2.3.2.3.2 Diamond dotted paper (DDP)

                                  Diamond dotted insulation paper is made of electrical insulating
                                  paper with diamond-shape epoxy resin dotted in both sides. it is a
                                  dry and crisp material at normal temperature (below35℃). This
                                  product has good dielectric properties and good mechanical
                                  properties after being heat-     solidification. It can be used as
                                  insulation material for coils and between layers of oil     immersed
                                  power and distribution transformers.
 Figure 2.18 Diamond
     dotted paper
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Akila Fernado, Dept. of Electrical Engineering                                 University of Moratuwa
Chapter 2                                                                   Training Experience


2.3.2.4 Transformer Winding

Most of the transformers produced at LTL are core type transformers and High Voltage winding
is wound over the Low voltage winding.

HV to LV winding ratio is taken by the simple equation: Np/Ns =Vp /Vs

But the number of turns in a winding is determined considering losses and costs associated with
the transformer.
When we reduce the number of turns to reduce the material cost, cross section area of the
conductors will have to be increased. It would lead to higher transformer losses, which should
not exceed the limits given in IEC standards for any reason.

The transformer designing software helps out to select the optimum transformer design to match
with the requirement of the customer. In the process of designing, capability of the winding to
withstand to various voltage stresses, over voltage conditions and mechanical forces developed
in the system are highly concerned. It should also be accurately insulated and contained enough
cooling surfaces for the oil to flow in between.

2.3.2.4.1 Low Voltage Winding (LV Winding).

The LV coil is wound in oval shape to control
the radial forces and this is wound by a
special winding machine. LV terminals are
prepared by copper bars and they are cut to
suitable sizes and drilled at one end.

The foil tension is adjusted by tightening the
holding strip of the axle on which reel of the
foil is mounted. The no: of turns can be
counted by the counter coupled to the
machine. Insulation is made by special paper,
Diamond Dotted Paper in the LV winding.
DDP is inserter in between conductor layers.                   Figure 2.19 LV winding
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Akila Fernado, Dept. of Electrical Engineering                             University of Moratuwa
Chapter 2                                                                       Training Experience

Cooling ducts are inserted at required places according to the design. Though Cu is preferred for
the winding at most of the time, Al is also utilizable for the purpose. Conductor foils are
sandwiched by layers of insulation paper called Diamond Dotted Papers (DDP) and pressboards
for better insulation. The LV leads are welded to the foil at beginning and ending by using TIG
(Tungsten Inert Gas) welding smoothly. The diagrams of LV are shown below.

                                                                     LV
                                                Core                 leads

                                 Cooling duct




                                                                   Fastening
                                                                    strips
                                    Press
                                    board


                                 Core
                                                                     Edge
                                                                     strip

                               LV Leads


              TOP VIEW                                 SIDE VIEW




       Figure 2.20 Top and side views of the LV winding



In most cases, copper foil is used as Low Voltage conductors instead of square conductors. The
reason is, when foil is used, very small short circuit vertical forces act on the clamping system
compared to a situation where, square conductors is used.

2.3.2.4.2 High Voltage Winding (HV Winding)

HV winding is wound on the LV winding with the idea of reducing leakage flux. To leave more
room for adjusting primary current density across the cross section of the foil winding HV
winding is wrapped leaving a gap on both edges of the foil. In the HV winding, the first and final
layers are insulated with a much thicker insulating material (or adding more layers of insulation).
This is to prevent surge breakdowns. Surge is due to mostly lightning. When a surge voltage is
induced it is distributed in the first few turns.


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Akila Fernado, Dept. of Electrical Engineering                                 University of Moratuwa
Chapter 2                                                                    Training Experience

Therefore a large voltage is there in the first few windings. So the insulation must be strong
enough to withstand high surges. As the HV is delta connected, both ends of the winding are
properly insulated. The layer insulation is made of high quality Diamond Doted Papers. The
tapping positions are made on HV side since the current flowing in the HV side is low than that
of the LV side. There are two types of HV winding machines in the factory.

      Manual controlled machine

      Numerical controlled machine


                                                 Numerical controlled machine is capable of
                                                 producing high quality HV windings and it is easy
                                                 and efficient as wire is not being handled
                                                 manually.
                                                 Manually operated machine, which consists of a
                                                 DC motor, uses an electronic device to control its
                                                 speed. Here signal is given by leg. Importantly
                                                 tension should be constant on the copper wire.




          Figure 2.21 HV winding

Before putting hand on the HV side winding, LV-HV barrier is wound around the low voltage
coil. Normally it is done using corrugated boards, press boards and diamond dotted papers. This
is essential as there shouldn‟t be any electrical paths connecting LV and HV windings. HV and
LV paths are connected to each other via a magnetic path.

Enamel insulated copper conductors are used for the HV winding. Each layer is insulated with
DDP papers and first and last two Layers are insulated more than the normal, to increase the
insulation between LV to HV and HV to Earth. This helps to prevent transformer from surge
breakdowns.



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Akila Fernado, Dept. of Electrical Engineering                              University of Moratuwa
Chapter 2                                                                    Training Experience

Transformer is a device mainly used for transmission and distribution purposes which deals with
the system voltage most of the time. Tapings are normally provided in the HV windings, in such
a way that rated voltage can be varied in steps of 2.5% of the nominal voltage. This would
facilitate to get required regulated distribution voltages at times when the supply line voltage
varies from the nominal voltage. The position where the nominal voltage is obtainable is called
the “nominal tap position”.

2.3.2.4.3 Special Features of Winding Design

       High short circuit strength due to use of epoxy dotted paper as inter-foil and inter layer
        insulation
       Accurately located taps in HV winding and LV foil design keep unbalanced ampere-turns
        to a minimum which make the design stronger under short circuit stresses.
       Minimum stress loads on the dielectric
       The layer-type winding results in a low ground capacity, which gives a nearly straight
        line surge distribution throughout the winding. A compact, high impulse-strength coil is
        the result.
       Low eddy losses
       More even heat distribution through the windings
       Automatic winding techniques can be used



2.3.2.5 Assembly Stage

                                    2.3.2.5.1 Assembly Stage I

                                    Assembling the built core and the windings together, stacking
                                    the upper yoke of the core, increasing the strength of the core
                                    by fitting timber clampers are the main functionalities of
                                    assembly stage 1. Special tightening steel strips are used to
                                    mount and strengthening the core,


Figure 2.22 stacking the upper
       yoke of the core
                                                                                        48 | P a g e
Akila Fernado, Dept. of Electrical Engineering                              University of Moratuwa
Chapter 2                                                                        Training Experience


2.3.2.5.2 Assembly Stage II

                                                 Here, appropriate tap selector is connected to the
                                                 core and prepares the vector group Connection to
                                                 match with the customer requirements. Even
                                                 before connecting the tap selector, voltage ratio
                                                 test, which is the first test carried out in a
                                                 transformer is done so that if there is an error in it,
                                                 it would be easier for them to correct before filling
                                                 transformer oil.

  Figure 2.23 conducting the Ratio Test
           after Complete Core



The wiring diagram is as follows for the HV side tap selector.




                       Figure 2.24 Tapping Positions
                                                                                           49 | P a g e
Akila Fernado, Dept. of Electrical Engineering                                 University of Moratuwa
Chapter 2                                                                  Training Experience

Using the tap changer rated voltage can be changed in steps of 2.5% of nominal voltage. There
are two types of tap changers “On Load Tap Changers” and “Off Load Tap Changers”. Electrical
connections are also made at this stage. On average, distribution transformers produced at LTL
are in the vector group Dyn11.

2.3.2.5.3 Dyn11

Dyn11 is the transformer winding pattern, high voltage windings are connected in delta
connection and low voltage windings are connected in star connection. Also, there is a phase
shift of 30° between HV and LV sides.




                     A1, C2




                                          B2, C1
      A2, B1




                     Figure 2.25 Transformer winding pattern



2.3.2.6 Drying Process

The entire finished core and the coil assembly will then be dried inside a special oven, which is
made for the exact purpose. The temperature and duration of the drying process ensures the
curing of epoxy coated insulation and removal of moisture within active parts in the assembly.


                                                                                      50 | P a g e
Akila Fernado, Dept. of Electrical Engineering                            University of Moratuwa
Chapter 2                                                                   Training Experience

Normally Oven maintains a temperature of 1200C and
Standard transformers like 11kV /415V and 33kV
/415 are kept 24 hours and 36 hours respectively.
Main aspect of drying is to remove moisture in the
materials used before filling oil. Because moisture is a
critical factor which disturbs the level of insulation of
a transformer which ultimately results in reducing the
life time.




                                                                 Figure 2.26 Drying Oven

Diamond Dotted Papers are capable of absorbing moisture and this process would evaporate the
excessive moisture left on the insulation boards. Other aspect is to heat provided melts the
diamond shape glue in DDP papers and they stick to winding layers. This increases the insulation
and as well as it strengthen windings.

2.3.2.7 Transformer Tank

2.3.2.7.1. Fabrication of a Transformer Tank

The Sealed type Transformer tanks are made of high quality pre formed corrugated fin walls
made out of cold rolled steel. It is ensured that finished tanks meet over pressure requirements &
permanent deformation limits specified by international standards. Fabrication plant of LTL is
located at sapugaskanda.

Most of the fabricated tanks are galvanized, to be in line with international standards and then
powder painted. Other non galvanized tanks will be shot blasted in order to remove all traces of
grease, rust welding slag and other impurities. Immediately after a shot blast, tanks are painted
with primer and finishing coats. All the items are then powder painted to provide a smooth
attractive final finish.




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Akila Fernado, Dept. of Electrical Engineering                             University of Moratuwa
Chapter 2                                                                     Training Experience

2.3.2.7.2. Shot blast

Shot Blasting removes the corroded particles deposited on the steel. Here tiny iron particles
come with compressed air through a horse and they knock the transformer tank at a high
velocity. The operator also has to be dressed in a special overall, which withstand to that high
velocity iron particles.

2.3.2.7.3. Powder Coating

Shot Blasted or galvanized Transformer tank has to be painted in such a way to protect tank from
corrosion. By powder coating the transformer gets a long lasting life by preventing not only from
corrosion, but also from external damages.

2.3.2.8 Finishing Department

The assembled core is taken out from the oven and mounted to the cover plate which will then be
put in to the transformer tank. According to the design it may be other way round as well, in that
way the cover will be fixed after depositing the core inside the tank.

The HV and LV bushings, external part of the tap changer, meters, other accessories according to
                                                                the customer requirements such as
                                                                breathers,    Integrated      Safety
                                                                Devices (ISD), etc will also be
                                                                fixed at the finishing department.
                                                                Tank is sealed using silicon and
                                                                put in to the vacume chamber and
                                                                kept there for a certain amount of
                                                                time (approximately 45 mins) at a
                                                                pressure of 0.7 mili bars to remove
                                                                vapor and air bubbles and fills oil
                                                                in to it.

                Figure 2.27 Vacuum Chamber




                                                                                         52 | P a g e
Akila Fernado, Dept. of Electrical Engineering                               University of Moratuwa
Chapter 2                                                                     Training Experience

2.3.2.9 Transformer testing

All transformers are subjected to the full range of specified tests as per international standards
such as IEC, BS and ANSI.

Following routine tests are carried out on each transformer before it is released for dispatch,

    Preliminary Insulation Resistance

    Vector Group

    Voltage Ratio

    Winding Resistance

    Oil Dielectric Strength

    Separate Source Voltage Withstand

    Induced Over - Voltage

    No Load

    Full Load

    Impedance Measurements

    Tank‟s pressure withstand ability


Following type/special tests are carried out either at the customer‟s request or as part of a
verification of a new design. All tests are performed within the factory except for dynamic short
circuit tests and impulse tests which are done at internationally accredited third party laboratories
such as KEMA, Netherlands.

The type/special tests are,

    Short Circuit

    Lightning Impulse Voltage

    Sound Level

    Temperature Rise




                                                                                         53 | P a g e
Akila Fernado, Dept. of Electrical Engineering                               University of Moratuwa
Chapter 2                                                                    Training Experience

2.3.3 Specific parameters

2.3.3.1 Cooling Methods of Transformer

The losses in a transformer should be less than 1% when comparing with the rating values of it.
These losses get converted into heat and thereby the temperature of the core, windings, oil and
tank will rise gradually when the transformer is in operation. Then the heat is dissipated into the
atmosphere from the transformer tank and the coolers. The cooling of transformer will help in
maintaining the temperature rise in various parts within the permissible limits.

In a transformer there aren‟t any rotating parts as in a normal machine to circulate the heat
generated. Therefore it is done by circulating oil within the transformer. When the normal
thermal convection is not sufficient, forced circulation is adopted. The small transformers have
adequate cooling surfaces to radiate all the heat caused by the losses. Cross section of the tank
(radiators) has to be increased with the capacity to provide sufficient space for the heat to get
dissipated.

The most commonly used method of cooling of distribution transformers as in LTL is ONAN
(Oil Normal Air Normal) where the oil in the transformer is circulated on account of natural
thermal heat. The oil takes away the heat from windings and gets cooled in radiators by natural
circulating of air.

2.3.3.2 Transformer Oil.

Transformer oil provides insulation and cooling hence its dielectric and cooling properties are as
important as anything. Based on this transformer oil properties can be classified as physical
chemical and electrical.

     Physical properties: - Viscosity, , Density

     Chemical properties: - Acidity, Corrosive Sulpher and

     Electrical properties: - Dielectric strength.

When transformer oil is new, its properties are excellent and up to the standards. But once it is
exposed to air, it readily takes up moisture. Any moisture or any particle contents will reduce the

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Akila Fernado, Dept. of Electrical Engineering                              University of Moratuwa
Chapter 2                                                                         Training Experience

dielectric strength of oil. Oil is the most critical factor that decides both the life time and
performance of a particular transformer. The main functions of oil are

    To create the acceptable level of insulation in conjunction with insulated conductors and
       coils.

    To provide a cooling medium capable of extracting heat without deterioration of
       insulation.

2.3.4. Transformer Protection.

The subject of transformer protection falls under several major headings, which are as follows.

    Protection of transformer against the effect of faults occurring when any of the parts of
       the system are short circuited.

            o To reduce high current additional reactance are inserted to the circuit
            o To reduce short circuit stress by adequate bracing of the transformer windings

    High-voltage high-frequency disturbances.



High-voltage and high frequency surges may arise due to arcing grounds at any time since
transformers are more of outdoor equipments. To overcome this issue, arching horns are used.
Arcing horns form a spark gap across the insulator with a lower breakdown voltage than the air
path along the insulator surface, so an overvoltage will cause the air to break down and the arc to
form between the arcing horns, diverting it away from the surface of the insulator. An arc
between the horns is more tolerable for the equipment, providing more time for the fault to be
detected.

    Pure earth fault

Earth fault has deferent effects according to whether the neutral point of the system is earthed or
isolated. Protection is earthing of the neutral point and used surge arrestors.




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Akila Fernado, Dept. of Electrical Engineering                                University of Moratuwa
Chapter 2                                                                     Training Experience


2.3.5. Transformer accessories.

2.3.5.1 Bushings of Transformers

Outdoor terminals of a transformer are supported with porcelain and oil filled bushings. There
are two types of bushings (Outdoor type and indoor type) and will be chosen according to the
requirements of the customer. The rated voltage, current, impulse voltage, high frequency
withstands, high voltage withstands, and any requirement for the accommodation of current
transformers determines the size of the bushing.

2.3.5.2 Pressure Relief Value.

The pressure relief value plays a significant role in the protection of a transformer while in
operation. The major faults inside the transformer may cause the instantaneous vaporization of
oil, leading to extremely rapid buildup of pressure .If this pressure is not relieved the transformer
tank will rupture and spilling oil over the wide area within few milliseconds .the consequent
damage and fire hazard possibilities are obvious. This device provides an instantaneous relieving
of dangerous pressure.

2.3.5.3 Conservator Tank

The conservator tank is fixed only to conservator type transformers. During the alternate load
cycles, the oil of the transformer expands and contracts. The conservator consist of an airtight
cylindrical metal drum which is supported horizontally on the transformer cover or neighboring
wall. The oil level goes up and down in the case of over load conditions and thus the size is
determined in such a way that to remains the certain amount of air in the conservator.

2.3.5.4 Buchholz Relay

The gas and oil actuated (Buchholz) relay has been used extensively for disconnecting the
transformer from the supply upon the occurrence of an internal fault or any other minor failure
which generates gases in sufficient quantities to operate the device and to actuate the controlling
circuit breakers.



                                                                                         56 | P a g e
Akila Fernado, Dept. of Electrical Engineering                               University of Moratuwa
Chapter 2                                                                     Training Experience

The gas operated relay is designed for this particular duty as most of the
internal faults that occur within the transformer generate gases. The gas
operated relay can only be fitted to Transformers having conservator
vessels, and is installed in the pipeline between the transformer and its
conservator tank. The relay comprises an oil-tight container fitted with
two internal elements, which operate mercury switches connected to
external alarm and tipping circuits.

                                                                                Figure 2.28 Buchholz
2.3.5.5 Level Indicator
                                                                                        Relay
This is used to identify the oil level of the transformer through external inspections. The
indicator may be provided with the alarm contacts, which give an alarm to the switchboard when
the oil level is dropped beyond the permissible level.

2.3.5.6 Temperature Gauge

This is provided to indicate the temperature of oil inside. It is a device made with alarm contacts
and of dial type operated by Bourdon gauge connected to a thermometer bulb located in the
region of hottest oil.

2.3.5.7 Tap Changer

Tap changer can be used for following purposes,

           Maintain the secondary voltage with a varying primary
              voltage
           To vary the secondary voltage
           To provide auxiliary secondary for a specified voltage
           Supply a reduced voltage for starting of rotating
              machines                                                   Figure 2.29 Tap Changer




                                                                                         57 | P a g e
Akila Fernado, Dept. of Electrical Engineering                               University of Moratuwa
Chapter 2                                                                   Training Experience


2.3.6. Analysis of the no-load harmonic content in distribution transformers.
[Project carried out by me]

2.3.6.1 Introduction

Harmonic currents and voltages are generated due to the non sinusoidal shape of their wave
forms due to various causes. In the case of transformers, these harmonic currents will increase
losses (Copper losses and core losses) and hence cause abnormal temperature rises which will
decrease the expected lifetime of a transformer. Load loss component        ) gets affected by the
square value of the current and thus contributes more for excessive heat generation in
distribution transformers.


On the other hand harmonic voltages that induce due to field harmonics, will build up additional
dielectric stresses on insulations which will lead to affect the level of insulation of the
transformer in the long run. Such conditions require either transformer de-rating to return to the
normal life expectancy or upgrading with a larger and more economical unit.


In the present times a greater awareness is generated by the harms of harmonic voltages and
currents produced by non-linear loads (which are capable of injecting harmonics in to the
system) such as diodes, thyristors. These combine with non-linear nature of the transformer core,
produce severe distortions in voltages and currents and increase the power loss.


Therefore the need for investigating the harmonic problem is obvious and is of great practical
significance in the operation of transformers.


2.3.6.2 Problem definition

Both No load and load losses are affected by the presence of harmonics in load currents.
Increment in no load losses in a distribution transformer due to harmonics is less compared to the
load loss, but has a significant contribution to the capitalization cost when operating in longer
term.



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Akila Fernado, Dept. of Electrical Engineering                             University of Moratuwa
Chapter 2                                                                   Training Experience

No-load loss which comprises of hysteresis loss and eddy current loss was analyzed under this
project. Ratio between the total area and the overlapped area of the core was calculated in each
capacity to confirm whether there is a relationship between the ratio (Total area: overlapped
area) and the no-load loss of a transformer. No-load test was conducted within a range of 200-
250 V at an increment of 10V to see the relationship between the applied voltage and the no-load
current (Magnetization current). This should give us an idea about how the variation of flux
density (B) and Magnetic field (H) would be at the no-load condition.


2.3.6.3 Project objective

Objective of the project was to analyze the No-load Harmonic content of transformers of
different capacities and to come up with a practical explanation on how the no-load loss could be
minimized by making appropriate adjustments in the core structure.

2.3.6.4 Methodology

    No-load test was conducted within a voltage range of (200V-250 V) on one or two
       randomly selected transformers from each standard capacities.
    Magnitudes and the effects of the Harmonics taking place were also be measured using
       the Power analyzer.
    Since we are only worried of the harmonics generated due to the core structure,
       harmonics in the system had to be reduced from the equation. This was done by
       measuring the system harmonics and reducing them from the no-load harmonic we
       obtained earlier. Harmonics of the system was
    Studied the relationship between flux density (B) and Magnetic field (H) by considering
       the relationship between applied voltage and no-load current.
    A research was done to identify the necessary adjustments which needs be done to the
       core structure in order to minimize the effect of transform formation.

2.3.6.5 Scope

Project was basically to minimize the effect of harmonics by focusing on the core structure.



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Akila Fernado, Dept. of Electrical Engineering                             University of Moratuwa
Chapter 2                                                                     Training Experience




2.3.6.6 Resources

    Oscilloscope
    Yokogawa PZ4000 Power analyzer
    Transformers with following capacities
       50kVA 33kV/415V                           400kVA 11kV/415V
       100kVA 11kV/415V                          400kVA 33kV/415V
       100kVA 33kV/415V                          630kVA 11kV/415V
       160kVA 11kV/415V                          630kVA 33kV/415V
       160kVA 33kV/415V                          800kVA 33kV/415V
       250kVA 11kV/415V                          1000kVA 33kV/415V
       250kVA 33kV/415V                          2500kVA 33kV/415V


Note: Project report with the obtained results will be submitted along with the training report.


2.3.7. Training experience

The training experience I had in LTL transformers (Pvt) Ltd was completely different from
LECO and CEB. High voltage system was a field I have been hardly exposed to. It was a
wonderful and exciting experience to get acquainted with novel systems to which we were not
aware of before. Production process of transformers, testing of transformers and being involved
in analytical projects were new and exciting experiences for us as 3rd year undergraduates which
made the whole training period an interesting one.

During my stay at LTL transformers, I managed to get a detailed learning about the production
process of transformers. This I consider as a unique experience since this is the only transformer
plant existing in sri lanka currently. I spent most of my time at the testing lab conducting daily
ongoing tests; including the special no-load test which was carried out for the necessity of my
project.




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Akila Fernado, Dept. of Electrical Engineering                              University of Moratuwa
Chapter 2                                                                     Training Experience

It was a great opportunity to study about high voltages tests, and testing equipments which I
would hardly get from another organization. Since high voltage is a section which we haven‟t
covered at the university yet, it was a nice experience for me to learn about it in a practical
background.

Apart from technical aspects, the experience I had in dealing with workers and staff was
undoubtedly remarkable. I consider this as one of the key features in our training outcomes as
this is the first time we are being exposed to such a working environment.

As a whole the training experience was truly unique and exciting, and I wish to thank all
technical and non technical staff who cause to make it a pleasant and a wonderful one.




                                                                                         61 | P a g e
Akila Fernado, Dept. of Electrical Engineering                               University of Moratuwa

				
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Dalage Lalani Shriyalatha Perera Dalage Lalani Shriyalatha Perera
About I'm a teacher.I would like to share my training experience with others.thanks