2.2. Training at Ceylon Electricity Board
I think Ceylon Electricity Board is the best place to train as an electrical engineering trainee.
The Ceylon Electricity Board (also abbreviated as CEB), is the largest electricity company in
Sri Lanka. With a market share of nearly 100%, it controls all major functions such as
electricity generation, transmission, distribution and retailing in Sri Lanka.
2.2.1. My Work Sites
I had two months Training in Ceylon Electricity Board. Information on worksites that I
worked during the training period is mentioned below in the table with names and
designations of key training personnel involved and time periods spent in each section.
Table 2.1 – Information on Worksites
Training Place Key Training Officer Time Period
Kothmale Power
Station
Mr. T.M.S.K. Thilakarathna
(Chief Engineer, Kothmale Power Station)
From 17/05/2010
To 30/05/2010
Kelanithissa Power
Station
Mr. Hendahewa
(Chief Engineer, Kalanithissa Power Station)
From 31/05/2010
To 06/06/2010
Kelanithissa
Combined Cycle
Power Station
Mr. N.A.F.G. Jayamaha
(Chief Engineer, Kelanithissa Combined Cycle
Power Station)
From 07/06/2010
To 13/06/2010
System Control
Centre
Mr. Lakshitha Weerasinghe
(Chief Engineer, System Control Center)
From 14/06/2010
To 20/06/2010
Generation
Planning & Design
Mr. S.H. Midigaspe
(Chief Engineer, Generation Planning &
Design)
From 21/06/2010
To 27/06/2010
Transmission
Operation &
Maintenance
(Anuradhapura
Region)
Mr. Hettiwattaa
(Chief Engineer, Transmission Operation &
Maintenance - Anuradhapura Region)
From 28/06/2010
To 11/07/2010
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2.2.2. Kotmale Hydro power station
2.2.2.1. Introduction
Kotmale hydro power station is one of the major power stations in the Mahaweli project. It is
an underground power station.
Figure 2.13 – Power house, Kotmale. The arrangement of basic components of the
power station. It consists of three generators 67 MW each.
2.2.2.2. Surge Chamber
The length of the tunnel is 7 km. It connects with the penstock which is 120 m long. There is
a surge chamber in between the tunnel and the penstock, which is there to protect the
penstock and the tunnel from high pressure situations. These high pressure situations occur
when shutting down a machine. In a machine tripping off, this situation is even worse. When
the main inlet valve closes the water pressure increases, then the water level in the surge
chamber increases and releases the extra pressure from the penstock and the tunnel.
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The power house consists of 3 Main Inlet
Valves of cylindrical type for three
generators. A hollow cylinder is there to
open or close the water flow. When the
cylinder is at horizontal position the water
flows through the cylinder and flow is
opened. When the cylinder is at vertical
position water flow is closed. It is operated
using hydraulic pumps.
Figure 2.14 – Surge Chamber
2.2.2.3. Turbines
Francis type turbines are used in the power
house and they are designed for a head of 201.5 m.
The water comes through the penstock is
directed to a spiral way with circular cross
section, whose diameter decreases gradually.
On the way the water goes out of the spiral way to
hit the turbine through guide vanes. There are 24
guide vanes in each turbine. Those guide vanes
are controlled by the governor with the help of
two servo motors. The action of servo motors
can be clearly seen when the machine is given
the Figure 2.15 – Turbine
frequency controlling task.
Power generated by the machine∝Water discharge
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2.2.2.4. Generators
Rated output of a generator 90000 kVA with power factor of 0.85. Generation voltage is 13.8
kV. One generator consists with a brushless exciter whereas the other two generators consist
with conventional static exciters.
Table 2.2 – Comparison between Static and Brushless Exciters
Static Exciter Brushless Exciter
Quick in response Slower in response
Carbon dust No Carbon dust
Maintenance is difficult Easy to maintenance
Deterioration
Brushless exciter is an AC machine, placed on top of the generator. It has a rectifying circuit
in its rotor. The power needed for excitation is taken from the rotating part itself, so no
brushes needed. But brushless exciters have brushes for protection purposes.
2.2.2.5. Transformer Yard
There are 9 single phase transformers in the transformer yard. The three generators are
equipped with three transformers for each. The secondary sides of the transformers are
connected in a way such that a star connection is formed. Using single phase transformers
other than one three phase transformer is suitable, because maintaining becomes easier and
also the replacing procedure is economical in some cases.
The ratings of the transformers are as follows.
Rated capacity - 30 MVA
Rated voltage - HV side 220 kV
LV side 13.8 kV
Rated current - HV side 20.54-248.6 A
LV side 2174 A
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Figure 2.16 – The transformer yard
2.2.2.6. The Switchyard
Kotmale switchyard consists with 2
switchyards. One is 220 kV switchyard
and the other is 132 kV switchyard which
is presently not in use. In the switchyard
SF6 circuit breakers and minimum oil
circuit breakers are used. We could
observe a repairing procedure of a
minimum oil circuit breaker.
The most important thing in this switch
yard is that the power that goes to
Biyagama through Kotmale Biyagama
line starts from here. Figure 2.17 – The Switchyard
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That line transfers a huge amount of power; therefore a fault in this line may be a reason for a
blackout. So the maintenance of this switchyard is very important.
The switchyard consists with following main power lines.
Incoming 220kV double circuit line from Victoria.
Outgoing 220k double circuit line to Biyagama.
Outgoing 220k single circuit line to Anuradhapura.
220 kV Double circuit line to Upper Kotmale. (Proposed.)
2.2.2.7. MW/Mvar Control
A synchronous generator has a capability curve of operation. When the generator violates the
curve, it trips off. So, variation of MW/Mvar must be done inside the curve. Amount of active
power is controlled by the governor. Reactive power is controlled by controlling the output
voltage. Automatic Voltage Regulator (AVR) is responsible of voltage control.
2.2.2.8. Maintenance of the Power Station
Maintenance of the power station is mostly done according to the specifications of the
manufacturer. There are maintenance programs held monthly, quarterly, half yearly and
annually. I got an explanation on maintenance from the ES (maintenance).
2.2.2.9. Synchronization
Before connecting any power plant to an electricity network, the output must be
synchronized. It means two waveforms of both sides must overlap. To synchronize, the
voltage, frequency, phase angle and the phase sequence of two waveforms must be identical.
Required voltage can be achieved by exciting. Frequency can be controlled by the governor.
So the phase angle and sequence must be checked when synchronizing and the supply must
be connected exactly at the correct point.
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2.2.3. Mini Hydro Power Station at Nillambe
On 21/05/2010 I visited Nillambe mini
hydro power station which adds 3.2
MW for the national grid. There are
two sets of machine units. Turbines are
of Francis type designed for a head of
110 m. The generator produces a rated
output of 1.6 MVA with the generation
voltage 6.3 kV at 0.8 power factor.
Voltage is then step-up to 33 kV and
connected to the distribution network.
Figure 2.18– The Nilambe Power Station
2.2.4. Kelanithissa Power Station
We learned about 20 MW gas and 115 MW gas turbines. Also we studied about,
Compressor
Combustion Process
Turbine
Generator & Excitation Methods
Description of 20 MW gas turbine,
Output Voltage – 11 KV
Power – 26690 KVA
Fuel –Diesel
Turbine Speed -5100 rpm
Generator Speed – 3000 rpm
2 pole, Cylindrical rotor
2.2.4.1. Kelanithissa Combined Cycle Power Station
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Combined Cycle Plant is a combination of gas turbine and steam turbine generator
(110MW+55MW). Inlet air drawn from air filters compressed and moved. Then air and fuel
are fired. Turbines are rotating from the use of hot gas. After that the exhaust gas goes to the
heat recovery steam generator (HRSG). Then HP and LP turbines rotates from the use of hot
exhaust gas.
2.2.5 System Control Center
A major disadvantage of Electricity, while comparing with the other types of energy is that it
cannot be stored in large amounts. So that the conventional and easiest way of supplying
electricity to the consumers is generate the needed energy at the same time they are
consumed. So it is essential to maintain the condition that demand equals supply. This is the
major function done by the System control centre.
CEB purchases power from private power producers. So it is important to select the suitable
power station to run at the suitable time, considering so many conditions. Economic,
agreements, irrigation and so on. System control centre consists with three main sections.
System Operations Branch controls power generation to match instantaneous demand.
Supply and demand balancing is achieved by maintaining system frequency within the range
49.5 – 50.5 Hz. I observed addition and rejection of generator units during day time. Plant
additions are done according to the weekly plan issued by the “Water Management
Secretariat”. Normally, base load is supplied by thermal power plants due to their low
flexibility.
I studied the Sri Lankan Power System network at the System Control Center.
There are two main hydro complexes in Sri Lanka, named Mahaweli complex (based on
Mahaweli River) and Laxapana Complex (based on Kelani River). Also there are some other
hydro power plants. The capacities of the plants are given below.
Table 2.3 – Hydro power plants & Capacities
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Hydro plant Installed Capacity (MW)
Mahaweli Complex
Victoria 70 x 3 210
Kotmale 67 x 3 201
Randenigala 61.5 x 2 123
Rantambe 25 x 2 50
Ukuwela 20 x 2 40
Bowatenna 40 x 1 40
Mahaweli Total 664
Laxapana Complex
Wimalasurendra 25 x 2 50
Canyon 30 x 2 60
Old Laxapana (8.3 x 3)+(12.5 x 2) 49.9
New Laxapana 50 x 2 100
Polpitiya 37.5 x 2 75
Laxapana Total 334.9
Other Hydro plants
Samanalawewa 60 x 2 120
Kukule 35 x 2 70
Udawalawe 2 x 2 4
Inginiyagala 12 12
Nilambe 1.6 x 2 3.2
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Other Hydro Total 209.2
Hydro Total 1208.1
Power stations of Mahaweli and Laxapana complexes are arranged in a cascade system and a
proper illustration with a diagram is given in Annex E and Annex F. Due to the cascade
arrangement the maximum usage of the potential of water is taken. The main objective of
Mahaweli complex is supplying water for agricultural purposes. But the main objective of
Laxapana complex is generating electricity.
Now Sri Lanka has a Thermal based generation system. The capacities of the plants are given
below
Table 2.4 – Thermal power plants owned by CEB
Thermal plant Installed Capacity(MW)
KPS GT 20x5 + 115 215
KCCP 108(GT) + 55(ST) 163
Sapu STG 1 16x4 64
Sapu STG 2 9x4 36
Sapu STG 3 9x4 36
Table 2.5 – Private Thermal power plants
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Thermal plant Installed Capacity (MW)
Lakdanavi 22.5
Asia Power 48
Barge 60
Aes Ccp 163
Ace Matara 24
Ace Horana 24
Ace Embilipitiya 100
Heladanavi 100
West Coast 300
Total 1355.5
2.2.5.1. Operations Planning Section
Operations planning section is responsible for planning the short term operations of the
power system. Before any maintenance process starts the system control centre must be
informed about it. So such processes are scheduled for certain duration.
2.2.5.2. Maintenance
Annual Maintenance processes are conducted on all the power stations to make sure the
power supply is reliable and safe. But if too many power stations are maintained
simultaneously, problems may occur due to the insufficient capacity. So these maintaining
processes are scheduled by system control centre such that there may not be any problem
with the capacity.
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The condition of power stations also must be checked. Such process is called Routine
Maintenance. They are also must be scheduled. Forced outages occur due to unexpected
break downs. Those power plants need repairs, but they cannot be planned. So there should
be a plan in system control centre to face such situation without doing harm for the
consumers.
Apart from the power stations, the transmission system also must be repaired from time to
time. Also faults in the transmission system must be expected. So system control centre has
plans to face such situations and repair the faults as soon as possible.
2.2.6. Generation Planning & Designing
2.2.6.1. Introduction
Electricity demand keeps increasing in Sri Lanka. So new supply enhancing projects must be
there to meet those requirements. In addition, depreciation and retirement of existing power
stations is also a reason for planning the generation projects in future. Ultimately the capacity
of the country is to be increased according to the plans. The generation plan is prepared
considering those factors and with the intention of supplying electricity to the consumers in a
reliable, stable and affordable manner. An econometric model is used in preparing the plan,
rather than thinking only about the financial benefits of selling electricity. A good generation
plan ensures a reliability of the entire power system.
The Long Term Generation Expansion Plan is prepared annually, and the planning horizon
covers fifteen years. In that process studies are done considering next twenty years.
Load forecasting, pre feasibility studies, feasibility studies, site surveys, finding funds are
also done in this branch in addition to generation planning.
2.2.6.2 Main Objectives of Generation Planning
To investigate the feasibility of new generating plants for addition to the system in
terms of the plant and system characteristics.
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To specifically investigate the future operations of the hydro-thermal system in
order to determine the most economical operating policy for reservoirs, hydro and
thermal plants.
To conduct system simulation studies to determine the economically optimum mix
of generating plants to meet the forecast demand and the acceptable reliability
levels in the 15 year period ahead.
To investigate the robustness of the economically optimum plan by analyzing its
sensitivity to changes in the key input parameters.
2.2.6.3 Demand Forecast
Demand forecasting is done to identify the changes in electricity demand and to identify the
related economic side of it. Three main sectors are considered in demand forecasting. They
are Domestic sector, Industrial and General Purpose sector and the other sector (Religious
Purpose and Street Lighting). Forecasting for the other sector is based on past demands. But
in domestic sector and Industrial and General Purpose sector some other independent
variables are considered. The data necessary for forecasting are taken from Central Bank,
Department of Census and Statistics and statistical unit of CEB. Those variables are Gross
domestic production per capita, past demand, Average electricity price, Gross domestic
production, Number of consumer accounts, previous year GDP.
2.2.6.4 Load Forecast scenarios
Forecast with demand side management.
Low load forecast. (Low population and low GDP growth; -1.2%)
Base Load forecast.
High load forecast. (High population and low GDP growth; +1%)
2.2.6.5 WASP Package
The software used in the generation planning branch is WASP (Wien Automatic System
Planning)-IV. It is internationally recognized software, distributed in over eighty countries
and fifteen international agencies.
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2.2.7 Transmission Planning
2.2.7.1. Introduction
In Sri Lanka majority of the power stations are located in remote places with respect to
Colombo and other load centers. Therefore the energy generated must be transmitted to a
long distant in an optimum way. The energy loss should be very low, and the voltage and the
frequency of the receiving end must not approach the undesired values. Stability and the
reliability are also very important in a transmission system. Presently in Sri Lanka 132 kV
and 220 kV transmission lines are used. The transmission planning branch is responsible for
planning and developing this transmission system. The planning horizon of the transmission
plan is 10 years.
2.2.7.2. Objectives of Transmission Planning
Finding out the transmission developments required to ensure reliable and stable
power system for the period of consideration and the planned implementation
dates.
Estimating the investment cost for these transmission developments.
2.2.7.3. The Importance of Transmission Planning
Transmission planning is important basically in following situations.
The components of the existing transmission system are expected to be expired in
recent future.
Connecting of a new power source to the system.
Connecting of a new huge load to the system.
To face the fault conditions in a better manner.
To make the system more stable and reliable.
To face the situations where the existing transmission system is expected to be
incapable of maintaining the desired voltage levels at the relevant points.
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2.2.7.4. Planning Criteria
To ensure quality and reliable supply under normal operation and under contingencies
following criteria are considered.
2.2.7.5. Voltage Criteria
Voltage Criteria defines the permitted voltage deviation at any live bus bar.
Table 2.6 – Voltage Criteria
Bus Bar Voltage.Allowable Voltage Variation. (%)
Normal. Single Contingency.
220 kV ±5 -10 to +5
132 kV ±10 ±10
Single Contingency - Outage of any one element of the system.
Double Contingency Condition - Simultaneous outages on two generator units, two
transformer units, two transmission lines or a combination
of them.
2.2.7.6. Thermal Criteria
The transmission network should not overheat due to overloading at steady state conditions.
The following steps are taken to maintain the network according to the criteria.
Enhance the grid substation capacities and construct new grid substations
Excess loads are transferred to adjacent grid substations, but those grid substations
must be capable of withstanding new loads.
2.2.7.7. Security Criteria
In this criteria the performance of the system under contingency conditions are considered. At
such situations the system must be able to withstand it without violating the voltage criteria.
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2.2.7.8. Stability Criteria
Stability criteria considers about the system stability during and after a system disturbance.
Transmission system should be able to withstand in following situations.
Three phase fault at any one overhead line terminal
Loss of any one generator unit, load rejection by loss of any transformer
2.2.7.9. Short Circuit Criteria
This criterion defines the maximum three phase circuit currents at the bus bars of grid
substations in order to protect the network.
Table 2.7 – Short Circuit Criteria
Bus Bar Voltage. System. Maximum Three Phase Fault Level.
132 kV and aboveOverhead 40.0 kA
Under ground 40.0 kA
33 kVOverhead 13.1 kA
Under ground 16.0 kA
11 kV Under ground 20.0 kA
2.2.7.10. PSS/E Software (Power System Simulator for Engineers)
PSS/E is the software is used for transmission planning. Using that software the system is
analyzed for the following four situations. Then the optimum results are identified.
Hydro Maximum Day Peak
Thermal Maximum Day Peak
Hydro Maximum Night Peak
Thermal Maximum Night Peak
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2.2.8 Transmission Operation & Maintenance (Anuradhapura Region)
2.2.8.1. Introduction
I was placed at Transmission operation and maintenance branch at Anuradhapura Region. I
went to seven grid substations in this time period. Therefore I got a very good chance to go
around the Sri Lanka. I visited Trinco, Valchchena, Pannala, Puttalam, Habarana, Old
Anuradhapura & New Anuradhapura Grid Substations. Grid substation is a place where 220
kV or 132 kV lines interconnect and step down that voltage to 33 kV for distribution. I
identified all the equipment, and also the functions of those equipments. I started to develop
software for Transmission Operation & Maintenance Division to improve the communication
of the way leaves process. I finalized the data entry form and data base form of the way
leaves software with the use of Java platform
2.2.8.2. Grid substation
Components of Grid Substation are,
Transformers
Circuit Breakers
Isolators
Surge Arresters
Current Transformer
Potential Transformers
Bus bars
Feeders
Protection Items
2.2.8.3. Circuit Breakers
Circuit Breakers can be categorized under operating voltages, quenching medium and
operating mechanism.
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Table 2.8 – Circuit Breakers
Operation voltage of Circuit
Breaker (kV)
Quenching medium Operating mechanism
220 SF6 Spring charge
132 SF6 Pneumatic or spring charge
33 SF6 or oil Spring charge
Circuit breakers are located in both line bays and transformer bays to switch on/off the
feeders purposely for maintenance or any other purposes and to ensure the protection of
equipment and stability of the system by the auto operation of breakers.
When closing, the circuit breaker pull rod goes up and completes the contact. 132 kV circuit
breaker must complete contact within 30 ms. Circuit breakers can operate manually or
remotely from control room. A huge electrical arc is produced at the operation of a circuit
breaker and it is quenched by the quench medium. Due to high dielectric strength SF6 is used
as quench medium. The transformer oil is also used for this but its dielectric strength is lower
than SF6 and need a lot of maintenance like filtering and testing of dielectric strength due to
formation of carbon. The spring operation mechanism moves pull rod using motor. But in
pneumatic mechanism it moves using the pressure difference.
2.2.8.4. Isolator
Isolator is a simple switch that cannot be operated when the particular line is loaded or will
be loaded with the operation of the isolator, since there is no medium of quenching the arcs
generated at such instant. By looking at the circuit breaker one cannot say it is open or closed.
But whether the line is closed or open can be seen by looking the isolator. Isolator should be
opened after switching of the relevant circuit breaker and it should be closed before the
circuit is originally closed by the circuit breaker as well.
2.2.8.5. Autotransformer
A transformer that contains an auto tap changer is called an autotransformer. The distribution
voltage is maintained using autotransformers at grid substation.
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Auto tap changer senses the secondary voltage of the transformer via a PT and changes the
tapping of the secondary coil using a diverter switch. In auto transformer 33 kV windings are
called Tertiary windings and they are connected in delta to reduce third harmonic effect. This
winding also can be used to improve power factor by connecting capacitor bank to windings.
2.2.8.6. Surge arrestors
Surges are generated due to lighting and switching operations. These surges damage high
voltage equipments. These surges are ground by surge arrestors.
2.2.8.7. Current Transformers
When a current is to be measured in a very high voltage circuit, an ammeter can’t be
connected directly to the circuit. In this case an ammeter is connected to the line through the
current transformer, which steps down the high value to low value. In a CT the primary
current isn’t controlled by the condition of the secondary circuit. In usual practice the primary
of a circuit is connected directly to one phase and the secondary is taken from the instrument.
Current transformers are also used to supply current to the protection relays.
2.2.8.8. Voltage Transformers
An instrument voltage transformer is small compared to the power transformer. It’s used to
connect the voltmeter for metering purposes. For voltages above 110 kV capacitor voltage
transformers are used since electromagnetic type is very expensive.
2.2.8.9. Protection
Another essential thing we came to know in Substation is Protection. We leant different
protection schemes.
Transformer Protection
Bus bar Protection
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2.2.8.9.1. Transformer Protection
The transformer faults can be categorized as,
Winding and terminal faults
Sustained or unclear external faults
Abnormal operating conditions such as over load, over voltage and over fluxing
Core faults
The following relays are used in transformer protection scheme.
Over current, Directional over current relays
Earth fault, restricted EF, Standby EF relays
Differential relay
Over fluxing relay
Winding/oil Temperature relays
Buchholz relay
2.2.8.9.2. Bus bar Protection
The standards construction for bus bars has been very high, with the result that bus faults are
extremely rare. Most common bus bar protection schemes are,
Differential protection
Fault bus protection
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