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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