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 – 2.1 – Information Information on Worksites Training Place
Key Training Officer
Time Period
Kothmale Power
Mr. T.M.S.K. Thilakarathna
From 17/05/2010
Station
(Chief Engineer, Kothmale Power Station)
To
Kelanithissa Power
Mr. Hendahewa
From 31/05/2010
Station
(Chief Engineer, Kalanithissa Power Station)
To
Kelanithissa
Mr. N.A.F.G. Jayamaha
Combined Cycle
(Chief Engineer, Kelanithissa Combined Cycle C ycle
Power Station
Power Station)
System Control
Mr. Lakshitha Weerasinghe
From 14/06/2010
Centre
(Chief Engineer, System Control Center)
To
Generation Planning & Design
Mr. S.H. Midigaspe (Chief Engineer, Generation Planning & Design)
30/05/2010
06/06/2010
From 07/06/2010 To
13/06/2010
20/06/2010
From 21/06/2010 To
27/06/2010
Transmission Operation &
Mr. Hettiwattaa
Maintenance
(Chief Engineer, Transmission Operation &
(Anuradhapura
Maintenance - Anuradhapura Region)
From 28/06/2010 To
11/07/2010
Region)
21 | P a g e
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 – 2.13 – Power 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.
22 | P a g e
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 – 2.14 – Surge 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. gradually. On the t he 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 – 2.15 – Turbine Turbine
frequency controlling task.
23 | P a g e
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 – 2.2 – Comparison 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 f or 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 24 | P a g e
Figure 2.16 – 2.16 – The 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 – 2.17 – The The Switchyard
25 | P a g e
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. Anuradhapura.
220 kV Double circuit line li ne 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 r esponsible 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.
26 | P a g e
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 t ype 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. – The Nilambe Power Station Figure 2.18 – The
2.2.4.
Kelanithissa Power Station
We learned about 20 MW gas and and 115 MW gas turbines. Also we studied about,
Compressor
Combustion Process
Turbine
Generator & Excitation Methods
Description of 20 MW gas turbine,
Output Voltage – Voltage – 11 11 KV
Power – Power – 26690 26690 KVA
Fuel – Fuel – Diesel Diesel
Turbine Speed -5100 rpm
Generator Speed – Speed – 3000 3000 rpm
2 pole, Cylindrical rotor
27 | P a g e
2.2.4.1. Kelanithissa Combined Cycle Power Station
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. 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 S ystem 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 – 49.5 – 50.5 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.
28 | P a g e
Table 2.3 – 2.3 – Hydro Hydro power plants & Capacities
Hydro plant
Installed Capacity (MW)
Mahaweli Complex
Victoria
70
x3
210
Kotmale
67
x3
201
Randenigala
61.5 x 2
123
Rantambe
25
x2
50
Ukuwela
20
x2
40
Bowatenna
40
x1
40
Mahaweli Total
664
Laxapana Complex
Wimalasurendra
25
x 2
50
Canyon
30
x 2
60
Old Laxapana Laxapana
(8.3 x 3)+(12.5 x 2)
49.9
New Laxapana
50
x2
100
Polpitiya
37.5 x 2
75
Laxapana Total
334.9
Other Hydro plants
Samanalawewa
60
x2
120
Kukule
35
x2
70
Udawalawe
2
x2
4
29 | P a g e
Inginiyagala Nilambe
12 1.6
12 x2
3.2
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 – 2.4 – Thermal Thermal power plants owned by CEB
Thermal plant
KPS GT
Installed Capacity(MW)
20x5 + 115
215
108(GT) + 55(ST)
163
Sapu STG 1
16x4
64
Sapu STG 2
9x4
36
Sapu STG 3
9x4
36
KCCP
30 | P a g e
Table 2.5 – 2.5 – Private Private Thermal power plants
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. 31 | P a g e
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 r epair 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.
32 | P a g e
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 ke y 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 load forecast. forecast. (Low population and low GDP growth; 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.
33 | P a g e
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 t o 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.
34 | P a g e
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 – 2.6 – Voltage Voltage Criteria Allowable Voltage Variation. (%) Bus Bar Voltage.
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.
35 | P a g e
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 t he network.
Table 2.7 – 2.7 – Short Short Circuit Criteria Bus Bar Voltage.
132 kV and above
33 kV
11 kV
System.
Maximum Three Phase Fault Level.
Overhead
40.0 kA
Under ground
40.0 kA
Overhead
13.1 kA
Under ground
16.0 kA
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
36 | P a g e
2.2.8
Transmission Operation & Maintenance (Anuradhapura Region)
2.2.8.1.
Introduction
I was placed at Transmission Transmission operation and maintenance maintenance branch at Anuradhapura 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.
37 | P a g e
Table 2.8 – 2.8 – Circuit Circuit Breakers Operation voltage of Circuit
Quenching medium
Operating mechanism
220
SF6
Spring charge
132
SF6
Pneumatic or spring charge
33
SF6 or oil
Spring charge
Breaker (kV)
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 it s 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. 38 | P a g e
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 b y 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 secondar y circuit. In usual practice the primar y 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
39 | P a g e
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
40 | P a g e
