Power Generation
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What are the main advantages of use of hydropower plants in a power system?
List the types of hydropower plant and their field of use
Define hydrograph and explain its use
Explain the function of following elements of hydropower plant
i.
ii.
iii.
iv.
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Penstock
Surge chamber
Forebay
Reservoir
State the field of use of steam power plants
Sketch the schematic diagram of a steam power plant and explain briefly function of
each of elements
What are the advantages of Gas Turbine station when compare with steam power plant
Explain briefly the operation of Gas Turbine power plant
What are the non-conventional power plants? Why these power plants are not being
used in bulk
Write a technical write-up on “Mini hydro power plants and their use in Sri Lanka
Explain how pumped storage power plant improve the load factor of the system
Explain operation of combined cycle power plant
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Certain power system consists of Run-off river plants, pumped storage power plants,
hydropower plants with ample storage, coal power plant, nuclear power plant, diesel
power plant. Allocate these power plants in a daily load curve. Give reasons for your
choice
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Explain briefly following types of hydropower plants and their field of use:
Run-off river plants
Reservoir power plants
Pumped storage plants
Write short notes on following components of hydropower plant:
Spillway
Forebay
Surge tank
What are the factors that should be considered when selecting location of a nuclear
power plant from following points of view?
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operation
safety
Describe the wind power generation in Sri Lanka with some technical characteristics.
What are the factors that should be considered when the type of power station is
selected?
Define following terms
1
i.
ii.
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Plant capacity factor
Plant use factor
How parameters in (b) effect to the size of the units, number of units, reserve capacity
and operating schedule of a power plant
Load characteristics and economical aspects
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Daily load of a certain power plant is given below
Time (hrs)
i.
ii.
iii.
Time (hrs)
0000- 0600
Demand
(MW)
2
1300-1700
Demand
(MW)
7.5
0600-0800
3.5
1700-1900
8.5
0800-1200
8
1900-2100
10
1200-1300
3
2100-0000
4.5
Sketch the load curve
Calculate energy generated per day
What is the Load factor of the plant
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List the major types of loads in an electrical system and sketch load curve for each
Define following terms:
i.
ii.
iii.
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Maximum load
Load factor
Diversity factor
State the advantage of diversity of the load in electrical system
Certain power system consist of four consumer groups: A,B,C and D. Their load
characteristics are as given below:
Consumer group A has maximum demand of 8 kW at 1900 hrs and the average load of
3.5 kW
Consumer group B has maximum demand of 6 kW and demand at 1900 hrs is 2.5 kW.
Load factor of the consumer group is 20%
2
Consumer group C ha maximum demand of 6 kW and demand at 1900 hrs is 3 kW.
Average load per day is 2.2 kW
Consumer group D has maximum demand of 15 kW and demand at 1900 hrs is 4 kW.
Load factor is 30%
The peak load of the system occurs at 1900 hrs.
Determine
i.
ii.
iii.
iv.
v.
vi.
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Diversity factor
Load factor of Consumer groups A and C
Average load of consumer groups B and D
Total Energy consumed by the system
Average load of the system
Load factor of the system
Certain electrical consumers’ monthly energy consumption is 600 kWh. Load factor of
the consumer is 40%. Tariff structure for the consumer is given below:
Rs. 300 per kW of the maximum load
Rs.11.00 per kWh of the consumed energy
I.
Calculate monthly electricity bill of the consumer
II.
What is the average cost per kWh?
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The gross electricity energy generation of Sri Lanka power system in 2009 is
amounted to be 9882 GWh. It has been recorded that the peak demand in this period as
1868 MW.
Calculate the load factor of the Sri Lanka power system
Explain why the utilities offer various Time-of-Day Tariff structures to the consumers,
especially industrial customers
A tariff system given below table is available to an industry metered at 400/230 V, which
draws an apparent power of 50 KVA at a lagging power factor of 0.85 throughout the day
and right throughout the year.
i.
ii.
What would be the best tariff system applicable to this industry?
Compute the amount of money that has to be allocated to pay the electricity bill
for the year 2011 under the best tariff system
(Assume the load pattern remains the same through out the year)
Assume that a stand by generator is available in this industry and is capable of supplying
the full load of the factory at least for a continuous period of four hours every day. If the
overall cost of generation including maintenance is estimated to be 8.00 Rs/kWh,
calculate the annual saving that can be achieved with the best tariff system.
3
Customer
Conditions
Category
Fixed
Maximum
Demand
Charge
Rs/kVA
Energy charge (Rs/kWh)
Rs/mont
h
per month
INDUSTRI
AL
metered at
400V/230
V
contract
demand
none
≤ 45 kVA
metered at
400V/230
V
contract
demand
675.00
>45 kVA
metered at
11kV or
above
INDUSTRI
AL/
HOTEL
INDU
STRIA
L
TIME-OFDAY
metered at
400V/230
V
contract
demand
charge
650.00
10.50
240.00
10.00
3000.00
9.00
3000.00
26.00 between 6.30pm-9.30
pm
650.00
3000.00
8.50 at other times
>45 kVA
metered at
11kV or
above
24.00 between 6.30 pm-9.30
pm
650.00
OPTION
3000.00
8.00 at other times
(Two Part)
An installation comprises of two 3 kW electric furnaces, eight 100 W lamps and ten 75 W
lamps. Average use of both furnaces is two hours daily and that of lamps is 100 hours
monthly. Determine the electricity bill under domestic tariff system.
Tariff structure:
4
0-30 Units @ 3.00
31-60 Units @ 3.70
61-90 Units @ 4.10
91-180 Units @ 10.60
Above 180 Units @ 15.80
What would be the monthly electricity bill if during dry session the full adjustment charge is
120% for over and above 5th block
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Certain shop consists of two 2.5 kW electrical kettles and eight 75 W fluorescent lamps.
The maximum demand of rest of electrical appliances is 4.6 kVA. The kettles operate
one hour per day and lamps 7 hours per day. The total energy consumes by the rest of
appliances per month is 25 units (kWh).
I.
What is the contact demand
II.
Calculate the monthly electricity bill of the shop under following tariff structure:
If the contact demand is less than 42 kVA
Fixed charge : Demand up-to 10 kVA : Rs. 30/=; Demand greater than 10 kVA: Rs. 230/=
Unit charge : Rs. 10.90
Overhead lines and underground cables
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Explain reason for using bundle conductor arrangements in transmission lines
Explain term “Transposition ” with related to the over head transmission line
Figure Q1 shows bundle arrangement of three-phase 230 kV, 50 Hz, 112 km long
transmission line. Outside diameter of a conductor is 7.77 mm and AC resistance at 200
C is 0.905 Ω/km. Temperature coefficient is 0.0044. The line is being operated at the
ambient temperature of 400 C.
9.5 m
9.5 m
0.35 m
Figure Q1
5
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Calculate
i.
ii.
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per-phase, per-unit length inductance, capacitance of the line
Per-phase resistance, inductive reactance and capacitive reactance
Sketch equivalent π – model of the transmission line and indicate parameters.
What changes of parameters would you expect, if the effect of earth was taken in to
consideration?
Explain term “skin effect” with related to the ac transmission lines
What is the purpose of use of bundle conductors in Extra High Voltage overhead
transmission lines?
Describe briefly effect of temperature to the performance of transmission lines
Conductor arrangement of three-phase 50 Hz overhead transmission line is shown in
figure Q1. Diameter of each conductor equals to 12.27 mm
12.5 m
0.4 m
0.4 m
0.4 m
Figure Q1
I.
II.
I.
II.
III.
IV.
Calculate per-phase, per-unit length inductance and capacitance of the transmission
line. Clearly state any assumptions that you make
If the length of the line is 200 km and calculate parameters calculate inductive reactance
and capacitive susceptance of equivalent π-model of the line
Briefly explain the materials that are used for insulators
List the types of insulators used and discuss their area of use
Explain term “flashover”
Figure Q1 shows an insulator string consisting four discs. Conductor voltage is 66 kV. If
C1=C2=12 C and C3=C4=7C, calculate
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a. Voltage distribution
String efficiency
C
C4
C
C3
C
C
C2
C1
Figure Q1
(a) List the types of insulators used in overhead lines
(b) Define following terms
i.
ii.
Flashover voltage
Puncture voltage
[3]
[4]
(c) List the methods of improving string efficiency and briefly explain one of them
[5]
(d) A 33 kV overhead line is hung using suspension type insulator consisting four units. The
mutual capacitance of the unit is 10 times the capacitance between the pin to earth.find:
i.
Voltage distribution across the units
ii.
String efficiency
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Transmission line performance
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220 kV, 50 Hz, 172 km long overhead transmission line delivers power to a load of 120
MW at 0.85 power factor lagging. Per-phase, per-unit length parameters of the line are
R=0.125 Ohm/km
X=0.49 Ohm/km
Y=3.3 x10-6 S/km
i. Calculate A,B,C,D constant of the line
ii. If receiving end voltage to be maintained at 220 kV, determine the sending end voltage
and current
iii. Calculate voltage regulation and transmission efficiency
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A 230 kV, three-phase 180 km long double circuit overhead transmission line
delivers power to a load of 250 MW at 0.95 power factor lagging. Per-phase
equivalent parameters of the line are:
r=0.098 Ω/km; x=0.42 Ω/km; b=2.5x10-6 S/km
The voltage at load is equal to 230 kV
(a) Calculate voltage, current, power and power factor at the sending end of the line [10]
(b) Determine voltage regulation and transmission efficiency
[4]
(c) Draw the phasor diagram to show all voltages and currents
[6]
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A 230 kV, 50 Hz, 150 km long overhead transmission line delivers power to a load of
100 MW at 0.8 power factor lagging. Per-phase, per-unit length resistance, inductive
reactance and shunt susepctance are 0.19 Ω /km, 0.32 Ω /km and 3.776 x10-6 S/km
respectively. If the voltage at load is maintained at 230 kV, using nominal πequivalent circuit calculate:
I. Sending end voltage, power and power factor
II. Voltage regulation and transmission efficiency
III. Required reactive power compensation at the receiving end of the line in order to
maintain voltage at both ends of the line at 230 kV
IV. If the line is opened at the receiving end and voltage at the sending end is equal to 230
kV state whether voltage increase/decrease or remain same when compare with sending
end voltage. Explain your answer
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List the various methods of voltage control and explain one of them briefly
8
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Explain why should not power system to be allowed to operate beyond the
permissible range of voltage variation
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A 230 kV, 50 Hz transmission line has length of 350 km. Distributed parameters of
the line are x=j0.428 Ohm/km, y=j3.26x10-6 S/km. A load at the receiving end
consumes 180 MW at 0.95 power factor lagging. Voltage at the receiving end is
maintained at 230 kV.
I.
II.
III.
Calculate surge impedance, propagation constant and surge impedance loading
Determine ABCD parameters of the line
Calculate sending end voltage, current, power and power factor.
Switchgear
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i.
Explain briefly method involved in interruption of electric arc of
DC circuits
ii.
AC circuits
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A three-phase short circuit occurs beyond the circuit breaker of a three-phase generator
at time t=0. After the time t=T1 the contacts of the breaker start to separate and at t=T2
the arc is successfully interrupted.
Sketch variations of current through the contacts of the breaker and voltage between the
contacts with the time and clearly indicate the followings:
i. Marking current
iv.
Restriking voltage
ii.
Breaking current
v.
Recovery voltage
iii.
Voltage across the arc
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Explain briefly arc interrupting mechanism in HRC fuses
A short circuit occurs at the distribution line which is protected by a HRC fuse. Sketch
first half cycle of prospective short circuit current and clearly indicate followings:
i.
Cut-off
ii.
pre arcing time
iii.
arcing time
iv.
total operating time
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Explain briefly function of each of following elements of power systems
I.
Circuit breaker
II.
Isolator
III.
Earthing switch
IV.
Lightning arrester
V.
Current and voltage transformer
Explain briefly following terms related to the circuit breaker
I.
Rated short circuit making current
II.
Rated short circuit breaking current
III.
Rated Operating sequence (duty cycle)
Explain briefly arch extinguish mechanism of following circuit breakers
I.
Air Break circuit Breaker
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II.
Minimum oil circuit Breaker
10
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