Power Chap I
Power Chap I
Power Chap I
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Outline
• Introduction
• Generation
• Transmission
• Distribution
• Utilization
Advantage of Electric Energy over
other form of energy
Electric Energy is:
• Flexible: Easily converted to light, heat,
mechanical energy…
• Cost: it is cheaper
• Transportation: easy & simple to transport,
cable
• Clean: has no smoke
What is power system?
• Power system: is a science which deals
with:
- Generation
- Transmission
- Distribution and
- Utilization of electric power system.
Goal of Power System
The goal of power system is:
To generate & supply reliable electric power
Energy
Converter
Energy
Storage
Transmission &
Distribution
Utilization
Generation
• Generators can be classified as AC/ DC
generators:
• Synchronous generator are the most common
used AC Generators.
• The max. capacity of a single generator
ranges up to 1GW , 34KV, due to insulation
problems.
• Electric Power can be generated from both
- Conventional and
- Non conventional (Renewable) Energy
sources
Generation
A. Conventional Energy sources
• Convectional energy sources are stocks of
energy sources which deplete (cannot
continuously re emerged) using natural process.
• Some of convectional Energy sources are:
- Fossil fuels (Coal, Oil, & Natural Gas) and
- Nuclear fuels
Coal
Nuclear Power Plant
B. Non-Conventional Energy
Sources
Non-conventional energy sources also called
Renewable energy.
Solar thermal route uses the sun's heat to produce hot water
or air, cook food, drying materials etc.
2. Horizontal-axis turbines, in
which the axis of rotation is
horizontal with respect to
the ground (and roughly
parallel to the wind stream.)
v. Three phase three wire system vi. Three phase four wire system
• The usual generation voltages are: 33KV, 11KV,6.6kv…
• The large network between power station and
consumers can be classified in to four parts. These are
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DC Transmission
The potential stress on the insulation is less it requires less insulation.
Less corona loss and reduced interference with communication circuits.
There are no stability problems and synchronizing difficulties.
DC Transmission
Disadvantages
Electric power cannot be generated at high d.c. voltage due to
commutation problems.
The d.c. voltage cannot be stepped up for transmission of power at high
voltages.
The d.c. switches and circuit breakers have their own limitations.
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AC Transmission
Now a day, electrical energy is almost exclusively generated, transmitted and
distributed in AC
Advantages
The power can be generated at high voltages.
The maintenance of AC sub-stations is easy and cheaper.
stepped up or stepped down by transformers with ease and efficiency.
Disadvantages
Required more copper
more complicated construction
An a.c. line has capacitance. Therefore, there is a continuous loss of power due to
charging current even when the line is open.
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Single phase and three-phase
Two types of systemtransmission
available
Single phase system
There will be only one phase,
Minimum amount of power can be transported.
Generating station and load station single phase
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Single phase and three-phase
Three phase system transmission
Poly-phase system where three phases are send together from the
generator to the load.
Each phase are having a phase difference of 120°, i.e. 120° angle
electrically.
Each of the three phases can be used as single phase.
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Constant power due to 3-phase
system
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Why Three Phase is preferred Over Single Phase?
The three phase power system has been adopted universally for
transmission of AC power because of the ff advantages
Higher power/weight ratio of alternators smaller, lighter and cheaper.
3-phase requires less copper or aluminum to transmit the same quantity
of power of a specific distance than a single phase system.
Three phase motors are self-starting due to the rotating magnetic
But a single phase motor is not self-starting, it requires a capacitor and
an auxiliary winding.
In Single phase systems, the instantaneous power is not constant and is
sinusoidal where as in a three phase the instantaneous power is always
the same.
Three phase supply can be rectified into dc supply with a lesser ripple
factor.
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Over View of Existing and Planned Interconnection
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Ethiopia is already connected to Sudan, Djibouti and the Border towns of
Kenya.
Ethiopia – Kenya 500 KV D.C. line under construction (up to 2000 MW)
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Two types of three phase circuit connections :
• Star Connection
There is four wire, three wires are phase wire and fourth is neutral which
is taken from the star point.
Star connection is preferred for long distance power transmission
because it is having the neutral point.
Balanced system: equal current will flow through all the three phases,
When the current will not be equal in any of the phase, then it is
unbalanced current
The line voltage is √3 times of phase voltage.
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Star Connection
Line voltage is the voltage between two phases in three phase circuit
phase voltage is the voltage between one phase to the neutral line.
the current is same for both line and phase.
V ab 3V an or V L 3V ph
I ab I an or I L I ph
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Delta Connection
Three wires alone and no neutral terminal is taken.
Preferred for short distance transmission due to the problem of
unbalanced current in the circuit.
V ab V an or V L V ph
I ab 3I a n or I L 3I ph
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In three phase circuit, star and delta connection can be arranged in four
different ways-
Reading assignment
Star-Star connection
Star-Delta connection
Delta-Star connection
Delta-Delta connection
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Distribution Systems
o The distribution system gives out the energy from the medium voltage
substations to customers’ location.
o It is composed of:
Medium Voltage (MV) lines (33kV, 15 kV)
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Load or Energy Sink
Load is the end equipment of the power system where the transmitted
electrical energy is converted to other forms of useful energy.
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Power, current and potential
transformers
A transformer
is a static machine
operates on ac supply and works on the principle of mutual
induction
Transformers can be categorized in different ways,
depending upon their purpose.
Power Transformer
Used in transmission network for stepping up or down the
voltage level.
It operates mainly during high or peak loads and has
maximum efficiency at or near full load.
Low voltage level power can be transmitted to the receiving
end
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This low voltage power if transmitted results in greater
line current which indeed causes more line losses
But Increase voltage level- decrease current – reduce
ohmic loss- reduction in cross sectional area of the
conductor- reduce capital cost- improves the voltage
regulation of the system- improve quality of power
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Instrument transformers
To reduce high voltages and current to small values which can be measured by
conventional instruments.
Are used in AC system for measurement of electrical quantities i.e. voltage,
current, power, energy, power factor, frequency.
Basic function of Instrument transformers is to step down the AC System voltage
and current.
Types of Instrument transformers
o Current Transformer (CT)(series transformer)
Used to step down the current of power system to a lower level to make it feasible
to be measured by small rating Ammeter (i.e. 5A ammeter)
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o Potential Transformer (P.T.)(parallel transformer)
Used to step down the voltage of power system to a lower level to make is
feasible to be measured by small rating voltmeter i.e. 110 – 120 V voltmeter
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Difference between C.T. and P.T.
Sl.
Current transformer (CT) Potential Transformer (P.T.)
No.
1 Connected in series with power circuit. Connected in Parallel with Power circuit.
4 Primary current depends on power circuit current. Primary current depends on secondary burden.
Primary current and excitation vary over wide range with Primary current and excitation variation are
5
change of power circuit current restricted to a small range.
One terminal of secondary is earthed to avoid the One terminal of secondary can be earthed for
6
insulation break down. Safety.
7 Secondary is never be open circuited. Secondary can be used in open circuit condition.
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o AC Power Review
Review of Phasors: The objective of phasor analysis is to simplify the analysis
of constant frequency ac systems.
o Instantaneous Power
v(t) = Vm cos(ωt + θv) p(t) instantaneous power is the power at any instant of
i(t) = Im cos(ωt + θi) time
P(t)= v(t)i(t) = VmIm cos(ωt + θvcos(ωt + θi)
v(t) instantaneous voltage
apply the trigonometric identity
i(t) instantaneous current
Ѳv and Ѳi are the phase angles of the voltage and
cos A cos B
1
cos( A B ) cos( A B ) current,
2 The
Vm andfirstIm
part
areisthe
constant or time
amplitudes (orindependent
peak values)
p (t )
1
VmI m
cos( v i )
1
VmI m
cos( 2 wt v i ) Its value depends on the phase difference between the
2 2
voltage and the current.
The second part is a sinusoidal function whose
frequency is 2ω, which is twice the a of the voltage or
current.
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Average power
The average power is the average of the instantaneous power over one
period.
The first integrand is constant, and the average of a constant is the same
constant.
The second integrand is a sinusoid.
We know that the average of a sinusoid over its period is zero
T
1
P (t )
T 0
p ( t ) dt
T T
1 1 1 1
P 2V m
I m
cos( v i ) dt 2V m
I m
cos( 2 wt v i ) dt
T 0
T 0
T T
1 1 1 1
P V m I m cos( v i ) dt Vm I m cos( 2 w t v i )dt
2 T 0
2 T 0
1
P VmI m
cos( v i )
2
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,
Complex power
using phasors V and I
S=1/2VI*=VrmsIrms* = 1
VmI ( v i )
m
2
1 1
VmI m
cos( v i ) j VmI m
sin( v i )
2 2
Apparent power
that the magnitude of the complex power is Active or real power
the apparent power
S V I rms rms
P =V I cos( )
rms rms v i
Z
V
I
V rms
I rms
( v i ) Reactive power
2
V rms
I
2
rms
Z
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Phasor Analysis
P V I cos
S V I
Q S si n V I si n
px (t ) Q si n 2 ( t v)
Q P P
cos cos a r c
P P
2
Q 2 S
V I
* Q= Reactive power (var, var,
Mvar)
S= complex power (VA,Kva, MVA)
Power factor (pf) = cosϕ
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Relationships between real, reactive and complex power
S co s , I co s
P P V
For a purely capacitive : the current leads the
Q V I si n voltage by 90°,
S si n 1 pf
2
Q S
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Real power supplied by the source is equal to the sum of the real powers
absorbed by the load and the real losses in the system
0 Pg en Pl oa d s Pl osses
Reactive power must also be balanced: The balance is between the sum of
leading and the sum of lagging reactive power producing elements
0 Q l ea di n g Q ca p s Q l a g g i n g Q i n du c
The total complex power delivered to the loads in parallel is the sum of the
complex powers delivered to each
0 S gen S l oa ds S l osses
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Example
I = 20-6.9 amps
*
S V I 100 30 20 6.9 2000 36.9 V A
36.9 pf = 0.8 l aggi n g
*
SR VR I 4 20 6.9 20 6.9
2
PR 1600W I R (Q R 0)
*
SL VL I 3 j 20 6.9 20 6.9
2
Q L 1200 v ar I X (PL 0 )
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Exercise
Calculate the average power absorbed by each of the five elements in
the circuit given.
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Exercise
Find current, voltage and apparent power for the ckt shown below
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Exercise
The apparent power entering a certain load Z is 250 VA at a power factor of 0.8
leading. If the rms phasor voltage of the source is 125 V at 1 MHz:
a) Determine Irms
b) P going into the load
c) Determine S into the load
d) Determine Z
e) The equivalent impedance of the load circuit should be of the form
Z = R + jωL or Z = R - j/ωC. Determine the value of L or C, whichever
is applicable
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Thank You!!!