Transcript Transmission of Electrical Energy

Transmission of Electrical Energy
Electrical energy is carries by conductors such as overhead
transmission lines and underground cables.
The conductors are usually aluminum cable steel reinforced (ACSR),
with steel core and aluminum conductors wrapped around the core. In
addition to phase conductors, a transmission line include one or two
steel wires known as ground or shield wires.
Cable lines are designed to be placed underground or under the
water. In cables, the conductors are insulated from each other and
surrounded by a protective sheath. They are more expensive than
overhead power lines.
Transmission lines are characterized by a series resistance and
conductance per unit length, and by a shunt capacitance per unit
length.
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Models:
Two-wire Conductor and a Coaxial Cable
R and L
Jacket
Braid
C
Shield
Dielectric
Conductor
ρl
 ρ : resistivity, l : length,A : cross- sectionalarea
A
 1
D
L    ln  H/m (r : radius of each conductor,D : distancebetween conductors)
 4
r
RDC 
C

D
ln 
r
( : dielectricconstant)
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Transmission Line Limits
• The maximum steady-state line current must be limited to prevent
overheating in the transmission line. The power loss in a
transmission line is approximately given by:
Ploss  3I R
2
L
• The voltage drop in a practical transmission line should be limited to
about 5%.
• The angle  in a transmission line should be less than 30o. This
limitation ensures the power flow in a transmission line is well below
the static stability limit, ensuring that the power system can handle
transients in a stable manner.
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Generation, Transmission, and Distribution
GENERATION
TRANSMISSION
Interconnection
Substation
Transmission
Substation
Distribution
Substation
2.4 kV
115 kV
G1
1-phase
120/240 V
345 kV
to
230 kV
3-phase
69 kV
765 kV
G2
Generators
DISTRIBUTION
Transmission
Substation
Heavy
industry
Medium
industry
Small-scale
industry and
residences
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Types of Power Lines
• Low-Voltage (LV) Lines: Provide power to building, factories, and
houses. The lines may be overhead or underground operating at
600 V or less.
• High-Voltage (HV) Lines: The voltage is usually between 2.4 kV
and 69 kV. Such systems are preferred in large cities. The
transmission lines spread out like fingers from one or more
substation to various loads such as campuses, shopping centers,
etc.
• Extra-High-Voltage (EHV) Lines: They connect the main
substations to the generating plants. Such lines operate at voltages
up to 800 kV and may be as long as 1000 km.
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Lightning Strokes
Charge separation takes place inside clouds, so that positive charges move to
the upper part of the cloud while negative charges stay below. The transfer of
electric charges sets up an electric field within the cloud. Also the negative
charges at the base of the clouds repels the free electrons on the ground below
and accordingly the ground becomes positively charged.
++++++
_____
____
++++
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Lightning and Transmission Lines
• When lightning makes a direct hit on a transmission line, it deposits
a large electric charge, producing an enormous over voltage
between the line and ground. The dielectric strength of air is
exceeded and the flashover occurs. The line discharge itself and the
over voltage disappears in less than 50 microsecond. Adequate
protection against lightning requires good grounding.
20 kA
69 kV
neutral
BIL
Insulating
Material
Solid Electrical Ground
(20 ohm)
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BIL: Basic Impulse Insulation
Example: A 3-phase 69 kV transmission line having a BIL of 300 kV is
supported on steel towers and protected by a circuit breaker. The
ground resistance at each tower is 20 ohm. The neutral of the
transmission line is also grounded at the transformer just ahead of the
circuit breaker. During an electric storm, one of the towers is hit by a
lightning stroke of 20 kA. Find the voltage across each insulator.
Vphase  69 kV/ 3  40 kV
T hecurrentflowing in thetower ground is zero.
Whenlightningstrikes,the voltageacross the
line resistanceis 20 kA  20 ohm  400 kV. T his
voltagewill appearacrosseach insulator.Since the
impulse exceedsBIL of 300 kV, a flashoverwill occur.
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