Supply Systems & Transmission of Electrical Energy
1- Choice of Supply Circuits
1-1 Feeding with a voltage equal to the generator voltage
The choice of the electrical
diagram for power plants depends on the nature of the loads and the importance
of continuous feeding to them, as well as on the flexibility in operation and
the ease of performing maintenance for electrical equipment, and there are many
executed diagrams, but in main forms that can be considered as one of the basic
plans or a mixture of them. These schemes are divided from one simple radiative
circuit to several overlapping circuits to maintain the continuity of
electrical supply, which means an increase in the use of protection systems and
thus an increase in economic cost with complexity in operation.a
Figure (1) shows several systems
of buses, which are connected to feeders with a voltage equal to the generation
voltage, and part (a) of the figure represents the simplest electrical diagrams
that can cut off the feed from the loads for a while. There is a circuit
breaker for the generator, as well as on each feeder to deliver loads and
protect in case of shortage. Each feeder also has a disconnecting switch and
its purpose is to protect back up. When servicing the open-ended cutting device
on the feeder. Most of the time, this switch connects to the ground to protect
and leak any remaining charge.
In Part B of the figure, two rods, cutting
devices and switches are used for loads that require continuous feeding, and
maintenance of the generator or rod cutting device can be performed without
interruption to feed and provide protection for people who perform maintenance
operations.
Parts (c) and (d) are less elastic than (b).
When using two rods for
distribution, a selector breaker can be connected to select the feeder rod, as
well as it works as a backup protection in case of failure of the feeder
cutting device.
Figure (1)
A bus-tie breaker is used when feeding more than one generator so that each
generator ends up on a part of the rod to avoid the difference in voltage in
value and angle on the two parts of the rod during reactor when the power is
interrupted from one of the generators, which causes circulating currents in
the circuit .
Parts (f) and (e) show a more
sophisticated scheme.
In all the above schemes, it is
necessary to have current and voltage transformers that feed the protection
devices. Also, measuring devices for electrical quantities such as current and
voltage and control units that enable the system operator to connect or
disconnect any electrical equipment. It is also necessary to have a ground bus to ground the feeders during
maintenance or repair. There may also be
fire walls between the parts of the rods to prevent the fire from
spreading to all parts.
1-2 Feed Through Voltage Lifting Transformers
In this case, the transformers
are used to raise the generating voltage, and there are several diagrams
between them Figure (2) where each generator can be connected to a transformer
and then to a feeder, part (a) of the figure. Or the generators are fed on a
rod from which feeders with a voltage equal to the generation voltage and the
other on transformers to raise the voltage, part (b) of the figure, and part (c)
of the same figure shows a diagram that allows the continuity of the feed all
the time, while Figure (D) allows flexibility in operation with the use of a
number of Less than the cut-off devices for state (C).
2- The importance of electric power transmission lines :
Transmission lines are used to transmit electrical energy from places of
generation, whether hydrothermal - gas- nuclear plants or others to the places
of electrical load centers. Lines - called "tie lines" - are also
used to connect electrical power
systems.
3- Systems and voltage of electrical power transmission lines :
3.1 Low voltage and DC transmission
Transmission by low voltage direct current
3.1.1 Radial system
This is the first method used to
transmit electrical energy, and Figure (3) shows a diagram of the system and
determines the area of the cable section based on the value of the current
allowed to pass through it (Current carrying capacity ), as well as the changes
in voltage at electrical loads not exceeding a certain percentage (5% - 6%) (Voltage
regulation). The main disadvantage of this system is the interruption of the
supply of loads to a distributor if it malfunctions.
Figure (2)
Figure (3)
3.1.2 Ring system
Figure (4) shows the diagram of
this system, in which it is possible to avoid interruption of the supply of loads
in case of a malfunction in the distributor, where the feed is done from the
other side, as well as the loop can be fed by more than one generator, as shown
in Figure (5).
Figure (4): Single-source ring feeding system
Figure (5): Ring feeding system from multiple sources
3.1.3 Wire system
In the case of large electrical
power transmission, this requires cross-section transmission lines and
therefore a high cost. Therefore, a wire convergence system can be used as
shown in Figure (6) where the N-1 circuit is fed through the first generator, and the N-2 circuit is fed through the second generator. Only the tie
line of the two generators is connected to each other and to the ground.
Therefore, the line carries N. The difference in current can be reduced to a
certain extent, as by reducing the cross-sectional area, the resistance of the
wire will increase, thus increasing the voltage difference and increasing the
value of the earth potential, and most often an electric line N with a
cross-sectional area equal to half of the cross-section area of the outer lines
(1-2) is used.
Figure 6: Tri-wire system
In order to determine the percentage of
savings in the material connected in a three-wire system to a two-wire system,
it is assumed that:
V: voltage between the wires in 2 – wire case I
and between the outer and the neutral in 3 - wire case II
P: Power transmitted
R1:
Resistance per unit length for case I.
R2:
Resistance per unit length of outer wires in case II
Resistance
of the neutral = 2 R 2 per
unit length
I in
2- wire case = P/V
Power
loss = 2 I 2 R = 2 (P/V) 2 R1
I in
3- wire case, and assume equal loads ,i.e I1 = I2, I through N=0
I
= P / (2V)
Power loss = 2 ( P /2V)2
R2
For equal power loss :
2 / (P/V )2 R1
= 2 ( P/2V)2 R2
R2
= 4 R1
Cross section of the outer is ¼
of that in the 2- wire
Copper ratio:
3 – wire = (2 x ¼ ) +
(1/2 x ¼ ) = 5 =
31.25
2 – wire 2 x 1 16
Reduction in case of using 3 –
wire = 68.75 %
If the neutral has the some
cross section as the outer, the ratio is :
3 – wire = (2 x ¼ ) + (1 x ¼ ) = 3 =
37.5 %
2 – wire 2 x 1 8
Reduction in case of using 3 –
wire = 62.5 %
If the loads on the outer lines
are not equal in a three-wire system, the savings will be reduced. Therefore,
it is preferable that the loads are the same in the two circuits.
3-2 Advantages of High Voltage
If the voltage is increased by a multiplier of m,
where m is greater than one, the amount of material used in the transmission
lines will decrease by 1/m2 for the same
value of the transmitted power and the loss of electrical power. If the voltage
increases by the multiplication factor of m, the current will decrease by a multiplier of 1/m for the same value of the
transmitted power, and since the loss of power is equal to the resistance
multiplied by the square of the current, it is for the same value of the lost
power (Ohmic loss) the resistance M2
will be the previous value and therefore
the material will be only 1/m2. However,
with the increase in the voltage of the transmission lines, it is necessary to
use towers, insulators and cutting devices in higher cost, so a technical and
economic study must be done to determine the optimal transmission voltage value
according to each system.
3.2.1 Transmission by alternating current
The use of AC current allows the possibility of raising the voltage using electrical power transformers, and there are many transmission systems that are explained in the following parts:
3.2.1.1
Single-sided system - two or three wires
Single phase Two and three - wire systems
Figure (7) shows the different cases used in the
transfer, where the secondary file of the transformer is shown in the figure.
In this case, too, a three-wire transmission is distinguished from a two-wire
transmission as is the case when using DC current.
Figure (7)
3.2.1.2 Bifacial system - three or four wires
Bifacial voltage generators have two orthogonal
coils, so the electric motive force generated in the two coils is perpendicular
to it, and Figure (8) shows a three- and four-wire system where the coils are
finally divided into two equal parts mid
point and the division points are connected.
Transportation using this system is very limited.
Figure (8): We are oppressed bifacial
3.2.1.3 Three-Phase System - Three or Four Wires
The tri-wire system represents the vast majority of
transmission systems. And connecting files in the form of a star or delta,
Figure (9).
Figure (9): Three-Sided System
In
order to compare the amount of material conducting the transmission lines in
this case and the transmission condition using constant current – 2 wires, it
is assumed that the maximum value of the voltage between the line and the
ground in both cases is equal.
Let E voltage between conductors in
d-c. , I is current
Power = E I
Loss = 2 R2 I
R : resistance of each wire per unit length for DC
system
For star connected 3 phase, maximum voltage E,
r.m.s value = E / Ö 2
I’ is the r.m.s. value of the line current
Power = 3/Ö 2 E I’ cos f
Loss = 3 R’ I’2
R’ : resistance per wire for 3 phase
system
For same transmitted power:
E I = 3/Ö 2 E I’ cos f
And same power loss:
2 R I2 =
3 R’ I’2
Hence R = R’ /
( 3 cos2 f )
Each wire in the 3 phase system has a cross section
= 1 / ( 3
cos2 f ) of that in the DC system.
As there are 2 conductors in the DC system and 3 in the 3 phase system , then
the DC system requires (3/2) / (1/ ( 3 cos2
f )) =
0.5 as much material.
3.2.1.4 Alternating Current Six Phase Systems
The hexagonal system consists of two connected
triangular systems so that each face of one of the triangular systems is
dislodged at an angle of 180 degrees from the opposite face in the second
triangular system. Figure (10) shows a hexagonal system consisting of two sets
of coils connected to a delta, part (a) shows the state of a hexagonal system
with 6 wires, and part (b) a hexagonal system with 7 wires as well as giving
the shape of the voltage vectors. 1) Shows a comparison between the different
systems of the AC state by imposing the same loads the same voltage value
between the wires and the same value of the cross-sectional area and the values
attributed to the case of one face with two wires.
Figure (10)
Table (1)
|
Voltage Drop (Approximate) |
Power Loss |
Amount of conductor |
Type of AC system |
|
|
1.0 |
1.0 |
1.0 |
2- wire |
Single Phase |
|
0.25 |
0.25 |
1.5 |
3-wire |
|
|
0.5 |
0.5 |
1.5 |
3-wre |
Two Phase |
|
0.25 |
0.25 |
2 |
4-wire |
|
|
0.25 |
0.25 |
2.5 |
5-wire |
|
|
0.167 |
0.167 |
1.5 |
3-wire * |
Three Phase |
|
0.5 |
0.5 |
1.5 |
3-wire ** |
|
|
0.167 |
0.167 |
2 |
4-wire |
|
|
0.042 |
0.042 |
3 |
6-wire |
Six Phase |
|
0.042 |
0.042 |
3.5 |
7-wire |
|
* Star voltage same as single phase
** Delta voltage same as single phase
3.2.2 DC Transmission
Recently, the use of continuous voltage
transmission lines, although it is still relatively limited, has increased, due
to the many features of constant current, as well as to the advances in power
electronics devices and systems that convert variable current into constant
current and vice versa, and the first use of these lines was for cables
extended under the seawater, and there are overhead lines. One of the
advantages of these lines is the lack of loss in the transmitted power, as
there is no loss in reactance due to its absence in the first place, as well as
the loss of voltage due to the same reason as before. There are no problems of
electrical equilibrium no matter the length of the electrical line, as is the
case when the line carries a declining current because the current is a
constant value. Variable current to constant current conversion plants and vice
versa also cause harmonic contamination in the areas surrounding these stations in the
network.
4- Multiple systems for feeding electrical networks
Figure (11) shows some feeding diagrams for the
transmission networks, and part A represents the fully sectionalized supply on
the rails and is characterized by a high
degree of reliability and flexibility , and the feeding is not interrupted if there
is a failure or maintenance in one of the lines one circuit where the other
circuit feeds and part (B) represents the feeding through a looped in supply
loop In this case, fewer cutting devices are used for case A, and also in this
case, it is possible to continuously feed loads even in case of failure or
maintenance in one of the lines, but the operation of the relays is more
difficult than in case (A).
The scheme in part C is less costly than in case A and
slightly lower than in case B and is used in case if the feed cuts are briefly
harmless. Adjusting the operation of relays is more difficult and removing a
short circuit may require more than one cutting device, and generally in large
networks it is a mixture of the previous schemes.
Figure (11)
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