Download Typical Transmission and Distribution Schemes

CHAPTER
1
Typical Transmission and
Distribution Schemes
Introduction, One line diagram, Standard voltages for transmission, Advantages of
high voltage transmission, Disadvantages of high voltage transmission, Equation of
volume of copper required and efficiency, Distribution system, AC systems and DC
systems, Advantages and disadvantages of AC and DC systems, Important points to
remember.
1.1 INTRODUCTION
Transmission and Distribution are the principle part of the power system. Transmission lines are the
connecting links between the electrical power generating stations and distribution system. The distribution
system connects all the loads to the transmission lines at substations. The substations perform switching
functions and voltage transformation.
In the beginning of the development of power system, the generation was DC only and the generators
were located at load centres. There was no need for transmission lines. With the development of transformer
which can change AC voltages from one level to another with high efficiency and also the availability of
electrical energy generated by water power, transmission of electrical energy over long distances become
feasible. This is because when the transmission voltage is increased, for the same power the I2R loss in the
lines gets substantially decreased. The poly phase motors which have better performance over single phase
motors, so the three phase transmission replaced DC and single phase systems. Presently only three phase
systems are used.
The power system consists of electrical power generation at 11 kV to 25 kV. The generating stations may
be hydroelectric power stations at places where potential of water is available or thermal power stations
near coal mines where water is available for cooling purposes. The generation voltage is stepped up to
transmission voltages of 220 kV to 400 kV with the help of transformers and the energy is transmitted to
load centre receiving stations at this voltage. At receiving stations it is stepped down to sub-transmission
voltages of 33 kV – 138 kV when it is supplied to large consumers. It is stepped down to 400/231volt by
distribution transformers and the power is distributed to medium and small consumers. The schematic
diagram of a simple power station is as shown in Fig. 1.1.
2 ELECTRICAL POWER TRANSMISSION AND DISTRIBUTION
Fig. 1.1 One line diagram of power systems
1.2 STANDARD VOLTAGES FOR TRANSMISSION
Consider two generators operating at 6.6 kV and 11 kV when there is a transmission line operating at
220 kV, the step up transformers are to be rated at 6.6/220 kV and 11/220 kV respectively. In order to reduce
the breakdown time there should be one spare transformer of each rating. Instead if generators are rated at
11 kV, then only one spare transformer can be used for replacement of any transformer in case of breakdown.
This saves the money on spares as well as storage space. When this argument is extended to all equipments
in the power system there is a need for standardising the voltages in the system. Such standardisation
reduces the cost of equipments, space for keeping the spares and also reduces problems in manufacture and
design. In our country the voltages used for different purposes are:
(a) 6.6 kV, 11 kV, 33 kV
Generation
(b) 110 kV, 132 kV, 220 kV and 400 kV
Transmission
TYPICAL TRANSMISSION
(c) 3.3 KV, 6.6 KV, 11 KV
(d) 400 V between the lines
231 V between line to neutral.
AND
DISTRIBUTION SCHEMES 3
Distribution of large consumers
Low voltage distribution
1.3 ADVANTAGES OF HIGH VOLTAGE TRANSMISSION
The following are the advantages of high voltage transmission:
(i) For a given power, if transmission voltage is increased the current carried by conductors decreases
(because P = VI cos Φ, power factor remaining constant, when V increases I decreases for same
power P).
(ii) When the current decreases, power loss in conductors I2R loss decreases and hence efficiency
increases.
(iii) When there is decrease in current the voltage drop IR or IZ decreases and improves the voltage
regulation.
(iv) Since the current density which the conductor can carry is same, cross section of the conductor
I˘
È
decreases Í a = ˙ which in turn decreases the volume of the conductor material.
d˚
Î
(v) The power transmitted by transmission lines is proportional to square of the transmission voltage.
As a consequence the overall capital cost of transmission decreases as the voltage increases.
Transmission of large power over long distances is economically feasible with high voltage
transmission only.
(vi) The power stations of large capacities are far away from load centres. This is because hydroelectric power stations are located purely on geographical considerations. Thermal power stations
are located near coal mines, nuclear power stations are located far away from thickly populated
areas. To transmit large power over long distances high voltage transmission is the only solution.
(vii) Interconnection of the power systems will be easier with high voltage transmission.
1.4 DISADVANTAGES OF HIGH VOLTAGE TRANSMISSION
Some of the disadvantages of the high voltage transmission are:
(i) Corona and radio interference: As the voltage increases the phenomenon of corona occurs.. Corona
also interferes with radio and television signals.
(ii) Line supports: For high voltage transmission conductors the ground clearance required is large.
Due to these the tower has to be big.
(iii) Construction difficulties: Erection of high voltage transmission towers requires high standard of
workmanship.
(iv) Insulation requirement: As the voltage increases the insulation requirement increases. Switching
surges are roughly four times the operating voltage. Hence the insulation required correspondingly
increases.
1.5 THE EQUATIONS THAT HIGHLIGHT THE ADVANTAGES OF HIGH
VOLTAGE OPERATION
Let the V and I be the phase voltage and current of a three phase system.
Then the per phase input power P = VI cos f Watts/phase
4
ELECTRICAL POWER TRANSMISSION
AND
DISTRIBUTION
The power loss in the circuit per phase p =
The Input power P =
I 2R
kW Ph
1000
VI cos f
kW Ph
1000
Then the current in the load I =
1000 P
Amps
V cos f
2
Ê 1000 P ˆ
R
¥
kW Ph
The power loss per phase p = Á
˜
1000
Ë V cos f ¯
Therefore the output O P = I P - Losses
Ê
1000 P 2 R ˆ
Then the O P = Á P - 2 2 ˜
V cos f ¯
Ë
Ê
1000 P 2 R ˆ
P
Á
˜
V 2 cos2 f ¯
Ë
O P
¥ 100 =
¥ 100
The percentage efficiency % h =
I P
P
1000 PR ˆ
Ê
After simplification % h = 1 - 2 2 ¥ 100
ÁË V cos f ˜¯
It is clear from the above equation that the percentage η varies directly with the system voltage and
so if the operating voltage is high the value of second term in the bracket is low and efficiency is high.
The increase in the operating voltage also reduces the power loss. When the operating voltage is
increased the operating current is decreased and hence the voltage drop is reduced and so improves the
voltage regulation.
1.6 TO SHOW THAT THE VOLUME OF CONDUCTOR MATERIAL
REQUIRED DEPENDS ON OPERATING VOLTAGE
Let the V and I be the phase voltage and current of a three phase system.
Then the per phase input power P = VI cosf watts/phase
The power loss in the circuit per phase p =
The input power P =
I 2R
kW/Ph
1000
VI cosf
kW/Ph
1000
Then the current in the load I =
1000 P
Amps
Vcosf
2
R
Ê 1000 P ˆ
¥
kW/Ph
The power loss per phase p = Á
˜
1000
Ë V cos f ¯
TYPICAL TRANSMISSION
The resistance of the conductor R =
AND
DISTRIBUTION SCHEMES 5
l
r
a
2
l
Ê 1000 P ˆ
¥
kW/Ph
The power loss per phase p = Á
˜
1000 a
Ë V cos f ¯
Area of the conductor a =
1000 P 2lr
V 2 cos2 f p
Volume of the conductor al =
1000 P 2 l 2 r
V 2 cos 2 f p
The above equation indicates that the volume of the conductor material is inversely proportional to
the square of the operating voltage per phase when the power to be transmitted over a fixed length is
taken as constant.
1.7 DISTRIBUTION SYSTEM
A distribution system consists of feeders, distributors and service mains. Figure 1.2 shows a distribution
system.
Feeders are conductors having large current carrying capacity connected from substation to distribution
transformer. Normally feeders are operated at high voltage say 11 kV.
Distributors are conductors from which current is tapped off for supply to the consumers and is
connected to secondary of the distribution transformer. Service mains are cables connected between
distributors and consumer premises.
Service
mains
Sub
station
Feeder
Distribution
Distribution
transformer
Fig. 1.2 Distribution system
1.8 TYPES OF TRANSMISSION AND DISTRIBUTION SYSTEMS
In the present scenario, the electrical energy is generated, transmitted and distributed in the alternating
form. This is because of the fact that the alternating voltages can be increased or decreased in magnitude
with the help and use of transformers. Whenever the DC supply is required the AC voltages are converted
to DC by using rectifiers and then can be transmitted or distributed. Hence the main division of
transmission and distribution are AC system and DC system.
6
ELECTRICAL POWER TRANSMISSION
AND
DISTRIBUTION
(1) AC System
The commonly used system for transmission of electrical power from generating station to the distribution
station is AC three phase three wire system or three phase four wire system. In three phase three wire
system the primary of the transformer is always connected either in star or delta. If it is in star connection
the neutral point is grounded. For large consumers like factories and industrial estates are directly
supplied from the primary distribution transformer. The secondary distribution transformers are always
connected in either star/star or delta/star so as to supply both single phase and three phase loads.
Advantages of AC system
(i)
(ii)
(iii)
(iv)
The cost of AC generation is very low when compared to the DC generation.
With the advent of transformers AC voltage can be raised or lowered to any desired value.
The maintenance of AC substation is very easy.
The motors running on AC system are simple in construction, low cost and decreases cost of
maintenance.
Disadvantages of AC system
(i)
(ii)
(iii)
(iv)
(v)
(vi)
Construction of AC transmission lines is more complicated than a DC line.
The voltage drop in AC lines is large because of the inductance effect of the line.
The copper requirement of AC line is more when compared to DC system.
In AC transmission the lines experience corona effect and corona loss.
In AC system of generation the synchronisation of alternators is required.
The charging current exists even when the line is open, because of the existence of shunt capacitance
between the lines and also between line and neutral.
(2) DC System
Though the AC systems are extensively used, there are few applications of DC supply alone. Some of
the applications of DC supply are for the charging of batteries, electroplating etc. To obtain the DC
supply, rectifiers or DC generators are used at substations or at load centres.
The systems are further classified as (a) Two wire DC system (b) Two wire DC with mid point
earthed and (c) Three wire DC system.
(a) Two Wire DC Systems: It is the simplest and direct output of the DC generators is given to the
loads with the help of busbars. A simple arrangement is as shown in Fig. 1.3.
(b) Two Wire DC with Mid Point Earthed: This type of connection is used when the required
output voltage is high. In this system of connection there are two lines conductors, the one having
+Vm with respect to mid point and the other is at –Vm with respect to mid point. So the total
voltage available at the two lines is 2 Vm with mid point earthed.
(c) Three Wire DC System: This type of connection is similar to the two wire DC with mid point
earthed. In this connection an additional wire from the mid point is drawn which makes the
system feasible to supply loads requiring low voltage and high voltage.
Advantages of DC system
(i) The cost of conductor material required is reduced from AC three phase system as it uses only two
conductors.
(ii) In DC systems there is no inductance and capacitance and so the voltage drop and power loss is
much lower compared to AC system.
TYPICAL TRANSMISSION
Vm
PM
2 Vm
L
O
A
D
PM
DISTRIBUTION SCHEMES 7
L
O
A
D
2 Vm
PM
PM
AND
Vm
DC two wire system
with mid point earthed
L
O
A
D
L
O
A
D
DC three wire system
Fig. 1.3 Two wire DC system and DC three wire system
(iii) Because of the low voltage drop and low power loss the voltage regulation improves and
transmission efficiency increases.
(iv) As the operating voltage is DC there is no skin effect and so the resistance of the conductor is low.
(v) For same voltage level the voltage stress on the insulation is less in a DC system and so the
insulation required is less in case of DC when compared to AC.
1.9 IMPORTANT POINTS TO REMEMBER
1. Advantages of high voltage transmission are:
(a) Reduction in copper loss, cross section of conductor decreases, reduction in volume of
conductor material, voltage drop decreases, voltage regulation improves,
(b) Increases the transmission efficiency and reduces the cost of power transmission over long
distances.
2. Disadvantages are corona loss and radio interference increases, line supports has to be stronger
and more insulation is required.
3. In AC transmission it will be three phase three wire system is used and in distribution three phase
four wire system is used so that both single phase and three phase loads can be supplied.
4. In the DC distribution there are three types of distribution schemes such as DC two wire, DC two
wire with mid point earthed and DC three wire system. When the higher operating voltages are
required DC two wire with mid point earthed and DC three wire systems are used. The main
advantage of this system is it requires a less conductor material, cost of distribution is reduced.
5. In DC system inductive and capacitive reactance values are zero, hence the voltage drop in the
line is less and regulation improves. There is no skin effect in the DC system and hence the
resistance of the conductor is reduced.