1. Generating station 2. Transmission network 3. Distribution Network
All these important networks are connected with the help of
conductors and various step up and step down transformers. A typical
transmission and distribution scheme is shown in the Fig. 1.
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| Fig. 1 Schematic Representation of a typical transmission distribution scheme |
A scheme shows a generating station which is located too far away
from cities and towns. It is generating an electrical power at 11 Kv.
It is required to increase this level for the transmission purpose.
Hence a step up transformer is used which steps up the voltage level to
220 Kv. This level may be 132 Kv, 220 Kv or more as per the requirement.
Then with the help of transmission lines and the towers, the
power is transmitted at very long distances. Design of the transmission
lines is based on the factors like transmission voltage level, constants
like resistance, reactance of the lines, line performance, interference
with the neighbouring circuits etc. Its mechanical features are
strength of the supports, sag calculations, tension etc. Transmission of
power by the overhead lines is very much cheaper. Similarly the repairs
also can be carried out comparatively more easily. The transmission is
generally along with additional lines in parallel. These lines are
called duplicate lines. Thus two sets of three phase lines work in
parallel. This ensures the continuity during maintenance and also can be
used to satisfy future demand. The power is then transmitted to the
receiving station via step down transformer. This transformer is 220/33
kv or 220/22 Kv transformer.
The power is then transmitted to the substations. A substation
consists of a step down transformer of rating 33 KV to 6.6 Kv or 3.3 KV.
The transfer of power from receiving station to the substation is with
the help of conductors called feeders. This is called secondary
transmission.
From the substations, power is distributed to the local
distribution centers with the help of distributors. Sometimes for bulk
loads like factories and industries, the distributors transfer power
directly. For the light loads, there are distribution centers consisting
of distribution transformers which step down the voltage level to 230 V
or 400 V. This is called primary distribution. In the crowded areas
like cities, overhead system of bare conductors is not practicable. In
such cases insulated conductors are used on the form of underground
cables, to give supply to the consumers. These cables are called service
mains. This is called secondary distribution.
This is the complete flow of an electrical power from the generating station to the consumer permises.
Let us study the line diagram of such a typical scheme of
transmission and distribution and discuss the various components and
voltage levels at the various stages in detail. The Fig. 2 shows the
line diagram of a typical transmission and distribution scheme.
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| Fig. 2 Line diagram of a typical transmission distribution scheme |
At the generating station, an electrical power is generated with
the help of three phase alternators running in parallel. In the scheme
shown, the voltage level is 11 KV but the voltage level may be 6.6 KV,
22 KV or 33 KV depending upon the capacity of the generating station.
After the generating station, actual transmission and distribution
starts. The overall scheme can be divided into four sections which are,
1. Primary transmission : It is basically with the help of
overhead transmission lines. For the economic aspects, the voltage level
is increased to 132 KV, 220 KV or more, with the help of step up
transformer. Hence this transmission is also called high voltage
transmission. The primary transmission uses 3 phase 3 wire system.
2. Secondary Transmission : The primary transmission line
continues via transmission towers till the receiving stations. At the
receiving stations, the voltage level is reduced to 22 KV or 33 KV using
the step down transformer. There can be more than one receiving
stations. Then at reduced voltage level of 22 KV or 33 KV, the power is
then transmitted to various substations using overhead 3 phase 3 wire
system. This is secondary transmission. The conductors used for the
secondary transmission are called feeders.
3. Primary Distribution : At the substation the voltage level is
reduced to 6.6 KV, 3.3 KV or 11 KV with the help of step down
transformers. It uses three phase three wire underground system. And the
power is further transmitted to the local distribution centers. This is
primary distribution, also called high voltage distribution. For the
large consumers like factories and industries, the power is directly
transmitted to such loads from a substation. Such bug loads have their
own substations.
4. Secondary Distribution : At the local distribution centers,
there are step down distribution transformers. The voltage level of 6.6
KV, 11 KV is further reduced to 400 V using distribution transformers.
Sometimes it may be reduced to 230 V. The power is then transmitted
using distribution, also called low voltage distribution. This uses 3
phase 4 wire system. The voltage between any two lines is 400 V. while
the voltage between any of the three lines and a neutral is 230 V. The
single phase lighting loads are supplied using a line and neutral while
loads like motors are supplied using three phase lines.
1.1 Components of Distribution
The distribution scheme consists of following important components :
1. Substation : Transmission lines bring the power upto the
substations at a voltage level of 22 KV or 33 KV. At the substation the
level is reduced to 3.3 KV or 6.6 KV. Then using feeders, The power is
given to local distribution centers.
2. Local distribution station : It consists of distribution
transformer which steps down the voltage level from 3.3 KV, 6.6 KV to
400 V or 230 V. Then it is distributed further using distributors. This
is also called distribution substation.
3. Feeders : These are the conductors which are of large current
carrying capacitor. The feeders connect the substation to the are where
power is to be finally distributed to the consumers. No tapping are
taken from the feeders. The feeder current always remains constant.
4. Distributors : These are the conductors used to transfer power
from distribution center to the consumers. From the distributors, the
tappings are taken for the supply to the consumers.
5. Service Mains : These are the small cables between the distributors and the actual consumers permises.
The interconnection of feeders, distributors and service mains is shown in the Fig. 3.
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| Fig. 3 |
There is no tapping on feeders. PQ, QR, RS and PS are the
distributors which are supplied are supplied by the feeder. The service
mains are used to supply the consumers from the distributors. Tappings
are taken from the distributors.
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