TRANSMISSION AND DISTRIBUTION
III Year / V Semester
Subject code : EE T 53
SYLLABUS
EE T53 TRANSMISSION AND DISTRIBUTION
Objectives:
To understand the structure of electric supply system and different
types of distribution systems,
To gain the knowledge of line parameters, skin effect, proximity
effect and corona in transmission lines.
To learn the performance evaluation of different types of
transmission lines by calculating transmission efficiency and voltage
regulation.
To study on the selection of cables and insulators for specific
applications and
To gain a thorough knowledge of the design aspects of rural and
town electrification schemes, HVDC and FACTS technology.
UNIT I: DISTRIBUTION SYSTEMS
Structure of electric power systems-one Line
Diagram-generation,
transmission
and
distribution
Systems-comparison of
distribution systems–radial and ring –two
wire dc, ac single phase and three phase
systems–current and voltage calculations in
distributors
with
concentrated
and
Distributed loads – Kelvin’s law for the
design of feeders and its limitations.
UNIT II: TRANSMISSION LINE
PARAMETERS
Resistance, inductance and capacitance of
single and three phase transmission linessymmetrical and unsymmetrical spacing–
transposition-single and double circuitsstranded and bundled conductors-application
of self and mutual GMD–Skin and Proximity
effect-inductive
interference-Coronacharacteristics.
UNIT III: PERFORMANCE OF
TRANSMISSION LINES
Development of equivalent circuits for short,
medium and long lines–efficiency and
regulation- Attenuation constant and phase
constant- surge impedance loading –power
circle diagrams for sending and receiving
ends-transmission capacity, steady state
stability limit–voltage control of lines -shunt
and series compensation.
UNIT IV: INSULATORS AND CABLES
Insulators–types and comparison–voltage
distribution
in
string
insulator–string
efficiency–Methods of improving string
efficiency–Stress and sag calculations–effect
of wind and ice– supports at different levels–
stinging chart-cables– types–capacitance of
cables–insulation resistance - dielectric stress
and grading- dielectric loss- thermal
characteristics- capacitance of three core
cables.
UNIT V: RECENT TRENDS IN
TRANSMISSION
Design of rural distribution, planning and
design of town electrification schemes–
comparison of EHVAC & HVDC system–
economic distance for HVDC–terminal
equipment for HVDC systems–description of
DC
transmission
system–planningadvantages-interconnection of HVDC & AC
systems–Introduction to FACTS technology
STRUCTURE OF THE POWER SYSTEM
DISTRIBUTION SYSTEM
i) Feeders.
(ii) Distributor
(iii) Service mains
(
CLASSIFICATION OF DISTRIBUTION SYSTEMS
(i) Nature of current
(iii) Scheme of connection
(ii) Type of construction
A.C. Distribution
D.C Distribution
Typical Primary Distribution System
Typical Secondary Distribution System
CONNECTION SCHEMES OF DISTRIBUTION SYSTEM
(i) a single line diagram of a radial system for d.c. distribution
•a single line diagram of radial system for a.c. Distribution
(i) Single line diagram of a radial system for D.C. Distribution
(ii) Single line diagram of radial system for A.C. Distribution
Single line diagram of ring main system for A.C. distribution
Advantages :
(a) There are less voltage fluctuations at consumer’s terminals.
(b) The system is very reliable as each distributor is fed via two feeders
Single line diagram of Interconnected system
Advantages :
 It increases the service reliability
 Any area fed from one generating station during peak load hours can be fed
from the other generating station.
 This reduces reserve power capacity and increases efficiency of the system.
REQUIREMENTS OF A DISTRIBUTION SYSTEM
Proper voltage
Availability of power on demand.
Reliability
Kelvin’s Law and its Limitations
The cost of conductor material is generally a very
considerable part of the total cost of a transmission line.
Therefore, the determination of proper size of conductor
for the line is of vital importance.
The most economical area of conductor is that for
which the total annual cost of transmission line is
minimum. This is known as Kelvin’s Law after Lord
Kelvin who first stated it in 1881.
The total annual cost of transmission line can be
divided broadly into two parts viz., annual charge on
capital outlay and annual cost of energy wasted in the
conductor.
(i) Annual charge on capital outlay. This is on account
of interest and depreciation on the capital cost of
complete installation of transmission line. In case of
overhead system, it will be the annual interest and
depreciation on the capital cost of conductors,
supports and insulators and the cost of their erection.
Now, for an overhead line, insulator cost is constant,
the conductor cost is proportional to the area of Xsection and the cost of supports and their erection is
partly constant and partly proportional to area of X
section of the conductor. Therefore, annual charge on
an overhead transmission line can be expressed as :
Annual charge = P1 + P2 a
...(i)
Where, P1 and P2 are constants and a is the area of
X-section of the conductor.
(ii) Annual cost of energy wasted. This is on account of
energy lost mainly in the conductor due to I2 R losses.
Assuming a constant current in the conductor
throughout the year, the energy lost in the conductor is
proportional to resistance. As resistance is inversely
proportional to the area of X section of the conductor,
therefore, the energy lost in the conductor is inversely
proportional to area of X-section. Thus, the annual
cost of energy wasted in an overhead transmission line
can be expressed as :
Annual cost of energy wasted = P3 / a
...(ii)
Where, P3 is a constant.
Total annual cost, C = exp. (i) + exp. (ii)
= (P1 + P2 a) + P3 / a
∴ C = P1 + P2 a + P3 / a
...(iii)
In exp. (iii), only area of X-section a is variable. Therefore, the
total annual cost of transmission line will be minimum if
differentiation of C w.r.t. a is zero.
Variable part of annual charge = Annual cost of energy wasted
Graphical illustration of Kelvin’s law
Limitations of Kelvin’s law.
(i) It is not easy to estimate the energy loss in the line without
actual load curves, which are not available at the time of
estimation.
(ii) The assumption that annual cost on account of interest and
depreciation on the capital outlay is in the form P1 + P2a is
strictly speaking not true. For instance, in cables neither the cost
of cable dielectric and sheath nor the cost of laying vary in this
manner
UNIT –II
Transmission
Line Parameters
INSULATORS