EEN-206
Power Transmission and Distribution
Lecture 5: Introduction
Dr. Deep Kiran
Voltage Levels Vs. Year
Voltage
Year
Level
12, 44, 60
Upto 1921
kV
1922 165
1923 220
1935 287
1953 330
1965 500 kV
1966 765 kV
1990 1100 kV
Indian Experience
Voltage
(kV) 1200kV
800kV
765kV HVDC
500kV
HVDC
400kV
220kV
1977 1990 2000 2002 2012 2017-18
Year
Development of 1200kV UHV AC
• 1200kV UHV AC, test line
charged at Bina, Madhya
Pradesh in October 2012.
• Has been Developed
Indigenously through Public
Private Partnership (PPP) with
35 Indian manufacturers in open
collaboration.
• 1200 kV connect the two cities
in Maharashtra (Wardha –
Aurangabad) commissioning is
expected soon.
Extra High Voltage
The International Electro-technical
Commission has classified the voltages into
the following levels (IEC 60038).
Low Voltage - upto 1 kV
Medium Voltage - 1 kV to 35 kV
High Voltage - 35 kV to 230 kV
Extra High Voltage - 230 kV to 800 kV
Ultra High Voltage - above 800 kV
Why High Voltage Transmission?
Improves the performance of
transmission lines.
Efficiency
Regulation
Reduces the requirement of
conductor material.
The installation cost of
transmission line per kilometer.
Increase in the transmission
capacity of the line.
Flexibility for future growth of
the system
Source: NASA Satellite Snapshot
Comparison of Right of Way Requirements
Why AC Generation and Transmission?
Economics!!! Easier and Cheaper to generate
AC power and transmit Voltage (and current)
transformation
Efficient utilization: Cheap and effective AC motors
Topological Difference
Transmission – Mesh (or Loop) (Why?)
Reliability: main concern
A contingency has system-wide effects
Distribution – Radial (Why?)
Economics!
A fault has local disruption
Protection issues
What is Power System
Generation, Transmission, Distribution, Utilization
Source: http://www.bravoprojects.co.in/transmission.php
Supply System
The electric supply system can be broadly classified into
DC or AC system
overhead or underground system
3-phase, 3-wire AC system is universally adopted for
generation and transmission of electric power
Distribution of electric power is done by 3-phase, 4-wire
AC system
Underground system is more expensive than the overhead
system
Structure of Power System
Comparison of AC and DC Systems
Advantages of DC transmission
Requires only two conductors as compared to three for AC
No inductance, capacitance, phase displacement and surge
problems in DC transmission=>Lesser voltage drop in a DC
transmission compared to AC=>better voltage regulation.
No skin effect in a DC system. Therefore, entire cross-section
of the line conductor is utilised.
Potential stress on the insulation is less in DC
Lesser corona loss and reduced interference.
Free from the dielectric losses and stability problem
Comparison of AC and DC Systems
Disadvantages of DC transmission
Electric power cannot be generated at high DC voltage due to
commutation problems.
DC voltage cannot be stepped up for transmission of power at
high voltages.
DC switches and circuit breakers have their own limitations.
Comparison of AC and DC Systems
Advantages of AC transmission
The power can be generated at high voltages.
The maintenance of AC sub-stations is easy and cheaper.
AC voltage can be stepped up or stepped down by transformers
with ease and efficiency.
Transmit power at high voltages and distribute it at safe
potentials.
Comparison of AC and DC Systems
Disadvantages of AC transmission
AC line requires more copper than a DC line.
Construction of AC transmission line is more complicated than
a DC transmission line.
Due to skin effect in the AC system, the effective resistance of
the line is increased.
AC line has capacitance. Therefore, there is a continuous loss
of power due to charging current even when the line is open.
Different possible systems of
transmission
DC system
DC two-wire
DC two-wire with mid-point earthed
Single-phase AC system
Single-phase two-wire
Single-phase two-wire with mid-point earthed
Three-phase AC system
Three-phase three-wire
Three-phase four-wire
Comparison of AC and DC Systems
P is the amount of power transmitted over same distance.
Vm is the maximum voltage for AC and DC system.
R1 , R2 and R3 are the resistances of each conductor for
DC, Single phase AC and Three Phase AC.
DC 2 wire (mid point
earthed)
AC 2 Wire (Single phase)
AC 3 wire (Three phase)
Comparison of AC and DC System
Comparison of AC and DC System
From above three equations, we have
Comparison of AC and DC System
The conductor cross-sectional areas for systems is
Therefore, ratio of conductor weights
Attendance
Overhead vs. Underground Systems
Overhead vs. Underground Systems
Overhead vs. Underground Systems
Other points: Safety, Flexibility, Current carrying capacity and
voltage drop, electromagnetic interference
Capacitance is predominant in the cables. This gives large charging
current which limits the length of the cables.
Therefore long distance transmission overhead lines are preferred.
Cables are mainly used at distribution levels.
Cables are preferred in following conditions:
Public safety involved and low interference is required
Large populated cities
Scenic beauty of city is important
Submarine crossing, and substation and transformer connections
Choice of Working Voltage
The cost of conductor is expensive in overhead
transmission and this is greatly affected by the choice of
the voltage level.
System 1 System 2
V1 V2 = mV1
I1 I2 = I1 / m
R1=V1/I1 R2 = V2/I2=m2R1
a1=k/R1 a2 =k/(m2R1) = a1/ m2
There will be a saving in cost of conductor material if the
power is delivered at higher voltages.
Choice of Working Voltage
Higher system voltages will entail more cost of insulation
in equipments like transformers, circuit-breakers,
lightning arresters, switches, etc.
Cost of insulation increases rapidly with increasing
voltages.
Therefore, for certain length of transmission line and
certain amount of power transfer the voltage level beyond
which it becomes uneconomical.
The question is how to select the transmission and
distribution voltages?
Choice of Working Voltage
If C is total annual cost as function of conductor cross
section A and working voltage V
C = f(A, V)
For minimum cost
δf / δA =0
δf / δV =0
This will result in most economical conductor cross
section and system voltage, if the function f can be defined
accurately.
Transmission Cost as Function of
Voltage Level.
Choice of Working Voltage
L kVA V is the line voltage in kV
V 5 .5
1.6 150 L is the line distance in km
L 3P P is the estimated
V 5 .5
1.6 100 maximum power in kW
per phase
Using these empirical relations preliminary estimate is
made.
Then voltage level is chosen by doing complete
economic study of existing interconnected system.
Transmission and Distribution Tower
Structures
