Electrical transmission line

ELECTRICAL
TRANSMISSION
Line
---By Dhananjay Jha, Engineer (E), SJVN

 Electrical transmission system is the
means of transmitting power from
generating station to different load
centres.

 gridmappowergrid_map.pdf

 Transmission Network
 Conductor
 Earthwire
 Inuslator
 Tower

 Network
 Transmission line model
 Characteristics
 Conductors
 Insulators
 Earthwire

 Generating Station
 Transmission Line
 Sub-Stations

 Associated system
 Voltage magnitudes & angle
 Active and reactive power

A transmission line can be represented by a 2-port network – a
network that can be isolated from the outside world by two
connections (ports) as shown:

If the network is linear, an elementary circuits theorem (analogous to
Thevenin’s theorem) establishes the relationship between the sending and
receiving end voltages and currents as
V AV BI
I CV DI
 
 
S R R
S R R
Here constants A and D are dimensionless, a constant B has units of ,
and a constant C is measured in siemens. These constants are sometimes
referred to as generalized circuit constants, or ABCD constants.

•Upto 80 Km
•Shunt capacitance is neglected and resistance & inductance are lumped
together.
•Therefore, IS = IR = I
Hence the ABCD constants for the short transmission line model, are
A

1
B 
Z
C

0
D

1

Considering medium-length lines (80 to 250 Km-long).
•The shunt admittance is also included for
calculations. However, the total admittance is
usually modeled ( model) as two capacitors of
equal values (each corresponding to a half of total
admittance) placed at the sending and receiving
ends.

The current through the receiving end capacitor can be found as :
Y
I V
2 2 C R
And the current through the series impedance elements is :
Y
I V  I
2 ser R R

From the Kirchhoff’s voltage law, the sending end voltage is
YZ
 
  2 1
S ser R C R R 2 R R ZI V Z I I V
V V ZI
    
 
    
The source current will be
Y Y
   
I I  I  I  I  I  V
  I Y V I
ZY
V
ZY
       
1 1 2 1 1
2 2 4 2 S C ser C C R S R R R R
  


Therefore, the ABCD constants of a medium-length transmission line are
1
ZY
2
1
4
1
2
A
B Z
ZY
C Y
ZY
D
 

 
   
 
 
If the shunt capacitance of the line is
ignored, the ABCD constants are the
constants for a short transmission line.

• For long lines, it is not accurate enough to approximate the shunt
admittance by two constant capacitors at either end of the line.
Instead, both the shunt capacitance and the series impedance must be
treated as distributed quantities.
• The voltages and currents on the line is found by solving differential
equations of the line.

d


 
sinh
'
tanh 2
'
2
Z Z
d
d
Y Y

d



Here, Z is the series impedance of the line,
Y is the shunt admittance of the line,
d is the length of the line,
 is the propagation constant of the line.
  yz
where y is the shunt admittance per kilometer and z is the
series impedance per km.

The ABCD constants for a long transmission line are
Z Y
' '
1
2
'
' '
' 1
4
' '
1
2
A
B Z
Z Y
C Y
Z Y
D
 

 
   
 
 

AC voltages are usually expressed as phasors.
Load with lagging power factor.
Load with unity power factor.
Load with leading power factor.
For a given source voltage VS and
magnitude of the line current, the
received voltage is lower for
lagging loads and higher for
leading loads.

In overhead transmission lines, the line reactance XL is normally much larger than
the line resistance R; therefore, the line resistance is often neglected. We consider
next some important transmission line characteristics…
1. The effect of load changes
Assuming that a single generator
supplies a single load through a
transmission line.
Assuming that the generator is ideal, an increase of load will increase a
real and (or) reactive power drawn from the generator and, therefore,
the line current.
1) If more load is added with the same lagging power factor, the
magnitude of the line current increases but the current remains at the
same angle  with respect to VR as before.

The voltage drop across the reactance increases but stays at the same
angle.
Assuming zero line resistance and source voltage to
be of constant magnitude:
Vs = VR + jXLI
voltage drop across reactance jXLI will stretch
between VR and VS.
Therefore, when a lagging load increases, the received voltage
decreases.
2) An increase in a unity PF load,
on the other hand, will slightly
decrease the received voltage at
the end of the transmission line.

3) Finally, an increase in a load with
leading PF increases the received
(terminal) voltage of the transmission line.

 In a summary:
1. If lagging (inductive) loads are added at the end of a line, the
voltage at the end of the transmission line decreases .
2. If leading (capacitive) loads are added at the end of a line, the
voltage at the end of the transmission line increases.

The voltage regulation of a transmission
line is
 VReg = (Vnl - Vfl )/ Vfl
where Vnl and Vfl are the no-load and full
load voltages at the line output.

2. Power flow in a transmission line
The real power input to a 3-phase transmission line can be computed as
, 3 cos Pin  VS IS S  3VLL S IS cosS
where VS is the magnitude of the source (input) line-to-neutral voltage and VLL,S is
the magnitude of the source (input) line-to-line voltage.
Similarly, the real output power from the transmission line is
, 3 cos 3 cos out R R R LL R R R P  V I   V I 
The reactive power input to a 3-phase transmission line can be computed as
, 3 sin 3 sin in S S S LL S S S Q  V I   V I 

And the reactive output power is
, 3 sin Qout  VRIR R  3VLL RIR sinR
The apparent power input to a 3-phase transmission line can be
computed as
, 3 3 in S S LL S S S  V I  V I
And the apparent output power is
, 3 3 out R R LL R R S  V I  V I

A simplified phasor diagram of a transmission line indicating that IS = IR = I.
Further it can be observed that the vertical segment bc can be expressed as
either VS sin or XLIcos. Therefore:
sin
cos S
L
V
I
X

 

Then the output power of the transmission line equals to its input
power:
V V
3 sin S R
L
P
X


Therefore, the power supplied by a transmission line depends on the angle between
the phasors representing the input and output voltages.
The maximum power supplied by the transmission line occurs when  = 900:
max
V V
3 S R
L
P
X

This maximum power is called the steady-state stability limit of the transmission
line. The real transmission lines have non-zero resistance and, therefore, overheat
long before this point. Full-load angles of 250 are more typical for real
transmission lines.

 Few interesting observations can be made from the power
expressions:
 The maximum power handling capability of a transmission
line is a function of the square of its voltage. For instance, if
all other parameters are equal, a 220 kV line will have 4 times
the power handling capability of a 110 kV transmission line.
 Therefore, it is beneficial to increase the voltage. However,
very high voltages is limit by other factors.

 The maximum power handling capability of a transmission line is
inversely proportional to its series reactance, which may be a
serious problem for long transmission lines. Some very long lines
include series capacitors to reduce the total series reactance and
thus increase the total power handling capability of the line.
 In a normal operation of a power system, the magnitudes of
voltages VS and VR do not change much, therefore, the angle 
basically controls the power flowing through the line. It is
possible to control power flow by placing a phase-shifting
transformer at one end of the line and varying voltage phase.

3. Transmission line efficiency
The efficiency of the transmission line is
P
P
100% out
in
  

4. Transmission line ratings
One of the main limiting factors in transmission line operation is its resistive
heating. Since this heating is a function of the square of the current flowing through
the line and does not depend on its phase angle, transmission lines are typically
rated at a nominal voltage and apparent power.
5. Transmission line limits
Several practical constrains limit the maximum real and reactive power that a
transmission line can supply. The most important constrains are:
1. The maximum steady-state current must be limited to prevent the overheating in
the transmission line. The power lost in a line is approximated as
Ploss  3IL2R
The greater the current flow, the greater the resistive heating losses.

2. The voltage drop in a practical line should be limited to approximately 5%. In
other words, the ratio of the magnitude of the receiving end voltage to the
magnitude of the sending end voltage should be greater than 95%.
This limit prevents excessive voltage variations in a power system.
3. The angle  in a transmission line should typically be  300 ensuring that the
power flow in the transmission line is well below the static stability limit and,
therefore, the power system can handle transients.

 Any of these limits can be more or less important in different
circumstances.
 In short lines, where series reactance X is relatively small, the
resistive heating usually limits the power that the line can supply.
 In longer lines operating at lagging power factors, the voltage
drop across the line is usually the limiting factor.
 In longer lines operating at leading power factors, the maximum
angle  can be the limiting f actor.

 Standard Voltage - 66,110,132, 220, 400 KV
or above
 Selection Criterion of Economic Voltage –
 Quantum of power to be evacuated
 Length of line
 Voltage regulation
 Power loss in Transmission
 Surge impedance level

Economic Voltage of Transmission of Power
L KVA
Nc
Empirical Formula
V
 5.5 
1.6 150 *
V = Transmission voltage (KV) (L-L).
L = Distance of transmission line in KM
KVA=Power to be transferred
Nc= Number of circuits

 Aluminium Conductor Steel Reinforced consists of a
solid or stranded steel core surrounded by one or more
layers of strands of 1350 aluminium. The high-strength
ACSR 8/1, 12/7 and 16/19 standings', are used mostly
for overhead ground wires, extra long spans, river
crossings, etc. The inner core wires of ACSR is of zinc
coated (galvanized) steel.

 Nomenclature :- Al/Steel/dia.
 Panther :- 30/7/3.0 mm
 Zebra :- 54/7/3.18mm
 Snowbird :- 42/3.98 + 7/2.21mm
 Moose :- 54/7/3.53mm

 AAC (All Aluminium Conductor)
 AAAC (All Aluminium Alloy Conductor)
 AL-59
 ACCC (Aluminium conductor composite core)

ACSR Moose AAAC Moose Al-59
Dia. (mm) 31.77 31.05 31.50
Cross sectional area
(sq-mm)
597 570 586.59
Ambient
Temperature
(deg.C)
40 40 40
Current carrying
capacity(A) at 75 C
728 699 759
At 95 C NA 952 976
SAG (m) at 85 C 13.26 14.15 14.52

 Mechanical Requirement
 Electrical Requirement
Mechanical Requirement
 Tensile Strength(For Tension)
 Strain Strength(For Vibration)

o Continuous current rating.
o Short time current carrying rating.
o Voltage drop
o Power loss
o Minimum dia to avoid corona
o Length of line

According to short time rating conductor size is given
by-
A 7.58* IF * t
Where A=area of conductor(mm2)
IF= fault current(KA)
t= fault duration(1 sec.)

Visual corona voltage in fair weather condition is
given by-
 (1  0.3)

 V0= corona starting voltage, KV(rms)
 r= radius of conductor in cm

D
 D= GMD equivalent spacing between conductors in cm
 m= roughness factor
= 1.0 for clean smooth conductor
=0.85 for stranded conductor
 

 

 
r
n
r
r
V 0 21.1
m log


Voltage gradient at the surface of conductor at operating voltage-



g 3




D
r
V
Log
n
0
Corona discharge form at the surface of conductor if g0≥ corona
starting gradient i.e.
g m r (1 0.3)
r
21.1
0

   
(rms kv/cm)

 River crossing
 Weight/ Dia. - Less Weight/Dia ratio conductor swing more.

In overhead transmission lines, the conductors are suspended
from a pole or a tower via insulators.

Insulator are required to support the line conductor and
provide clearance from ground and structure.
 Insulator material-
 High grade Electrical Porcelain
 Toughened Glass
 Fiber Glass

 System over voltage factors shall be
evaluated.
 Due to switching
 Power frequency
 Lightning

 Switching over-voltage :
 Switching off of long lines on no load
 Energizing lines of no load

 Power frequency over voltage
 Loaded line interrupted at one end
 Occurrence of fault
 Open line suddenly connected to load

Type of Insulator-
 Pin type insulator
 Suspension insulator
 Strain insulator

 Pin Type Insulator : Used for transmission and
distribution of electric power at voltages up
to 33 kV

 Suspension : For voltages greater than 33 kV,
it is a usual practice to use suspension type
insulators

 Strain : A dead end or anchor pole or tower is used
where a straight section of line ends, or angles off
in another direction.

 Disc insulator are joint by their ball pins and
socket in their caps to form string.
 No of insulator disc is decided by system
voltage, switching and lighting over voltage
amplitude and pollution level.
 Insulator string can be used either suspension
or tension.
 Swing of suspension string due to wind has to
be taken into consider.
Fig. single string
Fig. Double string

Earth wire provided above the phase conductor across the line
and grounded at every tower.
 It shield the line conductor from direct strokes
 Reduces voltage stress across the insulating strings during lightning
strokes
Design criterion:
 Shield angle
 25°-30° up to 220 KV
 20° for 400 KV and above
 Earth wire should be adequate to carry very short duration lightning
surge current of 100 KA without excessive over heating

EARTH WIRE
A  5 I  t
A= Area(in mm2) of conductor
I =current in KA
t = Time in second
Area of Steel Wire = 3*A(mm2)
 For EHV line it is suggested as 70 mm2 (7/3.66 mm).
 ACSR is used as earth wire (12/3.0 mm AL+7/3.0 mm steel)
 OPGW

 Optical Ground Wire
 Advantages :
 Serves the dual purpose of ground wire and
communication.
 High speed data transmission.


OPGW

 A Smart Grid is an electricity network that can
intelligently integrate the actions of all users connected
to it – generators, consumers and those that do both –
in order to efficiently deliver sustainable, economic
and secure electricity supplies. European Smart Grid
Technology Platform
 SG3 defines Smart Grids as the concept of
modernizing the electric grid. The Smart Grid is
integrating the electrical and information technologies
in between any point of generation and any point of
consumption. Smart Grid Working Group 3

 The aim of smart grid is to provide real-time
monitoring and control, and thus improve the
overall efficiency of the entire system apart from
inclusion of renewable energy resources into the
system.

 Presently, the Indian electricity system faces
a number of challenges:
 Shortage of power
 Power Theft
 Poor access to electricity in rural areas
 Huge losses in the grid
 Inefficient power consumption
 Poor reliability

 The Smart Grid is a transition of the present
energy system into a new era of reliability,
availability and efficiency.
 The smart grid vision involves a uniformly
integrated communication system with the present
power system.

 The India Smart Grid Task Force (ISGTF) is an
inter-ministerial group set up under the
chairmanship of Shri Sam Pitroda in September
2010 to serve as Government's focal point for
activities related to Smart Grid and to evolve a
road map for Smart Grids in India.

 Indian Smart Grid Forum (ISGF) was set up in
2010 to provide a mechanism through which
academia, industry, utilities and other stakeholders
could participate in the development of Indian
smart grid systems and provide relevant inputs to
the government’s decision-making.

 UHBVN,Haryana
 CESC, Mysore
 TSECL, Tripura
 KSEB, Kerala
 Electricity department-Govt. of Puducherry
 UGVCL, Gujrat
 AP CPDCL, Andhra Pradesh
 APDCL, Assam
 MSEDCL, Maharashtra
 CSPDCL, Chattisgarh
 HPSEB, Himachal Pradesh
 PSPCL, Punjab
 WBSEDCL, West Bengal
 JVVNL, Rajasthan

 HPSEB, Himachal Pradesh
 Project Area- Kala Amb
 No. of Consumers-650
 Total Cost- 17.85 Cr.
 MoP Share-Rs. 8.92 Cr.

 Benefits Envisaged
 Shifting peak load
 Reduction in penalties
 Reduction in outages
 Status- RFP (Request for Proposal) issued on
25.08.2014.

Electrical transmission line

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