Download different voltage selection criteria and insulation design of a transmis

International Journal of Advanced Technology & Engineering Research (IJATER)
DIFFERENT VOLTAGE SELECTION CRITERIA
AND INSULATION DESIGN OF A TRANSMISSION LINE FOR HV, EHV & UHV SYSTEM
Akhlaque Ahmad Khan1; Ahmad Faiz Minai2, Electrical & Electronics Engineering Department, Integral University, Lucknow [1][2]
Abstract
In this paper voltage selection and insulation design of a
transmission line are discussed. Electrical energy can be
generated centrally in bulk and transmitted economically
long distances. Further it can be adapted easily and efficiently to domestic and industrial application particularly for
lighting purposes and for mechanical work e.g. derives. The
system voltage in EHV system very much effect the capital
cost of transmission line. The weight of conductor material,
the efficiency of the line, the voltage drop in the line and
system stability depends upon system voltage. The choice of
voltage and insulation design therefore, a major factor in the
line designs.
tion in the comparison of competitive system in cost but the
case of maintenance, vulnerability to damage and public
amenity must also be taken into account. The table below
shows the comparative cost per mile for transmitting a given
amount of power using over head lines and under ground
cable:-
Introduction
The per capital consumption and electrical energy is a reliable indicator of a country’s state of development. Conventionally electrical energy is obtained by conversion from
fossil fuels (coal, oil, natural gas), nuclear and hydro
sources. Heat energy released by burning fossil fuels or by
fission of nuclear material is converted to electricity by first
converting heat energy to the mechanical energy and then
converting mechanical energy to the electrical energy
through generators. The earth has fixed non replenishable
resources of fossil fuels and nuclear materials. Hydro energy
though replenishable is also limited in terms of power. The
world’s increasing power requirements can only be partially
met by hydro sources. Further more ecological and biological factors place a stringent limit on the use of hydro sources
for power production.
The plots showing energy demand installed generating capacity indicates that the electrical energy requirement is an
exponential growth function and as per the slope of these
plots the continuous increase in power demand has been
doubling every seven years. As per the plots the installed
generating capacity in India must rise from 140 GMW in
1995, 1840 GW in 2007 and 3680GW in 2014 which in turn
would require a corresponding development in coal resources.
Among the different types of transmission systems are
overhead systems and cable systems. The primary considera-
ISSN NO: 2250-3536
Figure 1:- Power consumption per Capita
Table 1:- Comparative “Cost per Mile” for Transmitting a
given amount power using overhead line and under ground
line.
Voltage
KV
Cost of O/H Line
( 1000/Km).
Ratio
34
Cost of U/G Cable (
1000/Km.)
615
400
220
17
217
13
132
6.5
55
8
18
The generation voltages are usually 11KV &33KV and
since these are low for transmission over long distances so it
is step up by step up transformer to voltages upto 132KV,
220KV or 400KV and transmitted to bulk power sub-station.
Difficulty of getting power station sites near the consuming
centers makes it inevitable to transfer bulk of electrical energy through longer distances and is possible only by high
voltage transmission systems. Extra High Voltage (EHV)
above 220KV thus not only transmits power but utilized for
interconnection purposes also. High voltage is desired due to
various reasons such as power loss which is inversely pro-
VOLUME 2, ISSUE 2, MAY 2012
73
International Journal of Advanced Technology & Engineering Research (IJATER)
portional to both system voltage and power factor (p.f.) reduction in voltage drop in resistance, reduction in weight of
conductor material and enhanced efficiency of transmission.
Three main problems associated with EHV, namely radio
interference, line insulation and equipment insulation. The
various voltages adopted by different countries above
230KV are 275, 287, 345, 380, 400, 500, 735, 1100KV etc.
The highest voltage used in India is 400KV. Voltages above
765KV are called ultra high voltages (UHV).
Two main problems are involved which limits the large
amount of power to be transmitted over long distances by
A.C. systems. The first is the technical limitation and other
is the economic consideration and usually later governs the
final choice of design.
Recently by virtue of its various advantages over A.C.
transmission, high voltage direct current (HVDC) transmission is also become popular in some countries. A.C. is better
for generating and distributing point of view but D.C. is preferable for transmission over long distances. \
Voltage selection
A transmission line transmits electrical energy in bulk
from generating station to distributing station. The maximum generation voltage in developed countries is 33KV
while in India it is 11KV. The amount of power that has to
be transmitted through transmission line is very large, if this
power is transmitted at 11KV or 33KV, the line current and
power loss would be large. Therefore this voltage is stepped
upto higher value by using step-up transformers. The transmission voltages in India are 132KV, 220KV, 400KV &
765KV.
The system voltage in EHV system very much effect the
capital cost of transmission line. The weight of conductor
material, the efficiency of the line, the voltage drop in the
line and system stability depends upon system voltage. The
choice of voltage therefore, a major factor in the line designs.
The EHV systems are not only the power transporting system but fulfill the need of interconnecting the different EHV
systems so as to improve the reliability as well as efficiency
of the system. The EHV system here certain advantages such
as the voltage goes on increasing the current reduces, the
size of the conductor reduces and cost of conductor decreases. As current decreases line losses reduces thereby increasing the efficiency and voltage regulation increase.
Although much economy can be affected in EHV system,
the cost of insulation of conductor’s increases, the separation
or clearance between conductors is to be increased to avoid
electrical discharge. The problem of mechanical supporting
structure and right of way an acquisition become more diffi-
ISSN NO: 2250-3536
cult and expensive. The corona loss are also inevitable as
communication line and EHV lines are run as the same
tower the problem of radio interference with communication
circuit become very serious at EHV. Shunt reactance compensation (for improvement of p.f.) and series compensation
(for improvement of voltage regulation and stability) is required.
While selecting the transmission voltage the present and
future expectable voltage of other lines in vicinity of the line
under design are taken into account. The number of circuits
in EHV system can be one or two.
Different Criteria for Voltage Selection:-
First Criteria:According to Indian standard
Table 2:- Table for Voltage Selection
Distance (KM)
No. of Phases
Upto 8
Upto 16
Upto 64
Upto 116
Upto 240
Upto 480
Upto 800
3
3
3
3
3
3
3
Standard working voltage
(KV)
6.6
11
33
66
132
220
400
Second Criteria:
Imperical formula is given by
VL = 5.5√ ((L/1.6) + (P×1000/Cosø × NC × 150))
Where,
VL = Transmission line voltage in KV
L = Length of line in Kms. = 300Km
P = Power to be transmitted = 280MW
NC = Number of circuits
Cos ø =Power factor of load = 0.9 (lagging)
VL = 5.5√ ((300/1.6) + (280×1000/0.9 × NC × 150))
For,
NC = 1 (single circuit line)
VL = 261.55 KV
For,
NC = 2 (double circuit line)
VL = 192.4 KV
The standard line voltage near to this is 220KV.
Surge impedance Loading:A surge is the movement of charge along the conductor,
such surge are characterized by sudden very steep rise in
voltage followed by gradual decay in voltage. It is of very
short duration of the order of few micro seconds. The surge
VOLUME 2, ISSUE 2, MAY 2012
74
International Journal of Advanced Technology & Engineering Research (IJATER)
travel along the conductor in the form of waves. There are
generally two types of surge such as switching surge and
lightning surge.
The lightning surges are of particular interest due to their
large magnitude and different wave shapes. The surges reach
the transformer, switch gear and may damage them if they
are not properly protected.
A transmission line may be considered as generative capacitive reactive voltmeter in its shunt capacitance and consuming reactive volt-amperes in its series inductance. The
load at which the inductive and capacitive reactive voltamperes are equal and opposite is called surge impedance
loading (SIL).
This power transmitting capability of system is greater
than the actual power to be transmitted, hence, Double Circuit, 220KV line is selected.
V2/VC = I2 XL
Or,
V/I = √ (L/C)
= Z0
Surge impedance load of any line may be defined as power delivered by it to a purely resistive load equal to its surge
impedance.
SIL also gives the power transmitting capability, P (t). To
determine P (t) SIL is multiplied by factor (MF) obtained
from standard capability curve.
SIL = (VL)2/Z0
Where,
VL = Transmission line voltage in KV
Z0 = Surge impedance in ohm
= 200 ohm (for double circuit line)
= 400 ohm (for single circuit line)
The power transmitted decreases with length. To find out
the actual power that can be transmitted with stability SIL is
multiplied by MF (multiplying factor) which is obtained
from capability curve. The capability curve is plotted between MF and length (Km). From the capability curve the
value of MF is 1.44 corresponding to 300Km.
SIL = (VL)2 /Z0 MW
For Double Circuit Line:SIL = (220)2/200 MW
= 242 MW
P (t) = SIL × MF
= 242 × 1.44
= 348.48 MW
Figure 2:- Capability Curve
Design of insulation
The major aspect in designing a power transmission line is
choosing its insulation level which has a considerable influence on the cost as well as operating reliability. The influence of EHV systems is determined on the basis of possible
internal and external over-voltages. The recent advancements achieved in the design of circuit breakers and other
protective devices have reduced the severe effects of switching transients and power frequency over-voltages allowing
considerable reductions in the insulation level of EHV networks. The earthing of the system also plays an important
role in reducing the insulation.
Reduced insulation level results in large swing in the size
and cost of equipment. The system over-voltage factors are
therefore to be evaluated carefully while choosing the insulation levels.
Over-Voltage
In extra high voltage system types of over-voltages
likely to occur are:(a) Over voltages due to switching surges.
(b) Power frequency over voltages.
(c) Over voltages due to direct strokes of lightning
Switching Over Voltages
Over voltages due to switching surges are of an internal
origin and are generally oscillatory in nature. These
ISSN NO: 2250-3536
VOLUME 2, ISSUE 2, MAY 2012
75
International Journal of Advanced Technology & Engineering Research (IJATER)
over voltages are of a short duration and are generally
caused by the following switching operations.
(1) “Switching off” of long lines on no load.
(2) Interruption of low inductive current such as the
transformer magnetizing currents.
(3) Energizing lines of no load.
(4) Clearance of short circuits.
The most dangerous transient over-voltages are caused by
tripping of long lines under no load conditions. The length of
EHV lines and high capacitance due to bundled conductors
increases the probability of high switching surges. Switching
surges of 3 to 4 times the crest value of the normal phase to
neutral voltage are likely to occur as a result of breaker restriking.
Modern CBS are therefore designed for disconnecting unloaded lines without restriking which limits the over-voltage
to 2-2.5 times crest value of the normal phase to neutral voltage.
Dangerous switching over-voltages also occur by interruption of transformer magnetizing currents. The magnitude of
these over-voltages depends on the relation between the inductance and capacitance of the transformer. In the case of
EHV transformers the factor is usually 1.5. Higher voltages
generated can be absorbed by the lightning arrestors between
the transformer and the circuit breaker.
Relatively high over-voltages occur on energizing high
voltage lines and particularly a three phase rapid recourses.
In absence of damping and in the most unfavorable case of
reclosing when the applied voltage is at its maximum. The
voltage at the open end may reach double the crest value of
the phase to neutral voltage or even more.
Power Frequency Over Voltages
Operating frequency voltage rise occurs when:(1) A loaded line is interrupted at one end especially
when the line is long and the impedance is high.
(2) On occurrence of a single phase or two phases to
earth fault.
(3) An open line is suddenly connected to the source.
(4) At the free end of a line connected at one end only
(FERRANTI EFFECT).
A practical line sections of the range 300 500Km in length
which is usual at 220KV and 400KV, increase in voltage
during dropping of load does not exceed 1.3 to 1.5 times the
maximum normal phase to neutral voltage (rms). The value
of these over-voltages in practice does not depend on the
operating voltage but depends only on the length of the line
and the number of lines terminating at or emanating from a
station.
ISSN NO: 2250-3536
A wider knowledge of the magnitudes of lightning and internal over-voltages has justified the revision of insulation
requirements. Further with the advancements made in protective devices such as a non-restraining circuit breaker.
Circuit breaker with the provision of pre insertion resistors,
improved lightning arrestors and provisions of bus-car protection against earth faults in EHV stations have made it
possible to limit the effects of switching over-voltages to a
value of 2.5 for lines operating at 220KV.
The power frequency over-voltage factor has been reduced
to a value 1.3 for 220KV. The provision of shunt reactors on
EHV lines for the absorption of excessive reactive power
limits switching surge over-voltages and prevents abnormally high fundamental frequency and harmonic voltages.
Lightning Over-Voltage
Lightning is a very serious cause of over voltages on the
lines. These over voltages are produced in the lines either by
a direct stroke or by an induced stroke. The voltages due to
induced strokes are very much less as compared to those
produced by direct strokes. The important lightning surges
against which apparatus must be protected are caused by
direct strokes which originate either at the station or an
overhead lines leading to the section. These direct strokes
attain values as high as 4000KV with a high rate of voltage
rise (1000-10,000KV per micro second) and a high current
magnitude of (5000 to 200,000 amperes). These conditions
impose severe stresses on the station equipments and may
even be fatal for protective systems.
Surges that originate as direct strokes on the lines have to
propagate into the station are by far the most common and
are generally less severe. The lightning discharge wave is
very short and steep at the point at which it strikes and the
amplitude goes on decreasing under the action of skin-effect
of conductor and ground while it is propagating along the
line.
The frequency of occurrence of over voltages due to
thunder storms varies according to the Iso-Chronic level of
the region. Their amplitude depending on the presence of
earth wires and the quality of earthing.
A study in connection with the magnitude of lightning
currents has shown that currents upto 30,000 amperes account for 90% of the total number of observations recorded
and the currents of the order of 100 amperes exceed for only
1.5% of the observations. A value of 300,000 amperes of
lightning stroke current is generally accepted as the criterion
for determining the insulation level for EHV systems.
Power lines and switch yards can however, be protected
adequately against direct lightning strokes by means of
screening conductors, if the protective angle is 25-30 degrees and the tower footing resistance is limited to about 10
VOLUME 2, ISSUE 2, MAY 2012
76
International Journal of Advanced Technology & Engineering Research (IJATER)
ohm in the case of transmission lines and 0.5 to 1 ohm in the
case of substations.
d) The leakage of current to earth should be minimum
to keep the corona loss and ratio interference within
reasonable limits.
There are three types of insulators used for overhead lines.
(1) Pin type insulator
(2) Suspension type insulator
(3) Strain or tension type insulator.
Choice of Insulation Level for Transmission Lines
1. Pin Type Insulator
Insulation requirements are a function of internal and external over voltages. The number of insulators, swing angle,
weather conditions, phase spacing and clearance to tower
and ground wires determine the tower dimensions and cost.
Economy and practicability of design demand that the insulation strength b e kept a minimum and should provide basic
protection. One must strike a balance between the chances of
failure and the cost of greater insulation strength.
Multiplying Factors
The string insulation must be sufficient to prevent a flash
over from the stationary over voltages and switching surges
into account all the abnormal and prevailing circumstances
which decrease the flashover voltage such as rain, dust, insulator pollution, fog etc.
Some additional multiplying factors are therefore allowed
to account for contaminated, non-standard atmospheric conditions, polluted insulation surface etc. These factors influence the value of normal frequency flashover voltages more
than impulse flashover voltage. Further it becomes necessary
to make some allowance for safety margin for unforeseen
conditions.
Types of Insulators
Overhead line insulators are used to separate line conductors from each other and from supporting structure electrically. While designing an insulator the following points are
kept in view:a) The insulator should be able to withstand the over
voltages due to lightning, switching or other causes
under severe conditions in addition to the normal
working voltages.
b) It should posses’ high mechanical strength to bear
the conductor load under worst leading conditions.
c) It needs to have a high resistance to temperature
changes to reduce damage from power flashover.
The pin insulator is supported on a forged steel pin or bolt
which is secured to the cross arm of the supporting structure.
The conductor is tied to the insulator on the top groove on
straight line position and side groove in angle positions by
annealed binding wire of the same material as conductor. A
lead thimble is connected into the insulator body to receive
the pin.
Suspension Insulator
A suspension insulator consists of a number of separate
insulator units connected with each other by metal lines to
form a flexible chain or a string. The insulator string is suspended from the cross arm of the support. The conductor is
attached to the lowest unit.
An insulator assembly is shown in figure. Suspension insulator offers the following advantages:(a) Each unit is designed for operating voltage of about
11KV so, that a string can be assembled by connecting several units to suit the service voltage and weather conditions.
(b) In case the line is to operate on a higher voltage in future
to cope with the increasing load, additional units would be
introduced to the same string. In case of damage to one of
the units, only the damaged insulator but not the whole
string is replaced by one.
(c) The string is free to swing in any direction and therefore,
greater flexibility is provided. The tensions in the successive
spans are balanced. The lines can therefore be designed for
longer spans and higher mechanical loading.
(d)There is decreased liability to lightning disturbances if
the string is suspended from a metallic supporting structure
which works as a lightning shield for conductor.
Since the spring is hung from the support, the tower height is
to be increased. Greater spacing between the conductors is to
be provided to allow for swinging.
The types of suspension insulators in use are:
(a) Cap and Pin type
(b) Hewlett or interlink type.
Strain or Tension Insulators
ISSN NO: 2250-3536
VOLUME 2, ISSUE 2, MAY 2012
77
International Journal of Advanced Technology & Engineering Research (IJATER)
Strain or tension insulators are design for handling mechanical stresses at angle positions where there is a change
in the direction of the line or at termination of the lines. For
high voltage lines having longer spans and greater mechanical loading, suspension insulator strings are arranged in a
horizontal position. In case a single string is not sufficient to
take the load, two are more strings in parallel may be employed for higher conductor tensions.
Table 3:- Doultons Insulation Characteristic
S.No.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
No. of
units
(Disc)
1
2
3
4
5
6
7
8
9
10
11
Power frequency withstand
voltage
Dry
KV(rms)
78
135
185
235
275
315
365
415
455
495
530
Wet
KV(rms)
45
80
115
150
185
225
260
295
330
360
395
Impulse
withstand
test voltage
KV(Peak)
105
200
300
380
450
530
600
680
750
820
890
Switching Surge Over Voltage Factors
(1) System voltage = 220 KV
(2) Highest system voltage
=1.1×system voltage (for 220 KV)
= 1.05 ×system voltage (for 400 KV)
(3) Crest factor = 1.414
(4) Switching surge = 2.5 for 220 KV
= 2.0 for 400 KV
(5) Switching surge strength = 1.15
(6) Impulse flashover voltage = 1.15
(7) Contaminated surface, on standard condition = 1.1
(8) Overall factor of safety = 1.15
+ve impulse flashover voltage (crest 1.2/50 sec)
= Vpm × 1.414 × 2.5 × 1.5 × 1.15 × 1.15 × 1.1
= 6.5 Vpn for line at 220 KV
= (220/√3) × 6.5 = 825 KV
Corresponding to this voltage the number of discs required =
10.1 = 11 units
Power Frequency (50 HZ) Over Voltage
Factors
(1)System Voltage = 220 KV
ISSN NO: 2250-3536
(2) Highest System Voltage = 1.10
(3) Over voltage factor = 1.3 for 220 KV
= 1.5 for 400 KV
(4) Flashover Voltage = 1.15
(5) Non-standard atmospheric condition, contaminated
surface = 1.2
(6) Overall safety margin = 1.50
50HZ wet flashover voltage (rms)
= Vpn × 1.1 × 1.3 × 1.2 × 1.5 × 1.15
= 3 × (220/√3) = 380 KV
Number of discs corresponding to this voltage = 10.7 = 11
units
Figure3:- Characteristic of Power Frequency Over Voltage
Curve
Conclusion
The EHV systems are not only the power transporting system but fulfill the need of interconnecting the different EHV
systems so as to improve the reliability as well as efficiency
of the system. The EHV system here certain advantages such
as the voltage goes on increasing the current reduces, the
size of the conductor reduces and cost of conductor decreases. As current decreases line losses reduces thereby increasing the efficiency and voltage regulation increase.
References
[1]D. Povh, D. Retzmann, “Perspectives of Power system
interconnections” Siemens, Erlangen Germany
[2] FACTS Overview. IEEE and Cigré, Catalog Nr. 95 TP
108
[3] Muller, H.-C.; Haubrich, H.-J.; Schwartz, J.: Technical
Limits of Interconnected Systems.CIGRE Report 37-301,
Paris, Session 1992.
[4]UPSEB Statistics at a Glance, March 1988.
VOLUME 2, ISSUE 2, MAY 2012
78
International Journal of Advanced Technology & Engineering Research (IJATER)
[5]”Energy Statistics” Year Book, 1966 Published by
U.N. New York.
[6] J. Nagrath & D.P. Kothari “Modern Power System Analysis” Tata McGraw-Hill Education, 2003.
[7] H.P. Young “Electric Power System control” Edition 3,
Chapman & Hall, 1950.
[8] W.D. Stevenson “Elements of Power System Analysis”
fourth edition. TMH, New York 1982.
[9] Wadhawa, C.L. “Elements of Power System” New Age
International 2009.
[10] Ashfaq Husain “Electrical power system” Edition 2004,
Danpat Rai & Co, (Pvt.) Ltd. 1982.
Akhlaque Ahmad Khan was born in
Lucknow, Uttar Pradesh, India, on 01
July 1987. He Received the Bachelor’s
Degree from Integral University, Lucknow, U. P. India.
He is with the Integral University, as
Lecturer in the Electrical and Electronics
Engineering Department from August, 08.
Ahmad Faiz Minai was born in Lucknow, Uttar Pradesh, India, on November
2, 1985. He Received the Master’s Degree from Aligarh Muslim University,
Aligarh, U. P. India.
He is with the Integral University, as
an Assisstant Professor in the Electrical
and Electronics Engineering Department from July, 09.
ISSN NO: 2250-3536
VOLUME 2, ISSUE 2, MAY 2012
79