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A CONCEPTUAL DESIGN OF A
TRANSIMISSION LINE SITUATED
IN THE ISLAND OF MASBATE
Proposed Old
Masbate-Aror
oy
Transmission
LineDesign 1 ELEN 3254
Submitted By:
Moris I. Mascarias, BSEE V-1
Submitted To:
Engr. Jesus Bien
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SUMMARY
This document specifies the requirements for the construction of overhead transmission line
connecting the Old Masbate Diesel Power Plant and a proposed load end station situated at the
municipality of Aroroy in Masbate Province.
DISCLAIMER
This entire documentation is primarily intended for academic presentation. Calculations
presented herewith are based on data gathered as general facts in relation to the subject of its
study. Surveys that may be presented on foregoing parts of this document are mainly gathered
via research in various websites and in no way intended to be presented as absolute facts. -
Hence, the researcher is open to the possibility of conflict between the data to be presented and
the actual status of the case being posed in this study.
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Documentation of the Proposed
Old Masbate- Aroroy Overhead Transmission Line
CONTENTS
SCOPE ................................................................................................. 1
INTRODUCTION .................................................................................. 1
REFERENCES ....................................................................................... 2
DEFINITIONS ...................................................................................... 3
ROUTE SELECTION ............................................................................. 5
EASEMENTS .......................................................................................... 8
STRUCTURES ........................................................................................ 9
CONDUCTORS ..................................................................................... 16
STRUCTURE SPOTTING ...................................................................... 17
CORONA .............................................................................................. 22
EMF ....................................................................................................... 2
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SCOPE
For the construction of the proposed Old Masbate- Aroroy transmission line to be designed
having a 220 KV operating voltage, this document intends to make use of standard parameters
under the guidance of various accredited references and manuals to come up with an
appropriate and credible construct calculated for its specification. Whenever non-standard
parameters may be presented on the concluding parts, those are not contended as a certain
specification and its modification may be regarded appropriate.
INTRODUCTION
The project documented in this report is the Masbates220 kV Transmission Line that starts from
the Masbate Diesel Power Plant located at Barangay Tugbo in the municipality of Mobo, Masbate
and ends at municipality of Aroroy. The line traverses several barangays along the municipalities
of Mobo, Masbate City, Baleno, and Aroroy. The project would be consisted of five phases. The
phase I connects the line from Masbate Diesel Power Plant, the source of power, and Phase Vjoins to the load-end at its farthest point in Aroroy Town. The Transmission Line construction of
Phases II-IV would be consisted of segments of line of transmission poles situated in various
locations. The construction shall conform to the standards of the criteria as required by the listed
references. Drawings and supporting documents are provided as part of the Design Information.
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REFERENCES
The following documents are used to meet the standard criteria for the proposed construction of
the project featured in this study. These documents are presumed to be the latest guidelines
governing the codes and considerations in transmission line construction.
ENA EG-0 Power System Earthing Guide Part 1: Management Principles ENA
EG1 Substation Earthing Guide Energized Line Working with Polymer
Insulators for Voltages 60kV and AboveIEEE ESMOL
HB 102 Co-ordination of power HB 331 HandbookOverhead Line DesignIEEE Std 987 IEEE Guide for Application Of Composite Insulators
NS 167 Pole PositioningNS220 Guide for Overhead Transmission
ASCE Guidelines for Electrical Transmission Line Structural Loading (Manual No. 74)
1724E-204 Guide Specifications for Steel Single Pole and H-Frame Structures
McGrawHill, Steel Poles and Towers
1724E-214 Guide Specification for Standard Class Steel Transmission Poles
1724E-206 Guide Specification for Spun, Prestressed Concrete Poles and Concrete
Pole Structures
1724E-216 Guide Specification for Standard Class Spun, Prestressed Concrete
Transmission Poles
CL&Ps Transmission line route / configuration alternatives
McGrawHill The Linemans and Cable mansHandbook,
REA Design Manual for High Voltage Transmission Lines (REA
Bulletin 621)
BULLETIN 1724E-200 Designmanual for high voltage transmission lines
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DEFINITIONS
ACSR- Aluminum conductor steel-reinforced (ACSR) is a specific type of high-capacity,high-strength stranded conductor typically used in overhead power lines. The outer strands arehigh-purity 1350 or 1370 aluminum alloy, chosen for its excellent conductivity, low weight andlow cost.
Conductivity- Measure of a material's ability to conduct an electric current.
Corona- An electrical discharge brought on by the ionization of a fluid surrounding a conductorthat is electrically energized. The discharge will occur when the strength (potential gradient) ofthe electric field around the conductor is high enough to form a conductive region, but not highenough to cause electrical breakdown or arcing to nearby objects.
Easements- An easement is a non-possessory right of use or enters onto the real property ofanother without possessing it.
Efficiency - The useful power output divided by the total electrical power consumed a fractionalexpression
NESCNational Electrical Safety Code,published exclusively by IEEE, the NESC sets theground rules for practical safeguarding of persons during the installation, operation, ormaintenance of electric supply & communication lines & associated equipment. It contains thebasic provisions that are considered necessary for the safety of employees & the public under thespecified conditions.
Right of Way- Describes the legal right, established by usage or grant, to pass along a specificroute through grounds or property belonging to another, or a path or thoroughfare subject tosuch a right.
SagAn act of drooping as an effect of elevation.
Truss- A structure that "consists of two-force members only, where the members are organizedso that the assemblage as a whole behaves as a single object.
Mechanical Load -The force exerted on a body on surface while the force is a vector quantitythat possesses magnitude.
Tension- Describes the pulling force exerted by each end of a string, cable, chain, or similarone-dimensional continuous object, or by each end of a rod, truss member, or similar threedimensional objects.
Wind Loading- Analyzes effects of wind in the natural and the built environment and studiesthe possible damage, inconvenience or benefits which may result from wind.
ClearanceOpen space between two elements of a structure to aid in proper placement, tocompensate for minor inaccuracies in modification, or to allow unobstructed movement betweenparts.
ToleranceStructures potential to cope with changes in the following elements of its
surroundings such as a physical dimension, a measured value or physical property of a material,such as temperature, humidity, etc., and remain functioning.
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DESIGN BRIEF
A design brief is prepared to outline each section of the transmission line construction. It includes
the following information.
Project scope and description
Proposed route or end points
System loading requirements
Operating voltage
Line capacity (or in some cases, conductor size and material type for phase conductors)
Maximum design operating temperature of the conductors
Maximum design temperature under wind loading conditions
Minimum design operating temperature of the conductors
Permissible tower material, (e.g. concrete, wood, steel)
Permissible construction type, (e.g. H-pole, standoff insulators, etc.)
Allowance for additional circuits, if applicable
Conductor size and material type for overhead earth wire, where require whether the
overhead earth wire is not required for the full length of line whether an OPGW (optical
pilot ground wire) is required
Required Earthing of structures including maximum pole earth resistance and allowable
Earthing construction (for contestable projects the Client is generally responsible for the
Earthing study)
Protection requirements, where relevant to the scope of works
Any special conditions or arrangements already made for easements, right-of-way and
access to private land
Any special conditions or arrangements already made with the local council or roads
authority for lines on public roads.
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ROUTE SELECTION
In The process of analysis of the routing selection of the 69 KV transmission line from Mobo,Masbate up to its load end station at Aroroy Town, Several objectives are listed which areprimary considerations that advise the erection of the structure. These are;
Cost effectiveness and reliable system of transmission that interconnects to specified
substations and switching stations.
Maximization of the efficiency of the project
Minimization of adverse effects to sensitive environmental resources
Minimization of adverse effects to significant cultural resources
Minimization of adverse effects on designated scenic resources and heritage sites.
Minimization off conflicts with local and national land resource policies Minimization of the need to acquire property by eminent domain
Maintenance of public health and safety
In compliance with the aforesaid criteria, the project route is decided to be as shown below.
Line Route
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Proposed Project Area
The route is identified by avoiding, to such extent possible and applicable, areas where a
transmission line could create significant impacts such as:
Existing high-density residential areas.
Agricultural areas where center pivot irrigation systems are used.
Areas where horizontal clearances are limited because of trees or nearby structures.
Environmentally sensitive sites such as areas with threatened or endangered species of
Animals
Areas of Cultural Significance
Line Route
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The proposed project area is the most reasonable option concluded by the following
considerations.
Public and Social considerations
Distance of transmission centerline to homes and businesses
Distance and impact to public facilities, parks, and trails
Extent of tree and vegetation removal which should be minimized
Distance and to historical sites in the area
Environmental/Cultural considerations
Adherence to national and local regulations
Adherence to sound environmental principles
Avoidance of areas such as burial sites, wildlife protected areas, protected wetlands, andscientific research areas of threatened and endangered species of animals.
Engineering/Construction considerations
Adherence to sound engineering/construction principles
Safety
Reliability
Accessibility
Engineering Considerations
Suitable soil conditions
Required angle structures
Structure size
Span lengths
Potential total line length due to the proposed location
Special construction requirements
Cost effectiveness
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EASEMENTS
Pole positioning and location shall be in accordance with the regulation of Electric Power
research Institute. In this documentation, citation in Ausgrids NS 167, Pole Positioningalso
serves as a basis. Where possible, line routes shall follow existing roads, and be contained within
the reserved area for pole erection.
In the case of the proposed location, savings or other advantages due to reduction of distances
may be obtained by traversing private property or other land not dedicated as public roads. In
this case, negotiations with property owner shall be coordinated.
Several routes where vehicle access is not appropriate to conduct maintenance and repairs shall
be established a right of way.
Re-routing of small segments may be applied in instances of impossibility to establish a
right-of-way due to the unsatisfiable demands of the property owner.
Below are lists of easements permits and authorizations that are meant to be secured;
Private properties Easement from owner and permission to cutdanger trees and for traversing the respectiveproperties
Highways Permit or Agreement with National and localGovernment for the utilization of CentralNautical Highway
Other public bodies Authorization for each respective concerns
City and National Government Building Permits
Instances of joint and common use of poles ofother organizations
Permit or agreement
Wire crossing Permission of utility
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STRUCTURES
Cited references for the contents of the structure analysis are the ASCE Guidelines for Electrical
Transmission Line Structural Loading (Manual No. 74) and the 1724E-204, Guide Specifications
for Steel Single Pole and H-Frame Structures. The aforesaid manuals provide the major criteria
for the design consideration employed in this documentation.
In economic considerations for transmission lines 230 KV and below, wood structures is
apprehended as a more economic choice. However in the case of the Mobo-Aroroy project, in
which longer spans between pole structures should be employed from which heavier loading
situation would occur, steel structures may be considered more economical considering the long
term maintenance cost associated with wood structures.
Economic studies are conducted to determine by comparison, the structure configuration that
may be best employed. - Or materials to be used such as base pole class of wood, steel or
prestressed concrete. Similarly, such studies are also employed to determine efficiency in terms
of material costs, cost of foundations and erection and different structural heights that defines
the structures reliability.
Factors that define the structure reliability are defined as follows:
Strength: Horizontal spans are limited by cross brace, poles, etc. Vertical spans are
limited by cross arms structure strength. For H-frame structures, horizontal and vertical
spans are also limited by pullout resistance for H-frame structures.
Conductor Separation: Conductor separation is intended to provide adequate space
for line crew personnel on poles, prevention of contact and flashover between
conductors.
Clearances-to-Ground: Limits on spans are directly related to height of structures.
Insulator Swing: The ratio of horizontal to vertical span will be limited by insulator
swing and clearance to structure.
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For the purpose of the design featured in this study, which are subjected into various
parameters, primarily defined by the climate condition of the proposed location of the structure,
as well as the environmental concerns posed by the regulating bodies of forest preservation, the
use of steel structures shall be employed.
DESIGN CALCULATIONS:
For a 69 KV, steel transmission tower, the following preliminary values are assumed by
considering NESC heavy loading standards.
DESIGN OF STEEL TOWER
Conductor: ACSR galvanized stranded-steel ground wire. Three no. 1, hard-drawn strandedcopper, 120,000 volts.
Span = 400 ft.Nor. sag = 9 ft. 0 in. Nor. tension (60F.) = 570 lb.Max. Sag = 10 ft. 6 in. Max. Tension (0F., %-in. ice, 8 lb. Wind) =1960 lb.
Elastic limit, No. 1 wire = 2180 lb.
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Wind pressure on wires (%-in. ice, 8 lb. per square foot wind):
%-in. ground wire = 0.917 X 400 = 367 lb.No. 1 conductor = 0.885 X 400 = 354 lb.Wind on pole = 13 lb. per square foot X 1 times exposed area of windward side = 20 lb. perlineal foot.
With the above transverse loading and no broken wires, the compressive stress in the leg abovethe foundation is obtained by taking moments of the forces about the panel point 2 ft. 1 in. abovethe foundation.
Ground wire 367 lb. X 42.4 = 15,560 ft.-lb.Power wires 354 lb. X 39.9 = 14,120 ft.-lb.Power wires 354 lb. X 2 X 37.4 = 26,480 ft.-lb.
Wind on pole = 20 lb. X = 17,980 ft.-lb.
Total bending moment =15,560 ft.-lb. + 14,120 ft.-lb. + 26,480 ft.-lb. + 17,980 ft.-lb.= 74,140 ft.-lb.
Since the lever arm of the resisting forces = 1.9 ft.:
7 4,140 ft.-lb. (1.9 ft. X 2 legs) = 19,500 lb.
Vertical load-steel = 1700 lb.
Vertical load-wires and insulators = 1500 lb.
3200 lb. 4 legs = 800 lb.
Total compressive stress in each leg = 19,500 lb. + 800 lb. = 20,300 lb.
Since 1 L 3 X 3 X M = 1.44 sq. in,
Maximum unit stress in each leg = 20,300 1.44 = 14,100 lb. per square inch.
The transmission tower is to be designed with identical faces on its sides. - Hence, the leg is
restrained from buckling in one direction by the intersecting diagonals, so the maximum l/r will
be computed as follows;
l r = 44in. 0.92 = 48 or;
l r = 22in 0.59 = 37
The ultimate strength of the angle based on the greater value of l/r in the curve in Fig. 33 of
422
2
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McGrawHill, Steel Poles and Towersis 14,100 lb. per square foot; therefore;
Safety Factor = 35,000 14100 = 2.5
STRESS IN DIAGONALS
Assumptions are;
Ground wire = 367 lb. X 1 = 367
Conductors = 354 lb. X 3 = 1062
Wind on pole = 201b. X 6 = 120
Total = 1549 lb.
By web systems of two faces of McGraw Hills transmission poles and towers, the configurationof the tower design, will yield a shear stress of 775 lb. per face. The stress in the diagonal just
below the arm is to equal the shear multiplied by the secant of the slope.
Assuming a 45 slope for the diagonals;
Stress = 775 lb. X (1 cos 45) =1100 lb.
STRESS IN CROSSARMS
The cross arms should be designed for maximum wind loads on both spans or for the maximum
ice and wind loads on one span combined with a longitudinal load due to the breaking of the wire
in the other span. Ice loads are of minimal effect on the project location.- However, still included
in the calculation as set by standards.
CONDITION No. 1
Vertical load (%-in. ice on wires) - 0.770 lb. per foot X 400 ft. = 308 lb.
Insulator and pin = 25 lb. 333 lb.
= 333 lb.
333 lb. X 35 in = 11,660 in.-lb.
Weight of arm = 15 lb. per foot X X 12 = 860 in.-lb.
312
2
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Horizontal load (8 lb. per square foot wind on wire 0.885 lb. per foot X 400 ft.)
= 354 lb. X 12 in = 4,250 in.-lb.
Total bending moment = 16,770 in.-lb.
3in. X 3 in. X 5/16in. X i/c = 2 X 0.95 = 1.90.
Max. Unit stress in cross arm = 16,770 1.90 =8800 lb. per square inch.
CONDITION No. 2
Vertical load (3-in. ice on wires) - 0.770 lb. per foot X 200 ft. = 154 lb.
Insulator and pin = 25 lb.
179 lb.
179 lb. X 35 in =6270 in.-lb.
Weight of arm = 15 lb. per foot X X 12 =860 in.-lb.
Horizontal load (8 lb. per square foot wind on wire -0.885 lb. per foot X 200 ft.) =
177 lb. X 12 in =2120 in.-lb.
Total Bending Moment = 9250 in.-lb.
9250 1.90 = 4900 lb. per square inch,
Longitudinal load:
1960 lb. X X 1.50 = 3720 lb.
3720 1.94 = 1900 lb. per square inch,
3.1
2
2.85
1.50
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Maximum unit stress in cross arm = 4900 + 1900 = 6800 lb. per square inch.
The four legged Pratt Braced towers, designed for a 220 KV transmission line tower will be
employed in this in this project.
TOWER HEIGHT
For Minimum Permissible Ground Clearance (h1) for 220kV;
h1 = 7.01m
Maximum Sag (h2):
For the sag tension calculation for the conductor and earth wire, citation will be made with
provisions of IS: 5613:1985For the following combinations;
-100% design wind pressure after accounting for drag coefficient and gust response factor
at every day temperature
-36% design wind pressure after accounting for drag coefficient and gust response factor
at minimum temperature.
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For the conductors with higher aluminum content normally used for 220kV lines, increased sag
of 2 to 4% of the maximum sag value is allowed.
T22 [T2 + A E (t2- t1) - t1] =
From the above equation, we get sag tension of the conductor (T2).
Sag =
The following combinations of factors will be considered:No Wind, t2 = 00 C
No Wind, t2 = 750 C
No Wind, t2 = 320 C
Full Wind, t2 = 320 C
75% Full Wind, t2 = 320 C
Sag value for different temperatures and for different wind conditions are calculated and the
maximum value of the above combinations + 4% extra will gives the h2 of the conductor.
Vertical Clearance between Ground Wire and Top Conductor (h4): The same procedure isrepeated as done in finding sag of the conductor (h2) but only difference is instead of conductorproperties substitute earth wire properties.
H = h1 + h2 + h3 +h4 = 33.52 m.
FOUNDATION DESIGN
Tower foundations shall be capable of withstanding loadsspecified strength limit state and
serviceability limit state conditions. Pole embedment depths shall be is indicated in NS220,
Ausgrids Overhead Design Manual, and Sections 6.2 Foundations as cited.
AL P E
24T
AL P E
24T
PL
8T
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CONDUCTORS
Conductor materials which used for Mobo-Aroroy overhead transmission lines shall have the
following electrical and physical properties.
The conductor shall have a high conductivity for minimal losses
It should have tensile strength.
It should have a high melting point and thermal stability.
It should be flexible to permit us to handle easily and to transport to the site easily.
It should be corrosion resistance.
As cited in ACSR Manual, Aluminum Wires and Cables, conductor data sheet shall be the
following;
ACSR Code Name Zebra
No. of Conductor/Phase One
Stranding/ Wire Diameter 54/3.18mm AL + 7/3.18mm steel
Total Sectional Area 484.5 mm2
Overall Diameter 28.62 mm
Approximate Weight 1621 Kg/ Km
Calculated D.C Resistance at 20 0C 0.06915 ohm/Km
Minimum Ultimate Tensile Strength 130.32 KN
Modulus of Elasticity 7034 Kg/mm2
Coefficient of Linear Expansion 19.30 x 10-6/ 0C
Max. Allowable Temperature 750C
EARTHWIRE
By NESC Standard for Transmission Conductors, earth wires to be employed as lightning and
high voltage surge arresters for 220 KV line transmission towers shall have the properties as
summarized below.
Material of Earth wire Galvanized steel
No. of Earth Wire One or two *depending on the shielding angle as
required by the location
Stranding/ Wire Diameter 7- 3.15mm
Total Sectional Area 54.55 mm2
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Overall Diameter 9.45 mm
Approximate Weight 428 Kg/ Km
Calculated D.C resistance at 200C 3.375 ohm/Km
Min. Ultimate Tensile Strength 5710 Kg
Modulus of Elasticity 19361 Kg/mm2
STRUCTURE SPOTTING
The specifications of the route alignment of tower structures are profiled through the use of the
Google map imagery of the Masbate province. The output documentation shall be in the form of
digitized route alignment drawing by the utilization of the PLS-CADD, providing the digital
imagery of the terrain modeling along the selected route.
Following considerations are employed in the spotting of locations for the transmission towers
featured in this document;
The alignment of the transmission line shall be economical upon its construction and
access for its maintenance.
Routing of transmission line in protected /reserved forest area should be avoided. In case
that as such is not possible, the cutting down trees shal be kept at minimum.
The number of angle points shall be kept to a minimum.
The distance between the terminal points specified shall be kept shortest possible, and
consistent with the encountered terrain.
Low lying areas, river beds and earth slip zones shall be avoided to minimize risk to the
foundations and towers.
Ground level alignment shall be utilized.
Crossing of communication line shall be minimized and it shall be at right angle. Proximity
and parallel alignment with telecommunication lines shall be eliminated to avoid danger
of induction.
Areas subjected to flooding shall be avoided.
As provided by the REA, Design Manual for High Voltage Transmission Lines, clearances for
220 KV lines shall be of 7 meters at minimum. The ground clearance curve should not touch or
cross the ground line. Besides normal ground clearance, the clearance between power
conductor and objects like other power or telecommunication lines, houses, roads etc., shall also
be checked.
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While crossing over existing power lines, one of the crossing span of the new line is preferably
located near the existing power line, to take advantage of the higher height of the conductors
near the tower. This reduces the necessity of increasing the height of the towers of the new line
for obtaining the requisite clearance. Double suspension tension insulator strings, depending on
the type of the towers shall to be used in the new line on such crossings. Minimum clearances in
meters between lines crossing each other, specified for 220KV shall be 4.58 meters.
A tower schedule is prepared which contains all the information such as location numbers, type
of tower, span length, section length, sum of adjacent spans, weight spans as affected by of one
side as well as both sides under maximum and minimum sag conditions and angle of deviations.
SAG TEMPLATE CALCULATIONS
When calculating maximum sags for new conductors, allowances shall be made for conductorcreep and for minor errors in construction. This additional creep allowance does not have to be
applied to existing conductors which are being diverted or reconstructed.
Conductor: ACSR Zebra (420 mm2)
Construction: 54 Aluminums / 7 Steels / 3.18 mm
PARAMETERS:
Basic Span () : 350 meters
Ultimate Tensile Strength of Conductor (U.T.S.) : 13290 Kg
Overall diameter of the Conductor (d) : 28.62 mm
Weight of the Conductor (w) : 1.621 kg / m
Wind Pressure (P) : 83.38 Kg /m2
Coefficient of linear Expansion () : 19.3 106 per C
Youngs Modulus of elasticity (Final) (Ef) : 0.686 10 6 Kg / cm2
Youngs Modulus of elasticity (Initial) (Ei) : 0.4675 10 6 Kg / cm2
Maximum temperature (Ambient) : 50 C
Maximum temperature (Conductor) : 75 C
Minimum Temperature (Ambient) : (-) 2.5 C
Minimum Temperature (Conductor) : (-) 2.5 C
Normal Temperature : 32.2 C
Area of Cross section of Conductor (A) : 4.845 cm2
Factor of Safety (F.O.S.) (at 32.2 C) : 4
Factor of Safety (F.O.S.) (Otherwise) : 2
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Weight of Conductor per unit area () : = w = 1.621
A 4.845
= 0.334571723 Kg /m / cm2
Minimum Ground Clearance : 7.01 meters
CONDITION: I
Temperature = 32.2 CWind = NILFactor of Safety = 4
Working tension; T1 = U.T.S = 13290F.O.S 4
T1 = 3322.5 Kg
Working Stress; f1 = T1 = 3322.50A 4.845
f1 = 685.75851 Kg / cm2
Loading factor; q1 = P2 + w2 =1 (for no wind; P = 0)W
The working stress is determined by the following formula:
f12(f1k) = 22q12Ef24
k = f122q12Ef24 f12
k= 685.75851(350)2 (0.334571723)2 (1)2 (0.686 106)24 (685.75851)2
= 685.75851833.46049
k = (-) 147.70198
CONDITION: II
Temperature = 75 CWind = NILFactor of Safety = 4
Working tension; T1 = U.T.S = 13290F.O.S 4
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T1 = 3322.5 Kg
Working Stress; f1 = T1 = 3322.50A 4.845
f1 = 685.75851 Kg / cm2
Loading factor; q1 = P2 + w2 =1 (for no wind; P = 0)W
The working stress is determined by the following formula:
f12(f2(k- t Ef) = 22q12Ef24
f22[f2{147.70198(19.3 106) 42.8 (0.686 106)}]
= (350)2 (0.334571723)2 (1)2 (0.686 106)24
f22(f2+ 714.36542) = 3.919470757 108
f22= 555.553321 Kg / cm2
Working tension; T2= f2 A = 555.553321 4.84 = 2691.656 Kg / cm2
Maximum Sag; s = 2 q28 f2
= (350)2 (0.334571723) (1)8 555.553321
= 9.22 meters
CONDITION: III
Temperature = (-) 2.5 CWind = NILLoading factor; q3 = P2 + w2 = 1 (for no wind; P = 0)
W
Difference of temperature; t =2.532.2 =34.7 C
The working stress is determined by the following formula:
f32[f3(k t E f) = 22q32E f24
f32[f3{147.70198(19.3 10 6) (34.7) (0.686 106)}]
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= (350)2 (0.334571723)2 (1)2 (0.686 106)24
f32(f3+ 311.71908) = 3.919470757 108
f3= 851.8511701 Kg / cm2
Working tension; T3 = f3 A
= 851.8511701 4.845
= 4127.219 Kg / cm2
Factor of Safety = 13290 = 3.22, hence O. K.4127.2
Maximum Sag; s = 2
q38 f3
= (350)2 (0.334571723) (1)8 851.8511701
= 6.01 meters
For Sag calculation at any span, the following formula can be used.
Sag at any Span = Sag at Basic Span (Span Length)
2
(Basic Span) 2
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CORONA
Corona is defined as a self-sustained electric discharge in which the field intensified ionization islocalized only over a portion of the distance between the electrodes.
During unusual situations in the overhead transmission line when the intensity of the electric
field exceeds the dielectric strength of air, the area around the conductor experiences electricdrilling, which causes increased losses and apparent conductivity.
Corona phenomenon initiates as a hissing noise and is heard and ozone gas is formed which canbe detected and characterized by its color.
The object of this documentation was to obtain data for the choice of conductor to be used ona 220-kv. 60- Cycle line. The results show the effect of weathering of conductors, the type ofconductors of two different diameters, the effect of size of conductor strands and the method ofstranding.
Corona activity on the 220 kV transmission line can generate a small amount of sound energy.Corona also results in a small amount of power loss to the transmission line. The audible noiselevel can increase during foul weather conditions. Water drops may collect on the surface of theconductors and increase corona activity so that a crackling or humming sound may be heardnear a transmission line. Audible noise decreases with distance away from a transmission line.Corona is affected by the voltage of the line, the diameter of the conductor, and thecondition of the conductor and suspension hardware. The electric field gradient is therate at which the electric field changes and is directly related to the line voltage. Theelectric field gradient is greatest at the surface of the conductor. Large-diameterconductors have lower electric field gradients at the conductor surface and, hence,lower corona than smaller conductors, everything else being equal.
In this document the following solutions are proposed to be employed;
Minimize the voltage stress and electric field gradient. This is accomplished bymaximizing the distance between conductors that have large voltagedifferentials, using conductors with large radius, and avoiding parts that havesharp points or sharp edges.
Surface Treatments: Corona inception voltage can sometimes be increased byusing a surface treatment, such as a semiconductor layer, high voltage putty orcorona dope.
Employ proper geometric configuration of the conductors.
Eliminate unwanted voltage transients in the line, which can cause corona tostart.
By increasing the diameter of the conductor: Diameter of the conductor can beincreased to reduce the corona discharge effect.
Using Bundled Conductors, multiple conductors per phase are used in the 220 KV
line. This is a common way of increasing the effective diameter of the conductor,which in turn results in less resistance, which in turn reduces losses.
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MAGNETIC FIELDS
A double circuit 220kV transmission lines crosses several plane land surfaces in Masbateprovince and connects the Aroroy Substation and the Old Masbate diesel power plant inthe province. Each line is on one side of the tower and consist of three phase with eachphase consisting of a bundle of two conductors. The lines also have two conductors onthe top of the towers (called earth wires) that do not carry current but act as to shield
the line from lightning current surges.
Considering the EMF-Effect associated with the proposed design, the followingmeasures are conducted.
- The undergrounding of the power lines with the required transition enclosures: and
- Retaining the power lines above ground mounted on poles, with the slight realignmentof the easement.
Electric fields are created by the electric charges on high voltage equipment. They
diminish rapidly with distance and are shielded by common materials such as trees orbuildings. Electric fields have not been identified as a source of major safety issues.However, they can potentially cause a number of effects such as audible noise, RF noiseinterference and visible spark.Electric field strengths associated with 220 KV transmission lines typically are in therange of 1 - 3 kV/m under the power lines - contact shocks to occur. At the edge of thetransmission line easement, typically 20 to 30 meters from the power line poles, theelectric fields are typically in the range of 0.1 - 1.0 kV/m.
It is also possible for voltages to be induced in long metallic structures that are alignedso that they run parallel to the transmission lines. However, the induction is significantly
small; running through lengthy structures that makes it becomes dissipated andgenerally wontreach the bottom part that would affect the public.
There are a number of ways to reduce EMF effects: F irstly, as the magnitude of the EMFis inversely proportional to the distance from the current carrying elements, one canincrease the distance of the public from the conductors. Hence by increasing the widthof an easement, increasing the height of a transmission line or increasing the distanceto the boundary of the terminal station will reduce the magnetic field strengthmagnitude. Higher transmission poles will produce a lower EMF, so there is a potentialtrade-off between the height of a transmission pole and its effect on visual amenity andthe reduction in EMF. There are practical limits to the physical height of poles and sizeof the easements and substation sites so this solution, whilst generally practical, doesnot suit every situation.
More compact structures have lower EMF as better cancellation of the fields occurs if theconductors are close together but there are engineering limits to how close conductorscan be place to each other.
EMF shielding is possible using materials of high magnetic permeability. However, thissolution is expensive and usually only used to solve specific local problems.
Other solutions, such as current cancellat ion loops may offer alternative, but less provenoptions for addressing magnetic field problems.
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The various solutions are applied during detailed design. Generally speaking physicalseparation will provide appropriate EMF levels and this particularly applies totransmission lines through rural areas where the lines are well away from areas desedwith people.
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