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Transcript of transmission line
Under the Guidance of
Mr. P.S.PATIL Overhead transmission lines are the backbone in every electrical power transmission system.
Because the consumers are not concentrated at the location of the generation it is not possible to use power plants with high output. For the connection between the power plants and the consumers integrated network which consists of transmission line systems like overhead lines or power cable are in use. INTRODUCTION Transmission line transfers power from power rich centers to power deficient centers with minimum power loss.
Now-a-days design of transmission line is a very complex task, since it includes designing of constantly changing parameters.
Transmission line should transmit power over required distance economically and should satisfy the electrical and mechanical requirements prescribed in particular cases also designing includes very clear idea of efficiency, regulation and losses. INTRODUCTION Thus this project aims to develop a solution for designing transmission lines which will leave the manipulation and calculation part for computer and just by feeding some basic data we will get an optimized design of a new transmission line.
This software approach will provide a handy means for new ones entering in this field and will help them to solve complex problems in a lucid manner. AIM AND OBJECTIVE While designing transmission lines various parameters are to be considered.
They are as follows:
1) Size of conductors and spacing between conductors.
2) Calculation of line constants (ABCD parameters)
3)Calculation of corona loss.
4)Choice of method of grounding.
5)Calculation of radio interference.
6)Choosing number and type of type of insulators.
7)Effect of ice and wind-loading.
8)Electrostatic and electromagnetic effects.
9)Tower design: a)Height of tower.
b)Selection of line route.
c)Ground and conductor clearance.
d)Tower spacing ,span length.
10) Sag and tension calculation DIFFERENT PARAMETERS IN TRANSMISSION LINE When the alternating potential between conductor increases beyond a certain limit, a point is reached when a pale violet glow appears on the conductor surface, and accompanied by a hissing sound, this phenomenon is called as corona. CORONA It leads to the deterioration of insulation.
It also gives rise to radio interference.
When corona occurs it produces loss of power. It is very much affected by the size and spacing of the conductors. CORONA TYPES OF INSULATOR
Pin type insulator
Suspension type insulator
Strain type insulator
Number of insulator is decided by the system voltage, switching and lighting over voltage amplitude and pollution level. TYPES OF INSULATOR
1. Aditi Zade
2. Kiran Nagarikar
3. Minal Godbole
5. Shweta Gupta
6.Tejashree Shinde THANK YOU The voltages used as standards in India are 11KV, 22KV and 33KV for short transmission line, 66KV and 110KV for medium lines and 132KV, 166KV, 230KV and 400KV for long lines.
Now-a-days 765kV lines are also available.
Voltage is selected according to length of line and line loading. VOLTAGE SELECTION Depending on the type of circuit and transmission voltage, conductors vary in number and size. the basic parameters are weight per unit length, conductivity of materials, cross sectional area, cost of material.
The size of conductors selected depends on the length of transmission line, load on the line and the voltage of line. For the given loss of energy, the cross section of conductor and its weight is inversely proportional to the square of line. The loss on transmission line is 3I²R per unit length of the line. As the voltage increases, the cross section of the conductors reduces and hence reduces loss.
The choice of conductor depends upon receiving end current(Ir)
Current(Ir) = power(kW)/√3* vltg* P.F. Amps CHOICE OF CONDUCTORS Hard-drawn copper, hard-drawn aluminium, and steel-cored aluminium are most commonly used. Some of the important types of conductors are given as follows:
1.Hard-Drawn Copper Conductors
Copper of overhead lines is hard-drawn to give a relatively high tensile strength. It has a high electrical conductivity, long life, and high scrap value. Copper conductor is most suitable for distribution work.
2. Hard-Drawn Aluminium conductor or All-Aluminium Conductor (AAC)
The increasing trend in the cost of copper has resulted in replacement of copper and adoption of aluminium for transmission work. The cross section of aluminium conductor is 60% greater than that of copper and its weight is only 48.3% of that of copper conductor. Thus, handling, transportation and erection becomes economical. Corona effect is also reduced due to higher conductor diameter. TYPES OF CONDUCTORS 4. All-Aluminium Alloy Conductor (AAAC)
AAAC provide a good combination of tensile strength and conductivity. These conductors are known by different trade names. One of these alloys is ‘Silmalec’, which contains 0.5% silicon, 0.5% magnesium and the remainder aluminium.
5. Aluminium conductor alloy reinforced (ACAR)
ACAR has a central core of alloy of aluminium surrounded by layers of conductor aluminium. It gives better conductance. ACAR conductor has a smaller size and lesser weight as compared to ACSR conductor. 3. Aluminium Conductor Steel Reinforced (ACSR)
Conductors made of all-aluminium are not sufficiently strong mechanical for construction of long span lines. This deficiencies can be compensated by adding a steel core to the conductor. Such a conductor is called steel-cored aluminium (SCA) or ACSR. The important advantages of ACSR conductors are high tensile strength and light weight. TYPES OF CONDUCTORS The spacing of conductor depends on the lines and the span used, the conductors should not touch each other at sag conditions.
For high voltage lines, bundled conductors may be considered as an alternative to a single conductor for each phase or two , three or four sub-conductors belonging to the same phase may be used and the separation between sub-conductors may be adjusted as required by reactance and corona considerations. SPACING OF CONDUCTORS Resistance, inductance , capacitance and conductance due to leakage from shunt circuit , per unit length of line are the distributed constants of line. Resistance and inductive reactance forms impedance , shown in series with line , similarly capacitive reactance and conductance forms admittance shown as shunt with the line
Z=R + jX Y=G + jB
Using these constants we can find A,B,C and D constants which can be expressed as hyperbolic cosine and sine functions. CALCULATION OF LINE CONSTANTS (ABCD PARAMETERS) 3. Long lines: ABCD parameters for transmission line:
1. Short lines: A=1,B=Z, C=0, D=1
2. Medium lines: CALCULATION OF LINE CONSTANTS Where , E=kV to neutral(rms);
∂=air density factor;
r=radius of the conductor;
D=spacing between conductors;
Ed=disruptive voltage(kV rms); It can be calculated by Peek’s formula as, CORONA LOSS CORONA LOSS E/Ed Where , E=kV to neutral(rms);
r=radius of the conductor;
D=spacing between conductors;
F=factor which varies with ratio kW/km/conductor When the ratio E/Ed is less than 1.8, Peterson’s formula holds good and is given as Tower is a lattice structure that supports insulators , overhead transmission line conductors and overhead earth wire.
Towers are also used for supporting flexible bus bar , insulators etc.
Towers are three dimensional fabricated lattice structure made up by bolting/ welding the structural members of galvanized steel. TOWER DESIGN Tower design includes-
1) Height of tower
2) Ground clearance
3) Conductor clearance
4) Tower spacing
5) span length
6) Mechanical loading TOWER DESIGN Insulation is defined as the separation between conducting surfaces by means of non conducting (dielectric) material that would economically offer a high resistance to current.
Insulators may be fabricated from porcelain, toughened glass, fibreglass rods and sheds of polymer or silicone construction. INSULATORS There are various applications of insulators such as
Strain insulators are used at the line terminal or at anchor tower.
For voltage up to 11KV shackle insulator can be used for taking the strain off the line conductors.
Suspension type insulators are used for the above 33KV transmission line. APPLICATION OF INSULATOR The corona discharges emit radiations which may introduce noise signals in the communication channels, radio, TV receivers in the vicinity. This is called radio interference. Radio noise from overhead power lines is caused by corona on conductors and fittings, surface discharges on insulators and poor contacts in fitting and insulator strings.
The interference to radio and TV, due to extra high voltage transmission lines, having corona effect should be limited to tolerable limits.
The recommended surface gradients for EHV lines for satisfactory radio interference (RI)levels are different for urban and rural areas. RADIO INTERFERENCE The forces acting on the conductor are:
1. Conductor weight
2. Ice loading
3. Wind loading
The weight of conductor acts vertically downwards and depends upon type of conductor used. EFFECT OF LOADING ON CONDUCTORS Where, d= diameter of conductor with ice coating in metres
t= radial thickness of ice in metres Kgf/m ICE LOADING:
In snowy areas, ice is deposited on
the conductors .
The amount of deposition depends
upon the severity of weather conditions.
The weight per unit length of conductor
increases by ice deposition.
The ice loading can be calculated as:
Wi = πt(d+t)×913.5× EFFECT OF ICE LOADING Wind exerts horizontal pressure on the exposed surface.
Pressure of the wind is dependens upon its velocity.
Wind produces a transvers loading by exerting a pressure upon the projected area of conductor. EFFECT OF WIND LOADING The clearances to be provided between the lowest conductor of the line and the ground as per Rule 77 of the Indian Electricity Rules, 1956 are shown in the table. Span length is the distance between poles or towers.
When a conductor is suspended between two supports care should be taken so that there is not too much tension on conductor otherwise it will break. When a conductor is supported between poles or tower, it will sag or dip due to its own weight. But at the same time there should be adequate clearance between the lowest point of line and ground for safety.
If we increase the sag ,the tension will be reduced and the clearance from the ground will also be reduced, which will result in greater tower height. This makes the line more expensive . CALCULATION OF SAG AND TENSION Sag depends on following factors:
Weight of conductor: heavier the conductor, greater will be the sag.
Span length: as the distance between the supports increases, sag increases.
Tension : sag is inversely proportional to the tensile strength.
Temperature : with the rise in temperature, all metallic bodies expand, hence conductor length increases and hence the sag. CALCULATION OF AND SAG TENSION l=span length
w=weight per unit length of the conductor
H=tension in the conductor at the point of maximum deflection O
TB=tension in the conductor at the point of support B 1)CATENARY METHOD: There are two methods for calculation of sag and tension. They are as follows. 2) PARABOLIC METHOD: This method is applicable for span up to 300m and there is not much difference in span and conductor lengths. CALCULATION OF AND SAG TENSION Sag-tension calculations predict the behaviour of conductors based on recommended tension limits under varying loading conditions. These tension limits specify certain percentages of the conductor’s rated breaking strength that are not to be exceeded upon installation or during the life of the line.
These conditions, along with the elastic and permanent elongation properties of the conductor, provide the basis for determining the amount of resulting sag during installation and long-term operation of the line. Accurately determined initial sag limits are essential in the line design process. Final sags and tensions depend on initial installed sags and tensions and on proper handling during installation.
The final sag shape of conductor is used to select support point heights and span lengths so that the minimum clearances will be maintained over the life of the line. If the conductor is damaged or the initial sags are incorrect, the line clearances may be violated or the conductor may break during heavy ice or wind loadings. CACULATION OF SAG AND TENSION This project will be implemented through MATLAB software.
Presently we are dealing with the study of Electrical parameters. In future this project can further be extended by considering tower design, thereby studying mechanical parameters.
Also study of electromagnetic and electrostatic effect, method of grounding etc on overhead transmission line will be done. ACTIVITY PLANS M. V. Deshpande, “Electrical Power System Design”
Ashfaq Hussain, “Electrical Power Systems”
R. Rajshree, “Fundamentals of Power System”
S. Rao, “EHV-AC HVDC Transmission and distribution Engineering”
Nagrath and Kothari, “Modern Power System Analysis” REFERENCES