Project Report on

Name Class Section Roll No.

– Rajarshi Biswas – BEE - IV – A1 – 000610801017

Acknowledgement

We are heartily thankful to our supervisors whose encouragement, guidance and support from the initial to the final level enabled me to develop an understanding of the subject. Lastly we offer my regards to all of those who supported me in any respect during the completion of the project especially our guides Prof. P. K. Chattopadhyay, Prof. S. K. Goswami and Prof. R. N. Ganguly along with my fellow batchmates who provided valuable support and co-operation during the execution of project.

Introduction
The basic function of a transmission system is to transfer electrical power from one place to another or from one network to another network. A transmission system includes the terminal substations, transmission lines and intermediate substations. Transmission systems are required for
● Transfer of bulk power from large group of generating stations upto the main

transmission network ● System interconnection ● Transfer of power from main transmission from the main transmission network to secondary substations

The increasing need of transmitting greater and greater amounts of power over long distances has led to a continuous increase in transmission voltages. As a rule, larger the amount of power to be transmitted, higher is the requirement of transmission voltages. In India, the transmission voltages in use are 33, 66, 110, 132, 220 & 400 kV . The next higher voltage of 765/800kV has been selected for transmission of huge blocks of power from generating stations to load centres.

Necessity of EHV Transmission
Reduction of Electric Losses , increase in Transmission efficiency , inprovement of Voltage Regulation and reduction in Conductor Material requirement For transmission of given amount of power over a given distance through the conductors of a given material and at a given power factor as the transmission voltages increase,
● Line losses are reduced since losses are inversely proportional to the transmission

voltage. ● Transmission efficiency increase because of reduction in line losses ● The voltage regulation is improved because of reduction of percentage line drop ● Lesser conductor material is required being inversely proportional to square of transmission voltage

Economic consideration has led to construction of power stations of large capacity and so need of transfer of bulk power over long distances arose. Transmission of bulk power from generating stations to the load centre is technically and economically feasible only at the voltages in the EHV/UHV range. 2. Generating stations ( steam hydro and nuclear power stations ) are located in remote areas (far away from load centres) because of the reasons of economy, feasibility and points of view of safety and environmental conditions. EHV transmission is , therefore inevitable for transmission of huge blocks of power over long distances from these power plants to load centres. 3. Flexibility for future system growth. 4. Increase in Transmission capacity of Line P = Vs Vr sin δ / X where Vs and Vr are two terminal voltages, δ is load angle and X is the line reactance.
1.

Thus the power transmission capacity of a transmission line increases with the increase in transmission voltage. No doubt the cost of transmission line and terminal equipment also increases with the increase in transmission voltage rather than the square of the transmission voltage. Moreover there is also savings in the cost due to reduction in energy losses occurring in transmission lines. As a consequence the total cost of transmission decreases with the increase in transmission voltage.
5. Possibility of interconnections of Power Systems – It is practically not possible to have

interconnections of two or more power systems, which is necessary to achieve sharing of installed reserves and for development of integrated systems and grids, without EHV transmission. 6. Increase in Surge Impedance Loading - Load carrying capability of a line is expressed in the terms of surge impedence loading (SIL). Surge Impedence Loading is the power that a line carries when each phase is terminated by a load equal to the surge impedence loading of the line. For a transmission line , the surge impedence is given by Z c = where L and C are series inductance and shunt capacitance of per unit length. The surge impedence loading for a transmission line is given by 3V2/Zc, where V is the line to neutral voltage. It is evident that SIL varies as the square of operating voltage and therefore with the increase in voltage level, SIL itself increases. Thus power transfer capability of the line increases with increase with increase in voltage level. The surge impedence of a line can also be determined from its conductor configuration. 7. Reduction of Right of Way - In some countries “right-of-way” are paid for a rate proportional to the total width of transmission lines. Even in countries where right-ofway is not directly paid, there are usually strong pressure from public towards fewer and fewer transmission lines. With the passage of time right-of-way becomes either more costlier or difficult to obtainand therefore it is becoming necessary to have a fewer transmission lines operating at EHV/UHV . the worthnting point here is that with the increase in operating voltages, the number of circuit and requirement of land is considerably reduced.

765 kV – The next level of Transmission
Resource Conservation
●

A single-circuit 765-kV line can carry as much power as three singlecircuit 500-kV lines, three doublecircuit 345-kV lines, or six single-circuit 345-kV lines, reducing the overall number of lines and rights of way required to deliver equivalent capacity. high capacity of 765-kV can easily facilitate the efficient and economical integration of largescale renewable generation projects into the nation’s transmission grid. Typical 765-kV lines have a tower height of approximately 130-140 feet. This is 30-40 feet shorter than a typical double-circuit 345-kV tower.

● The

●

Performance and Design Efficiency
●

Power losses in a transmission line decrease as voltage increases. Since 765-kV lines use the highest voltage available in the United States, they experience the least amount of line loss. The greater transmission efficiency of 765-kV can be attributed mainly to its higher operating voltage (and thus lower current flow) and larger thermal capacity/low resistance compared to lower voltage lines. This also allows 765-kV lines to carry power over significantly longer distances than lower voltages.

●

● With up to six conductors per phase, 765-kV lines are virtually free of thermal

overload risk, even under severe operating conditions.
● By shifting bulk power transfers from the underlying lower-voltage transmission

system to the higher-capacity 765-kV system, overall system losses are reduced significantly.
●

New 765-kV designs have line losses of less than one percent, compared to losses as high as 9 percent on some existing lines. The overlay of a 765-kV system allows for both scheduled and unscheduled outages of parallel lower voltage lines without risk of thermal overloads or increased congestion.

●

Minimizing Costs
● Use of 765-kV technology allows transmission builders to take advantage of

economies of scale. A typical 765-kV line costs approximately $2.6 million(Rs. 1.2billion)/mile. For equivalent capacity, three 500-kV lines at a cost of $6.9 million/ mile or six 345-kV lines at a cost of $9.0 million(Rs. 4 billion)/mile would be required. In other words, 765-kV construction is only 29% of the cost of 345-kV and 38% of the cost of 500-kV for a comparable system.
●

Utilizing 765-kV results in a substantial reduction in system losses. For instance, a loss reduction of 250 megawatts, equates to saving as much as 200,000 tons of coal, and 500,000 tons of CO2 emissions on an annual basis.

● The addition of 765-kV systems relieves the stress on underlying, lower voltage

transmission systems, postponing the potential need for upgrades of these networks. This results in additional savings for end-use customers over time.

Our electric intensive society relies on the reliable delivery of power. By designing bulk power transmission systems to maximize efficiency and operational functionality, ETA is working to ensure that we can meet the energy needs of the nation’s electricity users in a responsible and cost effective manner.

Major components of an EHV Transmission

● ● ● ● ●

Tower – usually lattice steel towers Insulators Conductor – each conductor being stranded, ACSR Foundation and grounding- steel reinforced concrete foundation and grounding electrodes being placed in ground Shield conductors

The various points to be taken in consideration while transmission line design can be summarized as follows:

Line Design Practice
Present line design practice consists of following mandatory regulations of electrical safety. Climatic events have to be taken in consideration at specific locations. ● Contingency loading event of interest , i.e. broken conductors etc. have to be taken care of ● Special requirements and expectations
●

Structures
Lattice towers 2. Cantilever guyed poles and masts 3. Framed structures 4. Combination of above
1.

Insulators and Accessories
While selection of insulators various type of internal and external, electrical mechanical and environmental stresses have to be taken in consideration. Various stresses being developed are as follows:

Electrical Stresses on Electrical Insulation
Power frequency over-voltages ● Temporary over-voltages
●

Switching over-voltages ● Lightning over-voltages
●

Environmental Stresses
● ● ● ● ● ●

Temperature UV radiation Rain Icing Pollution Altitude

Conductors
The various transmission line parameters taken into consideration while designing are: Inductance and inductive reactance along with inductance of transposed 3 phase line. ● Capacitance and capacitive reactance while considering bundled conductors. ● Current carrying capacity or “ampacity”. ● Spiraling and bundle conductor effect.
●

Sag and Tension Calculations
The spans can be generally classified into level spans and inclined spans where conductor length is an important factor for calculating conductor slack. The various loading factors to be taken into consideration are: Wind loading ● Ice loading ● Combined wind and ice loading Sag change with combined effects of thermal elongation and elastic effects has to be taken into consideration too.
●

Corona and Noise
Corona modes can be classified into

1. Negative corona modes that includes trichel streamers , negative pulsations glow and

negative streamers 2. Positive corona mode includes burst corona, onset streamer, and positive glow. Main effects of corona discharges on overhead lines Corona losses ● Electromagnetic interferences ● TV interferences ● Audible noises So corona performances of HV lines and approach to control the corona performance lines are to be considered while selecting conductors.
●

Reactive Power Compensation
Shunt reactive power compensation includes voltage with the help of shunt capacitors and use of static VAR compensators. Series compensation involves establishing series capacitor banks. STATCOMS (static compensators) provide variable reactive power from lagging to leading but with no inductors or capacitors for VAR generators. Consists of GTO which produces alternating voltage sources in phase with transmission voltage and is connected to line through series inductors.

Our Goal is to transmit a power of 2000 MW over a distance of 500 Km.

Power Transformer
Generally Single phase units are employed i.e. a bank of three single phase transformer

Ratings

Voltage ratio

Tapping range percent

Percent Impedance Voltage

Cooling

630MVA, 800MVA

21/765

A - Off ckt taps/links +2 -6 (8 steps) 15.0 alternatively (tolerance allowed B - On load taps = + 10%) +2 -6 (16 steps)

ONAN/OFAF or OFAF or OFWF or ONAN/ODAF or ODAF/ODWF

Maximum flux density in any part of core and yoke at rated MVA, voltage and frequency : 1.9 tesla

Withstand capability for 25% above the rated voltage : 1 minute Withstand capability for 40% above rated voltage : 5 seconds Connections : HV Star, Neutral effectively earthed LV Delta Connection symbol : YN, dll (in 3 phase bank) Terminals :
a.

LV Terminals - 36 kV, 12500 amps. oil filled type bushing mounted on turrets, suitable for connections to bus bars in isolated phase bus ducts which shall have spacing of 1500 mm for each 210 MVA single-phase unit of the 630 MVA three phase bank. For each of 266.6 MVA single phase unit of 800 MVA, 3 phase bank, 2 Nos. 36 kV, 12500 Amp bushings per termination shall be used (Total 4 Nos. bushings).

b.

HV Terminal line end - 800 kV oil filled 1250 amps. condenser bushing with test tap. No arcing horns shall be provided.

Neutral ends - 36 kV porcelain bushing. No arcing horns shall be provided.

Auto Transformers

Three phase rating HV/IV/LV MVA

Voltage Ratio

Tapping range percent

Percent voltage impedence HV/IV

HV/LV

IV/LV

Cooling

315/315/105

765/220/33

+ 4.5% -7.5% 24 steps

12.5

40

25

630/630/210

765/400/33

+ 4.5 % -7.5% 24 steps

12.5

60

40

800/800/266.6

765/400/33

+ 4.5 % -7.5% 24 steps

12.5

60

40

1000/1000/333.3 765/400/33

+4.5% -7.5% 24 steps

14.0

65

45

ONAN/OFAF or ONAN/ODAF or ODAF ONAN/OFAF or ONAN/ODAF or ODAF ONAN/OFAF or ONAN/ODAF Or ODAF ONAN/OFAF or ONAN/ODAF or ODAF

Max. flux density in any part of core and yoke at rated MVA, voltage and frequency : 1.9 tesla

Withstand capability for 25% above the rated voltage : 1 minute Withstand capability for 40% above the rated voltage : 5 seconds Connection : HV/IV Star auto with neutral effectively earthed : LV-Delta Connection Symbol : YNa0, dll Short Circuit Level : 800 kV - 40 kA (rms) for 1 second 420 kV - 40 and 63 kA(rms) for 1 second 245 kV - 40 kA (rms) for 1 second Terminals:
a.

LV Terminals - 52 kV oil filled condenser bushings. The bushings shall be arranged in a line with 1000 mm spacing to allow mounting of phase to phase barriers. No arcing horns shall be provided.

b. IV terminal - 245/420 kV oil filled condenser bushings with test tap. No arcing horns

shall be provided.
c.

HV Terminal Line end - 800 kV oil filled condenser bushing with test tap. No arcing horns shall be provided

Neutral end - 36 kV porcelain bushing. No arcing horns shall be provided

Bushings
The voltage and current ratings, basic insulation level and creepage distance of the bushings shall be as follows: Voltage Rating kV(rms) 800 420 245 52 Basic Current Rating Creepage Distance Impulse Level (Amp) (mm) kV(peak) 1250 2000 1250 5000 16000 10500 6125 1800 2100 1425 1025 250 Switching Impulse Level kV(peak) 1500 1025 -

RIV at 508 kV (rms): 1000 µvolts (Max.) Corona Extinction Voltage : 508 (Min.) kV(rms) Partial discharge level pico coloumbs : 500 (Max.)

Selection of Conductor for Transmission

For efficient transmission, the power factor should be regulated at 0.9. Therefore, power transmitted; √3VICos Φ = 2000 MW → I = 1680 Amp Best suitable conductor in accordance to Industrial practice is ACSR Bersimis with rated current 2000 Amp. The physical characteristics of ACSR Bersimis is given below:

No of subconductor/phase Spacing between conductors Bundle Arrangement Stranding and Wire Diameter Overall Diameter Approx. mass Ultimate tensile strength AC Resistance AC Reactance

4 450 mm Horizontal Square 42/7 (4.57/2.54 mm) Al/Steel 35.04 mm 2.187 Kg/m 146.87 kN 0.052 Ω/Km 0.492 Ω/Km

Calculation of Line and Ground Parameters
Resistance of Conductors
Conductors used for e.h.v. transmission lines are always stranded. Most common conductors use a steel core for reinforcement of the strength of aluminium, but recently high tensile strength aluminium is being increasingly used, replacing the steel. The former is known as ACSR (Aluminium Conductor Steel Reinforced) and the latter ACAR (Aluminium Conductor Alloy Reinforced). A recent development is the AAAC (All Aluminium Alloy Conductor) which consists of alloys of Al, Mg, Si. This has 10 to 15% less loss than ACSR. When a steel core is used, because of its high permeability and inductance, power-frequency current flows only in the aluminium strands. In ACAR and AAAC conductors, the cross-section is better utilized

Effect of Resistance of Conductor The effect of conductor resistance of EHV lines is manifested in the following forms:
1. 2.

Power loss in transmission caused by I2R heating; Reduced current-carrying capacity of conductor in high ambient temperature regions.

This problem is particularly severe in Northern India where summer temperatures in the plains reach 50°C. The combination of intense solar irradiation of conductor combined with the I2R heating raises the temperature of Aluminium beyond the maximum allowable temperature which stands at 65°C as per Indian Standards. At an ambient of 48°C, even the solar irradiation is sufficient to raise the temperature to 65°C for 400 kV line, so that no current can be carried. If there is improvement in material and the maximum temperature raised to 75°C, it is estimated that a current of 600 amperes can be transmitted for the same ambient temperature of 48°C. The conductor resistance affects the attenuation of travelling waves due to lightning and switching operations, as well as radio-frequency energy generated by corona. In these cases, the resistance is computed at the following range of frequencies: Lightning—100 to 200 kHz; Switching—1000-5000 Hz; Radio frequency—0.5 to 2 MHz
3.

We notice the vast reduction in MW loss occurring with increase in transmission voltage for transmitting the same power. The above calculations are based on the following equations:
1. 2. 3.

Current: I = P/ 3V Loss: p = 3I2R = P2R/V2 Total resistance: R = L.r, where L = line length in km, r = resistance per phase in ohm/km

4.

Total above holds for

= 30°. For any other power-angle the loss is p = 3I2rL = E2r sin2 /(L.x2)

where x = positive-sequence reactance of line per phase.

Skin Effect Resistance in Round Conductors
It was mentioned earlier that the resistance of overhead line conductors must be evaluated at frequencies ranging from power frequency (50/60 Hz) to radio frequencies up to 2 MHz or more. With increase in frequency, the current tends to flow nearer the surface resulting in a decrease in area for current conduction. This gives rise to increase in effective resistance due to the 'Skin Effect'. The physical mechanism for this effect is based on the fact that the inner filaments of the conductors link larger amounts of flux as the centre is approached which causes an increase in reactance. The reactance is proportional to frequency so that the impedance to current flow is larger in the inside, thus preventing flow of current easily. The result is a crowding of current at the outer filaments of the conductor. The increase in resistance of a stranded conductor is more difficult to calculate than that of a single round solid conductor because of the close proximity of the strands which distort the magnetic field still further. It is easier to determine the resistance of a stranded conductor by experiment at the manufacturer's premises for all conductor sizes manufactured and at various frequencies.

Inductance of E.H.V. Line Configurations
Fig. 3.5 shows several examples of line configuration used in various parts of the world. They range from single-circuit (S/C) 400 kV lines to proposed 1200 kV lines. Double-circuit (D/C) lines are not very common, but will come into practice to save land for the line corridor. As pointed out in chapter 2, one 750 kV circuit can transmit as much power as 4-400 kV circuits and in those countries where technology for 400 kV level exists there is a tendency to favour the four-circuit 400 kV line instead of using the higher voltage level. This will save on import of equipment from other countries and utilize the know-how of one's own country. This is a National Policy and will not be discussed further…

Inductance of Multi-Conductor Lines—Maxwell's Coefficients
In the expression for the inductance L = 0.2 ln (2H/r) of a single conductor located above a ground plane, the factor P = ln (2H/r) is called Maxwell's coefficient. When several conductors are present above a ground at different heights each with its own current, the system of n conductors can be assumed to consist of the actual conductors in air and their images below ground carrying equal currents but in the opposite direction which will preserve the ground plane as a flux line. The flux linkage of any conductor, say 1, consists of 3 parts in a 3-phase line, due its own current and the contribution from other conductors. The self flux linkage is 11 = ( 0/2 ) I1 ln (2H/r). We may use the geometric mean radius instead of r to account for internal flux linkage so that we write 11 = ( 02 ) I1 ln (2H/Ds), where Ds = self-distance or GMR. For a bundle conductor, we will observe that an equivalent radius of the bundle, equation (3.12), has to be used. Now consider the current in conductor 2 only and the flux linkage of conductor 1 due to this and the image of conductor 2 located below ground. For the present neglect the presence of all other currents. Then, the flux lines will be concentric about conductor 2 and only those lines beyond the aerial distance A12 from conductor 1 to conductor 2 will link conductor 1. Similarly, considering only the current–I2 in the image of conductor 2, only those flux lines flowing beyond the distance I12 will link the aerial conductor 1. Consequently, the total flux linkage of phase conductor 1 due to current in phase 2 will be

while the inductance of each subconductor will be Lc = 1/i = ln(2 / ) ...(3.32) which is also N times the bundle inductance since all the sub-conductors are in parallel. The Maxwell's coefficient for the bundle is Pb = ln (2H/req), as for a single conductor with equivalent radius req.

Advantages of using Bundled Conductors

● The use of multiple conductors per phase having the same total area as a single

conductor will operate at lower temperature yielding lower resistances and losses for equal loads. ● Multiple conductors offer significant improvements in reactance over a single conductor of equal area. ● Bundled conductor lines have a higher capacitance to neutral as compared to single conductor line. Thus surge impedance reduces and thus maximum power transfer capability increases.

Sub – conductor Relative power transfer

1 1.0

2 1.3

4 1.6

8 1.7

Using quad bundled conductors, the line resistance changes drastically. The new line parameters are:

R ≈ 0.00019 pu X ≈ 0.00474 pu B ≈ 2.40644 pu Per unit values on 100 MVA base and per 100 km Therefore, equivalent resistance of 500 Km line ≈ 0.00608 Ω
● ● ● ●

Taking line current to be 1680 Amp, the copper loss comes out to be,

20 MW (approx.) Temperature Rise of Conductors and Current-Carrying Capacity
When a conductor is carrying current and its temperature has reached a steady value, heat balance requires

(Internal heat developed by I2R) + (External Heat Developed by Solar Irradiation) = (Heat Lost by Convection to Air) + (Heat Lost by Radiation)

Let Wi = I2R heating in watts/metre length of conductor Ws = solar irradiation in watts/metre length of conductor Wc = convection loss in watts/metre length of conductor and Wr = radiation loss in watts/metre length of conductor Then the heat balance equation becomes

Wi + Ws = W c + Wr
Each of these four terms depends upon several factors which must be written out in terms of temperature, conductor dimensions, wind velocity, atmospheric pressure, current, resistance, conductor surface condition, etc. It will then be possible to find a relation between the temperature rise and current. The maximum allowable temperature of an Al conductor is 65°C at present, but will be increased to 75°C . Many countries in the world have already specified the limit as 75°C above which the metal loses its tensile strength. The four quantities given above are as follows:
1. I2R heating. Wi = I2Rm watts/metre where, Rm = resistance of conductor per metre

length at the maximum temperature.

Rm =

R20

with α = temperature resistance coefficient in ohm/°C and R20 = conductor resistance at 20°C.

2.

Solar irradiation.

Ws = sa.Is.dm watts/metre where dm = diameter of conductor in metre, sa = solar absorption coefficient = 1 for black body or well-weathered conductor and 0.6 for new conductor, and Is = solar irradiation intensity in watts/m2. At New Delhi in a summer's day at noon, Is has a value of approximately 1000-1500 W/m2. [Note: 104 calories/sq. cm/day = 4860 watts/m2] 3. Convection loss.

Wc = 5.73

, watts/m2

where p = pressure of air in atmospheres, vm = wind velocity in metres/sec., and Dt = temperature rise in °C above ambient = t – ta. Since 1 metre length of conductor has an area of p dm sq. m., the convection loss is

Wc = 18. Δt.

, watts/metre

4.

Radiation loss. This is given by Stefan-Boltzmann Law Wr = 5.702 × 10–8 e(T4 - Ta4), watts/m2 where e = relative emissivity of conductor-surface = 1 for black body and 0.5 for oxidized Al or Cu, T = conductor temperature in °K = 273 + t and Ta = ambient temperature in °K = 273 + ta.

The radiation loss per metre length of conductor is

Wr = 17.9 × 10–8 e(T4 - Ta4) dm, watt/m.
The heat balance equation then becomes

I2Rm + saIsdm = 18Δt.

+ 17.9 e.dm [

4–

4]

Line Support Structure
The function of line support is to obviously support the conductors . line support must be capable of carrying the load due to insulators and conductors including the ice and wind loading on the conductors along wid the wind loading on conductor itself. The line supports are of various types including…
● ● ● ●

Wood Steel Reinforced concrete poles Lattice Steel towers

The main requirements of the line supports are:
1. High mechanical strength to withstand the weight of conductors along with wind loading, 2. 3. 4. 5. 6.

ice loading etc. Light in weight without the loss of mechanical strength Cheaper in cost Low maintainence cost Longer life Easy accessibility for painting and erection of line conductors

The choice of line supports for a particular situation depends upon the line span, cross sectional area , line voltage , cost and local conditions.

Lattice Steel Towers
Narrow base lattice steel towers are used for transmission at 11 kV and 33 kV and broad base lattice steel towers are used for transmission purposes at 66 kV and above. The broad base towers are mechanically stronger and have got longer life.Due to robust construction long spans (300m and above) can be used and are much useful for crossing fields, valleys, railway lines, rivers etc.
● Capable of withstanding most severe climatic conditions ● Immune from destruction by forest fires ● Risk of service interruptions , due to broken or punctured insulators is considerably

reduced owing to use of larger spans ● Lightening troubles are also minimized as each tower is a lightening conductor Steel towers are fabricated from painted or galvanized angle sections which can be transported separately and the erection don on the site. The height of the tower depends on the line voltage and the length of the span. Steel towers can be broadly classified into Tangent towers Deviation towers Tangent towers can be used for straight runs on the lines and upto 2° line deviation from the straight runs. The base of such steel towers may be square or rectangular. Insulators with these towers are usually suspension types
1. 2.

For deviations exceeding 2° special angle towers sometimes called deviation towers are used. Such towers have broader base and stronger members as they are to withstand the resultant force due to change in direction in addition to the forces the tangent towers are subjected to. Insulators used with these towers are of strain type. The cost of deviation tower is comparatively more than tangent towers as it is designed to withstand heavy loading as compared to normal tangent towers.

Conductor Configuration

Several conductor congurations are possible, but three configurations are most common i.e Horizontal Vertical Triangular In most of the cases horizontal and vertical configurations are employed from mechanical considerations, particularly when suspension inulators are being used. In horizontal configuration, all the conductors are mounted over one cross-arm. Though such an arrangement of conductors ned upports of smaller height but needs wider right of way. In certain conjested area where it is not possible to have horizontal arrangement, the conductors are placed in vertical formation( along the length of pole one below another). The drawbacks of vertical formations are taller towers and more lightening hazards.
● ● ●

In our concerned Project, we are going to use Horizontal Configuration. In unsymmetrical arrangement of conductors , the conductors are usually transposed at regular intervals in order to balance the electrical characteristics of various phases and prevent inductive interference with neighboring communication circuits.

Conductor Spacing
The spacing of conductors is determined by considerations partly mechanical and partly electrical. Larger spaing causes increase in inductance and hence the voltage drop, so that to keep the latter within a reasonable value the conductors should be as close together as is consistent with prevention of corona. The basic consideration regarding the minimum spacing between conductors is that the electrical clearances between the conductors under the worst conditions i.e maxium temperature and wind pressure , shall not be les than the limits of safety , particularly at the mid-spans. Owing to the of gusts of the wind, conductor has got tendency to move around in an electrical path, therefore in case osf suspension insulators, the minimum clearance to the supporting structure should be calculated with a 45° swing of the suspension string towards the structure. An empirical formula commonly employed for determination of spacing of conductors for an aluminium line is given below Spacing = (√S + V/150) m, where S= sag V= line voltage in kV Some typical spacing value are given below… Line voltage in kV Spacing in metres 0.4 11 33 66 132 220 400 765

0.2

1.2

2.0

2.5

3.5

6.0

11.5

14

Conductor Clearances
Line voltage in kV Clearance to ground in metres

0.4

11

33

66

132

220

400

765

4.6

5.5

6.1

6.1

6.1

7.0

8.4

12.4

Span Lengths
As the span length increases, the number of insulators and supports decreases resulting in decrease in cost but at the same time the height of the support will go up to allow for more sag and also the length of cross arms will have to be increased to take up increased spacing, this will cause increase in cost. Moreover the length of depends upon the working voltage , higher the working voltage of the system, the greater will be the economic length of the span owing to the higher relative cost of insulators to supports. Moreover the insulators constitutes the weakest part of a transmission line and reduction in number of towers per km with the use of longer span increases the reliability of line . and the only way to determine it is to calculate the total cost per km for a number of different span lengths, and the plot results to get most economical span length. With lattice steel towers the span length varies from 200-400m and for river crossings etc. exceptionally long spans upto 800 m or so have been successfully employed. In our concerned project the working length of the span os chosen to be 400 metres.
Due to the weight of the bundled ACSR conductors, there is a large sag within the towers.

Sag Calculation

Sag (S) = Where, = weight of the conductor per metre L = length of the span T = ultimate tensile strength

The conductor we are using is ACSR bersimis whose values for mentioned parameters are as follows : = 2.187 kg L = 400 m T =146.87 kN = 15 kgf Considering a factor of safety = 2 Te = effective tensile strength = 7.5 kgf

For that sag comes out be

S = 5.832m

Considering ice and wind loading
Let us assume Wind loading= 40 kg/m2 Ice loading = 1cm thick S= r r= √(

L2/ 8T c +

2 i)

+

w

2

r= resultant c= I= w=

loading

conductor loading ice loading wind loading

In such conditions the sag comes out to be

S=10.97m

Line Geometry
Type of Structure Configuration Minimum Ground Clearance Horizontal Conductor Spacing Height of the Tower Tower Span Span Length Phase to Earth Wire Clearance Lattice Steel Towers Single Circuit 12.4 m 15.3 m 45.0 m 40 .0m 400 m 12.4 m

Calculation of Inductance & Capacitance
Consider a 3 phase line with conductors A,B and C; each with radius “r” metres let the spacing between them be d1 , d2 and d3 respectively and the current flowing through them is IA, IB, IC respectively. For a BALANCED SYSTEM

LA = 2* 10-7*[ loge + loge √ d1 d3 + j√3 loge LB =2* 10-7 * [loge + loge √ d1 d2 + j√3 loge LC = 2* 10-7* [loge + loge √ d3 d2 + j√3 loge

] H/m ] H/m ] H/m

Now, we have considered horizontal arrangement of conductors, so three conductors are in same plane. d1 = d2 = d and d3 = 2d thus the value of inductance comes out as follows LA =2* 10-7* [loge + 0.5 loge2 - j√3 loge2 ] H/m LB = 2* 10-7*[ loge ] H/m LC = 2* 10-7*[ loge + 0.5 loge2 +j√3 loge2 ] H/m

In our design, d= 15 m

r’ = .7788 Thus putting those values

LA = 20 [7.838 – j 0.6] µH/m LA = 20 [7.838 + j 0.6] µH/m

LC = 1.35 µH/m

Leq = 1.4 µH/m = 1.568 mH/ Km

Capacitance
CN = Ds = 1.09(r’s3)1/4 =20.468 m Dm = 3 √Dab*Dbc*Dca =18.896m Putting in the values in the expression for capacitance CN = capacitance to neutral = 12.296 m Ic = charging current = 2Πf CN = 1.7 mA/Km

Corona
Introduction
Corona on transmission lines causes power loss, radio and television interference, and audible noise near the transmission line. At extra-high-voltage (EHV) levels (at 345 kV and higher), the conductor itself is the major source of audio noise, radio interference (RI), television interference (TVI), and corona loss. RI is a noise type that occurs in the AM radio reception, including the standard broadcast band from 0.5 MHz to 1.6 MHz. It does not take place in the FM band. Radio noise (RI or TVI) is usually expressed in mV/m or in dB above 1 m V/ m. The effects of corona in EHV transmission lines depend on a number of parameters that may not remain constant over a period, and the contributions of each add to the effects in a complex manner. The determination of the disruptive critical voltage requires the assignment of average values for the conductor irregularity factor that may vary considerably with the weathering effects on the conductor. In the Peterson1 expression for the fair weather corona loss, the corona factor that is a function of the ratio of the operating voltage to the disruptive critical voltage may also vary depending upon the operating voltage. Similarly, the radio and television interference levels depend upon the variations in radial distance from the conductor to the antenna and the line height. Some parameters are known with good accuracy and may be taken as constant, whereas there are others which are affected by errors of evaluation and may vary with time. The usual interest consists in evaluating the disruptive critical voltage and the radio noise due to corona with respect to these variable parameters. Many approaches are available for considering these parameter changes including worst-case analysis, sensitivity analysis, and Monte Carlo simulation. In this paper, sensitivity analysis of the disruptive critical voltage, visual critical voltage, corona loss, and radio noise (RI and TVI) is performed.

Sensitivity Analysis
A sensitivity analysis for the disruptive critical voltage, visual critical voltage, corona loss, RI and TVI is carried out. The normalized sensitivities with respect to the parameters on which these quantities depend are evaluated. The normalized sensitivity of a quantity with respect to a particular parameter on which it depends gives the change percentage in the particular quantity for one percent change in the parameter being considered. Table 1 gives the nominal values of the parameters used in this paper for sensitivity analysis.

Irregulari ty Factor, mv

Pressure , p (cm, Hg)

Irregulari ty Factor, m0

Conducto r Radius, r (cm)

Spacing, D (cm)

Temperatur e, T (° C)

Oper ating Voltage,

Supply Frequenc y, f (Hz)

0.9

74

0.9

1.5

550

35

V 765

50

Disruptive Critical Voltage
A transmission line should operate just below the disruptive critical voltage in fair weather, so that, corona only takes place during adverse atmospheric conditions. Therefore, the calculated disruptive critical voltage is an indicator of the corona performance of the line. However, a high value of the disruptive critical voltage is not the only criterion of satisfactory corona performance. The sensitivity of the conductor to foul weather and the fact that corona increases more slowly on stranded conductors than on smooth conductors should also be considered. According to Peek conductors, after making allowance for surface condition of the conductor by using the irregularity factor, the expression for the disruptive critical voltage (V d) is, V0 = 21.1 dm0r ln( ) kV…….. (1) Where, V0 r, D, m 0, is the disruptive rms critical voltage to neutral, kVs radius of conductor in centimeters; spacing between two conductors, cm; irregularity factor (0 < m0 £1) (1 for smooth, polished solid, cylindrical conductors; 0.93.0.98 for weathered, solid, cylindrical conductors; 0.87.0.90 for weathered conductor with more than seven strands; and 0.80. 0.87 for weathered conductor with up to seven strands and is the air-density correction factor.

d = 560 kV The air-density factor is:

d = 3.9211 (

)……… (2) where, p is the barometric pressure in centimeters of mercury and t is the ambient temperature, °C .

= 0.96

Visual Critical Voltage

The expression for the visual critical voltage, Vv , given by Peek is:

Vv = 21.1 δ mv r [1 +

] ln( ) …… (3) where Vv is the visual critical voltage in kilovolts, rms; mv , irregularity factor for visual corona ( ) 0 1 < £ mv (1 for smooth, polished, solid, cylindrical conductors; .93.0.98 for local and general visual corona on weathered, solid, cylindrical conductors; 0.70.0.75 for local visual corona weathered stranded conductors; and 0.80.0.85 for general visual corona on weathered stranded conductors.

It may be noted that the voltage equations (1) and (3) are for fair weather. For wet weather voltage values, the resulting fair weather voltage values should be multiplied by 0.80. For a three-phase horizontal conductor configuration, the factors 0.96 and 1.06 should be multiplied with the calculated disruptive critical voltage for the middle conductor and for the two outer conductors, respectively. The normalized sensitivities of both V0 and Vv with respect to the pressure, irregularity factor, and conductor radius, respectively are high and those with respect to the spacing and temperature are low. The values of V0 and Vv increase with an increase in the value of all parameters except the temperature. These values decrease with an increase in the value of the temperature. The smoother the surface of a given conductor, the higher is the disruptive voltage. For the same diameter, a stranded conductor is usually satisfactory for about 80%-85% of a smooth conductor. The air density factor, and hence V0 and Vv depend on the barometric pressure. The barometric pressure in turn is a function of the altitude.

Corona Loss
According to Peterson, the expression for the fair weather corona loss per phase or conductor, Pc is:

Pc = where d = conductor diameter, cm; f = frequency in Hz V = line-to-neutral operating voltage in kV = 442 kV

F = corona factor determined by test and is a function of ratio of V to V0. = 86 kW/Km/line

Thus corona loss for total line length of 500 Kms comes out to be 43 MW. The sensitivity values with respect to the operating voltage and supply frequency are quite high and with respect to the conductor, radius and spacing are low. Furthermore, the sensitivity with respect to the spacing is negative. As the spacing increases, the corona loss decreases. The corona loss is proportional to the frequency of the supply voltage. Therefore, the higher the frequency, the higher is the corona loss.

Radio Interference
The radio interference is a noise type that occurs in the AM radio reception, including the standard broadcast band from 0.5 MHz to 1.6 MHz. It does not take place in the FM band. Radio noise (radio or TV interference) is usually expressed in millivolts per meter or in decibels above 1 m V/m. As conductors age, radio noise levels tend to decrease. The RN is measured adjacent to a transmission line by an antenna equipped with a radio noise meter. The standard noise meter operates at 1 MHz (in the standard AM broadcast band) with a bandwidth of 5 kHz. For measurements in the RI range, a rod antenna usually determines the electric field, E and a loop antenna usually determines the magnetic field component H. The approximate value of the RI can be determined from the following empirical formula:

RI = 50 + Κ(Εμ − 16.95) + 17.3686 ln(

) + Fn + 13.8949 + FFW

where RI = radio noise in decibels above 1 m V/m at 1MHz K = 3 for 750-kV class, 3.5 for others, gradient limits 15 kV/cm-19 kV/cm; Em = maximum electric field at conductor in kV (rms)/cm D = (sub) conductor diameter, cm Fn = -4dB for single conductor (4.3422 ln (n/4) for n > 1 N = number of conductors in bundle D = radial distance from conductor to antenna, m H = line height, m R = lateral distance from antenna to nearest phase, m

FFW = 17 for foul weather; and 0 for fair weather.

RI is more sensitive to the maximum electric field Em, but less so to the line height and the lateral distance from the antenna. Although the RI is least sensitive to the parameter R, the lateral distance from the antenna, this parameter is the one that can be adjusted to keep the RI within the specified limit.

Television Interference

In general, power line RN sources disturbing television reception are due to non-corona sources. Such power line interference in the VHF (30 MHz-300 MHz) and UHF (300 MHz3000 MHz) bands is usually caused by sparking. Since, the sparks usually short out during rain, sparking is a fair weather problem rather than a foul weather one. The expression for the foul weather TVI in terms of the RI of a transmission line is:

TVI = RI – 20log10 [

] + 3.2

Where, TVI = television interference, in decibels (quasipeak) above 1 m V/m at a frequency f, MHz RI = radio interference in decibels (quasi-peak) above 1 m V/m at 1 MHz and at standard reference location of 15 m laterally from outermost phase F = frequency, MHz H = is the height of closest phase, m

The sensitivity study of TVI is carried out for an antenna location at 15 m from the threephase 345 kV line and for a TV channel signal where the carrier frequency is 83.25 MHz. It is observed that the sensitivity value of TVI with respect to the electric field is very high and that with respect to the conductor radius is moderately high. As conductors age, radio noise (RI or TVI) levels tend to decrease. Since, corona is mainly a function of the potential gradients of the conductors and the RN is associated with the corona, the RN as well as corona will increase with higher voltage, other things being equal.

Tower Footing Resistance
A lightning flash generally consists of several strokes which are lower charges, negative or positive, from the cloud to the ground. The first stroke is most often more severe than the subsequent strokes. The positive lightning is about 5% with magnitude between 11 kA and 171 kA. But the negative lightning is about 95% with magnitude between -10 kA and - 139 kA. The most lightning magnitude is between -10 kA and -50 kA. When lightning strikes a tower, a traveling voltage is generated which travels back and forth along the tower, being reflected at the tower footing and at the tower top, thus raising the voltage at the cross-arms and stressing the insulators. The insulator will flashover if this transient voltage exceeds its withstand level (backflash). Backflash voltages are generated by multiple reflections along the struck tower and also along the shield wire for shield lines at the adjacent towers. The backflash voltage across insulator for the struck tower is not a straightforward. The peak voltage will be directly proportional to the peak current. In order to reduce the number of flashovers on the lines, there are different methods to improve the lightning performance of lines i.e. improving critical flashover of insulators, reducing grounding impedance, installing shield wire for lines without shield wire and installing lightning arresters. The tower footing resistance is one of factors effected the back flashover voltage across the insulator in transmission system.

Lightning Source Model
The magnitude of a current impulse due to a lightning discharge is a probability function. Low discharge levels between 5 to 22 kA may result in a higher tendency for the lightning strike to pass by any shield wires and directly hit a phase conductor. The larger lightning impulse currents may tend to strike the tower top and lead to a back flashover. This study typically used stroke front time = 1.2 μs, tail time = 50 μs as lightning source.

Footing Resistance Model
The tower footing resistance for fast front surges is not well understood. The impulse ground resistance is less than the measured or calculated resistance because significant ground currents cause voltage gradients sufficient to break down the soil around the ground rod. A variable grounding resistance approximation can be applied which is surge current dependent as in

RT = where RT is tower footing resistance (ohm), Rg is tower footing resistance at low current and low frequency (ohm), I is surge current into ground (kA), Ig is limiting current initiating soil ionization (kA).

Ig =

where ρ0 is soil resistivity (ohm-meter), Eo is soil ionization gradient (about 300 kV/m).

Thus for our system, the tower footing resistance comes out to be about

12 Ω

Insulation Co-ordination
Insulation coordination of a system is correlation of the insulation of various components in the power system to the insulation of protective devices used for the protection of those equipments against over-voltages. The problem of insulation coordination involves not only the protection of equipment but the protectin of protective devices too. To achieve this a lightening arrestor must be applied on the system in such a way that it will discharge excessive voltage safely to the ground very quickly nad then restore itself as an insulator and protect the equipment insulation The insulation requirements are determined by considering the following :
1. Highest power frequency system voltage( continuous)- AC network has different

nominal power-frequency voltage level e.g 3.3 kv, 6.6 kv, 11kv, 33kv, 66kv, 132kv, 220kv, 400kv, 765kv rms continuous at 50hz. During light load period the power frequency voltage at the receiving end of transmission line rises. In a well regulated system, the permissible system allowed is called the highest system voltage. Thus each nominal voltage has certain corresponding highest system voltage 3.6 kv, 7.2kv, 12kv, 36kv, 72.5kv, 145kv, 245kv, 420kv, 800kv respectively. Each of the equipments are designed and tested to withstand the corresponding highest power-frequency system voltage without internal/external insulation failure. 2. Temporary power-frequency overvoltages - caused by load throw-offs, faults, resonance etc. The temporary power frequency overvoltages are of lesser peak, lesser rate of rise and of longer duration. Reactors/CBs/CTs Transformers Transmission Line Basic Insulation Level (kVpeak) Basic Switching Level (kVpeak) 1950 2400 2400

1550

1550

1550

Power 830 830 Frequency(kVrms) 3. Transient overvoltages surges caused by lightning, witching , restriking travelling waves etc. surges in power system are of comparatively , high rate of rise and last for few micro-seconds. Surges can cause spark-over and flash-over between phase and earth at weakest point, breakdown of gaseous/liquid and solid insulation, failure of transformers and rotating electric machinery For the safe operation of the equipment , it should have insulation strength equal to or greater than the basic insulation level and the protective equipment for a station/substation should be chosen to give the insulation good protection corresponding to the working of these

equipments as economically as possible. Insulation coordination consists of selecting insulation of various lines and equipment that have to be interconnected into a system for desired operational requirement. The system must be reliable and economical. The I.E.C. and Indian standards (or other standards) have only recommended certain values or proposed levels for coordinating insulation. But as transmission voltages and equipment insulation levels vary at e.h.v. levels, and there exist more than one insulation level for major equipment. Thus, in high lightning-prone areas or in systems with heavy switching-surge conditions, the selection of insulation levels will be different from areas with little or no lightning and with shorter lines. Normally, insulation systems are designed in a system for no flashovers, or if flashovers cannot be prevented such flashovers should be restricted to places where damage is not done, such as air gaps or in gap-type arresters. The flash-over should not disturb normal system operation and must occur in resealable insulation structures. The overvoltages that can cause damage are due to external origin, namely lightning, and operation of the system itself which are at power frequency, earth faults, and switching operations. We will consider here the insulation co-ordination principles based on lightning. These insulation levels are known as Basic Impulse Insulation Levels or BIL. Those based on switching-surge requirements are known as Switching Impulse Levels or SIL. The lightning arrester is the foundation of protection in e.h.v. ranges, which is selected for both lightning and switching-surge duty. It is usually of the magnetic blow-out (current limiting) gap type, or in recent years the gapless ZnO type. This is the peak value of impulse voltage as determined by the higher of the 1.2/50 ms spark-over value of the gap or residual voltage for standard 8/20 ms surge current in the 10 kA to 20 kA range. The latter applies to gapless type while both the voltages apply to gap type arresters. The lightning current passing through the arrester material is calculated as follows. Consider a travellong wave of voltage Vw crest, which is a w on a line with surge impedance Z. I ccompanied by current They strike an arrester whose duty is to hold the voltage across it constant at the protective level Vp. Now, by using Thevenin’s theorem, with the arester terminals open, the incident travelling wave will g w due to total reflection. The Thevenin impedance looking a voltage 2V ive through the open arrester terminals is equal to the Surge I mpedance Z of he line.
Therefore, with the arrester connected, the current through it will be

Ia = The maximum value the travelling-wave voltage Vw can reach is the flashover voltage of the line insulation. Also, it is assumed that Vp stays fairly constant at all current values discharged by the arrester.

Probability of Occurrence of Lightning Stroke Currents

The probability or the fraction of strokes reaching the ground whose magnitude is above a given anticipated value must be ascertained for the region in which the line will be run. Such experiments use ‘magentic links’ connected near tower tops and conductors whose magnetization intensity will depend upon the crest value of current. A typical probability curve is presented here.

Lightning Arresters and Protective Characteristics
Lightning arresters, also called 'surge absorbers' because they are also meant for switchingsurge protection, protect primarily major equipment such as transformers, rotating machines, shunt reactors, and even entire substations. Less expensive protective devices such as rod gaps can be used for protection of transformer and circuit-breaker bushings and open contacts. When substations have to be protected, they are located at the entrance of the incoming and outgoing lines. Modern surge absorbers for e.h.v. levels are designed to offer protection to equipment and lines for both lightning overvoltages and currents as well as switching surge over-voltages where the energy involved is much higher. Their characteristics will be described later. Arresters are of three important types and classified according to their internal structure. They are
1. 2. 3.

Gap type arrester without current-limiting functions Gap type arrester with current-limiting capability Gapless metal oxide varistors.

The first is commonly known by trade names such as Thyrite, Magnavalve, Autovalve, Miurite, etc., each one being associated with its manufacturer. The non-linear resistance material is usually sintered Silicon-Carbide (SiC) and is designed to dissipate the energy in short duration lightning-stroke current and the current at power frequency that will follow this current when the series gap conducts. The current is finally interrupted at a power-frequency zero. The second, or the current-limiting gap type, is of North-American and European design in which a magnetic action on the arc between the gap creates a lengthening of the arc with consequent large resistance capable of limiting the current. In such a design, the powerfrequency current can be extinguished prior to reaching a current zero. Such an arrester can perform switchingsurge duty also. The non-linear resistance is still SiC. The last one, the gapless MOV, is of recent origin, having been patented only in 1968 by a Japanese firm for lowvoltage low-current electronic circuitary but now is sufficiently well developed to handle e.h.v. requirements.

Protective Ratio - The most important property of a surge absorber is the 'protective ratio' which is defined as : Np = Rated Arrester Power - frequency Voltage, R.M.S. value Peak Impulse Insulation Level of Protected Equipment The selection of an arrester with a specified voltage rating is governed by the value of 'earthing coefficient' or the 'earth-fault factor'. These are defined as follows and are based on a single line to ground fault conditon. EC = Line- to-line voltage at arrester location RMS Value of healthy phase voltage at arrester location...(9.14) The earth-fault factor, EFF = 3 ´EC ...(9.15) and uses the line-to-ground voltage in the denominator of equation (9.14). The Indian standards and the I.E.C. use the EFF but arresters are still known by the earthing coefficient value.

Surge Impedance Loading (SIL) The surge impedance loading or SIL of a transmission line is the MW loading of a transmission line at which a natural reactive power balance occurs. Transmission lines produce reactive power (MVAR) due to their natural capacitance. The amount of MVAR produced is dependent on the transmission line's capacitive reactance (XC) and the voltage (kV) at which the line is energized. In equation form the MVAR produced is:

Transmission lines also utilize reactive power to support their magnetic fields. The magnetic field strength is dependent on the magnitude of the current flow in the line and the line's natural inductive reactance (XL). It follows then that the amount of Mvar used by a transmission line is a function of the current flow and inductive reactance. In equation form the Mvar used by a transmission line is: A transmission line's surge impedance loading or SIL is simply the MW loading (at a unity power factor) at which the line's Mvar usage is equal to the line's Mvar production. In equation form we can state that the SIL occurs when:

If we take the square root of both sides of the above equation and then substitute in the formulas for XL (=2pfL) and XC (=1/2pfC) we arrive at:

The term in the above equation is by definition the "surge impedance. The theoretical significance of the surge impedance is that if a purely resistive load that is equal to the surge impedance were connected to the end of a transmission line with no resistance, a voltage surge introduced to the sending end of the line would be absorbed completely at the receiving end. The voltage at the receiving end would have the same magnitude as the sending end voltage and would have a phase angle that is lagging with respect to the sending end by an amount equal to the time required to travel across the line from sending to receiving end. The concept of surge impedance is more readily applied to telecommunication systems than to power systems. However, we can extend the concept to the power transferred across a transmission line. The surge impedance loading or SIL (in MW) is equal to the voltage squared (in kV) divided by the surge impedance (in ohms). In equation form:

The value of the SIL to a system operator is realizing that when a line is loaded above its SIL it acts like a shunt reactor - absorbing MVAR from the system - and when a line is loaded below its SIL it acts like a shunt capacitor - supplying MVAR to the system.

Surge Arresters
The traditional method to reduce switching overvoltages is to install pre-insertion resistors in parallel with transmission line circuit breakers. The method proposed in this paper is to use only one set of surge arresters and then looking for the optimum point of installation by switching from both ends. According to the extensive amount of simulations performed in this research work if a surge arrester is installed at a point along the transmission line, it results in a local minimum for the switching overvoltage at that point. Moreover, there always exist two maxima at both sides of the local minimum, one of which is absolute maximum while the other is local. The trend is to reduce the absolute maximum. The absolute maximum reaches a minimum level when the two maxima at either side of the arrester are equal. The point of arrester installation satisfying this condition is the optimum installation point for switching at one end. For switching at the other end, there is another optimum installation point which is not necessarily the same as to the previous point. The final optimum is a point between these two sub-optimums. If the maximum switching overvoltage for the final optimum point is below the Protection Level of the line it is acceptable. Otherwise, another method like two sets of surge arresters or controlled switching by switchsynch relays must be used.
● ● ● ● ● ● ● ●

Type of Arrester Nominal discharge current (kA) Rated Arrester voltage (kVrms) Continuous operating voltage (kVrms) Maximum residual voltage at lighting impulse current of 20 kA (kVpeak) Maximum switching impulse residual voltage at 2 kA (kVpeak) Maximum steep current impulse residual voltage at 20 kA (kVpeak) Dynamic over voltage withstand capability for 3 peaks 0.3 sec. (kVpeak) 0.1 sec. (kVpeak) 1.0 sec. (kVpeak) 10.0 sec. (kVpeak) Discharge capability A. Transmission line class B. Minimum energy capability kJ/kV Maximum radio interference/partial discharge at 508 kVrms (µV/ pC)

Gapless (Metal Oxide) 20 624 485 1430 1280 1630

1240 1045 920 865 4 28 250/50

●

●

Insulator Flashover and Withstand Voltages
Under positive polarity lightning impulses, a standard " 10" disc (14.6 cm × 25.4 cm) shows a highly linear characteristic between spark-over voltage and number of discs. In fair or dry weather conditions, they can be expressed by the following values:

Time to Breakdown, ms 0.5 1.0 2.0 3.0 4.0 6.0 8.0 kV/disc, crest 188 150 125 110 105 97.5 92.5 1.20/50 ms wave, kV/disc = 87.5 kV crest Power Frequency - In fair weather the flashover voltage is 75 kV/disc, crest value, or 53 kV/disc, r.m.s. value. A standard disc has leakage distance of 31.8 cm (12.5 inches) over its surface. The usual creepage strength used is 1 kV/cm of leakage distance, r.m.s. value. Strings in Parallel - When lightning hits a line, many insulators are stressed in parallel. It has been found in practice from outdoor experiments that under lightning-type of voltage with extremely small wave-front (1 – 2ms), this point is not important as it definitely is under longer surges such as the switching surge. Therefore, single-string values for voltage can be used for flashover and withstand strength. Conductor-conductor Flashover - Under positive lightning wave the following empirical relation based upon experimental work can be taken: Flashover voltage Vcc = 590 kV/metre, crest

Circuit Breakers
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Type Installation No. of poles No. of trip circuits Rated short circuit making current. kApeak Value of pre-insertion resistor (Ω)

Pre- insertion time (ms) Rated operating duty cycle Rated break time (ms) Closing time (ms) Difference in the instants of opening of contacts - within a pole (ms) - between poles (ms) 12. Difference in the instants of closing of contacts between poles (ms) 13. Rated line charging current

: SF6 : Outdoor : 3 : 2 (independent) : 100 : 450 To limit switching surge overvoltage to 1.9 p.u. : 10 : 0-.3 second-CO-3 min.-CO : 40 : 100 : 2.5 : 3.33 : 10

The Circuit Breakers that are commonly used in switchyards are the GL318

GL318 Technical Characteristics
Absolute pressures of the SF6 gas mixture The table below gives the absolute pressures of the SF6 gas mixture : Minimum admissible temperature up to SF6 gas mixture absolute pressure at 20°C(68°F) pre - Filling rated absolute pressure for the insulation pae - Alarm absolute pressure for the insulation (pme+0.03 MPa) pme - Minimal absolute pressure for the insulation -20°C

0.85 MPa 0.74 MPa 0.71 MPa

Mass of SF6 gas = 103 kg

Circuit--breaker GL318 with spring operating mechanism
Description The circuit--breaker is made up of three poles. Each pole is made up of two interrupting modules connected by an electric connection. Each interrupting module is controlled by an operating device.

Interrupting module
Description The interrupting module is comprised of two interrupting chambers (1) -- in a ceramic envelope - equipped at each end with a corona ring (4) and an HV terminal (5). The interrupting chambers are laid out horizontally and attached, at their base, to a common housing (6). This housing contains the mechanism used to transfer the operating movement to the mobile

contacts of both chambers. The interrupting can also be equipped with capacitors (7).

Support column
Consisting of two, three or four ceramic insulators, the support column (2) allows the circuit--breaker to be ground--insulated and it also encloses the operating tie--rod which is attached to the interrupting chamber’smoving contacts. The support column can also be equipped with a corona ring (13).

Housing of the mechanism
A housing (3) -- situated at the base of the column -- contains the lever and crank assembly and which operates the moving contact. The SF6 filling and monitoring device (8) is also situated on the housing.

Interrupting chamber
Quenching medium - The quenching medium is pressurized SF6 gas or, in particular instances, pressurized SF6+CF4 gas mixture. Interrupting principle - The interrupting chamber is of thermal blasting type, using the energy from the arc, with an auxiliary autopneumatic. Description The interrupting chamber has been designed in such a way as to increase the mechanical resistance of the working part and take advantage of the low wear rate of the contacts subjected to the arc in SF6. The working part is enclosed in a leak tight ceramic

envelope, providing insulation between the circuit-breaker input and output. The chamber is made up of the following elements :
Mark 1 2 4 Component Envelope Fixed contact Moving conact It is worked by the operating mechanism and contains the blasting device. Information Can have a long creepage distance, depending on the pollution level.

Pole operation

Interrupting principle In “CLOSED” position the current passes through : -- the terminal (1), -- the fixed contact support (2), -- the main contacts (3), -- the moving contact (4), -- the moving contact support (5), -- the common housing (6), and follows the opposite chain on the other chamber and the terminal (1)

Opening

When an electrical or manual opening order is given, the energy accumulated in the opening spring (7) is released. The insulating tie--rod (8), directly activated by the opening spring (7), transmits the movement to the transfer mechanism (9) inside the housing which causes the contacts to separate simultaneously in both chambers.

Molecular sieve The arc has been extinguished. The SF6 molecules, separated by the arc, are re-formed instantaneously. Residual gases left over from the interruption operation are adsorbed by the molecular sieve situated at the base of the pole. A few powdery compounds are deposited in the form of dust which is quite harmless for the circuit breaker.

Special case of small currents For small currents (example : operating no--load lines, transformers or capacitor banks), the thermal energy of the arc is too low to provide enough excess pressure. Consequently, the conventional autopneumatic effect developing in the volume(Vp) is used mainly, to blast the arc.

Closing
Description

When an electrical or manual closing order is given, the energy accumulated in the closing spring (15) -- situated inside the operating device -- is released. This energy is transmitted directly to the pole operating shaft (16), to ensure closing. The release of the energy stored up in the closing spring (15) causes displacement of the moving parts, and so the closing of the interrupting chambers and also reloads the opening spring (7).

Switchyard Layout
1. Switching Scheme

: Double bus Double breaker

2.

Clearance (Minimum)

Phase to earth Phase to Phase Sectional Clearnace

: 6.4 m : 10.0 m : 45 m

3.

Bay details Bay width ii. Bus size (Tentative) a. Rigid bus/equipment Interconnecting bus
i. b.

: 45 m : 6” IPS tubular Aluminium pipe

Strung bus/Crossover bus

: Quad all Aluminium Tarantulla

iii. Bay Dimensions

Low level (Above ground) (m) Bus level Equipment Interconnecting level Crossover bus level Span length
12 12 27 15

High level (Above ground) (m)
27 12 39 90

Capacitor Voltage Transformer
In high and extra high voltage transmission systems, capacitor voltage transformers (CVTs) are used to provide potential outputs to metering instruments and protective relays. In addition, when equipped with carrier accessories, CVTs can be used for power line carrier (PLC) coupling.

Description
The capacitor voltage transformer type CCV is a single-phase instrument transformer composed of a capacitor divider and an electromagnetic unit (EMU). The capacitor divider consists of 3 capacitors units. Each capacitor unit is composed of an assembly of capacitor elements connected in series. A capacitor element consists of two electrodes (aluminium foil), separated by a mixed dielectric (film – paper – film). The elements are impregnated with synthetic liquid. From the electrical point of view, the capacitor divider is composed of a high voltage capacitor C1 and an intermediate voltage capacitor C2. C2 is located in the bottom of the bottom unit.

The electromagnetic unit, connected between the intermediate voltage terminal and the earth terminal of the capacitor divider, comprises essentially: ● a medium voltage transformer ● a compensating inductance ● a damping device

. 1-

Primary terminal 2 - Cast aluminum bellow housing 3 - Stainless steel expansion bellow 4 - Compression spring 5 - Insulated voltage connection 6 - Capacitor elements 7 - Insulator (porcelain or composite) 8 Voltage divider tap connection 9 - Cast-epoxy bushing 10 - HF terminal connection 11 - Ferro-resonance suppression device 12 - Secondary terminals 13 - Oil level sight-glass 14 - Aluminum terminal box 15 Intermediate transformer 16 - Oil/air block 17 - Oil sampling device 18 - Compensating reactor 19 -

Aluminum cover plate

Capacitor Stack
The capacitor stack is a voltage divider which provides a reduced voltage at the intermediate voltage bushing for a given voltage applied at the primary terminal. The capacitor stack is a multi-capacitor-unit assembly. Each unit is housed in an individual insulator. A cast aluminum cover is on top of the upper capacitor assembly and is fitted with an aluminum terminal. An adapter for mounting a line trap on top of the CVT can be provided with an optional (and removable) HV terminal. The capacitor units are mechanically coupled together by means of stainless steel hardware passing through the corrosion resistant cast aluminum housing. The mechanical connection also establishes the electrical connection between capacitor units. This facilitates field assembly of the CVT. Each capacitor unit is hermetically sealed; a stainless steel diaphragm (expansion bellow) preserves oil integrity by maintaining the hermetic seal while allowing for thermal expansion and contraction of the oil. The capacitor units operate in a practically pressure-free mode over a very wide ambient temperature range. The capacitor stack consists of a series of capacitorbelements. The dielectric spacers are a combination of kraft paper and polypropylene film. The ratio of paper/film is carefully determined to provide constant capacitance for a wide range of operating temperature. The aluminum electrodes are precision wound by microprocessor controlled machinery. The capacitor elements are connected with low inductance tinned copper tabs. The stack assemblies are hydraulically compressed and bound with epoxy fiberglass tape to obtain the optimum space factor for capacitance requirement and oil circulation. After assembly in the insulator, capacitor units are individually oven dried under vacuum and then impregnated with the processed synthetic oil.

Electromagnetic Unit (EMU)
The EMU is immersed in insulating oil and enclosed in an airtight metallic tank. The bottom capacitor unit is installed on the cover of the tank and is connected to the EMU through a tight bushing. The medium and top capacitor units are assembled on the bottom capacitor unit and connected in series. The adjustments of the transformer and the inductance with the capacitor divider are carried out in factory to answer the required specifications and do not have to be modified. The EMU steps down the intermediate voltage provided by the voltage divider to value suitable for relay andmetering applications. A series reactance cancels the phase shift induced during voltage transformation in the capacitor voltage divider. A set of internal taps is used for factory accuracy and phase angle adjustments to provide optimum performance. Over-voltage protection is provided by a protective gap connected in parallel to the series reactances. The inherent capacitance and iron-cored EMU of a CVT require the suppression of ferro-resonance.

The ferro-resonance suppression device (FSD) contains a saturable reactor, which acts like a switch, presenting a very high impedance under normal conditions and switching on a damping resistor across the secondary at a prescribed voltage, and switching off the damping load when voltage has normalized. The voltage sensitive switching strategy effectively suppresses ferroresonance without imposing a heavy permanently connected stabilizing burden on the unit, significantly improving the accuracy and the transient response performance of the CVT. No field adjustment of the unit is necessary. The EMU is housed in a cast aluminum base tank with a cast aluminum cover. The base tank is filled with treated mineral oil and hermetically sealed from the environment and from the synthetic oil in the capacitor units. A sight glass at the rear of the tank provides for easy oil level monitoring. No oil maintenance is necessary throughout the service life of the unit. An oil drain plug is provided on the base tank. No field adjustment of the unit is necessary.

Insulating Oil
We use insulating oils with excellent dielectric strength, ageing, and gas absorbing properties. The synthetic oil used for the capacitor units possesses superior gas absorption properties resulting in exceptionally low partial discharge with high inception/extinction voltage ratings. The oil used for the EMU is premium naphthenic mineral oil. The oil is filtered, vacuum dried and degassed with inhouse processing. It contains no PCB.

INSTRUMENT TRANSFORMERS
Current Transformers
Rated primary currents Rated secondary current Number of cores Rated continuous thermal current Parameter Core I Line/ Equipment Utilistation Protection Main- I 2000- Transformation ratio 1000- 500/1
1. 2. 3. 4. 5.

Accuracy Class Rated Burden Min.knee point voltage Vk Maximum Exciting Current at Rated Vk/2 Maximum Secondary Winding Resistance.

PS - 4000- 2000- 1000 volts 10-50-100 mA 15-5-2.5 ohm

A : 2000 A : 1 : 5 : 120% Core II Core III Line/ Equipment Metering Protection Main- II 2000- 2000- 1000- 1000- 500/1 500/1 0.2 PS ISF