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MODULE II
Corona
When an alternating potential difference is applied across two conductors whose spacing is large as compared to their diameters, there is no apparent change in the condition of atmospheric air surrounding the wires if the applied voltage is low. However, when the applied voltage exceeds a certain value, called critical disruptive voltage, the conductors are surrounded by a faint violet glow called corona.
The phenomenon of corona is accompanied by a hissing sound, production of ozone, power loss and radio interference. Electric power transmission practically deals in the bulk transfer of electrical energy, from generating stations situated many kilometers away from the main consumption centers or the cities. For this reason the long distance transmission cables are of utmost necessity for effective power transfer, which in-evidently results in huge losses across the system. Minimizing those has been a major challenge for power engineers of late and to do that one should have a clear understanding of the type and nature of losses. One of them being the corona effect in power system , which has a predominant role in reducing the efficiency of EHV(extra high voltage lines) which we are going to concentrate on, in this article. When an alternating current is made to flow across two conductors of the transmission line whose spacing is large compared to their diameters, then air surrounding the conductors (composed of ions) is subjected to dielectric stress. At low values of supply end voltage, nothing really occurs as the stress is too less to ionize the air outside. But when the potential difference is made to increase beyond some threshold value of around 30 kV known as the critical disruptive voltage , then the field strength increases and then the air surrounding it experiences stress high enough to be dissociated into ions making the atmosphere conducting. This results in electric discharge around the conductors due to the flow of these ions, giving rise to a faint luminescent glow, along with the hissing sound accompanied by the liberation of ozone, which is readily identified due to its characteristic odor. This phenomenon of electrical discharge occurring in transmission line for high values of voltage is known as the corona effect in power system. If the voltage across the lines is still increased the glow becomes more and more intense along with hissing noise, inducing very high power loss into the system which must be accounted for.
Factors Affecting Corona Effect in Power System
As mentioned earlier, the line voltage of the conductor is the main determining factor for corona in transmission lines, at low values of voltage (lesser than critical disruptive voltage) the stress on the air is too less to dissociate them, and hence no electrical discharge occurs. Since with increasing voltage corona effect in a transmission line occurs due to the ionization of atmospheric air surrounding the cables, it is mainly affected by the conditions of the cable as well as the physical state of the atmosphere. Let us look into these criterion now with greater details :
Atmospheric Conditions for Corona in Transmission Lines
It has been physically proven that the voltage gradient for di-electric breakdown of air is directly proportional to the density of air. Hence in a stormy day, due to continuous air flow the number of ions present surrounding the conductor is far more than normal, and hence its more likely to have electrical discharge in transmission lines on such a day, compared to a day with fairly clear weather. The system has to designed taking those extreme situations into consideration.
Condition of Cables for Corona in Transmission Line.
This particular phenomena depends highly on the conductors and its physical condition. It has an inverse proportionality relationship with the diameter of the conductors. i.e. with the increase in diameter, the effect of corona in power system reduces considerably.
Also the presence of dirt or roughness of the conductor reduces the critical breakdown voltage, making the conductors more prone to corona losses. Hence in most cities and industrial areas having high pollution, this factor is of reasonable importance to counter the ill effects it has on the system.
Spacing between Conductors
As already mentioned, for corona to occur effectively the spacing between the lines should be much higher compared to its diameter, but if the length is increased beyond a certain limit, the
of g (^) o .The value of g (^) o is directly proportional to air density. Thus the breakdown strength of air at a barometric pressure of b cm of mercury and temperature of tºC becomes δg 0 where
δ = air density factor = 273 t
Under standard conditions, the value of δ = 1.
Critical disruptive voltage, (^) V (^) c g 0 r ln^ dr
Correction must also be made for the surface condition of the conductor. This is accounted for by multiplying the above expression by irregularity factor mo.
Critical disruptive voltage, r V g mr^ d c ^0 0 ln kV/phase
where m (^) o = 1 for polished conductors = 0·98 to 0·92 for dirty conductors = 0·87 to 0·8 for stranded conductors
(ii) Visual critical voltage It is the minimum phase-neutral voltage at which corona glow appears all along the line conductors. It has been seen that in case of parallel conductors, the corona glow does not begin at the disruptive voltage Vc but at a higher voltage Vv, called visual critical voltage. The phase- neutral effective value of visual critical voltage is given by the following empirical formula
r
d r V (^) v mvg 0 r ( 1 0.^3 )ln kV/phase
where mv is another irregularity factor having a value of 1·0 for polished conductors and 0·72 to 0·82 for rough conductors. (iii) Power loss due to corona Formation of corona is always accompanied by energy loss which is dissipated in the form of light, heat, sound and chemical action. When disruptive voltage is exceeded, the power loss due to corona is given by:
P 241 105 f^25 dr V Vc ^2
^
kw/km/phase
Advantages and Disadvantages of Corona Corona has many advantages and disadvantages. In the correct design of a high voltage overheadline, a balance should be struck between the advantages and disadvantages. Below are the Advantages and disadvantages of Corona. Advantages Due to corona formation, the air surrounding the conductor becomes conducting and hence virtual diameter of the conductor is increased. The increased diameter reduces the electro- static stresses between the conductors.
Corona reduces the effects of transients produced by surges.
Disadvantages Corona is accompanied by a loss of energy. This affects the transmission efficiency of the line.
Ozone is produced by corona and may cause corrosion of the conductor due to chemical action.
The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal Voltage drop occurs in the line. This may cause inductive interference with neighboring Communication lines. Methods to reduce Corona Discharge Effect Corona can be avoided
- By minimizing the voltage stress and electric field gradient.: This is accomplished by using utilizing good high voltage design practices, i.e., maximizing the distance between conductors that have large voltage differentials, using conductors with large radii, and avoiding parts that have sharp points or sharp edges.
- Surface Treatments: Corona inception voltage can sometimes be increased by using a surface treatment, such as a semiconductor layer, high voltage putty or corona dope.
- Homogenous Insulators: Use a good, homogeneous insulator. Void free solids, such as properly prepared silicone and epoxy potting materials work well.
Insulating Material
The main cause of failure of overhead line insulator, is flash over, occurs in between line and earth during abnormal over voltage in the system. During this flash over, the huge heat produced by arcing, causes puncher in insulator body. Viewing this phenomenon the materials used for electrical insulator, has to posses some specific properties.
Properties of Insulating Material
The materials generally used for insulating purpose is called insulating material. For successful utilization, this material should have some specific properties as listed below-
- It must be mechanically strong enough to carry tension and weight of conductors.
- It must have very high dielectric strength to withstand the voltage stresses in High Voltage system.
- It must possesses high Insulation Resistance to prevent leakage current to the earth.
- The insulating material must be free from unwanted impurities.
- It should not be porous.
- There must not be any entrance on the surface of electrical insulator so that the moisture or gases can enter in it.
- There physical as well as electrical properties must be less affected by changing temperature.
There are mainly three types of insulator used as overhead insulator likewise
- Pin Insulator
- Suspension Insulator
- Strain Insulator In addition to that there are other two types of electrical insulator available mainly for low voltage application i.e. Stray Insulator and Shackle Insulator. Pin Insulator Pin Insulator is earliest developed overhead insulator , but still popularly used in power network up to 33KV system. Pin type insulator can be one part, two parts or three parts type, depending upon application voltage. In 11KV system we generally use one part type insulator where whole pin insulator is one piece of properly shaped porcelain or glass. As the leakage path
of insulator is through its surface, it is desirable to increase the vertical length of the insulator surface area for lengthening leakage path. In order to obtain lengthy leakage path, one, tower or more rain sheds or petticoats are provided on the insulator body. In addition to that rain shed or petticoats on an insulator serve another purpose. These rain sheds or petticoats are so designed, that during raining the outer surface of the rain shed becomes wet but the inner surface remains dry and non-conductive. So there will be discontinuations of conducting path through the wet pin insulator surface.
Fig 2.1- Pin Insulator
In higher voltage like 33KV and 66KV manufacturing of one part porcelain pin insulator becomes difficult. Because in higher voltage, the thickness of the insulator become more and a quite thick single piece porcelain insulator cannot manufactured practically. In this case we use multiple part pin insulator, where a number of properly designed porcelain shells are fixed together by Portland cement to form one complete insulator unit. For 33KV tow parts and for 66KV three parts pin insulator are generally used.
Designing Consideration of Electrical Insulator
The live conductor attached to the top of the pin insulator is at a potential and bottom of the insulator fixed to supporting structure of earth potential. The insulator has to withstand the potential stresses between conductor and earth. The shortest distance between conductor and earth, surrounding the insulator body, along which electrical discharge may take place through air, is known as flash over distance.
the greater demand by raising the line voltage than to provide another set of conductors. The additional insulation required for the raised voltage can be easily obtained in the suspension arrangement by adding the desired number of discs. (vi) The suspension type insulators are generally used with steel towers. As the conductors run below the earthed cross-arm of the tower, therefore, this arrangement provides partial protection from lightning. Disadvantages of Suspension Insulator
- Suspension insulator string costlier than pin and post type insulator.
- Suspension string requires more height of supporting structure than that for pin or post insulator to maintain same ground clearance of current conductor.
- The amplitude of free swing of conductors is larger in suspension insulator system, hence, more spacing between conductors should be provided.
Strain Insulator
When suspension string is used to sustain extraordinary tensile load of conductor it is referred as string insulator. When there is a dead end or there is a sharp corner in transmission line, the line has to sustain a great tensile load of conductor or strain. A strain insulator must have considerable mechanical strength as well as the necessary electrical insulating properties.
Fig 2.2- Strain Insulator
Shackle Insulator or Spool Insulator
The shackle insulator or spool insulator is usually used in low voltage distribution network. It can be used both in horizontal and vertical position. The use of such insulator has decreased recently after increasing the using of underground cable for distribution purpose. The tapered hole of the spool insulator distributes the load more evenly and minimizes the possibility of breakage when heavily loaded. The conductor in the groove of shackle insulator is fixed with the help of soft binding wire.
POTENTIAL DISTRIBUTION OVERA STRING OF SUSPENSION INSULATORS: A string of suspension insulators consists of a number of porcelain discs connected in series through metallic links. Fig. 2.3(i) shows 3-disc string of suspension insulators. The porcelain portion of each disc is in between two metal links. Therefore, each disc forms a capacitor C as shown in Fig.2.3(ii). This is known as mutual capacitance or self-capacitance. If there were mutual capacitance alone, then charging current would have been the same through all the discs and consequently voltage across each unit would have been the same i.e., V/3 as shown in Fig. 2.3(ii). However, in actual practice, capacitance also exists between metal fitting of each disc and tower or earth. This is known as shunt capacitance C1. Due to shunt capacitance, charging current is not the same through all the discs of the string [See Fig 2.3(iii)]. Therefore, voltage across each disc will be different. Obviously, the disc nearest to the line conductor will have the maximum voltage. Thus referring to Fig 2.3(iii), V 3 will be much more than V 2 or V 1.
The following points may be noted regarding the potential distribution over a string of suspension insulators: (i) The voltage impressed on a string of suspension insulators does not distribute itself uniformly across the individual discs due to the presence of shunt capacitance. (ii) The disc nearest to the conductor has maximum voltage across it. As we move towards the cross-arm, the voltage across each disc goes on decreasing. (iii) The unit nearest to the conductor is under maximum electrical stress and is likely to be punctured. Therefore, means must be provided to equalize the potential across each unit.
string efficiency is an ideal case for which the voltage across each disc will be exactly the same. Although it is impossible to achieve 100% string efficiency, yet efforts should be made to improve it as close to this value as possible. Mathematical expression. Fig. 2.3(iii) shows the equivalent circuit for a 3-disc string. Let us suppose that self capacitance of each disc is C. Let us further assume that shunt capacitance C1 is some fraction K of self capacitance i.e., C1 = KC. Starting from the cross-arm or tower, the voltage across each unit is V1,V2 and V3 respectively as shown. Applying kirchoff’s current law to node A I (^) 2 I 1 i 1 V 2 C V 1 C V 1 C 1
V 2 C V 1 C V 1 KC
V 2 V 1 ( 1 K )
Applying kirchoff’s current law to node B I (^) 3 I 2 i 2 V 3 C V 2 C ( V 1 V 2 ) C 1 V 3 C V 2 C ( V 1 V 2 ) KC V 3 (^) KV 1 V 2 ( 1 K ) V 3 (^) V 1 ( 1 3 K K^2 )
% 3 100 3
stringefficiency (^) VV
The following points may be noted from the above mathematical analysis: (i) If K = 0·2 (Say), then we get, V 2 = 1·2 V 1 and V 3 = 1·64 V 1. This clearly shows that disc nearest to the conductor has maximum voltage across it; the voltage across other discs decreasing progressively as the cross-arm in approached. (ii) The greater the value of K (= C1/C), the more non-uniform is the potential across the discs and lesser is the string efficiency. (iii) The inequality in voltage distribution increases with the increase of number of discs in the string. Therefore, shorter string has more efficiency than the larger one
String Efficiency and methods to improve String Efficiency The ratio of voltage across the whole string to the product of number of discs and the voltage across the disc nearest to the conductor is known as string efficiency i.e.,
n Voltageacrossdiscnear toconductor String Efficiency Voltageacrossthestring
where n = number of discs in the string.
String efficiency is an important consideration since it decides the potential distribution along the string. The greater the string efficiency, the more uniform is the voltage distribution. Thus 100% string efficiency is an ideal case for which the voltage across each disc will be exactly the same. Although it is impossible to achieve 100% string efficiency, yet efforts should be made to improve it as close to this value as possible.
Methods of Improving String Efficiency
The maximum voltage appears across the insulator nearest to the line conductor and decreases equalise the potential across the various units of the string i.e. to improve the string efficiency progressively as the cross arm is approached. If the insulation of the highest stressed insulator (i.e. nearest to conductor) breaks down or flash over takes place, the breakdown of other units will take place in succession. The various methods for improving the string efficiency are:
- By using longer cross-arms. The value of string efficiency depends upon the value of K i.e., ratio of shunt capacitance to mutual capacitance. The lesser the value of K, the greater is the string efficiency and more uniform is the voltage distribution. The value of K can be decreased by reducing the shunt capacitance. In order to reduce shunt capacitance, the distance of conductor from tower must be increased i.e., longer cross-arms should be used. However, limitations of cost and strength of tower do not allow the use of very long cross- arms. In practice, K = 0·1 is the limit that can be achieved by this method.
- By grading the insulators. In this method, insulators of different dimensions are so chosen that each has a different capacitance. The insulators are capacitance graded i.e. they are
Since a conductor is a flexible object with uniform weight per unit length, the geometric shape of a conductor strung on towers approximates that of a catenary. The sag of the conductor (vertical distance between the highest and lowest point of the curve) varies depending on the temperature and additional load such as ice cover. A minimum overhead clearance must be maintained for safety. Since the temperature of the conductor increases with increasing heat produced by the current through it, it is sometimes possible to increase the power handling capacity (uprate) by changing the conductors for a type with a lower coefficient of thermal expansion or a higher allowable operating temperature.
BUNDLE CONDUCTORS
For higher amounts of current, bundle conductors are used for several reasons. Due to the skin effect, for larger conductors, the current capacity does not increase proportional to the cross- sectional area; instead, it is only with the linear dimension. Also, reactance decreases only slowly with size. But the cost and weight do increase with area. Due to this, several conductors in parallel become more economical.
Bundle conductors consist of several parallel cables connected at intervals by spacers, often in a cylindrical configuration. The optimum number of conductors depends on the current rating, but typically higher-voltage lines also have higher current. There is also some advantage due to lower corona loss. American Electric Power is building 765 kV lines using six conductors per phase in a bundle. Spacers must resist the forces due to wind, and magnetic forces during a short- circuit.
Advantages At extra high voltage, the electric field gradient at the surface of a single conductor is high enough to ionize air, which loses power and generates both audible noise and interference with communication systems. The field surrounding a bundle of conductors is similar to the field that would surround a single, very large conductor—this produces lower gradients which mitigates issues associated with high field strength. When transmitting alternating current, bundle conductors also avoid the reduction in capacity of a single large conductor due to the skin effect. A bundle conductor also has lower reactance, compared to a single conductor. Additionally, bundled conductors cool themselves more efficiently due to the increased surface area of the conductors, further reducing line losses.
MECHANICAL DESIGN OF TRANSMISSION LINE
Sag in Overhead Transmission Line:
While erecting an overhead line, it is very important that conductors are under safe tension. If the conductors are too much stretched between supports in a bid to save conductor material, the stress in the conductor may reach unsafe value and in certain cases the conductor may break due to excessive tension. In order to permit safe tension in the conductors, they are not fully stretched but are allowed to have a dip or sag. The difference in level between points of supports and the lowest point on the conductor is called sag. Following Fig. 8.1 shows a conductor suspended between two equal level supports A and B. The conductor is not fully stretched but is allowed to have a dip. The lowest point on the conductor is O and the sag is S.
Fig 2.4- Sag in a transmission line The following points may be noted: (i) When the conductor is suspended between two supports at the same level, it takes the shape of catenary. However, if the sag is very small compared with the span, then sag-span curve is like a parabola. (ii) The tension at any point on the conductor acts tangentially. Thus tension T 0 at the lowest Point O acts horizontally as shown in Fig. (ii). (iii) The horizontal component of tension is constant throughout the length of the wire. (iv) The tension at supports is approximately equal to the horizontal tension acting at any point on the wire. Thus if T is the tension at the support B, then T = T 0. Conductor sag and tension. This is an important consideration in the mechanical design of overhead lines. The conductor sag should be kept to a minimum in order to reduce the conductor material required and to avoid extra pole height for sufficient clearance above ground level. It is also desirable that tension in the conductor should be low to avoid the mechanical failure of conductor and to permit the use of less strong supports. However, low conductor tension
(a) The weight wx of conductor acting at a distance x/2 from O. (b) The tension T acting at O. Equating the moments of above two forces about point O, we get,
2 Ty wx ^ x
T
y wx 2
2
The maximum dip (sag) is represented by the value of y at either of the supports A & B.
At supports A 2 x ^ l and y=s
8T
SagS wl^2
Effect of wind and ice loading - The above formulae for sag are true only in still air and at normal temperature when the conductor is acted by its weight only. However, in actual practice, a conductor may have ice coating and simultaneously subjected to wind pressure. The weight of ice acts vertically downwards i.e., in the same direction as the weight of conductor. The force due to the wind is assumed to act horizontally i.e., at right angle to the projected surface of the conductor. Hence, the total force on the conductor is the vector sum of horizontal and vertical forces as shown in
Fig 2.6- Effect of Ice and Wind Total weight of conductor per unit length is
wt ( w wi )^2 ww^2
Where w = weight of conductor per unit length = conductor material density × volume per unit length
wi = weight of ice per unit length = density of ice * volume of ice per unit length
= density of ice x 2 1
d t (^2) d (^2)
ww = wind force per unit length = wind pressure per unit area × projected area per unit length
= wind pressure x d 2 t 1
Vibration Damper Aeolian vibrations mostly occur at steady wind velocities from 1 to 7 m/s. With increasing wind turbulences the wind power input to the conductor will decrease. The intensity to induce vibrations depends on several parameters such as type of conductors and clamps, tension, span length, topography in the surrounding, height and direction of the line as well as the frequency of occurrence of the vibration induced wind streams. In the wake of wind power plants (up to 3 x diameter of the rotor behind the plant) the wind velocity will be reduced up to 0,5 of the velocity of the free wind stream, so that lower wind velocities could be expected more frequently here. That’s why the probability of a higher stresses for the conductors caused by wind-induced vibrations will be greater than without wind power plants. On the other hand the intensity of turbulences will increase which will hinder the arising of vibrations. The both important parameters for inducing vibrations, wind velocity and turbulence intensity, depends on the distance to the rotor and the height of it. The investigations showed an increasing of damage probability on OHTL due to the wake of wind power plants of the factor 2,5 to 3,5 between one and three rotor diameters behind the plant which will cause an equivalent decreasing of lifetime of conductors and earth wires. Stringing chart : For use in the field work of stringing the conductors, temperature-sag and temperature tension charts are plotted for the given conductor and loading conditions. Such curves are called stringing charts. These charts are very helpful while stringing overhead lines.
