12el32

Haresh Kumar (12El32) Dr. Syed Asif Ali Shah

QNO: 01. Differentiate between the percent and per unit system of representing of power system? Draw one line diagram showing the complete layout of power system by mentioning its each part.

Per unit System

Per-unit system is the expression of system quantities as fractions of a defined base unit quantity. Calculations are simplified because quantities expressed as per-unit do not change when they are referred from one side of transformer to the other. This can be a pronounced advantage in power system analysis where large numbers of transformers may be encountered. Moreover, similar types of apparatus will have the impedances lying within a narrow numerical range when expressed as a per-unit fraction of the equipment rating, even if the unit size varies widely. Conversion of per-unit quantities to volts, ohms, or amperes requires knowledge of the base that the per-unit quantities were referenced to.

Percentage System

In mathematics, a percentage is a number or ratio expressed as a fraction of 100. It is often denoted using the percent sign, "%", or the abbreviation "pct."

Percentages are usually used to express values between zero and one. However, it is possible to express any ratio as a percentage; for example, 111% is 1.11 and −35% is −0.35.

One Line Diagram


QNO: 02. What factors are considered while designing of transmission line briefly explain each following factors along with its justification?

1)Voltage level

Low voltage (LV) – less than 1000 volts, used for connection between a residential or small commercial customer and the utility.

Medium voltage (MV; distribution) – between 1000 volts (1 kV) and to about 33 kV, used for distribution in urban and rural areas.

High voltage (HV; sub transmission less than 100 kV; sub transmission or transmission at voltage such as 115 kV and 138 kV), used for sub-transmission and transmission of bulk quantities of electric power and connection to very large consumers.

Extra high voltage (EHV; transmission) – over 230 kV, up to about 800 kV, used for long distance, very high power transmission.

Ultra high voltage (UHV) – higher than 800 kV.

2)Conductor Type and Size

Types of Conductors:

There are four major types of overhead conductors used for electrical transmission and distribution.

AAC - All Aluminum Conductor

AAAC - All Aluminum Alloy Conductor

ACSR - Aluminum Conductor Steel Reinforced

ACAR - Aluminum Conductor Aluminum-Alloy Reinforced

For 36 kV transmissions and above both aluminum conductor steel reinforced (ACSR) and all aluminum alloy conductor (AAAC) may be considered. Aluminum conductor alloy reinforced (ACAR) and all aluminum alloy conductors’ steel reinforced (AACSR) are less common than AAAC and all such conductors may be more expensive than ACSR.

3)Voltage regulation In electrical engineering, particularly power engineering, voltage regulation is a measure of change in the voltage magnitude between the sending and receiving end of a component, such as a transmission or distribution line. Voltage regulation describes the ability of a system to provide near constant voltage over a wide range of load conditions. The term may refer to a passive property that result in more or less voltage drop under various load conditions, or to the active intervention with devices for the specific purpose of adjusting voltage.


4)Line regulation

Line regulation is the capability to maintain a constant output voltage level on the output channel of a power supply despite changes to the input voltage level.

5)Corona and Losses

When current is flowing through a conductor, the air surrounding the conductor gets ionized due to which some power loss occurs in the system. This phenomenon is known as corona loss and it can be explained more accurately. Besides air molecules, some free electrons are also present in the atmosphere. Now, when the potential across the conductors are increased, the voltage Gradient surrounding the conductor are also increases. Now for the electric field generated around the conductors, these free electrons get acceleration and move with a certain velocity. These moving electrons collide with the molecules present in the air and free some electrons from the outer radius of those molecules. And after some time, the air surrounding the conductor gets ionized and finally an electron avalanche takes place and a flash over may occur. Though this case is very rare because of the design of the Transmission line. This phenomenon of ionization of air surrounding the conductors is known as Corona loss.

6)Power load flow and System Stability

In power engineering, the power-flow study, also known as load-flow study, is an important tool involving numerical analysis applied to a power system. A power-flow study usually uses simplified notation such as a one-line diagram and per-unit system, and focuses on various forms of AC power (i.e.: voltages, voltage angles, real power and reactive power). It analyzes the power systems in normal steady-state operation. A number of software implementations of power-flow studies exist.

In addition to a power-flow study, sometimes called the base case, many software implementations perform other types of analysis, such as short-circuit fault analysis, stability studies (transient & steady-state), unit commitment and economic dispatch.In particular, some programs use linear programming to find the optimal power flow, the conditions which give the lowest cost per kilowatt hour delivered.

Power-flow or load-flow studies are important for planning future expansion of power systems as well as in determining the best operation of existing systems. The principal information obtained from the power-flow study is the magnitude and phase angle of the voltage at each bus, and the real and reactive power flowing in each line.

7)Load flow

A load flow study is especially valuable for a system with multiple load centers such a refinery complex. The power flow study is an analysis of the system’s capability to adequately supply the connected load. The total system losses, as well as individual line losses, also are tabulated. Transformer tap positions are selected to insure the correct voltage at critical locations such as motor control centers. Performing load flow study on an existing system provides insight and


recommendations as to the system operation and optimization of control settings to obtain maximum capacity while minimizing the operating costs. By this analysis we get the result of active power, reactive power, magnitude and phase angle.

8)System Protection

System protection is the art and science of detecting problems with power system components and isolating these components. Problems on the power system include:

1. Short circuits

2. Abnormal conditions

3. Equipment failures

9)Grounding

A high voltage system is designed for symmetric balance between impedances in the three phases and currents and voltages during normal operation. The neutrals of transformers and generators do not have any potential during normal operation, but in case of a fault with asymmetry or induced voltages the neutral has to be properly grounded. Grounding of system neutral has two important functions. It provides a reference for the electrical system and all components bonded to the system to ground. Grounding of neutral also establishes a current return path in case of a failure somewhere in the system so the fault can be detected. The neutral can be grounded in different ways and the method of neutral grounding will affect the fault currents and voltages within the system.

10)Insulation Coordination

Insulation Coordination in Power System was introduced to arrange the electrical insulation levels of different components in the electrical power system including transmission network, in such a manner.When any over voltage appears in the electrical power system, then there may be a chance of failure of its insulation system. Probability of failure of insulation is high at the weakest insulation point nearest to the source of over voltage. In power system and transmission networks, insulation is provided to the all equipment and components. Moreover failure of insulator at these points may cause bigger part of electrical network to be out of service. So it is desirable that in situation of insulator failure, only the easily replaceable and repairable insulator fails. The overall aim of insulation coordination is to reduce to an econ omically and operationally acceptable level the cost and disturbance caused by insulation failure. In insulation coordination method, the insulation of the various parts of the system must be so graded that flash over if occurs it must be at intended points. For proper understanding the insulation coordination we have to understand first, some basic terminologies of the electrical power system. Let us have a discussion.


11)Mechanical Design

Electrical Considerations for T.L. Design:

Low voltage drop

Minimum power loss for high efficiency of power transmission.

The line should have sufficient current carrying capacity so that the power can be transmitted without excessive voltage drop or overheating.

Conductivity of Conductor:

R = ρ.L/A, or

R = L/Ϭ. A

Where:

L: Conductor length.

A: Conductor crosses sectional area.

ρ: resistivity

Ϭ: Conductivity (Ϭ= 1/ρ)

The conductor conductivity must be very high

To reduce Conductor resistance R and hence reduce losses

PL= 3 I2 .R

Mechanical Considerations for T.L. Design:

The conductors and line supports should have sufficient mechanical strength:

To withstand conductor weight, Conductor Tension and weather conditions (wind, ice).

The Spans between the towers can be long.

Sag will be small.

Reducing the number and height of towers and the number of insulators.

12)Sag

Sag: It is defined as the vertical distance between the point where the line is joined to the tower and the lowest point on the line.


EFFECT OF SAG IN TRANSMISSION LINE

It Reduces Excessive Tension: While erecting an overhead line, it is very important that the conductorsare under safe tension. If the conductors are too much stressed between the supports (towers, utility poles),then the stress on the conductors may reach to anunsafe level and the conductor may break due to excessive pressure (i.e. tension). In order to permit safe tension in the conductors, the conductors (i.e. the transmission lines) are not fully stretched but are allowed to have a dip or sag. It Increases the Cost in Transmission Line when

Too Much:

The more space there is between the transmission towers, the more the transmission line will sag. If there is too much sag in a transmission line, it will increase the amount of conductor used, increasing the cost more than is necessary.

It Causes Power Failure: When a transmission line sag excessively, it is liable of causing power failure. An overheating electrical transmission line sagging into a tree sparked the greatest power failure in the Western United States in 1996. A similar incident is suspected to have caused the recent East Coast blackout.

13)Structural design

"Electrical Design of Overhead Power Transmission Lines" discusses everything electrical engineering students and practicing engineers need to know to effectively design overhead power lines. Co written by experts in power engineering, this detailed guide addresses component selection and design, current IEEE standards, load-flow analysis, power system stability, statistical risk management of weather-related overhead line failures, insulation, thermal rating, and other essential topics. Clear learning objectives and worked examples that apply theoretical results to real-world problems are included in this practical resource. "Electrical Design of Overhead Power Transmission Lines" covers: AC circuits and sequence circuits of power networks Matrix methods in AC power system analysis Overhead transmission line parameters Modeling of transmission lines AC power-flow analysis using iterative methods Symmetrical and unsymmetrical faults Control of voltage and power flow Stability in AC networks High-voltage direct current (HVDC) transmission Corona and electric field effects of transmission lines Lightning performance of transmission lines Coordination of transmission line insulation Amp city of overhead line conductors.

14)Conductor spacing and Ground clearance

The spacing of conductors is decided not only by the electrical consideration of the working voltage, but also by mechanical factors such as length of span, weight of conductors, prevalent wind direction, etc. With the increase of sag, the magnitude of the swing also increases and proper care is to be taken to maintain clearance under unfavourable condition. For suspension insulators, a greater spacing is required to allow for the swing of the insulator string.

For HT or EHV lines, with a voltage level of 66 kV or above the conductor spacing depends upon the configuration of the conductors.


15)Transmission Structures

Transmission structures are one of the most visible elements of the electric transmission system. They support the conductors used to transport electric power from generation sources to customer load. Transmission lines carry electricity over long distances at high voltages, typically between 115 kV and 765 kV (115,000 volts and 765,000 volts). There are many different designs for transmission structures. Two common types are:

Lattice Steel Towers (LST) consist of a steel framework comprised many structural components that are bolted or welded together.

Tubular Steel Poles (TSP) are hollow steel poles fabricated either as one piece or as several pieces fitted together.

16)Single and Double Circuit Transmission Lines, Bundle Conductor

Transmission lines which carry three phase power are usually configured as either single circuit or double circuit. A single circuit configuration has three conductors for the three phases. While a double circuit configuration has six conductors (three phases for each circuit). Double Circuits are used where greater reliability is needed. This method of transmission enables the transfer of more power over a particular distance. The transmission is thus cheaper and requires less land and is considered ideal from an ecological and aesthetic point of view. However, running two circuits in close proximity to each other will involve inductive coupling between the conductors.

17)Reliability and continuity


To ensure reliability, power supply manufacturers perform extensive tests on production units. Sometimes this involves burn-in or accelerated stress testing to weed out infant mortality [1]. In any case, typical tests include AC ripple, DC voltage levels, temperature, AC voltage and continuity.

These test parameters could be measured using a separate instrument for each measurement. A faster, more cost effective method would be the use of a single instrument to measure all the parameters, if one were available that could be easily and quickly switched to different measurement points and functions. A practical, cost effective solution that lies between these two extremes is described below.

Test Equipment Issues

Practical considerations preclude the use of a single instrument to measure all the required power supply parameters during production. This situation calls for a switching matrix and multiple measuring instruments. Still, the number of instruments should be minimized for reasons of cost, including capital expenditures, integration, operation, and maintenance expense. With the wide range of signals to be measured and switched, test system components must be carefully chosen to minimize these costs while assuring a high level of accuracy and test system throughput.

Qno:03 What are Transmission line parameters and hoe they effect the performance and efficiency of an electrical power system?

Transmission Line Parameters Resistance and Inductance:

The transmission lines are modeled by means of the parameters resistance, inductance, capacitance and conductance. Resistance and inductance together is called transmission line impedance. Also capacitance and conductance in parallel is called admittance Here we are not going to derive the formulas rather to develop some concepts about the transmission line parameters. It will help us understand the transmission line modelling and in analyzing the power system. In this article we will discuss about the line resistance and inductance. In the next article we will discuss about line capacitance and conductance.

Resistance:

The conductors of the transmission lines have small resistance. For short lines, resistance plays an important role. As the line current increases so do the ohmic loss (I2R loss). When the current exceed a certain value the heat generated due to ohmic loss starts to melt the conductor and the conductor becomes longer that results in more sag. The current at which this condition of conductor is irreversible is called thermal limit of conductor. Short overhead lines should be operated well within this limit.

The resistance R of a conductor of length 'l' and cross section 'a' is given by the formula


l

R = ρ ----

a

Here ρ is the resistivity of the conductor material which is a constant.

Transmission lines usually use ACSR conductors with spirally twisted strands. So the actual length of the conductor is about 2 % more than the ACSR conductor length. So from the above formula, the resistance of the line is proportionately 2% more than the conductor length. Another important factor is that when the frequency of current increases the current density increases towards the surface of conductor and current density at the center of conductor is less. That means more current flows towards the surface of conductor and less towards the center. This is well known skin effect. Even at power frequency (60/50 Hz) due to this skin effect the effective cross sectional area of conductor is less. Again from the above equation it is clear that the conductor resistance is more for higher frequency. So AC resistance of conductor is more than the DC resistance. Temperature is another factor that influences the resistance of conductor. The resistance varies linearly with temperature. The manufacturers specify the resistance of the conductor and one should use the manufacturers data.

Inductance:

For medium and long distance lines the line inductance (reactance) is more dominant than resistance. The value of current that flows in a conductor is associated with another parameter, inductance. We know that a magnetic field is associated with a current carrying conductor. In AC transmission line this current varies sinusoidally, so the associated magnetic field which is proportional to the current also varies sinusoidally. This varying magnetic field induces an emf (or induced voltage) in the conductor. This emf(or voltage) opposes the current flow in the line. This emf is equivalently shown by a parameter known as inductance. The inductance value depends upon the relative configuration between the conductor and magnetic field. Inductance in simple language is the flux linking with the conductor divided by the current flowing in the conductor. In the calculation of inductance the flux inside and outside of the conductor are both taken care of. The inductance so obtained is total inductance. Now onwards if not exclusively mentioned then inductance means total inductance due to conductor internal and external flux linkages. The symbol L is used universally to represent inductance. L is measured in Henry (H). It is usually expressed in smaller unit, milli Henry(mH). Manufactures usually specify inductance value per kilometer or mile.It should be noted that, in all the formulas below inductance L is in Henry per unit length and not simply Henry. Here few cases are depicted.

For a single phase line see the fig-A. The conductor inductance is

L = 2 * 10-7 ln ( D/r1' )


Here D is the distance between the centers of conductors.

r1' = r1 * e-(1/4) = 0.7788 r1

r1 is the actual radius of the conductor.

For a single phase line the return path also has inductance say L'. If the return conductor is of radius r2, then

L' = 2 * 10-7 ln ( D/r2' )

Therefore the total inductance of single phase circuit is Lt = L+L'rearranging we get Lt = 4 * 10 -7 ln [D / √ (r1

'. r2')]

Capacitance:

The last article was about line resistance and inductance. Now we will discuss about line capacitance and conductance. We already said that leakage current flows between transmission lines and ground and also between phase conductors. Leakage current flows to ground through the surface of insulator. This leakage current depends upon the suspended particles in the air which deposit on the insulator surface. It depends on the atmospheric condition. The other leakage current flows between the phase conductors due to the occurrence of corona. This leakage current also depends upon the atmospheric condition and the extent of ionization of air between the conductors due to corona effect. Both these two are quite unpredictable and no reliable formula exist to tackle these leakage currents. Luckily these two types of leakage currents are negligibly small and their ignorance has not proved to influence much the power system analysis for line voltage and current relationships. Here we will ignore the leakage currents so we will not show the leakage resistance. Inverse of this leakage resistance is called line conductance.Here rest of the article is about line capacitance. Like previous article on inductance here also I am not going to derive the formulas for capacitance for different line configurations rather to develop some concepts. As the flow of line current is associated with inductance similarly the voltage difference between two points is associated with capacitance. Inductance is associated with magnetic field and capacitance is associated with electric field.The voltage difference between the phase conductors gives rise to electric field between the conductors ( see Fig-A). The two conductors are just like parallel plates and the air in between the conductors is dielectric. So this arrangement of conductors gives rise to capacitance between the conductors. The value of capacitance depends on the configuration of conductors. We will discuss few configurations and the corresponding capacitance value.

When the line parameters for all the three phase conductors are nearly equal, then the line voltages at the other end of the line are more or less balanced. Of course the balanced three


phase system can be solved by considering any one phase and neutral. This is called per phase analysis. It should be remembered that here neutral does not mean the requirement of a neutral conductor for transmission. Although the above general formula for capacitance derived considering transposed lines, but it is often used for non-transposed lines to get approximate values.

Qno: 04:- What types of cables are used for AC and DC system of power ransmission? Also mention the advantages and disadvantages of HVDC system and enlist its components?

Types of Cables

In supply system different types of cables are used depending on the choice of system voltage. For voltages upto 66 kV, 3-core cable (i.e., multi-core construction) is preferred due to economic reasons. However, for voltages beyond 66 kV, 3-core-cables become too large and unwieldy and, therefore, single-core cables are used. The following types of cables are generally used for 3-phase service :

1. Belted cables — upto 11 kV

2. Screened cables — from 22 kV to 66 kV

3. Pressure cables — beyond 66 kV.

1) Belted Cables

These cables are used for voltages upto 11kV but in extraordinary cases, their use may be extended upto 22kV. Given shows the constructional details of a 3-core belted cable.

The cores are insulated from each other by layers of impregnated paper. Another layer of impreg-nated paper tape, called paper belt is wound round the grouped insulated cores. The gap between the insu-lated cores is filled with fibrous insulating material (jute etc.) so as to give


circular cross-section to the cable. The cores are generally stranded and may be of non-circular shape to make better use of available space. The belt is covered with lead sheath to protect the cable against ingress of moisture and mechanical injury. The lead sheath is covered with one or more layers of armouring with an outer serving.

2) Screened Cables

These cables are meant for use upto 33 kV, but in particular cases their use may be extended to operating voltages upto 66 kV. Two principal types of screened cables are H-type cables and S.L. type cable.

a) H-type cables

This type of cable was first designed by H. Hochstadter and hence the name.

Two principal advantages are claimed for H-type cables. Firstly, the perforations in the metallic screens assist in the complete impregnation of the cable with the compound and thus the possibility of air pockets or voids (vacuous spaces) in the dielectric is eliminated. The voids if present tend to reduce the breakdown strength of the cable and may cause considerable damage to the paper insula-tion. Secondly, the metallic screens increase the heat dissipating power of the cable.

b) S.L Type Cables

Given figure shows the constructional details of a 3-core S.L. (separate lead) type cable. It is basically H-type cable but the screen roundeach core insulation is covered by its own lead


sheath. There is no overall lead sheath but only armouring and serving are provided. The S.L. type cables have two main advan-tages over H-type cables. Firstly, the separate sheaths minimise the possibility of core-to-core breakdown. Sec-ondly, bending of cables becomes easy due to the elimina-tion of overall lead sheath. However, the disadvantage is that the three lead sheaths of S.L. cable are much thinner than the single sheath of H-cable and, therefore, call for greater care in manufacture.

3) Pressure Cables

For voltages beyond 66 kV, solid type cables are unreliable because there is a danger of breakdown of insulation due to the presence of voids. When the operating voltages are greater than 66 kV, pressure cables are used. In such cables, voids are eliminated by increasing the pressure of compound and for this reason they are called pressure cables. Two types of pressure cables viz oil-filled cables and gas pressure cables are commonly used.

a) Oil filled Cables

In such types of cables, channels or ducts are provided in the cable for oil circulation. The oil under pressure (it is the same oil used for impregnation) is kept constantly supplied to the channel by means of external reservoirs placed at suitable distances (say 500 m) along the route of the cable. Oil under pressure compresses the layers of paper insulation and is forced into any voids that may have formed between the layers. Due to the elimination of voids, oil-filled cables can be used for higher voltages, the range being from 66 kV upto 230 kV. Oil-filled cables are of three types viz., single-core conductor channel, single-core sheath channel and three-core filler-space.

b) Gas Pressure Cables

The voltage required to set up ionisation inside a void increases as the pressure is increased. Therefore, if ordinary cable is subjected to a sufficiently high pressure, the ionisation can be altogether eliminated. At the same time, the increased pressure produces radial compression which tends to close any voids. This is the underlying principle of gas pressure cables.

Advantages of HVDC Transmission


The high voltage d.c. transmission has the following advantages over high voltage a.c. transmission :

(i) It requires only two conductors as compared to three for a.c. transmission.

(ii) There is no inductance, capacitance, phase displacement and surge problems in d.c. trans-mission.

(iii) Due to the absence of inductance, the voltage drop in a d.c. transmission line is less than the a.c. line for the same load and sending end voltage. For this reason, a d.c. transmission line has better voltage regulation.

(iv) There is no skin effect in a d.c. system. Therefore, entire cross-section of the line conductor is utilised.

(v) For the same working voltage, the potential stress on the insulation is less in case of d.c. system than that in a.c. system. Therefore, a d.c. line requires less insulation.

(vi) A d.c. line has less corona loss and reduced interference with communication circuits.

(vii) The high voltage d.c. transmission is free from the dielectric losses, particularly in the case of cables.

(viii) In d.c. transmission, there are no stability problems and synchronising difficulties

Corona

When an alternating potential difference applied across two conductors exceeds a certain value, called critical disruptive voltage, the conductors are surrounded by a faint violet glow called corona. The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission line is known as corona. If the conductors are polished and smooth, the corona glow will be uniform throughout the length of the conductors, otherwise the rough points will appear brighter. With d.c. voltage, there is difference in the appearance of the two wires. The positive wire has uniform glow about it, while the negative conductor has spotty glow.

Corona Formation

When p.d. is applied between the conductors, potential gradient is set up in the air which will have maximum value at the conductor surfaces. Under the influence of potential gradient, the existing free electrons acquire greater velocities. The greater the applied voltage, the greater the potential gradient and more is the velocity of free electrons.

When the potential gradient at the conductor surface reaches about 30 kV per cm (max. value), the velocity acquired by the free electrons is sufficient to strike a neutral molecule with enough force to dislodge one or more electrons from it. This produces another ion and one or more free electrons, which is turn are accelerated until they collide with other neutral molecules,


thus producing other ions. Thus, the process of ionisation is cummulative. The result of this ionisation is that either corona is formed or spark takes place between the conductors.

Factors Affecting Corona

The phenomenon of corona is affected by the physical state of the atmosphere as well as by the conditions of the line. The following are the factors upon which corona depends :

(i) Atmosphere.

As corona is formed due to ionsiation of air surrounding the conductors, there-fore, it is affected by the physical state of atmosphere. In the stormy weather, the number of ions is more than normal and as such corona occurs at much less voltage as compared with fair weather.

(ii) Conductor size.

The corona effect depends upon the shape and conditions of the conduc-tors. The rough and irregular surface will give rise to more corona because unevenness of the surface decreases the value of breakdown voltage. Thus a stranded conductor has ir-regular surface and hence gives rise to more corona that a solid conductor.

(iii) Spacing between conductors.

If the spacing between the conductors is made very large as compared to their diameters, there may not be any corona effect. It is because larger dis-tance between conductors reduces the electro-static stresses at the conductor surface, thus avoiding corona formation.

(iv) Line voltage.

The line voltage greatly affects corona. If it is low, there is no change in the condition of air surrounding the conductors and hence no corona is formed. However, if the line voltage has such a value that electrostatic stresses developed at the conductor surface make the air around the conductor conducting, then corona is formed.

QNo: 05. Describe the formation of corona and discuss the factors which affect the corona?

Some ionisation is always present in air due to cosmic rays, ultra-violet radiations and radioactivity. Therefore, under normal conditions, the air around the conductors contains some ionised particles (i.e., free electrons and +ve ions) and neutral molecules. When p.d.is applied between the conductors, potential gradient is set up in the air which will have maximum value at the conductor surfaces. Under the influence of potential gradient, the existing free electrons acquire greater velocities.The greater the applied voltage, the greater the potential gradient and more is the velocity of free electrons. When the potential gradient at the conductor surface reaches about 30 kV per cm (max. value), the velocity acquired by the free electrons is sufficient to strike a neutral molecule with enough force to dislodge one or


more electrons from it. This produces another ion and one or more free electrons, which is turn are accelerated until they collide with other neutral molecules, thus producing other ions. Thus, the process of ionisation is cummulative. The result of this ionisation is that either corona is formed or spark takes place between the conductors.

Factors Affecting the Corona:

The voltage gradient for breakdown of the air is proportional to its density. In stormy weather (bad climate) the number of ions may be more than normal. As more number of ions present between the conductors means reduction of insulating property of the medium. So corona can occur at less voltage than critical breakdown voltage during bad climatic conditions than fair weather conditions.

Conductor:- The corona is conserved to be affected by the Size(diameter), shape(stranded or smooth) and surface condition (dirty or clean) of the conductor.

Corona decreases with the increase in the conductor diameter. This is due to the fact that increase in the conductor diameter reduces the electric field intensity.

For Stranded conductor corona is more compared to solid conductor. As the potential gradient of the stranded surface is more compared to the normal solid conductor for same power rating. For dirty surfaced conductor the break down voltage required for corona formation is less than clean surfaced conductor.

Spacing between Conductors:- With increase in the spacing between the conductors the electrostatic stresses reduces results in reduced corona effect.

Line Voltage:- At low line voltage there will be no corona. When the voltage applied increases breakdown voltages then corona formation takes place.

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