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Q.1 Corona is-

A. Partial breakdown of air
B. Complete breakdown of air
C. Sparking between lines
D. None of these
Ans: B

Q.2 Which of the following statements is true regarding corona-

A. Corona takes place at a voltage lower than breakdown voltage
B. Corona takes place at a voltage higher than breakdown voltage
C. Corona is a current phenomenon
D. Corona increases the transmission line efficiency
Ans: A

Q.3 Corona is accompanied by-

A.Violet visible discharge in darkness
B. Hissing sound
C. Vibration
D. Power loss
E. Radio interference
F. Ozone
G. All of the above
Ans: G

Q.4 Ozone effect can be detected by

A. Presence of ozone detected by odour
B. Hissing sound
C. Faint luminous glow of bluish colour
D. All of the above
Ans: D

Q.5 Corona loss increases with

A. Decrease in conductor size and increase in supply frequency
B. Increase in conductor size and decrease in supply frequency
C. Increase in both conductor size and supply frequency
D. Decrease in both conductor size and supply frequency
Ans: A

Q.6 The good effect of corona on overhead lines is to

A. Increase the lie carrying capacity due to conducting ionized air envelope around conductor
B. Increase the power factor due to corona loss
C. Reduce the radio interference from the conductor
D. Reduce the steepness of surge fronts
Ans: D

Q.7 Corona loss can be reduced by the use of hollow conductor because

A. The current density is reduced
B. The eddy current in the conductor is eliminated
C. For a given cross section, the radius of the conductor is increased
D. Of better ventilation in the conductor
Ans: C

Q.8 Bundled conductors reduce

A. Surface electric stress of conductor
B. Increases the line reactance
C. Decrease the line capacitance
D. All
Ans: A

Q.9 Corona loss in a transmission line is dependent on

A. Diameter of the conductor
B. Material of the conductor
C. Height of the conductor
D. None
Ans: A

Q.10 Corona occurs between two transmission conductors when they

A. Have high potential difference
B. Are closely spaced
C. Carry dc power
D. Both A and BAns: D

INTRODUCTION TO ELECTRICAL ENERGY

Energy is the basic necessity for the economic development of a country. Many functions necessary to present-day living grind to halt when the supply of energy stops. It is practically impossible to estimate the
actual magnitude of the part that energy has played in the building up of present-day civilisation. The availability of huge amount of energy in the modern times has resulted in a shorter working day, higher agricultural and industrial production, a healthier and more balanced diet and better transportation facilities.
As a matter of fact, there is a close relationship between the energy used per person and his standard of living. The greater the per capita consumption of energy in a country, the higher is the standard of living of its people.Energy exists in different forms in nature but the most important form is the electrical energy.
The modern society is so much dependent upon the use of electrical energy that it has become a part and parcel of our life. In this chapter, we shall focus our attention on the general aspects of electrical energy.

IMPORTANCE OF ELECTRICAL ENERGY

Energy may be needed as heat, as light, as motive power etc. The present-day advancement in science and technology has made it possible to convert electrical energy into any desired form. This has given electrical energy a place of pride in the modern world. The survival of industrial undertakings and our social structures depends primarily upon low cost and uninterrupted supply of electrical energy. In fact, the advancement of a country is measured in terms of per capita consumption of electrical energy.

Must Read: Electrical Engineering Interview Questions

Electrical energy is superior to all other forms of energy due to the following reasons :
(i) Convenient form.Electrical energy is a very convenient form of energy. It can be easily converted into other forms of energy. For example, if we want to convert electrical energy into heat, the only thing to be done is to pass electrical current through a wire of high resistance e.g., a heater. Similarly, electrical energy can be converted into light (e.g. electric bulb), mechanical energy (e.g. electric motors) etc.
(ii) Easy control.The electrically operated machines have simple and convenient starting, control and operation. For instance, an electric motor can be started or stopped by turning on or off a switch. Similarly, with simple arrangements, the speed of electric motors can be easily varied over the desired range.
(iii) Greater flexibility.One important reason for preferring electrical energy is the flexibility that it offers. It can be easily transported from one place to another with the help of conductors.
(iv) Cheapness.Electrical energy is much cheaper than other forms of energy. Thus it is overall economical to use this form of energy for domestic, commercial and industrial purposes.
(v) Cleanliness.Electrical energy is not associated with smoke, fumes or poisonous gases. Therefore, its use ensures cleanliness and healthy conditions.
(vi) High transmission efficiency.The consumers of electrical energy are generally situated quite away from the centres of its production. The electrical energy can be transmitted conveniently and efficiently from the centres of generation to the consumers with the help of overhead conductors known as transmission lines.

ELECTRIC ENERGY GENERATION UTILISATION INTERVIEW QUESTIONS & ANSWERS

If you are preparing for a technical interview for a lighting systems related products(LED, Lamps etc) company, then you have landed in a correct page.
This post will serve to refresh the Lighting concepts, illuminations, related laws and important terms.
Here we had collected some of frequently asked questions related to illumination.
n addition to that we had added Anna university previous years questions on EEE 8th sem paper Electric Energy Generation Utilisation
[1] Define luminous flux
It is defined as the total quantity of light energy emitted per second from a luminous body.
  • It is represented by symbol F and is measured in lumens.
  • The conception of luminous flux helps us to specify the output and efficiency of a given light source.
[2] What is meant by luminance?
It is defined as the luminous per unit projected area of either a surface source of light or a reflecting surface and is denoted by L.
[3] What are the laws of illumination?
Law of Inverse squares
Illumination at a point is inversely proportional to square of its distance from the point source and directly proportional to the luminous intensity (CP) of the source of light in that direction.
  • If a source of light which emits light equally in all directions be placed at the centre of a hollow sphere, the light will fall uniformly on the inner surface of the sphere.
  • If the sphere be replaced by one of the larger radius, the same total amount of light is spread over a larger area proportional to the square of the radius.
Lamber’s cosine law:
The illumination at a point on a surface is proportional to cosine of the angle which ray makes with the normal to the normal to the surface at that point.
[4] What is meant by candle power?
It is defined as the number of lumens given out by the source in a unit solid angle in a given direction.
It is denoted by CPCp = lumens /ω
[5] Define MHCP
The mean of candle power in all directions in the horizontal plane containing the source of light is termed as Mean Horizontal Candle Power
[6] What are all the sources of light?
As per the principle of operation the light sources may be grouped as follows
  1. Arc lamps
  2. High temperature Lamps
  3. Gaseous Discharge Lamps
  4. Fluorescent type Lamps
[7] Define utilisation factor
It is defined as the ratio of total lumens reaching the working plane to total lumens given out by the lamp utilisation factor  = [Total lumens reaching the working plane] / [Total lumens given out by the lamp]
[8] Name the various photometer heads
  1. Bunsen head or Grease spot photometer
  2. Lummer-brodhun photometer head
There are two types of lummer broadhun heads
  • Equality oc Brightness type photometer head
  • Contrast type photometer head
[9] What is polar curve?
  • In most lamps or sources of light the luminous intensity is not the same in all directions.
  • If the luminous intensity, ie, the candle power is measured in a horizontal plane about a vertical axis and a curve is plotter between candle power and the angular position, a curve obtained is called as horizontal polar curve.
  • The luminous intensity in all the directions can be represented by polar curves.
  • If the luminous intensity in a vertical plane is plotted against position, a curve known as vertical polar curve.
[10] Define space-height ratio
It is defined as the ratio of horizontal distance between adjacent lamps and height of their mountings.
Space-height ratio = [Horizontal distance between two adjacent lamps] / [Mounting height of lamps above working plane]
Thanks for reading …. Please leave your comments below….

Q. What is meant by knee point voltage?

A: Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated.(CT-current transformer).

Query:What is reverse power relay?

Resolution:Reverse Power flow relay are used in generating stations’s protection. A generating stations is supposed to fed power to the grid and in case generating units are off,there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay.

Q:What is the difference between Isolator and Circuit Breaker?

A:Isolator is a off load device which is used for isolating the downstream circuits from upstream circuits for the reason of any maintenance on downstream circuits. it is manually operated and does not contain any solenoid unlike circuit breaker. it should not be operated while it is having load. first the load on it must be made zero and then it can safely operated. its specification only rated current is given.But circuit breaker is onload automatic device used for breaking the circuit in case of abnormal conditions like short circuit, overload etc., it is having three specification 1 is rated current and 2 is short circuit breaking capacity and 3 is instantaneous tripping current.

Q:what is boucholz relay and the significance of it in to the transformer?

A:Boucholz relay is a device which is used for the protection of transformer from its internal faults, it is a gas based relay. whenever any internal fault occurs in a transformer, the boucholz relay at once gives a horn for some time, if the transformer is isolated from the circuit then it stop its sound itself other wise it trips the circuit by its own tripping mechanism.

Q:What is SF6 Circuit Breaker?

A:SF6 is Sulpher hexa Flouride gas.. if this gas is used as arc quenching medium in a Circuitbreaker means SF6 CB.

Q:Tell me in detail about c.t. and p.t. ?(Company:reliance)

A:The term C.T means current transformer,and the term P.T means potential transformer.In circuit where measurements of high voltage and high current is involved they are used there.Particularly when a measuring device like voltmeter or ammeter is not able to measure such high value of quantity because of large value of torque due to such high value it can damage the measuring device.so, CT and PT are introduced in the circuits. They work on the same principle of transformer, which is based on linkage of electromagneticflux produced by primary with secondary.They work on the ratio to they are designed.E.g if CTis of ratio 5000\5A and it has to measure secondary current of 8000A.then ANS=8000*5\5000=8Aand this result will be given to ammeter .and after measuring 8A we can calculate the primary current.same is the operation of PT but measuring voltage.

Q:Explain the working principal of the circuit breaker?

A:Circuit Breaker is one which makes or breaks the circuit.It has two contacts namely fixed contact & moving contact.under normal condition the moving contact comes in contact with fixed contact thereby forming the closed contact for the flow of current. During abnormal & faulty conditions(when current exceeds the rated value) an arc is produced between the fixed & moving contacts & thereby it forms the open circuitArc is extinguished by the Arc Quenching media like air, oil, vaccum etc.

Q:What is the difference between surge arrestor and lightning arrestor?

A:LA is installed outside and the effect of lightning is grounded,where as surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge.

Do you know what is corona effect?
First consider a conductor system to which voltage is applied. If the potential applied is raised, gradually, beyond a limit, a pale voilet glow will appear. This will be accompanied by a hissing noise indicating the ionization of surrounding air of the conductor surface with characterstic smell of ozone. This phenomenon is known as corona.
Simply we can say, phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission line is known as corona effect.
High electric field at conductor surface provides enough energy to the molecules of surrounding air that causes breakdown of air.
In case of a.c. Transmission ionized particles get attracted and repelled to conductor once during each half cycle. So, charge exists near the conductor surface. But in case of d.c. Transmission, negative and positive charges move respectively to the conductor of opposite polarity. So movement of charges between conductors is dependent on electric field distribution between them. But in case of a.c. It depends on potential gradient.
PHENOMENON OF CORONA – for an over head transmission system atomospheric air behaves practically like a perfect insulator when potential difference between conductors is small. If we increase the voltage, there is a corresponding increase in the electric field intensity.
If air is subjected to a uniform electric field intensity of peak value less than 30kv/cm, flow current between conductors is negligible. But when electric field intensity increases and reaches this critical value of 30kv/cm air ionizes and become conducting.
This breakdown accompanied by following phenomenon.
1. Hissing noise is produced.
2. A faint pale yellow glow appears which is visible in the dark.
3. Production of ozone gas.
4. Radio interference occurs.
5. There is a loss of power.
If all these phenomenon occurs then we say corona produced in a transmission.


Skin Effect In Transmission Line

In case of D.C., current density is uniform over cross section of conductors. On the other hand, in a conductor carrying A C there is a tendency of current to crowd near the surface of conductor that means
alternating current flowing through a conductor doesn’t distribute uniformly and concentrate near the surface of conductor.
In AC System no current flows through the core and entire current is concentrate on surface regions. Due to this effective area of conductor is reduced so ac resistance of conductor increases.
This phenomenon is known as Skin Effect as it causes concentration of current at the skin of conductor.
To explain skin effect, we assume conductor which is made up of a number of concentric cylinders. Magnetic flux linking a cylindrical  element near the centre of conductors is greater than that linking another element near the surface of conductor. It is due to the fact that former elements is surrounded by internal as well as external flux, while latter element by external flux only. So inner element will possess greater self inductance and therefore, will offer a large inductive reactance that outer element. Due to this difference in
inductive reactance current tends to crowd towards the surface or skin of conductor. So distribution of current over section of conductor will be non uniform. As show in fig.

Skin effect depends upon:
  • Type of conductor material
  •  Frequency
  •  Diameter of conductor
  •  Shape of conductor.
At low frequencies, this effect is negligible , in fact it is only of importance with high frequencies on with solid conductors of large cross section.Skin effect increases with increase in frequency, conductor diameter and permeability.
In, aluminium wire effect is same as in a copper wire of equal conductivity.
Skin effect is much smaller with stranded conductor than with solid conductors.
In parctice stranded conductors are invaraibly used for transmision and distribution purpose. This is used in order to overcome adverse effect of Skin-Effect.