RSMSSB JE Electrical Mock Test - 5 - Electrical Engineering (EE) MCQ

The correct answer is Ramnath Chowdhary

In News

• Ramnath Chowdhary is an international Alghoza player from Rajasthan who has made it to the India Book of Records for playing the Alghoza with his nose for 5 minutes and 5 seconds.

Key Points

• Ramnath Choudhary, an internationally renowned Algoza player from Rajasthan, has been recorded in the India Book of Records.
• He set a record by playing the Algoza with his nose for 5 minutes and 5 seconds.
• Choudhary is the only artist in the world who plays the Algoza with his nose and holds a record for having 12 feet long mustaches.
• He produces various tunes with the Algoza and sings stories of deities like Tejaji Maharaj, Bhairuji, Mataji, and Diggi Kalyan.
• In 2000, he impressed the then US President Bill Clinton, who invited him to America.
• He has since traveled to over a dozen countries, including the USA, Germany, Dubai, Australia, and Japan, showcasing his Algoza playing and mustaches.

The correct answer is Section 204.

Key Points

• As per the Rajasthan municipality act, 2009
• Section-204 Provision -
• Sewage and rainwater drains are to be distinct -
  • Whenever it is provided in the Act that steps may be taken for the effectual drainage of any premises,
  • The Municipality may require that there shall be one drain for offensive matter and sewage and
  • Another drain for rain-water and unpolluted sub-soil water, each emptying into separate, municipal drains or
  • Other places are set apart by the Municipality for the discharge of drainage or into other suitable places.

Additional Information

• ​Section-203 Provision -
  • Right of owners and occupiers of buildings or lands to drain into municipal drains.
• Section-205 Provision -
  • Right to carry drain through land or into drain belonging to other person-how and on what conditions to be authorized by the Municipality.
• Section-206 Provision -
  • Work, how to be carried out.​

The correct answer is Swaroop Singh.

• During the reign of Swaroop Singh, the last incident of Sati with the ruler is found.
  • In 1829, the Governor-General of India, Lord William Bentinck, had banned the practice of Sati in the English areas and made efforts to stop the native states as well.
  • Maharana Swaroop Singh wished that this practice should continue because Maharana was in favour of old practices.
  • The British talked to Maharana many times on this issue, but Maharana kept procrastinating and 16 years passed.
  • After all, Maharana Swaroop Singh, obeying the British, issued a decree in Mewar and banned the practice of Sati.

• Gilund is a village and an archaeological site located in the Rajsamand district of Rajasthan state in western India.
• It is one of five ancient sites excavated in the Ahar-Banas Complex which also includes the sites of Ahar, Ojiyana, Marmi, and Balathal.
• Out of the 111 reported sites found in the Ahar-Banas Complex, Gilund is the largest.
• The archaeological site was named after the present-day village, Gilund, and is locally known as Modiya Magari which means "bald habitation mound
• Gilund is located in the Banas Basin located within the middle of the Mewar Plain, which lies between the Aravalli Mountains and the Deccan Plateau.
• At the ancient site of Gilund, two mounds labeled as 'eastern' and 'western', measuring 45 ft and 25 ft respectively above the surrounding fields in height and covering an area of 500 X 250 yards were partially excavated by a team under the direction of B. B. Lal during 1959-60.
• Both sites have been occupied since the beginning of the Chalcolithic period, but it is evident that the eastern mound is taller due to a period of longer occupation.

• Gilund was occupied from approximately 3000-1700 BCE as the region’s largest chalcolithic site.
• These years of occupation are divided into three phases: Late Ahar-Banas 2000-1700 BCE, Middle Ahar-Banas 2500-2000 BCE, and Early Ahar-Banas 3000-2500 BCE.
• Here various housing structures have been uncovered, as well as large buildings with long parallel walls, workshops, refuse heaps, and an exterior wall surrounding the site.
• Structures were made from mud brick and burnt brick.
• The variety of stone stools and ground stones discovered also suggest early social complexity as they were used for grain processing and grinding.
• The most common artifacts found in Gilund were ceramics. Although few pieces of pottery and bones dating back to the Mesolithic period, it does not compare to the amount found at Bangor.

• The grasslands and pastures called Beed in the local language are found in Bikaner, Skiar, and Jodhpur.
• Grasses and their values have been recognized since time immemorial as the present-day cereals crops are the cultivated varieties of their wild ancestors.
• The use of grasses as food resources or as fodder has led to extensive breeding programs and improvement in pasture land.
• Grasslands are areas where the vegetation is dominated by grasses.
• However, sedge and rush can also be found along with variable proportions of legumes, like clover, and other herbs

• Rajasthan is a state in northern India. It covers 342,239 square kilometers or 10.4 percent of India's total geographical area.
• It is the largest Indian state by area and the seventh-largest by population.
• Area: 342,239 km²
• Governor: Kalraj Mishra
• Capital: Jaipur
• Population: 6.89 crores
• Chief minister: Ashok Gehlot

• The United States of Matsya, also called Matsya Union or Matsya Sangh, was the State of India which was formed on 18 March 1948 by the merger of four erstwhile princely states, Alwar, Bharatpur, Dholpur, and Karauli after accession.
• Shobha Ram Kumawat of the Indian National Congress was the first and last chief minister of the State from 18 March 1948 till 15 May 1949.
• Maharaja of Dholpur became its Rajpramukh.
• On 15 May 1949, the Matsya Union was merged with Greater Rajasthan, to form the United State of Rajasthan, which later became the state of Rajasthan on 26 January 1950.

Thus, the Matsya Sangh was established on 18.03.1948 in the first phase of Rajasthan integration.

• Gandhi Sagar dam-Madhya Pradesh
• Rana Pratap Sagar-Rajasthan
• Jawahar Sagar dam-Rajasthan
• Kota Barrage-Rajasthan

• Gandhi Sagar dam
• It is the first of the four dams built on the Chambal River, located on the Rajasthan-Madhya Pradesh border.
• Mainly situated in Madhya Pradesh state.
• Rana Pratap sagar dam
• This dam is located on the Chambal River near Rawatbhata in Chittorgarh district in Rajasthan.
• Jawahar Sagar dam
• It is the third dam in the series of Chambal Valley Projects, located 29 km upstream of Kota city and 26 km downstream of Rana Pratap Sagar dam.
• Kota Barrage
• It is the fourth in the series of Chambal Valley Projects, located about 0.8 km upstream from Kota City in Rajasthan.
• Water released after power generation at Gandhi Sagar dam, Rana Pratap Sagar dam, and Jawahar Sagar Dams, is diverted by Kota Barrage for irrigation in Rajasthan and in Madhya Pradesh through canals on the left and the right sides of the river.

• Madhya Pradesh, a large state in central India, retains landmarks from eras throughout Indian history.
• Begun in the 10th century, it's Hindu and Jain temples at Khajuraho are renowned for their carvings of erotic scenes, most prominently Kandariya Mahadeva, a temple with more than 800 sculptures.
• The eastern Bandhavgarh and Kanha national parks, noted Bengal tiger sanctuaries, offer guided safaris.
• Area: 308,245 km²
• Capital: Bhopal
• Chief Minister: Shivraj Singh Chouhan
• Population: 7.33 crores

The correct answer is V. K. Vashishtha.

Key Points

• "Bhagat movement: a study of the cultural transformation of the Bhils of southern Rajasthan" is authored by Vijay Kumar Vashishtha.
• Publisher: Shruti Publications, 1997, Jaipur.​

Additional Information

• Govind Guru started the " Bhagat Movement " in Rajasthan.
• For the development of the "Bhil community", he established the " Samp Sabha".
• The movement helped the community to stay within the boundaries of Hinduism.
• The movement developed in the region of Mewar, Durgapur, Gujarat, Vijaynagar, and Malwa.​​

Important Points

• Govind Guru was born in the Banjara family.
• He worked towards improving the moral character, habits, and religious practice of the " Adivasis".
• He also declared the " Rajput" and " Brahmin" communities as inferior because they degraded the women.

The correct answer is On arrival of son-in-law at Sasural.

Key Points

• Pawna Geet is the folk song of Rajasthan.
  • It is sung On the arrival of son-in-law at sasural.

Additional Information

• Some famous folk songs of Rajasthan.
  • Moria - In this folk song, there is anguish of such a girl, whose marriage relationship has been fixed but there is a delay in getting married.
  • Ghoomar - Ghoomar dance is a song sung by women on the occasion of Gangaur or Teej festivals, through which the heroine asks her beloved for decorative means.
  • Gorbandh - Gorbandh is the ornament of camel's neck. In Marwar and Shekhawati region, the song Gorbandh Nakhralo is sung on this ornament. This song describes the makeup of a camel.
  • Jhorawa - Popular song of Jaisalmer region which wife sings in separation of her husband.
  • Mumal - This is a popular song of Jaisalmer region, in which the beauty of princess Mumal of Lodrava is described. This is a graceful song.
  • Sithane - This is a slang song sung on the occasion of marriage, which is sung by women at the time of marriage for the purpose of laughing and addressing Samdhi and her other relatives.
  • Kaman - Kaman means - witchcraft. It is a song sung by women in rural areas to protect her husband from the witchcraft of another woman.
  • Peepli - Marwar is a song sung by women in Bikaner and Shekhawati region during the rainy season.
  • Dhola-Maru - This popular song of Sirohi region is based on the love affair of Dhola-Maru and is sung by Dhadhi people.
  • Lavani - Lavani means to invite. Lavani is sung in the context of the hero calling the heroine.

Auto-Transformer:

In an autotransformer, power is transferred through both conduction and induction processes.

EMF induced in the winding is proportional to the number of turns. Therefore, the secondary voltage can be varied by just varying a secondary number of turns. As winding is common in both circuits, most of the energy is transferred by means of electrical conduction and a small part is transferred through induction.

V2(I1 - I2) = Inductive transfer

V2I1 = Conductive transfer

At

The correct answer is option 4):(multi-meter)

Concept:

• A multimeter is a measuring instrument that can measure multiple electrical properties
• A multimeter is a combined unit of ampere meter, voltmeter and ohmmeter.
• It is used to check the conductors, the continuity of the circuit, measuring the voltage in the socket or the battery level of the car.
• The multimeters can not be used to measure electrical quantities such as frequency, charge, etc.
• All measurements are possible by using one meter only.
• Digital multimeters are more accurate than analog multimeters.
• The primary difference between the two is the display, an analog multimeter uses a needle to show the value, while a digital multimeter will show the results as numbers on a screen.
• The analog multimeter also exhibits low resistance and high sensitivity with scales down, hence a sudden change in the signal can be detected by an analog multimeter more swiftly than a digital multimeter.
• Analog multimeters are more bulky and costly than digital multimeters
• The figure shows that a multimeter used to check a diode

​

Additional Information

• A Clip-on meter or tongue tester is a clothespin-shaped instrument that can be clamped around a live wire in order to measure the current it's carrying. As a measurement principle, clamp meters detect the magnetic field emitted by current flowing in a wire in order to measure the current value.
• The power factor meter measures the power factor of a transmission system.

Methods of power factor improvement

• Synchronous Condensers: A synchronous motor operating without a mechanical load can be used to provide reactive power (leading power factor). It adjusts its excitation to compensate for the lagging power factor caused by inductive loads.
• Shunt Capacitors: Capacitors are connected in parallel with the load to provide reactive power compensation, which counteracts the inductive reactance of motors and transformers, improving the power factor.
• Static VAR Compensators (SVC): These are automated systems that use a combination of reactors and capacitors to dynamically control reactive power in real-time, which improves power factor under varying load conditions.
• Automatic Power Factor Controllers (APFC): These controllers automatically adjust the operation of capacitor banks based on real-time load conditions to maintain the desired power factor, avoiding penalties from the utility and ensuring efficient operation.

Explanation

• In a distribution system, to improve the power factor, synchronous capacitors (also known as synchronous condensers) are installed at the receiving end.
• This is because the receiving end is typically where the load is located, and loads (especially inductive loads like motors) tend to cause a lagging power factor. Synchronous condensers help by supplying reactive power, thereby improving the power factor and reducing losses in the system.

Kirchhoff's Current Law

According to Kirchhoff's Current Law, The total current entering a junction or a node is equal to the charge leaving the node. The algebraic sum of every current entering and leaving the node has to be zero.

Calculation

Applying KCL at node 'A', we get:

V_A = 12 V

I = -1 A

Synchronous motor

• Synchronous motors are capable of running at constant speed irrespective of the load acting on them.
• Unlike induction motors where the speed of the motor depends upon the torque acting on them, synchronous motors have got constant speed-torque characteristics.
• Therefore, from no-load speed to full-load speed, the torque of a 3-phase synchronous motor remains constant throughout.
• The constant speed characteristic is achieved by the interaction between a constant and rotating magnetic field.
• The rotor of a synchronous motor produces a constant magnetic field whereas the stator produces a rotating magnetic field.

The correct answer is option 1): 36

Concept:

• A cycloconverter converts a constant voltage, constant frequency AC waveform to another AC waveform of a different frequency. 3-phase to 3-phase cyclo converter:
• These types of cycloconverters are formed by using 3 three-phase to single cyclo converters linked together. The output of these converters can be connected in wye and delta. The circuit diagram is shown below.

• A 3-phase to single-phase converter uses 6 SCRs. For 3-ϕ to 3-ϕ cyclo converter, 36 SCRs are used.
• Applications: The cyclo converters are used in Cement mill drives Rolling mills Ship propulsion driver Water pumps Washing machines Industries, etc. 18 SCR, 3-ϕ to 3-ϕ cyclo converter It contains more harmonics than 36 SCRs cyclo converter.

Calculation:

I = E/ R+r
I = 10/4+1
I = 2
V = E - Ir
V = 10 - 2(1)
V = 8V
∴ The correct option is 3)

Concept:

To increase the range of a voltmeter, we need to the series resistance and it is given by

Where V is the required voltmeter range

Vm is the voltmeter range

Rm­ is the meter internal resistance

Note:

• To increase the ranges of ammeter, we need to connect a small shunt resistance in parallel with ammeters.
• To increase the ranges of a voltmeter, we need to connect a high series of multiplier resistance in series with voltmeters.

Additional Information

We can extend the range of ammeter by keeping a shunt resistance.

Here Rm = internal resistance of the coil

Rsh = Shunt resistance

I = Required full-scale range

Im = Full scale deflection of current

As the two resistances, Rm and Rsh are in parallel, the voltage drop across the resistance is equal.

‘m’ is called multiplying power

Bode plot:

• In electrical engineering and control theory, a Bode plot is a graph of the frequency response of a system. It is usually a combination of a Bode magnitude plot, expressing the magnitude (usually in decibels) of the frequency response, and a Bode phase plot, expressing the phase shift.
• The Bode magnitude plot is the graph of the function |H(s = jω)| of frequency ω (with j being the imaginary unit). The ω -axis of the magnitude plot is logarithmic and the magnitude is given in decibels, i.e., a value for the magnitude |H| is plotted on the axis at 20log10|H|
• The Bode phase plot is the graph of the phase, commonly expressed in degrees, of the transfer function H(s = jω ) as a function of ω . The phase is plotted on the same logarithmic ω-axis as the magnitude plot, but the value for the phase is plotted on a linear vertical axis.
• For many practical problems, the detailed Bode plots can be approximated with straight-line segments that are asymptotes of the precise response. Hence Bode plot as asymptotic plot.
• Bode plot is applicable for minimum phase system.

Minimum Phase system:

• A transfer function G(s) is minimum phase if both G(s) and 1/G(s) are causal and stable
• A minimum phase system does not have zeros or poles on the right-half plane and it does not have delay.
• Bode discovered that the phase can be uniquely derived from the slope of the magnitude for minimum-phase system.
• We can draw Bode plot for non-minimum phase systems, but the magnitude and phase-angle plots are not 'uniquely related'.
• For a Minimum phase system, the magnitude and phase-angle plots are uniquely related, that means if either one of them is specified over the entire frequency range, the other plot can be determined uniquely. This does not apply to NMP systems.

Additional Information

All pass systems:

An all-pass system is a system whose frequency response magnitude is constant for all frequencies.

Non-minimum phase network:

A system is said to be Non-minimum phase system if all the open loop poles and zeros are lies in right- half plane.

Transients:

Transients are very steep voltage steps that occur in electrical circuits due to the sudden release of a previously stored energy, either inductive or capacitive, which results in a high voltage transient, or surge being created.

This sudden release of energy back into the circuit due to some switching action creates a transient voltage spike in the form of a steep impulse of energy which can in theory be of any infinite value.

The resistor does not store the energy, they dissipate energy in the form of heat

The following points may be noted regarding the potential distribution over a string of suspension insulators:

• 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.
• The disc nearest to the conductor or line has a maximum voltage across it. As we move towards the cross-arm, the voltage across each disc goes on decreasing.
• 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.

Minimum oil circuit breakers (MOCB):

Minimum oil circuit breakers utilize oil as the interrupting media.

The features of designing MOCB are to reduce the requirement of oil, and hence these breakers are called minimum oil circuit breakers.

Unlike a bulk oil circuit breaker, a minimum oil circuit breaker places the interrupting unit in the insulating chamber at the live potential. The insulating oil is available only in the interrupting chamber.

Hence oil requirement will be less and hence MOCB has less volume of oil.

A minimum oil circuit breaker has the following disadvantages as compared to a bulk oil circuit breaker :

• Due to the smaller quantity of oil, the degree of carbonization is increased.
• There is a difficulty in removing the gases from the contact space in time.
• The dielectric strength of the oil deteriorates rapidly due to a high degree of carbonization.

Distributed winding:

For obtaining smooth sinusoidal emf waveform, conductors are placed is several slots under a single pole in a winding. This type of winding is known as distributed winding.

Although distributed armature winding of alternator reduces emf but still it very useful due to the following reasons

1. It reduces harmonic emf and so waveform is improved.

2. It also diminishes the armature reaction.

3. Even distribution of conductors, helps for better cooling.

4. The core is fully utilized as the conductors are distributed over the slots on the armature periphery.

Note:

Short pitch winding:

• It improves the waveform of generated emf i.e. generated emf can be made to approximate a sine wave more easily and the distorting harmonics can be reduced or totally eliminated.
• The disadvantage of using short-pitch winding is that the total voltage around the coils is somewhat reduced. In order to compensate for this reduction in Generated EMF, more number of turns, and therefore more copper is required.

Deflecting torque(T_d):

• The torque needed to move the pointer over a calibrated scale is known as deflecting torque.
• Deflecting torque can overcome the inertia of the moving system, controlling torque and damping torque.
• The deflecting torque in the analog measurement device is proportional to the current through the coil.

Controlling torque(T_c):

• It is to stop the pointer at the required final value.
• Controlling torque acts exactly equal and in opposite direction to deflecting torque.

Damping torque:

The pointer makes oscillations with respect to the final value, to eliminate these oscillations damping torque is required.

Key Points

• In meter, first produces T_d proportional to quantity to be measured and then T_c starts producing and rises until it balances T_d.
• At final steady-state T_c = T_d . So, no damping torque.
• When the current is removed, the deflecting torque T_d will become zero.
• But there is a presence of controlling torque in the opposite direction to the pointer movement, due to this controlling torque the pointer returns to zero.

Active transducers:

• Active transducers are those which do not require any power source for their operation.
• They work on the energy conversion principle. They produce an electrical signal proportional to the input (physical quantity).
• Piezoelectric, thermocouple, and photovoltaic cell transducers are some examples of active transducers.

Passive transducers:

• Transducers which require an external power source for their operation is called a passive transducer.
• They produce an output signal in the form of some variation in resistance, capacitance, or any other electrical parameter, which then has to be converted to an equivalent current or voltage signal.
• LVDT is an example of a passive transducer. LVDT is used as an inductive transducer that converts motion into an electrical signal.

Note:

LVDT (Linear Variable Differential Transformer):

• LVDT (Linear Variable Differential Transformer) is to convert the rectangular movement of an object to the equivalent electrical signal.
• LVDT is used to calculate displacement and works on the transformer principle.
• The LVDT alters a linear dislocation from a mechanical position into a relative electrical signal including phase and amplitude of the information of direction and distance.
• The working principle of the linear variable differential transformer or LVDT working theory is mutual induction; The dislocation is nonelectrical energy that is changed into electrical energy.​​

• The inner coil is the primary, which is excited by an AC source as shown.
• Magnetic flux produced by the primary is coupled to the two secondary coils, inducing an AC voltage in each coil.

​Applications:

• This transducer can also work as a secondary transducer.
• LVDT is used to measure the weight, force and also pressure.
• Some of these transducers are used to calculate the pressure and load.
• LVDT’s are mostly used in industries as well as servomechanisms.
• Other applications like power turbines, hydraulics, automation, aircraft, and satellites.

Concept:

Electrostatic force: It is the electric force on a static charge (Q) in a conductor due to an Electric field (E) and it is given as,

F_e = EQ

Magnetostatic force: It is the Magnetic force on a charge moving with a velocity (V) in a conductor due to Magnetic field (B) and it is given as,

F_m = Q (V × B)

Lorentz force:

If a moving charge is present in both electric field and magnetic field then the force on a charge in a conductor is known as Lorentz force.

F = F_e + F_m

F = EQ + Q (V × B)

This equation is known as the Lorentz force equation.

Note: All bold letters are vectors.

So, the electromagnetic wave experiences the Lorentz force which is the combination of the electrostatic force and magnetostatic force.

Induction motor modelled as a transformer

When all the rotor parameters are shifted to stator side induction motor circuit is given by

So, is the resistance which shows the power which is converted to mechanical power output or useful power.

Insulated Gate Bipolar Transistor (IGBT): It is a three-terminal power semiconductor device primarily used as an electronic switch. It is a 4 layer PNPN device that combines an insulated gate N-channel MOSFET input with a PNP BJT output in a type of Darlington configuration.

​Insulated gate Bipolar Transistor is also known as Conductivity-Modulated Field Effect Transistor.

The IGBT is developed by combining the characteristics of a BJT and a MOSFET.

Advantages:

• The insulated gate bipolar transistor (IGBT) is easy to turn ON and OFF.
• The switching frequency is higher than that of power BJT.
• It has a low on state power dissipation.
• It has simpler driver circuit.

Disadvantages:

• The switching frequency of insulated gate bipolar transistor (IGBT) is not as high as that of a power MOSFET.
• High turn-off time
• It cannot block high reverse voltages.

Important Points

Corona: The phenomenon of violet glow, hissing noise, and production of ozone gas in an overhead transmission line is known as the corona.

Factors Affecting Corona:
Atmosphere:

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

Conductor size:

• The corona effect depends upon the shape and conditions of the conductors.
• Corona loss varied along with the change in the diameter of the conductor.
• 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 an irregular surface and hence gives rise to more corona than a solid conductor.

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 a larger distance between conductors reduces the electrostatic stresses at the conductor surface, thus avoiding corona formation.

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.

Additional Information

Advantages of Corona:

• 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 electrostatic 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 neighbouring communication lines.

Important Points

Critical disruptive voltage: It is the minimum phase-neutral voltage at which corona occurs.

Consider two conductors of radius r cm and spaced d cm apart.

If V is the phase-neutral potential, then potential gradient at the conductor surface is given by:

• In order that corona is formed, the value of g must be made equal to the breakdown strength of air.
• The breakdown strength of air at 76 cm pressure and temperature of 25ºC is 30 kV/cm (max) or 21·2 kV/cm (r.m.s.).

Visual critical voltage: It is the minimum phase-neutral voltage at which corona glow appears all along the line conductors.

Methods of Reducing Corona Effect:

• It has been seen that intense corona effects are observed at a working voltage of 33 kV or above.
• The corona effects can be reduced by the following methods:

By increasing conductor size: By increasing conductor size, the voltage at which corona occurs is raised and hence corona effects are considerably reduced. This is one of the reasons that ACSR conductors that have a larger cross-sectional area are used in transmission lines.

By increasing conductor spacing: By increasing the spacing between conductors, the voltage at which corona occurs is raised and hence corona effects can be eliminated.

Concept:

String Efficiency of SCR: String Efficiency of SCR is the degree of capacity utilization of individual SCRs in a string of series / parallel connected SCRs. String efficiency is always less than 1.

String Efficiency

Where,

• V = Actual voltage of whole string
• I = Actual current of whole string
• V₁ = Voltage rating of one SCR
• I1 = Current rating of one SCR
• N = Total number of SCRs in a string
  

Derating Factor (DFR) of SCR: Derating Factor is the amount by which the string efficiency deviates from unity or 100%.

DFR = [1 – String Efficiency]

DFR = [100 – % String Efficiency]

• Derating Factor gives an idea of unused capacity available in a sting of SCRs.
• That's why it is a measure of reliability of string.
• The lesser the value of sting efficiency, the lesser will be voltage / current sharing by the individual SCRs.
• It means, more unused capacity will be available with the string and hence more DFR.
• But lesser string efficiency increases the cost of string.
• So that a compromise is made in between economy and reliability by properly designing a value of string efficiency.

Calculation :

Given, String efficiency = 80% = 0.8

∴ Derating factor (DFR) = 1 - 0.8

∴ DFR = 0.2