What is protective relay?

Answer: It is an electrical device designed to initiate

the isolation of a part of the electrical installation, or

to operate an alarm signal, in the event of abnormal

condition or a fault. In simple words relay is an

electrical device that gives signal to isolation device

(eg: Circuit Breaker) after sensing the fault and helps

to isolate the fault system from the healthy electrical

system

What are the different relays that employed for

protection of apparatus and transmission lines?

Answer: The relays that are usually employed for

protection of transmission lines include 

Over current relay

Directional relay

Distance relay

Under Voltage relay

Under-frequency relay

Thermal relay

Differential relay

Phase sequence relays

pilot relays

How the electrical power system protection is

divided?

Answer: The overall system protection is divided

into 

Generator protection

Transformer protection

Busbar protection

Transmission line protection and

Feeder protection

How relays are connected in the power system?

Answer: The relays are connected to the power

system through the current transformer (CT) or

potential transformer (PT). 

What are different types of principles of

operation of electro-mechaical relays?

Answer: Eletro-mechanical relays operate by two

principles. Electro-magnetic attraction and electro-

magnetic induction. In electromagnetic attraction

relay plunger is drawn to the solenoid or an armature

is attracted to the poles of the electromagnet. In case

of electro-magnetic induction, principle of operation

is similar to induction motor. Torque is developed by

electromagnetic induction principle

Action carried out by the relay and circuit

breaker during fault condition?

Answer: After the relay sensing the fault condition,

relay operates and close the trip coils. The effect of

this will be circuit breaker operate to open the

contacts.

What is the direction of rotation of rotor with

respect to the rotating magnetic field?

Answer: Rotating Magnetic Field is set up when 3

phase ac supply is provided to the stator windings of

the induction motor. Rotating Magentic Field rotates

at synchronous speed. Rotor rotates in the direction

of the rotating magnetic field

What is Slip In Induction Motor?

Answer: Speed of the Induction Motor will be

always less than synchronous speed and speed of the

motor further reduces when load is applied on the

motor. The difference between the speed of the

stator (synchronous speed Ns) and the actual speed

of the motor (N) is called slip of the induction motor

and is designated by letter 's'. It is represented in

RPM. However it is general tendency to represent as

fraction or percentage of synchronous speed.

Fractional Slip s = Ns- N

                            --------

                                Ns

What is the speed of the rotor magnetic field with

respect to rotor?

Answer: Currents flowing through the rotor

windings due to induction caused by stator will have

a frequency of ( f ', which is equal to sf where f is

the applied frequency to stator). This current flowing

through the rotor sets up a magentic field equal to s x

Ns   with respect to the rotor.

What is the relative speed between the fields set

up by the stator and rotor?

Answer: Stationary. The speed of the stator field

with respect to stator surface will be Ns and the

speed of rotor with respect to the stator surface will

be N. The speed of the magentic field of the rotor

with respect to the rotor will be s x Ns . Therefore the

speed of the rotor field with respect to stator surface

is equal to rotor speed of the motor and rotor field

with respect to rotor

N +  s x Ns = Ns ( 1-s ) + s x Ns =  Ns 

Therefore the relative motion between stator field

and rotor field will be stationary or zero.

What happens when Induction motor is loaded?

Answer:  During no load operation the relative speed

between stator magnetic field and rotor speed will be

less (means slip is less). Hence small emf is

generated which generates small currents and torque

will be generated  in such a manner sufficient to

overcome the friction and windage losses.

When motor is loaded (Load applied to rotor), motor

torque reduces as the load torque increases. Hence

motor slows down (slip increases). As the motor

slows down, the relative speed between rotor and

stator magnetic field increases. This results in

greater motor currents and generates greater motor

torque to overcome the load torque. Thus as the load

on the induction motor increases, motor slows down

until sufficient torque is generated to overcome the

applied load torque.

Why Induction motors are widely used in

Industries?

Answer: In Industries 80% of the motor drives used

are induction motors. Induction Motors are widely

used in Industries because of some features it

possess. They are:

Low Cost

Simple and Rugged Construction

Absence of Commutator 

Good power factor

Higher efficiency

Good Speed Regulation

What is the relation between between  Induction

motor and Transformer?

Answer: Induction motors are considered as

transformers with secondary winding rotating. This

is because both transformers and induction motor

operates in such a manner that power

is transferred to secondary windings (rotor) from

primary windings (stator) by mutual induction

principle.

Why the efficiency of the Induction motor is less

compared to transformer?

Answer: Induction motor and Transformer operates

on the same principle that power is transferred to

secondary by mutual induction. But in transformer,

flux will be traveled from primary to secondary

winding through the core of the transformer. In

induction motor flux travels from stator to rotor

through the air gap. Therefore power factor will be

less for induction motor compared to transformer

which also results in less efficiency for induction

motor compared to transformer.

Why Speed Control of Induction Motor is

difficult compared to DC shunt motor?

Answer: Speed Control of DC shunt motor can be

possible by providing resistance in the armature and

field circuits and varying the resistors speed changes

can be carried out over a wide range. On the other

hand speed control of induction motor is costlier and

bit tedious. 

Speed of Induction motors = (120x f x (1-s))/p. 

Therefore for speed control either supply frequency

should change or number of poles should change and

other methods can be employed. Care should be

taken that in the process efficiency and power factor

should be maintained. 

Explain the operation of induction motor in

simple words?

Answer: When 3 phase ac supply is given to stator of

the induction motor rotating magnetic field will be

generated. This magnetic field (flux) cuts the

stationary conductors of the rotor of induction

motor. Therefore an emf is induced by Faraday's law

of electro-magentism. As the rotor conductors are

short circuited, current starts flowing in the windings

of the rotor this produces a magnetic field. Due to

interaction of both the magnetic fields a torque will

be developed. If the torque generated is sufficient to

overcome the load torque rotor starts rotating. 

Difference between Solid Conductor and Stranded

Conductor

Solid Conductor:It consists of single piece of metal wire. It is cheap for manufacturing.Skin effect is higher in solid conductors as at higher frequencies current flow on the surface of the conductors results in the increase in the effective resistance. The main disadvantage of the solid wire is its more rigid property. It cannot be bent easilyStranded Conductors:Stranded wire consists of sub conductors touch each other. It is costlier to manufacture compared to solid wire. For the given current carrying capacity the size of the stranded conductor is large compared to solid wire. Different elements of strands can be wound together to get the transmission line of desired property (eg: ACSR conductor contains Aluminum and Steel stands wound together). Proximity and skin effect is reduced using stranded conductors.

Bundled Conductors and Composite Conductors

Difference

Composite Conductors:In Composite conductors sub conductors touch each other. Composite conductors are typically stranded conductors. In Composite conductors different

elements are used (In ACSR conductors aluminum has the properties of light weight, good conductivity and rutlessness and steel has the property of high tensile strength). Composite conductors are employed as they are flexible compared to solid conductor. Composite conductors reduce proximity effect and also reduces skin effect up to certain extent.Bundled Conductors:Bundled conductors are employed in Extra High Voltage (EHV) transmission as at higher voltages corona effect is significant. In bundled conductors sub conductors are placed as certain distance throughout the transmission lines. This reduces the corona discharge loss and interference with the communication lines nearby.

Corona Ring on Surge Arresters and other EHV

equipment

Have you ever seen a ring around the Surge Arrester or other EHV equipment? What is the purpose of the Ring around the Extra High Voltage (EHV) equipment? What is the use of big rings at EHV testing centers?Corona Ring:

This Metal Ring what we see in the substations and other transmission line network is "Corona Ring". At higher voltages such as in EHV transmission networks corona discharge effect is prominent. Corona Ring distributes the electrical potential gradient evenly and maintains the maximum value below the threshold limits of the corona effect. Corona Rings are employed at high altitude electrical equipment where density of the air is less. 

Reactive Power and Active Power flow in Power

System

Active Power:The power that is generated in the power system which is utilised in useful work (running the motor, lighting and heating energy)

Reactive Power:Reactive power is the power which will not do any useful work in the power system but helps the active power to flow. Without Reactive Power active power will not able to flow in the system.Different Power Flow In Power System:Active power in the power system depends on the power angle, means active power will flow in power system from bus having leading voltage power angle to lagging voltage power angle busbar. On the other hand, reactive power flow from the high voltage bus to low voltage bus in the power system, under this condition reactive power generated in the line (normally generated by alternator or shunt capacitance) is more compared to the reactive power absorbed (normally sink for reactive power will be series inductance) at the receiving end.

P (active power ) and delta (power angle) are closely coupled

Q (Reactive Power) and V (magnitude of the voltage) are closely coupled

Hence it is understood that for the flow of reactive power from one point to other in power system difference in the voltage should exist. Active power will flow in power system from leading power angle to lagging power angle even the voltage at receiving end is higher compared to sending end (still active power flows). 

Voltage and Frequency operating range (tolerance)

of Grid Supply in India

Frequency variation in the power system exists due to the mismatch between the supply of power and demand for the power. Voltage variations exist in the power grid is due to the mismatch in the reactive power between demand (MVAR) and available.In spite of all these variations there is certain limit for the operation limits (variations allowed) for voltage and frequency parameters dictated by the Grid Code. Any variations in the parameters (voltage and frequency) below operating limits considered as power grid is unhealthy and restoration steps will be taken to make the power grid healthy.In India according to Electricity Grid Code, the operation variation in the frequency and voltage allowed is given as below.Frequency Operating variation limits: 49Hz  to 50.5 HzVoltage operating variation limit:Voltage Rating          Maximum Voltage             Minimum Voltage  400 kV                       420 kV                               360 kV

  220 kV                       245 kV                               200 kV  132 kV                       145 kV                               120 kV

Islanding operation in Electrical Grid

Frequency and Voltage are two important parameters which decides the healthiness of the electrical grid at a particular instant of time.Frequency fluctuations occur in the electrical grid when there is mismatch between the amount of power generation and amount of power demand. Both under frequency and over frequency operation is harmful to the operation of power system. In electrical grid over frequency operation is a rare phenomenon.  Most of the cases we observe frequency drop phenomenon due to lack of power generation supporting the demand. Hence frequency in the power system drops below the rated.When the frequency tends to drop governor systems connected to the Turbo-Generators in electrical grid will act and allow more steam to flow to turbine and increase the speed. However there is limitation for this band. Beyond certain limit of operation if grid frequency is drop Islanding operation is employed in the grid.Islanding Operation:When the frequeny of the overall electrical grid is declining below the rated at pre determined frequency Ellectrical grid is divided into number of small islands which contains some generation units and load centers come out of the grid and operate together until the normal frequency is restored. This method of spliting the electrical grid in to small Islands is to protect the healthy part of the grid.

Methods to Improve Transient Stability

Transient Stability:Transient Stability is the ability of the power system to maintain the synchronism after the sudden large disturbance. These disturbances may be because of the application of faults, clearing of faults, switching ON and OFF surges in EHV systemTransient Stability Improvement methods: Transient stability of the system can be improved by increasing the system voltage. Increase in the voltage profile of the system implies increase in the power transfer ability.This helps in increasing the difference between initial load angle and critical clearance angle. Hence increase in power allows the machine to allows to rotate through large angle before reaching critical clearance angle.

Increase in the X/R ratio in the power system increases the power limit of the line. Thus helps to improve the stability

High speed circuit breakers helps to clear the fault as quick as possible. The quicker the breaker operates, the faster the fault cleared and better the system restores to normal operating conditions

By Turbine fast valving: One of the main reason for the instability in the power system is due to the excess energy supplied by the turbine during the fault period. Fast Valving helps in reducing the mechanical input power when the generator is under acceleration during the fault and hence improves the stability of the system

Use of Auto Re-closing: Majority of the faults in the power system will be momentary and can be self cleared.  Hence circuit breakers employed for fault clearance opens in sensing the fault with time delay of 2 cycles and re-closes after particular time to determine whether the fault is cleared. 

Some of the other ways to improve the transient stability are by employing lightning arresters, high neutral grounding impedance, single pole switching, quick Automatic Voltage Regulators (AVRs).

Properties of Sulfur Hexafluoride (SF6) gas

Sulfur hexafluoride gas (SF6) gas properties: Heavy, chemically inert, non toxic

No poisonous effect on the human body but decomposition products are poisonous

Color less and odor less

It is gaseous at normal room temperature and pressure

Density is about 6.6g/l at 20oC (5 times denser than air)

Critical temperature is at 45.6oC and can be liquefied by compression

Very good insulant with high dielectric strength

SF6 gas is electo-negitive ( tends to attract the free electrons and has the arc quenching property). Because of this main reason SF6 gas is used for arc quenching and insulation medium in circuit breakers.

The gas is highly stable

Unlike solid insulation materials, electrical breakdown of sf6 gas does not result in permanent deterioration of its properties

Decomposition occurs on the exposure to the electric arc. (Disassociation products will be SF2 and SF4 lower order fluorides)  

When does Proportional (P) Integral (I)

Derivative (D) Controllers are Used

When Proportional (P) Controller is Used:Proportional (P) Controller is used during the following conditions:

Load Changes are small

Offset (error exist due to difference between steady state value and desired value) can be tolerated

The process reaction rate is such as to permit a narrow proportional band. Since this reduces the amount of the offset

When Integral (I) Controller is Used:Integral (I) controller is used during:

When the offset must be eliminated

Integral saturation due to sustained deviation is not objectionable

When Derivative (D) Controller is used: Large transfer or distance velocity lags are present

It is necessary to minimize the amount of deviation caused by the system changes

When Proportional plus Integral (PI) Controller is used:Proportional and Integral action is employed when

System changes is small

Offset must be eliminated

No objection on the recovery duration

When Proportional and Derivative (PD) controller is used:PD action is employed when:

When the system changes are small

Improve the recovery time

When system lags are high

When Proportional, Integral and Derivative (PID) Controller is used:PID controller is used when system requires:

System changes are small

Offset must be eliminated

Fast recovery time

What are the different auxiliary systems in thermal

power plant?

Answer: Coal handling and storage, coal pulverizers,

water treatment plant, steam boiler, Draught

systems, Ash Handling systems, Steam turbine,

Circulation water system, Electrical Systems,

Control and Instrumentation, Pollution Control

equipment, and Fire Protection System

What is the purpose of Pulverizers?

Answer: Pulverizers serve two purposes, to dry the

coal and to grind the coal. They crush the coal to

size of 74microns

What are different types of Pulverizers?

Answer: Different types of Pulverizers are:

Ball tube mill

Ring roll mill

Hammer mill

Attrition type mill

What are different types of Steam boilers used in

Thermal Power Plants?

Answer: Different type of boiler used in thermal

power plants are:

Fire tube boiler

Water tube boiler

Natural Circulation boiler

Forced Circulation boiler

Once through boiler

What is the largest pump in the thermal power plant?

Answer: In thermal power plant Boiler Feed Pump

(BFP) is the largest pump, it delivers the water to the

boiler.

What are the major electric systems in Thermal Power Plant?

Answer: Major electrical equipment in thermal

power plant are

Turbine Generator

Exciter System

Generator Protection System

Generator Transformer

HT/LT switch gear

Electrical Switch-yard

What are the different Generator Protections employed in Thermal Power Plants?

Answer: Faults in the windings, Over load

protection, Over heating of windings or bearings,

Over speed protection, Loss of Excitation protection,

Motoring operation protection, Inadvertent

energisation, single phase or unbalanced current

protection, out of step operation protection, sub-

synchronous oscillations protection and earth fault

protectionHow Generator Transformer is cooled in Thermal Power Plant?

Answer: MVA power rating of the Generator

Transformer will be equal to the alternator. Hence

lot of heat will be generated while generator

transformer is under operation. Oil Forced Air

Forced (OFAF) type of cooling is employed for

generator transformerWhat are different types of circuit breakers employed in thermal power plant?

Answer: For low voltage operation

415/220V vacuum circuit breakers or air break

circuit breakers are employed. For voltage ratings

about 6.6kV and beyond SF6 circuit breakers are

employed.What type of cooling is provided for Generator in power plant?

Answer: Hydrogen gas cooling is employed for large

size generators because of better heat carrying ability

of the hydrogen. Hydrogen cooling is provided for

rotors and core of the generator. Water cooling is

provided for the stator of the alternator.Induction Machine Question and Answers:

Why Induction Motor name has come?

Answer: In Induction motor there is no electrical

contact to the rotor. Electrical currents are induced in

to the rotor circuit. Hence the name Induction motor

has come.

What happen when Induction motor run at

synchronous speed?

Answer: When the induction motor runs at

synchronous speed, there will be no induced emf in

to the rotor circuits. Therefore no rotor currents and

no torque is developed. Hence Induction motor will

come to halt.

What are different types of Induction Motors

available?

Answer: Induction motors are of two types based on

their construction

Squirrel cage Induction motor (As the rotor

winding will be like cage)

Slip Ring Induction motor or wound rotor

induction motor

What is the advantage of Slip Ring Induction Motor?

Answer: By providing external resistance to the

rotor circuits during starting of the motor starting

torque can be improved and starting currents can be

reduced

What is the advantage of Squirrel Cage Induction

Motor?

Answer: Low cost, rugged construction and less

maintenance are the advantages of squirrel cage

induction motors

What is the speed variation of the induction motor

from no load to full load?

Answer: The decrement in the speed of the small

rating Induction Motor from no load to full load will

be around 4 to 5 percent and 2 to 2.5 percent in the

case of large size induction motors

OPEN LOOP CONTROL SYSTEM:

Control System in which output quantity has no effect on the input quantity is called Open Loop Control System. Open Loop Control System has no facility to correct automatically the error generated in the output. From output of the system no feedback is given back to the input for correction. In Open loop control system the output can be varied by varying the input. But due the external disturbance system output may change. Any variation in the output from the desired once again attained by varying the inputs manually.

Advantages:

Open loop system is simple and economical

Construction of open loop system is easier

Open loop systems are generally stable

Disadvantages:

Open loop systems are inaccurate and

unreliable

The changes in the outputs due to external

disturbance are not corrected automatically

Closed Loop Control System:

Control system in which the output has an effect on

the input quantity in such a manner that the input

quantity will adjust itself based on the output

generated is called Closed loop Control System.

Open loop control system can be modified in to

closed loop control system by providing a feedback.

This feedback automatically corrects the changes in

the output due to external disturbance. Hence closed

loop control system is called automatic control

system.

Advantages:

Closed loop control systems are more

accurate even in the presence of non-linearities

The sensitivity of the system may be made

small to make the system more stable

The closed loop systems are less affected by

noise.

Disadvantages:

Closed loop control systems are costlier and

complex

The feedback in the closed loop system may

lead to oscillatory response

The feedback reduces the overall gain of the

system

Stability is the major problem in the closed

loop system and more care is needed to design a

stable closed loop system

Advantages of Bundled Conductors in Transmission

Lines

For transmission of more power for long distances to load centers Extra High Voltage (EHV) transmission is employed. Implementing Extra High Voltage has advantage of reduction in the copper losses and improves efficiency. However transmission of voltage beyond 300kV will poses some problems such as Corona effect which causes significant power loss and interference with communication circuits if round single conductor per phase is used. In order to reduce corona effect hollow round conductors are used. Keeping economical constraints other option is instead of using hallow round conductor it is preferable to use more than one conductor per phase which is called Bundled Conductors. For transmission of power beyond 400kV bundled conductors are employed.

Advantages of Bundled Conductors:Bundled conductors are primarily employed to reduce the corona loss and radio interference. However they have several advantages:

Bundled conductors per phase reduces the voltage gradient in the vicinity of the line. Thus reduces the possibility of the corona discharge. (Corona effect will be observed when the air medium present between the phases charged up and start to ionize and acts as a conducting medium. This is avoided by employing bundled conductors)

Improvement in the transmission efficiency as loss due to corona effect is countered.

Bundled conductor lines will have higher capacitance to neutral in comparison with single lines. Thus they will have higher charging currents which helps in improving the power factor.

Bundled conductor lines will have higher capacitance and lower inductance than ordinary lines they will have higher Surge Impedance Loading (Z=(L/C)1/2). Higher Surge Impedance Loading (SIL) will have higher maximum power transfer ability. 

With increase in self GMD or GMR inductance per phase will be reduced compared to single conductor line. This results in lesser reactance per phase compared to ordinary single line. Hence lesser loss due to reactance drop. 

High voltage Transmission Advantages and

disadvantages

Advantages: With increase in the transmission voltage size of the conductors is reduced (Cross section of the conductors reduce as current required to carry reduces).

As the reduction in current carrying requirement losses reduces results in better efficiency

Due to low current voltage drop will be less so voltage regulation improves

 Limitations: With the increase in the voltage of transmission, the insulation required between the conductors and the earthed tower increases. This increase the cost of line support

With increase in the voltage of transmission, more clearance is required between conductors and ground. Hence higher towers are required.

With increase in the voltage transmission, more distance is required between the conductors. Therefore cross arms should be long

What are the different substation components

Answer: Circuit Breakers, Disconnecting switches,

Grounding switches, Current Transformers, Potential

Transformers or Capacitor voltage Transformers,

Line Traps, Lightning Arrestors, Power

Transformers, Shunt Reactors, Current limiting

reactors, Station Buses and Insulators, Grounding

system, Series capacitors and shunt capacitors.

What are different types of busbar arrangement

Schemes in Substations?

Answer: Different switching schemes or busbar

arrangement scheme employed in switchyard are

Single Bus Scheme

Double bus single breaker scheme

Main and Transfer busbar scheme

Double bus single breaker scheme

Breaker and half scheme

Ring main bus scheme

Which type of bus arrangement is costlier and

more reliable?

Answer: One and half breaker scheme is most

reliable and costlier scheme. In one and half breaker

scheme 3 circuit breakers are provided for 2

busbars.  Hence the name is coined as One and half

breaker shceme

For distribution network which type of Switching

scheme is employed?

Answer: For distribution network Ring Main busbar

scheme is employed. It has the provision that if

power interruption occurs to one bus section due to

maintenance or fault, power can be fed the bus from

other side of the power system.

Factors on which material of station busbar depends?

Answer: Some of the factors on which station busbar

depends are:

Current Carrying capacity

Short Circuit Stresses

Establishing minimum electrical clearances

What are the different electrical clearances in

Substation?

Answer:

Ground Clearance: This is the minimum distance

between any live conductor and the earth or ground.

Phase Clearance: It is the clearance between the

different phases in a circuit or same phases in

different circuits

Sectional Clearance: This is the minimum

clearance between the live conductor and the limits

of work, where spacial clearance is related to the

safety of the operating personnel. The limits of work

section may be ground or the platform on which the

person is working.

Why EHV busbars are hallowed and cicular?

Answer: To avoid corona effect  

What are different Bus bar materials used in

Substation?

Answer: Busbars are generally made of copper and

Aluminum. Aluminim has the advantage of one third

the weight of the copper and also Aluminum

requires less maintenance and proper use of alloys

provide necessary rigidity required for bus material.

Aluminum is used widely in EHV and HV stations

What is the first equipment of the substation

connected to the incoming transmission line?

Answer: Lightning arrestor is the first equipment of

the substation connecting the incoming transmission

lines. It protects other substation equipment from

lightning and switching surges by suppressing them

to ground. Lightning arrestors also placed in the

substation between both the ends of major

equipment  such as power transformers for

protection. 

Different Levels of SCADA?

Answer: Different levels are SCADA systems are:

1. Field level devices and instruments

eg: Sensors

2. Remote Terminal Units (RTUs)

3. Communication system present

between the RTUs and Main Station

4. Main Control Station or Master

Station

5. Computer systems pertained to

processing the data for displaying to operator

Why SCADA systems are Implemented?

Answer: Some of the reasons or advantages why

SCADA systems are implemented are:

1. SCADA systems improves the

performance of the operation of the plant

2. SCADA systems provides better

protection to the equipment of the plant

3. SCADA systems

improves productivity of the personnel

4. Information receives very fast,

process the information and display it to

opertor in graphs and plots. Hence helps the

operator to take the decisions fast.

5. Provides better energy savings and

saves economy.

What is the capacity of CPU of Remote Terminal

Unit (RTU)?

Answer: Remote Terminal Units (RTUs) contain

generally microprocessor having 16 bit or 32 bit (eg:

68302 or 80386)

What is the Power Supply provided to the RTU?

Answer: Remote Terminal Units (RTUs) are

operated from 110V/240V + or - 10% and 50Hz or

12/24/48V + or - 10% DC Voltage

What are advantages of PLCs over RTUs?

Answer: Advantages of Programmable Logic

Controllers (PLCs) are they can be used for general

purpose can easily set for verity of functions. Some

of the advantages of PLCs over RTUs are: PLCs are

reliable, cheap, flexible compared to RTUs,

sophisticated control and troubleshooting in PLC is

easier. 

Squirrel cage Induction motor Advantages,

Disadvantages and Applications

Squirrel Cage Induction Motor:Advantages: Squirrel Cage Induction motors are cheaper in cost compared to Slip Ring Induction motors.

Requires less maintenance and rugged construction because of the absence of slip rings, brushes etc.

Squirrel Cage Induction Motors requires less conductor material than slip ring motor, hence copper losses in squirrel cage motors are less results in higher efficiency compared to slip ring induction motor

Squirrel cage motors are explosion proof due to the absence of brushes slip rings and brushes which eliminates the risks of sparking.

Squirrel Cage motors are better cooled compared to slip ring induction motors

Squirrel cage motors operate at nearly constant speed, high over load capacity, and operates at better power factor.

Disadvantages:

Main disadvantage of squirrel cage induction motor is that they have poor starting torque and high starting currents. Starting torque will be in the order of 1.5 to 2 times the full load torque and starting current is as high as 5 to 9 times the full load current. In slip ring induction motors this can be reduced by inserting  resistance during the starting of the machine and cutting down the resistance.

Squirrel cage induction motors are more sensitive to the supply voltage fluctuations. When the supply voltage is reduced machine draws more current on the other hand when the voltage increases it saturates the magnetic components of the machine. 

Speed control is not possible

The total energy loss during starting of squirrel cage motor is more compared to slip ring motors. This point is significant if the application involves frequent starting

Applications:Squirrel Cage Induction Motors are widely used in Industrial applications than slip ring induction motors due to cheaper in cost, rugged in construction, low maintenance. Squirrel Cage Induction Motors are suitable for applications where

the drive requires constant speed, low starting torque and no speed control drives.

Difference between Thermal and Nuclear Power

Plants

Thermal Power Plant:

In Thermal Power Plant Reactor follows Turbine. When load demand changes governor system of turbine operates accordingly depends on load demand by opening or closing steam value. Hence more or less inputs are taken to boiler according to the load demand.

Thermal Power Plants basically are peak load plants. However due to lack of power generation sources, thermal plants are forced to operate as base load plants.

Nuclear Power Plant:

In Nuclear Plants after reactor shut down still decay heat is produced due to the fission of the daughter nuclei and gamma radiation. Hence in nuclear power plant decay heat is to be removed continuously even the reactor is shutdown. Reactor cooling system must be continuously operated. 

In Nuclear Plants safety is prime criteria than production of electricity

Nuclear Power Plants are base load plants

In Nuclear Plants Turbine follows reactor. No variation of the turbine speed according to load demand. This is designed in order to avoid tripping of reactor.

Why Transformer rating is in KVA not kW

Rating of Electrical Transformer is in VA

(Volt Amperes ) because:

Maximum current carrying capacity of the

transformer is determined by the thickness of the

conductor and Maximum voltage supplied to the

winding depends on the insulation strength of the

conductor

Manufacturers of Transformers and alternators does

not know at what power factor the consumer uses the

machine and the nature of the load (capacitive,

resistive and inductive) connected to the machine.

Hence they rate the transformer depends on the

maximum current carry and the voltage to be

applied. This rating of the transformer corresponds

to apparent power of the machine (VA). Also iron

losses in transformer depends on the the voltage

applied and copper losses depends on the current

flowing through the winding. These losses are

independent of the power factor at which the

transformer operates.

On the other hand induction motors are rated with

real power (Watts) as the machine operating at

defined power factor at full load is

pre determined. Eg: Consider a transformer can

carry maximum current of 50A and max voltage

applied is 200V. Then

Power rating of the at full load (Unity power factor)

= 50x200 = 10kW Power rating of the at full load

(0.5 power factor)    = 50x200x0.5 = 5kW (This

means load having 0.5 power factor can connect

maximum of 5kW to the transformer. 5kW is the full

load of the transformer)

Mobile Substations Advantages and Applications

Mobile Substations are emergency power supply

substations employed temporarily to provide power

supply in the areas affected by the natural disasters

such as storms, cyclones and during wars. These

substations will have maximum rating of about

220kV.

Mobile Substation typically contains Trailer for unit,

Distribution Transformer, Cooling system for

Transformers,  Switchgear equipment (relays, circuit

breakers, isolators etc..)Advantages of Mobile Substation:

Mobility in service

Quick access for service

Less Maintenance cost

Minimum Clearance for utilization

Increase in Reliability

Maximum KVA with minimum height.

Application of Mobile Substations:

Mobile Substations are employed to provide

emergency power supply during natural disasters

and during wars

It also used to supply power when ordinary

conventional substation is under maintenance or

fault

It also employed for providing temporary

load supply to industrial plants, remote locations.

Factors affecting Size of Machine

Temperature and Machine size: Size of any motor, Generator or Transformer

depends on the Temperature rise in the machine. Temperature rise in the electrical machine beyond the designed value will damage the insulation of the motor. Temperature rise in the electrical machine is due to the heat generated due to losses. Current flowing in the windings in the machine generates heat. This generated heat should be dissipated else it damages the insulation of the motor.

For smaller rating motors, generators and transformers natural cooling is sufficient. However for larger rating machines heat generated will be more, hence generated heat should be removed with the help of coolents such as water either by natural circulation or forced circulation. Fans and blowers were also used to remove the generated heat.In  large alternators hydrogen cooling is employed to remove the generated heat

Low Voltage winding near core of the

Transformer

For explaining the operation of the transformer Low voltage winding (LV winding) and High voltage winding (HV winding) are wound on different limbs of the transformer for easy explanation. However in practical each limbs of the core will have both LV winding and HV winding wound together. 

In Transformer design LV winding will be wound close to the core of the transformer because the insulation provided between the LV winding and core of the transformer is quite less compared to the insulation provided between the HV winding and the core of the transformer which results in less cost for insulation and reduction in the size of the transformer for same MVA rating.

Hence due to this reason LV winding is placed near to the core of the transformer. 

On the core of the transformer insulation is placed and LV winding is wound. Once again insulation is placed between the LV winding and HV winding. 

Series and Shunt Reactors in Power system Shunt Reactor are used for reactive

compensation:During normal operation of an electrical power system, the transmission and distribution voltages must be maintained within a small range, typically, from 0.95 to 1.05 pu of rated value. Due to the load variations, shunt reactors and capacitors have been applied inpower systems to compensate excess reactive power (inductive for heavy load conditions, and capacitive for light load conditions). Shunt reactors are commonlyused to compensate the capacitive reactive power of transmission and distribution systems and thereby to keep the operating voltages within admissible levels.

Series Reactor for limiting the fault current.

DC component in Asymmetrical Short

Circuit Current ?

In symmetrical current the envelops of the peaks of the current waves are symmetrical with respect to zero axis. In asymmetrical currents the envelops of the peaks of the current waves are not symmetrical about the zero axis. If we plot short circuit waveform in oscillogram it appears that the short circuit currents are nearly asymmetrical for the first few cycles after the short circuit occurs and this asymmetrical behavior is maximum at the instant of the short circuit and then current waveform gradually becomes symmetrical after the few cycles of short circuit.

These asymmetrical alternating currents when considered as a single wave is difficult to interpret for circuit breaker applications and relay settings purpose. Complex formulation is involved for calculations of the circuit breaker applications and relay settings purpose. Hence this asymmetrical component is resolved in to simple components consists of (1) symmetrical ac component and (2) dc component. The sum of these two components at any instant is

equal to the magnitude of the total asymmetrical component of the ac wave.

DC component present is considered as offset and the magnitude of this component depends on the instant at which short circuit occur. It varies from zero to maximum value which is equal to peak of the asymmetrical component.

DC component have decaying nature with respect to time after the short circuit occur and dissipate as I2R loss in the circuit. The decrement or rate of decay of the dc component is proportional to the X/R ratio. If the ratio of reactance to resistance is infinite (zero resistance), dc component will never decay. On the other hand if the reactance of a line is zero, the decay takes place instantly.