1-What is an electrical energy?
It is an invisible form of
energy, whose presence cannot be seen, but can be felt by its various effects
like heating, lighting and chemical effect.
2-What are the different sources of electrical
energy/power?
Hydraulic power,
thermal power, Diesel power, Nuclear power, Wind power, solar power, geothermal
power and tidal power.
3-What is an electrical current?
Electrical
current is continuous flow of electrons in a conductor, in a particular
direction. It is measured in amperes (A).
4-What is Electromotive force (E.M.F)?
It is the force which
circulates or tends to circulate the electric current, in an electrical
circuit. In S.I unit it is measured in volts (V).
5-What is potential difference (PD)?
Potential difference
between any two points of an electrical circuit is the difference in electrical
pressure between these points.
6-What do you meant by voltage?
It is the measure of electrical pressure
between two points of an Electrical circuit
7-Define electrical resistance (R) of a conductor
It is the property of a
conductor, which opposes the flow of current through it. The unit of this is
Ohm (Ω).
8-Draw the relation between current, voltage and
resistance.
Voltage = Current X Resistance,
i.e. V = IR
9-State the laws of resistance (R).
Resistance of a conductor
is directly proportional to length (L) of the conductor. Inversely proportional
to the cross sectional area of the conductor.
R =L/A
10-What is the flow of current at 2 Ohm resistance,
when 230 V voltage is applied across a conductor?
Given that,
Current I =?, R = 2 Ω, V =
230 V
We know that, I =V/R
I = 230/2
I = 115 A
11-Find the resistance of a resistor, which takes 4A
current when connected to 110 V supply.
R = V/I
R = 110/4
R = 27.5 Ω
12-State Ohm’s Law.
Ohm’s law states that
current flowing through the conductor is directly proportional to voltage (V)
applied to the ends of the conductor and inversely proportional to Resistance
(R) of the conductor.
I = V/R
13-If a conductor of Resistance R0 at 0 °C is heated to
temperature t °C, then how do you calculate its resistance after heating?
Resistance of a conductor
at temperature t °C is given by,
Rt = R0 (1 + α t) Where
α is
temperature coefficient at t °C.
14-On what parameters the resistance of a conductor
depends?
Following factors affect
the resistance of a conductor:
- It directly varies as its length L
- It inversely varies as the cross section area A of the conductor
- It directly varies as the temperature of the conductor
Resistance R = L/A, where
L and A are length and area of a conductor.
15-Define electrical power.
The rate at which
electrical energy is expended or electrical work is done is called power (P).
The unit power is Watt.
Power = Work done
watt-sec./Time in second
P = VI X t/t i.e. VI Watts
P = I2R… Watts
16-A coil of 200 Ω resistances takes current of 10A.
Find the voltage applied and power consumed by the coil.
We know that, Power (P) = I2R
= 102 X 200 = 20000 Watts
Power consumed by coil is
20 KW (1 KW = 1000 Watts)
By ohm’s law, Voltage (V) =
IR = 10 X 200 = 2000 V
17-What is magnetic field?
The space or
region around a magnet, where magnetic flux exists is called as magnetic field.
18-Define reluctance.
Opposition to the creation
of magnetic flux in a magnetic circuit.
19-What is the phenomenon of electromagnetic induction?
The phenomenon by which an
E.M.F is induced in a conductor, due to change in the flux linking with the
conductor is called as electromagnetic induction.
20-What is mutual inductance (M)?
Mutual inductance between
any two coils, placed nearer, is the ability of one coil to produce an EMF in
the other coil, when the current in the first coil changes at the rate of 1
ampere per second.
21-What is Alternating current (AC)?
The current which changes
periodically, both in a magnitude and direction, at regular intervals of time
is called as alternating current (AC).
22-Explain the Power factor in an Electrical circuit.
Cosine of an angle (Փ) between
applied voltage and the total current in a circuit is called as power factor.
It is merely a number and is always less than unity.
Power factor = CosՓ
Power factor (cosՓ) = Active
power (KW) / Apparent power (KVA)
As per Indian Electricity rules it should be
always be greater than 0.9
23-What is apparent power?
It is the vector sum of
active and reactive powers, measured in KVA.
24-What is the significance of active and reactive
power?
In inductive loads like AC
motors, induction furnaces, transformers active power is required for
performing work and reactive power is to create and maintain electromagnetic
fields.
25-How do
you calculate the active power of DC, AC single phase and three phase power
supply?
DC active power = V X I
AC single phase power = V X I X Cos Փ
AC three phase power = √3 X V X I X Cos Փ
26-How do you calculate reactive power
for single and three phase of AC?
AC single phase power = V X I X Sin Փ
AC three phase power = √3 X V X I X Sin Փ
27-What is the significance of power factor?
Power
Factor has Following Advantages:
At unity or higher power factor, maximum power can be transferred
at same distribution capacity.
Reactive component of the network is reduced so the total current
in the system from the source.
I2R losses will reduce.
A high PF can help in utilizing the full capacity of the
electrical system.
28-How to improve power factor in an electrical system?
Addition of
power factor correction capacitors to the plant distribution system. They act
as reactive power generators and provide the needed reactive power to
accomplish KW of work.
29-Explain the terms leading, lagging, unity & zero
power factors.
Leading
Power Factor:
If the current leads
voltage by a certain angle, then that circuit is said to have a leading P.F,
capacitive circuit has leading P.F.
Lagging
Power Factor:
If the current in a
circuit lags the voltage by a certain angle, then that circuit is said to have
a lagging P.F, Inductive circuit has lagging P.F.
Unity
Power Factor:
If the current and voltage
are in a same phase in an electrical circuit, then that circuit is having zero
P.F, a pure resistive circuit has unity P.F.
i.e. P.F = Cos Փ = Cos 0 =
1
Zero P.F:
If the current in a
circuit leads or lags the voltage by an angle 90° then that circuit is said to
have zero P.F
Pure inductive and
capacitive circuits have zero P.F
i.e. P.F = Cos 90 = 0
30-What is a Transformer?
Transformer is
an electro-magnetic device, which transfers electrical energy from one circuit
to another circuit, either at the same voltage or at a different voltage, but
at the same frequency.
31-What are the different parts of transformer?
Transformer
has the Following Main Parts:
- LV and HV bush
- Oil conservator
- Radiator
- Dehydrating breather
- Earthing terminals
- Buchholz relay
- Temperature sensors and gauges
- Oil level indicators
32-What is the significance of Buchholz relay?
Buchholz relay is
connected in the pipe between the tank and the conservator. It sounds an alarm
when gases are generated in oil due to small discharge or arcing.
33-What is the purpose of Breather in transformer?
Breather
is filled with silica jell, when oil in the conservator rises, air is let out
through the breather, when the oil level is contracted during low loads/low
temperature, air is breathed in by the conservator through the breather, where
silica jell absorbs moisture and admits only dry air.
34-What are the different types of
transformer cooling system?
- ONAN: Oil natural air natural, used up to 55 MVA
- ONAF: Oil natural air forced, used in 30–60 MVA transformers
- OFAF: Oil forced air forced, used above 60 MVA
- OFWF: Oil forced water forced
- AN: Air
natural, used up to 1.5 MVA
35-What is current transformer (CT)?
Current
transformers are used for stepping down the AC current from higher value to
lower value for measurement, protection and control. This works on AC only.
36-What is voltage (potential) transformer (PT)?
Voltage
transformers are used for stepping down the AC voltage from higher value to
lower value for measurement, protection and control. This also works on AC
only.
37-What is switch gear system?
The equipments like
switches, fuses, circuit breakers, isolators, relays and control panels
associated with the fault clearing process in an electrical system are covered
by the term “Switch gear.”
38-What is a circuit breaker?
It is an automatic device
capable of making and breaking an electric circuit under normal and abnormal
condition such as circuits.
39-What are the various types of circuit
breakers?
- Air circuit breaker (ACB)
- Oil circuit breakers
- Minimum oil circuit breakers
- Air blast circuit breaker
- Sulphur hexafluoride (SF6) circuit breaker
- Vacuum circuit breaker (VCB)
40-Explain circuit breakers MCB, MPCB, MCCB, ELCB, RCCB
and Contactor.
MCB: Miniature
Circuit Breaker is an electro-mechanical device which guards an electrical
circuit from an over current that may effect from short circuit, overload or
imperfect design. Used for voltage level 230 and 440 V. Normally trip
characteristics are not adjustable. Can be used up to 100 Amps.
MPCB: Motor
power control circuit breaker, Protection against electrical faults such as
short circuits, line-to-ground faults and line-to-line faults. The MPCB can
interrupt any electrical fault that is below its breaking capacity when a motor
draws electric current above its name plate value for an extended period of
time. Overload protection is normally adjustable in MPCBs.
MCCB: Molded
case circuit breaker, the operation range is up to 1000 Amps. Trip
characteristics can be adjusted. Thermal/thermal magnetic operation.
ELCB: Earth
Leakage Circuit Breaker, this is used to protect the circuit from the electrical
leakage. When someone gets an electric shock, then this circuit breaker cuts
off the power at the time of 0.1 seconds for protecting the personal. This
circuit breaker connects the phase, earth wire and neutral.
RCCB: Residual
Current Circuit Breaker is essential current sensing equipment used to guard a
low voltage circuit from the fault. It comprises of a switch device used to
turn off the circuit when a fault occurs in the circuit. RCCB is aimed at
guarding a person from the electrical shocks. Fires and electrocution are
caused due to the wrong wiring or any earth faults. This type of circuit
breaker is used in situations where there is a sudden shock or fault happening
in the circuit. Whenever there is any ground fault, then it trips the circuit. These
are a very effective type of shock protection.
41-What are the different types of
motors?
- Induction motor
- Slip ring motors
- Direct current motors and
- Synchronous motors
42-Where the direct current motors are used?
The areas of application
where high starting torque and smooth acceleration over a broad speed range are
required.
43-How do you calculate synchronous speed of a motor?
Synchronous speed/RPM = (120
X frequency)/No. of poles.
44-What do you mean by slip?
The difference between
synchronous and full load speed is called slip.
Slip% = (Synchronous speed - Full load rated speed) X 100/Synchronous speed
45-What is neutral earthing?
It is connecting the
neutral point of three phase AC system to earth by a low resistance conductor.
46-Differentiate between neutral earthing and equipment
earthing.
Neutral earthing is
connecting a current carrying conductor to earth, neutral point of star winding
is connected to the ground.
Equipment
earthing is refers to the earthing of non-current carrying parts such as Motor
body, switch gear enclosures, transformer tanks etc. Equipment earthing
provides safety to personnel against severe shock from faulty electrical
equipment.
47-What are the various DC voltage levels used in power
plants?
- 24 V: Control
circuits, DC measuring instruments and static relays.
- 48 V: Telephone
communication, microwave protection and communication.
- 110 and
220 V: Protection systems, Emergency lighting, Operating mechanisms, trip
and closing coils, Emergency oil pump etc.
49-Explain Ampere hour (AH) capacity of battery?
The
capacity of the battery system is specified in terms of ampere hours (AH). It
is ampere hours which can be obtained from the charged batteries before
reaching the minimum voltage. It is the product of discharge current and
discharge time.
50-What is quick charging and trickle charging of
batteries?
Quick
Charging: During heavy loads, the battery gets drained quickly, the battery
charging equipment should have adequate rating to charge the batteries quickly.
Charging of battery within a hour is called as quick charging.
Trickle
Charging: The station battery is constantly connected to the load circuit. A
constant charging current is equal to the average load current provided to
compensate for continuous natural discharge. Such charging is known as trickle
charging.
51-What is MCC, PCC and Control panel in electrical
system?
MCC: It is the
motor control center. It provides power supply to all the motors in plant and
having circuit breakers of each individual motors in it. It receives main
incomer power from PCC.
PCC: It is the
power control center, where it receives main power from generator and
distributes to different MCC.
Control
Panel: Control panel consist of a controller. Controller may be PLC, DCS,
relay or some other type. It gives digital input signal to the MCC panel to
start the motor. Control panel works based on the PLC/DCS program or the relay
logic. Instruments are normally connected to control panel. Indications for the
interlocks also will be there in control panel.
52-What is Single line diagram (SLD) in an electrical
system?
SLD is a
simplified layout or notation for representing a three-phase power system. In
SLD electrical elements such as circuit breakers, transformers, capacitors, bus
bars and conductors are clearly shown by standardized schematic symbols.
53-What is a power cable? Explain colour
coding of cables.
A current carrying
conductor, enclosed within insulation system, fillers, screens, servings and
armors.
Colour coding:
For single phase supply:
- Phase: Red
- Neutral: Black
- Earth: Green
For three Phase supply:
- Phase : Red, Blue and Yellow (RBY)
- Neutral: Black
- Earth: Green
54- What is the
difference between wire and cable?
A wire is
a single conductor (material most commonly being copper or aluminum) while
cable is two or more insulated wires wrapped in one jacket. Multiple conductors
that have no insulation around would be classified as a single conductor.
55-List out the functions of Automatic Voltage
Regulator (AVR).
Controls the voltage of generator.
Controls the power factor of generator.
It limits the stator and rotor current.
It controls flux rates of generator.
AVR has control over eddy current losses.
Performs field flushing
& suppression.
56-What is the function of PMG and diodes
in AC generators?
The PMG (Permanent Magnet
Generator) is a system which is used for secondary exciting. The PMG provides
stable and reliable electric energy for AVR regardless the generator’s terminal
voltage. The generator with PMG excitation system can provide 300% rated
current during short-circuit, which occurs for 5–10 seconds.
The most
common function of a diode is to allow an electric current to pass in one
direction (called the diode’s forward direction), while blocking it in the
opposite direction (the reverse direction). As such, the diode can be viewed as
an electronic version of a check valve. This unidirectional behavior is called
rectification, and is used to convert alternating current (AC) to direct
current (DC).
57-What is synchronization in an
electrical system?
Synchronization
is the process of matching voltage, frequency and phase sequence of two
isolated electrical systems
58-Which methods are generally used for
synchronization?
Lamp
method and Synchronoscope method.
59-What are the
different protection systems provided for Generator?
- Generator Over current Protection
- Earth Fault Protection
- Generator Differential Protection
- Reverse Power Protection:
- Low Forward Power Protection:
- Negative Phase Sequence Protection:
- Over Voltage and Under Voltage Protection
- Over Frequency and Under Frequency Protection
- Rotor Earth Fault Protection
- Loss of Excitation Protection:
- Stator over Heating Protection
60-What is Corona? What is its effect on
transmission lines?
High voltage lines during
cloudy and rainy seasons create ionization of air surrounding the surface of conductor
giving audible, violet, visible, intermittent discharge called corona.
Corona
creates hissing sound, vibrations and power loss and radio interference
