Showing posts with label Electrical & Instrumentation. Show all posts
Showing posts with label Electrical & Instrumentation. Show all posts

60-Best Questions & Answers on Basics of Electrical for Power plant Mechanical Engineers

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

15-Emergencies in power plant operation

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