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

Batteries & troubleshooting


  

1-List down the applications of auxiliary DC power supply system in Power plants?
  •  Protections relays
  • Circuit breakers
  • DCS
  • Trip circuits
  • Signaling
  • Telephone system & communication
  • Emergency lighting
  • Transmitters & Positioners
  • Emergency oil pumps
  • Metering panels

2-List down the various DC voltages & their applications in power plants
Sl No.
Voltage
Applications
1
220 VDC & 110 VDC
Protection system
Emergency lighting
Trip & closing coil of circuit breakers
Operating mechanisms
2
48 VDC
Telephone communication
3
+/- 24 VDC
Control circuits
Measuring instruments
Static relays

3-What is the power source for DC supply?
Main power source is lead-acid storage battery & AC/DC rectifier set
4-What are the installation requirements for lead acid battery ?
  • The room should be isolated from other system
  • Room should have adequate lighting & ventilation
  • The room walls & floor should have acid proof tiles
  • Battery cells should be installed on insulated racks
  • Racks should be placed on insulator generally porcelain insulators
  • Cables should be acid proof
  • The DC bus bars should be of round or flat copper materials
  • Bus bars connections are made by soldering or brazing
  • The positive terminal of the battery is Red & negative terminal should be blue
30-selected QnA on power Transformers
5-How do you calculate the maximum discharge current of batteries?
Maximum discharge current = (Continuous load (W1) + Short time Load (W2)) / Rated Voltage
6-How do you specify the batteries?
Batteries have following specification:
  • Rated voltage
  • No.of cells
  • AH capacity
  • Arrangement of cells
  • Details of charging equipment

7-What do you mean by AH of batteries?
AH (Ampere hour) is the capacity of battery. It is the product of current and total time required for discharge. It depends on the magnitude of discharge current.
8-How do you decide the number of batteries for a particular battery bank?
No. of battery cells = Bus bar voltage / Voltage of one cell
9-What are the various charging methods employed for batteries?
  • Initial charging
  • Quick charging
  • Trickle charging
60-Basic questions & Answers on power plant electrical system

10-What do you mean by trickle charging? & what is its significance?
Trickle charging is constant charging current that is supplied to battery to compensate its discharge current.
Batteries get discharge when not in use. Hence to avoid sulphating, the batteries should be kept continuous charging by trickle charging.
11-What do you mean by primary cells (dry cell) & secondary cells?
Primary cells are non rechargeable cells & Secondary cells are storage batteries
12-What are anode, cathode & electrolyte in Lead-acid batteries?
  • Cathode: Sponge lead
  • Anode: Lead Peroxide (PbO2)
  • Electrolyte: Sulphuric acid & water

13-What is the voltage level of lead acid batteries that are generally employed?
It is 2 V
14-What is the voltage level of fully charged & fully discharged lead acid battery?
Voltage of fully charged cell is 2.5 V & 1.75 V for fully discharged cell
15-What will happen during battery discharge & charging of lead acid battery?
During discharging process, the lead & lead peroxide het change into lead sulphate. And during charging lead sulphate gets convert into lead & lead peroxide
16-How do you check the specific gravity (SG) of electrolyte of lead acid battery?
Specific gravity (SG) is checked with the help of Hydrometer
17-What is the SG of fully charged & fully discharged electrolyte
SG Fully charged electrolyte: 1.28 & 1.12 for fully discharged electrolyte
18-What is the proportion of Sulphuric acid & water in Lead acid batteries?
Sulphuric acid: 64% & Distilled water: 36%
19-How do make up electrolyte level in lead-acid batteries?
Lower level of electrolyte is due to evaporation of water.
If the lower electrolyte level is due to evaporation of water, then add distilled water. And if If the lower electrolyte level is due to spilling over bubbling, then add acid & water solution.
20-Explain the charging method of lead-acid battery?
During charging process current forced into battery from opposite direction to its normal direction of current flow.
That is during normal use current leaves from positive terminal & enters through negative terminal. During charging current enters through positive terminal & leaves through negative terminal
21-Why do the lead-acid batteries get discharge when they are not in use?
This is because internal discharge current due to impurities in the electrolyte
22-What electrolyte is used in Nickel-Iron batteries?
Potassium hydroxide
23-What is the voltage level & density of electrolyte of Nickel-Iron battery?
It is 1.2 V & 1.2
24-Which electrolyte is used for Nickel Cadmium batteries?
Potassium hydroxide
25-What precautions do you take to avoid over discharging of battery?
  • Over discharging can be avoided by
  • Regular charging of battery
  • Avoid excessive discharge rate
  • Avoid short-circuit currents & leakage current
  • Schedule charging & discharging
  • When not in use, battery terminals should be considered
Basics of power plant instrumentations
26-What do you mean by sulphated Battery?
Discharge battery with abnormal hardened lead sulphate on its plate is a Sulphated battery
27-What actions you will take if Battery is charging slowly?
  • Check battery connections & rectify if there are loose connections
  • Check charger capacity
  • Add distilled water or acid water solution if electrolyte is low
  • Check sulphatation of battery & rectify it
  • Increase current rating
  • Check connection that is positive lead of charger to positive of battery & negative to negative

28-What are the reasons for overheating of battery while charging?
  • Poor ventilation
  • High charging rate

Lower specific gravity of electrolyte
29- What are the reasons for overheating of battery while discharging?
High discharge rate
Separators between plates have damaged
30-What are the reasons for cells having unequal voltages?
  • Over discharge
  • Lower electrolytes level
  • Foreign material in electrolyte
  • Positive plate worn out



Questions Answers on basics of instrumentation for Power plant Mechanical Engineers

1. What is an instrumentation?
Instrumentation is the art of measuring a value of plant parameters.
Or
The technology which is used to measure and control the process system of plant is called instrumentation.
2. What is an instrument?
An instrument is a device that transforms a physical variable of interest (the measurand) into a form that is suitable for recording (the measurement).
3. What is measurand?
A physical quantity, property, or condition being measured. Often, it is referred to as a measured value.
4. What is a sensor?
The portion of a measurement system that responds directly to the physical variable being measured.
5. What are the various variables measured in a power plant?
Pressure, temperature, force, speed, frequency, current, voltage, resistance, time, displacement, acceleration, light intensity, density, pH, conductivity, flow etc.
6. What is calibration of a sensor?
The relationship between the physical measurement variable input and the signal variable (output) for a specific sensor is known as the calibration of a sensor.
7. Define true value.
The desired result of an experimental measurement.
8. Define error of an instrument.
The error of an instrument is defined as the difference between the true value and measured value.
 9. What is accuracy?
An expression of the maximum possible limit of error or the closeness of the agreement between the result of a measurement and a true value of the measurand.
10. What is an actuator?
An actuator is a device that is responsible for moving or controlling a mechanism or system.
11. Define the terms input and output of control system.
Input is an applied signal or an excitation signal applied to a control system from an external energy source in order to produce a specified output.
Output is the particular signal of interest or the actual response obtained from a control system when input is applied to it.
12. What are the various sub systems of instrumentation to control the power plant operation?
The efficient and reliable operation of the power plant requires following sub systems.
Monitoring Instruments: These include electromagnetic and electronic indicators, multi point/multi pen recorders, digital display units, recorders, drum level and flame indicators, ammeters, voltmeters etc. These are very essential for starting, running and shutdown of the units.
Transmitters and Sensors: These include, thermocouples, RTDs, pressure and level transmitters, differential transmitters. Generally transmitters are of two wire transmission principle and provide current signal of 4–20 mA.
Local Instruments or Field Instruments: These include pressure and temperature gauges, level switch, pressure switch, draft gauges, vibration switches etc.
Analytical Instruments: They include measurements required to ascertain chemical condition of process medium at various points. These include pH and conductivity analyzers, dissolved O2 analyzers, silica analyzers, smoke and density meters, SOx and NOx analyzers.
Analog and Digital Control System: The control systems in thermal power plant cover various closed loop control system. It employees sophisticated digital computers for process control. The control systems have
Interconnection with measurement systems, interlocks, protection, sequential control system annunciation and data acquisition system. Programmable Logic Control (PLC) and Digital Control System (DCS) find wide applications in modern power plants.
Control Modules: Control modules are provided to control the equipments.
Protection and Interlock Systems: The operation of the protection is accompanied by audible and visible annunciations to indicate primary causes for operation of protection. Both relay based and logic based methods are adopted.
Control Drives: Include pneumatic dampers, solenoid operated valves, control valves, motorized valves etc.
Power Supply Arrangement: The control and instrumentation devices are provided with power supply sources having back up/standby arrangements. Uninterrupted power supply modules (UPS), battery chargers, unit battery banks, station battery banks, normal/emergency switch gear supply power to the control and instrumentation.
13. What is a control system? What are the various controls used in boiler?
To control means to regulate, to direct or to command. Hence a control system is an arrangement of different physical elements connected in such a manner so as to regulate, direct or command itself or some other system.
  • Drum level control
  • Steam temperature control
  • CBD tank level control
  • Combustion control
  • Deaerator level and pressure control
  • Furnace draught control
  • Soot blower pressure control
14. What is an automatic control system?
Automatic control is to maintain a desired value of a parameter or the condition in a process system by measuring the existing value, comparing it to the desired value and employing the difference to initiate corrective action for reducing the difference.
15. What is control action? What are its different types?
Controller action is to maintain the controlled variables of the process at the desired value set by the operator.
Different Types of Control Actions:
Proportional control action
Integral control action
Derivative control action
16. What are the main components of DCS?
DCS system has following main units:
  • Main servers
  • Operation systems
  • Firewall
  • Controllers
  • Digital input and output modules
  • Analog input and output modules
  • Field terminal assembly
17. What is server?
Server is a master computer whose main purpose is to handle request for data from other peripheral computers.
18. What is redundancy of a system?
Redundancy means supporting simultaneously or back up for any online system. This term is often used in DCS, where redundant server is needed. Redundant server works as main server when main server fails. Similarly we have redundant controller also.
19. What is the difference between consoles and flex stations?
In server Architecture of DCS, there are two types of communication depending on the configuration of the client. There are 2 types of clients – Flex Station and Console station.
Flex station talks to server for data exchange. So, if the server fails, the flex stations do not receive anything and go blind.
The console stations directly talk to controller for IO status, set points, and alarm set points. So, if the server is down, still the plant can be visualized on these computers, dynamic data exchange is possible, you can start/stop the motors, and you can open/close the valves through console stations. (Motors can be started/stopped only if they are connected through hardwired IOs/connected to the controller serially).
20. What are fire wall in DCS?
A firewall is a security device that can be a software program or a dedicated network appliance. The main purpose of a firewall is to separate a secure area from a less secure area and to control communications between the two.
21. What do you mean by controller? What are its functions?
A controller is a comparative device that receives an input signal from a measured process variable, compares this value with that of a predetermined control point value (set point) and determines the appropriate amount of output signal required by the final control element to provide corrective action within a control loop. An Electronic Controller uses electrical signals and digital algorithms to perform its receptive, comparative and corrective functions.
An electronic sensor (thermocouple, RTD or transmitter) installed at the measurement location continuously sends an input signal to the controller. At set intervals the controller compares this signal to a predefined set point. If the input signal deviates from the set point, the controller sends a corrective output signal to the control element. This electric signal must be converted to a pneumatic signal when used with an air operated valve.
22. What are the different types of standard signals used in Industrial Instrumentation measurement?
Standard signals used in instrumentations are 4–20 mA, 0–5 VDC, 1–5 VDC, 24 VDC etc.
23. Why 4 mA and 20 mA are used instead of 0 mA and >20 mA in signal measurement?
For error detection 4 mA is good, If instrument is faulty, it can be easily identified. If the output is 0 mA it will be difficult to identify whether it is a faulty signal or a good signal.
20 mA is used as maximum because the human heart can withstand up to 30 mA of current only. For safety reason 20 mA is chosen as maximum value.
24. What is pressure?
It is the force per unit area.
Pressure = Force/Area
25. What is absolute pressure?
Absolute pressure is the sum of atmospheric pressure and gauge pressure.
Absolute Pressure = Gauge Pressure + Atmospheric Pressure
26. What are the various pressure detectors?
Bourdon tubes, bellows, capsules, diaphragms, strain gauges and manometers.

27. Explain the piezoelectric and capacitive methods of pressure measurements in transmitters.
Piezoelectric Method:
These are certain small crystalline materials which generate voltage when they are subjected to forces or stress. Crystalline materials when stressed or forced they develop electric charges, these charges are measured and converted.
Example: Naturally available Quartz, synthetic crystals, Rochelle salts and barium titrate.
Capacitive Method:
In this method the mechanical movement of mechanical material like diaphragm changes the capacitance of an electronic component.
28. What is pressure transmitter?
A pressure transducer, often called a pressure transmitter, is a transducer that converts pressure into an analog electrical signal.
29. What is the difference between pressures switch and pressure transmitter?
Pressure Switch is an electromechanical device which measures the pressure in a system and when the pressure reaches too high or too low of a given set point, the device will switch meaning it will open orclose the circuit that powers a certain devices like creating alarms, start or stop of pump, open or close of valve etc.
Pressure Transmitter is also an electromechanical device which senses the pressure and sends signal to remote device for further measurement and control. For an example continuous measurement of steam pressure, lube oil and control oil pressure measurement etc.
30. What is the difference between pressure transducers and pressure transmitters?
Transmitters are current output devices and transducers are voltage output devices.
31. What are the different types of temperature sensors used in power plant?
RTD, Thermocouples, thermometer, infrared sensors, bimetallic devices.
32. Which effect is used in thermocouple?
See back effect is used in thermocouple.
33. What are active and passive sensors?
Active sensors generate an electric current in response to an external stimulus which serves as the output signal without the need of an additional energy source. Such examples are a diode, piezoelectric sensor, and thermocouple.
Passive sensors require an external power source to operate, which is called an excitation signal. The signal is modulated by the sensor to produce an output signal. For example, a thermistor does not generate any electrical signal, but by passing an electric current through it, its resistance can be measured by detecting variations in the current or voltage across the thermistor.
34. What is the function of thermo-well in thermocouples or temperature sensors?
Thermo-well is used to protect the thermocouple from harmful atmosphere, corrosive fluid, and physical damage.
35. How do you identify J, K and S type thermocouples on their physical appearance?
  • J Type Thermocouple: Positive wire is white and negative wire is red.
  • K Type Thermocouple: Positive wire is yellow and negative wire is red.
  • S Type Thermocouple: Positive wire is red and negative wire is white.
36. What are the different types of RTDs used in power plant?
2 wire, 3 wire and 4 wire RTDs are generally used.
37. What is RTD PT-100?
RTD PT-100 has resistance of 100 ohms at temperature 0 °C.
38. What is the change in resistance at every 1 °C rise in temperature of RTD PT-100?
0.384 ohm resistance will rise/lower at every 1 °C rise/lower in temperature.
39. What are the different types of flow meters used in power plant?
Following are the Flow Meters used in Power Plant:
  • Venturimeter
  • Orifice meter
  • Flow nozzle
  • Turbine flow meter
  • Mass flow meter
  • Magnetic flow meters
  • Rotameters
 40. What are the different types of orifice plates? Explain.
Different Types of Orifice Plates are:
Concentric: These plates are used for ideal liquid as well as gases and steam service. Concentric holes are present in these plates that’s why it is known as concentric orifice.
Segmental: This plate has hole in the form of segment of the circle. This plate is used for colloidal and slurry flow measurement.
Eccentric: This plate has the eccentric holes. This plate is used in viscous and sherry flow measurement.
41. Why negative voltage is used in turbine vibration probes?
The voltage found between the sensor tip and the target is a negative DC voltage. The negative bias voltage used by proximate may be due to its safety. The negative voltage may vary while adjusting the gap that can be checked by the signal conditioner used by the probe.The extreme end of the gap gives a negative DC voltage of 24 and its zero when the gap between sensor and target is almost lapping.
42. What is load cell?
Load cell is transducer which converts a force into an electrical signal. It comprises of strain gauge which is connected to Wheatstone bridge circuit in load cell.
43. What is a control valve? What are different types of control valves?
A control valve is a device capable of modulating flow at varying degrees between minimal flow and full capacity in response to a signal from an external control device. The control valve, often referred to as “the final control element,” is a critical part of any control loop, as it performs the physical work and is the element that directly affects the process.
44. What is the function of positioner?
A positioner is a device put into a valve to ensure that it is at a correct position of opening as per the control signal. An I/P converter only sends the opening/closing request to valve but cannot confirm its position.
                Positioner senses the valve opening through a position feedback link connected to valve stem which is its input signal. I/P converter output is its set point input. The difference between these two is the error signal based on which the positioner positions the valve to correct position to reduce error to zero. Hence positioner is nothing but a pneumatic feedback controller.
45. What do you mean by direct acting and reverse acting control valves?
If the controller output increases when the measurement value rises, it is a direct-acting controller, and if controller output decreases when the measurement rises, it is a reverse-acting controller.
     46. What is the function of an actuator in control valve?
 An actuator is a pneumatic powered device which supplies force and motion to open or close a control valve.
4   47.What is Solenoid Valve? What are its main types?A solenoid is electrically operated valve. It consists of solenoid coil in which magnetic plunger moves. This plunger is connected to the plug and tends to open or close the valve.
There are two types of solenoid valves:
                Normally Open
                Normally closed
Questions & Answers on batteries
48.What are the input and output signals of Turbine governor?
Governor Inputs:
  • Power supply 230 V AC/110 V AC
  • 4–20 mA MW signal
  • Main steam inlet pressure
  • 3rd Extraction pressure
  • 4–20 mA HP and LP governor valve demands
  • Turbine speed
Governor Outputs:
  • Control signals to HP and LP actuators
  • Light fault and heavy faults
  • Electrical and mechanical over speed
Also read basic questions & answers on power plant electrical system

                 

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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