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

Questions & Answers on Power Transformers


1-What do you mean by power transformer?

It is a static Electro-magnetic machine which transforms alternating current from one AC voltage to another AC voltage at same frequency & at the same apparent power (KVA).

2-What is the principle of Power transformer operation?

Power transformers work on the principle of electromagnetic induction. Which states that, EMF induced in a closed conducting circuit when the magnetic flux linking with that circuit changes in time.

3-What is the main function of a power transformer?

Generally it is used for stepping up or stepping down of Voltage to desired level

4-What are the various parts of power transformer?

General arrangement of Power Transformer

  • Casing
  • Core
  • Primary & secondary windings
  • LV & HV bushings
  • Radiators & cooling system
  • Conservator
  • Breather
  • Protection devices like Buchholz relay, relief valves, temperature sensors

5-Why does the oil conservator is placed at higher elevation?

Oil conservator is placed at a slightly higher level than that of the tank. It accommodates the contraction & expansion of oil level during lower & higher loads respectively. At higher load, oil temperature rises and hence level in the conservator rises & at lower load, oil temperature decreases & level in conservator drops down.

The above cushioning in oil level is by cushioning bag present in conservator, the air cushion in the conservator permits expansion & contraction of the oil tank without contact with moist air.

6-What is the function of breather in Transformers?

Breather is installed in a pipe from conservator. One end is connected to air cushion bag in the conservator, other end is connected to external air.

Breather is filled with dry silica jel, generally pink in colour. When oil in the conservator rises, air is let out through the breather. During low load when oil level in the conservator decreases, air is sucked into the cushion bag through breather. Silica jet absorbs the moisture & lets only dry air. Wet silica jells are blue in color.

7-What is the function of Buchholzs relay?

It is fitted in the pipe between conservator tank & main oil tank. It operates by gas during arcing or short circuits

8-What are the various cooling methods employed in Power transformers?

  • ONAN: Oil Natural & Air Natural
  • ONAF: Oil natural & Air forced
  • OFAF: Oil forced & Air forced
  • OFWF: Oil forced & water forced
  • AN: Air natural

9-What is the function of Pressure relief valve in Transformers?

It is fitted on tank to vent out the gases formed in oil & hence protects the transformer from explosion

10-What do you mean by small, medium & large transformers?

  • Small transformers: < 5 KVA
  • Medium transformers: 5 to 500 KVA
  • Large transformers :> 1 MVA

11-What do you mean by core type transformer?

In this type of transformers, winding surround the limbs of core

12-What do you mean by the Shell type transformer?

In this type of transformers, core surrounds the major portion of the windings

13-What is the relation between voltage, current and number of turns on coils in a transformer?

We have following relation:

Vp/Vs = Np/Ns = Is/Ip

Where, Vp and Vs are Voltage on primary and secondary side.

Np and Ns are No. of turns on primary and secondary coils.

IP and Is are Primary side current and secondary side current of a transformer.

14-A 10 KVA single phase 2200/220 Volts Transformer has 60 turns on secondary side, then calculate Primary current, no. of turns on coil and secondary side current.

Given that,

Vp = 2200 V

Vs 200 V

Ns 60

Transformer rating 10 KVA

We know that Vp/Vs = Np/Ns

Np 60 X 2200/220 = 600 turns

Further, KVA rating of transformer is (Vp Ip)/1000 and (Vs X Is)/1000

10 2200 Ip/1000

Therefore, current on primary side Ip 10 X 1000/2200 = 4.54 Amps

Similarly current on secondary side Is 10 1000/220 45.45 Amps

15-A Power transformer’s input voltage is 11 KV & output voltage is 110 KV,then calculate the number of turns on secondary side, if Primary side winding has 25 turns

We have

V1 / V2 = N1 / N2

11 / 110 = 25 / N2

N2 = 250 Nos

16-A power transformer input voltage is 11 KV & output voltage is 220 KV, then calculate the secondary side current if it has 2300 Amps of current on primary side?

We have

V1 / V2 = I2 / I1

11 / 220 = I2 / 2300

I2 = 115 Amps

17-What are the materials of composition of lamination cores?

Laminations are thin 0.2 to 0.3mm thick silicon sheets. These are further coated by varnish or insulation oxide.

18-Why do the laminations are made up of silicon steel sheets?

Higher the silicon content in steel sheets increases the resistivity & reduces the eddy current losses. But silicon percentage is restricted up to 3.25% avoid brittleness of sheet.

19-What are the various tests carried out on Transformers?


  • IR value measurement
  • Magnetic balance test
  • Magnetizing current test
  • Capacitor measurement
  • Ratio test
  • Vector group test
  • Induced voltage test
  • Temperature rise test
  • BDV test
  • No load current test
  • No load loss test

20-What is the significance of magnetizing current test?

  • This test is performed to locate the defects in the magnetic core structure,
  • To detect the shifting of winding, failure
  • To detect the problems in tap changers

21-What are factors which affect IR value of transformer?

  • Surface condition of the terminal bushings
  • Quality of oil
  • Quality of winding insulation
  • Temperature of oil
  • Duration of application & value of test voltage

22-What are the various protections given for power transformers?

  • Differential protection for earth faults
  • High oil temperature & high winding temperature protection
  • Over current protection
  • Over fluxing protection
  • Protection against fire
  • Protection against lightening
  • Buchholz relay for gas & arcing protection
  • Pressure relief valve

23-On what factors transformer loading depends on?

  • Transformer current
  • Winding temperature
  • Oil temperature

24-What are the effects of Transformer over loading?

Overloading results into

  • Cellulose insulation becomes mechanically weak & resulting in winding failure
  • Oil gets rapidly oxidized

25-What is the function of tap changer in Power transformers?

Tap changer is for adjusting the secondary voltage

26-What do you mean by No-load current?

The current flowing through the terminal of a winding when rated voltage is applied at rated frequency the other winding being left open circuited

27-What do you mean by no-load losses in Power transformers?

The active power absorbed when rated voltage at rated frequency is applied to the terminals of one winding, with other winding being left open circuited

28-What parameters of Transformer oil are tested yearly?

Transformer oil qualities are recommended as per IS-335-1963

  • Density
  • Kinematic Viscosity
  • Flash Point
  • Pour point
  • Neutralization Number (Acidity)
  • Sludge
  • Moisture content
  • Dissolved Gas Analysis (DGA)
  • Dielectric Dissipation test
  • Interfacial tension
  • Break down Voltage

29-What is the importance of Transformer BDV test?

BDV test is done to detect moisture, dirt & conductive particles in the oil. The BDV value should be more than 50 KV

30-Briefly explain the BDV test of transformer oil?

This test applies an AC voltage of frequency 40 to 60 Hz through two polished electrodes having diameter 12.5mm to 13 mm with oil gap around 2.5 to 4.0 mm.Rise in voltage between the electrodes is at uniform rate of 2 KV/sec.Thus voltage is increased from zero to till breakdown.

The final value will be arithmetic mean of 6 consecutive tests



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.
220 VDC & 110 VDC
Protection system
Emergency lighting
Trip & closing coil of circuit breakers
Operating mechanisms
48 VDC
Telephone communication
+/- 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 instrumentations for Power plant Mechanical Engineers

1. What is an instrumentation?
Instrumentation is the art of measuring a value of plant parameters.
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


15-Emergencies in power plant operation

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