DC Motors Application in Thermal Power Plant

In thermal power plants dc motors are employed for certain control and critical emergency operations which are supplied by dedicated batteries. DC motors operate as backup drives for normal ac drive systems when ac power supply to the plant is lost.

In thermal power plant, the dc motors finds applications for performing control functions such as

• Turbine governor motor
• Governor limit setting
• Motor operated rheostats
• Emergency lubrication for the turbines (main, boiler feed pumps)
• Generator (H2 oil seal).
• DC motor operated valves

The emergency type services are of short duration. The emergency lube oil pump running time depends on the turbine-generator run-out time with vacuum or broken vacuum. The Hydrogen gas seal oil pump may be operated for 3 to 4 hours. The turbine-generator emergency bearing oil pumps provide lubrication, the emergency seal oil pumps maintain Hydrogen seal in case ac power is unavailable. The dc motor operated valves usually perform essential services, i.e., one ac and one dc operated valve in parallel or in series with each other will assure opening or closing, respectively.

DC motors employed in thermal plants are classified in to two types based on the type of application.

• DC motors carrying out Control function 
• Dc motors carrying out Emergency function

Control functions:

This category consists of the turbine governor motor, governor limiting setting, motor operated rheostats, etc. These motors are small, about 1/8 hp or less. They are operated quite infrequently for short duration.

Emergency functions:

This category consists of turbine-generator emergency (lubrication) bearing oil pumps and emergency seal oil pumps. Such pumps may also be provided for steam turbine drives of feedwater pumps, fans, and other large loads. The lack of lubrication during a shutdown without ac power will ruin the linings of the bearings and damage the shaft. 

Hydrogen seal oil pump is provided to prevent the escaping of hydrogen (for large turbine-generators hydrogen cooling is provided for efficient cooling) from the casing by providing a tight seal with high pressure oil.

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Three Phase Winding Transformer Advantages

Generally in power system mostly two winding transformers are employed. But three winding transformers are employed because of some advantages: 

• The most common reason for having a three winding transformer is to provide a delta connection tertiary winding
• To limit the fault level on the low voltage system of the transformer by dividing the LV infeed (in order to provide double secondary windings) 
• Providing tertiary winding helps to interconnect different power system operating at different voltages (Three winding transformer helps provide power supply at two different secondary voltages, 220kV/11kV/6.6kV transformer can able to provide power at two different voltage levels (11kV and 6.6kV) 
• To regulate the voltage and reactive power of the system by providing synchronous capacitor connected to one of the terminals of the transformer 

Why Delta winding prefer:-

It is always desirable to have one delta connection winding in the three phase transformer as delta connected three phase winding will offer low impedance path for the three phase currents. Also the presence of delta connected three phase winding allows to circulate the current around the delta winding in the event of unbalance loading condition 

Although power system designers aims to avoid use of star/star transformer in power system but cases will arise when the phase shift between the star/delta and delta/star is not applicable such as in the power station supplying power to auxiliary system. Therefore it is common practice to have a star/star with delta tertiary three winding transformer supplying power to the plant auxiliary system

B/H Curve of the magnetic material (core of the transformer) is not linear. Is a sinusoidal voltage (flux) is applied across the primary winding, the magnetizing current obtained will not be sinusoidal in nature and consists of fundamental component and several harmonics. Third harmonic components predominate with several other higher harmonic components. If there is no delta connected winding, or if the star connections of the transformers are not grounded, the line to earth capacitance currents supply system lines supply the harmonic components. If the harmonic components cannot flow in any one of these paths then, secondary voltage will be distorted.

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SCADA System Applications

Applications of SCADA:

SCADA (Supervisory Control and Data Acquisition) systems have a wide range of applications. SCADA technology is widely used where the process systems spread in wide range of area away from control center. These process system applications require control, monitor and frequent intervention for better operation of the system. In these applications the process system can be controlled by Master Station through two way communication. Control applications of SCADA includes opening and closing switches, circuit breakers, valves, and starting and stopping of the motors etc.

Applications in power plant control:

A group of Hydro and Gas generation plants are to be started when the load demand exceeds the generating capacity, these plants are considered as peak load plants because these plants can start in no time and deliver power to the grid. These plants will be located in the remote locations. These plants can be controlled by opening and closing the valves of the turbine so that they can deliver the power in peak conditions and can be kept on standby during normal load conditions.

Applications in Oil and Gas Industry:

Many process control parameters and equipments such as fluid measurement equipment, gathering systems and pumps which are spread over the wide area in the field. Control and monitoring applications include turning on and off the motors and pumps, gathering meter information of various process parameters (pressure, viscosity, flow rate..) continuously and takes certain decisions. This can be done through SCADA systems which monitor and control the processes.

Applications in pipelines:

Pipelines carrying oil, gas, chemicals and water which are located at varying distance from the plant needs continuous monitor and control. Control includes opening and closing of valves, starting and stopping the pumps.  Monitoring the flow rate, and other parameters to avoid leakage in the pipelines by acquiring the data and carrying out suitable controls if required through SCADA systems

Applications in Power Transmission:

Electrical power transmission which is spread over thousands of kilometers can be controlled by opening and closing of the circuit breakers and other functions, this is done in master control substation which can control the other substations through SCADA systems.

Application in Irrigation system:

Irrigation systems which are spread over wide area can be controlled by closing or opening the valves, gather the meter values of amount of water supplied and can control from the master station if required.

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SCADA Systems Interview Questions and Answers

SCADA Interview Questions 2013:

Different Levels of SCADA?

Answer: Different levels are SCADA systems are:

1. Field level devices and instruments eg: Sensors
2. Remote Terminal Units (RTUs)
3. Communication system present between the RTUs and Main Station
4. Main Control Station or Master Station
5. Computer systems pertained to processing the data for displaying to operator

Why SCADA systems are Implemented?

Answer: Some of the reasons or advantages why SCADA systems are implemented are:

1. SCADA systems improves the performance of the operation of the plant
2. SCADA systems provides better protection to the equipment of the plant
3. SCADA systems improves productivity of the personnel
4. Information receives very fast, process the information and display it to opertor in graphs and plots. Hence helps the operator to take the decisions fast.
5. Provides better energy savings and saves economy.

What is the capacity of CPU of Remote Terminal Unit (RTU)?

Answer: Remote Terminal Units (RTUs) contain generally microprocessor having 16 bit or 32 bit (eg: 68302 or 80386)

What is the Power Supply provided to the RTU?

Answer: Remote Terminal Units (RTUs) are operated from 110V/240V + or - 10% and 50Hz or 12/24/48V + or - 10% DC Voltage

What are advantages of PLCs over RTUs?

Answer: Advantages of Programmable Logic Controllers (PLCs) are they can be used for general purpose can easily set for verity of functions. Some of the advantages of PLCs over RTUs are: PLCs are reliable, cheap, flexible compared to RTUs, sophisticated control and troubleshooting in PLC is easier. 

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BHEL electrical Interview questions

BHEL electrical questions:

BHEL interview questions are listed below.

1. Explain about the Torque-speed and Torque-Slip Characteristics of Induction motor?

2. Methods to improve the power factor of Induction motors?

3. Conditions to operate transformers in parallel?

4. Why power transformers are designed for maximum efficiency at full load?

5. Resolve EX-oR gate using Universal gates (nand or nor)?

6. What is the use of capacitor in single phase Induction motor?

7. Explain about the 'V' and inverted 'V' curves of synchronous motor?

8. What is power system stability?

9. Explain about different switching schemes employed in power plant (busbar arrangements)?

10. What is step potential and touch potential?

11. What is string efficiency of insulators?

12. What is electrical corona? conditions that affect the corona?

13. What is surge impedance loading of a line? What happen when load of line is equal to surge impedance loading?

14. Why we use test signals in control systems to find the response of the system?

15. Explain P, I, PI, PD, PID controllers?

16. Differences  open loop and closed loop system

17. What is demand factor, load factor, load curve?

18. How speed of dc shunt motor can be controlled?

19. Different braking methods of dc motor?

20. Applications where dc motors out rule the ac motors?

21. what is the difference between micro processor and micro controller?

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BHEL 2013 General Engineering Previous Paper

BHEL ET general engineering paper 2013:

1. Chlorine acts as bleaching agent in the presence of ___

2. Satellites receive power from ___ (Solar cells)

3. Rotometer is used to measure the ______ (flow rate of fluids and gases)

4. For ideal gas PV/RT is ____ (Unity)

5. Strain Gauge is used to measure ____ (pressure)

6. Type of flow when Reynolds number of fluid is high _____ ( Turbulent)

7. When Iron ball, Wooden ball and cork ball is dropped in to liquid which will reach the bottom first__ (Iron ball)

8. Hook's law is applicable upto the _____ (proportionality limit)

9. Moderator used in nuclear reactor______ (To slowdown the fast neutrons by collision)

10. Fuel used in fast breeder reactor_____ (PU 239 )

11. Which of the following is not an electromagnetic wave____ (Ultra sonic waves)

       a) X ray b) gamma rays c) Cathode rays d) Ultra sonic waves

12. Compression ratio of petrol engine is ____

13. Coplanar forces are _______ (forces acting along the same plane)

14. What is Bakelite ____

15. Bernoulli principle explains about the _____ (energy conservation)

16. When the Length of the simple pendulum is Doubled what happens to energy (Question is something like that) ___

     a) E b) E/2 c) 4E d) 8E

17. LVDT is used to___

18. One question is on Fraunhofer lines?

19. Above curie temperature magnetic materials act as__

20. Synchronous motor has similar construction to ___ (Alternator)

21. Which of the motor is best used for speed control ___ (DC Shunt motor)

22. Variables that define the properties of the gas__ (Question  is something like that)

      a) Pressure b) Volume c) Temperature d) All the above

23. Why boron is used in Stainless steel___ (To reduce corrosion)

24. Definition of Center of Buoyancy

25. Which is hardest __

 a) Cast Iron b) Mild iron c) other options

26. Why rain drop is spherical__ (Surface Tension)

27. Which of the following has low conductivity __

    a) Copper b) Aluminum C) Steel d) Air

28. Which of the following cannot be used as Permanent magnet __

29. One simple question on angular velocity___
30. Magnetic north pole in uniform magnetic field align towards___ (Geometric south of the earth)
31. All electrical machines are __ (electro mechanical conversion devices)

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BHEL 2011 Electrical Previous Question Paper

BHEL Executive Trainee (ET) Electrical previous paper:

Number of Questions:26

I am providing some of the objective questions appeared in the BHEL recruitment exam which i remember. Please correct me if i am wrong

1. MHO  relay is used for the protection of: 

a) Long distance Transmission line

b) Medium length lines

c) Short transmission lines

d) None of the above

2. Reactance Relay is

a) Voltage restrained directional relay

b) Directional restrained over current relay

c) Voltage restrained over current relay

d) None of the above

3. Which relays are doesn't affect by power swings

4. When a transmission line connected in parallel to other transmission line

a) Stability decreases and fault current decreases

b) Stability increases and fault current decreases

c) Stability decreases and fault current increases

d) Stability increases and fault current increases

5. Software interrupts in 8085 microprocessor:

6. Number of address and Data lines in 8085 microprocessor

7. When Halt instruction is executed in microprocessor

8. RL circuit was given and asked to find the value of Inductor

9. Circuit with resistor values are given and asked to solve the equivalent resistance (quite easy circuit given)

10. Small circuit was given and asked to solve the current flowing through the resistor (simple circuit)

11. In the presence of earth capacitance value of  transmission lines ___ (Increases)

12. L-G fault phase difference between positive and negative sequence currents

13. For complete protection of 3 phase line ___ ( Two phase and One earth fault relay is sufficient )

14. During slip test in synchronous motor the voltage across the field wingdings is ___ (zero)

15. Reactive power in synchronous machine is limited by ___ (field current and power angle )

16. For changing the direction of rotation in dc motor armature is used because __

17. Which type of DC motor has negative voltage regulation __

18. Auto transformer voltage transformation ___ (HV-LV)/HV

19. Asked to find out linear system from different equations ___

20. SCR turns off when __ (current falls below holding current )

21. AC voltage converter ___

22. BP register pointing data in __ (Stack Segment )

23. Question related to 8259 and ICW

24. Semi Closed or completely closed rotor bars are employed to __

25. Asked to find phase margin for an open loop transfer function

26. Matrix A, B,C and D is given asked to find transfer function

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BHEL 2013 Electrical Interview Questions

2013 Interview Questions:

These are some of the interview questions asked in BHEL 2013 Electrical Interviews. Questions are collected from the Net and some of my friend's interviews.

Be prepare to answer some basic questions such as

• Tell me about yourself
• What do you know about BHEL
• Why do you want to join BHEL
• Final Year Project  Some basic questions from Bio-data.

Electrical Questions:

1. What are the different conditions for parallel operation of Transformers?
2. What are V and inverted V curves in Synchronous motor and their significance?
3. Explain electrical corona? and methods to reduce the electrical corona?
4. Explain about P, PI and PID controller? Their advantages and disadvantages?
5. What are the different braking methods used in dc motors?
6. What is water hammering phenomenon?
7. Advantages of DC motors over AC motors?
8. What is Load factor, Demand factor, Installed capacity, spinning reserve?
9. Which power plants can be used as a peak load plants. Reason?
10.What is String efficiency in insulators?
11. What is Transient stability and methods to improve transient stability?
12. What is single line diagram and the assumptions required?
13. What is Step Potential and Touch potential in substations?
14. What are different switching schemes employed in substations?
15. How Speed of DC shunt motor is controlled?
16. What is the difference between Microprocessor and Microcontroller?
17. What is SIL and its significance?
18. Realize Ex-OR using NAND gates?
19. What is Race around condition?
20. Why single phase induction motor is not self starting and methods to start single phase induction motor?

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Battery Interview Question and Answers 2014

What is primary cell and secondary cell?

Answer: Battery which can be discharged only once is called primary cell ex: Manganese Zinc and alkaline Secondary cells are the battery which can be charged and discharged for a number of times ex: Lead acid and Nickel cadmium

Do temperature have effect on battery?

Answer: Temperature is one of the major factors that have impact on battery performance, shelf life, charging and battery voltage control. At higher temperature batteries will have more chemical activity compared to batteries which operate at the designed temperature

What factors that decide the VRLA battery to completely charged from fully discharged condition?

Answer: Some of the important factors that determine the duration of the discharged battery to charge are: Duration of the discharge occurred Temperature at which battery is operating Size and efficiency of the charger

Can VRLA batteries be enclosed in sealed containers?

Answer: VRLA batteries should not be enclosed in the sealed containers. Oxygen and hydrogen gases are produced in the battery, however they once again convert back to water. But during over charging these gases escape out of the cell. If the batteries are enclosed inside a sealed container, the pressure inside the container due to hydrogen and oxygen increases and may explode

What is stratification in the battery?

Answer: During charging of the battery acid is produced at the plate surface and during discharging of the battery acid will be consumed, due to this nonuniform concentration heavy parts of the electrolyte settles at the bottom and lighter parts of the electrolyte floats on the top. This phenomenon is call stratification. Stratification affects the life of the battery.

What is end of life of the battery?

Answer: If a battery cannot able to hold proper charge or if the battery capacity is falls below 80% of its rated capacity then it is considered as the end of life of the battery. Many factors affect the life of the battery Charging and discharging cycles Temperature Leakage of the gases or electrolyte corrosion of the plates over discharge rates

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Valve Regulated Lead Acid (VRLA) Batteries Advantage:

What is Batteries-

A container consisting of one or more cells, in which chemical energy is converted into electricity and used as a source of power.

Types of Batteries:-

Nickel Cadmium (NiCd) — mature and well understood but relatively low in energy density. The NiCd is used where long life, high discharge rate and economical price are important. Main applications are two-way radios, biomedical equipment, professional video cameras and power tools. The NiCd contains toxic metals and is environmentally unfriendly.

Nickel-Metal Hydride (NiMH) — has a higher energy density compared to the NiCd at the expense of reduced cycle life. NiMH contains no toxic metals. Applications include mobile phones and laptop computers.

Lead Acid — most economical for larger power applications where weight is of little concern. The lead acid battery is the preferred choice for hospital equipment, wheelchairs, emergency lighting and UPS systems.

Lithium Ion (Li‑ion) — fastest growing battery system. Li‑ion is used where high-energy density and lightweight is of prime importance. The technology is fragile and a protection circuit is required to assure safety. Applications include notebook computers and cellular phones.

Lithium Ion Polymer (Li‑ion polymer) — offers the attributes of the Li-ion in ultra-slim geometry and simplified packaging. Main applications are mobile phones.

Valve Regulated Lead Acid (VRLA) Batteries are popularly termed as maintenance free lead acid batteries. Each cell in VRLA battery will have a value (automating operating) to allow the gas to escape if developed by the over charging the cells and to hinder the entrance of oxygen from the air into the cell

These VRLA batteries are also called sealed lead acid batteries and also recombinant batteries. Oxygen which evolve during discharging at the positive plate of the battery recombine with the hydrogen which  evolve at the negative plate of the cell to from water. This results in prevention of the water loss and unlike flooded batteries water is not needed to be added periodically. The valve present in VRLA cell acts as a safety valve which opens when the concentration of hydrogen gas inside the battery increases dangerously high. Some of the advantages of VRLA cell is listed below:

Advantages:

• VRLA battery is maintenance free and no periodic water refilling is not required
• Reduced demand on ventilation
• The capacity range of VRLA battery is high (12000 Ah)
• VRLA batteries will have longer life. Life span of this batteries will be around 10 years
• Feasibility for float charge operation with the same voltage of the cells, to continue in using existing charging equipment
• VRLA batteries are economical

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