tstransco AE electronics Preparation

Notification for tstransco Telecom exam is out and there will be lots of doubt about what to study and how to score more. First topic and scoring but most people forget is the general awareness topic, it covers Telangana related topics history geography and current scenario, latest national affairs and English and aptitude.  This can be concentrated in last one week to make sure you will not forget at the last moment. If we go across the previous question papers there can be an idea of the general questions.

When comes to the subject point of view as the syllabus covers the entire gate syllabus. Concentrate on the topics from where questions can be asked more. As the Transco deals with transmission lines it may contain electromagentic questions, communication and signals and systems can be asked more and is the important are. The edc, analog and digital circuits a general overview may be sufficient if we go through the previous gate and ies papers we come across lots of theoretical questions which may be asked.  Maths there can be basic questions again the previous gate papers questions can be more than sufficient. Major concentration area can be electromagnetics, communication, signals and systems.

Primary water and cold gas temperature control

Primary water and cold gas temperature are two important parameters to cool the stator windings and the rotor of the generator respectively. The primary water flows through the hollow stator windings of the generator thereby absorbing the heat from the stator windings. This primary water is then cooled using the DM water through the plate heat exchangers which are used for cooling the Primary water. The cooling of the primary water depends upon the primary water temperature control valve whose setpoint is some constant value above the setpoint of the cold gas temperature control valve


The cold gas or the hydrogen gas is used to cool the generator rotor the hydrogen gas is cooled again with the DM water from various coolers. The cold gas temperature control valve set point is some constant plus the temperature of the cold gas. 

Turbine overspeed trip or protection

In a thermal power plant turbine overspeed trip is one of the most important protection which saves the turbine to control the overspeed when the unit suddenly trips.

There are various reasons for turbine over speed such as sudden load throwoff, load rejection, boiler trip, turbine trip, generator trip. When the load throwoff and rejection happens on the grid the turbine speed tend to raise to supply the additional load requirement due to load throwoff or rejection. In this this case the load rejection relay (LSR) gets operated and drain the hydraulic oil which closes the HP and IP turbine control valves partially for sometime to control the turbine speed.

When the turbine trips sudden due to boiler trip, turbine protection trip or generator breaker open the turbine tends to get overspeed due to large steam available during this case the HP and IP emergency stop valves full close and reduce speed of the turbine.

The overspeed at which the turbine to be tripped is generally 10 percent above the normal rotating speed. There will be three types of overspeed trip. They are mechanical trip, logical trip and finally electrical first mechanical trip if it fails then goes for logical trip, if it also fails the sure tripping is the electrical trip

For mechanical trip there will be a setting at the first bearing of the turbine where a spring will get acted at overspeed and which drains the oil supply to the stop valves there by resulting in turbine trip.

At the first bearing there will be trip oil pump which is attached to a spindle and this spindle is attached to a small element which is attached to a spring. When the turbine exceeds the maximum speed then the trip oil pump will push the spindle inside, the element will then pull the spring inside which is connected to a limit switch, which acted as the spring moves inside draining the auxillary oil which closes the stop valves resulting in turbine trip.

FET

FET stands for field effect transistor. As the name stands it works as transistor using the effect of the transistor. The FET is unipolar device unlike BJT which is a bipolar device. In FET only the majority charge carriers are responsible for the transistor action. There are various types of FET . They are
1. JFET
2.MOSFET

the above two are again subdivided into n-type and p-type FET's. The source and the drain indicate the type of the FET.

Actuators in thermal power plants

Actuators are known as movers which is used in opening and closing of the valves. There are various types of actuator they are pneumatic, hydraulic and electrical. Pneumatic and hydraulic actuators in the entire set they are more precise compared to the electrical actuators. The high end applications use hydraulic as they operate more accurately as per the command extended and the chances of faulty operation or extent of damage or regular repairs are very less.

The pneumatic actuators are used in medium but important areas they are less accurate and less costly compared to the hydraulic actuators and servicing of these actuators is to be done regularly. 

The electric actuators are less accurate and very cheap among the three and are used in higher number in the thermal power plants.

The hydraulic actuators are generally used in very important areas of thermal power plant where high and fast response is needed such as hpbypass and lpbypass. These are very crucial and there accurate response saves plant under shutdown and startup. The malfunction in these areas provides disastrous results for a thermal power station.

The pneumatic actuators are generally used in lesser but important applications such as feed water station and drain valves of the heaters.

The electrical actuators are used in every area of a thermal plant and are used individually or in support of the hydraulic and pneumatic actuators.

The selection of the actuators mainly depend on the reliability to work in various temperature areas as the thermal plant consists of very high temperatures. It also depends on the reliability and the durability. Specially actuators are used in combination with the control valves.

The major problems always arises with the electrical actuators as these contains number of electrical components which will be damaged more frequently compared to the pneumatic and hydraulic actuators or control valves. The major problems that arises are limit switch adjustment, pcb card problem, transformer card damaged, fuse blown off, cable shortage.

BIPOLAR JUNCTION TRANSISTOR (BJT)

The transistor is a three terminal semiconductor device which is used for multiple applications such as amplifier, switch etc...

Bipolar Junction transistor or BJT uses two charge carriers for it's operation holes and electrons. The three terminals of BJT are emitter, base and collector. The emitter is heavily doped semiconductor and moderately sized, the base is lightly doped semiconductor and is smaller in size, the collector is moderately doped and is larger in size among the three terminals. There are two types of BJT based on the semiconductor material used for the three regions. They are
1) NPN
2) PNP

NPN is the most commonly used BJT.  the base is p-type semiconductor while the emitter and collector are n-type semiconductor materials. The P type semiconductor is sandwiched or placed between two n-type semiconductors. The BJT is used in three basic configurations they are

1)Common base where base is common between emitter and collector. This is used as buffer.
2) Common emitter where emitter is common between input and output , this configuration is most commonly used and have universal applications such as amplification and other important circuits.
3) Common collector where collector is common between input and output.

The charge carriers usually flow from emitter to collector and the current direction is indicated by the arrow Head at the emitter end. In PNP transistor holes are the majority carriers while electrons are minority carriers where as in NPN holes are minority carriers and electrons are majority carriers.

Control and instrumentation in a thermal power plant

Control and instrumentation is considered as the heart of any major power plant, steel factories or space stations as the major problems can be avoided using the control and instrumentation equipment. Few advantages we get with control and instrumentation in a thermal power plant are below:

1. We can avert major damages caused due to raising temperatures by putting a setpoint for temperature which is measured using RTD  or TC
2. The opening and closing of the control valves, actuators or dampers is done through commands. The position of these valves can be known using the position feedback installed in respective areas.
3. There are open and close feedback control systems which take decisions based on the feedbacks received.

The equipments as part of the control and instrumentation are

1. RTD
2. THERMOCOUPLE
3. PRESSURE TRANSMITTERS
4. LEVEL TRANSMITTERS
5. PRESSURE SWITCHES 
6. LEVEL SWITCHES
7. TEMPERATURE SWITCHES
8. SOLENOIDS
9. AIR FILTER REGULATORS.
10. I TO P CONVERTERS
11. CONTROL VALVES
12. ACTUATORS
13. CONDUCTIVITY , PH AND OTHER TRANSMITTERS ANS ANALYZERS
14. RELAYS
15. COILS
16. MCB 
17. DIODE CIRCUITS
18. DP TRANSMITTERS
19. DP SWITCHES

RTD stands for resistance temperature detector which detects the temperature of the location where these RTD is installed by varying it's resistance. There are 2,3,4 wire RTD which improve the accuracy of temperature detected. 4 wire RTD is most accurate one.

The TC stands for thermocouple is used to generate millivolts voltage based on the temperature at the junction of the thermocouple. There will be temperature converters for thermocouples which are used to convert these millivolts into temperatures. Generally thermocouples are used to detect temperatures from 200-600 deg c temperatures in a thermal power plant.

Level transmitters are used to find the level of liquid in a particular tank or vessel. For example level of the boiler drum indicates the amount of water present in the drum, if sufficient level is not maintained in the drum it should be properly filled using the boiler feed pumps. If level transmitters are not present then it will be difficult to maintain the drum level with in specified range.

Conductivity and PH transmitters are used to find the conductivity and PH value of the liquid respectively. Conductivity/PH transmitters are mainly used in the primary water system of the generator to find the conductivity of the primary water and proper NAOH dosing of the primary water needs to be done if the conductivity moves into acidic region. Where as the conductivity/Ph analysers are used in the swas system.

Control valves are one of the main areas of the thermal power plant which will be operated frequently depending on the conditions set by the operator and in auto mode. There are two two types of control valves. They are Hydraulic and pneumatic control valves. The pneumatic control valve is operated using the air pressure and it contains air filter regulators, I to P CONVERTERS, positioners, booster relays, air lock relays and feedback unifs. The air filter regulators is is used to filter the air and regulate the pressure of the air supplied for operation. The air lock relay is used to hold the control valve in the same position if the power supply goes off. The I to P converter is used to provide air supply to the valve based on the current signal or operator signal.

The hydraulic control valves operate based on the amount of oil pressure supplied to the valve. These contains oil pumps and are very fast in response compared to the pneumatic control valves and are very costly and used in applications where high precision and accuracy are needed such as hpbypass and lpbypass unit and hydraulic governor. For other applications pneumatic control valves are used.

Light emitting diode

Light emitting diode or LED is similar to the working of the normal PN junction diode where the generation and recombination of the charge carriers takes place. But during the recombination process while the charge carriers move from conduction band to the valence band, there is a light emitted in the visible light region which comes in various colors depending on the type of the semiconductor material used. This is the working principle of the LED.

The normal semiconductor material is not used for the construction of the LED. The GALLIUM ARSENIDE is used for the construction of the LED.

Temperature control valves in turbine region of thermal power plant

   There are many areas where temperature control valves are used but specially and most important of it's application are present in the turbine area of a thermal power plant. There are four major areas where we use these temperature control valves two of these are used to maintain the temperature of the water and two of these are used to maintain the temperature of the oil. They are
1) cold gas temperature control valve
2) primary water temperature control valve
3) Main oil tank temperature control valve
4) HPCF temp control valve.

The main oil tank temperature control valve is used to maintain the oil temperature to the range defined, if the temperature is more that the setpoint the valve is closed and the oil is passed through the coolers and gets cooled, if the temperature is less compared to the setpoint the valve is opened and the temperature is raised.

The temperature control valve of hpcf is operated in the similar manner compared to the main oil tank temperature control valve as the oil needs to be maintained in specific temperature range if not maintained the expansion of the shaft will be uneven and results is defects arised in the longer run and also during the valve opening and closing.

Primary water temperature control valve is used to maintain the temperature of the primary water in the defined setpoint range, the primary water is used to cool the stator windings in the generator.

The cold gas temperature control valve is used to maintain the temperature of the cooling water which is used to cool the hydrogen gas. The hydrogen gas is used for cooling the rotor.

GATE EDC QUESTION TOPICS AND PSU

              EDC  is one of the most important topics in GATE and there will be a number of question from 1 mark to 2 marks to linked answer type questions. EDC is vast topic and if prepared covers from basic to the large circuits.The following are the areas from which EDC question comes in most of the GATE and PSU. The most scoring subject for an ece student is EDC

1. Doping questions like calculation of the doping concentration, doping ratio.
2. calculation of the depletion width, electric field, mobility and basic circuits realted to Diodes
3. NPN, PNP circuits mainly their biasing and current calculations and all important circuits objectives and their formulae CE, CB and CC configurations. -- Linked answer questions
4. FET realted questions mainly deals the same topics as BJT but are not in the same quantity as BJT.
5. Various types of diodes and their equations.
6. Silicon controlled rectifier( Less in Gate)
7. Difference among quantities such as direct and indirect bandgap, avalance and zener breakdown etc.. ( Objective part) Engineering services.
8. Questions on formation of bands and their theory. Objective part both in PSU and GATE, ESE.
9. Do not ever leave the topics of BIPOLAR JUNCTION TRANSISTOR.
10. Known standard values like mobility of hole and electron, their charge etc....

All the best to GATE aspirants.

ZENER BREAKDOWN and zener vs avalanche

               Zener breakdown occurs in specially developed heavily doped diode of N.Type and P.Type impurities. Zener diode specially designed to operate in the reverse bias region specially in the breakdown region  and is used in circuits of voltage regulators. The symbol of zener diode is shown below

                A Zener diode works as normal PN Junction diode in the forward bias, so it is generally used in reverse bias so in general the cathode is shown with the positive sign and anode is shown in negative. As the electrons move from the anode to cathode the current is measured from cathode to anode.

A zener diode skeleton is as shown

The Typical characteristics of a zener diode in both the forward and the reverse bias in comparison with the normal PN Junction diode is shown above. In the forward biasing condition there exists a cutin voltage after which the current starts passing through the diode and the concept of the forward biasing is same for the Zener diode and normal PN junction diode. Now consider the case of the reverse bias as the doping is very high and the depletion width is very small, there exists a very high electric field in the depletion region. 

                                                 Electric field is inversely proportional to distance

Now as we slowly increase the reverse bias region, initially there will not be any current and as we still increase the reverse bias voltage, at a particular voltage the electric field now present along with the applied field is very high so that the electric field removes the electrons from their orbits and there by creating the free charge carriers which results in a short circuit between the cathode and the anode, this condition is said to be the Zener breakdown. The zener breakdown occurs much before the avalache breakdown and as compared to the avalnche breakdown, here there does not exist the process of electrons hitting other atoms and dislodging the charge carriers, the device will be safe. As there are no collisions there will not be any heating effect on the device.

The voltage at which the breakdown occurs is called the zener breakdown. Now if we remove the biasing the zener diode will return to its normal state.

The zener diode has negative temperature coefficient as the temperature raises the electrons will already be available at some raised energy level and a small voltage is required for the breakdown whereas in the avalanche breakdown has a positive temperature coefficient as the temperature raises the avalanche breakdown voltage increases, as the temperature increases the collisions will happen at low kinetic energy and difficult to dislodge charge carriers at that energy so it requires more voltage for avalanche breakdown.

Difference between valence band and conduction band

            Generally the valence band and conduction band concept comes into picture during the discussion of semiconductors. These concepts also come in the case of conductors and insulators, but in the conductors the valence band and conduction band are said to be merged and there is no difference between the two, while in the case of insulators the valence band and conduction band are at greater distance and no charge transfer takes place between them.

The valence band is the outer most band of the atom where the bonding takes place between two similar or dissimilar atoms and molecules are formed or different elements are formed. The charge carriers in the mainly electrons present in this band are the bonded electrons. In semiconductors specially P-Type the electrons in the valence band takes place in conduction process in the name of the Hole. So P type semiconductor valence band has free charge carriers ( As a result of generation and recombination in the valence band)  and conduction takes place in the valence band

The conduction band is where when the bonding takes place and if any charge carrier is not in the control of its nucleus are roams around the material but within the material and is said to be free charge carrier and takes place in the conduction process. In semiconductors, the additional charge carriers which are not in the vicinity of their nucleus is found in the N Type semiconductors. The conduction process takes place in the conduction band in the N Type semiconductor.

The important aspect to consider is the valence exists and the conduction band is an imaginary band which is assumed to develop the concept of the semiconductor physics and which is found to be in good resemblance with practical results

. 

Avalanche breakdown

Breakdown of the PN Junction diode occurs in the reverse bias condition. As we keep on increasing the reverse bias voltage for the PN junction diode the charge carriers will move away from the depletion region into N side and P side, this will increase the depletion width further. Now during this time the minority carriers which arise in the junction that are resulting in the reverse saturation current due to the generation and recombination of charge carriers. These minority carriers generated in the depletion region will have increased velocity which proportional to the increased reverse bias. At a particular voltage the velocity if these charge carriers is so high that these charge carriers when they hit the atoms or ions present in the depletion region, they liberate or dislodge further charge carriers from the ions/ atoms. (Diode biasing)

This process will continue and looks like multiplication of charge carriers takes place and finally the depletion region behaves as a perfect conductor and this condition is known to be breakdown of the PN junction diode. The device once reaches this condition will lose its properties and is permanently damaged as infinite current flows through the device in a short span. There are special types of diodes designed to operate in the breakdown region such as Zener diode. The PN Junction diode under the breakdown condition are done when the P type and N type semiconductors are lightly doped whereas in case of the zener diode the N Type and P Type are heavily doped.  This is also called as avalanche breakdown.

Avalanche breakdown

PN Junction Diode in forward and reverse biasing

The PN juntion diode is the basic electronic device after which large number of electronic devices were invented. PN junction diode normally used under the forward bias as it allows current to flow through it only under this condition, while in reverse bias also a minute amount of current flows through the device but it is negligible and is not used in reverse bias. Special types of diodes are used to work under the reverse bias condition for specific application such as Zener Diode.

When we consider the forward bias we consider

1) Ideal case

2) Practical case

Ideal case in which there will not be any resistance or any junction voltage under zero biasing and when a forward voltage is applied the current will suddenly raise.But ideal cases are never true in the field of electronics.

In practical case there will already be a voltage across the junction. Now the forward biasing the PN means P connected to positive side and N is connected to the negative side, the junction voltage will be opposite direction of forward bias. So initially there will not be any current passing through the junction. Increase a bit forward voltage still no current, continue this at a particular voltage called as the cutin voltage of the diode which is approximately equal value to the junction voltage current starts to flow through the diode.  While increasing the voltage under forward biasing the the charge carriers which are separated by the junction voltage will start moving towards the junction due to the potential applied now when the potential applied is slightly greater than the junction potential then the carriers will cross the junction leading to current. As we further increase the forward voltage the current  increases in exponential manner.

Under the reverse biasing condition , the P side is connected to the negative terminal and N side connected to the positive terminal which leads to the carriers moving away from the junction and the current produced will always be zero in ideal case and practically a reverse saturation current exist.

Difference between hole and electron

           Electron and hole are the two charge carriers that are present in a semiconductor material. The electron is the majority carrier in an N-type semiconductor and hole is the majority carrier in P-type Semiconductor. The N-type and P-type semiconductors are called as extrinsic semiconductors as these are doped with the external impurities i.e., P-type has trivalent impurities while N-type has penta-valent impurities. The intrinsic semiconductor (pure form of semiconductor) is doped with impurities to form P type and N type semiconductors. The intrinsic semiconductor is said to behave as an Insulator at absolute zero (0 deg c). The electron and hole have same mass and charge but electron has negative charge while hole has positive charge.

• Electron is negatively charged and its value is given as 1.6022 x 10-19 Coulomb.
• Hole is positively charged and its value is given as 1.6022 x 10-19 Coulomb.
• Mass of electron and hole:  9.1 x 10-31 kg.

         A hole is said to be absence of electron in the valence band, then how will a physically non existing element have mass and charge. This can be understood as follows. Consider the diagram below in the valence band of a P-Type semiconductor.

             Assume semiconductor at room temperature there will be continuous breakdown and creation of covalent bonds in the valence band which is generation and recombination of electrons and holes consider a bond breakage takes place and electron will move from one location and occupy another vacant location in the valence band as follows. In the series of diagrams the location of electron moving from right to left can be seen which resembles as if a hole is moving from left to right.

               From the above series of diagrams an electron movement in the valence band is considered as hole movement in opposite direction and thus a physically non existing material has mass and charge. In semiconductor materials hole is always considered on valence band and electron is always considered in conduction band.

PN JUNCTION DIODE

                     A PN junction is the basic of many electronic circuits and is considered to be the basic semiconductor device. The PN junction consists of a P type semiconductor and an N type semiconductor and it is formed in many ways one way is to take an extrinsic semiconductor device and then start doping equal amount of P type and N type impurities one from each side and allow uniform doping across the semiconductor, as the impurities get settled inside and at the center there will be one side p and other side n type as in fig 1 a. One more way to form a pn junction is take one P type semiconductor and another N type semiconductor and the join them together and heat at the junction as in fig 1 b.

As we know that the P side semiconductor consists of majority of holes and acceptor ions and the N Type consists of the majority electrons and donor ions. Their view is shown in the figure 2

At the junction now the electrons from the N Side will move towards the holes in the P type semiconductor  as a result the holes and the electrons get recombined and the process continues until no further electron and hole pair recombination is possible at a current temperature. The recombination process is shown below in the figure 3

As the recombination process continues at the junction, there will be a situation reached when no further recombination is possible, at that moment the area around the junction of P and N  is completely free of the free charge carriers i.e., the region is depleted of the charge carriers hence the region is called as depletion region, which is shown below in fig 4

The depletion region consists of the electric field due to the acceptor ions on the N side and the Donor ions on the P side due. As the depletion region is free of charge carriers it just works as an insulator and on both sides of depletion region there are charge carriers i.e., conductors, hence the depletion region is also used as capacitance and is famously called as Depletion Capacitance. The voltage across the depeletion region is called cut in voltage. This is how PN Junction is formed.

HYDRAULIC GOVERNOR

            Hydraulic governor in a thermal power plant is the system with the help of which the steam entering into the turbine is controlled thereby controlling the load of a thermal power plant. There are two types of governors

1) Mechanical Hydraulic governor

2) Electro hydraulic governor 

             The important functions of the hydraulic turbine governor are as follows:

• To start, maintain and adjust unit speed for synchronizing with the running units/grid. The speed should be maintained to attain the frequency of the grid
• The system frequency is maintained during the running i.e., after synchronization by adjusting turbine output to load changes, as the frequency on the grid changes this will be change the speed of the rotor as the frequency and speed are directly related.                                               Speed = 120 * Frequency/Poles

• To adjust output of the unit in response to operator or other supervisory commands this is done by providing the command through the interface which indeed will adjust the pressure of oil supplied to the control valves of HP control valve and IP control valve, as the valves are turbine valves are controlled by the oil the valves will be acted based on the oil pressure.
• To perform normal shut down or emergency over speed shut down for protection, as the speed of the turbine gets past the limit, the over speed device must act and cutoff the oil supply to the valves of the turbine which will be closed and results in unit shut down.

Governing system in a thermal plant includes the following 

1. Speed sensing elements 
2. Governor control
3. Hydraulic pressure supply system.

During the shut down of the system if any work is done on the hydraulic governor, then the governing system must be calibrated again as per the desired characteristics so that the valves i.e., HP and IP control valves does not arise problems. The characteristics will be taken at particular temperature of the oil in the control fluid tank, that is set or designed for, otherwise the characteristics will be differ during operation.

 Electro hydraulic governor is the one which uses digital controller. Apart from that both the Electro hydraulic governor and mechanical hydraulic governor will perform same operation. If Electro hydraulic governor fails then mechanical governor will take charge but vice versa is not possible and results in unit shutdown.

Causes of shut down of a thermal power plant

Major causes for shut down of a thermal power plants are:

1)Boiler tube leaks is the major causes in a stable thermal power plant due to long run or due to entry of unwanted materials the boiler tubes will get holes and the water consumption if comes to uncontrollable level the unit will be shut down manually and is a planned shut down, which is a safe shut down.
2)PA fan failure which is due to cutoff oxygen to the boiler results in flame failure.
3)Improper handling during single ID fan or FD fan failure
4)Loss of all fuel condition due to negative pressure of fuel flow not sufficient or due to flame scanners fault 
5)Due to lower drum level
6)Due to turbine temperatures
7)Due to cooling water temperature
8)Due to primary water temperature
9)Due to temperatures at turbine and generator areas
10)Due to electrical failure
11)Due to liquid in main leads
12)Improper response during BFP trip, as there will be turbine driven boiler feed pumps and motor driven boiler feed pumps and when the turbine driven pumps get tripped off the motor driven pumps will enter into service automatically during which the feed water control should be taken into manual mode if not acted properly during this condition the unit will get tripped out
13)Due to failure of hydraulic governor which is the heart of the operation of the turbine as the hydraulic governor supplies the oil to the control valves in the predefined fashion to operate as per the operator commands.
14)Due to control fluid pump failure
15)Due to improper control of main steam pressure.
16)Due to sudden opening of HPBypass either due to pressure or due to mal function
17)Due to LSR ( Load shedding relay) acted and plant not responded properly
18)Due to more turbine vibrations, in general the turbine vibrations are considered serious and if real can make shaft deform or misalign, but some times the vibrations may be faulty due to spike due to which careful study needs to be done before putting vibration trip into service
19)Due to axial shift crossing limits , if unit gets tripped due to axial shift condition, before starting the turbine there needs to be a thorough study regarding the case as turbine tripping under the axial shift condition is rare and is an indication of misalignment.
20)Due to flow in the bushing below defined value
21)Flame failure condition which occurs during the low load conditions, in the low load conditions even though the flame exists it will be unstable and the flame scanners will be unable to detect the flame due to which the flame failure condition arises and the boiler is tripped. During the low load condition proper maintenance of air flows and the fuel supplied in the elevations is most important.

Reverse Saturation current

            The reverse saturation current which is in general heard in the Electronics subjects has the basic definition taken or derived from the PN junction diode. PN Junction diode is the basic of semiconductor physics  or micro electronics from where the electronic circuits are derived. Reverse saturation current is observed when the PN junction is put under the reverse biasing condition , that is the P type is connected to the negative terminal and the N type is connected to the positive terminal.

             Under this condition the electrons in the N side which are the majority carriers will move away from the PN Junction deeper into the N side, the holes in the P Type which are majority carriers will move to the P side. This makes the area nearer to the junction depleted of the free charge carriers and the depletion region i.e., region free from the free charge carriers will be increased. Now it will become difficult for the charge carriers to cross the increased depletion region.

             If the region is depleted of the free charge carriers then how can the current be available in the reverse bias. In any biasing condition the breakdown of the bonds and creation of the electron hole pairs take place at room temperature. The electron hole pairs that are generated in the depletion regions as bonds are also present in the depletion region, the hole will be pulled towards the  N type as the depletion region of   N- Type contains the donor ions which are positive and the electrons generated will be pulled towards the P type as the region consisting of acceptor ions which are of negative charge, as the electron hole pair formation and recombination takes almost at a constant rate at a given temperature the current also is maintained constant and hence is called the saturation current as this is produced in the reverse bias, this is called as reverse saturation current. 

Forced Draft Fan or FD Fan

              Forced Draft fan or FD fan is another most important part of the boiler. The PA fan will carry the powdered coal from mills to the boiler while the ID fan will remove the gases after the combustion of the coal. The FD is also called as secondary air system which is used to provide sufficient amount of oxygen to the boiler so that combustion of coal will go smoothly. If sufficient amount of air is not supplied by the FD fan or excess amount of air is supplied then the combustion of coal will not be proper and the unburned coal will settle on the walls of the boiler. The continuous settlement of the coal powder on the walls of the boiler will lead to the formation of clinkers. 

            Clinkers are formed due to piles of unburned material in the boiler, improper burning of coal is not the only reason for formation of clinker, it may also be due to the entry of foreign particles into the boiler. All the particles other than coal are termed as foreign particles of boiler. Due to the formation of the clinkers, the ash which is resulted due to the burning of coal will settle on the clinker and starts to build into the boiler and occupy the space of the fireball. (Fireball is the area of the flame present in the boiler and if looked through the peep hole it will resemble as ball hence termed as fireball) . As the area of the fireball is occupied by the clinker and if it goes unnoticed it will result in the termination of the fire which means system is tripped.

            Operators must ensure proper working of the FD Fan and the ID fan to make sure the boiler is maintained at the proper negative furnace pressure The load of the FD fan should be reduced if any of the ID fans goes out of service( In general two ID and two FD fans are used now a days) , the negative pressure created by the ID fan is reduced, the boiler will move into the positive furnace pressure zone which results in the fire coming out of the boiler thereby reducing the lifetime and efficiency of the boiler. 

            If any of the FD fan goes off then operators should reduce the load on the ID fan as the FD fan provides air into the system, the amount of air removed by the ID fan will be more, which results in the more negative pressure in the boiler, if goes unnoticed, the negative pressure will result in the flame loss leading to unit trip. This the importance of the FD fan and its relation with ID Fan.

Primary Air fan or PA Fan

Primary air fan also called as the PA fan is the considered the most important part of the boiler in the thermal power plant. In a thermal power plant where there are two PA fan , two FD fan and two ID fans are present and if any one of the ID fan or FD fan is tripped off, then by maintaining the air flow the unit can be saved and kept in running status. But if any one PA fan goes off it will be difficult to save the plant. This is the importance of the PA fan.

Primary air fan starts from along the mills area of the boiler and flows all the way to the boiler. While moving from the mills to the boiler it carries the fine particles of the coal which is powdered in the mills to the boiler and supplies the much needed fuel for the thermal power plant. The amount of fuel flow to the boiler depends on the proper working of the primary air fan.

Primary air fan is used to supply cold air and hot air. The air to be supplied depends on the condition of the coal. If wet coal got powdered in the mill, as the wet coal is of heavy weight it cannot be carried easily to the boiler. So hot air is also provided to the mills area to dry the wet coal and carry the coal to the boiler. Thereby reducing the wastage of the coal.

 A loss in the PA fan for a live plant (if two PA)will cut off the fuel flow to the boiler and it will not provide sufficent time to the operators to handle the plant as it almost cuts half of fuel and as the reaction time is minute the unit cannot be saved.

HPBYPASS FLOWS

              HPBYPASS flow, the flow in the hpbypass line in the system is called as HPBYPASS flow. In general during the running condition of a thermal power plant there will not be any flow in this region and hpbypass flows are zero. During the unit shutdown or trip condition the hpbypass valves are present and flow is established. Then why do we need to know about the hpbypass flows?

             Monitoring the HPBypass flows during the unit running condition is very much important and the flows are maintained in the feedwater loop. If in running condition HPbypass flows are established then there is some amount of steam lost from the system and is bypassed from the HP bypass. So this additonal amount of steam is to be supplied to compensate for the flow in the HP bypass line and it should be fed through the feedwater loop and hence the flows are added to the HP bypass loop.

case study:
Suddenly there is rise in the feed water flow and the level of drum started to increase very fast and there is no manual intervention and the Hpbypass valve is fully closed.

Solution:
As the hpbypass valve is fully closed, first isolated the hpbypass flow condition from the feedwater loop by making its value zero. Then went to the location, the flows are generally calculated using the Differential pressure transmitter (DP) , found out that there is a leak in impulse line of the DP transmitter beause of which there is high pressure maintained on one side and low pressure on the other which transmitter assumed to be flow and raised the feedwater flow. Closed both the impulse line and rectified the leak and working fine.

Induced Draft Fan

                ID Fan stands for induced draft fan. ID Fan is very important auxiliary of the boiler in the thermal power plant. It is used to remove the waste gases from the boiler after the combustion of the coal in the boiler. The ID Fan sucks the gases from the boiler and releases to the outside environment after collection of the ash and dust particles in  the electrostatic precipitators through the chimney. The ID fan creates the negative pressure so that the smoke inside will be suck through the path provided. ID fan are synchronous motors and works on the principle of VFD (Variable frequency drive)

                 A variable-frequency drive (VFD) (also termed adjustable-frequency drive, variable speed drive, AC drive, micro drive orinverter drive) is a type of adjustable-speed drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and voltage. The speed of the fan should be varying depending on the supply of the air to the boiler for combustion and always the boiler should maintain the negative pressure to avoid the flame to come out of the boiler which is done by the ID fan.

                 Proper operation of ID Fan is very much important as the postive pressure in boiler will reduce the life time of the boiler and flame will get out of the boiler and also more negative pressure in the boiler  will put off the flame and interrupts the plant operation also leads to the reduction of life time of the boiler.VFD is very much important to save a lot of energy as the speed of the fan can be varied according to the demand, if VFD is not present and a fixed speed device is used the speed will remain same even though the demand is not there. This wastes a lot of  power.

HEATER EXTRACTIONS IN STEAM TURBINE

Extraction lines to heaters will be regularly known topic for a electrical engineer who works on the improvement of the efficieny of a thermal power plant.

In general there are two types of heaters present in the thermal power plant for improving the efficiency. They are

• LP heaters
• HP heaters

           LP Heaters as the name implies this is the heater which gets heated through low pressure steam. Now where does this low pressure steam come from, this low pressure steam is the steam at the LP Turbine for which  a line or an extraction is provided to the LP Heaters to heat the water present in the Heater. As the extraction line is derived from steam at LP turbine the name is provided as LP heaters. Generally in plants above 500 MW there will be three LP heaters and they lie in the system after the CEP's. This is the initial level of heating provided to the water which helps in improving the efficiency of the thermal power plant.

           HP heaters as the name implies is the heater which is heated with the help of high pressure steam. The high pressure steam is in general the HP exhaust or also called as CRH (Cold reheat line) which enters the boiler for reheating purpose. There will be two or more heaters in a plant above 500 MW. These heaters will be available in the Feed water line cycle. This is second level of hetaing which improves the efficiency of a thermal power plant.

Control fluid in thermal power plant

             In thermal power plants for controlling purposes the fluid used is called as the control fluid. This control fluid is situated in a separate room called the control fluid room which has atleast  two pumps out of which only one pump will be running once and in case of failure the other pump takes care and will be automatically gets on based on the low pressure or on failure of the other pump.

The control fluid is used for different purposes such as ,

• HP secondary fluid
• IP secondary fluid
• Auxillary fluid
• Trip fluid
• start up fluid
• Primary Fluid

Control oil is transmitted to various locations such as

1. Governing Rack
2. Control valves  like IPCV and HPCV etc...

               The control fluid is used in the operation of the valves and the pressure of the fluid determines by the amount of the valve opening for which command is provided by the hydraulic governor or electro hydraulic governor and are defined by characteristics of the hydraulic governor. The control fluid must be maintained at specific temperature so that the predefined characteristics are maintained and there will be no ambiguity during the operation.

For maintaining the temperature of the control fluid and get the defined characteristics there is a cooler and heater provided in the control fluid system. If the temperature raises above the set point the oil is circulated through the cooler and if temperature is very less the control fluid is heated through the heater available.

SEAL STEAM SYSTEM

         Seal steam system is used to prevent the leakage of the steam from the gland seals. Even though carbon-ring seals and labyrinth seals are installed to minimize the leakage the super heated steam is can leak through small pores present and as the steam is of very high temperature of more than 500 degrees, it is very dangerous to the people working around as the surrounding temperatures raises. If steam leakage takes place, a lot of steam is wasted and the performance of the system is degraded. The LPT (Low pressure turbine) is connected to the condenser where the vacuum is maintained.

           If leakage is present the vacuum will pull the outside air into the system which has different temperatures due to which deformations can take place in the metal areas, also if perfect sealing is not maintained the vacuum present inside will be reduced and if the vacuum is not present, the ability to pull the steam towards the condenser will not be done properly and the steam may get condensed in the LPT area which is not at all desirable which affects the plant performance. The vacuum maintenance is very much important which is why there will be a set point up to which vacuum to be maintained. This is why seal steam is very important in turbine which will seal the glands and avoid any steam leakage and protect the system. The excess seal steam which enters the system will taken care by the leak steam system which moves the excess steam out of system.

STARTUP IN THERMAL POWER PLANT

There are two types of startup's in a thermal power plant. They are 

1) Cold startup

2) Warm startup.

The procedure for the both the process is same but the time taken will be more for a cold startup and less for a a warm startup. First let us see the general procedure and then differentiate between the two.

1) The oil guns are fired in lowest elevation and the oil used is light oil as there the heavy oil cannot catch up fire directly, initially the light oil is fired.

2) Once sufficient temperature is reached then the heavy oil will also be fired. 

3) Steadily the oil firing is done at different elevations i.e., heights. This process will go on until sufficient temperature is reached to introduce the coal. The oil required will be maintained in the pump house called the fuel oil pump house. Sufficent level of oil stocks should be mainained as per the necessity and proper maintenance of the Oil guns to be done

4) As sufficent temperature is reached the along with the oil coal is also introduced into boiler through different corners.

5) During the whole process the BFP( Boiler feedpump) will be providing the water to boiler and the generated steam is bypassed from entering the turbine through the HPBYPASS and LPBYPASS. once sufficent criteria is maintained the ESV( Emergency stop valves) of the HP Turbine is opened. Then the turbine is rolled to 360 RPM. 

6) After meeting all the criteria to allow the steam to enter the turbine the turbine is rolled to rated speed and the the unit is synchronised and the steam enters into the turbine by opening the control valves.

7) Once unit is synchronised the load in raised consistently until the requirement or rated capacity as needed.( Requirement < Rated capacity)

The main difference between cold and warm startup is the oil firing and the coal firing will take more time and process will be done slowly in cold start up to ensure the proper heating is done at all levels and equipments as all the equipments will be having less temperatures.

CONDENSATE EXTRACTION PUMPS

               The Condensate extraction pumps i.e., CEP's play a very important role in power plants. In  steam turbine, after the steam has completed its work in the turbine it will be condensed in the condenser. The condensed steam present in the condenser has to utilised again as this is a cyclic process .

               The CEP' s are used to drive the condensed steam and through the condensate spray station which consists of LP heaters that are used to heat the water to improve the efficiency of the thermal power plant. Through the LP heaters the feed water will reach the deaerator.

               The suction of the CEP's is taken from the hotwell which situates below the condenser.The CEP's can easily drive the water as vaccum is maintained in the condenser to extract the steam from the steam turbine.The cycle from condenser to the deaerator is called the feedwater cycle.

               The losses that takes place due to evaporation or any leaks will be pumped in through the hotwell make up pumps. As the suction to the CEP is hotwell there should be sufficient level maintained in the hotwell. If sufficient level is not maintained in the hotwell the CEP's will get tripped out as there is no sufficent water to drive.

HMI in thermal power station

          HMI Stands for Human Machine Interface. HMI plays a vital role in many areas from education to research, Hospitals etc.. which simplifies the task of the employees as well as the people who gets the service. The logics to be implemented in servers or Distributed processing units which are communicated to the operator workstation through the network switches and through the fibre optic cable.

          Consider a Thermal power plant where a huge number of equipments and instruments are available which also requires a greater amount of man power. In olden days when there is not much develoment towards the HMI side people physically has to go towards the instruments and start the devices. But as technology has been upgraded lot of softwares have come into place. people provide the signals from the system where software for human machine interface is installed and get the feedback. Now if people want to start a machine or a device we simply sit at the system/computer and give a command.

          With the help of the feeback that we receive which is displayed on computer screens, if any problem arises people are able to recognise the specific nature of the problem and will be able to solve in quick time which previously takes huge days.

          Another advantage which is also available is the response time, a command is enough to avert any damage if system goes out of its defined criteria.
         

How to crack bank clerk

Hi all,

As you all know now the bank exam is divided in prelims and mains.

Prelims is easy to crack as it has
Aptitude
reasoning
english


For cracking prelims you should be good in atleast two.

When it comes to mains you have a general awareness topics.

1) Score to the maximum in General awareness and computer section
2) Try to score as many as possible in reasoning and aptitute
3) Cross the margins in english
4) Search for questions where you can complete fast.
5) Do not waste much time on lengthy questions as time is precious.
6) A good 2 month preparation is enough to crack it.

ALL THE BEST FOLKS

Carrer opportunities for B.tech freshers.

Hi All,

After completing your b.tech there are various career opportunities you can choose.

1) Go for higher studies.
2) Bank Jobs PO/CLERK/SO.
3) SSC CGL
4) SOFTWARE
5) CORE
6) FACULTY
7) GOVERNMENT JOBS IN CORE OR GENERAL
8) Start a business.

How to crack IES Electronics Telecommunication

Hi friends,

When compared to GATE cracking Engineering services exam is bit difficult.  You can follow the following:

1) Prepare for the Descriptive exam first. Take 4 to 5 subjects in each paper and be thorough in them.
2) Have a overview on the remaining topics.
3) Practice is a must and need for different topics.
4) Try to score maximum marks in the objective as in descriptive we cannot expect the marks.
5) Maintain neatness while attempting descriptive as paper valuation is very strict.
6) Do not write unnecessary stuff.
7) Prepare for atleast one year.

HOW TO CRACK GATE ECE

Hi Friends,

Cracking GATE is of great importance for every engineering student. Now a days it also a mandatory exam to appear for large number of PSU's. Through GATE you may get an interview call for the P.SU's based on your merit, you can opt for higher studies M.tech or M.E. Also gate is mandatory for P.Hd selection in reputed institutes.

How to crack GATE?

This is a question people regularly ask and search based on my experience i can give few tips related to ECE section.

1) Start preparation atleast six months before exam for a better score.
2) Start with the subjects which you are familiar with.
3) Try to solve maximum objective problems for which lot of materials are available.
4) My suggestions are Digital electronics/ microprocessors , control systems, Signals and systems, Maths, Electronic devices and circuits,  Networks are must to get some what better score.
5) Students usually leave the Electromagnetics topic but if you are confident atelast once then you can score maximum in this topic.
6) Communication is yet another area which is a bit complicated but better practice more and more question of different types to boost confidence , Many of the linked questions comes from this area.
7) Do not neglect the general topic as this boosts your score.
8) If your sole aim is GATE  Do not deviate or write exams of low competiton or of complexity of GATE so that you may lose.


Refer books for EDC subject part EDC Books

chess

Hi

Chess is a two-player strategy board game played on a chessboard, a checkered gameboard with 64 squares arranged in an eight-by-eight grid.^[1] Chess is played by millions of people worldwide, both amateurs and professionals.

Each player begins the game with 16 pieces: one king, one queen, two rooks, two knights, two bishops, and eight pawns. Each of the six piece types moves differently, with the most powerful being the queen and the least powerful the pawn. The objective is to 'checkmate'^[2] the opponent's king by placing it under an inescapable threat of capture. To this end, a player's pieces are used to attack and capture the opponent's pieces, while supporting each other. In addition to checkmate, the game can be won by voluntary resignation by the opponent, which typically occurs when too much material is lost, or if checkmate appears unavoidable. A game may also result in a draw in several ways.

buy a Magnetic chess board

HOW TO UNDERSTAND EDC

Hi Friends,

EDC is a very interesting topics of the engineering. Where people have to use their imaginative power to understand the topics. Teachers who teach the subject must have greater imagination so that the students understand it in a good manner. Once if you understand the basics of  EDC specially about the hole and electron, PN Junction and Transistors the life will be very easy in future topics like analysing the elctronic circuits, integrated circuits etc...

This is one topic where if you understand the topic once then you will never get any doubt and even if you get the doubts you will yourself clarify.

Please do follow the NPTEL videos along with books to understand the subject and master it.

Best books for EDC

Hi Friends,

There are many books for EDC but these are few important books

1) For objective EDC by J B GUPTA

2) ELECTRONIC DEVICES AND CIRCUITS BY BOYLESTED

3) ELECTRONIC DEVICES AND CIRCUITS BY JACOB MILLMAN

4) ELECTRONIC DEVICES AND CIRCUITS BY BELL

5) MICROELECTRONICS BY SEDRA AND SMITH

6) SEMICONDUCTOR PHYSICS AND DEVICES BY DONALD NEAMEN

7) PHYSICS OF SEMICONDUCTOR DEVICES BY SM SZE

First go with millman and boylested and then go for remaining books for better knowledge

B.TECH ECE

Hi Friends,
           
            Being a B.Tech graduate in ECE or electronics is a special feeling. There is something interesting in the learning and some speciality in the topics where we have to put our imagination into practice also there is a lot of innovative things and innovations to be done.  In the current world technology is moving at a faster pace whom do we credit, it exists with the people in the field of electronics. The pride will always remain with the Students of ECE.

           But how far we are able to apply the things that we learn into practice or atleast how many are working in the field of their study. May be Less than 10% are working in their concerned fields. Who created this problem?


           It lies with parents, children and the people who govern us. In many educational institutions there is no sufficient faculty or instruments, laboratories , at-least books. How are these institutions getting permission and how are the parents sending their children to these institutions and how are children not get any idea about their future in the college.

         After completing the B.Tech in ECE lakhs of students do not know what is their next area where to go. Specially ECE students do not have their core jobs or govt jobs just like others. All the students start questioning yourself then question your faculty and get quality education that helps you in your future. Just being the Pride of an ECE student does no good.