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.