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.