Why Armature is placed on Stator for Synchronous machines

Opeation principle of synchronous machine is quite similar to dc machine. However difference exists in the synchronous machine as there is no need to rectify the time varying emf generated in the armature winding by commutator. Hence in place of commutator slip rings are provided in synchronous machine.

For alternators we see in big power plants armature will be on stator and field winding will be on rotor. Some of the questions to be answered is what happen when the field is placed on stator and armature is on rotor like dc machine? What are the merits and demerits in placing armature on stator?

Why Rotating Magnetic field:

Like dc machine armature of the machine can be placed on the rotor and still we can operate the synchronous machine. Main problem arises for large rating machines when the armature is placed on the rotor shaft large currents are to be collected by the brushes of the machine at higher voltages, which results in sparking and wear and tear of the brushes. More than the above reasons the cost of the brushes employed will be huge as it has to able to carry large currents. Therefore for large rating machines (500kVA to 750MVA) armature is placed on the stator which requires no brushes to collect the large currents. For lower rating machines (up to 250kVA) armature of the machine can be placed on the rotor as the current to be collected will be small.

Salient Pole vs Non Salient Pole Synchronous Generator Difference

Salient Pole Synchronous Alternator:

• Salient pole Generators will have large diameter and short axial length
• Pole shoes cover 2/3 of the pitch
• Salient Poles are laminated in order to reduce eddy currents
• They are used in hydraulic turbines or diesel engines
• Salient pole generators will have typical speed about 100 to 375 rpm.
• As the  speed of the water turbine is slow  hence more number of poles are required to attain the frequency. Therefore Salient pole machines will have typically number of poles will be between 4 to 60.
• Cheaper compared to cylindrical rotor machines for speeds below 1000rpm.
• Causes excessive windage losses
• Flux distribution is not uniform due to the presence of salient poles, hence emf waveform generated is not good compared to cylindrical machine 
• Salient Pole Synchronous Generators are employed in Hydro-Power plants.

Non-Salient pole Synchronous Alternator:

• Non-Salient pole generators will have smaller diameter and longer axial length
• They are used for High speed operation (typically speed will be 1500 and 3000 rpm)
• Better in dynamic balancing because of absence of salient poles.
• Less windage loss
• Robust construction and noiseless operation
• Nearly sinusoidal flux distribution around the periphery, therefore gives a better emf waveform than salient pole machine
• No need to provide damper windings (except in special case to assist the synchronising) because the field poles themselves acts as efficient dampers.
• Non-Salient pole generators are used in Thermal,Gas and in Nuclear Power plants.

Distributed Control System (DCS) in Power Plant

Usage of Single Computer for control of all the process variables of plant would result in the loss of control action in the plant if  failure occurs to the master single computer. Also, time required for accessing large data, processing the received data and controlling of the entire plant by single computer will takes more time and slows down the process system

Distributed Control System:

In distributed control system, different process systems in the plant will have their control system loop. Thus in distributed control system number of individual control loops will be present. Several Microprocessors  were employed at the different locations of the power plant and dedicated to provide services for individual process system of the plant and monitor and control each process of the plant (This is unlike SCADA master-slave architecture, in DCS all microprocessors will be masters). Every processor at different locations of the plant acquire the data from its process system, process the received data and provides control functions and also stores the required data. These microprocessors are connected to each other and also connected to the station master computer through which operator can see the information and take control actions if necessary. Control system in power plant will have distribution in both topologically and functionally. Functional distribution such as operation of boiler side values, fans, turbine values, boiler oil firing, boiler coal firing for carrying out such different functions Microprocessors  are distributed. 

Topological distibutuion is achieved by employing closed loops for operation of different equipment such as boiler steam A equipment, boiler steam B equipment, boiler oil firing AB/CD/EF elevations will have dedicated control loop

This type of distribution results in increase in speed and reliability of the control system.

Why Efficiency of Thermal Power Plant is low (30-35)%

Thermal efficiency of Steam Power Plant:

In steam power plant overall efficiency will be in the order of about 30%. In case of modern thermal power plats where super critical parameters (pressure and temperature) are employed and by employing number heat savig devices the power plant efficiency will be raised to around 40%. Quality of the coal also plays significant role in improving the efficiency of the Thermal Plant.

Some the losses occurring i the steam power plants are:

Loss at Boiler - 15%

Loss at the Turbo-Generator

Heat Rejection to the condenser - 55%

Alternator loss                             - 1%

From above mentioned figures we understand that more than 50% of total heat combustion is lost as heat rejected to the condenser. This loss cannot be avoided as heat energy cannot be converted into mechanical energy without (some heat rejected) drop in temperature (From the second law of thermodynamics-A fraction of heat can only be converted to mechanical work and remaining heat must be rejected). Steam in the condenser is at lowest temperature.

Thermal efficiency of the power plant mainly depends on three factors.

a) Pressure of the steam entering the turbine

b) Temperature of the steam entering the turbine

c) Pressure in the condenser

Thermal efficiency of the plant can be improved by increasing the temperature and pressure of the steam. For this reason high temperature and pressure is used and also thermal efficiency is improved by decreasing the pressure in the condenser

Also efficiency can be improved by reheating the steam between the turbine stages

Thermal Power Plant Advantages and Disadvantages

Thermal power plat has the following merits and demerits:

Advantages:

• Fuel used is cheaper
• Smaller space is required compared to hydro power plant
• Economical in initial cost compared to hydro plants and running costs are less compared to gas plants or diesel plants
• Thermal plants can be placed near load centers unlike hydro and nuclear plants. Hence transmission of power losses can be minimized
• Thermal plants are able to respond to the load demand more effectively and supports the performance of the electrical grid
• Steam plants can withstand for overload for certain extent

Disadvantages:

• Higher maintenance and operational costs
• Pollution of the atmosphere
• Huge requirement of water 
• Handling of coal and disposal of ash is quite difficult and requires large area
• Gestation period (period for commissioning of plant) takes long time
• Efficiency of thermal plant is quite less (30-35%)
• Operational cost of thermal plant is more costlier compared to hydro and nuclear plant

Thermal Power Plant Interview Questions-Answers

What are the major electric systems in Thermal Power Plant?

Answer: Major electrical equipment in thermal power plant are

• Turbine Generator
• Exciter System
• Generator Protection System
• Generator Transformer
• HT/LT switch gear
• Electrical Switch-yard

What are the different Generator Protections employed in Thermal Power Plants?

Answer: Faults in the windings, Over load protection, Over heating of windings or bearings, Over speed protection, Loss of Excitation protection, Motoring operation protection, Inadvertent energisation, single phase or unbalanced current protection, out of step operation protection, sub-synchronous oscillations protection and earth fault protection

How Generator Transformer is cooled in Thermal Power Plant?

Answer: MVA power rating of the Generator Transformer will be equal to the alternator. Hence lot of heat will be generated while generator transformer is under operation. Oil Forced Air Forced (OFAF) type of cooling is employed for generator transformer

What are different types of circuit breakers employed in thermal power plant?

Answer: For low voltage operation 415/220V vacuum circuit breakers or air break circuit breakers are employed. For voltage ratings about 6.6kV and beyond SF6 circuit breakers are employed.

What type of cooling is provided for Generator in power plant?

Answer: Hydrogen gas cooling is employed for large size generators because of better heat carrying ability of the hydrogen. Hydrogen cooling is provided for rotors and core of the generator. Water cooling is provided for the stator of the alternator.

Emergency Auxiliary Equipment in Thermal Power Plant

Power required for avoiding the damage to the equipment in case of power failure is called survival power. Consider a total grid collapse, which results in tripping both offsite power (electrical power coming from the grid to the thermal power plant) and onsite power (power generated by the thermal power plant). During this emergency condition some electrical power is required to operate certain pumps and other auxiliaries to keep the major equipment in the thermal power plant in safe condition

Some of the auxiliaries which require power during emergencies in thermal power plant are

• Turbine emergency lube oil pump: This is battery driven centrifugal oil pump designed to operate at 40% of required oil to turbine and ensures the turbine coast down smoothly
• Lubricating oil pumps 
• Compressors for Air blast circuit breakers
• Emergency lighting
• Control room controls
• Communication and telemetry equipment in the power plant

These emergency auxiliaries operate when power black out occurs and protect the major equipment of the plant. This load will be supplied by battery banks in thermal power plant. This is the major reason why battery  banks are provided in thermal power plants. This load will be around 0.25% to 0.3% of the total unit capacity.

Difference between Thermal and Nuclear Power Plants

Thermal Power Plant:

• In Thermal Power Plant Reactor follows Turbine. When load demand changes governor system of turbine operates accordingly depends on load demand by opening or closing steam value. Hence more or less inputs are taken to boiler according to the load demand.
• Thermal Power Plants basically are peak load plants. However due to lack of power generation sources, thermal plants are forced to operate as base load plants.

Nuclear Power Plant:

• In Nuclear Plants after reactor shut down still decay heat is produced due to the fission of the daughter nuclei and gamma radiation. Hence in nuclear power plant decay heat is to be removed continuously even the reactor is shutdown. Reactor cooling system must be continuously operated. 
• In Nuclear Plants safety is prime criteria than production of electricity
• Nuclear Power Plants are base load plants
• In Nuclear Plants Turbine follows reactor. No variation of the turbine speed according to load demand. This is designed in order to avoid tripping of reactor.