Thermal Power Plant

What are the main circuits in the Thermal Power Plant?

Answer:Thermal Power plant consists of four main circuits, they are:

·                     Feed water and steam flow circuit

·                     Coal and ash circuit

·                     Air and gas circuit

·                     Cooling water circuit

Steam power plant works on which cycle?

Answer: Steam power plant works on the principle of Rankine Cycle

Why Thermal Power Plants are always situated by the side of rivers or lakes?

Answer: Thermal Power Plants are situated close to the rivers or lakes because to meet the large quantity of the water requirement. Water is required in the steam power plants

·                     steam in the boiler

·                     for cooling purposes such as in condensers

·                     as carrying medium such as in disposal of ash

·                     for drinking purpose

What is the purpose of deaerator in steam power plant?

Answer: The presence of dissolved gases like oxygen and carbon dioxide in water makes the water corrosive, as they react with metal and forms iron oxide. The solubility of the gases in the water decreases with increase in the temperature and becomes zero at boiling point or saturation point.The purpose of the deaerator is to reduce the dissolved oxygen content in the condensate ( feed water) by increasing its temperature. These gases are removed in the deaerator  by heating the feed water to saturation temperature through the bled steam from the turbine.

Why cooling towers require in the Steam power plant ?

Answer: Cooling towers cool the warm water discharged from the condenser and feed the cooled water back to the condenser. There are two types of cooling methods provided in the cooling towers, they can be either wet type or dry type

Wet type Cooling towers: In wet cooling towers warm water from the condenser are made to spray on slats or horizontal bars and air is passed from bottom, as the water splashes down from one slat to other due to gravity air and water mixes and heat is rejected in to the air

Dry type Cooling tower:

 Dry type towers are employed where the cooling water is not available in plenty. In dry type warm water from condensate is made to flow through the finned tubes over which cooling air is passed. Heat is rejected to air as water is cooled

Why are feed water heaters are used?

Answer:Regenerative Feedwater heaters are employed in steam power plants to improve the cycle efficiency. Also it increases the steam flow rate and reduces the steam flow to the condenser. They raise the temperature of the feed water before it enters the economizer

What is the Thermal efficiency of steam power plant?

Answer: Thermal efficiency of steam power plant is defined as the ratio of heat equivalent of mechanical energy transmitted to the turbine shaft to the heat of combustion. Generally Thermal efficiency of the steam power plant will be in the range of 30-35%

What is the overall efficiency of the Thermal Power Plant or Steam Power Plant?

Answer: Overall efficiency of the system is defined as the ratio of heat equivalent of electrical output to the heat of combustion. Generally Overall efficiency of the steam plant will always be less than the thermal efficiency of the steam plant, it will be of the order of 29-33%

Overall efficiency of steam plant is determined by multiplying the thermal efficiency of the plant with efficiency of the generator (electrical efficiency)

Why the Thermal efficiency of the steam power plant is quite low?

Answer: In Steam power station, more than 50% of the total heat of combustion is lost as heat rejected to the condenser and the loss is unavoidable as the heat energy cannot be converted in to mechanical energy with out a drop in temperature. Steam in the condenser is at lowest temperature. This is the reason that the thermal efficiency of the power plant is quite low.

On what factors efficiency (thermal) of the steam plant depends?

Answer: Efficiency of the thermal plant depends on three factors, they are

1.             pressure of steam entering the turbine

2.             temperature of the steam entering the turbine

3.             pressure in the condenser

Thermal efficiency increases with increase in temperature and pressure of the steam entering the turbine. For this reason high temperature and pressure are used. Thermal efficiency is effectively increased by decreasing the pressure in the condenser, so pressure in the condenser is kept as low as possible.Thermal efficiency also increases by reheating the steam between turbine stages

Indian Engineering Services (IES) Objective Questions and Answers:

1) A self excited d.c shunt generator, driven by its prime mover at the rated speed fails to build up the voltage across its terminals at no load. What reason can be assigned for this? (IES 2006) 

a) The initial shunt field mmf does not assist the residual magnetism

b) The field circuit resistance is higher than the critical resistance

c) One of the inter-pole connection is removed

d) Brush axis slightly shift from the geometrical neutral axis of the machine

2) Wave winding is employed in a dc machine of: (IES 2006)

a) High current and low voltage rating

b) Low current and high voltage rating

c) High current and high voltage rating

d) Low current and low voltage rating

3) The resultant flux density in the air gap of a synchronous generator is the lowest during: (IES 2006)

a) Open circuit b) Solid short circuit c) Full load d) Half load

4) If the load of an Induction motor is increased from no load to full load, its slip and the power factor will respectively (IES 2006)

a) decrease, decrease

b) decrease, increase

c) increase, decrease

d) increase, increase

5) A single phase Induction motor is running at N rpm. Its synchronous speed is Ns. If its slip with respect to forward field is 's', what is the slip with respect to the backward field is: (IES 2006)

a) s  b) –s c) (1-s)  d) (2-s)

6)  Match the following: (IES 2003)

List I                                                List II 

A. DC Motor                        1. Circle Diagram

B. DC Generator                  2. V-Curves

C. Alternator                        3. Open circuit characteristics

D. Induction Motor                4.Speed-Torque characteristics

Codes:

         A          B          C         D

a)      4           3          1         2

b)      3           4          2         1

c)      4           3          2         1

d)      3           4          1         2

7)  A smaller air gap in a polyphase induction motor helps to: (IES 2004)

a) reduces the chances of crawling

b) Increases the starting torque

c) reduces the chances of cogging

d) reduce the magnetising current

8) If the supply voltage of the induction motor is reduced by 10%. By what percentage approximately will the maximum torque decreases? (IES 2004)

a) 5% b) 10% c) 20% d) 40% 

 Answers:

(1) b (2) b (3) b (4) d(5) d (6) c (7) d (8) c

 1) A self excited d.c shunt generator, driven by its prime mover at the rated speed fails to build up the voltage across its terminals at no load. What reason can be assigned for this? (IES 2006) 

a) The initial shunt field mmf does not assist the residual magnetism

b) The field circuit resistance is higher than the critical resistance

c) One of the inter-pole connection is removed

d) Brush axis slightly shift from the geometrical neutral axis of the machine

2) Wave winding is employed in a dc machine of: (IES 2006)

a) High current and low voltage rating

b) Low current and high voltage rating

c) High current and high voltage rating

d) Low current and low voltage rating

3) The resultant flux density in the air gap of a synchronous generator is the lowest during: (IES 2006)

a) Open circuit b) Solid short circuit c) Full load d) Half load

4) If the load of an Induction motor is increased from no load to full load, its slip and the power factor will respectively (IES 2006)

a) decrease, decrease

b) decrease, increase

c) increase, decrease

d) increase, increase

5) A single phase Induction motor is running at N rpm. Its synchronous speed is Ns. If its slip with respect to forward field is 's', what is the slip with respect to the backward field is: (IES 2006)

a) s  b) –s c) (1-s)  d) (2-s)

6)  Match the following: (IES 2003)

List I                                                List II 

A. DC Motor                        1. Circle Diagram

B. DC Generator                  2. V-Curves

C. Alternator                        3. Open circuit characteristics

D. Induction Motor                4.Speed-Torque characteristics

Codes:

         A          B          C         D

a)      4           3          1         2

b)      3           4          2         1

c)      4           3          2         1

d)      3           4          1         2

7)  A smaller air gap in a polyphase induction motor helps to: (IES 2004)

a) reduces the chances of crawling

b) Increases the starting torque

c) reduces the chances of cogging

d) reduce the magnetising current

8) If the supply voltage of the induction motor is reduced by 10%. By what percentage approximately will the maximum torque decreases? (IES 2004)

a) 5% b) 10% c) 20% d) 40% 

 Answers:

(1) b (2) b (3) b (4) d(5) d (6) c (7) d (8) c

Transformer

Why Low Voltage (LV) winding is placed near to the transformer core?

Anwer: For a given conducting material insulation required depends on the voltage. Hence if High Voltage (HV) winding is placed near to the transformer core, more insulation is required to insulate between the transformer core and the High Voltage (HV) winding. This results in increase in the cost of the insulation material and also size of the transformer increases significantly. Thus Low Voltage (LV) winding is placed near the core which requires less insulation between the core and LV winding.

What are the different insulation materials used in power transformers?

Answer: In power transformers the primary insulation medium and cooling medium is transformer oil. It serves the purpose of both insulation and cooling. Apart from transformer oil different insulation materials employed are oil impregnated paper, press board, wood, mica, and asbestos.

What is Transformer Breathing?

Answer: When transformer under full load, power transformer oil present in the transformer heats up and gets expand. During this process gas at the top of the oil gets expel out in to the conservator present at the top of the power transformer along with hot oil and cool oil from conservator comes down. This process is called breathing out of the transformer.

When the load on the power transformer is removed or during no load condition, transformer oil cools and air is drawn in to the transformer. This is called breathing in of the transformer. The incoming air may consists of moisture which should be removed else results in the deterioration of the dielectric strength of the transformer oil. Hence the air entering the power transformer is made to pass through the breather where moisture is removed from air through silica gel

Transformer Testing methods

Transformer Tests:

Tests on the transformers are carried for verifying the capabilities of transformer to withstand Thermal stresses, Dielectric stresses, short circuit electro-dynamic stresses and environmental stresses.

Routine tests:

These are carried out on every transformers before dispatch to ensure that it is in accordance with the specifications.Some of the tests come under routine test include:

·                     Measurement of the winding resistance

·                     Verifying the polarity of the windings

·                     Measurement of load losses and the impedance voltages

·                     Measurement of no load losses and no load no load current.

·                     Electrical tests at the power frequency

Type Tests:

Type tests are performed on the first transformer of one type and are intended to check the design characteristics. It is presumed that every transformer would also comply with the type test, since its design is identical.

·                     Temperature rise test

·                     Tests of ability to withstand full wave impulse

·                     Tests for switching impulse withstand

·                     Noise level test

Special Test:

Special tests are conducted in the presence of the purchaser or his representatives as specified in the tender

·                     Partial discharge test

·                     Checking the level of Radio interference voltage

·                     Vibration test

·                     Test on ability to withstand Short -circuit current

·                     Measurement of Noise level

Voltage Regulation of Transformer

Voltage Regulation:

When the secondary of the transformer is loaded (from no load to full load), the secondary voltage changes even though primary supply voltage is held constant at rated value.

% Voltage Regulation = (V_{2(no load)} - V_{2 (full load)})*100 / (V_{2(no load)})

With the rated voltage applied to the primary winding, the secondary  terminal voltage varies with the load current and power factor even the primary voltage is kept constant. This variation is called voltage regulation.

Voltage regulation depends on the voltage drop in the impedance of the transformer, load current and load power factor.

Leading and Lagging power factor regulation:

·                     For lagging power factor load the secondary voltage decreases with increase in the load current (when the transformer is loaded). Thus for lagging power factor loads, the regulation is positive ( voltage drop observed as the load current increases). 

·                     For leading power factor load, the secondary voltage increases slightly with increase in the load current. This type regulation. Thus for leading power factor loads, the regulation is negative (raise in voltage as load current increases) 

For better performance of the system voltage regulation should be as minimum as  possible i.e,  Change in secondary voltage with load should be as minimum as possible when primary voltage is kept constant.

Salient Pole Generators in Hydro Power Plant

Salient pole Synchronous generators and induction generators are used in hydroelectric power plant to convert the mechanical energy output of the turbine to electrical energy. Normally in hydro plants synchronous generators are employed for generating electricity, however for small power plant generator applications (plants less than 5 MVA generation capacity) Induction generators are used because of  their lower cost due to the absence of the exciter, voltage regulator, and synchronizer which should be associated  with synchronous generators.

Salient pole Synchronous generators are used in hydoelectric  plant because:

The majority of hydroelectric installations utilize salient pole synchronous generators. Salient pole machines are employed  because the hydraulic turbine in the hydroelectric plants operates at low speeds compared to steam plants, therefore generators in hydro plant requires  large number of field poles to produce the rated frequency. Rotor of synchronous generator requires  salient poles is mechanically best suited for low-speed applications, compared to round rotor machines which are employed in horizontal axis high-speed applications in turbo-generators. Round rotor or non salient pole synchronous generators are used in Thermal power plant because of high speed turbine. 

Questions : Hydroelectric Power Plant

What is Forebay and its importance?

Answer: Forebay serves as regulating reservoir storing water temporarily during the light load period and providing the water for initial increase of the load during which water in the canal is being accelerated. In simple words Forebay may be considered as enlarged body of water just above the intake to store water temporarily to meet the hourly load fluctuation. When the hrdroelectric plants are located just at the base of the dam, no forebay is required because the reservoir itself serves the purpose of forebay. However, where the plants are situated away from the storage reservoir forebay is provided.

What is Penstock?

Answer: It is the closed conduit or pipe which connects the forebay or surge tank to the scroll case of the turbine. Penstocks are built of steel or reinforced concrete. The thickness of the penstock must be adequate to withstand both the normal hydrostatic pressure and also sudden surges both above and below normal caused by fluctuations in the load and in emergency conditions.

 What is water hammering phenomenon?

Answer: When the load on the generator reduces suddenly, governor mechanism will close the turbine gates and thus create an increased pressure in the penstock. This results in ' water hammering phenomenon' and may need pipe (penstock) of extra ordinary strength to withstand the pressure otherwise the penstock will burst. To avoid this positive water hammering pressure some means are provided to take the rejected flow. This job will be carried out by small storage tank or reservoir for receiving the rejected flow. This storage reservoir or tank is called as 'surge tank'.

What is Trash Racks?

Answer: Trash Racks are long, flat vertical bars spaced accordingly such that it provides the minimum passage width to the water through the turbine. They are employed to prevent the ingress of floating and other material to the turbine

What is Tailrace?

Answer: Water after doing the useful work in the turbine is discharged to the tail race. It is the path through which water will flow down stream after running the turbine.Tailrace storage will acts as reservoir for the down stream projects for generating electricity or for irrigation purposes