Why efficiency in Thermal Power Plant is Low?

Almost 50% of the heat generated is lost at the condenser as heat rejection. It is unavoidable as with out heat rejection it is not possible to convert heat energy into mechanical energy and drive the turbine without drop in temperature. Therefore majority of the loss takes place in the condenser. Thus efficiency of the thermal power plant is between 30-35%.

On What Cycle does Thermal Power Plant operate?

Thermal Power plant works on the principle of Rankine cycle

Why Generation Voltage in Thermal Power Plant is between 11kV to 33kV?

The current carrying conductor cross section depends upon the magnitude of the current it is carrying and insulation strength of the conductor depends on the maximum voltage it can withstand. Therefore while designing the generator an optimum value is chosen between the amount of the current and voltage conductor can withstand.

What are the different circuits in Thermal Power Plant?

Some of the major circuits in the thermal power plant are:

·         Coal and Ash circuit

·         Air and gas circuit

·         Cooling water circuit

·         Feed water and steam flow circuit

How efficiency of Thermal Plant can be improved?

Some of the methods by which the efficiency of the thermal plant can be improved are:

·         By increasing the temperature and pressure of the steam entering the turbine

·         By reducing the pressure in the condenser

·         By reheating the steam between different stages between the turbine

Advantages and Disadvantages of Thermal Power Plant?

Advantages:

·         Thermal Power Plants can be operated near the load centers unlike hydro and nuclear plants

·         Requires less space compared to hydro plants and cost of construction is less

·         Running or operating costs are less compared to diesel or gas plants

·         Can able to handle over load for certain period of time

Disadvantages:

·         Emits green house gases and causes pollution

·         Coal and Ash handling requires large area

·         Efficiency is low

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Discuss the criterion for choice of voltage for transmission and distribution

Criterion For Choice Of Voltage For Transmission And Distribution.

We know that the power W =VI , so for the same power if voltage is increased then current will decrease. If suppose power required is 500 W and voltage is 250 V then current I will be equal to 500/250 =2A.

Now if voltage is increased from 250V to 500V and the power required is same then I = 500/500 =1 A

Now, if the voltage is still increased to say 1000V for the same power then I = 500/1000 =0.5 A

From the above examples, we see that if the voltage is doubled, the current will be halved and when it is quadrupled then current is reduced to 1/4th of its initial value. Thus we conclude that if voltage is increased to n times then the current will be reduced to 1/nth times for the same power.With the reduction of the current to 1/nth times, the conductor area will also be reduced to 1/nth times of its original area for the same current density, Hence less material is required when the voltage is increased.

We also know that when the current passes through any conductor, there is loss of power in that particular conductor according to the relation, I2 R. As the loss is proportional to the square of the current. So if the current is reduced to ½ value , then the loss will be reduced to 1/4th its original value. Hence the efficiency of the transmission line and all others equipments associated with the line will increase and more power will be available for use.

When current is passing through a conductor there will be a voltage drop according to the relation V=IR. So, when the current is reduced the drop of the voltage is less in the line, of course with the same cross sectional area of the conductor.

With the reduction of cross sectional area, considered the main advantage of transmitting electrical energy at very high voltage viz 132kV , 220kV or even 400kV.

But in case of distribution system such high voltage is dangerous, so distribution voltage is generally 400/230V.

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What are the advantages of high voltage transmission? Give its limitations also.

Ans: High voltage transmission is subdivided into HVAC and HVDC transmission systems.

(i) HVAC transmission: Advantages of HVAC transmission are as follows: As the voltage is increased, the current carried by the conductors decreases. The i2R losses correspondingly get reduced. However the cost of transmission towers, transformers, switches and circuit breakers rapidly increases with increase in voltage, in the upper ranges of a.c. transmission voltages.

(ii) HVDC transmission:

Advantages They (HVDC lines) are economical for bulk power transmission. The voltage regulation problem is much less in DC since only IR drop is involved. There is easy reversibality and controllability of power flow through a DC link. Also there is considerable insulation economy.

Limitations: The systems are costly since installation of complicated converters and DC switchgear is expensive. The converters require considerable reactive power. Lack of HVDC circuit breakers hampers network operation. Moreover there is nothing like DC transformer; voltage transformation has to be provided on the a.c. sides of the system.

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