Transmission Supply system (Ac and Dc)

 

Transmission Supply system (Ac and Dc)

The transfer of electric power from a power station to consumer’s premises is know as “Electric supply System”.

An Electric Supply System mainly comprises of transmission and distribution lines.

The transmission lines are necessary for

1.      Bulk power transfer from large group of generating stations to the main load centres.

2.      For system interconnection.

3.      For transfer of power from main transmission network to distribution substations.

The distribution lines will transfer the power from substations to different consumer’s premises.

Transmission of power can be done by two ways, namely

a)      A.C System

b)     D.C System

Every system has its own advantages and disadvantages but in usual practice A.C Transmission systems are used.

Relative Advantages of D.C and A.C Transmission systems

Advantages of D.C Transmission system:

1.      It requires only two conductors for transmission of power. It is also possible to transfer the power using single conductor by using earth as a return conductor. Hence, the cost of the conductor material is reduced.

2.      Potential stresses developed on the insulation in the D.C system is 70% as compared to A.C system. Hence, it requires less insulation.

3.      No inductance, capacitance, phase displacement and surge problems.

4.      There is no skin effect; hence full cross-section of the conductor is utilized.

5.      No stabilizer is required for transmission of power over long distances because the voltage drop in the line is less.

6.      The effect of corona is reduced by using earth as negative conductor and the lines of positive conductor.

7.      Construction of D.C lines is easy as compared to A.C lines.

8.      Underground cables can be used because of less potential street and neglisible dielectric loss.

9.      For a give load and sending end voltage, the voltage regulation is better in D.C

10. Less interference with communication lines.

Disadvantages of D.C Transmission system

1.      Electrical power cannot be generated at high voltage because of commutation problem.

1.    Voltage can not be stepped up.
2.    It requires costly switch gear, as the low resistance interruption method can not be used for  the circuit breakers.
3.    The cost of convertion, inverting stations are very high.
4.    It requires converting equipment at sending end and inverting equipment at receiving end substations.

Advantages of A.C Transmission system

1.      Electrical power can be generated at high voltages (3.3KV, 6.6KV and 11KV) easily.

2.      The voltage generated can easily be stepped-up by using transformer and also it can be stopped down by using step-down transformer.

3.      The coast of A.C Switch gear is less.

4.      Maintenance of A.C Substation is simpler.

5.      The transformer substations are much efficient that the motor-Generator sets Used in D.C systems.

Disadvantages of A.C Transmission system

1.      Construction of A.C transmission lines is not easy as compared D.C Transmission lines.

2.      Amount of conductor mater