AC Secondary Transmission:

AC Secondary Transmission:

~ From the receiving station, power is then transmitted at 33kV by underground cables (or sometimes by overead lines) to vanous substatons (S.S) located at different points in thei city.
~ This is known as secondary or low voltage transmission.
~ At the substation (SS) this voltage is further reduced from 33kV to 3.3 kV. using step down transformers.

AC Primary Distribution:
~ The output of substation at 3.3 kV can be directly given to ai customer whose demand exceeds 50 kVA using special feeders. i This is primary distribution.

AC Secondary Distribution:
~ The secondary distribution is done at 440 or 230 V.
~ The reduction in the voltage level from 3.3 kV to 440/230 V is done by the step down transformers at the distribution substations.
~ The most common system for secondary distribution isi 440/230 V, three phase, four wire system.
~ The single phase residential load is connected between any one line and the neutral where as three phase, 440 V. motor load is connected across three phase lines directly.
~ The standard frequency for a.c. working is 50 Hz in India. For single phase traction applications, the frequencies used are 25 Hz or 16.67 Hz.

Advantages of AC System :

1. High voltage can be built-up.
2. The changes in the voltage level as per requirement can be done using step-up and step-down transformers.
3. Maintenance cost of substations and generation cost of AC system 1s low. i
4. The motors used are simple in construction and have low maintenance.
5. Mantenance of substation is cheap.i

Disadvantages of AC Systems:

1. The inital set up is very expensive.
2. The resistance offered by the lines is high due to skin effect and thus leading to voltage drop.
3. The AC lines are more sensitive to corona.
4. The AC line even show losses due to reactance offered by the line i
5. The speed of altemator (ac generator) requres to be controlled
Disadvantages of DC Transmission:
1. Expensive converters.
2. The power transmitted can be used at lower voltage only.
3. Voltage transformation is not easier in case of DC and hence it has to be done on the AC side of the system.
4. Circuit breaking for multi-terminal lines is difficult. 4

Applications of DC Transmission:

The comparison of AC and DC transmission in terms of economics and technical performance, leads to following areas of applications for DC transmission :
1. Long distance power transmission on a large scale.
2. Under ground or under water cables.
3. A synchronous interconnection of AC systems operating at different frequencies or where independent control of systems is desired.