The a.c. system which is very commonly used for the transmission of power till substations and local distribution centre is three phase three wire system. While for the secondary distribution, the universally adopted a.c. system is three phase four wire system.
1.1 Three Phase Three Wire System
The three phase three wire system may be star or delta connected. If it is star connected, then its neutral is grounded. The Fig. 1 shows the scheme of three phase three wire system for the primary distribution. The large consumers like factories which need bulk supply are directly supplied from the substations. The power is also distributed to other substations and distribution centres.
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| Fig. 1 Three phase three wire scheme |
1.2 Three Phase Four Wire System
The fourth wire in this system is neutral and hence the transformer secondary in such system is always star connected. This system is generally preferred for the secondary distribution. The single phase loads are connected between one of the three lines and a neutral while the three phase loads can be given three phase supply directly, alongwith the provision of neutral for the internal distribution.
The Fig. 2 shows the three phase four wire system.
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| Fig. 2 Three phase four wire scheme |
The voltage between any of the lines and a neutral is 230 V while the voltage between any two lines is 400 V.
1.3 Advantages of A.C. System
The different advantages of a.c. system, which is commonly used now-a-days are :
1. It is possible to build up high a.c. voltage levels, using high speed a.c. generators of large capacities.
2. The cost of such a.c. generators is very low.
3. The a.c voltages can be raised or lower as per the requirement. The generated voltage levels of 11 KV, 22 KV etc. are raised upto 220 KV for transmission purpose. This is not possible in case of d.c.
4. The maintenance of a.c. substations is very easy.
5. The high voltage levels can be lowered to safe potential using step down transformers for the distribution purpose.
6. The motors running on a.c. are simple in construction, cheaper and require less attention from maintenance point of view.
7. The a.c. supply can be converted to obtain d.c. wherever required.
8. The maintenance of a.c. substations is cheaper.
9. The transforming a.c. substations are much efficient than d.c. system using rectifiers.
1.4 Disadvantages of A.C. System
The various disadvantages of a.c. system are,
1. The construction of a.c. transmission line is more complicated than a d.c. line.
2. The resistance of a.c. line is higher due to the skin effect causing more voltage drop.
3. The drop is also due to the inductance of a.c. line, causing loss of power.
4. When the line is open, the charging current flows due to the capacitance of an a.c. line which also causes loss of power continuously.
5. The copper requirement for a.c. line is more than a d.c. line.
6. The a.c. lines are more sensitive to corona than a d.c. line.
7. The problem of synchronization of alternators exists in case of a.c. system.
8. The speed of alternators in a.c. system is not economical and required to be controlled within very low limits.