Structure of Power System

Power transmittability increases and transmission losses decreases with increasing voltage level. The larger the blocks to be transmitted and the greater the distance over which they must be wheeled, the higher must be the operating voltage chosen.

We can distinguish the following transmission levels from the lowest voltage class as shown in the figure.

• Distribution Level (Secondary and Primary)
• Sub-transmission Level
• Transmission and Pool Level

Distribution Level :

    These circuits are the finest meshes in the network. Distribution voltage levels are classified into two types.

        1. Primary or Feeder Voltage (For instance 33 kV)

        2. Secondary or Consumer Voltage (For instance 230 V per phase)

    The distribution circuits fed from the distribution substations supply energy to the domestic or medium sized (small industrial and commercial) customers. Distribution Engineering constitutes the major part covering the problems of overhead or underground service, metering, switching and protection.

Sub-transmission Level :

    The sub-transmission line distributes energy to a number of distribution substations in a certain geographical area at a voltage level that typically varies between 33 kV and 132 kV. It receives the energy directly from the generator bus in the generating station or via bulk power substations. Large customers are served directly from those stations.

    The role of a sub-transmission system is mainly the same as that of a distribution system except that it serves a larger geographical area and distributes energy in larger blocks at higher voltage levels. In many systems there are no clear demarcation lines between sub-transmission and transmission systems. Increased load density makes it necessary and economical to superimpose a new and higher voltage grid on the existing one.

Transmission and Pool Level :

    This handles the largest blocks of power. Also it interconnects all the generator stations and all the major loading points in the system. The energy can be routed in any desired direction on the various links of the transmission system to the best overall operating economy via, inter ties. Transport of energy can take place to or from other power systems belonging to the same power pool. It tends to appear as a loop structure. So it has more path combinations that better suits for the purpose of the transmission level.

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