Design Considerations for Primary Distribution

Design Considerations for Primary Distribution

The design of primary distribution is based on the following considerations,

1. Selection of voltage : The commonly used voltage levels for primary distribution are 33 kV, 11 kV and 6.6 kV. The selection depends on the available load estimation of the area. The highest voltage which is economically justified is selected, keeping in consideration, the future possible load demand.

The economic comparison over the period of time considered for the planning, decides the proper voltage level. The number of plans based on different possible voltage levels are prepared, considering a period of 8 to 10 years. The cost is compared by dividing the schemes into the steps to be taken at particular years. All the schemes are identical in capacity and reliability. The voltage level giving most economical scheme is selected.

The economical voltage level is selected based on an available empirical formula given by,

V = 5.5 √ 0.62 L + 3p/100

where V = Line voltage in kV, L = Distance in km

P = Maximum power per phase to be delivered in kW

The standard higher voltage nearest to the value given by above formula is generally selected. The value must be selected such that the line voltage regulation remains well within the limits.

2. Choice of scheme : The choice of scheme is based on the various considerations such as economy, reliability, safety from consumer point of view, efficiency, load to be supplied and overall appearance.

For low and medium load density locations, a radial distribution system is used. If the reliability of supply is of great importance from the load point of view then parallel feeders are used. For high load density areas ring main distribution scheme is used.

The number of feeders to be used depends on the various factors such as total load to be supplied, permissible voltage drop, the load density, the distance upto secondary distribution, proper locations for secondary distribution etc.

3. Size of feeders : The size of the feeder conductor is determined on the basis of,

i) Current carrying capacity.

ii) Permissible voltage drop in the feeder.

iii) Economic consideration.

Each of these considerations may give different sizes. Basically the feeder size is decided based on current carrying capacity and then modified based on voltage drop and economic consideration. The current carrying capacity depends on the conductor losses and surroundings. The current carrying capacity is decided for a maximum working temperature of 75 °C. Standard tables are available for the current carrying capacities of different ACSR conductors for various operating temperatures for both overhead lines as well as underground cables. Using the table, maximum current capacity of conductor and its tentative size is determined. The tables for conductor resistances and inductances are also available. Hence the voltage drop in the feeder due to rated current can be determined. If the drop is small then it can be adjusted by selecting the turns ratio of the transformer at the secondary distribution. But if voltage drop is high then alternative size must be selected so as to reduce the voltage drop. The size is then compared with the size obtained from Kelvin's law for economical aspects.

Review Question

1. Discuss the design of primary distribution system with respect to

i) Selection of voltage ii) Choice cf scheme iii) Size cf feeders.