Primary Distribution Design - Types And Advantages


Primary Distribution Design - Types And Advantages

Primary distribution design is a problem which is comes in mind when generation ans are situated far outside from the load areas. Following are the factors affecting primary distribution design.

(1) Voltage Level / Magnitude

Voltage magnitude are affected by Standards

  • Local regulations and
  • Physical relation with power system.

In India voltage level of primary distribution systems are 11 KV, 22 KV or 33 KV. power  loss is directly proportional to the square of the voltage. General percentage variation of power loss and voltage drop with respect to voltage shown in figure.
Primary Distribution Voltage Graph
Primary Distribution Voltage vs Power Loss Graph

If voltage level selected is larger than it permits to increase the length of distribution line but then we have to increase the clearance distance with increase in ratings of transformer and other equipments which increases the cost.

Voltage level depends on the load values and load densities which is a function of load concentration in given areas.

(2) Conductor Size

Conductor size depends on the current carrying capacity. Current carrying capacity can be determined from the transformer rating which is in KVA. Transformer rating depends
on the load connected to it.

(3) Voltage Drop

As per IEEE standard total allowable voltage drop in the distribution system is 8%.

Out of 8%, in primary distribution system, voltage drop allowable is 3% while in secondary system allowable voltage drop is 5%. 

If voltage drop exceeds above these values then following preventive measures are need to be taken.
  • Use of capacitors
  • Use of voltage boosters
Replacement of exciting conductors by newer low resistance conductors.
  • Increase the system voltage level.
  • Re-networking of the circuit.
Out of above preventive measures, the selection of measure is taken depending on
the cost required.

(4) Types Of Distribution System

In general following types of distribution systems are commonly used :
  1. Radial system
  2. Loop system and
  3. Primary network system.
Selection of type of distribution system is taken by taking considerations like cost, reliability level, operating efficiency, voltage regulation, network flexibility and load balancing etc.

Proper selection of distribution system gives following advantages :
  • Lower I²R losses.
  • Good voltage regulation.
  • Larger or improvement in reliability level.
  • Need of larger rated transformer reduced.
  • Investment cost needed is lower.
  • Transfer of load from highly loaded feeder to lower loaded feeder can be possible.
  • Flexible layout etc. 

(5) Size, Nature And Location Of Large Loads

(6) Size, Nature And Location Of Small Loads

(7) Future Load Growth Of The Area.

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