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Electrical Power Distribution System

The main function of an electrical power distribution system is to provide power to individual consumer premises. Distribution of electric power to different consumers is done with much low voltage level.
Distribution of electric power is done by distribution networks. Distribution networks consist of following main parts
Distribution substation,

Primary distribution feeder,

Distribution Transformer,

Distributors,

Service mains.





The transmitted electric power is stepped down is substations, for primary distribution purpose. Now these stepped down electric power is fed to the distribution transformer through primary distribution feeders. Over head primary distribution feeders are supported by mainly supporting iron pole (preferably rail pole). The conductors are strand aluminum conductors and they are mounted on the arms of the pole by means of pin insulators. Some times in congested places, underground cables may also be used for primary distribution purposes.
typical power distribution system
Distribution transformers are mainly 3 phase pole mounted type. The secondary of the transformer is connected to distributors. Different consumers are fed electric power by means of the service main. These service mains are tapped from different points of distributors. The distributors can also be re-categorized by distributors and sub distributors. Distributors are directly connected to the secondary of distribution transformers whereas sub distributors are tapped from distributors. Service main of the consumers may be either connected to distributors or sub distributors depending upon the position and agreement of consumers. In this discussion of electrical power distribution system, we have already mentioned about both feeders and distributors. Both feeder and distributor carry the electrical load, but they have one basic difference. Feeder feeds power from one point to another without being tapped from any intermediate point. As because there is no tapping point in between, the current at sending end is equal to that of receiving end of the conductor. The distributors are tapped at different points for feeding different consumers; and hence the current varies along their entire length.

Radial Electrical Power Distribution System

In early days of electrical power distribution system, different feeders were radially come out from the substation and connected to the primary of distribution transformer directly.
radial electrical power distribution system
But radial electrical power distribution system has one major drawback that in case of any feeder failure, the associated consumers would not get any power as there was no alternative path to feed the transformer. In case of transformer failure also, the power supply is interrupted. In other words the consumer in the radial electrical distribution system would be in darkness until the feeder or transformer was rectified.



Type of Electrical Power Distribution Systems:

Electrical power is distribution either three wires or Four wires (3 wire for phases and 1 wire for Neutral). Voltage between Phase to Phase Called Line Voltage and Voltage between Phase and Neutral is Called Phase Voltage.
This Forth wire may or may not be distributed in Distribution System and Same way this neutral may or may not be earthed
Depending of this neutral condition (Earthed-not Earthed-access-not access) there are various type of earthing System.
The neutral may be directly connected to earth or connected through a resistor or a reactor. This system is called directly earthed or Earthed System.
When a connection has not been made between the neutral point and earth, we say that the neutral is unearthed.
In a network, the earthing system plays a very important role. When an insulation fault occurs or a phase is accidentally earthed, the values taken by the fault currents, the touch voltages and over voltages are closely linked to the type of neutral earthing connection.
A directly earthed neutral strongly limits over voltages but it causes very high fault currents, here as an unearthed neutral limits fault currents to very low values but encourages the occurrence of high over voltages.
In any installation, service continuity in the event of an insulation fault is also directly related to the earthing system. An unearthed neutral permits service continuity during an insulation fault. Contrary to this, a directly earthed neutral, or low impedance-earthed neutral, causes tripping as soon as the first insulation fault occurs.
The choice of earthing system in both low voltage and medium voltage networks depends on the type of installation as well as the type of network. It is also influenced by the type of loads and service continuity required.
The Main objectives of an earthing system are Provide an alternative path for the fault current to flow so that it will not endanger the user, Ensure that all exposed conductive parts do not reach a dangerous potential, Maintain the voltage at any part of an electrical system at a known value and prevent over current or excessive voltage on the appliances or equipment.
Different earthing systems are capable of carrying different amounts of over current. Since the amount of over current produced in different types of installation differs from each other, required type of earthing will also differ according to the type of installation. so in order to ensure that the installation goes with the existing earthing system or else to do any modification accordingly, we need to have a proper idea of the present earthing system. It would enhance the safety as well as the reliability
As per IEC 60364-3 There are three types of systems:
(1) Unearthed System:
IT System.
(2) Earthed System:
TT
TN (TN-S, TN-C, TN-C-S).
The first letter defines the neutral point in relation to earth:
T = directly earthed neutral (from the French word Terre)
I =unearthed or high impedance-earthed neutral (e.g. 2,000 Ω)
The second letter defines the exposed conductive parts of the electrical installation in relation to earth:
T =directly earthed exposed conductive parts
N =exposed conductive parts directly connected to the neutral conductor