Distribution System Design - Online Student Guide

Table of Contents

Distribution System

The system which delivers power to electrical consumers at their place of consumption(in customer location and capacity to meet the customer peak demand and energy demand) for local use in reliably usable form (stable voltage quality, higher reliability) is the distribution system. The distribution system is the bridge between the transmission system and the consumer’s meters.

Transmission and Distribution

Transmission

Transmission is of bulk power over a larger distance at the voltage of 66kV-1100KV. The transmission system is often designed and operates in the network.

Distribution is of relatively small power over a small area at 33KV and below includes the utilization voltage. The distribution system is often designed and operates in a radial.

For the distribution design, we considered the following aspects:

utilization voltage
Demand of consumers
Distributes
voltage regulation
personal safety

pic source:https://www.electricalindia.in/electrical-distribution-network-for-smart-cities/ — Pic source: https://www.electricalindia.in/electrical-distribution-network-for-smart-cities/

Some common terminology and system equipment of the distribution system:

service transformer: Lowers the voltage at the utilization level
Secondary transformer
Load center: Area served by a particular distribution transformer.
Primary distribution
Substation: Primary distribution lines receive power at this point.
service area: Service area is the area serviced by a particular substation or feeder.
Sub transmission line: The line which fed power to the primary transformer at substations.

Primary and secondary distribution system

Primary distribution system: It operates at a voltage higher than the utilization voltage. The most commonly used primary distribution voltages are 33KV, 11KV, 6.6KV, and 3.3KV. Primary distribution is carried out by a 3-Phase 3-wire system.

Secondary Distribution system: It is the part of the distribution system at which the consumers utilizes the electrical energy. Secondary distribution is carried out at 400/230v,3-phase,4-wire system.

Distribution system design includes Distribution transformer design, primary distribution design, and substation.

Selection of Distribution transformer

Selection of transformer size
Location(must be located at the center of gravity)
voltage level
configuration
Load density
Load center size
sitting (center of gravity)
In urban areas go for the maximum transformer size and are usually higher due to high load density.
In Rural the maximum size of LT is limited by power loss and voltage drop and Transformer sizes are usually smaller due to low density and load coincident.

Selection of LT line route

Short radial length to minimize the voltage drop and line loss.
maximum number of households should be covered.
Avoids placing poles in flooded or land.
Minimize deforestation as much as possible.
Minimize the width of the river crossing.
Must be Routed along feasible roads, foot trail/hilly roads; facilitates the maintenance of the line.

Substation sitting

Availability of land for the necessary equipment.
cost of land.
opportunity for the Feeder gateway.
cost of preparing the site.
cost of transporting material.
Right of way for transportation of substation equipments.

Area Substation selection

Service area
Input/output voltage level
sitting
sizing
source grid substation
High voltage and low voltage side configuration(HT bus bar arrangement, No of LV feeders , No of Transformers, etc)

Distribution Lines

Mainly distribution line is classified into:

Single phase/3 phase
Feeder layouts

Radial distribution
Loop distribution
Network distribution
construction: overhead/underground

Single phase/3-phase prospective

Exactly how and where the transition from a 3-phase to a 1-phase circuit takes place in a distribution system depends upon planner preferences and other situations. There are commonly two popular methods for the 3-phase to 1-phase transition:

American system
European system

American system

uses relatively small 1-phase transformer.
useful in spare thin populated area where there is low consumption of energy per user.
Low capital investment
Loss is reduced
The secondary voltage level in USA for residentals service is 120V/240V.

Lower wattage devices(Lighting )=120V
Higher wattage devices(refrigerator)=240V

The diagram for 1-phase 3-wire system is shown below:

European system

use relatively large transformer,often 3 phase.
Less number of transformer is sufficient.
used in dense populated area where there is more consumption of energy per user.
Individual consumers may also be provide with single phase service from three phase secondary circuit.
The only the difference between American and European system is that the utilization voltage in European system is 250V and 120V in American system.

Single wire earth return power Distribution

Single phase ground return are mostly used in India,USA ,china up to 66KV.
This type of distribution needs high quality of earthing at each load center(earthing resistance=0.5ohm in sub station).
It is used where the distance between load center/consumer is very high.

Underground and overhead Lines

An overhead lines are the structure of distribution system that transmits the electrical
energy over a large distances which consists of one or more conductors suspended by
towers or poles. The overhead (OH) lines are mainly bare conductors. The distribution
system that transmits electrical energy through cables buried in ground is called
underground distribution system. The underground (UG) systems mainly uses power
cables to transmit electrical energy. The UG lines are used due to following reasons:

Reliability is very high.
(2-10) times costly than O/H lines.
Extreme weather condition (snow, wind) in distribution areas.
Densely populated urban areas having right of way problem.
Can be used near submarine crossings.
It does not have interference with communication lines.
There is no any hazards due to lightning discharges.
High Public Safety.

The OH lines are used due to following reasons:

It is widely used in rural areas.
Overhead system is very cheap as no insulation coating is used over the conductors i.e. the conductors
used are bare conductors.
Faults can be easily detected and maintenance is very simple.
The amount of insulation required is less because air provides necessary insulation.
For same amount of power, less conductor size is used.

Connection Scheme of Distribution System

A feeder is an electrical distribution circuit fed from a single source point (breaker or fuse) at the substation. It operates at primary distribution voltage and disseminates power through a service area which is its feeder service area. There are basic three types of feeder systems:
– Radial System
– Loop System
– Network System

Radial Feeder Systems

Radial feeders originate from the secondary bus(KV) of a distribution substation.
Power flows only in a single direction.
The feeder and sub-feeders are 3 phase ,4 wire circuits.
This type of feeder is most commonly used to supply small residential, commercial, and industrial areas.

Advantages
– Low cost: this is the least expensive type of distribution(most economical).
– Simplicity of analysis: It is easiest to analyze and operate the system.

Disadvantages

– Less reliable: Any equipment failure will interrupt service to all consumers
downstream from it.Because there is only one path between the substation and the customer.Thus, if any elements along the path fails,a loss of power delivery results).

Loop System

The loop system consists of a distribution design with two paths between sources and customers.
This layout is often called the European system.

Advantages
– High reliability than radial system(since feeder and loop components have sufficient reserve capacity to serve the load).
Disadvantages
– In terms of complexity, a loop feeder system is only slightly more complicated than a radial system.
– More power loss than in radial and parallel feeder. and cost is high than radial system.

Network System

Feeder ring mains are energized from two or more two generating stations or substations.
Multiple paths between all points in the network.

Advantage

high level of reliability.
The loss of any source will not interrupt the flow of power to any customers.
The multiple failure of sources can occur with little or no interruption.