Distribution system design

Distribution system

The system which deliver 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 distribution system.Distribution system is the bridge between the transmission system and the consumers meters.

Transmission and Distribution

Transmission

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

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

Some common terminology and system equipment of distribution system:

1. service transformer:Lowers the voltage at utilization level
2. Secondary transformer
3. Load center:Area served by particular distribution transformer.
4. Primary distribution
5. Substation: Primary distribution lines receives power at this point.
6. service area:service area is the area service by particular substation or feeder.
7. Sub transmission line:The line which fed power to the primary of transformer at substations.

Primary and secondary distribution system

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

Secondary Distribution system:It is the part of 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 area 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 size 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 household should be covered.
• Avoids placing poles in flooded  or land.
• Minimize deforestation as much possible.
• Minimize width of river crossing.
• Must be Routed along the feasible roads,foot trail/hilly road;facilitates the maintenance of line.

Substation sitting

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

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:

1. Single phase/3 phase
2. Feeder layouts

• Radial distribution
• Loop distribution
• Network distribution
• construction :overhead/underground

1.Single phase/3-phase prospective:Exactly how and where the transition from 3-phase to 1-phase circuit takes place in a distribution system depends upon planner preference’s and other situations.There are commonly two popular method for 3-phase to 1-phase transition:

1. American system
2. European system

1.American system:

1. uses relatively small 1-phase transformer.
2. useful in spare thin populated area where there is low consumption of energy per user.
3. Low capital investment
4. Loss is reduced
5. 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:

2.European system:

1. use relatively large transformer,often 3 phase.
2. Less number of transformer is sufficient.
3. used in dense populated area where there is more consumption of energy per user.
4. Individual consumers may also be provide with single phase service from three phase secondary circuit.
5. 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

• Radial feeders originates from 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 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 consists of a distribution design with two paths between sources and customers.
• This layout is often called European system.

• Feeder ring mains is energized from two or more than 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.

Disadvantages

•  Network systems costs considerably higher and much more complicated analysis and
  operating procedures.