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• Distribution system


  The electrical system between the sub station fed by the transmission system and the consumers meters

  
• Parts of power distribution system

  • Feeders
  • Distributors
  • Service mains

  
• Feeder


  A conductor which connects the sub-station (or Localized generating station) to the area where the power is distributed
• Distributor


  A conductor from which tappings are taken For supply to the consumers
• Service mains


  A small cable which connects the distributor to the consumer's terminals
• Classification of distribution systems

  • By nature of current (DC, AC)
  • By type of construction (overhead, underground)
  • By scheme of connection (radial, ring main, interconnected)

  
• Primary distribution


  Carries the load at higher than utilization voltages from the substation (or other source) to the point where the voltage is stepped down to the value at which the energy is utilized by the consumer

  
• Secondary distribution


  Includes that part of the system operating at utilization voltages, up to the meter at the consumer's premises

  
• Types of primary distribution systems

  • Radial systems
  • Loop systems
  • Primary network systems
• Radial system


  Separate feeders radiate from a single substation and feed the distributors at one end only
• Ring main system


  The primaries of distribution transformers form a loop
• Types of secondary distribution systems

  • Individual transformer - single service
  • Common secondary main
  • Banked secondaries
  • Secondary networks

  
• Individual transformer - single service


  Applicable to certain loads that are more or less isolated, such as in rural areas where consumers are far apart and long secondary mains are impractical
• Common secondary main


  Takes advantage of diversity between consumer's loads and demands
• Banked secondaries


  A single-feeder low-voltage network, with the secondary along a section or grid to which the transformers are connected
• Secondary networks


  Created by connecting together the secondary mains fed from the transformers supplied by two or more primary feeders
• Electric power distribution is the portion of the power delivery infrastructure that takes the electricity from the highly meshed, high-voltage transmission circuits and delivers it to customers

  
• Primary distribution lines are "medium-voltage" circuits, normally thought of as 600 V to 35 kV
• At a distribution substation, a substation transformer takes the incoming transmission-level voltage (35 to 230 kV) and steps it down to several distribution primary circuits, which fan out from the substation
• Close to each end user, a distribution transformer takes the primary-distribution voltage and steps it down to a low-voltage secondary circuit commonly 120/240 V

  
• The distribution infrastructure is extensive; after all, electricity has to be delivered to customers concentrated in cities, customers in the suburbs, and customers in very remote regions

  
• Elements of a substation

  • Primary power lines
  • Ground wire
  • Overhead lines
  • Transformer for measurement of electric voltage
  • Disconnect switch
  • Circuit breaker
  • Current transformer
  • Lightning arrester
  • Main transformer
  • Control building
  • Security fence
  • Secondary power lines
  • Wave traps
• Types of substations

  • Transmission substation
  • Distribution substation
  • Collector substation
  • Converter substations
  • Switching station
• Transmission substation

  Connects two or more transmission lines
• Distribution substation


  Transfers power from the transmission system to the distribution system of an area

  
• Distribution substation typically operates at medium voltage levels, between 2.4 kV-33 kV
• Major components of a distribution substation

  • Supply line
  • Transformers
  • Bus-bars
  • Switchgear
  • Out-coming feeders
  • Switching apparatus (switches, fuses, circuit breakers)
  • Surge voltage protection
  • Grounding

  
• Supply line


  Distribution substation is connected to a sub-transmission system via at least one supply line, which is often called a primary feeder
• Transformer


  Transforms the supply line voltage to distribution level voltage
• Supply line


  Primary feeder that connects the distribution substation to the sub-transmission system
• Supply line


  • Can be an overhead line or an underground feeder, depending on the location of the substation
  • Connected to the substation via high voltage disconnecting switches to isolate lines for maintenance or repair
• Transformer


  Steps down supply line voltage to distribution level voltage
• Transformer classification factors


  • Power rating
  • Insulation
  • Voltage rating
  • Cooling
  • Winding connections
  • Voltage regulation
• Transformer power rating


  Expressed in kilovolt-amperes (kVA) or megavolts-amperes (MVA), indicates the amount of power that can be transferred
• Transformer insulation


  Can be liquid (mineral oil, nonflammable or low-flammable liquids) or dry (ventilated, cast coil, enclosed non-ventilated, sealed gas-filled)
• Transformer voltage rating


  Governed by the sub-transmission and distribution voltage levels the substation is connected to, indicated by the manufacturer

  
• Transformer cooling


  Dictated by the transformer power rating and maximum allowable temperature rise, can be self-cooled, force air, force oil and air, or force oil and water
• Transformer winding connections


  Can be delta (each phase winding connected in a triangle) or wye (each phase winding connected to a neutral point)
• Transformer voltage regulation


  Achieved through voltage tap regulators that can change the transformation ratio to maintain nominal voltage at customer service points
• Busbar


  Used to carry large current and distribute current to multiple circuits within switchgear or equipment