Power System Fundamentals

1
Power System Fundamentals

• EE 317
• Lecture 10
• 27 October 2010

2
Distribution System Overview

• Role of the Substation
• What is a Bus?
• What is a Feeder?
• What does Radial mean?
• What are the methods used by engineers to
  increase reliability in the distribution system?

3
Bahria Town Grid Station
4
Roles of a Substation

• What are they?

5
Voltage transformation

• Why?
• Typical levels 230,138,69, 34.5 or 23 kV in
• And 12kV or 4 kV out

6
System Protection

• Substation Breakers what do they protect?
• The Public
• Substation Transformers
• Switchgear

7
System Reliability

• Isolate Faults trip a feeder or a bus as opposed
  to an entire substation

8
TYPICAL Sub Station
9
Elements of sub station

• Substations generally have switching, protection
  and control equipment and one or more
  transformers.
• In a large substation,circuit breakers are used
  to interrupt any short-circuits or overload
  currents that may occur on the network.
• Smaller distribution stations may use recloser
  circuit breakers or fuses for protection of
  distribution circuits.
• Substations do not usually have generators,
  although a power plant may have a substation
  nearby. Other devices such as capacitors and
  voltage regulators may also be located at a
  substation.

10
Functional Types of sub stations

• Transmission substation
• A transmission substation connects two or more
  transmission lines
• Distribution substation
• A distribution substation transfers power from
  the transmission system to the distribution
  system of an area
• Collector sub station
• In distributed generation projects such as a wind
  farm, a collector substation may be required
• Switching sub station
• A switching substation is a substation which does
  not contain transformers and operates only at a
  single voltage level.

11
Layout of a Typical Substation

• Hi-voltage Breaker
• Switchgear
• Station Transformer
• Switchgear
• Bus
• Feeder Switchgear (and/or breakers)
• Single primary feeders
• Multiple primary feeders

12
High Voltage Breaker

• A circuit breaker is an automatically-operated ele
  ctrical switch designed to protect an electrical
  circuit from damage caused by overload or short
  circuit. Its basic function is to detect a fault
  condition and, by interrupting continuity, to
  immediately discontinue electrical flow. Unlike
  a fuse, which operates once and then has to be
  replaced, a circuit breaker can be reset (either
  manually or automatically) to resume normal
  operation. Circuit breakers are made in varying
  sizes, from small devices that protect an
  individual household appliance up to
  large switchgear designed to protect high voltage
  circuits feeding an entire city.

13
Switch Gears

• Switchgear for low voltages may be entirely
  enclosed within a building. For transmission
  levels of voltage (high voltages over 66 kV),
  often switchgear will be mounted outdoors and
  insulated by air, though this requires a large
  amount of space. Gas insulated switchgear used
  for transmission-level voltages saves space,
  although it has a higher equipment cost. Oil
  insulated switchgear generally takes up more
  space, and is being phased out to eliminate the
  risk of an oil spill.

14
Station Transformers

• Station Transformers come in varying sizes,
  depending on site requirements, but are normally
  connected to the network through overhead
  connections which results in exposed HV or LV
  bushings that necessitate a fence or other
  protective enclosure for personnel safety and
  equipment security.

15
Primary Radial Feeders

• Layout
• 3 ? to any customer locations requiring it as
  well as to other key points to maximize
  reliability
• Sectionalizer locations
• Feeder Tie locations
• Single phase to all other locations (balancing
  load connected to each phase)

16
Sectionalizing Switches

• Switches WITHIN a feeder service area
• Isolate Faults
• Allow majority of feeder customers served to be
  put back into service during an extended repair

17
Feeder Tie Switches

• Switches BETWEEN two different feeders to
  interconnect them
• Allow an adjacent feeder region to be served by a
  feeder that is still in service when a substation
  problem or feeder fault which can be isolated
  from the substation side has disrupted service on
  an entire feeder

18
Distribution Service

• Primary Feeder serves Distribution pots (or
  transformers)
• Normally Fused
• Switchable Transformer
• Serves Secondary circuit
• distribution voltage 120/240 V
• Houses/streetlights connected to secondary
• Service Entrance Cable to Home/Meter

19
Distribution Transformers

• Because of connection to overhead, open wire
  feeders these transformers are often exposed to
  lightning strikes
• To minimize insulation breakdown and transformer
  failure lightning arrestors are added

20
Distribution Equipment

• Lightning arrestor
• Bushing Insert
• 11-kV Insulator
• Fused Cutout
• Terminating Dead-End
• 66- kV insulator
• 3-? 0.4 kV spacer
• Secondary capacitor