Power System Fundamentals
EE 317
Lecture 10
27 October 2010
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?
Bahria Town Grid Station
Roles of a Substation
 What are they?
Voltage transformation
 Why?
 Typical levels 230,138,69, 34.5 or 23 kV in
And 12kV or 4 kV out
System Protection
 Substation Breakers : what do they protect?
 The Public
 Substation Transformers
 Switchgear
System Reliability
 Isolate Faults: trip a feeder or a bus as opposed
to an entire substation
TYPICAL Sub Station
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.

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.
Layout of a Typical Substation
Hi-voltage Breaker
 Switchgear
 Station Transformer
 Switchgear
 Bus
 Feeder Switchgear (and/or breakers)

Single primary feeders
Multiple primary feeders
High Voltage Breaker

A circuit breaker is an automaticallyoperated electrical 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.
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.
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.
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)
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
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
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
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
Distribution Equipment
 Lightning arrestor
 Bushing Insert
 11-kV Insulator
 Fused Cutout
 Terminating Dead-End
 66- kV insulator
 3- 0.4 kV spacer
 Secondary capacitor