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Power Distribution Systemsand Its Characteristics
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Distribution System Characteristics
The Electric Power System is usually divided into
three segments, which are generation,
transmission, and distribution.
In a broad definition, the distribution system is
that part of the electric utility system between the
bulk power source and the customers serviceswitches.
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Distribution System Characteristics
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Distribution System
A distribution system includes the following
components
1. Sub-transmission system;
2. Distribution substations;
3. Distribution primary feeders;
4. Distribution transformers;
5. Secondary circuits;
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Distribution System
Some distribution system engineers define the distributionsystem as that part of the electric utility system between
the distribution substations and the consumers service
entrance.
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Distribution Substation
Distribution substations consist of an almostinfinite number of designs considerations:
Load density;
High side voltage;
Low side voltage;
Land availability
Reliability requirements;
Load growth;
Voltage drop
Emergency conditions;
Cost and losses
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Distribution Substation
A Typical distribution substation arrangement.
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Distribution Substation
The voltage of the high side bus can be anywhere from34.5 kV all the way to 345 kV and beyond.
The average or preferred high side voltage level is
approximately 115 to 138 kV.
The average substation consists of two transformers withan impedance of approximately 10 percent (0.1 p.u.).
The low voltage bus in a multiple transformer substation is
usually split (contains a normally openbreaker or switch)to alleviate circulating currents as well as reduce the short
circuit current seen by the system.
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Distribution Substation
Normally, Two or more feeders are connected toeach bus through a feeder breaker.
On smaller substations where short circuit levels
are lower, a recloseris sometimes used instead ofa breaker.
Short circuit levels at the terminals of the low
voltage bus are generally kept at 12 000 amperes
or less although there are many systems wheremuch higher levels can be found.
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Distribution Substation
A typical substation may include the followingequipment:
Power transformers
Circuit breakers
Disconnecting switches
Station buses and insulators
Current limiting reactors
Shunt reactors Current and potential transformers
Capacitor voltage transformers
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Distribution Substation
A typical substation may include the followingequipment:
Coupling capacitors
Series capacitors
Shunt capacitors
Grounding systems
Lightning arrestors/gaps
Line traps Protective relays
Station batteries
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Distribution Substation
The electrical and physical arrangements of theswitching and busing at the sub-transmission
voltage level are determined by the selected
substation scheme.
The selection of substation scheme is based upon
Safety
Reliability
Economy
Simplicity
Other considerations
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Distribution Substation
The most commonly used substation bus schemes
Single bus scheme
Double bus double breaker scheme
Main and transfer bus scheme
Double bus single breaker scheme
Ring bus scheme
Breaker and a half scheme
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Substation Location
The location of a substation is dictated by:
The voltage levels,
Voltage regulation considerations
Sub-transmission costs
Substation costs
Cost of primary feeders, main feeders and distribution
transformers. It is also restricted by other non technical factors.
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Substation Location
The following rules should be observed inselecting an ideal location for a substation.
Locate the substations closer to the load centers
Locate substations such that proper voltage regulation
can be obtainable without taking extensive measures
Proper access for incoming sub-transmission lines and
out going primary feeders and also allow future growth.
Should provide enough space for the future substation
expansion.
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Substation Location
The following rules should be observed inselecting an ideal location for a substation.
Should not be opposed by land use regulations, local
ordinances and neighbors.
Should help to minimize the number of customers
affected by service discontinuity.
Other considerations such as adaptability, emergency
etc.
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Distribution Feeders
There are various feeder designs that can improvecustomer service:
Simple radial feeder (system)
In this system, a given secondary feeder is fed from only one
transformer and one primary cable. Each substation operates independently and there is no
duplication of equipment.
No extra tie cables, circuit breakers or transformers with largereserve capacityare needed.
System investment is usually the lowest of all circuitarrangements.
For a well-planned and well-maintained system, the servicereliability of the radial system is usually high.
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Distribution Feeders Simple radial feeder (system)
The radial system is exposed to many interruption possibilities
(the most important of which are those due to primary
overhead or underground cable failure or transformer failure).
The loss of a primary cable or transformer will cut off service
to the affected loads until repairs can be made.
This is often the reason for design engineers to select one of
the other circuit arrangements, which can minimize production
shutdown.
The system will be satisfactory only if the interruptions
frequency is very low and if there are ways to operate the
system without planned outages.
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Distribution Feeders A typical Simple radial system
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Distribution Feeders Expanded radial system
May be applied to larger loads by using a radial primary
distribution system to supply a number of unit substations
located near the centers of load, supplying the load throughradial secondary systems.
This would result in better voltage regulation and in lower
power loss than in a system using heavy low-voltage feeders of
extensive length
If the transformers are installed without individual primary
protection, a fault in any one transformer will result in loss of
the entire system.
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Distribution Feeders Typical Expanded Radial System
(a) (b) (c)
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Distribution Feeders
Secondary Selective System
This is the next widely used system to radial system
A pairs of unit substations are connected through a normally open
secondary tie circuit breaker. Use of this type of system result in reliability increase.
It providesflexibilityin operation, particularly when equipment is
being maintained or serviced.
Any part of a primary feeder, or transformer and associated
equipment, can be de-energized for inspection or maintenance
without service interruption.
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Distribution Feeders
Secondary Selective System
(a) (b)
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Distribution Feeders
Secondary Selective System
Transformers used in this system are often equipped with cooling
fans to provide additional capacity during emergency operations.
The tie breaker should be interlocked with the transformersecondary breakers to prevent the transformers from being
operated in parallel.
Parallel operation of the transformers would increase the available
secondary short-circuit current and would risk the loss of power to
both secondary busesin the case of a transformer fault or primarycable fault.
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Distribution Feeders
Primary Selective System
This system is another way of reducing the time required to restore
voltage to a load in the event of loss of a primary feeder as
compared with a radial system. In this system, two or more primary feeders are provided.
Two primary feeders are extended to each transformer, and
selector switches are provided so that any transformer may be
connected to either of the two primary feeders.
Eachprimary feedermust have sufficient capacity to carry themaximum load that may be connected at one time.
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Distribution Feeders
Primary Selective System
Under normal operation:
The system is operated with the load divided approximatelyequally on each primary feeder.
When a fault occurs on one primary feeder, there will be aninterruption of power to the load connected to that feeder.
The interrupter switch connected to the faulted feeder will beopened and the respective transformer will be reconnected to theenergized feeder.
The two switches associated with one transformer should bemounted in separate individual metal enclosures in order that a de-energized feeder may be safely maintained while the other feeder isenergized.
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Distribution Feeders
Primary Selective System
The primary selective system has a higher first costthan a radial
system and is usually lower than that of a comparable secondary
selective system. The service reliability of this system lies between that of a radial
system and that of a secondary selective system.
This scheme is in popular use on many underground systems.
It also offers little remedy for computer problems caused by
temporary faults to the overhead system.
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Distribution Feeders
Looped Primary System
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Distribution Feeders
Looped Primary System
In Figure (a) looped primary system:
The primary loop is fed by a single medium - voltage circuit
breaker. One loop-sectionalizing interrupter switch is located at the primary
of each transformer.
One section of the primary loop is connected directly to the
primary of each transformer.
In case of a fault in a transformer or on the primary loop, theprimary circuit breaker will open and clear the fault.
After the fault is located, the two interrupter switched at the ends
of the faulted section are opened and the remainder of the loop is
again energized.
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Distribution Feeders
Looped Primary System
In Figure (b) looped primary system:
Two medium-voltage circuit breakers are used, one on each end of
the loop, and two interrupted switches are used at each transformerlocation.
With this arrangement, the only time that any part of the load is
without service for a extended period is when a fault takes place in
a transformer or its secondary bus.
Any fault on a section of the primary loop will not affect thetransformers supplying their respective loads.
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Distribution Feeders
Looped Primary System
The disadvantage of this arrangement lies in a complex and time-
consuming operation in locating a faulton the primary loop or on any
transformer, because the entire system must be de-energized for a time,
and the time required to restore service may also be lengthy.
When the load centers are located relatively far apart, the looped
primary system will cost little more than a comparable radial system.
The initial cost can be reduced by using fused interrupter switchedinstead of medium-voltage circuit breakers.
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Distribution Feeders
Secondary Network System
This system can provide a very high degree of service reliability to all
the loads.
The difference with the secondary selective system is in the way thesystem is operated.
In the SSS, the tie circuit between secondary buses is normally open
and each transformer supplies its own load.
In the SNS, the secondary buses are tied together and the transformers
operate in parallel to supply the entire load. In the SNS, the transformer secondary switching and protective device
is a special low-voltage power circuit breaker known as a network
protector.
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Distribution Feeders
Secondary Network System
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Distribution Feeders
Secondary Network System
Under normal condition
The total load is shared by all of the transformers operating in parallel .
For a Fault in a primary feeder or in a transformer or if voltage fail The power flow from the secondary bus to the transformer will cause
the network protector to open, thus disconnecting the transformer from
the secondary bus.
The remaining energized transformers will continue to supply power
to the bus, and there will be no interruption of power to the loads.
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Distribution Feeders
A Spot Network Arrangement
In this system, there are two or more transformers connected to a
single bus through network protectors.
The transformers and primary feeders must have sufficient capacity tocarry all loads connected to the bus with one transformer out of
service.
For larger installations, a system having a primary selective and
secondary network arrangementcan be an advantage.
Each of the primary feeders must have sufficient capacity to supply theentire load that is connected to it.
If two primary feeders are used to supply the primary selective
network system, half of the transformers would normally be connected
to each feeder with adjacent transformers on different feeders.
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Distribution Feeders
A Spot Network Arrangement
In case of a primary feeder fault, the fault is isolated from the system
by automatic tripping of the primary feeder circuit breaker and all the
network protectors associated with the faulted circuit. Maintenance switching of primary feeders can be done without
customer interruption or involvement.
Spot networks are common in downtown, high density areasand are
being applied frequently in outlying areas for large commercial
serviceswhere the supply feeders can be made available. This system also represents the most compact and reliable arrangement
of components and is the most reliable for all classes of loads.
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Distribution Feeders
A Spot Network Arrangement
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Factors Effecting Primary Feeder
Rating
The nature of the load connected.
The load density of the area served.
The growth rate of the load.
The need for providing spare capacity for emergencyoperation.
The type and cost of circuit construction employed.
The design and capacity of the substation involved.
The type of regulating equipment used.The quality of the service required.
The continuity of the service required.
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Factors Effecting Feeder Routing
Future load growth.
Load density.
Physical barriers.
Voltage drops.
development patterns.
Feeder configuration.
Total cost.
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Factors Effecting Number of Feeders
Conductor size.
Voltage drop.
Substation capacity.
Primary voltage levels.
Load density.
Feeder length.
Feeder limitations.
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Factors Effecting Conductor Size
Selection
Power losses.
load growth rate.
load forecast.
voltage drops.
Transformer rating.
Conductor rating.
Total cost.
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Unit Substations
Unite substations are small, self-contained, metal-clad units,usually installed in residential areas where large sites are
unobtainable.
They are usually well landscaped, and their incoming and
outgoing feeders are placed underground.Primary feeders from one substation extend to meet those
from other, adjacent unit substations, this is just to make sure
a continuous supply in case of a unit substation out of service.
Each substation, therefore, must be designed with sparecapacity to enable this transfer of load to made during
contingency conditions.
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Unit Substations
A Unite substation consists of the following sections.
1. Primary section: This provides for the connection of one or
incoming high voltage circuits, each of which may or not be
provided with a switching device or a switching and interruptingdevice.
2. Transformer section: This includes one or more Transformers with
or without automatic load-tap-changing equivalent.
3.Secondary section:This provides for the connection of one or more
secondary feeders, each of which is provided with a switching and
interrupting device.
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Unit Substations
Different Type of Unite substations.
Radial:One primary feeder to a single step-down Transformer with a
secondary section for connection of one or more outgoing radial
feeders. Secondary selective:Two step-down Transformers, each connected to
a separate primary source. The secondary of each transformer is
connected to a separate but through a suitable switching and protective
device.
Primary selective and primary loop:Each step-down transformerconnected to two separate primary sources through switching
equivalent to provide a normal and alternate source. Upon failure of
the normal source, the Transformer is switched to the alternate source.
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Unit Substations
Different Type of Unite substations. Secondary spot network:
Two step-down Transformers, each connected to a separate primary
source.
The secondary side of each transformer is connected to a common bus
through a network protector that is equipped with relays to trip the
protector on reverse power flow to the Transformer and phase sequence at
the Transformer secondary.
The bus has provisions for one or more secondary radial feeders.
Distribution network:
A single step down transformer having its secondary side connectedto a bus through a network protector, which is equipped with relays
to trip it on reverse power flow and reclose upon restoration of the
correct voltage, phase angle, and phase sequence at the Transformer
secondary.
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Unit Substations
Different Type of Unite substations.
Duplex:
Two step down Transformers, each connected to a separate primary
source. The secondary side of each Transformer is connected to a radial feeder
These feeders are joined on the feeder side of the power circuit breaker by
a normally open tie circuit breaker.
This type is used primarily on electric utility primary Distribution
Systems.
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Unit Substations
Advantages Unite substations.
Unit substation has been widely accepted for industrial power
distribution mainly because the engineering of the components is
coordinated by the manufacturer. The cost of field labor, and installation time are greatly reduced, and it
is safer to operate.
The operating costs are reduced due to the reduced power losses from
shorter secondary feeders.
Besides, a unit substation is very flexible and easy to expand. Unit substations are available for either indoor or outdoor locations.
Primary unit substations may be located outdoors, particularly when
the primary supply is above 34.5 kV.
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