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UnitUnit--IIIIDistribution FeedersDistribution Feeders
by
N.Ramchander,Asst Professor,EEED-BVRIT.
by
N.Ramchander,Asst Professor,EEED-BVRIT.
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CONTENTS
CONTENTS
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Introduction Distribution System-Classification Connection Schemes of primary feeders
Primary Distribution feeder rating Primary Distribution voltage levels
Primary feeder loading Secondary Distribution Systems
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INTRODUCTION
INTRODUCTION
Classical Structure of Power System
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Distribution System-Classification
1. Primary distribution, which carries the load at higher thanutilization voltages from the substation (or other source) to the poin
where the voltage is stepped down to the value at which the energy
is utilized by the consumer.
Primary distribution systems include three basic types:
1. Radial systems, including duplicate and throwover systems
2. Loop systems, including both open and closed loops
3. Primary network systems
2. Secondary distribution, which includes that part of the systemoperating at utilization voltages, up to the meter at the consumers
premises.
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Secondary distribution systems operate at relatively low utilizationvoltages and, like primary systems, involve considerations of servicereliability and voltage regulation. The secondary system may be of fourgeneral types:
1. An individual transformer for each consumer; i.e., a single service
from each transformer.
2. A common secondary main associated with one transformer from whicha group of consumers is supplied.
3. A continuous secondary main associated with two or moretransformers, connected to the same primary feeder, from which a groupof consumers is supplied. This is sometimes known as bankingof
transformer secondaries.
4. A continuous secondary main or grid fed by a number of transformers,connected to two or more primary feeders, from which a
large group of consumers is supplied. This is known as a low-voltageorsecondarynetwork.
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Factors Affecting Selection of feederFactors Affecting Selection of feeder
The nature of the load connected
The load density of the area served
The growth rate of the load
The type and cost of circuit employed The design and capacity of the substation
involved
The quality of the service required The continuity of the service required
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Radial Systems
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Advantages:
Simplest and lower cost
Simple in planning, design, andoperation.
Most widely used from ofdistribution system(99%-inINDIA)
Disadvantages:
it is less reliable than other two
If any branch along this pathfails, a loss of power delivery
results.
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Loop Type Feeders
which essentially provide a two-way primaryfeed for critical consumers.
the supply from one direction fail, the entireload of the feeder may be carried from the
other end this type of system may be operated with the
loop normally open or with the loop normallyclosed.
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Open Loop System
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In the event of a fault, the section of the primary
on which the fault occurs can be disconnected at
both its ends and service re-established to the
unfaulted portions by closing the loop at the pointwhere it is normally left open, and reclosing the
breaker at the substation (or supply source) on the
other, unfaulted portion of the feeder.
The disconnecting devices between sections are
manually operated and may be relativelyinexpensive fuses, cutouts, or switches.
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Closed LoopSystem
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This type of systems are used, where a greater degree
of reliability is desired. Here, the disconnecting devices are usually the more
expensive circuit breakers.
The breakers are actuated by relays, which operate
to open only the circuit breakers on each end of the
faulted section, leaving the remaining portion of theentire feeder energized.
when the fault is located, the disconnecting devices
on both ends of the faulted section may be openedand the unfaulted sections reenergized by closing the
proper circuit breakers.
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Primary Network Systems
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Primary network is a system of interconnected
feeders supplied by a number of substations. The primary network system supplies a load fromseveral directions.
In general losses in a primary network are lowerthan those in a comparable radial system due toload division.
The reliability and the quality of service of theprimary network arrangement is much higher than
radial and loop arrangements. However, it is more difficult to design and
operate than the radial or loop type.
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Tie LinesTie Lines
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A tie line is a line that connects two supplysystems to provide emergency service to onesystem from another.
Usually, a tie line provides service for area loadsits route as well as providing for emergency
service to adjacent areas or substations.
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Low Voltage DistributionLow Voltage DistributionSystem(LVDS)System(LVDS)
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High Voltage DistributionHigh Voltage DistributionSystem(HVDS)System(HVDS)
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Factors Affecting the FeederFactors Affecting the Feeder
voltage levelvoltage level The feeder voltage level is important factor
affecting the system design, cost and operation. Some of the design and operational aspects of
the primary voltage levels depend upon the: Feeder length
Feeder loading
No of distribution S/Ss and its rating
No of Primary lines
No of customers effected by a specific failure
System maintenance practice
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Voltage Square rule:Voltage Square rule:
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For a given percentage of voltage drop, the length of feeder and
loading are direct functions of the feeder voltage levels.
Ex:- if feeder voltage level is doubled for the same permissibledrop, the power capacity of the feeder is four times and length
of the feeder can be increased four times.
thereforevoltage square factor = (VLN
new) length ratio X load ratio
(VLNold )
length ratio = new length/old length
load ratio = new feeder loading/old feeder loading
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g{t~ Y t|xg{t~ Y t|x