radial, loop double and grid form systems
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Primary distributionsystem is that partof the electricdistribution system
between distributionsubstation anddistributiontransformers.
It is madeup of circuits calledprimary feeders ordistribution feeders.
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In this system all the sections/branches of afeeder are fed from a single source in afixed direction
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Load
Load
Load
Generator
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In this it is shownthat there are twoprimary feeders
from which furthersubfeeders supply tothere respectiveloads. If any one ofthe feeder becomes
faulty then supplythrough that feederhas to cut so noreliability
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Merits Reasons
Simple in design, construction,operation and maintenance
Suitable size of conductor.Very simple protection schemes.Maintenance easy.Easy fault current calculations.
cheapest system No extra length of line andswitching arrangements required.Less capital cost as different sizesof conductors can be used.As no alternative arrangementsare necessary so shortest route
can be adopted hence cost savingAs used in rural areas so largespans can be used which furtherreduces cost
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Demerits ReasonsVery poor service reliability In case of fault on a feeder the
supply to consumers from thosefeeders is cutoff and restored onfault removal.
Excessive voltage drop and linelosses
Due to passage of total currentthrough the common main sectionof the feeder, voltage drop(IR)and line losses will be more ascompared to the system whichfacilitates distribution of loadcurrents into two or more paths
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It is considered more suitable for the areaswith low density of load involving longdistances and requiring low degree ofavailability.
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5
132KV
4
Grid Station
11KV
3
2
Distribution
Transformer
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5
3
11KV
2
Distribution
Substation
0.4KV
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What is closed loop system? If a HT line emanating from a Grid station and
covering the load area in shape of a loop,
finishes at the same Grid station returning to
same breaker or another independent breaker it
is called closed loop and the system is calledclosed loop system.
In this system each transformer has two
sectinalizers, one on either side of the
transformers tap-off point.
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Typical WAPDA closed loop system
Bus bar
Station circuit breaker
Sectionalizing
switch closed
transformer
LoadSectionalizing switch
closed
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Merits of closed loop system1. This system is more reliable than radial system.
In case of fault, breaker at grid station opens.
After tracing the fault the faulty section is isolated, and supply to
remaining section is restored.
The consumers dont have to wait very long till the rectification of
the fault as in the case of Radial system. In case of two simultaneously faults, the consumers have to wait
much till the rectification of the fault of at least one section. But
possibility of two simultaneously faults is less.
It is usually preferred that there must be two breakers instead of
one.
2. Less voltage drop, less line losses As compared to radial system the voltage drop and line losses is less.
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Application
It is usually recommended for HIGH density areas wheredistances are small. preferably for urban areas.
This system can be equally useful for rural areas whereeconomy so permits. But to ensure effectiveness, the loop
should be of small size.
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OPEN LOOP SYSTEM
In a closed-loop system with two breakers
(one at each end), if one of the two
sectionalizers is kept normally open, thesystem is named as open-loop system.
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ADVANTAGES
Economical
Availability of service is improved Locating fault is easy
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Double Supply System
Additional radial feeder is provided side by
side to feed the same consumer.
Using automatic change over switches.
Duplicate supply system or parallel
feeders.
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Substation Bus
CircuitBreakerSectionalizing
switch
FUSE
TRANSFORMERS
LOAD
LOAD
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Merits and DemeritsAdvantages Disadvantages
Higher service reliability. Very Expensive Extra feeders are installed which
doubles the cost. Cost of sectionalizers. Additional length of line needed
to connect the feeder to thetransformer.
Less line losses. Reduced Safety
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GRID FORM OF H.V SYSTEM
In this system all the distribution transformers of a load area, areinterconnected providing normally two or more alternatives. while
the whole system is fed from two or more bulk supply sources.
But availability can not be improved through
manual operation of sectionalizer to isolate faulty section during
emergency as it will take quite a considerable time to identify thefaulty section and to determine the better alternative.
As manual operation may result in wrong
operation due to complexity of the system so to have a real benefit
of this system, automatic switching arrangements with sophisticated
protection schemes should be provided including establishment of aremote control center.
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C.BC.BC.B C.B
C.BC.BC.BC.BC.BC.B C.BC.B
C.B C.B C.B C.B
C.B C.B C.B C.B C.B C.B C.B C.B C.B C.B C.B
C.B C.B C.B C.BC.B C.BC.B C.B C.B C.B C.B
C.BC.B
STEP DOWN
TRANSFORMER
C.B C.B
11KV BUS BAR
132KV BUS BAR
DISTRIBUTION
TRANSFORMER
C.B C.B C.B C.B
400 VOLT
400 VOLT
SINGLE LINE DIAGRAM OG GRID FORM OF H.V-DISTRIBUTION
SYSTEM
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Merits Most reliable because normally no possibility of failure of
supply and availability is almost 100%.
In case of two simultaneous faults, service to the
consumers will not be affected at all, or effect will be
limited.
Demerits It is most costly system.
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Applications The system being the most reliable, and at the same time
the most costly, is recommended for the areas of highest
priority of supply with very high load density.
If the economy for a county is not a problem, this system
is considered the most suitable one for urban areas,
specially the big cities