radial, loop double and grid form systems

8/6/2019 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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2


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In this system all the sections/branches of afeeder are fed from a single source in afixed direction

3

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

1

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

radial, loop double and grid form systems

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