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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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A Typical distribution substation arrangement.

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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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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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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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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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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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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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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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(a) (b)

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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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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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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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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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Looped Primary System

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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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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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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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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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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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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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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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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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