LDPP 01 - E

Power Distribution Power Distribution Systems and Its Systems and Its Characteristics IICharacteristics II

Distribution FeedersDistribution FeedersThere are various feeder designs that can improve customer 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 large reserve capacity are needed.

System investment is usually the lowest of all circuit arrangements.

For a well-planned and well-maintained system, the service reliability of the radial system is usually high.

Distribution FeedersDistribution 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.

Distribution FeedersDistribution Feeders A typical Simple radial system

Distribution FeedersDistribution 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 through radial 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.

Distribution FeedersDistribution Feeders Typical Expanded Radial System

(a) (b) (c)

Distribution FeedersDistribution FeedersSecondary 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 provides flexibility in 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.


(a) (b)


Under normal operation: The tie breaker open and each transformer supplies own load. For a fault occur in a transformer or its primary feeder, or if a

transformer or primary feeder is de-energized for maintenance, the transformer secondary switching device or breaker is to be opened and the tie breaker is to be closed.

The load connected to both buses is supplied by the energized transformer.

Each transformer and its primary feeder must have sufficient capacity to carry the total load.


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 transformer secondary 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 buses in the case of a transformer fault or primary cable fault.

Distribution FeedersDistribution FeedersPrimary 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.

Each primary feeder must have sufficient capacity to carry the maximum load that may be connected at one time.



Under normal operation: The system is operated with the load divided approximately

equally on each primary feeder. When a fault occurs on one primary feeder, there will be an

interruption of power to the load connected to that feeder. The interrupter switch connected to the faulted feeder will be

opened and the respective transformer will be reconnected to the energized feeder.

The two switches associated with one transformer should be mounted in separate individual metal enclosures in order that a de-energized feeder may be safely maintained while the other feeder is energized.


The primary selective system has a higher first cost than 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.

Distribution FeedersDistribution FeedersLooped 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, the

primary 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.


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 transformer location.

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 the transformers supplying their respective loads.


The disadvantage of this arrangement lies in a complex and time-consuming operation in locating a fault on 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 switched instead of medium-voltage circuit breakers.

Distribution FeedersDistribution FeedersSecondary 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 the system 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.


This is an electrically operated circuit breaker provided with relays arranged to trip the breaker on reverse power flow to the transformer and to reclose the breaker when normal voltage conditions return to the primary of the transformer.

Separate low-voltage fuses are usually installed on the load side of the network protector to mitigate damage to the protector and to protect the low-voltage system on the occurrence of high-magnitude fault current.



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.

Distribution FeedersDistribution FeedersA 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 to carry all loads connected to the bus with one transformer out of service.

For larger installations, a system having a primary selective and secondary network arrangement can be an advantage.

Each of the primary feeders must have sufficient capacity to supply the entire 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.


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 areas and are being applied frequently in outlying areas for large commercial services where 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.


Factors Effecting Primary Feeder Factors Effecting Primary Feeder RatingRating

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 emergency operation.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.

Factors Effecting Feeder RoutingFactors Effecting Feeder RoutingFuture load growth.Load density.Physical barriers.Voltage drops.development patterns.Feeder configuration.Total cost.

Factors Effecting Number of FeedersFactors Effecting Number of FeedersConductor size.Voltage drop.Substation capacity.Primary voltage levels.Load density.Feeder length.Feeder limitations.

Factors Effecting Conductor Size Factors Effecting Conductor Size SelectionSelection

Power losses.load growth rate.load forecast.voltage drops.Transformer rating.Conductor rating.Total cost.

Unit SubstationsUnit SubstationsUnite 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 spare capacity to enable this transfer of load to made during contingency conditions.

Unit SubstationsUnit SubstationsA 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 interrupting device.

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.

Unit SubstationsUnit SubstationsDifferent 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 transformer connected 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.


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 connected to 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.


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.

Unit SubstationsUnit SubstationsAdvantages 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.

Documents

LDPP 01 - E