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Al i Ahmed Khali l

20452928Assignment # 8

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1. Explain briefly the different secondary distribution system configurations. Your answer shouldaddress the following:

a. The single line diagram of each configuration.

b. The main advantage of each configuration.

c. The disadvantages of each configuration.d. The degree of reliability of each configuration.

Separate Service System

In this system each customer is served by a separate transformer and secondary connection. This

configuration is rarely applied, as it is used only for light load rural areas.

Radial System

Single Line Diagram

Advantage

Radial systems are simple to operate and they are the least expensive systems.

Disadvantage & Degree of Reliability

The main disadvantage of this configuration is that it is not that reliable. In case of fault

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Secondary-Bank System

Single Line Diagram

Advantage

1. Improving the system voltage regulation, reliability and supply continuity.

2. Providing parallel feeding paths for motor-starting currents. Thus, reducing the system voltage

drops and light flickers caused by motor starting.3. Flexibility for meeting the load growth by sharing loads between transformers without the

need for increasing certain transformers capacity.Disadvantage

1. To identify the blown fuse careful requirements are necessary.2. Difficult secondary fuse coordination.3. Difficulty in performing transformer load management. This is due to the fact that, the design

requires equal loading for each transformer in the secondary-banking configuration.Degree of Reliability

It is more reliable than the radial system.

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Spot Network

Single Line Diagram

Advantage

This configuration allows power service even during multiple faults on distribution transformers.

Disadvantage

In this configuration there is parallel operation of transformers, so there is a possibility of reverse

power flow. To prevent the reverse flow, protector devices with sensitive reverse power relays areimplemented. This increases the additional cost

Degree of Reliability

This configuration is fed from more than one feeder to provide the highest reliability in industrial,high-loading density areas and large commercial applications.

Secondary-Network System

Advantage

1. It has the most reliability of other secondary distribution system configurations.

2. The loss of any transformer or feeder will not affect customer power continuity.Disadvantage

1. It has the highest cost.

2. Its need far complex protection.Degree of Reliability

The degree of reliability is highest in this configuration.

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2. Discuss the main components of the secondary distribution system. Your answer should considerthe following issues:

a. The secondary system voltage level.

b. The design consideration of the secondary system.c. The degree of reliability of each component in the secondary system.

Secondary System Voltage Level

1. For residential loads: single-phase three-wires are used with a voltage of 120/240 V. The

120 V is used for the lighting and the 240 V for the large household appliances.

2. For residential and commercial loads with high load densities: a grid secondarynetwork uses single-phase three-wire 120/208Y V. The 120 V is used for lighting and the208 V for the large household appliances.

3. For spot networks in high-density high-rise residential buildings, for commercial

loads and for industrial loads: the voltages used are 277/480Y V. The distributionnetwork is made of four-wire three-phase system, where the 277 V voltage level is used

for the fluorescent lightning and the single-phase loads.

Design Consideration

The secondary system is made of single-phase for residential applications and three-phase forcommercial and industrial applications with high load densities. The secondary distribution systemcan be fed from [1]:

1. Small number of large size distribution transformers.

2. Large number of small size distribution transformers.

The main components of the secondary distribution system are

Secondary Mains.

Customer Service.

Customer Meters.Limiters.Network Protector (NP).

High-voltage switch.

Secondary Mains

All secondary mains wires are three-phase four-wire wye-connected conductors with solidly

grounded neutrals. These secondary mains are either overhead or underground

Customers Service

The customer service connection to the secondary distribution circuit depends mainly on thesecondary system type (overhead or underground), the secondary system load density, and the utilityregulations and requirements (over current protection).

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Customer Meters

Customer meters are used to measure the customer energy consumption to determine the revenue forthe electric utilities. For residential customers, they are billed for kWh consumed. For commercial

and industrial applications, they are billed for their power consumption, demand charges and powerfactor penalties.

Limiters

The limiter is a high capacity fuse with a restricted copper section. It is installed in each secondary

main phase conductor at each junction point. The choice of implementing a limiter is based on:

1- The minimum system service interruption.2- The limiters cost has to be less than the cost of cable damage.

Network Protector (NP)

Network protectors are used to connect distribution transformers to the secondary-network. NPs

consist of an air CB type using a closing and tripping mechanism. This mechanism is controlled by aphasing relay, a network master and backup fuses.

High Voltage Switch

The high-voltage switch is a three-position switch located at the high-voltage side of the networktransformer as shown in Fig. 4. Position (1) is for grounding the primary feeder; position (2) is for

normal operation and position (3) is for disconnecting the network transformer.

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Degree of Reliability of each Component

Table 1 shows the necessary operation of protective devices for different fault conditions (locations).