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Distribution System Introduction: Distribution system discuss how the power that is generated by the power stations are supplied to the consumers. i.e. industrial purpose or home use etc. Distribution system includes the main distribution substation(132/66KV) and sub distribution substations (large66/33KV small (33/11kV) and distribution system directly to consumers (11/0.4KV) The figure below gives an idea of the distribution system
Types of Distribution system according to voltage range According to voltage levels distribution system are classified into two. They are:- 1) Primary Distribution system Primary distribution the voltages range 2.4KV to 69KV. Actually primary distribution includes the distribution from high voltage substation (132/66KV) to (66/33KV) to (33/11KV) distribution substation 2) Secondary Distribution system Secondary distribution is low voltage distribution that is three phase 440V and single phase two wire 240V or may be 220V. In secondary distribution voltage is stepped down from 11KV to 440V(three phase) and 220(single phase)
Primary Distribution system Primary distribution includes the following topic:- 1) Substation Arrangement 2) Types of Systems 3) Primary Distribution Feeders Arrangement 4) Primary Network So to built a primary distribution network we have to fulfill those criteria Substations Arrangement:-A simple substation arrangement consists of one incoming line and one transformer. More complicated substation arrangements result when there are two or more incoming lines, two or more power transformers, or a complex bus network. Some substation arrangement are shown by the figure.
Layout of substation arrangement Above the substations we select (G) type arrangement because of the following advantages:- -It has two incoming line capacity with breakers placed in line -It has a re-closer -Transformer protection capability is high because of an extra breakers in front of it - Also power by pass capability in case of transformer failure.
Substation Bus Arrangements A bus is a junction of two or more incoming and outgoing circuits. The most common bus arrangement consists of one source or supply circuit and one or more feeder circuits. It is the most important part of the distribution as well as the total power system. So we have carefully take the decision which bus bar is suitable for our system. The bus bar arrangements are classified into the following categories – (a) Single bus bar scheme (b) Single sectionalized bus bar scheme (c) Double bus bar single breaker scheme (d) Double bus bar double breaker scheme (e) Main and transfer bus scheme etc.
Single bus bar scheme Advantages: 1)Simple construction and low cost 2)Less maintenance and simple operation Disadvantages: If a fault occurs on the bus bar there is complete shutdown in the system resulting a large fault currents Single sectionalized bus bar scheme Advantages: 1)Fault on bus bar will not cause complete Shutdown. 2)Repair and maintenance can be done without shutdown the system. Disadvantages: Cost is high than that of single bus bar system
Double bus bar single breaker scheme Advantages: 1)Flexibility of operations is increased 2)Non-synchronized systems can be used, to supply outgoing circuits 3)Fault on bus bar will not cause complete Shutdown 4)Repair and maintenance can be done without shutdown the system Disadvantages: 1) Cost of equipment is more 2) Operation is more complex Double bus bar double breaker scheme Advantages: 1)More Flexibility and more protection is used 2)Because of Two breakers no hampering in system continuity in case of repair or maintenance 3)Fault on bus bar will not cause complete Shutdown Disadvantages: 1) Cost of equipment is more 2) Operation is more complex
Decision According to advantages and disadvantages of above bus bar we have to select one bus bar arrangement for our desire system. So we chose double bus bar single breaker system because of the following Reasons:- • It is more reliable than single bus bar and operation is less complicated than double bus bar double breaker system. • We also take cost into considerations
Types of Primary Feeder System 1)Tie Feeder: The main function of a tie feeder is to connect supply source to load. It may connect two substation buses in parallel to provide service continuity for the load supplied from each bus. 2) Loop Feeder: A loop feeder has its ends connected to a source (usually a single source), but its main function is to supply two or more load points in between. Each load point can be supplied from either direction; so it is possible to remove any section of the loop from service without causing an outage at other load points.
3) Radial Feeder : A radial feeder connects between a source and a load point, and it may supply one or more additional load points between the two. Each load point can be supplied from one direction only. 4)Parallel Feeder: Parallel feeders connect the source and a load or load center and provide the capability of supplying power to the load through one or any number of the parallel feeders. Parallel feeders provide for maintenance of feeders without interrupting service to load.
Decision From above four types of feeders we chose the parallel feeder. Because of the following reasons:- • More reliable and flexible • It provide for maintenance of feeders without interrupting service • High protection capability
A feeder includes a main or main feeder which usually is a three phase four wire circuit and branches, which usually are single or three phase circuits tapped off the main. A given feeder is sectionalized by re-closing devices at various locations in such a manner as to remove as little as possible of the faulted circuit so as to hinder services to as few consumers as possible. This can be achieved through the co-ordination of the operation of all the fuses and re-closers. There are various and yet interrelated factors affecting the selection of a primary feeder rating. Examples are: 1)The nature of the load connected 2)The load density of the area served 3)The growth rate of the load 4)Providing spare capacity for emergency operations 5)The type of regulating equipment used 6)The quality of service required 7)The continuity of service required.
Types of Primary Feeder Distribution Now we discuss about the types that are used in primary distribution feeder. There are the five types of primary distribution feeder arrangement which is given below:- 1) Radial type primary feeder 2) Radial Type primary feeder with tie and sectionalizing switches 3) Radial type primary feeder with express feeder and back feed 4) Loop Primary-Radial Distribution System 5) Primary Selective Distribution System
Decision Although the cost is a little high because of primary cables and switchgear but because of the following advantages we chose the primary selective distribution feeder arrangement. 1) Protection against losses 2) Each section can be operated separately 3) Provides normal and alternate sources operation 4) No discontinuity in case of maintenance
Selection of Primary Feeder Voltage Level When we are about to select the voltage level we have to consider the following things:- 1. Primary feeder length 2. Primary feeder loading 3. Rating of distribution substations 4. Number of transmission lines 5. Number of customers 6. System maintenance practices etc
Secondary Distribution System The voltage levels for a particular secondary system are determined by the loads to be served. The utilization voltages are generally in the range of 120 to 600 V. Secondary Distribution Design Considerations 1)Choice of voltage:For secondary distribution system the voltage chosen is the standard voltage used in the country at the consumer’s level, i.e. 440/415V three phase for motor loads and 240V/220V single phase for lighting loads etc 2) Conductor size:The conductor size is chosen mainly on the basis of the permissible voltage drop in the distribution section under consideration. The secondary distributors are designed for 6% voltage drop from the transformer to the last consumer in the system.
Types of Systems 1) Conventional Simple-Radial Distribution System Advantages: Operation and expansion is simple and Reliability is high Disadvantages: In case of maintenance service interrupted 2) Expanded Radial Distribution System Advantages: a) Operation and expansion is simple and Reliability is high b) Capability to work with lager loads Disadvantages: a) In case of maintenance service interrupted
3) Secondary Selective-Radial Distribution System Advantages: a) Operation and expansion is simple and Reliability is high b) Capability to work with lager loads c) In case of maintenance no service interrupt Disadvantages: a) Cost is high 4) Secondary Network Distribution System Advantages: a) Operation and expansion is simple and Reliability is high b) Capability to work with lager loads c) In case of maintenance no service interrupt d) Network Protection is used Disadvantages: a) Cost is high
Final Decision For Primary Distribution • Substation Arrangement ( Double breaker with automatic re-closer) • Substation Bus arrangement (Double bus bar with double breaker) • Types of Primary feeder Systems ( Parallel feeders)
Primary Distribution Feeders Arrangement For Secondary Distribution System • Types of Systems (Expanded Radial Distribution System)
