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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 Class 8 and 10 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 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. 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 Generator Single Line Representation 10 to 30 KV of Step-up T/F Power System 132 KV, 220 KV, 400 KV. Transmission EHV Step down T/F 66 KV, 33 KV OR 11 KV Distribution HV Receiving Station Step Down T/F To large Industries at 11 kv or 33 kv or 66 kv consumer of HV level 440V (Utilization) Consumers of LV level 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. The following connection schemes of distribution system are generally employed: (1) Radial system (2) Ring main system (3) Interconnected system 1.Radial Distribution system - only one/single path is connected between each Distribution and substation is called radial Distribution system. - Fault occurs either on feeder or a distributor, all the consumers connected to that distributor will get affected. - In India, 99% of distribution of power is by radial distribution system only. Advantages- - Its initial cost is minimum - Simple in planning, design and operation. - Useful when the generation is at low voltage. - Station is located at the center of the load Disadvantages- - • Distributor nearer to the feeding end is heavily loaded. - The consumers at the far end of the feeder would be subjected to series voltage fluctuations with the variations in load. (2 ) Ring main system - Feeder covers the whole area of supply in the ring fashion and finally terminates at the substation from where it is started. - Closed loop form and looks like a ring. Advantages- - Less conductor material is required as each part of the ring carries less current than in the radial system. - • Less voltage fluctuations. Disadvantage- - It is difficult to design when compared to the designing of a radial system. Interconnected system-In this system, the feeder is energised by two or more than two generating station or sub station. Advantages- -There is more flexibility. -Reliability is more. If fault occur on one section, supply can be continued by other route. -Good voltage regulation is achieved. -Size of substation is less compared to the radial system. 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 coordination 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. 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 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 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. 1) Conventional SimpleRadial 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 Single line diagram 13.8 KV 13.8 KV 13,800/120V 50/51 13,800/120V 51N 3-100/5 50/51 51N 2 1 3-100/5 1 2 T T 500MVA 13.8KV 1200A 500MVA 13.8KV 1200A 1000 KVA 13,800 - 480 Y/277V 3-1500/5 A WHM 1000 KVA 13,800 - 480 Y/277V 3-1500/5 V T A WHM V T 480/120V 480/120V 1600A 1600A 1200A 200A 100A 400A M 150 KVA 480 - 208Y/120Y 600A 600A 75HP Feeder 1 Feeder 2 200A M Feeder 3 150HP 112.5 KVA 480 - 208Y/120Y It requires much lesser space and hence leaves room for designer for providing other useful information in the drawing It is very versatile and comprehensive because it can depict very simple DC circuits, or very complicated three-phase system Simple, hence requires much lesser time for reader to understand the basic system design One-line diagram 3-line diagram Provide a basic roadmap to the interconnections of the electrical system, and serve as a building block from which all types of system analyses are based. Prepared as a result of further working on the basis of single line diagrams as they provide details for electrical wiring connections. Part of initial plant electrical design. Is usually a part of the initial tender document. Part of detailed design document. Is prepared usually after the tendering stage i.e. before manufacture. Mainly used for working out panel schedules, load schedules, fault analysis, protection system deign Used for control designing circuit diagrams, control circuits, phase sequencing, differential relay settings, metering transformer connections etc. Simplified notation for representing a three-phase power system. Since the loads on the three phases are identical , any one phase can be used for representing either of the phases Here all three conductors of the three phase system are shown individually. Details of the power and the control circuits are also shown as per actual field connections. 31 ELECTRICAL POWER MEASUREMENTS 32 Wattmeter A AC Source I(t) V(t) + W One - phase two - wire Load V . + Single Wattmeter Method 33 34 Single-Phase Two-Wire System The voltage and current detected by the METER are the voltage and current applied directly to the Load. The indication on the Meter is the POWER being dissipated by the load. Wattmeter 1 A L1 AC Source + W I(t) V V(t) One - phase three - wire Load . N + V(t) I(t) L2 V W A + Wattmeter 2 Two Wattmeter Method PT = W1 + W2 35 36 Single-Phase Three-Wire System (Split Phase) The voltage and current detected by the METERS are the voltage and current applied directly to the Load. The indication on EACH METER is the power being delivered by the LINE to which the meter is connected. The total power dissipated by the load is the ALGEBRAIC SUM of the two indications. Blondel Theorem Blondel theory states that total power is measured with ONE LESS wattmeter than the number of WIRES. 1-P 2-W 1 Wattmeter 1-P 3-W 3-P 3-W 2 Wattmeters 2 Wattmeters 3-P 4-W 3 Wattmeters 37 vcn vca 120o 120o vbc n van 120o vab vbn 38 a b vab van c vbn vbc vca Four - Wire Three - Phase System vcn n Vl-n = 120 / 277 Volts Vl-l = 3 * Vl-n Vl-l = 208 / 480 Volts 39 A a AC Source b Wa + V A Wb + V van c vbn A Three Wattmeter Method + V vcn n Wc Four - Wire Three - Phase Load + PT = W a + W b + W c 40 41 Three-Phase Four-Wire System The three meters use the FOURTH wire as the common voltage REFERENCE. Each meter indicates the PHASE power. The TOTAL POWER for the three phases is the ALGEBRAIC SUM of the three meters. In essence, each meter measures a SINGLE PHASE of the three phase system. a vab vca Three - Wire Three - Phase System b vcb c 42 43 Remember Blondel’s Theory . . . total power is measured with ONE LESS wattmeter than the number of WIRES. 44 Three - Phase Three - Wire System With Two Meters A a Wa + V V AC Source vac vab b A + Wb Three - Wire Three - Phase Load + + vcb c V A Two Wattmeter Method Wc + + PT = W a + W b Three-Phase Three-Wire System The wattmeters used for this connection each measure the PHASE CURRENTS The measured voltages are the LINE-TO-LINE values, NOT Phase Voltage. Thus the indications on each of the meters IS NOT the power delivered by the PHASE of the measured current. This configuration is a very NON-INTUITIVE connection! 45