Distribution System od power system engi

PS

1. Basic power distribution system

2. Distribution system: That part of power system which distributes electric power for local use is known as distribution system. In general, the distribution system is the electrical system between the sub-station fed by the transmission system and the consumers meters. It generally consists of feeders, distributors and the service mains.

3. Components of distribution system:

4. 1.Feeder: A feeder is a conductor which connects the sub-station (or localised generating station) to the area where power is to be distributed. Generally, no tappings are taken from the feeder so that current in it remains the same throughout. The main consideration in the design of a feeder is the current carrying capacity.

5. 2.Distributor: A distributor is a conductor from which tappings are taken for supply to the consumers. In Fig. , A B , BC , CD and DA are the distributors. The current through a distributor is not constant because tappings are taken at various places along its length.

6. 3.Service mains: A service mains is generally a small cable which connects the distributor to the consumers’ terminals.

7. Classification of distribution system: A distribution system may be classified according to ; 1. Nature of current: According to nature of current, distribution system may be classified as, (A) d.c . distribution system ( B ) a.c . distribution system.

8. 2. Type of construction: According to type of construction, distribution system may be classified as, ( a ) overhead sys- tem ( b ) underground system

9. 3. Scheme of connection: According to scheme of connec - tion , the distribution system may be classified as, ( a ) radial system ( b ) ring main system ( c ) inter-connected system.

10. A.C Distribution system: Now-a-days electrical energy is generated, transmitted and distributed in the form of alternating current. A.C voltage can be step up or step down easily with the help of transformer. The a.c . distribution system is classified into ( i ) primary distribution system and ( ii ) secondary distribution system.

11. 1.Primary distribution system (3 phase 3 wire system): When the distribution is some what higher then utilization level then the system is called primary distribution. The most commonly used primary distribution voltages are 11 kV, 6·6 kV and 3·3 kV.

12. Primary distribution system: Due to economic considerations, primary distribution is carried out by 3- phase, 3-wire system .

13. Electric power from the generating station is transmitted at high voltage to the substation located in or near the city. At this substation, voltage is stepped down to 11 kV with the help of step-down transformer. Power is supplied to various substations for distribution or to big consumers at this voltage. This forms the high voltage distribution or primary distribution.

14. 2.Secondary distribution system(3 phase 4 wire system): When the distribution voltage is at utilization level then it is called secondary distribution. The secondary distribution employs 400/230 V, 3-phase, 4-wire system.

15. Secondary distribution system:

16. The primary distribution circuit delivers power to various substations, called distribution sub- stations. At each distribution substation, the voltage is stepped down to 400 V and power is delivered by 3-phase,4-wire a.c . system. The voltage between any two phases is 400 V and between any phase and neutral is 230V.

17. D.C Distribution system: For certain applications, d.c . supply is absolutely necessary. For this pur - pose, a.c . power is converted into d.c . power at the substation by using converting machinery e.g. rectifiers. The d.c . supply from the substa - tion may be obtained in the form of ( i ) 2-wire or ( ii ) 3-wire for distribution.

18. 1.2-wire d.c . System: As the name implies, this system of distribution consists of two wires. One is the outgoing or positive wire and the other is the return or negative wire. The loads such as lamps, motors etc. are connected in parallel between the two wires.

19. 2-wire d.c system: This system is never used for transmission purposes due to low efficiency but may be employed for distribution of d.c . power.

20. 2.3-wire d.c system: It consists of two outers and a middle or neutral wire which is earthed at the substation. The voltage between the outers is twice the voltage between either outer and neutral wire. V between any outer and the neutral and 2V between the outers. Loads requiring high voltage ( e.g ., motors) are connected across the outers.

21. 3-wire d.c system: whereas lamps and heating circuits requiring less voltage are connected between either outer and the neutral.

22. Connection schemes of distribution system: According to method of connections distribution system is classified as : Radial system Ring main system Interconnected system

23. (a) Radial system:

24. In this system separate feeders radiates from the substation to each area and it feeds the distributor at one end only. This is the simplest design of distribution system and has the lowest initial cost. This system is used where power transferred is low and the distance between substation and load is low.

25. LESS RELIABILITY: As the consumers are fed by single radial feeder only , therefore fault on the feeder or distributor causes complete shut down of the consumers who are on other side of fault. Owing to above disadvantage this system is used only for short distances.

26. (b) Ring main system: This consist of interconnection of primary of different distribution transformers which form a loop through the area to be supplied. The feeder is closed on itself. The feeder SMOQS forms a complete ring.

27. Ring main system:

28. The advantages of such arrangement is that: It offers a greater reliability of supply. In the event of a fault on any section of the feeder, say at “F” the supply to all consumers can continue to be available by isolating the faulty section between ‘S’ & ‘M’. There are less voltage fluctuations at consumer’s terminal.

29. (c) Interconnected system: When the feeder ring is energized by two or more than two substations then it is called as interconnected system. Fig. Shows the single line diagram of interconnected system where the closed feeder ring DPCOBRAQD is supplied by substation s1 & s2 at D & C.

30. Interconnected system:

31. It has following advantages: It offers greater reliability of supply. It reduces reserve power capacity & increases efficiency.

32. Requirements of good distribution system: There are certain requirements of distribution system to provide the continuous electrical energy to consumer with minimum fluctuations. These requirements are as follows:

33. LOW VOLTAGE FLUCTUATIONS: The variations of consumer’s terminal voltage should be a low as possible. This variation should be within +/-6 of the rated voltage at consumer’s terminal. If the voltage variations goes beyond this limit, certain equipments may damage.

34. 2. AVAILABILITY OF POWER ON DEMAND: An electrical energy cannot be stored, therefore it should be supplied whenever there is required by the consumer. To maintain the continuity and to fulfil the requirement of electrical power, previous load curve should be studied by supply authorities and should predict the future demand.

35. 3. RELIABILITY: Reliability is very important factor; because everywhere electrical energy is used such as industries, commercial complexes; irrigation purpose, domestic appliances etc. Therefore supply authority should provide electrical power with good quality and continuously.

36. 4. STABILITY: Fault on the neighbouring system should not affect the stability of distribution system. Stability and reliability can be improved by interconnected system, automatic switch gear system etc.

37. Design considerations: Design of feeders and distributors requires careful consideration to fulfil above requirements. Design consideration for feeder: Current carrying capacity: While designing the feeder, current carrying capacity is the most important factor.

38. The current carrying capacity indicates the cross sectional area required by the conductor. 2. Type of conductor: The type of material depends on the service provided. Generally in urban & suburban areas, A.C.S.R conductor is used as a feeder.

39. 3. Distance : While designing of feeder distance plays an important role because it suggests the total voltage drop in the feeder by which voltage drop in the feeder can be compensated by means of voltage regulating equipment at substation.

40. 4. Cost : Cost plays an important role to choose the type of construction i.e. Overhead or underground. Generally in the city areas where safety is important criteria, underground cables are used as feeder. In rural areas as the cost is main criteria, Therefore overhead conductor is used as feeder.