Classification of distribution systems
- 1. CLASSIFICATION OF DISTRIBUTION SYSTEMS Presented by Anumula shanthi Dept of EEE
- 3. DISTRIBUTION SYSTEM: DEFINITION: ✶ 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 substation fed by the distrubution system and the consumers meters. It generally consists of feeders, distributors and the service mains.
- 4. ✶ 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. ✶ II.) DISTRIBUTOR: ✶ A distributor is a conductor from which tappings are taken for supply to the consumers. In Fig.AB, 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. While designing a distributor, voltage drop along its length is the main consideration since the statutory limit of voltage variations is ± 6% of rated value at the consumers’ terminals. (iii) Service mains: A service mains is generally a small cable which connects the distributor to the consumers’ terminals.
- 6. Classification of Distribution Systems: ✶ A distribution system may be classified according to ✶ (i) Nature of current. ✶ (ii) Type of construction. ✶ (iii) Scheme of connection.
- 7. ✶ (i) Nature of current. According to nature of current, distribution system may be classified as ✶ (a) d.c. distribution system (b) a.c. distribution system. Now-a-days, a.c. system is universally adopted for distribution of electric poweras it is simpler and more economical than direct current method. ✶ (ii) Type of construction. According to type of construction, ✶ distribution systemmay be classified as ✶ (a) overhead system (b) underground system. The overhead system is generally employed for distribution as it is 5 to 10 times cheaper than the equivalent underground system. In general, the underground system is used at places where overhead construction is impracticable or prohibited by the local laws. ✶ (iii) Scheme of connection. ✶ According to scheme of connection, the distribution system may be classified as (a) radial system (b) ring main system (c) interconnected system.
- 8. A.C. Distribution Now-a-days electrical energy is generated, transmitted and distributed in theform of alternating current.Alternating current in prefered to direct current is the fact that alternatingvoltage can be conveniently changed by means of a transformer. High distrubution and distribution voltages have greatly reduced the currentin the conductors and the resulting line losses.The a.c. distribution system is the electrical system between the stepdown substation fed by the distrubution system and the consumers’ meters.
- 9. CLASSIFICATIONS OF AC DISTRIBUTION SYSTEMS: ✶ The a.c. distribution system is classified into (i) primary distribution system (ii) secondary distribution system
- 11. PRIMARY DISTRIBUTION: ✶ It is that part of a.c. distribution system which operates at voltages somewhat higher than general utilisation than the average low-voltage consumer uses. ✶ The most commonly used primary distribution voltages are 11 kV, 6·6kV and 3·3 kV
- 13. ✶ Primary distribution is carried out by 3-phase, 3-wire system. ✶ Fig. shows a typical primary distribution system. ✶ Electric power from the generating station is transmitted at high voltage tothe substation located in or near the city. At this substation, voltage isstepped down to 11 kV with the help of step-down transformer. ✶ Power issupplied to various substations for distribution or to big consumers at thisvoltage. This forms the high voltage distribution or primary distribution.
- 16. Secondary distribution systems ✶ It is that part of a.c. distribution system employs 400/230 V, 3-phase, 4-wire System. Fig shows a typical secondary distribution system. ✶ The primary distribution circuit delivers power to various substations, called distribution substations. The substations are situated near the consumers’ localities and contain step down transformers. ✶ 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 230 V.
- 18. ✶ The single phase domestic loads are connected between any one phase andthe neutral,. ✶ Motor loads are connected across 3-phase lines directly.
- 19. DC DISTRIBUTION:✶ For certain applications, d.c. supply is absolutely necessary. d.c. supply is required for the operation of variable speed machinery (i.e., d.c. motors storage battery. ✶ For this purpose,a.c. power is converted into d.c. power at the substation by using converting machinery e.g.,mercury arc rectifiers, rotary converters andmotor-generator sets. ✶ The d.c. supply obtained in the form of ✶ (i) 2-wire or ✶ (ii) 3-wire for distribution.
- 20. 2-WIRE DC 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 negativewire. ✶ The loads such as lamps, motors etc. are connected in parallel between thetwo wires as shown in Fig. ✶ This system is never used for distrubution purposes due to low efficiency but may be employed for distribution of d.c. power.
- 21. 2- WIRE DC SYSTEM:
- 22. 3- WIRE DC 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. ✶ The principal advantage of this system is that it makes available Two voltages at the consumer terminals, ✶ V between any outer and the neutral and 2V between the outers. ✶ Loads requiring high voltage (e.g., motors) are connected across the outers, whereas lamps and heating circuits requiring less voltage are connected between either outer and the neutral.