K.S.R. POLYTECHNIC COLLEGE TIRUCHENGODE DISTRIBUTION AND UTILIZATION Department of Electrical and Electronic Engineering V A KUPPUSAMY, M.E., MISTE., Sr. Lecturer / EEE K.S.R. POLYTECHNIC COLLEGE TIRUCHENGODE – Prepared by DISTRIBUTION Unit - I

DISTRIBUTI ON UNIT – I

SUB – STATION The electrical power is generated, transmitted and distributed in the form of alternating current. Electrical power is produced at the power houses which are far away from the consumers. It is delivered to the consumers through a large network of transmission and distribution. At many places in the line of the power system, it is desirable and necessary to change some characteristics of electric supply. This is accomplished by suitable apparatus called sub-station. INTRODUCTION Sub - Station3

SUB – STATION Sub - Station4 The assembly of apparatus used to change some characteristic (voltage, ac to dc, frequency, p.f etc) of electric supply is called sub station. Sub-stations are important part of power system. The continuity of supply depends upon the successful operation of sub-stations. The following are the important points to be considered while laying out a sub-station. It should be located at a proper site i.e., at the load centre. It should provide safe and reliable arrangement. The capital cost should be minimum..

Classification of Sub – Station Sub stations may be classified according to Service requirement. Constructional features (or) design. 1. According to Service requirement A sub-station may be called upon to change voltage level or improve power factor or convert a,c power into d.c power etc. according to the service requirement substations may be classified into. Sub - Station5

a) Transformer sub-station The sub station which change the voltage level of electrical supply are called transformer sub stations. These sub stations receive power at some voltage and deliver it at some other voltage. Transformer will be the main component in such sub stations. Most of the sub-stations in the power system are of this type. Sub - Station6

b) Switching sub-station These sub station do not change the voltage level ie incoming and outgoing line have the same voltage. However they simply perform the switching operations of power lines. Sub - Station7

c) Power factor correction sub-station These sub station improve the power factor of the system. Such sub station are generally located at the receiving end of transmission lines. In these sub stations, synchronous condensers are used for power factor improvement. Sub - Station8

d) Frequency changer sub-station The sub station which change the normal supply frequency are called frequency changer sub station such a frequency change may be required for industrial utilisation. Sub - Station9

f) Industrial sub-station Big industrial consumer need bulk power. The sub station which supply power to individual industrial concerns are known as industrial sub station. Sub - Station10

2. According to construction features A sub station has many components like circuit breakers, switches, fuses, instruments etc. These components must be housed properly to ensure continuous and reliable service. According to constructional features, the substations are classified as, a.Indoor sub station. b.Outdoor sub station. c.Under ground sub station. Sub - Station11

a. Indoor sub station In these sub stations all the equipment's are installed within the sub station buildings. These sub stations are usually designed for a voltage range of 11KV, 33KV, and 66KV. Sub - Station12

b. Outdoor sub station For voltage beyond 66KV equipment's are installed outdoor. It is because for such voltages the clearances between conductors and the space required for switches, circuit breakers and other equipment becomes so great. Hence in these substation all the equipment's are mounted in out door. Outdoor sub stations are further classified in to pole mounted substation and foundation mounted sub station. Sub - Station13

a. Pole mounted substation This is an outdoor sub station with equipment installed overhead on H-pole or A-pole structure. It is the cheapest form of substation for voltages not exceeding 11KV. In pole mounted sub stations, upto 100KVA capacity transformers are used. Sub - Station14

b. Foundation mounted substation In these sub stations above 100 KVA transformers are mounted over a concrete foundation. Suitable fences are arranged for safety. Sub - Station15

Underground substation In thickly populated areas, the space available for equipment and building is limited and the cost of land is high. Under such situations, the sub-station is arranged in the underground. Sub - Station16

Gas Insulated substation A Gas insulated substation is an electrical substation in which the major structures are contained in a sealed enclosure with SF 6 gas as insulating medium. High voltage conductor, switch gears, instrument transformers, the bus bar and all other equipment are housed in metal enclosures. Filled with SF 6 gas at 4 to 6 times the atmospheric pressure. Sub - Station17

11KV / 400V Distribution Substation Sub - Station18

SUB – STATION EQUIPMENT The equipment required for a transformer sub-station depends upon The type of sub-station Service requirement. The degree of protection required. A transformer sub – station has the following main equipment's. Bus Bar. a)Single bus-bar arrangement. b)Single bus-bar system with sectionalisation. c)Double bus-bar arrangement. Sub - Station19

Bus - Bars Bus-bars are copper or aluminium bars and operate at constant voltage. The incoming and outgoing lines in a sub- station are connected to the bus-bars. The most commonly used bus-bar arrangements in sub-station are: Single bus-bar arrangement. Single bus-bar system with sectionalisation. Double bus-bar arrangement. Number of lines operating at the same voltage have to be directly connected electrically, bus bars are used as the common electrical component. Sub - Station20

Insulators The most commonly used material for the manufacture of insulator is porcelain. There are several type of insulator (eg. Pin type, suspension type, post insulator etc.) Their use in the sub-station will depend upon the service requirements. The insulators serve two purposes. They support the conductors and provide necessary insulation between conductors and supports to avoid leakage current. Sub - Station21

Isolators This is accomplished by an isolating switch or isolator. An isolator is essentially a knife switch. It is designed to open a circuit under no load. That is isolator switches are operated when no current flows in the line in which it is connected. It is often desired to disconnect a part of the system for general maintenance and repairs. Sub - Station22

Lightning arrestor In order to protect transmission line, transformer and other equipment from lighting arrestors are used. Sub - Station23

Circuit Breaker It is so designed that it can be operated manually (or by remote control) under normal conditions and automatically under fault conditions. For automatic operation, a relay circuit is used with a circuit breaker. Equipment which can open or close a circuit under normal as well as fault conditions. Sub - Station24

Power Transformers Used in a substation to step-up or step- down the voltage. Except at the power station, all the subsequent substations use step-down transformers to gradually reduce the voltage Sub - Station25

Instrument Transformers Operate at high voltages and carry current of thousands of amperes. Measuring instruments and protective devices are designed for low voltages (generally 110V) and currents (about 5A). Therefore, they will not work satisfactorily of mounted directly on the power lines. This difficulty is overcome by installing instrument transformers on the power lines. a)Current transformer (C.T) b)Potential transformer (P.T) Sub - Station26

Current Transformers (C.T) A current transformer is essentially a step up transformer which steps down the current in a known ratio. The primary of this transformer consists of one or more turns of thick wire connected in series with the line. The secondary consists of a large number of turns of line wire and provides for the measuring instruments and relays. Sub - Station27

Potential Transformers (C.T) It is essentially a step down transformer and steps down the voltage in a known ratio. The primary of this transformer consists of a large number of turns of fine wire connected across the line. The secondary winding consists of a few turns and provides for measuring instruments and relays a voltage which is a known fraction of the line voltage. Sub - Station28

Metering and Indicating Instruments There are several metering and indicating instruments (e.g ammeters, voltmeters, energy meter etc.) installed in a sub-station to maintain watch over the circuit quantities. The instrument transformers are invariably used with them for satisfactory operation. Sub - Station29

Miscellaneous equipment In addition to above, there are following equipment in a substation. Fuses. Carrier – current equipment. Sub-station auxiliary supplies. Sub - Station30

Carrier current equipment This equipment is installed in the substations for communication, relaying telemetering or supervisory control. This equipment is suitably mounted in a room known as carrier room and connected to the high voltage power circuit. Sub - Station31

Substation Auxiliary supply In addition to the above mentioned equipment’s, substation also contains some auxiliary equipment’s and circuits. They are, Lighting in the switch yard, control rooms. Emergency lighting. Measuring instruments. Relays. Trip coils and closing coils. Protection system. Control Circuits. For giving supply to these equipments and circuits auxiliary supply system is necessary. Sub - Station32

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Bus bars The bus bar is a bare conductor. Shape is rectangular, square, round tubes or solid bars Made up of aluminium. Main conductor from which a number of connections are made. Bus bars are 5 to 6 meters of length. Aluminium is less weight and cheaper and excellent corrosion resistance. Single bus-bar arrangement. Single bus-bar system with sectionalisation. Double bus-bar arrangement. Sub - Station35

Single bus-bar arrangement A single set of bus-bar is used for the complete generating station. All the transformers, generators and feeders were connected to this single bus bar. The generators are connected to bus bar through isolators and circuit breakers. Similarly the outgoing feeders are also connected to the bus-bar through isolators and circuit breakers. This type of arrangement is used for D.C stations and small A.C stations. Sub - Station36

Single bus-bar system with sectionalisation arrangement A single set of bus-bar is divided into sections. Any two sections of the bus bar are connected by a circuit breaker and isolators. If a fault occurs on any section of the bus, that section can be isolated without affecting the supply from other sections. Also repairs and maintenance of any section of the bus-bar can be carried out by de- energising that section only. Sub - Station37

Double bus-bar system This system consists of two bus bars, one main bus bar and another auxiliary bus bar. The incoming and outgoing lines are connected to the two bus bars through circuit breaker and isolator. At normal condition, the incoming and outgoing lines are connected to the Main bus bar, and the auxiliary bus bar is kept as reserve bus bar. In case of repair or maintenance of main bus bar, the continuity of supply to the circuit can be maintained by transferring the circuit to the auxiliary bus bar. Sub - Station38

Sectionalised Double bus-bar system This scheme auxiliary bus bars are used with the sectionalised main bus bar. In this method of connection, any section of the bus bar can be isolated for maintenance work. It will be noted that the auxiliary bus bar is not sectionalised because this is not necessary and is expensive. Sub - Station39

Ring bus-bar system The ends of the bus bars are returned to form a ring. In this system each feeder is supplied from two paths. By a doping this scheme of connections, the alternators connected to any one of the bus bar section can be used for supplying the load to the feeders on any section. Sub - Station40

Distribution System A part of power system which distributes electric power from the substation to the consumers is known as distribution system. Sub - Station41 Requirements of Distribution System A considerable amount of effort is necessary to maintain an electric power supply within the requirements of various types of consumers. Some of the requirements of a good distribution system are proper voltage, availability of power on demand and reliability. 1.Proper Voltage. 2.Availability of power on demand. 3.Reliability. 1.Inter connected system. 2.Reliable automatic control system. 3.Providing additional reserve facilities.

Parts of Distribution System A distribution system consists of three major parts. They are Feeder, Distributors and Service mains Sub - Station42 Feeder Which connects the substation to the area where power is to be distributed. No tappings are taken from the feeder. The current in it remains the same throughout Main consideration in the design of feeder is the current carrying capacity.

Distribution System A distribution is a conductor from which tappings are taken for supplying to the consumers. AB, BC, CD and DA are the distributors. Sub - Station43 Service Mains A service main is generally a small cable which connects the distributor to the consumer terminals. Sub Station Ring Main Distributor Feeder A B D C Service Mains

Classification of Distribution Systems A distribution system may be classified according to Sub - Station44 1. Based on type of supply a)D.C distribution system. b)A.C distribution system. 2. Based on Character of service voltage a)Low tension distribution (LT) (400V). b)High tension distribution (HT) (11KV). 3. Based on type of construction a)Overhead distribution system. b)Underground distribution system. 4. Based on number of wires a)Two wire distribution. b)Three wire distribution. c)Four wire distribution. 5. Based on Scheme of connection a)Radial distribution system. b)Ring distribution system. c)Inter connected distribution system.

Various system of Power Distribution The power is distribution in two methods. 1. D.C system2. A.C system Sub - Station45 1. D.C system a)D.C two wire. b)D.C two wire with mid point earthed. c)D.C three wire. 2. A.C system a)Single phase two wire. b)Single phase two wire with mid point earthed. c)Single phase three wire. B. Two phase A.C system a)Two phase four wire. b)Three phase three wire. C. Three phase A.C system A. Single phase A.C system a)Three phase three wire. b)Three phase four wire.

Comparison of cost of conductors in A.C and D.C system While comparing the cost of conductors of various systems the following assumption to be made. The power transmitted by each system is same. The distance over which the power transmitted is same. The line losses in the system are same. The maximum voltages between any conductor and earth is the same in all cases. In three wire system, the loads should be balanced. Sub - Station46

Sub - Station47 D.C two wire system with one conductor earthed

Connection Scheme of Distribution System All distribution of electrical energy is done by constant voltage system. In practice the following distribution circuits are generally used. Sub - Station48 1. Radial System In this system separate feeder radiate from a single sub station and feed the distributors at one end only. The radial system is employed only when power is generated at low voltage and the sub station is located at the centre of the load.

Sub - Station49 2. Ring Main System In this system, the primaries of distribution transformers form a loop. The loop circuit starts from the substation bus-bars, makes a loop through the area to be served, and returns to the substation. The single line diagram of ring main system for a.c. distribution where substation supplies to the closed feeder LMNOPQRS. The distributors are tapped from different points M, O and Q of the feeder through distribution transformers. Advantages Less voltage fluctuations at consumer terminals. The system is more reliable because each distributor is fed by two feeders. In the event of fault on any section of the feeder the continuity of supply is maintained.

Sub - Station50 3. Inter Connected System When the feeder ring is energised by two or more than generating stations or substations, it is called inter-connected system. The single line diagram of interconnected system where the closed feeder ring ABCD is supplied by two substations S 1 and S 2 at points D and C respectively. Distributors are connected to points O, P, Q and R of the feeder ring through distribution transformers. Advantages It increases the service reliability. Any area fed from one generating station during peak load hours can be fed from the other generating station. This reduces reserve power capacity and increases efficiency of the system.

Sub - Station51 1. A.C. DISTRIBUTION CALCULATIONS A.C. distribution calculations differ from those of d.c distribution in the following respects : In case of d.c system, the voltage drop is due to resistance alone. However, in a.c. system, the voltage drops are due to the combined effects of resistance, inductance and capacitance. In a d.c system, additions and subtractions of currents or voltages are done arithmetically but in case of a.c system, these operations are done vectorically. In an a.c. system, power factor (p.f.) has to be taken into account. Loads tapped off form the distributor are generally at different power factors. There are two ways of referring power factor viz. 1.It may be referred to supply or receiving end voltage which is regarded as the reference vector. 2.It may be referred to the voltage at the load point itself. There are several ways of solving a.c. distribution problems. However, symbolic notation method has been found to be most convenient for this purpose. In this method, voltages, currents and impedances are expressed in complex notation and the calculations are made exactly as in d.c. distribution.

Sub - Station52 2. METHODS OF SOLVING A.C. DISTRIBUTION PROBLEMS In a.c. distribution calculations, power factors of various load currents have to be considered Generally the power factors are referred in two ways. Power factor referred to receiving end voltage. Power factor referred to load voltage. i). Power factor referred to receiving end voltage Consider an a.c. distributor A B with concentrated loads of I 1 and I 2 tapped off at points C and B. Taking the receiving end voltage V B as the reference vector, let lagging power factors at C and B be cos θ 1 and cos θ 2 w.r.t. V B. Let R 1, X 1 and R 2, X 2 be the resistance and reactance of sections AC and CB of the distributor.

Sub - Station53 Impedance of section AC = Z AC = R 1 + jX 1 Impedance of section CB = Z CB = R 2 + jX 2 Load current at point C= I 1 I 1 = I 1 (cos θ 1 – j sin θ 1 ) Load current at point B = I 2 I 2 = I 2 (cos θ 2 – j sin θ 2 ) Current in Section CB = I CB I 2 = I 2 (cos θ 1 – j sin θ 2 ) Current in Section AC = I AC I AC = I 1 + I 2 = I 1 (cos θ 1 – j sin θ 1 ) + I 2 (cos θ 2 – j sin θ 2 ) Voltage drop in Section CB, V CB = I CB. Z CB = I 2 (cos θ 2 – j sin θ 2 ) (R 2 – j X 2 ) R 2 + jX 2 R 1 + jX 1 I 1 cos θ 1 I 2 cos θ 2 BCA

Sub - Station54 Voltage drop in Section CB, V CB = I CB. Z CB = I 2 (cos θ 2 – j sin θ 2 ) (R 2 – j X 2 ) Voltage drop in Section AC, V AC = I AC. Z AC = [I 2 (cos θ 2 – j sin θ 2 ) I 2 (cos θ 2 – j sin θ 2 )] (R 2 – j X 2 ) Sending end Voltage V A = V B + V AC + V CB Sending end Voltage I A = I 1 + I 2 Sending end Voltage V A = cos θ S

Sub - Station55 I2X2I2X2 I2R2I2R2 I AC X 1 I AC R 1 I1I1 I2I2 I AC α θ2θ2 β θsθs θ1θ1 VBVB VCVC VAVA R 2 + jX 2 R 1 + jX 1 I 1 cos θ 1 I 2 cos θ 2 BCA

ii) Power factor referred to respective load voltages Sub - Station56 I2X2I2X2 I2R2I2R2 I AC X 1 I AC R 1 I1I1 I2I2 I AC VBVB VCVC VAVA

ii) Power factor referred to respective load voltages Sub - Station57 I2X2I2X2 I2R2I2R2 I AC X 1 I AC R 1 I1I1 I2I2 I AC VBVB VCVC VAVA

Therefore, the neutrals are at the same potential and voltage across each impedance is same and equal to phase voltage whether the circuit is balanced or unbalanced. The three phase currents or line currents can be determined by dividing the phase voltage by the impedance of the concerned phases. Sub - Station58 3 – Phase, 4 wire Star – Connected Unbalanced Load circuits IBIB IYIY IYIY IBIB IRIR B Y R I N = I R + I Y + I B B` Y` R` IBIB IYIY VPVP In 3 phase, 4 wire, star-connected load circuits the star points of load and the generator are tied together through neutral wire of zero impedance. The current in neutral wire can be determined by applying Kirchhoff’s first law at star point N. According which I N + I R + I Y + I B = 0 or current in neutral wire, I N = - (I R + I Y + I B )

In case of a balanced three – phase, four – wire system when the neutral is disconnected, no change is produced. But in case of unbalanced 3 – phase, 4 – wire system, when the neutral is disconnected, the loads which are connected between any two conductors and the neutral are connected in series and potential difference across the combined load become equal to line voltage. The potential difference across each load is thus changed as per rating of the load. Sub - Station59 Consequence of Disconnection of Neutral in Three Phase Four wire system

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