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CHAPTER 1 Typical Transmission and Distribution Schemes Introduction, One line diagram, Standard voltages for transmission, Advantages of high voltage transmission, Disadvantages of high voltage transmission, Equation of volume of copper required and efficiency, Distribution system, AC systems and DC systems, Advantages and disadvantages of AC and DC systems, Important points to remember. 1.1 INTRODUCTION Transmission and Distribution are the principle part of the power system. Transmission lines are the connecting links between the electrical power generating stations and distribution system. The distribution system connects all the loads to the transmission lines at substations. The substations perform switching functions and voltage transformation. In the beginning of the development of power system, the generation was DC only and the generators were located at load centres. There was no need for transmission lines. With the development of transformer which can change AC voltages from one level to another with high efficiency and also the availability of electrical energy generated by water power, transmission of electrical energy over long distances become feasible. This is because when the transmission voltage is increased, for the same power the I2R loss in the lines gets substantially decreased. The poly phase motors which have better performance over single phase motors, so the three phase transmission replaced DC and single phase systems. Presently only three phase systems are used. The power system consists of electrical power generation at 11 kV to 25 kV. The generating stations may be hydroelectric power stations at places where potential of water is available or thermal power stations near coal mines where water is available for cooling purposes. The generation voltage is stepped up to transmission voltages of 220 kV to 400 kV with the help of transformers and the energy is transmitted to load centre receiving stations at this voltage. At receiving stations it is stepped down to sub-transmission voltages of 33 kV – 138 kV when it is supplied to large consumers. It is stepped down to 400/231volt by distribution transformers and the power is distributed to medium and small consumers. The schematic diagram of a simple power station is as shown in Fig. 1.1. 2 ELECTRICAL POWER TRANSMISSION AND DISTRIBUTION Fig. 1.1 One line diagram of power systems 1.2 STANDARD VOLTAGES FOR TRANSMISSION Consider two generators operating at 6.6 kV and 11 kV when there is a transmission line operating at 220 kV, the step up transformers are to be rated at 6.6/220 kV and 11/220 kV respectively. In order to reduce the breakdown time there should be one spare transformer of each rating. Instead if generators are rated at 11 kV, then only one spare transformer can be used for replacement of any transformer in case of breakdown. This saves the money on spares as well as storage space. When this argument is extended to all equipments in the power system there is a need for standardising the voltages in the system. Such standardisation reduces the cost of equipments, space for keeping the spares and also reduces problems in manufacture and design. In our country the voltages used for different purposes are: (a) 6.6 kV, 11 kV, 33 kV Generation (b) 110 kV, 132 kV, 220 kV and 400 kV Transmission TYPICAL TRANSMISSION (c) 3.3 KV, 6.6 KV, 11 KV (d) 400 V between the lines 231 V between line to neutral. AND DISTRIBUTION SCHEMES 3 Distribution of large consumers Low voltage distribution 1.3 ADVANTAGES OF HIGH VOLTAGE TRANSMISSION The following are the advantages of high voltage transmission: (i) For a given power, if transmission voltage is increased the current carried by conductors decreases (because P = VI cos Φ, power factor remaining constant, when V increases I decreases for same power P). (ii) When the current decreases, power loss in conductors I2R loss decreases and hence efficiency increases. (iii) When there is decrease in current the voltage drop IR or IZ decreases and improves the voltage regulation. (iv) Since the current density which the conductor can carry is same, cross section of the conductor I˘ È decreases Í a = ˙ which in turn decreases the volume of the conductor material. d˚ Î (v) The power transmitted by transmission lines is proportional to square of the transmission voltage. As a consequence the overall capital cost of transmission decreases as the voltage increases. Transmission of large power over long distances is economically feasible with high voltage transmission only. (vi) The power stations of large capacities are far away from load centres. This is because hydroelectric power stations are located purely on geographical considerations. Thermal power stations are located near coal mines, nuclear power stations are located far away from thickly populated areas. To transmit large power over long distances high voltage transmission is the only solution. (vii) Interconnection of the power systems will be easier with high voltage transmission. 1.4 DISADVANTAGES OF HIGH VOLTAGE TRANSMISSION Some of the disadvantages of the high voltage transmission are: (i) Corona and radio interference: As the voltage increases the phenomenon of corona occurs.. Corona also interferes with radio and television signals. (ii) Line supports: For high voltage transmission conductors the ground clearance required is large. Due to these the tower has to be big. (iii) Construction difficulties: Erection of high voltage transmission towers requires high standard of workmanship. (iv) Insulation requirement: As the voltage increases the insulation requirement increases. Switching surges are roughly four times the operating voltage. Hence the insulation required correspondingly increases. 1.5 THE EQUATIONS THAT HIGHLIGHT THE ADVANTAGES OF HIGH VOLTAGE OPERATION Let the V and I be the phase voltage and current of a three phase system. Then the per phase input power P = VI cos f Watts/phase 4 ELECTRICAL POWER TRANSMISSION AND DISTRIBUTION The power loss in the circuit per phase p = The Input power P = I 2R kW Ph 1000 VI cos f kW Ph 1000 Then the current in the load I = 1000 P Amps V cos f 2 Ê 1000 P ˆ R ¥ kW Ph The power loss per phase p = Á ˜ 1000 Ë V cos f ¯ Therefore the output O P = I P - Losses Ê 1000 P 2 R ˆ Then the O P = Á P - 2 2 ˜ V cos f ¯ Ë Ê 1000 P 2 R ˆ P Á ˜ V 2 cos2 f ¯ Ë O P ¥ 100 = ¥ 100 The percentage efficiency % h = I P P 1000 PR ˆ Ê After simplification % h = 1 - 2 2 ¥ 100 ÁË V cos f ˜¯ It is clear from the above equation that the percentage η varies directly with the system voltage and so if the operating voltage is high the value of second term in the bracket is low and efficiency is high. The increase in the operating voltage also reduces the power loss. When the operating voltage is increased the operating current is decreased and hence the voltage drop is reduced and so improves the voltage regulation. 1.6 TO SHOW THAT THE VOLUME OF CONDUCTOR MATERIAL REQUIRED DEPENDS ON OPERATING VOLTAGE Let the V and I be the phase voltage and current of a three phase system. Then the per phase input power P = VI cosf watts/phase The power loss in the circuit per phase p = The input power P = I 2R kW/Ph 1000 VI cosf kW/Ph 1000 Then the current in the load I = 1000 P Amps Vcosf 2 R Ê 1000 P ˆ ¥ kW/Ph The power loss per phase p = Á ˜ 1000 Ë V cos f ¯ TYPICAL TRANSMISSION The resistance of the conductor R = AND DISTRIBUTION SCHEMES 5 l r a 2 l Ê 1000 P ˆ ¥ kW/Ph The power loss per phase p = Á ˜ 1000 a Ë V cos f ¯ Area of the conductor a = 1000 P 2lr V 2 cos2 f p Volume of the conductor al = 1000 P 2 l 2 r V 2 cos 2 f p The above equation indicates that the volume of the conductor material is inversely proportional to the square of the operating voltage per phase when the power to be transmitted over a fixed length is taken as constant. 1.7 DISTRIBUTION SYSTEM A distribution system consists of feeders, distributors and service mains. Figure 1.2 shows a distribution system. Feeders are conductors having large current carrying capacity connected from substation to distribution transformer. Normally feeders are operated at high voltage say 11 kV. Distributors are conductors from which current is tapped off for supply to the consumers and is connected to secondary of the distribution transformer. Service mains are cables connected between distributors and consumer premises. Service mains Sub station Feeder Distribution Distribution transformer Fig. 1.2 Distribution system 1.8 TYPES OF TRANSMISSION AND DISTRIBUTION SYSTEMS In the present scenario, the electrical energy is generated, transmitted and distributed in the alternating form. This is because of the fact that the alternating voltages can be increased or decreased in magnitude with the help and use of transformers. Whenever the DC supply is required the AC voltages are converted to DC by using rectifiers and then can be transmitted or distributed. Hence the main division of transmission and distribution are AC system and DC system. 6 ELECTRICAL POWER TRANSMISSION AND DISTRIBUTION (1) AC System The commonly used system for transmission of electrical power from generating station to the distribution station is AC three phase three wire system or three phase four wire system. In three phase three wire system the primary of the transformer is always connected either in star or delta. If it is in star connection the neutral point is grounded. For large consumers like factories and industrial estates are directly supplied from the primary distribution transformer. The secondary distribution transformers are always connected in either star/star or delta/star so as to supply both single phase and three phase loads. Advantages of AC system (i) (ii) (iii) (iv) The cost of AC generation is very low when compared to the DC generation. With the advent of transformers AC voltage can be raised or lowered to any desired value. The maintenance of AC substation is very easy. The motors running on AC system are simple in construction, low cost and decreases cost of maintenance. Disadvantages of AC system (i) (ii) (iii) (iv) (v) (vi) Construction of AC transmission lines is more complicated than a DC line. The voltage drop in AC lines is large because of the inductance effect of the line. The copper requirement of AC line is more when compared to DC system. In AC transmission the lines experience corona effect and corona loss. In AC system of generation the synchronisation of alternators is required. The charging current exists even when the line is open, because of the existence of shunt capacitance between the lines and also between line and neutral. (2) DC System Though the AC systems are extensively used, there are few applications of DC supply alone. Some of the applications of DC supply are for the charging of batteries, electroplating etc. To obtain the DC supply, rectifiers or DC generators are used at substations or at load centres. The systems are further classified as (a) Two wire DC system (b) Two wire DC with mid point earthed and (c) Three wire DC system. (a) Two Wire DC Systems: It is the simplest and direct output of the DC generators is given to the loads with the help of busbars. A simple arrangement is as shown in Fig. 1.3. (b) Two Wire DC with Mid Point Earthed: This type of connection is used when the required output voltage is high. In this system of connection there are two lines conductors, the one having +Vm with respect to mid point and the other is at –Vm with respect to mid point. So the total voltage available at the two lines is 2 Vm with mid point earthed. (c) Three Wire DC System: This type of connection is similar to the two wire DC with mid point earthed. In this connection an additional wire from the mid point is drawn which makes the system feasible to supply loads requiring low voltage and high voltage. Advantages of DC system (i) The cost of conductor material required is reduced from AC three phase system as it uses only two conductors. (ii) In DC systems there is no inductance and capacitance and so the voltage drop and power loss is much lower compared to AC system. TYPICAL TRANSMISSION Vm PM 2 Vm L O A D PM DISTRIBUTION SCHEMES 7 L O A D 2 Vm PM PM AND Vm DC two wire system with mid point earthed L O A D L O A D DC three wire system Fig. 1.3 Two wire DC system and DC three wire system (iii) Because of the low voltage drop and low power loss the voltage regulation improves and transmission efficiency increases. (iv) As the operating voltage is DC there is no skin effect and so the resistance of the conductor is low. (v) For same voltage level the voltage stress on the insulation is less in a DC system and so the insulation required is less in case of DC when compared to AC. 1.9 IMPORTANT POINTS TO REMEMBER 1. Advantages of high voltage transmission are: (a) Reduction in copper loss, cross section of conductor decreases, reduction in volume of conductor material, voltage drop decreases, voltage regulation improves, (b) Increases the transmission efficiency and reduces the cost of power transmission over long distances. 2. Disadvantages are corona loss and radio interference increases, line supports has to be stronger and more insulation is required. 3. In AC transmission it will be three phase three wire system is used and in distribution three phase four wire system is used so that both single phase and three phase loads can be supplied. 4. In the DC distribution there are three types of distribution schemes such as DC two wire, DC two wire with mid point earthed and DC three wire system. When the higher operating voltages are required DC two wire with mid point earthed and DC three wire systems are used. The main advantage of this system is it requires a less conductor material, cost of distribution is reduced. 5. In DC system inductive and capacitive reactance values are zero, hence the voltage drop in the line is less and regulation improves. There is no skin effect in the DC system and hence the resistance of the conductor is reduced.