Power System-III By J.SOMLAL, M.Tech.,(Ph.D), MISTE, MIETE, Assistant Professor, EEE Department, K L University, Vaddeshwaram, Vijayawada, Andhra Pradesh-522502 Web: http://www.kluniversity.in Mobile No:9989743653 ELECTRIC TRACTION What is Traction? The act of drawing or pulling, as by an elastic or spring force. Traction refers to the maximum frictional force that can be produced between surfaces without slipping. TRACTION is resulting from a specific FRICTION coefficient (i.e. friction between rubber and ground) combined with area of ground covered by the tire – FOOTPRINT combined with vehicle WEIGHT pressing a tire onto the ground. TRACTION is a resistance between tire and ground. Why do we need traction? Well, that question comes up every once in a while. Its a good question? If our feet do not find a surface with good grip (traction) our legs and feet could not move us forward. Same story for a car - if the ground would not provide enough resistance (traction) the force generated in the engine would not be able to move the car forward. The scenario is quite simple. The more traction can be made available the more torque can be generated to move more weight. Requirements of an IDEAL Traction System The following are some of the important requirements of the driving equipment used for traction purposes: The coefficient of adhesion(sticking) should be high so that high tractive effort at start is possible and rapid acceleration of the train can be obtained. It should be possible to overload the equipment for short periods. The wear(consume by use) caused on the shoes, wheel tires and the track should be minimum. It should be possible to use regenerative braking so that on descents it should be possible to generate energy and feed back to the supply system. It should be pollution free. Traction System 1. Non-Electric Traction System: Does not use electricity at any stage. Ex: Steam Engine Drive, Internal Combustion Drive. 2. Electric Traction System: If electric supply is used for driving a locomotive, the system is known as electric traction. Involves use of electricity at some stage or other. Ex: Battery Electric Drive, Diesel Electric Drive, etc. Electric traction is the most efficient of all other systems and is going to be the future system to be adopted by almost all countries of the world. Types of Electric Traction Vehicles which receive electric power from a distribution network fed at suitable points from either a central power station or substations suitably spaced. It is further subdivided into 1) Systems operating with d.c. such as trolley buses, tramways and railways. 2) Systems operating with a.c. such as railways. The group consists of self contained locomotives i.e. they generate their own power. It is further subdivided into 1) Diesel electric trains and ships 2) Petrol electric trucks and lorries 3) Battery driven road vehicles DIESEL ELECTRIC TRACTION The diesel electric locomotive uses a diesel engine to drive an electric generator, which then supplies the current to traction motors, which are geared directly to the locomotive's wheels. In India, the diesel locomotives were introduced in 1945 for shunting purposes on broad gauge section and in 1956 for main line services on medium gauge section. The diesel electric locomotives employed in practice are of the following types: 1. Main line diesel electric locomotive having engines of output not exceeding 1500 kW and speeds of 160 kmph. 2. Shunting diesel electric locomotive having an engine of 225 to 375 kW output and speed between 25 to 50 kmph. 3. Diesel electric multiple units stock of which each motor has an engine of 135 to 150 kW output and train is capable of having speeds between 80 to 110 kmph. 4. Diesel electric rail car having an engine of 75 to 450 kW output which may operate as a single car or car with one or more trailer coaches. In diesel electric system used for traction, electric motors are used for driving the locomotive which are fed by a d.c. generator driven by diesel engine mounted on the same locomotive as shown by the schematic diagram in fig.1 ADVANTAGES The initial investment required is low as compared to direct electric traction since there is no need of overhead structure distribution system and equipments. Due to its higher acceleration and retardation, the schedule speed over a given route will be higher. It can be put into service at any moment since hardly any time is required to start up the engine and put it on duty. The power loss in speed control is very low because it can be carried out by field control of generator. Its overall efficiency is higher than that of steam locomotive about 25%. There is no interference with the adjoining communication lines. Since a diesel electric locomotive is a self contained unit and does not requires any overhead structure hence it can be used on any route. DISADVANTAGES Its overload capacity is limited. The life of the diesel engine is comparatively shorter. In addition to motor generator set, special cooling system is required for cooling the diesel engine also. Its running and maintenance costs are high. For the same power output, diesel electric locomotive is costlier than steam or electric locomotive. Regenerative braking cannot be used with such types of drives. ELECTRIC LOCOMOTIVE It is the most widely used traction system in which the vehicle draws electrical energy from a distribution system fed at suitable points from a central power station or substation. In India both AC and DC type of electrified train systems operate today. 1500 V DC based train system is mostly operating in Mumbai area. It is being converted to 25 kV AC system. Rest of the India where routes are electrified mostly operates under 25 kV AC overhead wire. The electric locomotives are of two types A.C. locomotive D.C. locomotive In case of d.c. locomotive d.c. motors are used for traction. The fig.1 shows a schematic block diagram of d.c. locomotive. It basically consists of a step down transformer, a full wave rectifier with filters and d.c.motors. The fig.2 shows an a.c. type electric locomotive. The following are the advantages and disadvantages of electric system: ADVANTAGES Since electric motors are used as the drives , the system is clean and pollution free. Starting torque is high, so high acceleration is possible. Speed control is very simple. Braking is simple and efficient. Electric braking is used in this case which is superior to mechanical braking used by steam and diesel locomotives. It is possible to apply regenerative braking which has the following advantages Above 80% of energy spent during ascent(upward movement) is pumped back during descent. Less maintenance of brake shoes, wheels, tyres and rails on account of less wear and tear. An electric locomotive requires much less time for maintenance and repairs than a steam locomotive; and hence can be kept in service for 95% or more of the working day if desired. Its maintenance and repair cost is about 50 % of that of steam locomotive. The electric locomotive can be put into service immediately whereas steam locomotive requires about two hours getting up steam and be ready for service. The centre of gravity of electric locomotive is lower than that of steam locomotive due to which it is able to negotiate curves at comparatively higher speeds. DISADVANTAGES The most important factor against electric traction is high capital outlay on overhead supply system. Therefore, unless heavy traffic is to be handled electric traction becomes uneconomical. Power failure for few minutes can cause disruption of traffic for hours. The electric traction system is tied to electric routes only. Hence it cannot be used on any of the routes. In case of A.C. traction the communication lines running along the track experience considerable interference from power lines. The communication lines therefore must either be removed away from the track or replaced by special expensive cables (this increases the capital cost outlay by 15%) What are the voltages used for electric traction in India? Voltages used are 1.5kV DC and 25kV AC for mainline trains. Calcutta had an overhead 3kV DC system until the '60s. The 1.5kV DC overhead system (negative earth, positive catenary) is used around Bombay (This includes Mumbai CST - Kalyan, Kalyan - Pune, Kalyan - Igatpuri, Mumbai CST - Belapur - Panvel, and Churchgate Virar). Conversion to 25kV AC has already been done on the Titwala-Kasara section; next to be converted are Khapoli-Vangani, Vangani-Thane, and Titwala-Thane. The Madras suburban routes (Madras-Tambaram in the '60s, extended later to Villupuram) used to be 1.5kV DC until about 1967, when it was converted to 25kV AC (all overhead catenary supply). The 25kV AC system with overhead supply from a catenary is used throughout the rest of the country. The Calcutta Metro uses 750V DC traction with a third-rail mechanism for delivering the electricity to the EMUs. The Calcutta trams use 550V DC with an overhead catenary system with underground return conductors. The catenary is at a negative potential. The Delhi Metro uses 25kV AC overhead traction with a catenary system on the ground-level and elevated routes. System of Track Electrification Presently, following four types of track electrification systems are available: Direct Current System: DC at 600-750 V is universally employed for tramways in urban areas and for many suburban railways while 1500-3000 V dc is used for main line railways. Low voltage dc system is undoubtedly superior to single phase ac system for heavy suburban services. (1) Single Phase AC System: In this system ac series motors are used for getting the necessary motive power. The voltage employed for distribution network is 15 to 25 Kv at 162/3 or 25 Hz, which is stepped down on the locomotive to a low voltage (300 to 400 V) suitable for supplying to single phase ac series motors. (2) Three Phase AC System: In this system 3-phase induction motors operating at 3300 to 3600 V systems consist of two overhead wires and track rail for the third phase and receives power either directly from the generating station or through the transformer substations. (3) The Composite Systems: Such systems incorporate good points of two systems while ignoring their bad points. Two such composite systems presently in use are: A. Single Phase To Three Phase System or Kando System: In this system single phase hv ac system is employed for distribution purposes and 3-phase induction motors for getting the necessary driving power in order to have the advantage of low cost of single phase overhead distribution system together with the desirable characteristics of 3-phase induction motors (at low frequency 3 phase induction motor develops high starting torque without excessive current). Speed control is also conveniently achieved by varying the supply frequency. This system is likely to be developed in future. B. Single Phase To Direct Current System : This system combines the advantages of hv ac distribution system and dc series motors for traction. The voltage used for overhead distribution system is 25 kv at normal supply frequency of 50 Hz. The locomotive carries transformer and converting machinery to step-down the voltage and convert into dc. This system of track electrification using 25 kv, 50 Hz, single phase ac supply has been adopted for all future track electrification in India. The advantages of such a system are light overhead catenaries owing to lower currents, less number of substations (usually spaced at 50-80 km distances), flexibility in the location of substations, simplicity of substation design, lower cost of fixed installations, higher adhesion coefficient and higher starting efficiency. The draw-backs of this system are unbalancing effect on the supply and interference to telecommunication circuits. Fortunately both of these undesirable effects can be minimized. Typical Railway Services S.No Parameter of Comparison Urban or City Service Sub-Urban Service Main Line Service 1 Acceleration 1.5-4Kmphps 1.5-4Kmphps 6-8Kmphps 2 Retardation 3-4Kmphps 3-4Kmphps 1.5Kmphps 3 Max. Speed 120Kmph 120Kmph 160Kmph 4 Distance between stations 1Km 1-8Km >10Km 5 Long free Special remarks Free running period is Free running running and if any absent and coasting period period is absent coasting period is small. and coasting s. Acceleration period is long. and braking periods are small comparatively. TRAIN MOVEMENT-SPEED TIME CURVES A train is to be run most optimally as for as possible. for this it is necessary to know the speed time curves so that it is possible to find out what energy must be supplied to the train to perform a particular job. The slope at any point of the curve gives the acceleration or retardation of the train at that instant of time. The area between the curve and the time axis up to a certain time gives the distance covered by the train till that instant of time. Positive slope-Acceleration; Negative slope-Retardation. (i) Acceleration while notching up or constant Acceleration: During this period of run (0 to t1), Rst is gradually cut off so that the motor current is limited to a certain value and the voltage across the motor is gradually increased. (ii) Speed curve running: From t1 to t2, the current starts decreasing with the increase in speed. (iii) Free running(t2-t3):This period occurs when the power output from the driving axels balances the rate at which energy is expended against the resistance to motion. (iv) Coasting period(t3-t4): At the end of free running period , supply to the motors is cut off and train is allowed to run under its own momentum. (v) Braking period(t4-t5):At the end of coasting period, brakes are applied to bring the train to stop. Crest Speed, Average Speed And Schedule Speed Crest speed is the maximum speed (Vm) attained by a train during the run. Average speed is the mean speed from start to stop i.e. the distance covered between two stops divided by the actual time of run is called the average speed. Schedule speed is the ratio of distance covered between two stops and total time of run including the time of stop. The schedule speed of a given train when running on a given service (i.e. with a given distance between stations) is affected by (i) acceleration and braking retardation (ii) maximum or crest speed and (iii) duration of stop. Direct Current Motor Control CONTROLLING DC Motors: For a DC motor, Torque T=0.1592Φ[Z Ia /A]P N-m =KΦIa For shunt motor: Φ is constant for a constant supply voltage, so, the torque is directly proportional to the current drawn by the motor. For series motor: Φ α Ia, therefore T α I2. Eb=V-Ia r= ΦZNP/60A N=(60A/ΦZP)(V-Ia r) • THE SIEMENS ELECTRIC RAILWAY laid down at the Berlin Exhibition of 1879. The short line was about 600 yards long, and current was drawn from a third rail between the track which acted as the return to the dynamo. The locomotive hauled a maximum of some thirty passengers at a speed of about four miles an hour. This historic illustration shows one of the earliest trainloads of passengers, seated upon three carriages.