GANDHINAGAR INSTITUTE OF TECHNOLOGYBranch: Electrical

Subject: Utilization of Electrical Energy and Traction (2160907)

ALA PRESENTATION on,

Topic: AC Electrification System

PREPARED BY: Darshil Shah (140120109050)

GUIDED BY: Prof. Rahish Silavat

Sem: 6

INDEX

Introduction Types of AC electrification system Single phase AC system Three phase AC system Composite system Single phase to Three phase or Kando System Single phase to Direct Current system Conclusion References

INTRODUCTION Electrification technique is very much vast application to ensure the better efficiency and reliability

of the Traction system. By applying this Electrification system there are no any kinds of pollution and any kind of unnecessary maintenance is required.

A Railway Electrification System supplies electric power to railway trains and trams without an on-board prime mover or local fuel supply. Selection of an electrification system is based on economics of energy supply, maintenance, and capital cost compared to the revenue obtained for freight and passenger traffic.

Different systems are used for urban and intercity areas; some electric locomotives can switch to different supply voltages to allow flexibility in operation.

So, according to this here topic can also bifurcate in further classifications:

1. DC Electrification system 2. AC Electrification system

Single phase System Three phase System

TYPES OF AC ELECTRIFICATION SYSTEM

1. Single phase AC system

2. Three phase AC system

3. Composite system

4. Single phase to Three phase system

5. Single phase to DC system

SINGLE PHASE AC SYSTEM In this system, AC series motors are used for getting necessary motive power. The voltage employed

for distribution network is 15,000 to 25,000 volts at 6 (2/3) 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 by means of a stepdown transformer carried on locomotive.

The distribution network may be fed directly, at high voltage, from generating station when the extent of electrification is within a radius of about 30 km from the generating station. For longer distances the economic voltage for the power transmission system is higher than that desirable for traction system, and therefore, transformer substations are employed for feeding the distribution network.

Low frequency operation of overhead transmission line is reduced the line reactance and hence the voltage drop directly.

If, electric supply is taken from industrial frequency network, which is usually the case, substations in addition to transformers have necessarily the frequency converter to convert 3- phase, 50 Hz into single phase 6 (2/3) or 25 Hz as required. AC system is invariably adopted for mainline service where the cost of overhead structure is more important and rapid acceleration and retardation is not so important as for suburban railways.

THREE PHASE AC SYSTEM In this system, 3 phase Induction motors operating at 3000 to 3600 volts and normal

frequency 6 (2/3) Hz are employed for getting the required motive power. The three phase induction motors are simple and robust in construction and have high operating efficiency & property of automatic regenerative braking without requiring any additional equipment.

The substations receives power from high voltage transmission line at power frequency & step down the voltage & change the frequency. Since, two trolley wires per track are necessary, the line voltage of the distribution system has to be limited to values below the highest voltage used for single phase railways, and as the 3 phase induction motors can be built economically for operation at moderately high voltage, the line voltage is usually chosen so that the motors may be supplied directly from trolley wires.

This system employed in hilly areas, where output power required is high & regeneration on large scale is possible.

Disadvantage is two overhead contact wires and constant speed characteristic of induction motors this system is no longer likely to be adopted.

COMPOSITE SYSTEM Efforts have been made to combine the advantages of DC/AC and 3 phase/1 phase system and

composite system have been evolved. The single phase AC system is preferable for the view point of distribution and contact wire

system. It can be converted either three phase AC or DC at low voltage in locomotives. At present there are two composite systems are used:

1. Single phase to Three phase system (Kando system)

2. Single phase AC to DC system

COMPOSITE SYSTEM

Single phase to Three phase system

Single phase to DC system

SINGLE PHASE TO THREE PHASE SYSTEM

In this system, single phase high voltage AC system is employed for distribution purpose and three phase induction motors for getting the necessary motive power in order to have the advantage of low cost of single phase overhead distribution system together with desirable characteristics of 3 phase Induction motors.

The voltage used for distribution network is 15000 volts at 50 Hz. Locomotive carries a phase converter for converting single phase to three phase supply at relatively low frequency.

It is convenient to use for some railway tractions.

SINGLE PHASE TO DIRECT CURRENT SYSTEM

In this system, the advantage of two system are combined together by employing high voltage AC 1 phase system for overhead distribution system is obtained together with desirable characteristics of DC series motors.

Basically this system is adopted on 25 kV and 50 Hz. It consist numerous advantages over DC system as enumerated below:

The line current for given demand of power is reduced on account of high system voltage. It causes reduction in cross-section of overhead conductor which leads to a savings in cost of supporting structures & their foundations being lighter in weight & simpler. The weight of copper require for track km for the 1500 V DC system is 3700 kg and for 3000 V DC system is 3300 kg whereas for AC system is 1600 kg.

On account of high voltage substations are spaced at longer distances (50 to 80 km) whereas the substations are spaced at 12 to 30 km in case of 3000 V DC system and 5 to 12 km in case of 1500 V DC system. It affects substantial savings in capital and expenditure on traction substation & transmission line on account to reduce no. of S.S. and reliability.

SINGLE PHASE TO DIRECT CURRENT SYSTEM (cont…)

A very special feature of the AC locomotive In comparison with the DC locomotives is the greater amount of adhesion obtained with AC locomotives. In case of AC rectifier locomotives employing DC traction motors, coefficient of adhesion is obtained as much as 45% against the value of 27% in case of DC locomotives.

The greater adhesion is obtained because of possibility of connecting all DC series motors in parallel permanently and controlling their speed by varying the voltage across each motor.

The starting efficiency is high of AC locomotive than DC locomotives as in case of AC supply voltage for starting reduced by means of ON load tap changer installed either primary or secondary side of transformer where in case of DC it is obtained by starting Resistance.

There are some drawbacks with AC locomotives such as unbalanced imposed on power supply and interference with neighboring communication lines. It can be minimized by taking suitable precautions as:

SINGLE PHASE TO DIRECT CURRENT SYSTEM (cont…)

Taking of supply for the traction substations at very high voltages (110 or 132 kV) and from a supply system having high capacity.

Balancing of traction load equally on all the three phases which is possible by connecting different traction substations across different phase in rotations.

Employing Scott-T connection to three/two phase transformer in traction substation. Replacing open aerial communication lines by lead or aluminium sheathed underground

cables and earthing the sheathing of cables at regular intervals. Provide isolations to transformers and keep voltage upto 60V under operating conditions.

COMPARISION OF AC 1- PHASE, 3- PHASE SYSTEM & COMPOSITE SYSTEM

Single phase AC system Three phase AC system Composite system

Single phase AC supply is required

Three phase AC supply is required

Transformation is required

High starting torque Less starting torque Less starting torque

AC Series motor is used 3 phase Asynchronous motor is used

First use of DC motor and then use of 3 phase Induction Motor

2500-5000 V from Generating station to Receiving station

3000-3600 V from Generating station to Receiving station

15000 V for single phase to three phase conversion

High efficiency Less efficiency Moderate efficiency

It is used for small (50-80 km) distances

for hilly areas for long distances

REFERENCE

www.Wikipedia.com/three-phase-traction-system www.electronics-hub.com/electrical-traction www.elprocus.com/what-is-an-electric-locomotive-systems-and-their-types-in-india

CONCLUSION According to the AC electrification system, we are come to know that AC motors are good

and efficient to handle the constant speed at longer distance, also according to previous methods 1 phase AC Induction motors are efficient for the traction for AC Locomotives.

THANK YOU FOR LISTENING