Electric Traction: History and Current Collection | Electrical Engineering

In this article we will discuss about the history and current collection systems of electric traction.

History of Electric Traction:

Electric traction was first introduced in Britain in the year 1890 at 600 V dc using third rail for urban and suburban services. For its application to long distance main-line service the developments were towards increasing the operating voltage and as a result the line rail system utilising 1,200 V dc was tried in Manchester-Bury line.

This system of supply was not satisfactory due to inherent drawbacks, such as insufficient protection provided by the wooden planks and choking up of grooves by snow during winter season, and therefore, overhead contact system was adopted employing 1,500 V dc later on. This system is still used for suburban and main lines in many countries including our country.

In India electric traction was first introduced for suburban service in Bombay in 1925 and in Madras in 1931. It was later extended to Poona and Igatpuri. The system adopted was 1,500 V dc. The power used to be supplied to traction motors at 1,500 V dc from rotary converter substations. For 1,500 V supply two 750 V rotary converters, each of 2,500 kW capacity were employed in series.

Mercury-arc rectifiers were introduced for the first time for supplying dc power when Madras-Tambaram route was electrified. The rectifiers employed were multianode continuously evacuated steel tank type. Later on grid controlled mercury-arc rectifiers were used. Multianode air-cooled sealed type steel tank rectifiers having on-load tap changing arrangement for variation of voltage have also been employed. Nowadays solid state devices are finding increasing use in these dc substations.

After about 20 years, electric traction was introduced on the Howrah—Burdwan and Sheoraphuli-Tarakeshwar branch lines on 3,000 V dc, as at that time there was a general trend towards adopting of 3,000 V dc for electric traction.

The early applications of the three phase system were to Swiss mountain railways, and the first application to main­line railways (operating at 3,000 V) was in 1902. Subsequent development of the 3-φ system have been entirely for main­line electrification, and the principal applications are in Northern Italy.

The single phase ac system operating at low frequency (16 2/3 Hz) found extensive applications in Switzerland, Germany, Austria and Scandinavia for suburban as well as main-line services. In America about 800 km (on the Pennsylvania and New Haven lines) are electrified on the single phase system, but at 25 Hz frequency and 1,100 V voltage. The single phase 50 Hz system has also been adopted for the electrification of the Katanga lines in the Belgian Congo.

Kando system (single phase-three phase system) was developed in Hungary in 1932.

From the reports obtained from French National Railways on relative merits and demerits of ac and dc systems it was found that ac system employing commercial frequency of 50 Hz was advantageous both financially and operationally. In 1957 Railway Board decided to adopt 25 kV, 50 Hz single phase system of electrification for all future schemes.

This system gives considerable saving in the cost of overhead equipment, substations and locomotives. The size of the overhead system is reduced because of adoption of high voltage leading to saving in the cost of supporting structures and their foundations. On account of the high voltage, the traction substations can be spaced at longer distances.

AC substations are simpler than dc substations because they have only step- down transformers and necessary switchgear. However, ac traction systems have some drawbacks also like induced voltage in aerial signaling equipment and interference with neighbouring communication lines, and therefore, it will need some remedial devices to overcome such drawbacks.

In this system 25 kV, 50 Hz single phase system is employed for supplying power to the locomotives. The locomotive draws power from the overhead equipment (OHE) through sliding contact by means of pantograph.

The locomotive essentially consists of:

(i) Air circuit breaker, provided on the roof of the locomotive, mainly to disconnect the locomotive from hv supply in the event of fault in the equipment and to isolate the locomotive equipment at a voltage change point or phase change point in the overhead equipment such as before entry into the neutral section,

(ii) Step-down transformer to step down the supply voltage to utilization level,

(iii) On-­load tap changer for providing variable voltage

(iv) Converting machinery for converting ac into dc,

(v) Smoothing chokes for reducing the magnitude of ac component of rectified undulating dc and

(vi) Dc traction motors.

Current Collection Systems of Electric Traction:

There are mainly two systems of current collection for loco­motives, tramways or trolley buses viz.: 1. Conductor Rail System 2. Overhead System.

Current collection from overhead system is far superior to that from the conductor rail system. This is because both theoretically and experimentally current collection is more difficult from a rigid body than from an elastic one. Further the insulation of the third rail at high voltages used on single phase ac traction would also be impracticable and endanger the safety of the personnel.

1. Conductor Rail System:

The conductor rail system is employed at 600 V for suburban services on account of its relative cheapness and easier inspection and maintenance. The supporting structure does not interfere in the visibility of the signals. Also, it is not necessary to protect the rail from accidental contact by plate players etc., except at stations and in sidings. In this system, current is supplied to the electrical operated vehicle either through one rail conductor or through two rail conductors.

In case of one rail conductor the track rail is employed as the return conductor. The advantage of using insulated return rail is elimination of electrolytic action due to return currents on other public services buried in the vicinity of the railway tunnels. The rails are mounted on insulators parallel with the track rails at a distance of 0.3 to 0.4 m from the running rail with their upper most surface acting as contact surface and are fed at suitable points from substations.

Conductor rail with a side running contact, as shown in Fig. 15.2 (a), is also employed. Some railways employ an under-running contact. Latter systems are supposed to be more protective against accidental contact. The wearing of the rail conductor is due only to the friction of the collector shoes and pitting which usually occurs at starting when the current drawn is very large. The rails are designed from electrical considerations rather than mechanical.

The main considerations are:

(i) Electrical conductivity

(ii) Cost,

(iii) Wearing qualities

(iv) Contact surface available for the collector shoes and

(v) Shape and size of the conductor rail, keeping in view the type of insulators used.

A special steel alloy is used for the rails for economy reasons. The composition is such that it provides high conductivity and at the same time satisfies all other requirements. A typical composition is iron 99.63%, carbon 0.05%, manganese 0.2%, phosphorous 0.05%, silicon 0.02% and sulphur 0.05%. It has a resistance of about 12.0 x 10^-8 Ω-m at 18°C (about seven times of that of copper and about one quarter or one-fifth of that of the running rails).

To reduce the voltage drop at joints, conductor rails are bonded together by short length flexible copper conductors riveted or welded to the rails. The conductor rail is not fixed rigidly to the insulators in order to take care of the contraction and expansion of rails. To prevent creepage due to friction of the collector shoes, however, it is anchored at intervals of 100 to 150 metres.

The current is conveyed from the conductor rail to the train equipment by means of collector shoe (flat steel shoe about 20 cm in length and 7.6 cm in width) which presses on to the rail with a force of about 15 kg. In case of top contact the necessary contact force is obtained by gravity and in case of side and under- running contacts springs are employed to obtain necessary contact force. The current which can be collected is about 300 to 500 A.

Sometimes it may not be convenient to have the conductor rail on the same side of the track therefore it is desirable to provide shoes on both sides of the locomotive or train. Also, there are going to be gaps in the rails at points and crossings, at least two shoes must be provided on each side so as to avoid discontinuity in the current flow. This system is suitable for heavy current collection, top contact system for voltage up to 750 V and side contact system up to 1,200 V.

2. Overhead System:

This system is adopted when the trains are to be supplied at high voltage (1,500 volts or above). In this system with high operating voltage the trains requiring high power may be supplied through conductors of relatively small cross section and the collection of the current required by a train can be done by a collector with a sliding contact.

Overhead con­struction is universal for all ac railways and is also used with dc tramways, trolley buses and locomotives operating at voltages of 1,500 volts and above. In all these cases the running rails are utilized as the return conductor, therefore, with dc and single phase system only one overhead wire is required for each track.