Power Supply for Electric Traction Interview Questions and Answers:

1. What is the arrangement of power supply on Kanpur-Tundla section ?

Ans. 132 kV supply is taken at one point from the supply authorities and then it is distributed by the railway authorities.

2. Name the current collection systems employed for electric locomotives, tramways or trolleys.

Ans. There are mainly two systems of current collection for locomotives, tramways or trolleys viz. (i) conductor rail system and (ii) overhead system.

3. The design of overhead system is more complex as compared to conductor rail system. Why ?

Ans. Because great difficulty is experienced in ensuring that the current collector and contact wire remain in contact at a fairly even pressure.

4. Why current collection from overhead system is considered superior over conductor rail system ?

Ans. Current collection from overhead system is considered superior over conductor rail system 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.

5. What is intermediate category ?

Ans. Compound catenary construction consists of three wires, all in the same vertical plane. The upper wire is catenary wire, which is insulated from the supporting structures. From this wire another wire, known as intermediate catenary, is supported by droppers clipped to both wires. The third wire i.e., trolley wire is maintained in definite and constant tension by means of automatic tightening gear.

6. What are the functions of dc traction substations ?

Ans. The function of dc traction substations are to (i) step down the transmission voltage to the utilization level (ii) convert it into dc and (iii) sectionalizing the overhead equipment. Such substations, therefore, consist of step-down transform­ers, converting machinery, ac circuit breaker, dc circuit breaker and smoothing and metering equipment.

7. Why separate feeding systems are required for the trolley wire and the track rails ?

Ans. Separate feeding systems are required for the trolley wire and track rails in case of tramways in order to comply with the following regulations.

1. The voltage at the trolley wire is not to exceed 550 volts and that at generating stations (or sub­stations) 650 volts.
2. The trolley wire shall be divided into sections not exceeding 1.6 km in length.
3. The potential difference between any two points of the track rail return systems shall not exceed 7 volts.
4. The potential difference between any point on the rail and earth should not exceed 4 volts.

8. What for TPC stands ?

Ans. TPC stands for traction power controller.

9. How mimic diagram enables the TPC to visualize the whole of the power supply system ?

Ans. A mimic diagram depicting diagrammatically the entire OHE system as well as different substations and control posts is provided at each remote control centre to provide actual visual position of supply system to traction power controller. The mimic diagram is generally not lit up but whenever there is any change in the interruptor position at the switching post, corresponding point on the diagram gets illuminated and there is an alarm also to draw the TPC attention. The availability of the electrical supply to each and every section of OHE is monitored by the PTs at switching stations. Thus entire electrical supply system remains under the control of TPC who is fully equipped with facilities for instantaneous control of any switching operation for quick isolation of the faulty sections and restoration of power supply to the adjoining healthy sections and also for the isolation of sections for maintenance work anywhere in the system that may extend upto 250 km in every direction of the centre.

10. Why are insulated overlaps installed ?

Ans. Insulated overlaps are installed mainly to provide isolating facilities required for the operation and maintenance of OHE.

11. How insulated overlap is different from uninsulated overlap ?

Ans. Uninsulated overlap. In case OHE is made continuous, which is possible by splicing of conductors, long lengths of it would be affected whenever there is a fault or damage. It will also need long time for rectification of damage. It is, therefore, essential to divide the length of elementary section into a number of sub-elementary sections each of which is separated from its adjacent one by means of what is known as an uninsulated overlap. In an uninsulated overlap, two contact wires belonging to two adjacent sub-elementary sections are run parallel to each other for at least one span and electrically connected to each other by means of jumpers.

Insulated Overlap. Insulated overlaps are installed chiefly for providing isolating facilities required for the operation and maintenance of OHE. In case two lengths of OHE belong to two adjacent elementary sections, there will be insulated overlap span. Insulated overlaps are also provided at feeding posts, sub-sectioning posts and booster stations. Insulated overlaps are similar to uninsulated overlaps except for the following:

1. In order to avoid accidental contact between the contact wires, gap between them is kept more in case of insulated overlap.
2. Electrical continuity between the two sections is maintained by means of jumper connections in series with isolator or interruptor. With such an arrangement, a section can be made dead and work can be carried out on the other equipment away from the overlap.
3. The anchoring portion of the catenary and contact wire is separated from the rest of the portion by means of insulators provided at a distance of 2.5 m from the supports and within overlap span. The tail ends of the anchoring OHEs are connected to the other nearby equipment so as to prevent it from attaining any other potential.

12. Why neutral section is provided in the OHE of ac traction system but not in dc traction system ?

Ans. In dc traction, supply to OHE can be provided from both ends so that voltage drop can be reduced and in such a case substations operate in parallel. In case it is felt necessary, say for sake of convenience in maintenance, to have some boundary of supply between two substations, it can be had simply by means of insulated overlap. There will be no damage in this case even if pantograph bridges the OHE. But in single phase ac traction above arrangement is not permissible because adjacent substations tap different phases of the three-phase system so as to equalise load on the three phases. So momentary passing of pantograph under insulated overlap will result in short circuit between two phases of the supply system and damage the pantograph and OHE. This situation is avoided by interposing a small length of OHE, called the neutral section. It is insulated from both sides and is not connected to any source of power supply. It’s main function is to allow physically smooth and electrically sparkless passage of pantograph from one section to the other. As per Indian Railway practice, neutral section is located in the middle of two feeding posts and in front of sectioning and paralleling post.