By Deepu Dharmarajan
Posted 3 weeks ago

Neutral Zone and Its impact for Signalling and Rollingstock

Rail Electrification

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Neutral Zone and Its impact for Signalling and Rollingstock

  1. General

AC electrified railway system is one in which single phase electrical energy is supplied to trains by means of an Overhead Line Equipment (OLE) system, comprising of a contact wire supported by droppers from a catenary wire and associated support and registration equipment. Depots commonly incorporate a trolley wire system, consisting of a single auto-tensioned contact wire without catenary. Lets consider 25kV AC traction system for the purpose of this study. The system is energised at a nominal system voltage of 25kV 50Hz AC 132/25kV AC TRACTION POWER SUPPLY SYSTEM.

On the 25kV side, each transformer could be connected to a double pole 25kV isolating switch / earth switch, from which one conductor is connected by 25kV cable to the adjacent 25kV Feeder Station and onward to the 25kV AC OLE. The other terminal could be connected via the 25kV Feeder Station return current busbar to the track running rails and return conductors (where installed) to create a multiple earthed system.

Under normal operating conditions the voltage on the low voltage side may rise to 27.5kV phase to earth, this being an equivalent voltage to that of a 48kV phase to phase three phase system.

To separate the electrical phases at each of the Feeder Stations, neutral sections are installed in the OLE. The pantograph traverses the neutral section in the power off state.

Sectioning of the OLE is achieved by Track Sectioning Cabins (TSC), insulated overlaps and section insulators.

Current collection by trains is obtained by means of a pantograph mounted on the roof of the rolling stock. The pantograph head runs on the underside of the contact wire to achieve a smooth, arc* free current collection.

Note: *- Flow of current through an air gap between a contact strip and a contact wire usually indicated by the emission of intense light and heat.

Traction current drawn from the overhead contact/catenary wires is returned to the supply point through the traction rails and return conductors.

The following are some of the traction return systems utilised:

  • Booster transformer with return conductors;
  • Return conductor only (booster-less); and
  • Rail/earth wire return.

An earth wire electrically connects each overhead line structure. The earth wire is connected to the traction rail at prescribed intervals. Where structures cannot be conveniently connected to the earth wire each structure is connected directly to the traction rail by means of a traction bond.

2. Definition of Neutral Section

Section of a contact line provided with a sectioning point at each end, to prevent successive electrical sections with a differing phase, being connected directly together by the passage of current collectors. The neutral section is a dead zone and therefore, the locomotive has to negotiate the section in momentum. In nutshell neutral zones are made to achieve the isolation between different power sources and to minimise the synchronisation task in between individual power system.

Figure 1: Neutral Section

The locomotive is switched off while negotiating the neutral section to avoid flash over at the time of exit and re-entering the live zone. For this track magnets can be utilized to switch off and ON for unattended operations. Some railways use warning board to enable driver to switch off and on.

The locomotive negotiates the neutral section in its own momentum. Therefore, the location is always chosen so that the physical terrain should not cause in convenience in the momentum of the train.

OLE designers ensure neutral zones are placed away from stopping signal and level crossing and automated trains ensure trains are not stopped in neutral zone, and train can coast through the neutral section. Design also ensure neutral zones are placed in the up gradient but on a flat or down gradient and away from sharp curve as it will not provide sufficient straight length to accommodate the neutral section. 

3. Neutral Section Detection

 When the train run into neutral section where there is no high voltage power supply is available without any precaution, a sudden disconnection of high voltage power supply may disturb the incoming power system and as well as the equipments of rolling stock. A detection method is required to detect the neutral section before entering it, to smoothly negotiate by managing various loads. So, in order to protect the train from the undesired arc between Pantograph and Over Head Catenary line, it is desired that when the train passes through the neutral section the traction power is automatically cut-off via VCB before entering the neutral section and is automatically connected back after passing the neutral section as stated before. This requirement is achieved by the Neutral Section Detection System.

The Neutral Section Detection System can detect the marking of the neutral section and can inform onboard Train Control and Monitoring System (TCMS) to minimise the traction load and to open the vacuum Circuit Breaker(VCB). This system can also control the VCB in case of absence of TCMS.

To satisfy the power control requirement when passing over the neutral zone, there are two components required. First component is to be situated on the track side which gives indication about the incoming neutral section to the train. Second component is underframe mounted train borne equipment, which receives the indication signal sent by the track side equipment.

The generic location of train borne receiver in TP car and track magnet location on the either side of the track.

This system is referred to as “Automatic Power Control (APC)” to identify the neutral section.

 4. System Composition

The APC system comprises of two components: the track side equipment (Inductors or Track side Magnets) and train borne equipment (APC Receiver or NSD Antenna). The track side equipment is a means of providing south polarity magnetic field of the required strength and pattern at the required height, location. The train borne equipment is a means of detecting the magnetic fields of the track side equipment.

4.1  Inductor (Trackside Magnet)

The inductor (Track side Magnet) is installed on the one side of the track, at both end of the neutral section. The South polarity face of the magnet (Inductor) is in sky facing position while North polarity of the magnet (Inductor) is in earth facing position. 

4.1.1 Location of APC Trackside Magnet

Automatic Power Control (APC) track side magnets shall be situated each side of neutral sections. The distance (D) from the centre of the neutral section to the APC magnet shall be calculated as follows.

4.2  Receiver (Train Mounted APC receiver)

The APC receiver is mounted below the underframe of the train, there is one receiver per each car with pantograph on the same side of the trackside magnet & train. The train mounted APC receiver is fixed to the under frame.

Whenever train passes through the desired magnetic field produced by the track magnet which are mounted on the railway track, the APC receiver detects the magnetic field and sends feedback to TCMS to control the VCB.

Refer the Figure 2 you can see APC receiver mounted under the car where pantograph is installed and Track magnets installed on the track

Figure 2: APC Receiver and Track magnet

4.3  Method of Operation

Refer to Figure 2. Red dots are the two APC receiver mounter under the car frame. APC receivers are mounted on the same car where Pantograph & auxiliary power equipment are installed. Two red rectangular boxes are the track magnet installed on track at same side as the APC receiver. One of them will be at the entry of the neutral section and other is on the exit of the neutral section.

While the train reaching the neutral section either driver will put the master handle in coasting position, or the system   will put in coasting position for an unattended train.

APC receiver is positioned in original status with South contact opened and neutral section relay in the train will be on dropped position.

When APC receiver detect the first track magnet, a signal will be sent to the activate neutral section relay, Train Control Management System detect the APC receivers high signal and send a signal to the Train Control Unit to ramp down the tractive effect. Same thing happens when it detects the second magnet. Once the train passes the neutral section area after a pre set time Train Control Management system resets APC receiver.

4.4 Interface between Rollingstock and Overhead Electrification Team

Horizontal distance between the centre of pantograph and the APC receiver is known as LAR which shall be obtained from the Rollingstock supplier to do the assessment with various speeds and scenarios to ensure train can coast through the neutral zone.

4.5 Interface between Rollingstock, Overhead Electrification& Signalling Team

All three discipline shall gather necessary information for their discipline to ensure that no train will be stuck on the neutral section with all scenarios.

 

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