Electrical power is generated in three phases with each phase having a phase shift of 1200. The utility power supply system also expects equal loading on all phases. Traction power system works on single phase 25kV traction system. It now becomes important to use all the three phases equally and convert them into one phase traction supply. This is achieved by feeding single phase supply to the adjacent traction substation which is separated by a neutral section. Each feed covers a distance of about 30-40Km feeding all the trains in that zone. By the theory of probability, equal numbers of trains in feed zone are expected, drawing same load current. In this way un-balancing on utility power supply system is avoided
Neutral Section is defined as short insulated dead overhead equipment separating sectors fed by two adjacent substations which are normally connected to different phases.
The neutral section is a dead zone and therefore, the locomotive has to negotiate the section in momentum. 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, warning board at 500M, 250M and final board for switching OFF and ON are provided. The assistant loco pilot speak out the approaching neutral section loudly to alert the loco pilot. Loco pilot also acknowledges the voice by repeating the approaching neutral section and is in readiness to act so that he should not enter the neutral section with DJ/VCB in ON condition.
Location of Neutral Section
The locomotive negotiates the neutral section in its own momentum. Therefore the location is always so chosen that the physical terrain should not cause inconvenience in the momentum of the train. These are (1) Away from stopping signal and level crossing (2) Away from up gradient preferably on flat or down gradient (3) Away from from sharp curve as it will not provide sufficient straight length to accommodate the neutral section.
Types of Neutral section
There are three types of neutral section namely (1) Conventional Type (2) Short Neutral Section – Section Insulator and PTFE type (3) Dynamic Neutral Section or ASNS type
The arrangement consists of five spans of overhead equipment in which the central span has to be strictly of 49.5 m length. This span gives an effective neutral section of 41 m length by proper selection of staggers. Neutral section of 41 m has been designed for three locomotives attached in the front of a train.Dead zone is created by physical separation of dead zone (thick green line) with the live zone of yellow and blue phase . The dead zone provides mechanical continuity for the pantograph movement to support the upward force. The dead zone is charged by the pantograph of the leading locomotive with yellow phase during entering the neutral zone and by blue zone at the time of exiting the neutral section. Length of the neutral section is chosen as 41 m so that accidentally the dead zone is not charged with Y and B phase simultaneously. The advantage of the system is its maintenance free but the main disadvantage is possibility of a locomotive getting struck in the dead zone due to alarm chain pulling or bad driving of loco pilot. This will require a assisted loco be sent to pull the train.
Short Neutral Section
A short neutral section is of 5 m in which the dead zone is electrically insulated by section insulator with mechanical continuity. With section insulators the speed is restricted to 100 Kmph if the runners of the section insulators are in trailing direction or 70 Kmph otherwise. Short neutral section of ceramic beaded resin bonded glass fiber rod insulators are used instead of air gap. These neutral sections are fit for speed up to 200 Kmph.
Dynamic Neutral Section or ASNS type: To be continued