PSI Technical Fundamentals — Traction Distribution Notes

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Traction substations step down incoming EHV supply (220/132/110/166 kV) to 25kV for feeding OHE, using traction transformers most commonly rated 13.5 MVA (ONAN) or 10.8 MVA (ONAN) — though the real RDSO reference schematic (Drg. No. ETI/PSI/702) shows figures like 132/25kV, 13.5/21.16 MVA in practice.

Voltage Regulation — Real Limits

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Exam Focus — The Real Bus-Bar Voltage Window

Permissible bus-bar voltage variation at grid substations is +10% and -5% — i.e. between 27,500V and 23,750V. Transformer tappings are set to keep voltage as HIGH as possible without exceeding 27.5kV at the feeding post at any time. This asymmetric window (+10/-5, not a symmetric ±) is a genuinely easy detail to get backwards.

25kV Supply at Traction Substations

On the secondary side: one transformer circuit breaker and one feeder circuit breaker, with associated double-pole isolators, arranged so full operational flexibility is ensured.
The traction substation is designed for REMOTE operation, with feeder change-over facility via isolator/bus coupler.
One end of the transformer's secondary winding is solidly earthed at the substation, connected to track/return feeder through the buried rail.

Feeding & Sectioning Arrangements

Since single-phase traction load causes unbalance on the Supply Authority's 3-phase system, power for adjacent substations is tapped off across DIFFERENT phases of the grid in cyclic order — which is exactly why a Neutral Section must exist between substations, ensuring a pantograph never bridges two different phases.

Voltage Unbalance Type	Permissible Limit
Instantaneous	5%
2 hours	3%
Continuous	2%

Sectioning Structure — Real Definitions

The Real Hierarchy: Sector → Sub-Sector → Elementary Section

OHE is sectioned at intervals of 10 to 15 km along the route for rapid fault isolation and maintenance — at each such point, switching station interruptors (usually rated 600A) are provided.
The shortest section isolable by opening interruptors alone is called a 'sub-sector.' Each sub-sector further divides into smaller 'elementary sections' via off-load manually-operated isolator switches.
Feeding Post (FP): each feeder supplies OHE on one side through interruptors controlling individual lines — a two-track line has FOUR interruptors at each Feeding Post.
Sectioning & Paralleling Post (SP): situated roughly midway between Feeding Posts, marking the demarcation between zones fed from different substation phases — this is where the Neutral Section actually sits.

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Exam Focus — Why Neutral Section Warning Boards Exist

Since the Neutral Section always remains 'dead,' warning boards are placed in advance specifically to remind the driver to open the loco circuit breaker (DJ) BEFORE approaching it, coast through, and only switch it back to 'close' on the other side. Special care is taken to locate neutral sections on level tangent track, well away from signals and level-crossing gates — placing one at a bad location could strand a train mid-neutral-section unable to draw power.

Power Supply for Signalling

To ensure reliable 240V AC for signalling, a 25kV/240V auxiliary transformer (tapping the 25kV OHE) is provided at: every wayside station (for CLS), level crossings more than 2km from a railway station, at IBH (Intermediate Block Huts), and at all power supply installations. If power block affects both OHE sub-sectors feeding the signal supply, colour-light signals go out of operation — but since this is rare and typically under an hour, no additional Electrical Department arrangement is made; instead, S&T keeps portable generator sets ready, with standby diesel generators at large stations with heavy shunting.

Since traction load fluctuation affects the 240V signalling supply, S&T provides static-type voltage regulators to limit voltage fluctuations to ±5% — installed in separate kiosks, inside the remote control cubicle, ASM's room, or cabins depending on load-centre position.