The traction supply chain from grid to pantograph passes through several distinct installation types, each with a specific role in feeding and sectioning the OHE. Knowing the difference between them is a frequent AEE exam theme.
Installation Types
TSS — Traction Sub-Station
- Receives power from the state/regional grid (typically 132/220 kV) and steps it down via traction transformers to the 25 kV / 2×25 kV OHE supply.
- TSS locations are spaced along the route based on load and permissible voltage drop.
Feeding Post / SSP (Sub-Sectioning Post) / SP (Sectioning Post)
- Sectioning Post (SP) — located midway between two TSS, allows the OHE to be split into isolable sections for maintenance without de-energizing the whole feed.
- Sub-Sectioning Post (SSP) — provides finer sectioning between an SP and TSS, further limiting the section affected by an isolation or fault.
- The general principle: more sectioning points mean a smaller section is affected by any single fault or maintenance block.
Exam Focus
A frequent question pattern asks you to order TSS, SP, and SSP by function or position along a section — know that SP sits midway between TSS pairs, and SSPs sit between an SP and a TSS.
Sectioning Units, Smallest to Largest
- Elementary section — the smallest sectioning unit of the OHE, isolable manually via a section isolator.
- Larger groupings — sub-sector, sector, and feeding zone — are progressively bigger, and are often remote-controlled rather than manually operated.