Traction Substation (TSS) Works for Metro & Railway Projects

1. What is a Traction Substation?

A Traction Substation (TSS) converts utility power (typically 66kV, 132kV or 220kV grid supply) to the traction supply voltage required by trains. For metro systems in India (DMRC, Pune Metro, Chennai Metro, etc.), the typical configuration is:

• Incoming: 66kV or 132kV from the state grid or a dedicated receiving substation
• Transformation: 66kV / 25kV Scott-connected or V-connected traction transformers
• Traction supply: 25kV AC to the Overhead Equipment (OHE) for standard AC metro systems
• 3rd rail (DC): Some older metro lines (DMRC Blue Line, Yellow Line) use 750V DC 3rd rail via rectifier substations

KVPE has executed traction substation and Auxiliary Substation (ASS) works for Delhi Metro across multiple corridors, including the Janakpuri to Mukundpur and South Extension projects.

2. Types of Traction Power Systems in Indian Metro

3. Civil & Site Preparation

TSS civil works are typically in a constrained station area or on metro viaduct land. Key elements:

3.1 TSS Building

• Single or double-storey RCC structure housing transformer yard (outdoor or indoor), GIS room, control room, and battery room
• Transformer plinth: Oil-resistant finish, oil sump (volume ≥ 110% of total transformer oil) with gate valve to storage pit
• Cable basement: Below GIS/switchgear room for HV and LV cable routing
• Ventilation: Forced ventilation in transformer room (even for dry-type transformers), natural ventilation minimums for control room

3.2 Earthing and Bonding

Traction substations have a particularly complex earthing design due to the traction return current requirements. Two separate earth systems are typically maintained:

• Power earth (PE): For transformer neutrals, equipment enclosures — conventional sub-surface earth grid
• Traction earth: Rail return current bonded to traction transformer neutral — specific bonding design to avoid stray current interference with signalling

3.3 Cable Routes

HV feeder cables from the main receiving substation to the TSS are laid in underground trenches or in cable tunnel sections of the metro viaduct. All cable routes must be coordinated with metro civil and signalling teams to avoid interference with safety-critical systems.

4. Traction Transformer Installation

The traction transformer is the heart of the TSS. Typical ratings for metro TSS: 25MVA to 40MVA (per transformer), with two transformers in N+1 configuration for reliability.

4.1 Transformer Types Used

• Oil-immersed (ONAN/ONAF): Used in outdoor transformer yards. Lower cost but requires oil sump and fire-fighting provisions
• Dry-type (Cast Resin): Used in indoor/underground substations. Fire-safe, no oil, but higher cost and limited to lower ratings (typically up to 10MVA at 25kV)
• Scott connection: Converts 3-phase grid supply to two single-phase 25kV outputs (alpha and beta sections) for balanced traction loading

4.2 Unloading and Positioning

Traction transformers (25–60 tonnes) arrive by heavy-transport trailer. Unloading requires:

• Hydraulic gantry or mobile crane (50T–100T capacity depending on transformer weight)
• Steel rollers and skids for moving transformer into position within transformer bay
• Plinth levelled to ±3mm tolerance
• Bushings removed for transport (separately crated) — refitted on-site in a controlled, dry environment

4.3 Oil Processing

Oil-immersed transformers undergo oil filling on-site:

1. Vacuum-dry the transformer tank for minimum 24 hours
2. Fill with filtered, degassed transformer oil under vacuum
3. Oil sample taken for dielectric strength test (minimum 60kV/2.5mm per IS 1866)
4. Buchholz relay, OTI, WTI, PRD installed and calibrated
5. Transformer heated to 60–65°C and oil topped up (thermal expansion accounted for)

5. HV Switchgear (66kV GIS/AIS)

The 66kV (or 132kV) incoming switchgear in a TSS is typically GIS (Gas Insulated Switchgear) due to space constraints in metro stations. A typical TSS 66kV switchgear bay includes:

• Incoming feeder from receiving substation (busbar coupler if dual bus)
• Bus bar protection CT
• Transformer feeder bay with circuit breaker, CTs, VTs, earthing switch
• Bus section coupler (for double busbar configurations)

Installation follows the GIS assembly and SF6 gas filling methodology described in the GIS Substation Installation Guide.

6. LV Equipment & DC Systems

The 25kV traction output is fed to the Overhead Equipment (OHE) or 3rd rail via:

• 25kV feeder panels: 25kV vacuum circuit breakers for each OHE section, with overcurrent and earth fault protection
• Neutral section management: Electronic controls to manage the neutral section between adjacent TSS feeding zones
• Auxiliary supply: Station auxiliary transformer (typically 33kV/415V or 11kV/415V) for station loads, escalators, lifts, ventilation
• DC battery system: 110V DC VRLA battery (typically 200–400 Ah) for protection and control backup

7. Protection, Relay & SCADA Integration

Traction substation protection is more complex than conventional power substations due to the dynamic, single-phase loading from train movements. Key protection elements:

7.1 Transformer Protection

• Differential protection (87T) — primary defence against internal faults
• Overcurrent and earth fault (50/51, 50N/51N) on HV and LV sides
• Buchholz relay (oil-immersed transformers) — detects internal faults via gas accumulation
• Temperature protection (OTI/WTI) with alarm and trip settings

7.2 Feeder Protection

• Distance protection (21) on 25kV feeders — detects faults anywhere along the OHE up to the remote TSS
• Overcurrent (50/51) — backup for distance protection
• Auto-reclosure (79) — restores supply automatically after transient faults

7.3 SCADA Integration

All TSS equipment is integrated into the metro's Centralised Traffic Control (CTC) or SCADA system. Communication protocols are typically IEC 61850 or DNP3. Integration requires:

• Mapping of all protection relay signals to SCADA points
• Logical device configuration in IEC 61850 Server (relay side)
• Factory Acceptance Test (FAT) at relay manufacturer for SCADA compatibility
• Site Integration Test (SIT) with metro's control room team

8. Third Rail Power Interface

For DC 750V 3rd rail systems (as used in original DMRC corridors), the TSS includes rectifier equipment:

• Rectifier transformer: Converts 33kV or 11kV AC to a lower AC voltage suitable for rectifier input (typically 600–800V AC)
• 12-pulse rectifier: Converts AC to DC 750V — 12-pulse design to minimise harmonic injection into the grid
• DC switchgear: 750V DC circuit breakers with rapid-acting protection (fast-acting fuses or solid-state breakers) for short-circuit protection
• DC feeder cables: Low-impedance, high-current cables (typically 630mm² or 1×240mm² bundled) from rectifier to 3rd rail feed points

9. Testing & Commissioning

TSS commissioning is conducted in close coordination with the metro authority's Operations & Maintenance (O&M) team. The commissioning sequence:

10. Safety in Metro Environments

Metro substation work has additional safety layers beyond standard electrical site safety:

• Metro authority PTW: All electrical isolation and energisation requires a formal Permit to Work from the metro authority's OCC (Operations Control Centre), not just the contractor's own PTW system
• Track exclusion zones: Any work within 2 metres of live tracks requires a Track Possession — obtained from OCC with train movements suspended on that section
• 3rd rail safety: 750V DC is lethal. Specific isolation procedures, rubber insulating mats, and insulated tools mandatory at all times near 3rd rail equipment
• OHE isolation: 25kV OHE must be earthed before any work under or near it — use of hot-line tools prohibited within 2 metres without earth clamps
• Station integration: Civil and station fit-out teams are working simultaneously in the same space — interface safety plans and daily coordination meetings are mandatory