GIS Substation Installation: Site Preparation to Commissioning

1. What is a GIS Substation?

A Gas Insulated Substation (GIS) uses sulphur hexafluoride (SF6) gas as the insulating medium instead of air. All high-voltage components — circuit breakers, bus bars, disconnectors, earthing switches, current transformers, voltage transformers, and cable sealing ends — are housed in hermetically sealed metal enclosures filled with SF6 gas at pressures typically between 0.3 and 0.6 MPa.

The result is an extremely compact installation footprint: a 220kV GIS bay can occupy as little as 15–20% of the space required by a conventional Air Insulated Substation (AIS) of the same voltage. This makes GIS the preferred choice for urban substations (metro rail, power utilities in cities), high-pollution environments, and locations where land is at a premium.

KVPE has executed GIS substation projects for Delhi Metro Rail Corporation (DMRC), BSES Rajdhani, and several DISCOMS across India at voltages from 33kV to 132kV.

2. GIS vs AIS: Key Differences

3. Installation Phases Overview

4. Phase 1 — Civil & Site Preparation

Civil work is the critical-path activity for GIS projects. Delays here cascade directly into the equipment installation schedule. Key civil elements include:

4.1 Foundation and GIS Hall

The GIS building must be designed specifically for GIS installation. Key requirements:

• Floor load: Minimum 15 kN/m² for GIS module transport and erection
• Crane provision: An overhead EOT crane (typically 5–10T for 33–132kV GIS) with adequate headroom for vertical module lifts
• Floor trenches: Cable trenches between GIS bays and to cable basement; trench widths per OEM GIS layout drawing
• Anchor bolt layout: GIS base frames are anchored to embedded bolts — anchor bolt layout must be cast precisely to OEM drawings. Tolerance: ±2mm
• Humidity and dust control: GIS hall must maintain <60% relative humidity. Provide HVAC from day of GIS module delivery
• Access hatches: Cable basement hatches sized for cable drum-end access and sealing end installation below GIS

4.2 Earthing Grid

The earthing system for a GIS substation requires careful design due to the high fault current interruption capability of GIS circuit breakers (up to 40–63kA). Requirements:

• Earth grid conductors: 50mm² HDGS flat or 75mm² copper conductor as per design
• GIS enclosure earthing: Each GIS module enclosure earthed at minimum two points
• Cable sheath earthing: At cable sealing ends, cable sheaths bonded to earth grid
• Earth pit resistance: Target <1Ω. Test with fall-of-potential method before backfilling

4.3 Control Room & Battery Room

The control room houses protection panels, DC distribution boards, SCADA servers, and communication equipment. The battery room (48V or 110V DC) provides backup power for protection, control, and communications during a grid outage. Temperature control (18–25°C) is mandatory for battery longevity.

5. Phase 2 — Equipment Receiving & Inspection

GIS equipment is transported in sealed modules pressurised with dry nitrogen (typically 0.02–0.05 MPa above atmosphere) to prevent moisture ingress. On delivery:

• Check transport nitrogen pressure on each module — any module showing zero pressure is suspect and must be quarantined pending OEM inspection
• Inspect for external damage: dents, cracked flanges, damaged density monitors, missing seals
• Verify desiccant condition in any modules that were opened for inspection at factory
• Check all accessories: bolts, gaskets, gas hose assemblies, earthing straps — per OEM packing list
• Raise a Non-Conformance Report (NCR) for any discrepancy before accepting delivery

6. Phase 3 — GIS Module Assembly

Assembly is performed by OEM-trained technicians or experienced GIS erection engineers. The environment during flange connections is critical:

6.1 Pre-Assembly Requirements

• Dew point inside the GIS hall: below −40°C (use desiccant dehumidifiers, running 24/7)
• Dust-free working area — no grinding or cutting during module interconnections
• All personnel to wear clean-room-grade lint-free gloves during flange work
• Tools to be cleaned and pre-inspected

6.2 Module Interconnection Sequence

1. Position GIS modules on base frames per OEM layout. Level using precision jack bolts (tolerance ±1mm)
2. Tighten anchor bolts to OEM specified torque
3. Purge nitrogen from connecting compartments — confirm with pressure gauge
4. Open flanges, inspect internal surfaces — wipe with lint-free cloth and isopropyl alcohol if required
5. Install new O-ring gaskets — never reuse gaskets from factory
6. Torque all flange bolts to OEM specification in cross-pattern sequence
7. Record compartment designation, bolt torque, gasket batch number in installation log

6.3 Cable Sealing Ends

EHV cable connections to GIS use plug-in cable sealing ends (CSE). The CSE is factory-fitted to the cable by qualified cable jointers before the GIS end connection is made. The CSE interfaces with the GIS cable connection module via a standardised conical or cylindrical plug-in interface (typically IEC 62271-209). This interface is gas-tight and must be assembled strictly per OEM procedures.

7. Phase 4 — SF6 Gas Filling

SF6 gas handling is a specialised operation requiring certified technicians and calibrated equipment. SF6 is a potent greenhouse gas (GWP of 23,500 relative to CO2) and must not be released to atmosphere.

7.1 Evacuation

1. Connect vacuum pump to GIS compartment via gas valve
2. Evacuate to <0.1 mbar (100 Pa) and hold for minimum 1 hour
3. Verify no pressure rise during hold period (indicates leak or moisture) — if pressure rises, identify and rectify before proceeding

7.2 Gas Filling

1. Connect certified SF6 gas cylinder to GIS gas valve via regulator
2. Fill slowly to rated gas density (as per OEM density-temperature chart)
3. Allow temperature equalisation for minimum 30 minutes
4. Verify final gas density on density monitor
5. Record cylinder serial number, gas quantity, final pressure, temperature in gas register

7.3 Leak Detection

Each filled compartment is tested with a calibrated SF6 gas leak detector (sensitivity: 1 ppm minimum). A full circumferential check of all flanges, gas valves, and density monitors is performed. Any leaks above 0.1% per year (measured) are rectified immediately. All results entered in the SF6 gas register, which becomes part of the official handover documentation.

8. Phase 5 — Protection & Control Wiring

Each GIS bay is controlled via a Local Control Cubicle (LCC) mounted adjacent to the GIS. The LCC contains the bay control unit, interlocking logic, and local indication. Upstream, the protection relays are housed in the control room panels.

8.1 Wiring Sequence

• Inter-panel cabling: Control room to LCC (typically 4C × 2.5mm² FRLS cables in dedicated cable trays)
• CT/PT secondary circuits: Screened cables (2C × 2.5mm² FRLS) with screen grounded at one end only
• Trip/close circuits: Segregated from monitoring circuits, routed separately to avoid common-mode interference
• Fibre optic links: OEM-specified for Bay Control Unit to SCADA communication

8.2 Protection Relay Setting Upload

Protection relay settings are prepared by the protection engineer and reviewed by the client's engineer before upload. Settings cover overcurrent protection (50/51), earth fault (50N/51N), differential protection (87), and auto-reclosure (79). Settings are uploaded via relay front port and verified on-screen before energisation.

9. Phase 6 — Testing & Commissioning

Commissioning of a GIS substation is a structured, witnessed process. All tests must be documented and signed by the commissioning engineer and client representative.

9.1 High Voltage Testing

After assembly and gas filling, GIS compartments are tested per IEC 62271-203 / IEC 62271-1:

• AC withstand test: Applied at 80% of factory test voltage for 1 minute. No flashovers or partial discharge above threshold permitted
• Partial Discharge (PD) measurement: Using UHF or acoustic PD detectors built into GIS — baseline PD reading established and recorded
• DC withstand (for cable sections): EHV cables connected to GIS tested per cable manufacturer specification

9.2 Protection Relay Injection Testing

Each protection relay is injection tested using a relay test set (e.g., Omicron CMC). Primary injection is performed where feasible — secondary injection at minimum. Results verified against relay setting sheet. Trip and alarm circuits verified to operate correctly.

9.3 Interlocking Verification

Every interlocking condition in the GIS is verified by functional testing — attempting illegal switching sequences and confirming the interlocking prevents the action. Key interlocks: Bus coupler with bus bar earthing, circuit breaker with disconnectors, earthing switch with disconnector.

9.4 Battery and DC System

Battery capacity test: Battery discharged to minimum allowable voltage at rated current — capacity verified ≥100% of rated Ah. DC distribution: voltage levels at panel busbars verified (typically 110V ±10% or 48V ±10%).

9.5 First Energisation

First energisation (or "charging") is done under controlled conditions. The sequence is: HV cable charged first from the incoming source, then GIS bus bar, then each feeder bay. All protection systems are live. The commissioning engineer, protection engineer, and client representative are present. Magnetising inrush (if a transformer is being energised) is monitored.

10. Phase 7 — Handover & Documentation

A GIS substation handover package includes:

• As-built single line diagram and panel GA drawings
• As-built cable schedules and wiring diagrams
• Factory test reports for all major equipment
• Site commissioning test reports (signed)
• SF6 gas register (compartment-wise, initial fill quantities, cylinder serial numbers)
• Protection relay settings register (all relays, version numbers)
• Operation & Maintenance (O&M) manuals for GIS, transformers, protection relays, SCADA
• Spares list and recommended spares inventory
• Training completion certificates for client O&M personnel

11. Safety Requirements

GIS substation construction and commissioning carries specific safety requirements:

• Electrical isolation: All work on energised HV equipment requires Written Permission to Work (PTW) with formal isolation and earthing — no exceptions
• SF6 hazards: SF6 itself is non-toxic but displaces oxygen — adequate ventilation mandatory in enclosed GIS rooms. In the event of an internal arc, SF6 decomposes into toxic by-products — self-contained breathing apparatus (SCBA) required for re-entry after any internal fault
• Crane operations: GIS modules can weigh 500kg to several tonnes — all lifts require a Lift Plan and trained riggers
• HV testing safety: During AC withstand testing, the test area is barricaded and only authorised personnel within the test zone. Warning signs and barriers per IS 5216
• Working at height: GIS module assembly at upper levels requires scaffolding or MEWP — safety harness mandatory