1. Introduction

2. Role of Outdoor VTs in Oil and Gas Substations

• Metering and Billing: Accurate voltage measurement is essential for calculating energy consumption, ensuring fair billing between operators and grid providers, and monitoring power usage efficiency (e.g., in pipeline compressor stations).

• Protection Systems: VTs feed voltage signals to protective relays, which detect faults (e.g., overvoltage, undervoltage, phase imbalance) and trigger circuit breakers to isolate faulty sections, preventing equipment damage or downtime.

• Control and Automation: In modern smart substations, VTs provide real-time voltage data to SCADA (Supervisory Control and Data Acquisition) systems, enabling remote monitoring, load balancing, and predictive maintenance of pumps, compressors, and refining equipment.

• Safety Interlocks: In hazardous areas, VTs support safety systems that shut down operations if voltage anomalies indicate a risk of fire or explosion.

3. Environmental Challenges in Oil and Gas Substations

3.1 Corrosion

• Offshore platforms: Saltwater mist and high humidity accelerate metal oxidation, particularly for unprotected steel components.

• Onshore oil fields: Hydrogen sulfide (H₂S), a byproduct of oil extraction, is highly corrosive to copper, aluminum, and insulation materials.

• Refineries: Chemical vapors (e.g., sulfur dioxide, chlorine) from processing units attack metal surfaces and degrade organic insulators.

3.2 Temperature Extremes

• Desert locations: Daytime temperatures can exceed 60°C, while nighttime drops to 0°C, causing thermal expansion/contraction that stresses seals and insulation.

• Arctic operations: Temperatures as low as -40°C can freeze insulating oils or brittleness plastic components.

• Refinery yards: Proximity to furnaces or exhaust stacks creates localized high-heat zones (up to 80°C).

3.3 Mechanical Stress

• Vibration: Pumps, compressors, and drilling equipment generate continuous vibration, which can loosen connections, damage windings, or crack ceramic insulators.

• Shock: Explosions (e.g., from gas leaks) or equipment malfunctions produce shock loads, requiring robust structural design.

• Wind and weather: Offshore platforms face hurricane-force winds (up to 150 km/h) and wave impacts, while desert substations endure sandstorms that abrade surfaces.

3.4 Hazardous Atmospheres

4. Design Features of Outdoor VTs for Oil and Gas Applications

4.1 Insulation Systems

• Oil-Immersed Insulation:

• Mineral oil or synthetic esters (e.g., Midel 7131) provide excellent dielectric strength and heat dissipation.

• Sealed tanks (hermetically sealed or with expansion bellows) prevent moisture and gas ingress, critical for H₂S-prone areas.

• Additives (e.g., antioxidants) resist oil degradation under high temperatures or chemical exposure.

• SF₆ Gas Insulation:

• Sulfur hexafluoride (SF₆) is inert, non-flammable, and offers superior dielectric properties, making it ideal for compact, high-voltage (≥132kV) applications.

• Gas-tight enclosures with pressure monitoring prevent leaks, essential for explosive zones.

• Solid Insulation (Dry Type):

• Epoxy resin or silicone rubber insulators resist moisture and chemical attack, suitable for low-voltage (≤33kV) or corrosive environments.

• Avoided in high-heat areas, as solid insulators trap heat more effectively than oil or gas.

4.2 Enclosure and Material Selection

• Housings:

• Stainless steel (316L grade) is preferred for offshore or H₂S environments, as it resists pitting and crevice corrosion better than 304 stainless steel.

• Fiberglass-reinforced plastic (FRP) offers lightweight corrosion resistance for onshore desert applications but lacks the mechanical strength of metal.

• Aluminum, treated with chromate conversion coatings or powder coatings, provides a cost-effective alternative in moderate corrosion zones.

• Insulator Bodies:

• Porcelain with a glazed finish resists sand abrasion and tracking (surface discharge due to pollution) but is brittle.

• Composite insulators (silicone rubber sheds bonded to a fiberglass core) combine flexibility (vibration resistance) with hydrophobicity (water repellency), ideal for salt spray or heavy pollution.

4.3 Explosion Protection

• Flameproof Enclosures (Ex d): Designed to contain explosions and prevent flame propagation to the external atmosphere. Enclosures are made of thick-walled steel and use flame paths (narrow gaps) to cool escaping gases below ignition temperature.

• Increased Safety (Ex e): Components are sealed, and electrical connections are reinforced to prevent sparking. Used in Zone 2 (infrequent gas presence).

• Intrinsically Safe (Ex i): Secondary circuits are designed to limit energy (voltage <30V, current <300mA) to levels that cannot ignite flammables, suitable for low-power metering applications.

4.4 Thermal Management

• Heat Sinks: Aluminum fins on enclosures enhance heat dissipation in high-temperature areas.

• Heating Elements: Low-wattage heaters (thermostatically controlled) prevent oil freezing in arctic environments.

• Thermal Expansion Compensation: Bellows or diaphragms in oil-filled VTs accommodate volume changes from temperature swings, preventing tank deformation.

4.5 Vibration and Shock Resistance

• Winding Reinforcement: Windings are impregnated with epoxy or varnish and clamped tightly to prevent movement under vibration.

• Flexible Connections: Copper braids (instead of rigid busbars) between windings and terminals absorb vibration stress.

• Shock-Mounted Bases: Rubber or spring mounts isolate the VT from structural vibrations in compressor stations or drilling rigs.

5. Key Selection Criteria for Oil and Gas Substations

5.1 Voltage Rating

• Primary Voltage: Must match the substation’s system voltage (e.g., 33kV for gathering stations, 132kV for transmission links to refineries).

• Secondary Voltage: Typically 110V (phase-to-neutral) or 220V (phase-to-phase) for compatibility with metering and relay equipment.

• Overvoltage Withstand: Must tolerate temporary overvoltages (e.g., 1.5× rated voltage for 1 minute) from lightning strikes or switching operations.

5.2 Accuracy Class

• Metering: Class 0.2 or 0.5 for billing and energy management, ensuring ≤0.2% ratio error at rated voltage.

• Protection: Class 3P or 6P, prioritizing linearity during faults over absolute accuracy (e.g., ≤3% error at 10% overvoltage).

• Special Note: In HVDC links (used in offshore wind-oil grid integration), VTs must maintain accuracy across DC and harmonic components.

5.3 Pollution and Creepage Distance

• Creepage Distance: The total surface length of the insulator from high-voltage to ground. For pollution class IV (severe, e.g., offshore), creepage distance ≥25mm/kV (rated voltage).

• Shed Design: Composite insulators with deep, spaced sheds prevent water bridging and pollution accumulation.

5.4 Short-Circuit Withstand

• Short-Time Current Rating: Typically 25A for 1 second (for 110V secondary) to withstand fault currents from connected equipment.

5.5 Compliance with Industry Standards

• IEC 61869-3: Specifies requirements for outdoor VTs, including insulation levels, temperature ranges, and mechanical tests.

• API RP 540: Oil and gas industry guidelines for electrical equipment in hazardous locations, covering installation and maintenance.

• NORSOK M-001: For offshore installations (Norwegian sector), mandates corrosion resistance and reliability under extreme marine conditions.

• ATEX/IECEx: Certification for explosion protection in hazardous zones.

6. Installation and Maintenance Best Practices

6.1 Installation Considerations

• Location: Mount VTs away from direct sources of heat (exhaust stacks), vibration (compressors), or chemical spray (washdown areas). Elevate them 300mm above ground to avoid standing water.

• Mounting: Use stainless steel brackets with anti-corrosion coatings. For offshore platforms, ensure mounts are welded to the structure to resist wind loads.

• Wiring: Secondary cables should be shielded and routed in sealed conduits (e.g., PVC or stainless steel) to prevent moisture or gas ingress. Avoid sharp bends that could stress connections.

• Grounding: A dedicated low-impedance ground (≤5Ω) prevents voltage surges and ensures safety. Use copper-clad steel ground rods in corrosive soils.

6.2 Maintenance Protocols

• Visual Inspections: Quarterly checks for corrosion, insulator damage (cracks, tracking), oil leaks, or loose connections. Clean insulators with deionized water (offshore) or dry brushing (deserts) to remove salt/dust.

• Electrical Testing:

• Insulation Resistance: Annual megger tests (≥1000MΩ at 5kV) to detect moisture in windings.

• Ratio and Phase Error: Every 3–5 years to ensure metering accuracy, using portable test sets.

• Oil Analysis: For oil-immersed VTs, test for moisture (<20ppm), acidity (<0.1mg KOH/g), and dielectric strength (>30kV) annually. Replace oil if degraded.

• Explosion-Proof Checks: For Ex d enclosures, inspect flame paths for damage or corrosion annually—even minor scratches can compromise explosion containment.

• Thermal Imaging: Bi-annual scans to detect hot spots in connections, indicating loose terminals or winding faults.

7. Case Study: Offshore Platform VT Application

• Salt spray and 95% humidity (corrosion risk).

• Zone 2 hazardous area (occasional methane leaks).

• Wind speeds up to 160 km/h and vibration from subsea pumps.

• Type: Oil-immersed, hermetically sealed with 316L stainless steel tank.

• Insulators: Composite (silicone rubber) with 30mm/kV creepage distance (pollution class IV).

• Explosion Protection: Ex d enclosure (flameproof) for Zone 2 compliance.

• Accuracy: Class 0.2 (metering) and 3P (protection) in a dual-winding design.

• Thermal Features: Heaters (activated below -5°C) and expansion bellows for temperature swings (-20°C to +50°C).

• Insulation resistance >5000MΩ.

• Oil moisture <10ppm, no acidity.

• No corrosion on stainless steel components.

• Accurate measurements (±0.15% error) and reliable protection during a 33kV line fault.

8. Future Trends: Smart VTs for Digital Oilfields

• Condition Monitoring Sensors: Embedded sensors measure oil moisture, gas pressure, and winding temperature, transmitting data via 4G/LoRa to SCADA systems for predictive maintenance.

• Digital Outputs: Ethernet or IEC 61850-9-2LE (sampled values) for direct integration with digital relays, eliminating analog signal losses.

• Self-Diagnostics: Built-in logic to detect faults (e.g., insulation degradation) and send alerts, reducing reliance on manual inspections.

9. Conclusion