What is required of testing for oil-immersed transformers?

1. Oil Condition Inspection: Core Indicator of Insulation & Cooling Performance

Key Inspection Contents & Standards

• Oil Level Check:
  • Observe the oil level gauge: The oil level should match the ambient temperature (e.g., at 20°C, the oil level should be between the “20°C” upper and lower marks on the gauge).
  • Abnormality Handling: If the oil level is too low, check for oil leakage (e.g., flange seals, valve connections) and add qualified insulating oil; if too high, release excess oil to the normal range (avoid overflow).
• Oil Color Check:
  • Normal Color: Transparent light yellow (new oil) or light brown (in-service oil, no obvious turbidity).
  • Abnormality Judgment: Dark brown, black, or turbid oil indicates severe oxidation or contamination (e.g., metal particles, moisture), which reduces insulation performance—sample the oil for chromatographic analysis and replace it if necessary.
• Oil Temperature Check:
  • Focus on the top oil temperature (measured by the temperature controller): The maximum allowable temperature rise is ≤65K (e.g., if the ambient temperature is 30°C, the top oil temperature should not exceed 95°C).
  • Abnormality Handling: If the temperature rises abnormally (e.g., exceeds 100°C without overload), check the cooling system (e.g., radiator blockage, fan failure) and core/winding for faults (e.g., multi-point grounding).
• Oil Leakage Check:
  • Inspect key parts: Flange connections, oil valves, radiator joints, and bushing bases.
  • Abnormality Handling: For minor leakage, tighten the bolts or apply oil-resistant sealant; for severe leakage (e.g., tank cracks), shut down the transformer and repair by welding (using low-hydrogen electrodes).

2. Physical Shape Inspection: Prevent Internal Structural Damage

Key Inspection Contents & Standards

• Tank & Bushing Inspection:
  • Check the transformer tank for no bulging, depression, or cracks; check high/low voltage porcelain bushings for no fragmentation, chipping, or discharge marks (e.g., white powdery traces).
  • Abnormality Handling: If deformation or cracking is found, stop operation immediately—internal core/windings may be damaged (e.g., short circuit due to bushing cracking), and conduct a comprehensive internal inspection after shutdown.
• Accessory Shape Check:
  • Inspect radiators, oil conservators, and gas relays for no deformation or displacement; ensure the oil conservator’s breathing valve is unobstructed.

3. Operating Sound Inspection: Intuitive Reflection of Internal Status

Key Inspection Contents & Standards

• Normal Sound: A low, uniform “hum” (caused by magnetic flux vibration in the core), with no obvious fluctuations or abnormal noises.
• Abnormal Sound Judgment & Handling:
  • Sharp “Buzzing”: Indicates core multi-point grounding (circulation current causes core vibration) or loose core clamping—measure the core ground current and eliminate the grounding fault.
  • Crackling/Popping Sound: Indicates partial discharge (e.g., winding insulation breakdown, bushing internal discharge)—shut down immediately to avoid short circuits, and conduct an insulation resistance test.
  • Mechanical Vibration Noise: Indicates loose accessories (e.g., radiator bolts, fan brackets)—tighten the loose parts after confirming no load impact.

4. Shell, Arrester & Oil Valve Inspection: Ensure Safety & Operational Flexibility

Key Inspection Contents & Standards

• Shell Grounding Check:
  • Ensure the shell’s grounding wire is intact and tightly connected (no rust or loose bolts); measure the grounding resistance (≤4Ω).
  • Abnormality Handling: If the grounding wire is broken or resistance is too high, replace the grounding wire and re-ground to prevent electric shock risks.
• Lightning Arrester Inspection:
  • Check the arrester for no physical damage; the discharge counter should display normally (no continuous discharge records).
  • Abnormality Handling: If the arrester has discharge marks or the counter is faulty, replace the arrester to avoid overvoltage damage to the transformer.
• Oil Valve Inspection:
  • Check the oil drain valve, oil sampling valve, and radiator valve for no leakage; ensure the valves can be opened/closed smoothly (no jamming).
  • Abnormality Handling: For jammed valves, disassemble and clean them; for leaking valves, replace the valve core or seal ring.

CHH Power’s Practical Inspection Tips

• Regular Recording: Record inspection data (oil level, temperature, sound description) daily to track trends (e.g., gradual oil level drop may indicate slow leakage).
• Tool Assistance: Use a stethoscope to listen for subtle abnormal sounds, and a thermal imager to detect local overheating (e.g., bushing base temperature exceeding 100°C).