How to deal with high temperature of oil-immersed transformer?

1. Economical Operation of Oil-Immersed Transformers: Based on Loss Characteristics

(1) Core Losses vs. Copper Losses

• Iron Loss (Core Loss): Caused by alternating magnetic flux in the iron core (hysteresis loss + eddy current loss). It is basically constant during operation, regardless of load changes (only related to input voltage and frequency).
• Copper Loss (Winding Loss): Caused by current flowing through the winding conductor (I²R loss). It changes with load—the higher the load current, the greater the copper loss.

(2) Most Economical Operating State

• Iron loss = Copper loss (this state is called the “economic load rate,” typically 50–70% of the rated load).
• At this point, total loss (iron loss + copper loss) is minimized, and the transformer’s temperature rise stays within the normal range (≤65K for top oil temperature, per IEC standards)—avoiding overheating and extending service life.

(3) CHH Power’s Optimization Suggestion

• Avoid long-term low-load operation (e.g., <30% rated load), which wastes energy due to high iron loss ratios.
• Avoid long-term overload operation (e.g., >120% rated load), which causes excessive copper loss and overheating.

2. Key Fault Diagnosis & Handling for Oil-Immersed Transformers

(1) Fault 1: Poor Contact of Taps/Switches

• Diagnosis Method:
  1. Gas Relay Frequent Action: Poor contact causes local arcing, generating gas that triggers the gas relay (light gas alarm).
  2. Winding DC Resistance Measurement: Use a DC resistance tester to measure the resistance of each tap position. An abnormal resistance value (e.g., higher than the standard unbalance rate of ≤2%) indicates poor contact.
• CHH Power’s Handling Process:
  1. Power off the transformer and drain the insulating oil to expose the tap switch.
  2. Clean the contact surface (remove oxide or dirt) with fine sandpaper, then apply a thin layer of conductive grease (compatible with transformer oil) to reduce contact resistance.
  3. Reassemble, inject qualified insulating oil, and re-measure DC resistance to confirm the unbalance rate meets standards.

(2) Fault 2: Inter-Coil Short Circuit

• Diagnosis Method:
  1. Abnormal Internal Sound: A sharp “buzzing” or “crackling” sound (instead of the normal low hum) indicates arcing from short circuits.
  2. Gas Relay Frequent Action: Short circuits generate high temperatures, decomposing oil into gas (e.g., hydrogen) and triggering alarms.
  3. Bridge Measurement of Winding DC Resistance: A significant drop in DC resistance (e.g., >5% compared to historical data) confirms inter-coil short circuits.
• CHH Power’s Handling Process:
  1. Mild Short Circuit (Few Turns Affected):
     • Open the tank, lift the winding, and reprocess the insulation between the shorted coils (use high-temperature epoxy resin to repair insulation layers).
  2. Severe Short Circuit (Multiple Turns Damaged):
     • Replace the entire winding with CHH Power’s factory-manufactured windings (using the same material: oxygen-free copper wire + Class H insulation paper).
  3. Post-Repair Verification: Perform a low-voltage impulse test to confirm no residual short circuits, then inject oil and test dielectric strength.

(3) Fault 3: Short Circuit Between Core Silicon Steel Plates

• Diagnosis Method:
  1. Abnormal Sound: A dull “humming” sound (caused by eddy current increase in the core).
  2. Insulation Resistance Drop: Measure the core-to-ground insulation resistance with a 2500V megohmmeter—values <100MΩ indicate possible short circuits.
  3. Oil & Gas Analysis: Short circuits cause local overheating, increasing dissolved gas in oil (e.g., methane, ethylene).
  4. Air Vibration Test: Tap the core lightly; uneven vibration suggests silicon steel sheet adhesion (short circuit).
• CHH Power’s Handling Process:
  1. Drain oil and open the tank to expose the core.
  2. Use a thin blade to separate the shorted silicon steel sheets, then apply a layer of core insulation paint (compatible with high temperatures) to restore insulation between sheets.
  3. Reassemble the core, retighten clamping devices, and re-measure core insulation resistance to ensure it meets standards.

3. Handling of Oil Shortage & Cooling System Faults

(1) Oil Shortage Handling

• Problem Impact: Insufficient oil level exposes the winding to air, reducing insulation performance and increasing the risk of partial discharge.
• CHH Power’s Solution:
  1. Confirm the oil level (via the oil level gauge) and check for leaks (e.g., flange seals, valve connections) to avoid re-shortage.
  2. Add qualified insulating oil (same brand and type as the original oil) that has passed the dielectric strength test (≥35kV/2.5mm) and moisture test (≤15ppm).
  3. Add oil to the “normal” oil level mark (corresponding to ambient temperature), then run the transformer at 50% load for 2 hours to ensure oil circulation.

(2) Cooling System Fault Handling

• Additional Note: For indoor oil-immersed transformers, ensure the inlet/outlet of the transformer room is unobstructed. Clean accumulated dust monthly to maintain air circulation—blocked vents can increase indoor temperature by 5–10°C, indirectly raising oil temperature.