Maintenance Regulations for Mining Transformers

　　I. Scope of Application

　　This regulation applies to the periodic maintenance, fault repair, and testing operations of oil-immersed and dry-type transformers in mining scenarios. It is compiled in accordance with standards such as the Coal Mine Safety Regulations (2016 Edition) and Guide for Maintenance of Power Transformers (DL/T 573-2021).

　　II. Maintenance Cycles and Classification

　　A. Overhaul (Requiring Core/Cover Lifting)

　　Cycle: Initial overhaul within 5 years of commissioning, followed by overhauls every 5 years. Overhauls shall be conducted earlier in case of export short circuits, internal fault diagnosis, or abnormal operating conditions.

　　Content: Includes comprehensive disassembly and maintenance of the core, windings, tap changer, oil tank, and accessories, as well as insulation oil treatment and electrical testing.

　　B. Minor Repair (Without Core Lifting)

　　Cycle: At least once a year, focusing on external inspections, oil replenishment, and replacement of wearing parts.

　　Content: Bushing cleaning, oil level inspection, replacement of desiccant in the breather, maintenance of the cooling system, etc.

　　III. Preparation Before Maintenance

　　A. Data Collection

　　Collect defect records, historical maintenance reports, oil sample analysis data, and electrical test reports.

　　B. Site Survey

　　Confirm the layout of lifting equipment, power access points, and safety precautions (e.g., rainproof and fireproof facilities).

　　C. Tools and Materials

　　Specialized Tools: Vacuum oil filter, pressure oil filter, megohmmeter (2500V), DC resistance tester.

　　Consumables: Qualified insulation oil, sealing gaskets, lock bolts, desiccant (silica gel).

　　IV. Key Steps for Overhaul

　　A. Oil Drainage and Disassembly

　　Drain the transformer oil completely and flush the oil tank and accessories (do not use hot water when oil temperature ≥50℃).

　　Remove bushings, oil conservators, and explosion vents, and seal pipe interfaces to prevent foreign objects from entering.

　　B. Core Inspection

　　Core: Check the grounding status; the insulation resistance between the core and clamping piece shall be ≥10MΩ (measured with a 1000V megger). Repair areas where the insulating paint on silicon steel sheets has peeled off.

　　Windings: Evaluate the insulation grade (R60/R15 absorption ratio ≥1.3), clean oil duct impurities, and wrap damaged conductors.

　　Tap Changer: Test the DC resistance (deviation ≤2%) and inspect the contact surface finish of the contacts.

　　C. Assembly and Oil Filling

　　Reassemble according to the pre-disassembly structure, with bolt tightening torques conforming to specifications (e.g., M12 bolt torque 8.8-10.8N·m).

　　Fill with oil under vacuum to the +40℃ mark on the oil level gauge, maintaining a vacuum degree ≤133Pa for ≥4 hours.

　　D. Testing and Acceptance

　　Routine Tests: DC resistance, insulation resistance, transformation ratio, dielectric loss factor (tanδ ≤0.5%).

　　Type Tests: Power frequency withstand voltage test (for 35kV transformers, test voltage 72kV for 1 minute).

　　V. Core Content of Minor Repair

　　A. External Cleaning

　　Clean dust from the bushings and radiator surfaces, and inspect the porcelain bushings for cracks or discharge traces.

　　B. Oil Level and Quality

　　Adjust the oil level to the -20℃ mark and take an oil sample for analysis (withstand voltage ≥35kV, moisture content ≤30ppm).

　　C. Accessory Maintenance

　　Replace discolored silica gel and test the operating value of the pressure relief valve (opening pressure 55kPa±5%).

　　Check the cooling fan vibration value (≤4.5mm/s) and bearing lubrication.

　　D. Grounding and Protection

　　Verify that the(enclosure) grounding resistance is ≤4Ω and test the heavy gas action flow rate of the Buchholz relay (1.0±0.2L/min).

　　VI. Safety Specifications

　　A. Moisture Prevention Measures

　　Control the exposure time of the core: ≤16 hours when relative humidity ≤65%, ≤12 hours when humidity ≤75%.

　　B. Fire and Explosion Prevention

　　Equip the maintenance area with dry powder fire extinguishers and prohibit open flame operations; control the oil tank deformation during vacuuming to ≤15%.

　　C. Personnel Qualifications

　　Maintenance personnel shall hold high-voltage electrician certificates and be familiar with the Electrical Safety Work Regulations (for power plants and substations).

　　VII. Handling of Special Scenarios

　　A. Dampness Drying

　　Adopt the vacuum hot oil circulation method, controlling the oil temperature ≤85℃ and vacuum degree ≤13Pa.

　　B. Oil Quality Deterioration

　　Implement vacuum oil filtering until the withstand voltage reaches ≥50kV; replace the adsorbent if necessary (silica gel regeneration temperature 120℃).

　　VIII. Record Keeping and Archiving

　　Fill in the Transformer Maintenance Record Form, including maintenance date, items, replaced parts, and test data.

　　Archive materials: maintenance reports, oil sample analysis sheets, and electrical test curves, with a retention period until the next overhaul.

　　Supplementary Provisions: For matters not covered in this regulation, refer to the Coal Mine Electromechanical Equipment Integrity Standards and the equipment manufacturer's technical manuals.