Mantenimiento preventivo de transformadores: muestreo de aceite, DGA y pruebas de resistencia de aislamiento

1. Scope & Purpose

This guide outlines preventative maintenance procedures for liquid-filled power transformers, specifically focusing on critical diagnostic tests: transformer oil sampling for Dissolved Gas Analysis (DGA), and insulation resistance testing. These procedures are applicable to distribution and power transformers ranging from 500 kVA to 50 MVA operating in industrial and utility environments. Regular execution of these tests provides insight into the transformer’s internal health, identifies nascent faults, and prevents catastrophic failures, minimizing unscheduled downtime and expensive repairs. This guide supports compliance with IEEE C57 series standards, NETA ATS/MTS specifications, and NFPA 70B recommended practices for electrical equipment maintenance.

2. Safety Precautions

WARNING: HIGH VOLTAGE AND STORED ENERGY HAZARD. Improper procedures can result in severe injury, arc flash, electrocution, or death. Adhere strictly to all site-specific lockout/tagout (LOTO) protocols as per OSHA 29 CFR 1910.147 and NFPA 70E standards. Ensure all sources of electrical energy to the transformer are de-energized, tested, and locked/tagged out before commencing work. Verify zero energy state using an appropriately rated voltage detector. Confirm the absence of hazardous stored energy in capacitor banks or other components. Allow sufficient time for transformer core de-magnetization. Verify proper grounding of the transformer before work begins.

WARNING: HOT OIL AND CHEMICAL HAZARD. Transformer insulating oil can be hot, causing burns. Used oil may contain PCBs or other hazardous contaminants. Wear appropriate personal protective equipment (PPE) as specified below. Handle oil samples with care. Dispose of all contaminated materials and oil per environmental regulations.

Personal Protective Equipment (PPE) Mandatory:

• Arc Flash Suit (Minimum CAT 2, as determined by site-specific arc flash analysis, NFPA 70E compliant)
• Insulated Rubber Gloves (Rated for System Voltage + Leather Protectors, ASTM D120)
• Safety Glasses or Face Shield (ANSI Z87.1)
• Hard Hat (ANSI Z89.1)
• Steel-Toe, Electrical Hazard (EH) Rated Boots (ASTM F2413)
• Flame-Resistant (FR) Clothing (OSHA 29 CFR 1910.269)
• Chemical-Resistant Gloves (for oil handling)

3. Tools & Materials Required

4. Pre-Maintenance Inspection Checklist

5. Step-by-Step Procedure

5.1 Transformer De-Energization and LOTO

1. Verify Operational Status: Confirm the transformer is currently in service or scheduled for outage.

2. Isolate All Energy Sources: Initiate site-specific LOTO procedure. Open all primary and secondary breakers, disconnect switches, and any auxiliary power feeds to the transformer. Common mistake: Forgetting auxiliary power feeds.

3. Verify Zero Energy: Using a rated voltage detector, test all primary and secondary terminals for absence of voltage. Test the voltage detector before and after use on a known live source. Common mistake: Not testing the voltage detector.

4. Ground the Transformer: Apply approved temporary grounding equipment to all primary and secondary windings. Ensure grounding cables are securely clamped to the transformer terminals and to the station ground grid. Visual indicator: Secure, tight ground connections with no visible gaps.

5. Apply Locks and Tags: Place personal LOTO devices on all isolating points. Complete LOTO tags with date, time, name, and reason for outage.

6. Allow for De-magnetization: Wait a minimum of 15 minutes after de-energization before proceeding with oil sampling or insulation resistance tests to allow the transformer core to de-magnetize and residual charges to dissipate. This is critical for accurate insulation resistance readings. Common mistake: Rushing the de-magnetization process.

5.2 Transformer Oil Sampling for DGA

1. Prepare Sample Bottles: Unwrap the two 1L amber glass DGA sample bottles. Inspect for cleanliness and ensure seals are intact. Mark bottles clearly with transformer ID, date, time, and technician name. Ensure a plastic 1L bottle is also ready for general oil quality tests.

2. Locate Sample Valve: Identify the sampling valve, typically located near the bottom of the transformer tank or on a radiator connection. Clean the area around the valve thoroughly with a clean, lint-free rag and industrial degreaser. Allow to dry completely. Visual indicator: Valve area is free of dust, dirt, and oil residue.

3. Attach Drain Hose: Thread a clean, oil-resistant PVC hose onto the sampling valve. Direct the other end into a waste oil container. Ensure a snug fit to prevent leaks.

4. Flush the Valve and Hose: Slowly open the sampling valve and allow approximately 1 to 2 liters (0.25 to 0.5 gallons) of transformer oil to flush through the hose into the waste container. This removes stagnant oil and any contaminants from the valve and hose. Visual indicator: Flushing oil appears clear and free of sediment or air bubbles.

5. Collect DGA Sample (Glass Bottle 1): Partially close the valve to achieve a slow, steady stream. Immediately immerse the end of the hose to the bottom of the first amber glass bottle. Fill the bottle slowly, ensuring no air bubbles are trapped during filling. Overfill slightly, then slowly withdraw the hose, allowing excess oil to spill. Immediately cap and seal the bottle tightly to prevent air ingress. Common mistake: Trapping air bubbles in the sample. Air bubbles introduce atmospheric gases, corrupting DGA results.

6. Collect DGA Sample (Glass Bottle 2): Repeat step 5 to collect a second DGA sample in the second amber glass bottle. This provides a backup in case the first sample is compromised or requires retesting.

7. Collect General Oil Sample (Plastic Bottle): Use the plastic bottle for other routine oil tests (e.g., dielectric strength, moisture, acidity). This sample does not require the same air exclusion rigor as DGA. Fill to the shoulder of the bottle.

8. Close Valve and Clean Up: Close the sampling valve tightly. Remove the hose. Clean any spilled oil from the transformer and surrounding area. Use the torque wrench to ensure the sampling valve cap is tightened to 20-25 Nm (15-18 ft-lbs). Visual indicator: No oil drips from the sampling valve.

9. Package and Ship Samples: Securely package all samples in appropriate shipping containers. Complete all required lab submission forms. Ship samples immediately to an accredited laboratory for analysis. Expedited shipping is recommended for DGA samples to maintain sample integrity.

5.3 Insulation Resistance Testing (Megger Test)

1. Verify LOTO and Grounding: Re-confirm that the transformer is de-energized, locked out, tagged out, and properly grounded as per Section 5.1. Critical: Do not proceed if LOTO or grounding is compromised.

2. Record Ambient Temperature: Use the infrared thermometer to measure and record the ambient air temperature and the surface temperature of the transformer tank. Insulation resistance values are temperature-dependent and require correction to a standard temperature (e.g., 20°C/68°F) for accurate trending. Common mistake: Not recording temperature, leading to misinterpretation of results.

3. Prepare Transformer for Test: Disconnect all external primary and secondary connections from the transformer bushings. Ensure the transformer windings are completely isolated. Clean bushing surfaces with a lint-free rag to remove any surface contamination that could affect readings.

4. Connect Megohmmeter: Connect the megohmmeter leads as follows:

   • Test 1 (High Voltage Winding to Ground): Connect the ‘LINE’ (H.V.) terminal of the megohmmeter to one of the primary (H.V.) bushings. Connect the ‘EARTH’ (Ground) terminal of the megohmmeter to the transformer tank ground connection. If applicable, connect the ‘GUARD’ terminal to the other H.V. bushings to eliminate surface leakage currents.
   • Test 2 (Low Voltage Winding to Ground): Connect the ‘LINE’ (H.V.) terminal to one of the secondary (L.V.) bushings. Connect the ‘EARTH’ terminal to the transformer tank ground connection. If applicable, connect the ‘GUARD’ terminal to the other L.V. bushings.
   • Test 3 (High Voltage Winding to Low Voltage Winding): Connect the ‘LINE’ (H.V.) terminal to one of the primary (H.V.) bushings. Connect the ‘EARTH’ terminal to one of the secondary (L.V.) bushings. If applicable, connect the ‘GUARD’ terminal to any remaining disconnected windings.

   Ensure all connections are clean, tight, and secure. Visual indicator: Leads are firmly attached and not loose.

5. Perform Insulation Resistance Test (1-Minute Reading): Select the appropriate test voltage for the transformer’s insulation class (e.g., 500V for L.V. systems, 1000V or 2500V for M.V. systems, 5000V for H.V. systems). Apply the test voltage for exactly one minute. Record the insulation resistance reading in megaohms (MΩ) at the 1-minute mark. Common mistake: Not applying voltage for the full minute, resulting in unstable readings.

6. Perform Dielectric Absorption Ratio (DAR) Test: After the 1-minute reading, continue applying the test voltage for another nine minutes (total of 10 minutes). Record the insulation resistance reading at the 10-minute mark. The Dielectric Absorption Ratio (DAR) is calculated as the 10-minute reading divided by the 1-minute reading (IR_10min / IR_1min). A DAR value ≥ 1.3 for oil-filled transformers is generally considered acceptable, with higher values indicating drier insulation. If the insulation system is significantly capacitive, a Polarization Index (PI) test (10-minute reading divided by 1-minute reading) should be considered, extending the test duration. Reference: IEEE Std 43-2013.

7. Discharge Windings: After each test, allow the transformer windings to discharge fully through the megohmmeter’s internal discharge circuit. This may take several minutes for larger transformers. Verify zero voltage on the windings using the multimeter before touching any terminals. Critical: Failure to discharge can lead to severe shock.

8. Record Results and Restore: Document all insulation resistance readings, DAR values, test voltage, and ambient/transformer temperatures. Disconnect megohmmeter leads. Reconnect all transformer external connections, ensuring appropriate torque values for all bolted connections (refer to OEM specifications, typically 40-50 Nm / 30-37 ft-lbs for main terminals). Remove temporary grounding equipment and LOTO devices per site procedure. Restore transformer to service as per operational protocols.

6. Post-Maintenance Verification Checklist

7. Troubleshooting Guide

8. Recommended Maintenance Schedule

9. Spare Parts Reference

For a complete selection of transformer spare parts and related components, consult the UNITEC-D E-Catalog.

10. References

• IEEE Std C57.106-2015: IEEE Guide for Acceptance and Maintenance of Insulating Oil in Equipment.
• IEEE Std 43-2013: IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machinery.
• NFPA 70E: Standard for Electrical Safety in the Workplace.
• NFPA 70B: Recommended Practice for Electrical Equipment Maintenance.
• OSHA 29 CFR 1910.147: The Control of Hazardous Energy (Lockout/Tagout).
• NETA ATS-2021: Standard for Acceptance Testing Specifications for Electrical Power Equipment and Systems.
• NETA MTS-2023: Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems.
• Transformer OEM Documentation (refer to specific manufacturer manuals for detailed torque specifications and procedures).