Dielectric Loss Testing for Instrument Transformers and Bushings: Small Oil Volume, Big Challenges

Most technical literature on dielectric loss testing focuses on power transformers, which hold thousands of liters of oil. Yet instrument transformers, bushings, and tap changers also contain insulating oil—often less than 50 liters—and their oil quality is equally critical. Testing these small-volume devices with an insulating oil dielectric loss tester presents unique challenges that standard procedures do not address.

Why Small-Volume Equipment Deserves Attention

Current transformers (CTs), voltage transformers (VTs), and oil-filled bushings are essential for protection and measurement. Their oil volumes range from 5 to 50 liters, compared to 5,000 to 50,000 liters for power transformers. Because of the small volume, a fixed amount of contamination has a proportionally larger effect. A few grams of moisture that would be negligible in a large transformer can saturate the oil in a bushing. Additionally, these devices often operate at high electrical stress relative to their size, accelerating oil degradation.

The Sampling Problem

Small-volume equipment presents severe sampling difficulties:

• Limited access: Many CTs and bushings have no drain valve. Sampling requires disassembly or syringe extraction through a fill port.
• Minimal sample volume: A bushing may hold only 5 liters. Drawing 2 liters for flushing leaves insufficient oil for continued operation.
• Risk of air ingress: Removing oil creates a vacuum that draws in moist air. This can damage the device.
• Non-representative samples: Oil at the top of a bushing may differ significantly from oil at the bottom.

For these reasons, many utilities avoid oil testing on small equipment entirely—a practice that leads to surprise failures. An insulating oil dielectric loss tester requiring only 40 mL of sample enables testing where larger-volume methods cannot be used.

Recommended Sampling Approach for Small Equipment

When testing CTs, VTs, or bushings, follow these modified procedures:

1. Use a gas-tight glass syringe (50-100 mL) with a luer lock fitting.
2. Flush the sampling port with 20-30 mL, discarding the flush.
3. Draw the test sample slowly to avoid air entrainment.
4. Cap the syringe immediately and test within 4 hours.
5. If the device has a drain valve, use gravity flow but limit flushing to 500 mL.
6. Record the oil temperature at sampling for reference.

For critical bushings, consider installing a permanent sampling port during the next outage to simplify future testing.

Interpretation Limits for Small-Volume Equipment

Small oil volumes degrade faster and tolerate less contamination. Apply stricter limits than for power transformers:

• New oil in CT/VT/bushing: tan δ ≤ 0.001 at 90°C.
• Acceptable service condition: tan δ ≤ 0.005.
• Caution: tan δ 0.005-0.010. Plan replacement at next outage.
• Critical: tan δ > 0.010. Replace oil or replace device.

Note that for some older bushings, oil replacement is impractical or impossible. In these cases, tan δ trending becomes the primary indicator for scheduling replacement of the entire bushing.

Moisture: The Dominant Failure Mechanism

In small-volume equipment, moisture ingress is the leading cause of elevated tan δ. Sources include:

• Degraded gaskets and seals.
• Thermal breathing cycles drawing in humid air.
• Cellulose insulation decomposition releasing water.
• Incomplete drying during manufacturing or repair.

For CTs and bushings, moisture content above 20 ppm is a serious concern, and above 40 ppm indicates imminent risk of dielectric breakdown. Karl Fischer titration should accompany every dielectric loss measurement. An insulating oil dielectric loss tester with resistivity measurement adds value here—low resistivity combined with high tan δ strongly indicates moisture.

Case Example: Bushing Failure Prevention

A 145 kV oil-filled bushing showed tan δ of 0.014 at 90°C with moisture of 45 ppm. The utility had no historical data for this bushing. Comparison with sister bushings on the same circuit showed tan δ of 0.003 and moisture of 12 ppm. The anomalous bushing was replaced during a scheduled outage. Post-mortem inspection revealed degraded gaskets and paper insulation moisture penetration. Without the dielectric loss test, the bushing would likely have failed in service within 12-18 months.

Special Considerations for Tap Changers

Load tap changer (LTC) oil is separate from main tank oil and requires independent testing. LTC oil is subject to arcing during tap changes, generating carbon particles and acids. Tan δ in LTC oil rises rapidly after switching operations. Test LTC oil every 6 months or after 5,000 operations. Limits are more permissive than main tank oil because arcing by-products are expected, but rising trends still indicate the need for oil change or contact inspection.

Testing Logistics for Field Teams

When planning small-equipment testing with a portable insulating oil dielectric loss tester, account for:

• Sample volume: Carry syringes and small containers suitable for 40-100 mL samples.
• Access equipment: Ladders, platforms, or bucket trucks may be needed for bushings.
• Time per device: Allow 30-45 minutes per CT/VT/bushing including sampling and documentation.
• Batch testing: Collect samples from multiple devices and test them together to improve efficiency.
• Data management: Record device serial number, oil volume, and last oil change date for trending.

Conclusion: Do Not Neglect the Small Players

Instrument transformers and bushings fail less frequently than power transformers, but when they fail, the consequences are severe—protection loss, flashover, and collateral damage. Testing their oil with an insulating oil dielectric loss tester requires modified sampling techniques and stricter interpretation limits, but the effort is justified. Small oil volume means small sample requirements, and modern testers handle 40 mL samples with full accuracy. Add these devices to your oil testing program, and prevent the failures that catch operators by surprise.