Transformer Oil Filtration Rest Period: Standards and Safe Operation

After transformer oil filtration, a rest period is vital before energizing the equipment. This stage allows trapped moisture, gases, and fine impurities to settle and dissipate while neutralizing static charges generated during filtration.

1. Standard Rest Periods

The required rest time depends on oil quality, filtration method, environment, and voltage level.

Recommended durations:

Medium/low-voltage transformers: 12–24 hours

High-voltage transformers (≥110 kV): 24–48 hours

Ultra-high-voltage or new/overhauled units (≥500 kV): 48–72 hours

Rule: Longer rest means better oil stability and safer commissioning—avoid shortening the process to meet deadlines.

2. Test-Based Evaluation

Time alone isn’t sufficient. After resting, sample and test the oil. Commissioning is allowed only if key indicators meet standards:

Indicator Standard

Breakdown voltage ≥40 kV (≥110 kV units); ≥35 kV (≤35 kV units)

Moisture content ≤15–20 mg/L (≥110 kV); ≤10 mg/L (≥500 kV)

Dielectric loss (tan δ) Within specified limits

Gas content <1% for high-voltage units

If results fail, extend the rest period or re-filter the oil.

3. Operational Essentials

Before Resting

Continue circulation for 15–30 minutes after heating stops to discharge static charge.

Seal all valves immediately to prevent air and moisture ingress.

During Resting

Temperature: Keep above 20 °C to promote degassing.

Humidity: Maintain below 65% RH; avoid humid or rainy conditions.

Oil level: Check periodically—minor drops are normal; top up with qualified oil if needed.

Before Commissioning

Verify oil tests (especially breakdown voltage and moisture).

Inspect seals, bushings, and coolers for leaks.

Vent trapped gas through dedicated valves.

4. Recommended Operating Procedure

Complete oil filtration

Continue circulation 15–30 minutes

Seal transformer system

Rest for at least 24 hours

Maintain suitable temperature and humidity

Take samples and test indicators

Confirm qualified oil test report

Inspect all seals and vents

Begin operation with close monitoring

Summary

The rest period after oil filtration is a crucial stabilization phase that ensures transformer safety and insulation reliability. Proper temperature, humidity control, sealing, and test verification guarantee stable oil quality and safe startup.

Never skip or shorten the rest phase—scientific testing and sufficient standing time are the foundation of reliable transformer operation.