Transformer Tightness Test with Vacuum: Step-by-Step Procedure, Leakage Calculations

The transformer tightness test with vacuum (also called vacuum drop test or leakage test) is a quality assurance procedure that verifies the integrity of transformer tanks before oil filling. This test ensures that no air leaks exist in the tank, preventing moisture contamination and ensuring reliable transformer operation. When a vacuum is applied to an evacuated transformer, any air leakage can draw humid air into the tank, damaging the paper and winding insulation. A successful vacuum drop test confirms that leakage is below the acceptable limit of 150 mm Hg liters per minute (or 20 m³Pa/min).

Why Transformer Tightness Testing Matters

When you’re commissioning a new power transformer or after major maintenance, one of the most important pre-operational checks is ensuring the transformer tank is completely airtight. This is where the vacuum tightness test comes into play.

Imagine a transformer tank as a sealed container that will hold transformer oil and house copper windings surrounded by paper insulation. If there’s even a tiny hole or crack in this tank, when you draw a vacuum on the empty tank, humid air will be sucked inside along with moisture. When this humid air expands as the vacuum is released, the moisture condenses on the cold windings and core, soaking into the paper insulation and severely reducing insulation strength.

This is why manufacturers and electrical engineers perform a final tightness test with vacuum before oil filling. It’s a preventive measure that saves transformers from catastrophic failure years down the road.

Vacuum Drop Test Principle

What Happens During Vacuum Application?

When you apply a vacuum to a transformer tank without oil, several physical phenomena occur:

1. Air Leakage: Any holes, cracks, or loose connections in the tank allow atmospheric air to enter.
2. Moisture Release: When air is drawn into a vacuum, it expands and its temperature drops. This expansion and cooling causes water vapor in the air to condense.
3. Insulation Contamination: If the core and windings are cold (which they are before oil filling), this condensed moisture gets absorbed into the paper insulation layers.
4. Reduced Insulation Resistance: Once moisture penetrates the paper, the insulation resistance drops dramatically, compromising the transformer’s dielectric strength.

The Leakage Test Concept

The vacuum drop test works on a simple principle: Measure how quickly the vacuum pressure drops over time. If the tank is perfectly sealed, the pressure should remain constant or drop minimally due to gas de-absorption from the insulation. If the tank has leaks, the pressure will drop more noticeably as air leaks in.

By comparing two pressure readings taken 30 minutes apart (after allowing one hour for de-absorption), we can calculate the leakage rate and determine if it exceeds acceptable limits.

Complete Procedure for Transformer Tightness Test with Vacuum

Equipment Required

Before starting the test, gather these essential instruments and tools:

• Vacuum Gauge: McLeod type (mechanical) or electronic type with reading range of -1 kPa (-10 mm mercury) to better
• Vacuum Pump: Capable of creating 50 mbar (5 kPa) vacuum
• Dry Nitrogen or Dry Air: For breaking the vacuum at the end
• Measuring Tape or Ruler: For tank measurements if needed
• Pressure/Vacuum Test Kit: With all necessary valves and connectors
• Thermometer: To monitor ambient temperature
• Log Sheet: To record all readings and observations

Step-by-Step Test Procedure

Step 1: Connect the Vacuum Gauge

• Locate a suitable valve on the top of the main transformer tank (not on the conservator)
• Connect your vacuum gauge to this valve using appropriate connecting tubing
• Ensure all connections are tight to prevent air leakage during the test
• Important: Vacuum application and measurement must be performed only on the top of the main tank, not on side connections or the conservator

Step 2: Connect the Vacuum Pump

• Connect your vacuum pump to another opening on the transformer tank or cover
• This is a separate connection from the gauge
• Ensure the pump connection is also secure and leak-free
• The pump should be capable of achieving at least 50 mbar (5 kPa or approximately 2 mm of mercury) vacuum

Step 3: Evacuate the Transformer Tank

• Start the vacuum pump and gradually reduce the tank pressure
• Continue evacuation until you reach 50 mbar (5 kPa or about 2 mm Hg)
• Do not attempt full vacuum on transformers. The specified pressure is only 50 mbar, not atmospheric evacuation
• For large transformers (above 20 MVA), this evacuation process may take several hours
• For smaller transformers, it might take 30 minutes to an hour

Step 4: Close the Valve and Stop the Pump

• Once you reach the target pressure of 50 mbar, close the vacuum valve firmly
• Stop the vacuum pump
• Disconnect the pump from the transformer (the gauge remains connected)
• Do not disturb any connections for the next hour

Step 5: Wait for Gas De-absorption (60 Minutes)

• Wait for one full hour after closing the vacuum valve
• This waiting period is crucial because it allows dissolved gases trapped in the insulation material to slowly escape and be absorbed from the tank atmosphere
• Without this de-absorption period, you would get false readings due to gases coming out of the insulation
• During this time, ensure the transformer is not disturbed and keep the ambient temperature as stable as possible

Step 6: Record the First Pressure Reading (P1)

• After exactly 60 minutes, carefully read the vacuum gauge
• Record this value as P1 (first pressure reading)
• Note the units on your gauge (typically mm Hg or kPa)
• Write down the exact time and ambient temperature
• Example: P1 = 1.5 mm Hg at 2:30 PM, Ambient Temp = 28°C

Step 7: Wait Another 30 Minutes

• Do not disturb the system
• Wait for exactly 30 more minutes
• Continue to monitor and record any environmental changes (temperature variations)
• Keep the tank in a stable condition

Step 8: Record the Second Pressure Reading (P2)

• After the additional 30 minutes, take the second pressure reading
• Record this value as P2 (second pressure reading)
• Note the time and ambient temperature
• Example: P2 = 2.1 mm Hg at 3:00 PM, Ambient Temp = 28.5°C

Also, read Transformer Tightness Test With Dry Air or Nitrogen

Calculation of Leakage Rate

We need to determine how much air leaked into the tank during the 30-minute interval between readings P1 and P2.

Formula for Leakage Rate Calculation

The standard formula used in transformer testing is:

\(\text{Leakage Rate} = \frac{V \times (P_2 – P_1) \text{ [mmHgLiters]}}{30 \text{ Minutes}}\)

Where:

• V = Volume of transformer tank in liters (from the rating plate)
• P2 = Second pressure reading in mm Hg
• P1 = First pressure reading in mm Hg
• Time interval = 30 minutes

Practical Example

Given Data:

• Transformer tank volume: 1,000 liters (as mentioned on the rating plate)
• First reading after 60 minutes: P1 = 1.5 mm Hg
• Second reading after 30 minutes: P2 = 2.1 mm Hg

Calculation:

\(\text{Leakage Rate} = \frac{1000 \times (2.1 – 1.5)}{30}\)

\(\text{Leakage Rate} = \frac{1000 \times 0.6}{30}\)

\(\text{Leakage Rate} = \frac{600}{30}\)

\(\text{Leakage Rate} = 20 \text{ mmHgLiters/minute}\)

Unit Conversion Reference

In the above example, the leakage rate of 20 mmHg·liters/minute is equivalent to approximately 2.67 m³Pa/min (cubic meters-pascal per minute).

Standard conversion factors:

• 1 mm Hg·Liter/min ≈ 0.1333 m³Pa/min
• 1 m³Pa/min ≈ 7.5 mm Hg·Liter/min

Acceptance Criteria

Allowable Leakage Limits

According to international standards (IEC, CBIP, and IEEE guidelines), the maximum acceptable leakage for transformer tanks is:

• 150 mm Hg liters per minute OR
• 20 m³Pa/min (cubic meters-pascal per minute)

If your calculated leakage rate is less than or equal to these limits, the test is ACCEPTED ✓

Interpreting Test Results

What to Do If Test Fails

If your calculated leakage rate exceeds the acceptable limit (150 mm Hg·L/min), the tank has leaks that must be found and repaired:

1. Release the vacuum (break vacuum using dry nitrogen or dry air to prevent moisture ingress)
2. Inspect the tank thoroughly for visible cracks, holes, or loose connections
3. Common leak locations:

• Tank seams and welds
• Around valve openings and bushings
• Connection points for radiators or cooling equipment
• Loose or improperly sealed lifting lugs
• Small holes in flat surfaces

1. Repair the identified leaks using appropriate welding, sealing compounds, or tightening procedures
2. Repeat the entire vacuum drop test from the beginning to verify that leaks have been eliminated

Important Considerations and Best Practices

Temperature Effects on Readings

Temperature changes during the test can affect your pressure readings. If the ambient temperature drops during the 90-minute test period, the gas inside the tank will contract naturally, showing a pressure drop even without any actual leaks.

Best Practice:

• Perform the test during stable temperature conditions (early morning or evening is ideal)
• If temperature change exceeds ±2°C during the test, repeat the test
• Correct readings for temperature variations if necessary

Preventing Moisture During Testing

The whole point of this test is to prevent moisture from entering the tank. Therefore:

1. Never break the vacuum with humid air – Always use dry nitrogen or thoroughly dried compressed air
2. Break vacuum slowly – Rapid depressurization can draw in moisture-laden air
3. If the test is interrupted, start over from the beginning – Don’t attempt to resume mid-test

Comparison with Other Transformer Tank Tests

The vacuum drop test is one of several tank integrity tests. Here’s how it compares:

Real-World Troubleshooting: Common Issues

Issue 1: Vacuum Won’t Hold (Rapid Pressure Drop)

Symptom: Pressure drops from 50 mbar back to atmospheric within minutes

Causes:

• Valve connections not properly tightened
• Gauge connection leaking
• Major crack or hole in the tank

Solution:

• Check and tighten all visible connections
• Inspect the tank visually for obvious damage
• Consider calling in an expert if you suspect structural damage

Issue 2: Slow, Consistent Pressure Drop (Possible Small Leak)

Symptom: Pressure gradually increases from 1.5 mm Hg to 5 mm Hg over 30 minutes

Causes:

• Small crack or hole somewhere in the tank
• Pinhole leaks in welds
• Loose valve packing

Solution:

• Use soap and water to spray the entire tank (solution will bubble at leak location)
• Mark all suspected leak locations
• Have them welded or resealed and test again

Conclusion

The transformer tightness test with vacuum might seem like a technical formality, but it’s actually a critical gateway before your transformer enters service. By ensuring the tank is completely sealed before oil enters, you’re preventing moisture contamination that could cut transformer life in half.

Whether you’re a maintenance engineer at a power station, a transformer manufacturer’s quality inspector, or an electrical consultant commissioning new equipment, understanding this test and properly executing it is essential for transformer reliability and safety.

Remember: A perfect vacuum drop test result (leakage ≤ 150 mm Hg·L/min) is your assurance that the transformer will serve reliably for decades to come.