Dry-Type Transformer Common Faults: Diagnosis & Solutions Guide

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Dry-type transformers are widely used in indoor installations due to their fire‑safe, maintenance‑free design. However, like any electrical equipment, they can experience faults over time. This guide covers seven common faults in dry‑type transformers – from zero insulation resistance to discharge noise – and provides practical diagnostic steps and solutions to keep your transformer running reliably.

1. Zero Insulation Resistance Between Core and Ground

Symptom: The core‑to‑ground insulation resistance measures zero, indicating a direct metallic connection.

Possible Causes:

Burrs or metal wires carried into the core during manufacturing
Damaged base insulation causing contact between core and base
Metal objects (e.g., screws, debris) fallen into the low‑voltage coil, bridging the pull plate and core

Solutions:

Use a lead wire to probe the channel between the low‑voltage coil and the core – remove any foreign objects.
Check and repair base insulation.
If the problem persists, use a welding machine with the ground lead connected to the grounding plate and tap the base with a welding rod (current ≈ 250A). A single tap often clears the metallic bridge.

2. Limit Switch Failure or Abnormal Operation

Symptom: The limit switch does not activate when the enclosure door opens, or it activates incorrectly.

Function: The limit switch is a safety device that triggers an alarm when any enclosure door is opened while the transformer is energized. Its contacts should close immediately upon door opening.

Solution:

Inspect the limit switch mechanism for mechanical damage or misalignment.
Check the electrical circuit – ensure the alarm relay and wiring are intact.
Replace the limit switch if faulty.

3. Temperature Controller Malfunction

Symptom: The temperature controller fails to:

Trigger an over‑temperature alarm when temperature exceeds the set point
Trip the transformer when temperature exceeds the trip set point

Diagnosis:

Stop the transformer.
Verify the set temperature values on the controller – are they accurate?
Check the temperature sensing element (thermocouple or RTD) – is it correctly positioned?
If settings are incorrect, reset them; reposition the sensor if necessary.

If issues persist, contact the manufacturer‘s customer service for further assistance.

4. Reduced Insulation Resistance of Core to Ground

Symptom: Insulation resistance drops below acceptable values (typically < 1 MΩ per kV of operating voltage).

Cause: The most common cause is moisture ingress – insulation materials absorb moisture, reducing their resistance.

Solution:

Place an iodine‑tungsten lamp (or other controlled heat source) under the low‑voltage coil.
Bake continuously for 12 hours to drive out moisture.
Re‑measure insulation resistance – it should return to normal levels.

Prevention: Keep the transformer in a dry environment (relative humidity < 70%) and avoid long‑term outages.

5. Discharge Noise During Power‑Frequency Withstand Voltage Test

Symptom: Crackling or sparking sounds during the acceptance test (power‑frequency withstand voltage test).

Possible Causes & Solutions:

Cause	Solution
Pull plate discharges at clamp tightening point	Tighten the clamp to ensure good conductivity between pull plate and clamp.
Spacer block creepage (especially in 35kV products)	Apply enhanced insulation treatment (e.g., add insulation layers or replace with higher‑grade material).
High‑voltage cable connection improper (too close to tap changer or angle connection tube)	Increase insulation distance and re‑tighten bolts.
Dust or particles on inner wall of high‑voltage coil (moisture absorption reduces insulation)	Clean the coil surface with dry compressed air or a soft brush.

6. Discharge Between Enclosure and Ground Rail During Power‑Up Surge

Symptom: Sparking occurs between the aluminum alloy panels of the enclosure and the ground rail when the transformer is energized.

Cause: Poor conductivity between enclosure panels – indicating a grounding failure.

Solution:

Use a 2500V megohmmeter to break down any insulating oxide layer on the panels.
Alternatively, scrape off the paint film at each panel connection point.
Connect all panels with copper wire for reliable grounding.

7. Burnt Angle Connection Tube

Symptom: Blackened or charred area on the high‑voltage coil angle connection tube.

Diagnostic Steps:

Carefully inspect the blackened part.
Use a knife or scraper to remove the black carbon layer.
If the red paint underneath remains intact, the coil insulation is likely undamaged.
Perform a transformer ratio test – if the ratio is normal, the coil is not short‑circuited.

Conclusion: The burn is caused by an external short circuit that produced an arc and burnt the angle connection tube – not an internal coil fault. Replace the tube and correct the external issue.

Summary: Fault Prevention Tips

Fault Area	Prevention Measure
Core insulation	Keep transformer dry; avoid metal debris during installation
Limit switch	Test monthly during routine inspections
Temperature controller	Verify set points annually; calibrate sensors
Insulation resistance	Monitor with periodic megohmmeter tests; avoid damp environments
Discharge during testing	Maintain cleanliness; use proper insulation distances
Enclosure grounding	Ensure all panels have firm electrical continuity
Angle connection tube	Protect against external short circuits with proper overcurrent protection