News

 

I. A Comprehensive Overview of Common Issues with Existing Transformers

 

The following is The Six Most Common Types of Failures , covering more than 80% of actual cases in the existing market:

 

Core principle: For transformers in service for more than five years, oil leakage ranks first, followed by insulation aging, core-related issues, and tap-changer failures. Bushing flashovers frequently occur in humid and dusty environments, while excessive temperature rise is often a “complication” of other faults.

 

• Quick Reference Guide to Six Common Problem Categories

 

Serial Number	Major Problem Categories	Typical presentation	Core Reason	Handling method
1	Oil leakage	Oil leakage at the main body or connection flange; continuous drop in oil level.	Seal gasket aging and cracking; uneven bolt tightening torque.	Replace the sealing gasket; tighten the bolts; replenish oil and reset.
2	Insulation aging/moisture ingress	Insulation resistance decreases year by year; oil color darkens and turns black; light gas alarms trip frequently.	Long-term thermal aging; moisture ingress due to poor sealing; oil degradation	Oil sample chromatographic analysis; vacuum oil filtration or oil replacement; drying treatment
3	Multi-point grounding of the core	Core grounding current > 1 A; abnormal oil temperature rise; increased acetylene in oil chromatography.	Core burrs and foreign objects causing short circuits; insufficient insulation clearance of clamping components.	Capacitor discharge surge suppression; series current-limiting resistor; core lifting and repair
4	Tap changer failure	Inadequate voltage regulation; “crackling” discharge sound; DC resistance imbalance; contact burnout.	Poor contact between contacts; accumulation of oxide film; insufficient spring pressure	Repeatedly rotate to remove the oxide film; measure the DC resistance; replace the burnt contacts.
5	Casing flashover/discharge	“Squeaking” sounds in overcast, rainy, and foggy weather; surface tracking discharge marks; localized heating	Surface contamination leading to moisture absorption; cracks in porcelain components; poor grounding of the end screen.	De-energize and clean bushings; apply RTV anti-fouling flashover coating; replace damaged bushings.
6	Oil temperature too high	The upper oil temperature remains continuously above the limit (e.g., oil temperature > 85°C); the ammeter reading exceeds the range.	Overload; poor heat dissipation (blocked air ducts, fan failure); internal winding short circuit	Reduce the load; clean the radiators; install auxiliary fans; perform maintenance during a power outage.

 

 

III. Detailed Analysis of Each Question

 

1. Oil Leaks—The Number-One Killer in the Existing-Stock Market

 

On-site performance

This is the most common problem on site and occurs in virtually every aging transformer. Specifically, it manifests as:

 

Visible oil seepage or oil droplets are observed on the transformer body, flange sealing surfaces, bushing roots, or drain valves.

The oil level indicator shows a continuous drop in oil level, and the oil conservator’s oil level is abnormal.

In severe cases, oil stains are visible on the ground, creating a safety hazard.

 

Root cause

Over long-term operation, sealing gaskets may age, harden, crack, or even rupture; alternatively, uneven bolt-tightening forces during installation can lead to seal failure. In addition, porosity in cast-iron components and poor weld quality can also result in leakage.

 

Handling Plan

Minor leakage: Inspect and tighten the sealing bolts immediately upon detection, clean the leaking surface, and monitor the situation.

Seal gasket aging: Handle according to the standard procedure of “shut down and relieve pressure—clean the sealing surfaces—replace the seal gasket—repressurize and inspect—add oil and reset.”

Porosity or weld defects: These can be repaired using oil-injected welding, but stringent fire-prevention measures must be implemented; for porosity in steel plates, the surface can also be repaired by overlay welding with a steel plate.

Batch sealing points: It is recommended to include them in the major overhaul plan and systematically replace all sealing components of the entire transformer.

 

2. Insulation Aging and Moisture Absorption—An Invisible “Time Bomb”

 

On-site performance

This is the most common deep-seated problem in transformers with long service lives:

 

Insulation resistance measured with a megohmmeter was found to have decreased significantly year by year, accompanied by an increase in leakage current.

The transformer oil darkens in color, turning black; in severe cases, it develops a burnt odor.

Increased water content and elevated acid value in the oil, accompanied by the appearance of abnormal components in the insulating oil chromatography analysis.

Frequent light-gas signal alarms; in severe cases, differential protection trips.

 

Root cause

Prolonged operation at high temperatures leads to the natural aging of insulating paper and insulating oil; inadequate sealing allows moisture to ingress into the equipment, resulting in insulation dampness; as the oil degrades, its insulating properties further deteriorate, creating a vicious cycle.

 

Handling Plan

Diagnostic Preliminary Steps: Collect oil samples for chromatographic analysis, measure insulation resistance and the absorption ratio (an absorption ratio less than 1.3 indicates insulation moisture), and measure the dielectric dissipation factor (tan δ) to locate insulation moisture defects.

Mild oil degradation: vacuum oil filtration to remove moisture and impurities, restoring the oil’s dielectric properties.

Severe oil degradation: Replace all transformer oil and flush the tank with qualified new oil.

Severe aging of the insulating paper: requires disassembly for major overhaul, replacement of insulating components, and re-impregnation with varnish.

Routine maintenance: Regularly replace the desiccant in the respirator to ensure a tight seal and prevent moisture ingress.

 

3. Multipoint Grounding of the Core—The Most Common Internal Fault

 

On-site performance

This is the third most common high-frequency issue among all transformer faults:

 

The core grounding current has increased significantly (normally it should be less than 1 A; in severe cases, it can reach 10 A or even higher).

Oil temperature has risen abnormally, and acetylene and total hydrocarbon levels in the oil chromatography analysis have increased significantly.

The operating sound has become noisy and chaotic, sometimes accompanied by unusual noises.

If left unaddressed for a prolonged period, severe conditions can lead to localized overheating of the core, resulting in insulation damage to the windings and deterioration of the transformer oil.

 

Root cause

Causes such as incomplete removal of burrs during manufacturing, deposition of metallic debris during operation, damage to the insulating pads between the clamping pieces and the core, or core warping that results in contact with the enclosure can lead to the formation of a second—or even multiple—grounding points on the core, thereby inducing circulating currents and overheating.

 

Handling Plan

Unstable grounding: A sliding rheostat may be connected in series with the working ground conductor to limit the current to below 1 A as a temporary transitional measure.

Caused by burrs or foreign particles: this can be eliminated using the capacitor discharge impact method, which does not require lifting the cover and thereby reduces outage time.

On-site live-line diagnosis: Use the open-circuit voltage method to initially locate the grounding point, and monitor the core grounding current and the open-circuit voltage.

When the fault cannot be eliminated: lift the cover for inspection, locate the short-circuit point and repair it, then restore reliable single-point grounding of the core.

 

4. Tap Changer Faults—A Common Culprit Behind Voltage Regulation Failure

 

On-site performance

The tap changer is the “moving joint” of a transformer and also a frequent failure point:

 

The adjustment feels stiff and sluggish, or the output voltage is abnormal after adjustment.

A "crackling" or "squeaking" discharge sound is heard during operation.

When measuring DC resistance, the resistance values at each tap position are significantly unbalanced or excessively high.

In severe cases, the contacts may melt and fuse, resulting in ground faults and even triggering heavy-gas protection tripping.

Three-ratio method analysis of oil chromatography often indicates high-temperature overheating or discharge characteristics.

 

Root cause

Oxide films and oil contamination on the contact surfaces increase contact resistance; insufficient spring pressure or uneven roller pressure can also contribute to poor contact; furthermore, long-unused tap changers may experience coating delamination and reduced spring force. Once the tap changer contacts become poorly engaged, they cannot withstand the mechanical and thermal stresses of short-circuit currents, making them highly susceptible to developing into serious faults.

 

Handling Plan

Contact oxidation: During maintenance, the no-load tap changer shall be operated through at least five full cycles to remove the oxide film.

Faulty contact diagnosis: Measure the DC resistance at each tap position; the three-phase unbalance ratio must be within the acceptable range.

Minor contact burn: Remove the contact, grind and polish it, then reuse.

Severe contact burnout: the contacts or the entire tap changer must be replaced.

Mandatory inspection: After maintenance or tap-changing of the on-load tap changer, the DC resistance must be measured to verify that the moving and stationary contacts inside the tap changer are in good contact before the equipment can be put into service.

 

5. Bushing Flashover Discharge—The Most Easily “Visible” Hidden Hazard

 

On-site performance

The most obvious sign of a casing failure is readily apparent during routine inspections:

 

On overcast, foggy days or during damp mornings, the surface of the bushing emits a “squeaking” or “crackling” discharge sound.

The surface of the bushing exhibits obvious tracking marks—branch-like white or black patterns.

Infrared temperature measurement can detect localized abnormal temperature increases in the bushing.

In severe cases, surface flashover on the bushing can occur, leading to a single-phase-to-ground short circuit and even tripping.

Cracks or breakage may occur in porcelain bushings.

 

Root cause

When dust and dirt accumulate on the surface of bushings, their conductivity increases dramatically under humid conditions, leading to a rise in surface leakage current and ultimately causing flashover. In addition, manufacturing or installation defects in bushings, aging of porcelain components, poor grounding of the end screen, and adverse weather conditions such as rain or snow can also trigger discharge. For dry-type transformers, similar issues are often caused by environmental pollution and dust accumulation.

 

Handling Plan

Regular cleaning: During power outages, clean the surface of the bushings to remove dirt, dust, and debris, and verify that the grounding is in good condition.

Anti-pollution flashover treatment: After cleaning, apply a long-lasting RTV anti-pollution flashover coating to the surface of the bushing, which can significantly enhance its resistance to pollution-induced flashover.

Replace the bushing: If the bushing exhibits cracks, damage, or excessive dielectric loss, it must be replaced promptly.

Inspection precautions: Check the bushing for oil leakage, surface tracking, abnormal heating, and discharge sounds. If discharge or an abnormal temperature rise is detected, immediately de-energize the equipment.

 

6. Overhigh Oil Temperature—A “Warning Signal” for Multiple Faults

 

On-site performance

Oil temperature is a comprehensive health indicator, and any abnormal increase must be taken very seriously:

 

The top-oil temperature continuously exceeds the permissible limit (for oil-immersed transformers, the typical limit is a top-oil temperature not exceeding 85°C, with a temperature rise not exceeding 55 K).

The ammeter pointer exceeds the preset limit, and the transformer itself is visibly overheating.

The oil level rises abnormally as a result, and in severe cases the protective device trips.

Increased exhalation volume from the respirator; in severe cases, a burnt odor may be detected.

 

Root cause

The most direct cause is operation under overload, particularly during summer peak demand periods or when there is severe three-phase load imbalance. In addition, failures in the cooling system—such as blocked air ducts, damaged fans, or excessively high ambient temperatures—are also common causes. At a deeper level, internal faults like winding short circuits and poor oil quality can lead to abnormal overheating.

 

Handling Plan

Immediate troubleshooting: First, verify whether the system is operating under overload; if so, immediately reduce the load or adjust the three-phase balance.

Thermal management inspection: Clean dust from the transformer’s surface, clear ventilation ducts, inspect fan operation, and install auxiliary cooling fans as necessary.

External Cooling: For transformers operating in high-temperature environments, measures such as shading and enhanced ventilation can be implemented.

Internal fault troubleshooting: If the oil temperature remains high after eliminating overload and cooling issues, the power must be shut off to perform oil chromatography analysis and an internal inspection to determine whether there is a winding short circuit or core fault.

Real-time monitoring: Increase the frequency of infrared temperature measurements for heavy-load equipment, and conduct specialized inspections when the load reaches 90% or more.

 

IV. Quick Diagnostic Mnemonic (For On-Site Use by Customers)

 

To facilitate team recall and on-site use, you can remember the following mnemonic:

 

First, check whether the oil level is leaking; second, listen for any unusual noises.

Third, check whether the temperature rise is significant; fourth, inspect the bushings for any signs of damage or discoloration.

Fifth, smell to see if there is a burnt odor; sixth, test the insulation to see if it is intact.

A sudden change in oil color is a major issue; use chromatographic analysis to pinpoint the problem.

 

This six-step methodology—“inspect, listen, measure, examine, smell, and test”—constitutes the core technical framework for the “free transformer health check” mentioned last time. When you arrive on-site at a customer’s location, walk through these five steps one by one, and supplement them with the checklist of common issues and their corresponding solutions, the level of professionalism you demonstrate will be the strongest rationale for why the customer is willing to entrust the job to you.