Insights into analyzes and diagnosis of Transformers
Transformers are at the heart of modern energy supply systems. Their reliability and durability are crucial for keeping power grids stable and avoiding outages. However, behind their seemingly indestructible facade are many sensitive components that need to be checked and maintained regularly.
To prevent damage and cost-intensive transformer failures in the long term, it is important to understand the background: Why is the transformer oil and electrical conditions being tested? What role does paper insulation or moisture in oil play? What are the advantages of visual inspections and IR measurements on the transformer?
As a rule, the data available from the network operator represents the initial package for the first assessments of the transformer. The quality of this data varies greatly from case to case, so further steps may be necessary.
The second level of assessment is the standard in the field, while the third level is only mostly performed in specific situations where it is justified by the results of the previous assessments.
1st evaluation level: Historical data
- Initial data provides a first diagnosis, but this is often inaccurate or outdated, so further tests are usually necessary
- Data sources:
- Main transformer data from nameplate and transformer book
- Maintenance history
- All available historical measurement data (visual inspection, oil analysis, DGA, electrical measurements, IR thermographic)
2nd evaluation stage: Data from transformers in operation
- On-site diagnosis usually provides all the necessary data for a good assessment of the transformer
- Data source:
- Tests in operation: visual inspection, oil analysis, IR thermographic
- This diagnosis is sufficient to assess the condition of the transformer
3rd evaluation level: Data from out-of-service transformers
- Necessary if the previous level 1&2 diagnostics indicate this or if more detailed information is required, but provide more profound insights into the specific condition of the transformer
- Data source:
- Out-of-service tests: winding resistance, magnetizing current, short-circuit impedance, SFRA transformer, winding ratio, PDC/FDS measurement, power factor / tan δ etc.)
Further information:
- The oil analysis provides the most comprehensive information about the condition of the transformer.
- The oil analysis can be carried out while the transformer is in operation.
- The results are available quickly and cost-effectively.
The visual inspection is used as part of periodic maintenance. This can be carried out during operation and is a decisive indicator of the specific condition.
- The purpose of the visual inspection is to check the external condition of the transformer for signs of damage, wear or leaks
- The inspection includes the inspection of housings, connections, cooling systems, pipes and other components
- Signs of external damage such as cracks, dents or corrosion may indicate internal problems.
- Leaks of oil or coolant may indicate leaking seals or cracks in the transformer housing, which must be repaired to ensure the safety and reliability of the transformer
- The following periodic visual inspections are recommended, mainly on the following components:
- Measured values of the device displays
- Cooling systems
- Control wiring
- Paintwork
- Bushing and surge arrestor
- Insulators
- Connection points of the bushing
- Sealing test (leaks)
- Insert of the dehydrating breather
Thermography is a recognized non-destructive test based on standards (ISO 9712/ISO 473, ISO54191) and guidelines (VdS, VATh). This must be carried out by level 1 certified employees and evaluated by level 2 certified employees.
How does an IR thermography work?
- The thermal imaging camera (IR measurement) records the thermal electromagnetic radiation of the target resolved into frequencies and displays them
- On the transformer under load (IB ≥ 30% IN) to detect hot-spots and temperature differences
Why use IR?
- Detection during operation of the transformer
- Documentation of system statuses and potential risks
- Early detection of weak points and damage (external)
- Increased system availability and reliability
- Avoidance of consequential damage
- Reducing the risk of fire and accidents
What do you want to determine with IR?
- Electronic faults (poor connection, component overloading, phase imbalances, etc.)
- Heat accumulation
- Fluid flow problems (clogged or blocked pipes, radiator,...)
- Mechanical problems (oil level, mechanical stress, wear, ...)
- In operation or not (radiator,...)
Oil level in the bushings
Increased temperature at some fan motors
Expansion tank fill level
Increased temperature at a bushing connection
- Cooling Medium
Transformer oil absorbs the heat generated during operation and efficiently transfers it away from critical components such as windings and the core. This cooling function helps maintain safe operating temperatures and prevents thermal damage. - Electrical Insulator
The oil acts as an insulating barrier between energized parts, enhancing the dielectric strength of the transformer. It prevents electrical discharges and short circuits, ensuring stable and safe performance under high voltage conditions. - Lubricant
Moving parts within the transformer, such as tap changers, rely on the oil for lubrication. This reduces friction, minimizes wear, and contributes to the longevity and smooth operation of mechanical components. Information Carrier
Transformer oil serves as a diagnostic medium by carrying information about the internal condition of the transformer. Through techniques like dissolved gas analysis (DGA), it enables early detection of faults, supports predictive maintenance, and helps avoid costly failures.The various analysis methods are described in more detail in the chapter "Which important oil characteristics can be analyzed?
Consists of cellulose
- Serves as an insulator for the cables. Insulates many times better than just oil
- However, paper is very sensitive to temperature and moisture
- Paper ages at different rates due to differences in winding thicknesses and wear due caused by temperature differences or load profiles
Among other things, furans are produced when the paper decomposes
- The furan 2-FAL (2-furaldehyde) is meausered in trasformer oil
Replacing the paper is too expensive and is therefore constantly checked.
- Paper defines the service life of a transformer
Most of the moisture is bound in the paper, with only a few percent contained in the oil.
- If the paper is too damp, the insulation of the paper decreases
- Moisture accelerates paper decay, as paper decay in turn produces moisture
- Generally a value of 200 is considered deep-of-life
The properties of transformer oil can be investigated in many ways (gases, paper quality, oil quality):
Inspection on site
In the field, the transformer can be checked for the following criteria
- Oil temperature compared to ambient temperature
- Oil is taken from different oil compartments of the transformer tank (e.g. transformer, bushings, tap-changer)
- Visual inspection of the oil
- Oil level and oil flow control
Laboratory analysis
In the laboratory, the oil can be analyzed according to the following criteria and the condition can be assessed using common standards such as oil quality IEC 60422, gas-in-oil IEC 60599 and paper CIGRE TB 779.
The usual detection limits for laboratory analyses are 0.05-0.1 mg/kg (blue). Reliable interpretation is achieved from ~0.3 mg/kg (yellow).
Physical oil analysis:
| Oil quality - IEC60422 | ||
| Commonly Performed | Infrequent | Exeption |
| Acidity | Oxidation Stability | PCB Content |
| Color | Sediment & Sludge | Corrosive Sulphur |
| Water content | Particle Count | Aromatic Content |
| Breakdown voltage | Flash Point | Biodegradability |
| Dissipation factor | Pour Point | Toxicity |
| DC Resistivity | Density | |
| Interfacial Tension | Viscosity | |
| Inhibitor Content |
| Oil quality: Show service life & maintenance parameters as table | |
| Moisture in oil | -Dielectric properties -Paper moisture |
| Interfacial tension | -Mud failure -Solids content |
| Color number | -Oil ageing progress |
| Dissipation factor (Tan D oil) | -Oil ageing progress |
| Inhibitor | -Protection against oxidation |
With the help of physical oil analysis, a diagnosis of the oil quality can be derived (e.g. breakdown voltage or water content):
Furan
Chengdong analysis has established itself in the market for assessing the condition of paper and is used in the standard „Cigre A2-107 "Identification of early-stage paper degradation by methanol.
The furan/methanol analysis helps to determine the condition of the paper and thus the actual ageing of the transformer.
The quality of the oil indicates its condition and suggests potential maintenance actions, such as reconditioning, regenerating, or re-inhibiting. Analysing the dissolved gas can indicate potential transformer faults that may require repair.
Dissolved Gas Analysis (DGA)
Analysis of dissolved gases in insulating oil for early detection of overheating, arcing and insulation problems in transformers.
What is analyzed?
- The transformer is not in an optimal condition when certain gases are produced
- Hydrogen (H₂) occurs with all thermal and electrical faults in the transformer; key gas for partial discharges
- Methane (CH₄) Heat gas; occurs mainly with thermal faults up to 300 °C and partial discharges
- Ethane (C₂H6) Heat gas; occurs mainly at temperatures between 300 °C and 700 °C and with stray gassing
- Acetylene (C₂H₂) Arc gas; it is produced by discharges with open arcs in oil
- Carbon monoxide (CO) is produced during the thermal decomposition of transformer paper or cellulose insulating materials
Interaction of Buchholz relay and DGA
- Through the combination of continuous monitoring, regular DGA analyses and protection by the Buchholz relay, operators can rectify problems at an early stage and avoid failures and damage.
- The Buchholz relay triggers in the event of a fault and increased gas generation. The DGA analysis supports early fault detection and diagnostics.
- Supplementary diagnostics: The Buchholz relay can both trigger an alarm and move to the open position of the transformer. It supplements the DGA by reacting quickly in the event of gas formation, while the DGA examines the exact gas composition for fault analysis.
The advantage of DGA in operation
- Early fault detection: Potential damage such as overheating or arcing can be detected before major failures occur
- Avoidance of breakdowns: Reduction of downtime through timely maintenance or repair
- Cost efficiency: Minimization of repair and replacement costs through preventive measures
- Improved operational safety: Greater reliability and protection of the transformer and the entire system
- Trend monitoring: Long-term analysis enables creeping changes in the condition of the transformer to be detected.
Forms of representation of potential fault types
Methods such as the Duval triangle and the pentagon diagram enable diagnosis by classifying certain gas concentrations into specific fault types, which are described in more detail in IEC 60599, for example.
- Dissolved water
- Hydrogen bound to the hydrocarbon molecules that make up the oil.
- From a saturation of 100% of the moisture in oil, free water forms and endangers operation.
- Emulsified water
- Supersaturated in solution, but has not yet completely broken away from the oil. It normally gives the oil a milky appearance.
- The water is also located at the molecular level between the hydrocarbon molecules.
- As this is a thermodynamically unstable state, water droplets can form spontaneously at any time.
- Free water
- Also supersaturated in solution, but at a high concentration to form water droplets and breaker from the oil. The warmer the oil is, the more water can dissolve in it.
The effect of water
- Water affects the breakdown voltage of the insulation, the temperature at which water vapor bubbles are created and the loss-of-life of the insulating materials.
- The breakdown voltage of oil decreases significantly if the water content exceeds two to three percent by weight.
- Dry transformers (less than 0.5 % water in the paper) are much less susceptible to the development of water bubbles
- In this case, emergency loading may be possible at hot-spot temperatures below 180 °C with little risk of bubbling. In contrast, a wetter transformer with 2.0 % moisture in the paper runs in danger of forming water bubbles at hot-spot temperatures of only 139 °C under the same conditions.
- Excessive moisture accelerates the ageing of the paper insulation, with the loss-of-life being directly proportional to the water content. The deterioration of the paper insulation results from the weakening of the hydrogen bonds of the molecular chains of the paper fibers.
- Thermal Failure Modes:
- Accelerated cellulose aging:
- Increased dissolved carbon oxides (CO, CO2)
- Measured by the degree of polymerization (DP)
- Influenced by temperature, oxygen, moisture, sludge
- Nominal service life temperature limit: IEC (98 °C at 20 °C ambient temperature), IEEE (110 °C at 30 °C ambient temperature)
- Hot metal:
- Increases values of methane (CH4), ethane (C2H6), ethylene (C2H4)
- Metal temperature limit: 140 °C
- Hot metal covered with cellulose: Elevated levels of metal gases and carbon oxides
- Bubbling: Caused by rapid temperature rise leading to moisture imbalance and bubbling
- Accelerated cellulose aging:
- Tests and diagnostics:
- DGA: Key gases: carbon oxides, hydrocarbons, hydrogen
- Furan analysis: Indicates depolymerization (2-furfuraldehyde and related compounds)
- Physical properties of the oil: Moisture, acidity, interfacial tension, breakdown voltage, oxygen content
- Moisture in the cellulose: Derived from oil moisture content by dielectric frequency response
- Moisture content in the windings: Values 0.5–1% is considered dry, up to 2% is acceptable, above that is moderately moist, etc.
- DC resistance of the winding: Indicates potentially poor line connections if the deviation is > 1 % of the FAT
- IR thermography: detecting excess temperatures, fill levels and other thermal abnormalities
| Formed gas | Icon | Temperature | Required energy [kJ/mol] |
| Hydrogen | H2 | ~120 | 338 |
| Methane | CH4 | 338 | |
| Acetylene | C2H4 | >700 | 960 |
| Ethylene | C2H4 | 720 | |
| Ethan | C2H6 | 607 |
- Design Information:
- Paper type: Influences the loss-of-life of the cellulose (kraft paper vs. thermally upgraded cellulose)
- Oil preservation system: Influences the DGA interpretation and the moisture/oxygen values
- Temperature profile: Essential for assessing thermal performance in relation to load and ambient temperature
- Core superheat calculation: Determines the risk of H2/C2H6 gas formation at core hot spots > 130 °C
- Bubbling temperature: Calculates the moisture in the cellulose (critical for overload capacity)
- IEC60076/7: The service life is halved with a temperature increase of 6 °C
- 20 °C to 90 °C, increase in reaction speed by a factor of 128
- Temperature increases of 10°C, double the reaction rate
[1] „What assessment levels are there for the transformer?” und [1] „Illustrations for „What assessment levels are there for the transformer?“:
SUPPORTING ASSET MANAGEMENT WITH DATA-BASED SOLUTIONS, UNIVERSITY OF WUPPERTAL, in Zusammenarbeit mit Maschinenfabrik Reinhausen (GCC-Cigre-Paper_MR&BUW.pdf)
[1]: „Further text for „What assessment levels are there for the transformer?“:
CIGRE WG A2.34, „Guide for Transformer Maintenance“ presented at the WG A 34, 2011, Vo. 445
[2] „What does a visual inspection of a transformer involve?” :
8 Periodic Inspections of a Substation Transformer (8 Periodic Inspections of a Substation Transformer _ EEP.pdf)
[3] „Why is an IR measurement valueable?”: Andrei Weimer. ISO 9712/ISO 473, ISO54191 und Richtlinien VdS,VATh
[4] „What are the tasks of transformer oil?“:?“: Erwin Hilbert – Öl-Kompendium
[4] „Further text for „What are the tasks of transformer oil?“” Dr. Alexander Alber:
Infobase - TTissue27_Adv_MR_single.pdf - Ansicht nach Jahr (sharepoint.com) / Das Duvaldreieck in 3 Minuten erklärt: Reinhausen / MergedFile / 702873_filipic_stefan_2019.pdf
[4] „Further text and illustration (diagram) for What are the tasks of transformer oil?“”: IEEE C57.104 (2019)
[5] „What is the task of paper in a transformer?”: Erwin Hilbert
[6] „Which important oil characteristics can be analyzed?“: Collaboration TDI, TSC, A2
[7] „Why is moisture in oil dangerous for transformers?“:
Understanding water in Transformer Systems, Lance Lewand (Understanding_Water_in_Tranformer_Systems_-_Lance_Lewand.pdf). TDI Erwin Hilbert
[7] „Illustration for Why is moisture in oil dangerous for transformers?”:
Understanding water in Transformer Systems, Lance Lewand (for the formulas).
[8] „llustrations of thermal images of a transformer”: TSC Philipp Birgmeier
[8] „What does a thermal recording look when there is a problem with transformers?”:
CONDITION ASSESSMENT OF POWER TRANSFORMERS, CICRE (A2-49 Health Index Draft September 2017.pdf). TST Andrei Weimer, TDI Erwin Hilbert
