A las 2:47 a.m. en punto, la alarma en la sala de control sonó: el análisis de gas disuelto (DGA) para el transformador de potencia de 10 MVA, 33/11 kV había saltado de un normal 60 ppm de gas combustible a 340 ppm en seis horas. El operador de la planta tiene tres opciones: limpiar toda la unidad y terminar el turno con una pérdida en la producción, continuar la operación normal con la esperanza de no activar el relé Buchholz, o comenzar a realizar diagnósticos. El diagnóstico de fallas en transformadores de potencia es exactamente el campo de estudio que lleva a diferenciar entre los tiempos de inactividad programados para mantenimiento y las interrupciones forzadas que le quitan a la empresa $50,000 o más en ingresos perdidos, costos de transporte y compra de electricidad de emergencia. En este artículo, se puede encontrar información sobre los tipos de fallas más comunes, sus métodos de detección y las instrucciones paso a paso para realizar diagnósticos de fallas en la práctica.
Explicamos la causa física de cada tipo de falla, los dispositivos de diagnóstico responsables de detectarlas, los fondos aproximados necesarios para equipos de prueba y diagnósticos de terceros, y el trabajo de mantenimiento que permite evitar la mayoría de las fallas mucho antes de que lleguen al interruptor automático.
En términos simples, solucionar una falla en un transformador de potencia significa seguir un procedimiento claramente definido basado en el conocimiento, destinado a identificar, medir y rectificar fallas en transformadores de más de aproximadamente 1 MVA, mediante pruebas como DGA (análisis de gas disuelto), pruebas de resistencia de aislamiento, pruebas de resistencia de devanados y pruebas de aceite de transformador. Se estima que 80% de los transformadores defectuosos fallan debido a fallas eléctricas causadas por ruptura del aislamiento, descarga parcial o impacto de sobretensión, siendo las fallas térmicas y relacionadas con la máquina la gran mayoría del resto. Lo que debemos tener en cuenta es que no debemos quitar la tapa de un transformador hasta que los resultados de DGA y las pruebas eléctricas hayan mostrado cuál es el subsistema que falla.
¿Cuáles son las fallas más comunes en transformadores de potencia?
Los transformadores son equipos maduros que se conocen por ser muy fiables y pueden durar entre 30 y 40 años si se construyen correctamente y se enfrían adecuadamente. La falla de los transformadores generalmente se clasifica en una de cinco categorías, siendo la primera la ruptura del aislamiento de los devanados, que causa alrededor del 35-40% de las fallas de dicho equipo. Esto está relacionado con cortocircuitos entre vueltas, cortocircuitos entre diferentes capas, así como fallas a tierra que ocurren debido a la deterioración del aislamiento de papel. Además, las debilidades en los terminales también son responsables del 10-15% de las fallas, y esto es particularmente peligroso debido a la posibilidad de que aceite caliente altamente inflamable sea rociado desde los terminales cuando ocurre una falla. Las fallas de los cambiadores de tomas bajo carga se comparan con los terminales, ya que representan un porcentaje similar de las fallas, siendo los problemas de forja una de las principales razones, ya sea por desgaste de contacto, contaminación del aceite en el compartimento del cambiador de tomas, o fallas en el motor. Los defectos del núcleo y del circuito magnético pueden presentarse en forma de pernos sueltos o circulación de corriente causada por daños en el aislamiento entre las laminaciones, lo que lleva a un sobrecalentamiento local y pérdidas excesivas en vacío. Además, los problemas relacionados con el aceite del sistema de enfriamiento, como radiadores obstruidos, bombas y ventiladores que no funcionan, bajo nivel de aceite o altos niveles de humedad, conducen a una degradación térmica que inicia la destrucción de partes dentro del transformador.
Cómo se desarrollan las fallas en los transformadores: Modos y mecanismos de falla
La existencia de fallas requiere un largo proceso de degradación que ocurre con el tiempo para que aparezca una falla. La mayoría de las fallas en devanados y aislamiento resultan de tensiones térmicas. Según la regla de envejecimiento aplicada en la industria, la vida útil del aislamiento se reduce a la mitad cuando su temperatura supera entre 6°C y 10°C por encima de su temperatura nominal de 98 °C al utilizar papel térmicamente mejorado. Por ejemplo, operar un transformador de 10 MVA bajo una carga de al menos 20% durante el verano consumirá años de vida útil del aislamiento, incluso si el transformador no se desconecta.
Las tensiones eléctricas conducen a su propio proceso de fallo. Las descargas parciales que aparecen como micro-descargas a través de burbujas, vacíos y material de aislamiento dañado pueden llevar a una degradación lenta del aislamiento. En el caso de un devanado de 33 kV, se debe tener en cuenta que una actividad de descarga parcial de 100 pC ya es una señal de advertencia. Y si hay descargas parciales en el transformador de 1000 pC, el transformador experimentará un fallo que podríamos observar en un par de semanas. La presencia de humedad puede empeorar ambos procesos. En el caso de un transformador que tiene un nivel de humedad de 2 % en su aislamiento de papel, el papel puede envejecer aproximadamente el doble de rápido que en un transformador sin influencia de humedad, lo que lleva a la destrucción del aislamiento. Debido a la presencia de humedad en el aceite, la resistencia dieléctrica puede disminuir de 40 kV (calificación mínima según IEC 60156) a menos de 20 kV, lo que es suficiente para provocar un fallo. La fuerza mecánica conectada a las corrientes de cortocircuito en el devanado puede cambiar y aflojarlo, por lo que incluso si el transformador sobrevive al primer caso, el segundo fallo a través puede finalmente dañar si ocurre en seis meses entre los fallos. Tenga en cuenta que reconocer esos mecanismos proporciona una base muy buena para el diagnóstico de fallos en transformadores.
Tipos de fallos, señales de advertencia y causas raíz
| Tipo de fallo | Participación típica de fallos | Señales de advertencia tempranas | Causas raíz más comunes |
|---|---|---|---|
| Fallo de devanado / aislamiento | 35–40% | Aumento de DGA acetileno/etileno, desviación de relación, aceite caliente | Sobrecalentamiento, humedad, PD, daño por sobretensión, sujeción suelta |
| Fallo de bushing | 10–15% | Grietas visibles, fugas de aceite, cambio de capacitancia, PD en el bushing | Daño en porcelana, entrada de humedad, contaminación |
| Fallo de OLTC / cambiador de tomas | 10–15% | Ruido inusual al cambiar de toma, aumento de resistencia de contacto, oscurecimiento del aceite | Desgaste de contacto, aceite carbonizado, desgaste mecánico |
| Fallo de núcleo | 8–12% | Aumento de pérdida en vacío, puntos calientes locales, zumbido inusual | Laminaciones sueltas, aislamiento de núcleo dañado, corrientes circulantes |
| Fallo del sistema de enfriamiento / aceite | 10–15% | Alta temperatura del aceite superior, bajo nivel de aceite, paradas de bomba/ventilador | Radiadores bloqueados, bombas fallidas, fugas, entrada de humedad |
| Sobrecarga / evento del sistema externo | 5–10% | Carga alta sostenida, calentamiento armónico, disparo de protección | Sobrecarga sostenida, armónicos, desequilibrio de voltaje |
La tabla contiene dos piezas importantes de información sobre el proceso de solución de problemas. Una que vale la pena mencionar es que los datos que muestran la “participación de fallos” se recopilan de investigaciones combinadas sobre diferentes fallos en la industria realizadas por CIGRÉ e IEEE, que han identificado las variaciones en estas cifras según la clase de voltaje de los transformadores. Por ejemplo, los transformadores de clase de distribución sufren de muchos más fallos por sobrecarga que los transformadores que transmiten más de 110 kV sufren de fallos dieléctricos y de OLTC. Además, el aceite aislante de un transformador es un mensaje oculto, ya que indica el tipo de fallo que está ocurriendo.
Síntomas de fallo vs. causas probables
| Síntoma observado | Fallo más probable | Confirmar con | Sospechosos secundarios |
|---|---|---|---|
| Disparo repentino del relé Buchholz | Fallo interno severo (de vuelta a vuelta, núcleo) | DGA + resistencia de devanado + prueba de relación | Liberación rápida de humedad, acumulación de gas |
| Aumento gradual de temperatura a carga constante | Degradación del sistema de refrigeración | Verificación del flujo de aceite, corriente de bomba/ventilador, DGA | Degradación del aceite, mayor temperatura ambiente, obstrucciones |
| Alta pérdida en vacío y zumbido fuerte | Daño en el núcleo o mala aislamiento de laminación | No-load loss test, core ground current | Mechanical looseness, DC magnetization |
| Decreasing insulation resistance | Moisture ingress or contamination | Insulation resistance test, oil breakdown voltage | Paper aging, oil sludge |
| Voltage ratio out of tolerance | Winding shorted turns or OLTC damage | Turn ratio test (TTR), winding resistance | Tap changer misalignment |
| Oil colour darkening and sludge | Thermal degradation of oil | Acid number (IEC 62021), interfacial tension | Overheating, oxidation, contact with air |
This table of symptoms is purposely conservative: every symptom indicates a key suspect, but skilled troubleshooters would not leap to conclusions about anything. For example, a Buchholz trip could arise just as easily from a broken cooling pipe that introduces air into the container but involves no electrical failure at all. Thus, the following diagnosis sequence always has to confirm with measures beforehand, prior to any decision being made about opening the container.
Key Diagnostic Tests & Acceptable Limits
| Test | Standard Reference | Typical Acceptable Range | When to Run |
|---|---|---|---|
| Dissolved gas analysis (DGA) | IEC 60599 / IEEE C57.104 | Total combustible gas < 720 ppm; key gas ratios normal | Annual; immediately after any trip or abnormal event |
| Insulation resistance (IR) / PI | IEEE 43 | Polarization index ≥ 2.0 for dry paper; IR per kV rating | Annual; before energizing after maintenance |
| Winding resistance | IEC 60076-1 | Phase imbalance ≤ 2% between phases | After trips, OLTC inspections, suspected shorted turns |
| Turn ratio test (TTR) | IEC 60076-1 | Deviation ≤ 0.5% from nameplate | Annual; after reconnection or OLTC work |
| Oil breakdown voltage (BDV) | IEC 60156 | ≥ 40 kV for new oil; ≥ 30 kV acceptable in service | Each oil sample; before oil reclamation |
| Moisture in oil / paper | IEC 60814 / Karl Fischer | < 2% moisture in paper; < 10 ppm water in oil (typical) | Annual; after suspected moisture ingress |
| Partial discharge measurement | IEC 60270 | PD level below manufacturer acceptance (often < 100 pC at HV) | Factory tests; after major insulation disturbance |
| Frequency response (SFRA) | IEC 60076-18 | Low deviation index across frequency sweep | After through-faults or transport; baseline comparison |
These limits are working numbers, rather than evitable ones, since a transformer that always worked under 5% moisture equivalent indicators could still work properly — however, the trend matters more than any separate number. Thus, you need to keep all the testing results available for a year-to-year comparison, because a 40% increase in hydrogen content in two instances is far more important than a normal absolute value.
Where Faults Hit Hardest: Applications & Critical Assets
Priorities in troubleshooting vary across applications as the costs of failures differ from one application to the other. In electricity generation and transmission, one transformer with a rating of 110 kV/50 MVA could cost up to $ 400,000 and even a million dollars, and it could take up to 1 season to replace it. In an industrial environment, the same 10 MVA device would usually have a backup unit in place, and the goal, therefore, would be triaging the failure, figuring out whether it could be repaired on the site or needs to be serviced offsite. For renewable energy devices, similar to wind farms, transformers are exposed to street cycle and they face different issues connected with OLTC damage, flashover and insulation failures, not caused by single accidents. Places like data centers and hospitals will be focusing on making the availability issues the priority, and this is why they will resort to paying much more than usual for the repairs.
As for networks, companies operating in it will choose the risk management strategy and will be dealing with high-risk failures every 6-12 months and low-risks failures every 2-3 years. Thus, the same transformer or the same fault may receive completely different treatment, depending on the usage context of the technology.
Diagnostic Tools, Brands & Price Ranges
| Tool / Service | Representative Brands | Typical Price Range | Notas |
|---|---|---|---|
| DGA sampling kit & lab test (per sample) | Doble, SGS, local utilities labs | $80–$250 per sample | Full gas chromatography panel |
| Portable DGA monitor (online) | Kelman (GE), Serveron, Qualitrol | $15,000–$45,000 | Continuous monitoring of key gases |
| Insulation resistance tester (5–10 kV) | Megger, Fluke, Doble | $1,500–$6,000 | For IR, PI, DAR measurements |
| Winding resistance meter (micro-ohmmeter) | Megger, Vanguard, DV Power | $4,000–$12,000 | Low-resistance measurement to 0.1 µΩ |
| Turn ratio tester (TTR) | Megger, Vanguard, Doble | $5,000–$15,000 | Full 3-phase TTR with OLTC test |
| SFRA test set | Doble, OMICRON, Megger | $20,000–$60,000 | Winding deformation analysis |
| Third-party diagnostic service (full site battery) | Doble, Intertek, local certified labs | $5,000–$25,000 per visit | DGA + electrical + oil tests on one unit |
Depending on factors such as location, manufacturer and uniqueness, prices may differ, so it may be safer to consider them as planning values rather than actual prices. For example, one of the valid ideas for budgeting a small industrial fleet is to purchase a medium-range insulation resistance tester and winding resistance meter first, which costs from $6,000 to $18,000 together; DGA samples can then be sent to a lab and third-party inspection for SFRA and PD can only be done when any dysfunction appears. There are many global players on the market such as Doble, OMICRON, Megger, Fluke and Qualitrol, so it is possible to be sure that they can serve as a benchmark. Nevertheless, it should be noted that Jiangsu Subian Electric Power and other Chinese companies have already started to produce the transformers these diagnostic devices assess, thus introducing IEC 60076-related distribution and power transformers from 10 kVA to 100 MVA to the market at reasonable prices and lead times. For comparing diagnostic methodology used by various companies, it is necessary to say that a company producing transformers can also provide its clients with interpretation of testing results.
Building a Troubleshooting Workflow: 8 Steps
Any serious troubleshooting activity must adhere to the same structured sequence, as this is how seasoned technicians get into trouble.
- Establish the site and obtain data; validate the trip or alarm, isolate the transformer, and take note of any relay targets, SCADA logs, load history, and ambient temperature for the last two days, before undertaking any resetting of protection.
- Perform a DGA straight away; if oil is available, take a sample for dissolved gas testing; fingerprinting (IEC 60599) tends to indicate overheating, PD or arcing, before any electrical test is conducted.
- Carry out electrical inspections in a specific order: insulation resistance and polarization indices first (simple and fast); followed by windings resistance, then turns ratio; compare all readings with the base line or name plate values.
- Assess the condition of the oil; breakdown voltage, moisture, acidity and dielectric strength will tell you if the oil itself is the culprit or the victim.
- Analyze gases based on key ratios; hydrogen indicates PD, ethylene and methane signal thermal failure, acetylene in excess of 5 ppm indicates arcing or over-heating.
- Make a decision as to whether it’s minor, moderate or severe; referencing IEEE C57.104 or IEC 60599 condition.
- Check first before opening; only carry out tank opening after it has been determined where the fault is by means of a physical inspection — the entry into the tank costs $5,000 to $20,000.
- Make sure everything is documented; in every asset history file record every result, every decision made as well as every action taken with repairs.
Preventive & Predictive Maintenance Practices
The trouble-shooting that does not have to be done is the best trouble-shooting There are four layers that make the transformer maintenance effective. The first layer is routine inspection of the transformer which includes the visual inspection, heating indicator reading, oil level and leaking checking and hearing functionality monitoring on a monthly or quarterly basis. The second layer is the periodic testing that includes annual DGA, insulation resistance testing, winding resistance testing and oil breakdown voltage (BDV) testing which should take place twice a year for transformers above 20 years old or have undergone overloading. The third layer is the predictive monitoring which includes DGA monitor installation, winding temperature sensor installation and some moisture sensor installation for the very expensive transformers that can cost around $15000-$50000 but the money is paid off quickly. The last layer is the oil maintenance that allows to maintain the breakdown voltage (BDV) above 30 kV and acid number below as a result increasing the service time of the oil.
It is also important to mention two rules of maintenance that should be memorized and followed. The first one is that notes must be taken as the development trends matter more than thresholds when making decisions. The second one is that the targeted DGA should be conducted for the transformers with the emergency event even though the transformer is functional as there may be some damage that is not defined by alarm sign.
| Maintenance Activity | Typical Frequency | Key Checks | Typical Cost per Visit |
|---|---|---|---|
| Routine visual inspection | Monthly / quarterly | Oil leaks, oil level, temperature gauges, sound, desiccant colour | $100–$300 (in-house) |
| Annual DGA + oil sampling | Annually (6 months for aging units) | Key gas ratios, BDV, moisture, acid number | $80–$250 per sample |
| Annual electrical test battery | Annually | IR/PI, winding resistance, turn ratio, no-load/load loss if warranted | $1,000–$4,000 in-house |
| Cooling system service | Every 6–12 months | Fan/pump operation, radiator cleaning, blocked fins | $500–$2,000 |
| Breather / desiccant service | Every 6 months | Silica gel condition, oil seal level | $100–$500 |
| OLTC inspection (oil-filled compartment) | Every 2–5 years or by tap-change count | Contact wear, oil carbonization, mechanism timing | $1,500–$5,000 |
Frequently Asked Questions
How often should I perform DGA on a power transformer?
For a healthy transformer, DGA should be utilized at least once a year, as stated in IEEE C57.104. However, this frequency should be changed to every 6 months in case of units that are over 20 years old, units that have been overloaded or units that already have gases present in moderate levels. In case of assets that are very valuable, the installation of the DGA monitor system that constantly samples gases should be considered (this equipment costs 15,000-$45,000). It is quite probable that this type of installation will pay off if at least one fault can be detected in time.
What does acetylene in DGA indicate, and how much is too much?
Acetylene (C₂H₂) is produced only under extreme conditions associated with arcing and very high heat (higher than 700 °C). IEEE C57.104 states that the content of acetylene in the liquid must not go above the level of 5 ppm, otherwise, it must be treated as moderate to high condition and investigated. Taking measurements above the limit of 20 ppm may lead to transformer shut down for additional electrical testing since serious damages such as tank rupture may occur following any type of arcing fault that develops for a few days.
Can I troubleshoot a transformer without opening the tank?
Yes. Usually, DGA, insulation resistance measurement, winding resistance measurement, turn ratio measurements, and oil tests correctly identify the location of the problem in the unit without entering the tank. It costs $5,000-20,000 to open a transformer for internal inspection. Therefore, it is necessary to conduct all the access tests before checking the tank.
What is the difference between a polarization index of 1.5 and 3.0?
The polarization index (PI) shows the relationship between the insulation resistance readings that were taken 10 minutes and 1 minute after the initial voltage application. The value lower than 1.5 suggests that the insulation is wet or dirty and requires further drying out. The PI range of 1.5-2.0 is marginal, while the 2.0-3.0 range is acceptable. Any value above 3.0 indicates that the insulation is dry and clean.
How much does a power transformer fault investigation typically cost?
If a third-party diagnostic investigation is performed, DGA plus electrical tests plus oil analysis plus SFRA would cost between 5,000-25,000 per transformer, depending on its power rating, site accessibility, and scope of work. It is cheaper to conduct the tests with one’s own appliances since in this case the only investment would be expertise and not money.
References
- IEC 60076-1: Power transformers – Part 1: General — the foundational international standard for power transformer rating, testing, and performance.
- IEC 60599: Mineral oil-filled electrical equipment in service – Interpretation of dissolved and free gases analysis — the reference for interpreting DGA gas ratios and fault types.
- IEEE C57.104: Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers — the standard condition-category framework used in gas interpretation.
- IEEE 43: Recommended Practice for Testing Insulation Resistance of Rotating Machinery — widely referenced for insulation resistance and polarization index limits.
- CIGRÉ — international council whose reliability surveys provide the statistical basis for transformer failure distribution data.
- Electrical4U – Dissolved Gas Analysis of Transformer Oil — a practical introductory reference on DGA interpretation.
- Jiangsu Subian Electric Power – Power Transformer Manufacturer — IEC 60076-compliant transformer manufacturer offering design data and support for fault investigation.
Conclusion
It is not just luck or the age of a professional, but the right sequence of actions that will help in eliminating faults in power transformers. Start with the process of DGA followed by insulation and winding resistance tests and totaling the results against established norms in order to make wise decisions about your further actions on the problem. In fact, many faults can be repaired at the early stage, thus saving a lot of time and funds in the long run.
- Carry out DGA regularly; at least once a year; if the unit is aged or stressed, at least every six months.
- Ensure keeping a record of the results of the testing as the dynamics of the process is the most important indicator rather than the absolute number.
- Never start inspecting the transformer until it is well known what the cause of the problem is.
- Allocate from $1500 to $6000 for the testing of one transformer as one avoided accident will cover this sum for for the period of time.
- Partner with a manufacturer who can help you with the data that you get for the issues; for example, Jiangsu Subian Electric Power which produces transformers that follow the IEC 60076 standards.