{"id":10825,"date":"2026-08-29T23:43:13","date_gmt":"2026-08-29T15:43:13","guid":{"rendered":"https:\/\/q8bbzjnpm5.wpdns.site\/?p=10825"},"modified":"2026-08-29T23:43:13","modified_gmt":"2026-08-29T15:43:13","slug":"practical-guide-to-troubleshooting-power-transformer-faults","status":"publish","type":"post","link":"https:\/\/subian-electric.com\/de\/news\/practical-guide-to-troubleshooting-power-transformer-faults\/","title":{"rendered":"Praktischer Leitfaden zur Fehlersuche bei Leistungstransformatoren"},"content":{"rendered":"

At precisely 2:47 a.m., the alarm in the control room sounded off \u2013 the dissolved gas analysis (DGA) reading for the 10 MVA, 33\/11 kV power transformer had jumped from a normal 60 ppm of combustible gas to 340 ppm within six hours. The plant operator has three options: cleanse the whole unit and finish the shift with a loss in production, continue normal operation with the hope not to trip the Buchholz relay, or start carrying out diagnostics. Power transformer fault diagnostics is exactly the field of study that leads to differentiating between the scheduled maintenance downtimes and forced outages that take away $50,000 or more from the company in lost revenue, transport cost, and emergency electricity purchase. In this article, one can find the information about the most common types of faults, their detection methods, and step-by-step instructions for doing fault diagnostics in practice.<\/p>\n

We explain the physical cause of every type of fault, the diagnostic devices responsible for detecting them, the approximate funds necessary for testing equipment and third-party diagnostics, and the maintenance work that allows avoiding the majority of faults long before they reach the circuit breaker.<\/p>\n

In simple terms, troubleshooting a power transformer fault means following a clearly defined knowledge-based procedure aimed at identifying, measuring and rectifying transformer malfunctions in transformers of more than about 1 MVA, by means of such tests as DGA (dissolved gas analysis), insulation resistance testing, winding resistance testing and transformer oil tests. An estimated 80% of the faulty transformers fail due to electrical faults caused by insulation breakdown, partial discharge or surge impact, with thermal and machine-related failures accounting for the vast majority of the rest. What we should keep in mind is that we must not remove the cover of a transformer until the results of DGA and electrical tests have shown what the failing subsystem is.<\/p><\/blockquote>\n

\"Practical<\/p>\n

\n

What Are the Most Common Power Transformer Faults?<\/h2>\n

Transformers are mature pieces of equipment which are known to be very reliable, and can last for about 30 to 40 years if they are built properly and are properly cooled. Failure of transformers usually falls into one of five categories, the first being winding insulation breakdown which causes around 35-40% of the failures of such equipment. This is connected with short circuits between turns, short circuit between different layers, as well as ground faults which occur due to deterioration of paper insulation. In addition, weaknesses in bushings are also responsible for 10-15% of failures, and this is particularly dangerous due to the possibility of highly flammable hot oil being sprayed from bushings when a failure occurs. Failures of on-load tap changers are compared to bushings as they account for a similar percentage of the failures, with forge problems being one of the main reasons \u2014 whether it is due to contact wear, oil contamination in the tap changer’s compartment, or motor drive breakdowns. Defects of core and magnet circuit can take place in the form of loose bolts or circulation of current caused by damage to insulation between laminations leading to local overheating and excessive no-load losses. In addition, problems related to cooling system oil, such as blocked up radiators, malfunctioning pumps and fans, low oil level, or high moisture levels lead to thermal degradation that starts the destruction of parts inside the transformer.<\/p>\n

How Transformer Faults Develop: Failure Modes & Mechanisms<\/h2>\n

The existence of faults needs a long process of degradation that occurs over time for a fault to appear. Most windings and insulating failures result from temperature stresses. According to the aging rule applied in the industry, the lifespan of insulation is halved when its temperature exceeds 6\u00b0C to 10\u00b0C above its rated temperature of 98 \u00b0C when using thermally improved paper. For instance, running a 10 MVA transformer under a load of at least 20% for the summer will consume years of insulation life even if the transformer will not trip.<\/p>\n

Electrical stresses lead to their own failure process. Partial discharges that appear as micro-discharge through bubble, void, and damaged insulation material can lead to slow breakdown of insulation. In the case of 33 kV winding, it should be noted that partial discharge activity of 100 pC is already a warning sign. And if there are transformer partial discharge of 1000 pC, the transformer will experience failure we could observe in couple of weeks. Presence of moisture can worsen both processes. In a case of transformer that has 2 % moisture level in its paper insulation, the paper can age about twice as fast as in a transformer without moisture influence which leads to destruction of insulation. Because of a moisture presence in the oil, the dielectric strength can be decreased from 40 kV (minimum rating according to IEC 60156) to less than 20 kV, which is enough to make failure happen. The mechanical force connected to short circuit currents on the winding can change and loosen it, so even if the transformer survives first case, the second through-fault can finally damage if it occurs in six months time between the faults. Please note that recognizing those mechanisms gives a very good basis for diagnosis of transformer faults.<\/p>\n

Fault Types, Warning Signs & Root Causes<\/h2>\n\n\n\n\n\n\n\n\n\n\n
Fehlerart<\/th>\nTypical Share of Failures<\/th>\nEarly Warning Signs<\/th>\nMost Common Root Causes<\/th>\n<\/tr>\n<\/thead>\n
Winding \/ insulation failure<\/td>\n35\u201340%<\/td>\nRising DGA acetylene\/ethylene, ratio deviation, hot oil<\/td>\nOverheating, moisture, PD, surge damage, loose clamping<\/td>\n<\/tr>\n
Bushing failure<\/td>\n10\u201315%<\/td>\nVisible cracks, oil leaks, capacitance change, PD at bushing<\/td>\nPorcelain damage, moisture ingress, contamination<\/td>\n<\/tr>\n
OLTC \/ tap changer fault<\/td>\n10\u201315%<\/td>\nUnusual tap-change noise, contact resistance rise, oil darkening<\/td>\nContact wear, carbonized oil, mechanical wear<\/td>\n<\/tr>\n
Core fault<\/td>\n8\u201312%<\/td>\nIncreased no-load loss, local hot spots, unusual hum<\/td>\nLoose laminations, damaged core insulation, circulating currents<\/td>\n<\/tr>\n
Cooling \/ oil system fault<\/td>\n10\u201315%<\/td>\nHigh top-oil temperature, low oil level, pump\/fan trips<\/td>\nBlocked radiators, failed pumps, leaks, moisture ingress<\/td>\n<\/tr>\n
Overload \/ external system event<\/td>\n5\u201310%<\/td>\nSustained high load, harmonic heating, protective trip<\/td>\nSustained overload, harmonics, voltage imbalance<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The table contains two important pieces of information regarding the troubleshooting process. One worth noting is that the data showing the \u201cshare of failures\u201d are gathered from combined research into different failures in the industry carried out by CIGR\u00c9 and IEEE, which have identified the variations in these figures according to the voltage class of the transformers. For example, the distribution class transformers suffer from far more overload failures than transformers transmitting over 110 kV suffer from dielectric and OLTC failures. Moreover, a transformer\u2019s insulating oil is a hidden message, as it indicates the type of failure occurring.<\/p>\n

\"Fault<\/p>\n

Fault Symptoms vs. Likely Causes<\/h2>\n\n\n\n\n\n\n\n\n\n\n
Observed Symptom<\/th>\nMost Likely Fault<\/th>\nConfirm With<\/th>\nSecondary Suspects<\/th>\n<\/tr>\n<\/thead>\n
Sudden Buchholz relay trip<\/td>\nSevere internal fault (turn-to-turn, core)<\/td>\nDGA + winding resistance + ratio test<\/td>\nRapid moisture release, gas accumulation<\/td>\n<\/tr>\n
Gradual temperature rise at constant load<\/td>\nCooling system degradation<\/td>\nOil flow check, pump\/fan current, DGA<\/td>\nOil degradation, higher ambient, blockages<\/td>\n<\/tr>\n
High no-load loss and loud hum<\/td>\nCore damage or poor lamination insulation<\/td>\nNo-load loss test, core ground current<\/td>\nMechanical looseness, DC magnetization<\/td>\n<\/tr>\n
Decreasing insulation resistance<\/td>\nMoisture ingress or contamination<\/td>\nInsulation resistance test, oil breakdown voltage<\/td>\nPaper aging, oil sludge<\/td>\n<\/tr>\n
Voltage ratio out of tolerance<\/td>\nWinding shorted turns or OLTC damage<\/td>\nTurn ratio test (TTR), winding resistance<\/td>\nTap changer misalignment<\/td>\n<\/tr>\n
Oil colour darkening and sludge<\/td>\nThermal degradation of oil<\/td>\nAcid number (IEC 62021), interfacial tension<\/td>\nOverheating, oxidation, contact with air<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

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.<\/p>\n

Key Diagnostic Tests & Acceptable Limits<\/h2>\n\n\n\n\n\n\n\n\n\n\n\n\n
Test<\/th>\nStandard Reference<\/th>\nTypical Acceptable Range<\/th>\nWhen to Run<\/th>\n<\/tr>\n<\/thead>\n
Dissolved gas analysis (DGA)<\/td>\nIEC 60599 \/ IEEE C57.104<\/td>\nTotal combustible gas < 720 ppm; key gas ratios normal<\/td>\nAnnual; immediately after any trip or abnormal event<\/td>\n<\/tr>\n
Insulation resistance (IR) \/ PI<\/td>\nIEEE 43<\/td>\nPolarization index \u2265 2.0 for dry paper; IR per kV rating<\/td>\nAnnual; before energizing after maintenance<\/td>\n<\/tr>\n
Winding resistance<\/td>\nIEC 60076-1<\/td>\nPhase imbalance \u2264 2% between phases<\/td>\nAfter trips, OLTC inspections, suspected shorted turns<\/td>\n<\/tr>\n
Turn ratio test (TTR)<\/td>\nIEC 60076-1<\/td>\nDeviation \u2264 0.5% from nameplate<\/td>\nAnnual; after reconnection or OLTC work<\/td>\n<\/tr>\n
Oil breakdown voltage (BDV)<\/td>\nIEC 60156<\/td>\n\u2265 40 kV for new oil; \u2265 30 kV acceptable in service<\/td>\nEach oil sample; before oil reclamation<\/td>\n<\/tr>\n
Moisture in oil \/ paper<\/td>\nIEC 60814 \/ Karl Fischer<\/td>\n< 2% moisture in paper; < 10 ppm water in oil (typical)<\/td>\nAnnual; after suspected moisture ingress<\/td>\n<\/tr>\n
Teilentladungsmessung<\/td>\nIEC 60270<\/td>\nPD level below manufacturer acceptance (often < 100 pC at HV)<\/td>\nFactory tests; after major insulation disturbance<\/td>\n<\/tr>\n
Frequency response (SFRA)<\/td>\nIEC 60076-18<\/td>\nLow deviation index across frequency sweep<\/td>\nAfter through-faults or transport; baseline comparison<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

These limits are working numbers, rather than evitable ones, since a transformer that always worked under 5% moisture equivalent indicators could still work properly \u2014 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.<\/p>\n

Where Faults Hit Hardest: Applications & Critical Assets<\/h2>\n

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.<\/p>\n

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.<\/p>\n

Diagnostic Tools, Brands & Price Ranges<\/h2>\n\n\n\n\n\n\n\n\n\n\n\n
Tool \/ Service<\/th>\nRepresentative Brands<\/th>\nTypischer Preisbereich<\/th>\nAnmerkungen<\/th>\n<\/tr>\n<\/thead>\n
DGA sampling kit & lab test (per sample)<\/td>\nDoble, SGS, local utilities labs<\/td>\n$80\u2013$250 per sample<\/td>\nFull gas chromatography panel<\/td>\n<\/tr>\n
Portable DGA monitor (online)<\/td>\nKelman (GE), Serveron, Qualitrol<\/td>\n$15.000\u2013$45.000<\/td>\nContinuous monitoring of key gases<\/td>\n<\/tr>\n
Insulation resistance tester (5\u201310 kV)<\/td>\nMegger, Fluke, Doble<\/td>\n$1,500\u2013$6,000<\/td>\nFor IR, PI, DAR measurements<\/td>\n<\/tr>\n
Winding resistance meter (micro-ohmmeter)<\/td>\nMegger, Vanguard, DV Power<\/td>\n$4,000\u2013$12,000<\/td>\nLow-resistance measurement to 0.1 \u00b5\u03a9<\/td>\n<\/tr>\n
Turn ratio tester (TTR)<\/td>\nMegger, Vanguard, Doble<\/td>\n$5.000\u2013$15.000<\/td>\nFull 3-phase TTR with OLTC test<\/td>\n<\/tr>\n
SFRA test set<\/td>\nDoble, OMICRON, Megger<\/td>\n$20,000\u2013$60,000<\/td>\nWinding deformation analysis<\/td>\n<\/tr>\n
Third-party diagnostic service (full site battery)<\/td>\nDoble, Intertek, local certified labs<\/td>\n$5,000\u2013$25,000 per visit<\/td>\nDGA + electrical + oil tests on one unit<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

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.<\/p>\n

\"Building<\/p>\n

Building a Troubleshooting Workflow: 8 Steps<\/h2>\n

Any serious troubleshooting activity must adhere to the same structured sequence, as this is how seasoned technicians get into trouble.<\/p>\n