What Tests and Maintenance Are Required for Transformers?

Maintenance is required because transformers are among the most critical equipment in a facility, and problems such as oil deterioration, insulation aging, loose connections, overheating and accessory faults develop in them gradually over time. Left unnoticed, these small issues can turn into major failures and long outages: a neglected connection problem, insulation weakness or oil deterioration affects not only the transformer itself but the continuity of the entire facility it feeds. The purpose of a maintenance program is therefore to continuously monitor the transformer's electrical, thermal and mechanical health and to catch small deterioration before it becomes a failure, rather than reacting after the event. This is achieved through visual inspection, oil and insulation tracking, electrical tests, accessory and cooling checks, thermal inspection and disciplined record keeping.

Depending on the application, transformer testing includes DGA, oil quality tests covering moisture, dielectric strength and acidity, the transformer turns ratio test, winding resistance measurement, insulation resistance, power factor or tan delta and capacitance assessments, bushing checks including capacitance and power factor measurements on high-voltage units, thermal inspection and OLTC verifications. These electrical and oil-based tests are combined with the maintenance side: visual inspection of the tank, radiators, conservator, silica gel breather and connections on oil-immersed units, cleaning and airflow checks on dry-type units, cooling system verification covering fans, pumps and control logic, and checks of protection accessories such as the Buchholz relay, pressure relief devices and temperature indicators. The exact package is chosen according to transformer type and criticality, and results are archived for trend tracking.

DGA is important because it is one of the strongest methods for understanding thermal and electrical stresses inside a transformer at an early stage. By analyzing the gases dissolved in the transformer oil, it can reveal developing internal faults such as overheating, partial discharge, arcing and insulation deterioration before they grow into failures that take the unit out of service. This makes it a cornerstone of the maintenance program for oil-immersed transformers, alongside oil quality parameters such as moisture, dielectric strength and acidity. The value of DGA multiplies with repetition: results archived over successive samples show how gas levels are changing, and it is this change over time, rather than any single report, that exposes a slowly developing internal problem while there is still time to plan intervention.

The ratio test shows whether the transformer's winding ratio complies with the nameplate value and the tap positions. A deviation from the expected ratio may indicate winding problems, an incorrect tap position or internal connection defects, which makes this simple measurement a meaningful window into the transformer's internal condition. It provides especially valuable information at particular moments: after maintenance work, after any intervention on the OLTC and whenever suspicious operating conditions raise questions about the unit. Because tap changers alter the effective ratio by design, the test is evaluated against the specific tap position, not against a single number. Read together with winding resistance results and previous measurements of the same transformer, the ratio test helps separate harmless variation from genuine internal change requiring closer inspection.

The winding resistance test is performed because it provides important data about winding continuity, the health of internal connections, soldered and connection areas and, especially, tap changer contact quality. Deterioration in any of these areas shows up as abnormal resistance behavior long before it becomes a visible failure. Interpretation depends on comparison: abnormal increases are noticed far more easily when values are compared between phases or against previous measurements of the same transformer, which is why taking a single measurement in isolation is not sufficient and trend tracking is essential. In transformers with an on-load tap changer, the measurement doubles as an OLTC diagnostic, since worn or deteriorating contacts reveal themselves through resistance differences across tap positions. Results belong in the maintenance archive with the rest of the test history.

Bushings are checked separately because they are among the most sensitive external insulation elements of a transformer, and a bushing failure can be critical enough to take the entire unit out of service. During maintenance, the first points to check are surface contamination, cracks, oil leakage, damage to the porcelain or composite body, terminal looseness and thermal anomalies at the connections. Beyond the visual review, bushing capacitance and power factor measurements are important diagnostic tools for monitoring internal condition, especially on high-voltage transformers, because dielectric deterioration inside a bushing is invisible from outside. Thermal camera inspection supports this heading as well, since a bushing terminal running hotter than its neighbors is often the first field sign of looseness or increased resistance that deserves immediate attention.

OLTC maintenance is critical because the on-load tap changer is one of the parts of a power transformer exposed to the highest mechanical and electrical stress: it moves, switches and carries current while the rest of the transformer stands still. If it is neglected, voltage regulation may deteriorate and the risk of serious internal failure increases. The maintenance program therefore monitors tap changing behavior, the motor drive group, auxiliary contacts, mechanical synchronization, the condition of the diverter compartment in oil-type units and the required electrical checks. Related tests reinforce this work: the ratio test verifies behavior across tap positions, and winding resistance measurement exposes tap changer contact problems. OLTC records belong in the same maintenance archive as the rest of the transformer history so wear trends can be followed.

Yes, the maintenance approach for dry-type transformers differs from oil-immersed units because there is no oil system to analyze. The focus shifts to cleaning and air circulation: cooling ducts clogged with dust and surface accumulation reduce cooling efficiency, raise winding temperature and shorten transformer life. Regular cleaning, connection tightness checks, monitoring of resin surface cracks and verification of the temperature monitoring systems stand at the center of the program. Where a forced-air system exists, the health of the fans and unobstructed airflow are essential checks. Visual inspection covers the resin surfaces, body, ventilation ducts, dust accumulation and overheating marks. The common ground with oil-immersed units remains: connection checks, thermal assessment and record keeping apply to both, but oil analysis is replaced by cleanliness and cooling discipline.

A thermal camera detects abnormal heating on a transformer before it can be seen or felt, making it a very efficient maintenance tool. The most valuable inspection points are bushing terminals, cable connections, busbar contact points, radiator connections, OLTC connections and the interiors of auxiliary panels, where hot spots caused by looseness or increased resistance appear early. Interpretation relies on comparison rather than absolutes: what matters is not a single temperature value but the differences between similar phases and similar connections, since a clear deviation is what signals a developing problem. A thermal anomaly is often the first visible field sign of a growing fault. Thermal images should be archived with the rest of the maintenance records so that heating trends can be followed from one inspection to the next.

Record keeping is important because many transformer problems are understood not through a single measurement but through change over time. DGA results, oil test reports, ratio and winding resistance measurements, bushing assessments, thermal images, OLTC maintenance records and accessory checks should all be archived regularly. With this history, slowly developing deterioration becomes visible before a fault occurs: gas levels creeping upward in successive DGA samples, winding resistance drifting away from earlier values or a connection warming a little more at each thermal scan are all trends that individual reports cannot reveal. Trend tracking therefore provides far stronger decision support than any standalone result, guiding decisions about deeper diagnostics, planned intervention or continued operation, and turning the maintenance program from a series of snapshots into a continuous picture of transformer health.