What Tests and Maintenance Are Required for Transformers?

Transformers are among the most critical system equipment that ensure electrical energy is transformed safely and efficiently. Therefore, the tests and maintenance required for transformers are not processes that come to the agenda only when a fault occurs. The main purpose is to continuously monitor the electrical, thermal and mechanical health of the transformer and to notice small deterioration before it turns into a major failure. Because a neglected connection problem, insulation weakness or oil deterioration in a transformer can affect not only the equipment itself, but also the continuity of the entire facility to which it is connected. For related context, see What Is a Transformer Differential Protection Relay? What Does It Do, How Does It Work and Why Is It Used?.

The first step of maintenance is always safety. Before working on a transformer, the related section should be taken out of service with the correct switching sequence, all connections should be made safe and absence of voltage should be confirmed. Especially in oil-immersed power transformers, cutting electrical energy alone is not sufficient; internal tank pressure, hot surfaces, oil level and auxiliary systems should also be considered. Therefore, transformer maintenance should be planned in accordance with the discipline of working on primary equipment. For related context, see What Tests and Maintenance Are Required for Transformer Differential Protection Relays?.

Visual inspection is the basis of transformer maintenance. In oil-immersed transformers, oil leakage, radiator condition, tank body, paint damage, rust, bushing surfaces, connection terminals, conservator, silica gel breather, Buchholz connections and cooling equipment should be visually inspected. In dry-type transformers, resin surfaces, body, ventilation ducts, dust accumulation, cracks, connection tightness and overheating marks should be checked carefully. Although visual inspection seems simple, the first sign of many serious failures is seen here. For related context, see What Is a Transformer Substation? What Does It Do, How Does It Work and Which Sections Does It Consist Of?.

In oil-immersed transformers, oil condition is one of the most critical headings of the maintenance program. Dissolved Gas Analysis, or DGA, is one of the strongest methods for understanding thermal and electrical stresses inside the transformer at an early stage. Through analysis of gases dissolved in oil, developing faults such as overheating, partial discharge, arcing or insulation deterioration can be detected before they grow. In addition, moisture, dielectric strength, acidity and general quality condition of the oil are important for transformer service health. For related context, see What Tests and Maintenance Are Required for MV Cable Terminations?.

One of the most basic headings among electrical tests is the transformer turns ratio test. This test checks whether the winding ratio of the transformer complies with the nameplate value and tap positions. Ratio deviation may indicate winding problems, incorrect tap position or internal connection defects. Especially after maintenance, after OLTC intervention or in suspicious operating conditions, the ratio test provides very valuable information.

Winding resistance measurement is also one of the basic tests required for transformers. The winding resistance test provides important data about winding continuity, internal connections, soldered or connection areas and especially tap changer contact health. Abnormal increases can be noticed more easily when values are compared between phases or with previous measurements of the same transformer. Therefore, taking a single measurement alone is not sufficient; trend tracking is very important here.

Insulation and dielectric health assessments are critically important for long transformer service life. Tests such as insulation resistance, power factor or tan delta and capacitance help identify signs of moisture, contamination, aging and deterioration in the main insulation system. Especially in power transformers, the dielectric condition between winding-earth and between windings tells much more than a simple visual check. Therefore, an appropriate dielectric test package should be included in the maintenance plan according to transformer type and criticality.

Bushings are among the most sensitive external equipment of a transformer and should be evaluated separately during maintenance. Surface contamination, cracks, oil leakage, porcelain or composite body damage, terminal looseness and thermal anomaly are the first points to check. In addition, bushing capacitance and power factor measurements are important diagnostic tools for monitoring bushing health, especially in high-voltage transformers. Because a bushing failure can create results critical enough to take the entire transformer out of service.

In transformers with an on-load tap changer, or OLTC, the maintenance program becomes even more important. Because in many power transformers, the OLTC system is one of the parts exposed to the highest mechanical and electrical stress. Tap changing behavior, motor drive group, auxiliary contacts, mechanical synchronization, diverter compartment condition in oil-type units and required electrical checks should be monitored carefully. If the OLTC is neglected, transformer voltage regulation may deteriorate and the risk of serious internal failure may increase.

Cooling system control directly determines transformer life. In oil-immersed transformers, fans, pumps, radiators, thermometers, alarm and trip contacts, automatic control logic and auxiliary supplies should operate correctly. If the fan or pump group does not start on a loaded transformer on hot days, insulation life can be reduced significantly. In dry-type transformers, cleanliness of cooling ducts, health of fans if a forced-air system exists and unobstructed airflow are very important.

Protection and auxiliary accessories are also a natural part of maintenance. Buchholz relay, pressure relief devices, oil level indicators, temperature indicators, moisture removal system, alarm contacts and auxiliary wiring should be checked regularly. Although these elements do not appear as large as the main transformer, they are often the first parts that warn of an approaching fault. Therefore, not only the main tank but all auxiliary systems of the transformer should be evaluated as a whole.

Thermal camera inspection is a very efficient maintenance tool for transformers. When bushing terminals, cable connections, busbar contact points, radiator connections, OLTC connections and auxiliary panel interiors are thermally inspected, hot spots caused by looseness or increased resistance can be detected early. The important point here is not a single temperature value, but evaluating differences between similar phases and similar connections. A thermal anomaly is often the first visible field sign of a growing problem.

In dry-type transformers, unlike oil-immersed transformers, the maintenance approach focuses more on cleaning and air circulation. Cooling ducts clogged with dust and surface accumulations reduce cooling efficiency; this increases winding temperature and shortens life. Therefore, regular cleaning, connection tightness checks, monitoring of surface cracks and verification of temperature monitoring systems should be at the center of the maintenance plan for dry-type transformers.

One of the most important parts of maintenance is record keeping. DGA results, oil test reports, ratio and winding resistance measurements, bushing assessments, thermal images, OLTC maintenance records and accessory checks should be archived regularly. Because many transformer problems are understood not through a single measurement, but through change over time. When trend tracking is performed, slowly developing deterioration is noticed before a fault occurs. In summary, the tests and maintenance required for transformers consist of visual inspection, oil and insulation health tracking, ratio and winding resistance tests, dielectric assessments, bushing and OLTC checks, cooling system verification, thermal inspection and record management carried out together. If transformer tests, maintenance planning, MV/HV connections and operational safety in your facility will be evaluated together, it is possible to proceed in an integrated way with transformer maintenance and testing, HV/MV testing, maintenance and repair, LV/MV/HV project design and consultancy and HV operation responsibility services.