What Tests and Maintenance Are Required for Power transformers?

Power transformers systems that convert the voltage level of electrical energy and are among the most critical equipment in terms of energy continuity of the facility. For this reason, the required tests and maintenance in the power transformers are not just about looking for leaks from the outside. The main purpose is to ensure safe operation by evaluating the transformer windings, magnetic core, isolation system, oil, bushinglerini, cooling system, protection equipment and connection points together. The correct approach to power transformer maintenance is not to intervene after the malfunction occurs, but to recognize the failure tendency early.

The first step in maintenance is always safety. Before working on power transformer, the transformer should be de-energized, the relevant MV and LV sides should be safely separated, the absence of voltage should be verified, and the necessary grounding operations should be carried out. Since transformer is a piece of equipment that carries significant energy on both the medium voltage and low voltage sides, incorrect maneuvering, incomplete grounding, or improper connection during maintenance can pose a serious risk to personnel and equipment. Therefore, the testing and maintenance process must be carried out with the correct occupational safety procedure.

Visual inspection is the foundation of power transformer maintenance. Transformer tank, radiators, bushings, oil level gauge, thermometers, fans, cable connections, gasket areas, grounding connections and protection equipment should be visually examined. Findings such as oil leakage, rust, corrosion, paint blistering, bushing cracks, insulator contamination, loose connection, tarnishing, abnormal sound or odor should be taken seriously. A seemingly small leak or connection looseness in the transformer can turn into a major failure over time.

In oil-lubricated power transformers, transformer oil serves as both insulation and cooling. Therefore, the oil level, oil color, oil leakage and general condition of the oil should be checked regularly. Low oil level may cause the winding and core heat to not be adequately transported and the insulation safety to be weakened. Oil leakage is not just a loss problem. It can also lead to moisture and air ingress. Therefore, the oil condition in oily transformers is one of the most critical topics of maintenance.

Transformer oil tests give invaluable information about the internal health of the power transformer. Parameters such as dielectric strength testing, moisture content, acidity, interfacial tension, and overall oil quality help assess the oil's insulating properties. Deterioration in oil values may indicate aging or contamination in the transformer's internal insulation system. For this reason, oil analysis is not a one-time but a maintenance data that should be monitored at regular intervals.

DGA i.e. Dissolved gas analysis is one of the most important advanced diagnostic methods in oily power transformers. Thermal stresses, arc events, partial discharge or insulation deterioration in the transformer can cause different gases to form in the oil. The type and amount of these gases help to understand what kind of failure tendency in the transformer has. DGA result should be evaluated together with past values and other tests, not for making a hasty decision alone.

Bushing controls are of particular importance in power transformer maintenance. The bushings ensure that the energized ends of the transformer are safely removed from the tank. Serious risk can occur if there are cracks, contamination, oil leakage, surface marks, signs of partial discharge, loose connections, or abnormal heating in these areas. Bushing failures can often have sudden and severe consequences. For this reason, bushing health should be monitored with both visual control and, where appropriate, capacitance and tan delta measurements.

Insulation resistance measurement is a basic control within power transformer tests. The general condition of the insulation is evaluated by measuring the insulation resistance at the winding-ground, between the windings and at the appropriate connection points. This test provides preliminary information about moisture, contamination, or insulation weakness. In large power transformers the polarization index can also be monitored. However, since the insulation resistance result is affected by temperature, humidity, and previous test conditions, it should not be interpreted in isolation but in conjunction with trend following.

Winding resistance testing is done to assess the condition of transformer windings and connection points. Obvious resistance difference between phases can warn of loose connection, winding problem, tap changer contact, or connection failure. Especially in transformers with OLTC or tap changers, winding resistance measurement at different tap positions provides valuable information. This test helps to understand whether the main current-carrying paths of the transformer are healthy.

The turns ratio test checks whether the primary and secondary voltage ratio of the transformer is correct. Also known as TTR testing, this process is used to evaluate the accuracy of winding ratio, connection group, and tap positions. Incorrect turns ratio can mean a winding problem, connection error, or tap changer mismatch. In commissioning, maintenance, and post-failure control processes, the turns ratio test is an important validation step.

Tan delta and capacitance measurement is used to evaluate the insulation system of the power transformer in more detail. This test reveals dielectric losses and capacitive behavior within the insulation. Windings, bushings and some insulation sections can be examined with this method. Increase in tan delta value. It can warn of moisture, aging, contamination or insulation deterioration. This test is an essential diagnostic method, especially in critical transformers, empowering maintenance decisions.

OLTC maintenance is a separate heading for the power transformers with a tap changer under load. OLTC works to regulate the transformer output voltage under load and includes a mechanical-electrical contact structure. Due to the arcing and mechanical movement that occurs during the stage transitions, OLTC can wear out over time. For this reason, contact status, transition resistances, mechanical operation, engine mechanism, oil status and control circuit should be checked regularly. OLTC malfunctions can directly affect of the transformer operation.

The cooling system must be checked during maintenance. Radiators, fans, oil pumps, fan contactors, temperature relays, thermostats, and wiring connections should be inspected. Whether the fans are engaged, the cleanliness of the radiator surfaces, the adequacy of oil circulation, and the correct operation of temperature gauges should be evaluated. Lack of cooling raises the winding temperature and accelerates insulation aging. Therefore, the cooling system is one of the most important auxiliary systems that determine the life of the transformer.

Protection equipment should be tested separately in power transformer maintenance. Buchholz relay, pressure relief device, oil level gauge, oil temperature gauge, winding temperature gauge, fan control system, alarm contacts and trip outputs should be checked. On the electrical protection side, differential protection, overcurrent, ground fault and temperature protections must work correctly. If the protection equipment is not working, the transformer may not be able to separate the fault from the system before it escalates.

Thermal camera inspection is a very useful method in power transformer maintenance. During the thermal control under load, bushing connections, cable lugs, busbar connections, radiators, fan zones and panel connections can be examined. A significant temperature difference between phases or abnormal heating at the junction may mean a loose connection or an increase in contact resistance. A thermal camera helps to catch unnoticed heating problems early.

Grounding connections should be checked regularly. Transformer tank, neutral point, cable shields, surge arrester connections, and protection grounds should have a secure and low-resistance connection. A loose or broken ground connection can weaken personnel and equipment safety in the event of a fault. In particular, the continuity of equipotential connections in transformer substation should be reviewed.

The maintenance approach in dry-type power transformers differs from that of oil-lubricated transformers. Oil analysis is not performed in these transformers; instead, winding surface cleanliness, ventilation, temperature sensors, fan system, insulation resistance, connection tightness and dust-moisture effect come to the fore. Dirt and dust accumulation in dry type transformers can cause superficial leaks and heating. For this reason, the cleanliness of the environment where dry type transformers are located and air circulation are of great importance.

Connection tightness and mechanical controls should not be neglected in power transformers. Primary and secondary connections, bushing terminals, busbar connections, cable lugs, fan circuits, control cables and protection contact connections should be checked. Loose connections can cause heating, arcing, and equipment damage over time. Proper torque control and joint integrity should be assessed during maintenance.

At the end of the maintenance, all test results should be recorded. Insulation resistance, polarization index, winding resistance, turns ratio, tan delta, capacitance, oil tests, DGA results, thermal camera findings, temperature records, alarm history and interventions should be archived regularly. The most valuable approach in power transformer maintenance is trend following. Because rather than a single measurement, it shows the change of values over time the transformer's true health.

In summary, the required tests and maintenance in the power transformers. It consists of safe operation preparation, visual inspection, oil level and leakage check, transformer oil tests, DGA analysis, insulation resistance measurement, winding resistance test, turns ratio test, tan delta and capacitance measurement, OLTC maintenance, bushing check, cooling system check, protection equipment tests, thermal camera inspection, and verification of ground connections. When this maintenance approach is implemented regularly, transformer failures are detected earlier, equipment life is extended and the energy continuity of the facility becomes more secure.