Power Transformer Operation and Maintenance

Many people think that transformer operation and maintenance only begins after the manufacturer delivers the goods. In fact, it starts on the day the equipment arrives and lasts throughout the entire life of the equipment. Anyone who has managed transformers for a long time understands that whether the equipment will break down often does not depend on the manufacturing process, but more on the operation and maintenance methods. Only by clarifying what to check before power is supplied, which signals to monitor during operation, how to arrange maintenance, and how to determine whether to shut down when there is an abnormality can the reliability and life of the transformer be guaranteed. Let’s go through the running and maintenance from the beginning in this order.

Daily operation monitoring of Power Transformer Operation

When running this thing, the signals that are monitored daily actually follow a pattern. In summary, these include load current, voltage, temperature, cooling equipment, noise, vibration, local overheating, odor, discharge traces, and alarm events. Dry equipment and liquid-immersed equipment each have their own focus: dry transformers should focus on ventilation, dust accumulation, condensation and winding surface conditions, while liquid-immersed equipment should pay more attention to oil temperature, oil level, pressure, leakage and casing, and perform dissolved gas analysis when necessary.

Temperature is the most intuitive and most prone to problems. However, when the temperature is abnormal, don’t just focus on the temperature gauge itself. At the same time, check whether the load, ambient temperature, fan or pump is normal, ventilation is blocked, the three-phase current is unbalanced, there are harmonics, and the connection points are heated. These are often the root causes. The same applies to abnormal noise. It can come from over-excitation, harmonics, loose magnetic cores or structures, fans or pumps, or even external resonance. The internal fault cannot be determined by the sound level alone.

Another point that is easily overlooked is that overload capacity is not a fixed percentage. Whether it can be overloaded, how much it can be overloaded, and how long it can last must be evaluated together with the effects of initial load, ambient temperature, hot spot temperature, cooling status, duration, and insulation aging; a single number cannot be simply applied.

Planning and implementation of Preventive Maintenance of Power Transformer

The most taboo aspect of maintenance is applying the same fixed periodic table to all equipment. Maintenance plans should be tailored to product type, importance, environment, load, manufacturer requirements, and historical status. Maintenance of dry products typically includes infrared inspection, cleaning, ventilation and fan inspection, connection fastening, insulation appearance, temperature control alarm, and necessary insulation or ratio testing; liquid-immersed products are covered with leak and oil level inspection, cooling equipment, casing, tap changers, insulation fluid testing, dissolved gas analysis, and temperature meter inspection.

Taking dry rectifier transformers as an example, it is recommended to have power outages and maintenance every six months in harsh environments, and the maintenance interval in normal environments should not exceed one year. Power outage maintenance involves inspecting coils, cores, star-sealing wires, tap terminals, and fasteners; removing dust; maintaining fans and bearings; checking tap switches; and retesting insulation resistance. The dust screen and the air outlet of the cabinet top fan must also be inspected and cleaned regularly. The cleaned dust screen must be dried and then put back in. Putting it back in with moisture will only sow hidden dangers.

The real purpose of preventative maintenance is to resolve problems before they become malfunctions, and this depends on how the diagnostic results are used. Any test data should be placed in trends and benchmarks: compared with historical values, compared with other phases of the same model and other equipment, and confirmed with multiple pieces of evidence. A transformer should not be directly deemed scrapped if the insulation resistance of a single transformer is low or the value of a gas is abnormal without review. Trends are far more important than single readings.

Several security boundaries in operational maintenance

Some safety boundaries must be emphasized separately because if stepped on, the consequences are more serious than equipment damage. Adjusting the unexcited tap must be done after a complete power outage, isolation, and power testing, and the three-phase tap positions must be consistent. After the protection action, it cannot be put back into operation until the cause of the fault is clearly found. Moisture-affected products can be dried using infrared lamps, ovens, hot air, or short-circuit methods, but the heating rate must be controlled, and the insulation resistance must be continuously measured until the readings stabilize and meet the standards several times in a row, at which point drying is considered qualified. Before re-energizing after each maintenance, it is also necessary to confirm that the tools and temporary grounding have been removed, the wiring and taping are correct, the personnel have been evacuated, and protection and cooling have returned to normal.

Abnormal signal identification and safe shutdown

Abnormal temperature rise, odor, discoloration, noise, vibration, leakage, discharge traces, protective actions, and sudden changes in trend data are all signals that require further diagnosis, rather than conclusions about a particular malfunction. Dust, condensation or conductive contamination on the surface area of dry products may cause creepage, local overheating or even insulation breakdown; partial discharge is often caused by air gaps, tip electric fields, contamination, cracks or interface defects, and long-term development will gradually erode the insulation. Infrared imaging is a good way to detect relative hot spots, but it is affected by load, surface emissivity and observation conditions. The temperature difference between phases and connection points must be compared while recording the load and environment. The absence of hot spots at low load does not mean that they must be normal at peak times.

There are two situations in which power must be cut off immediately: one is a fault that endangers the safety of the transformer and the protective device refuses to move; the other is a fire or explosion of nearby equipment that threatens the transformer. After the shutdown, the equipment must remain out of service until the cause of the fault is found out. Repeated trial delivery cannot be used to determine whether the equipment is still usable.

Treat operation and maintenance as a closed loop for management, including pre-power supply verification, in-operation monitoring, regular maintenance, and decisive shutdown in case of abnormalities. Many transformer failures can actually be blocked out of the door in advance.