Power Transformer Operation: Monitoring, Cooling and Safety

When many people first take over a transformer, they will simply understand the operation as closing the gate to deliver power, and then take a look at it every now and then. Anyone who has actually managed equipment in a substation or workshop knows that operation is something that must be monitored from beginning to end: what to check before power is supplied, which signals to observe during operation, what to do if the temperature is high, how to judge if there is an unusual noise, and under what circumstances the plant must be shut down immediately. Only by streamlining these aspects can the equipment be considered safe and long-lasting. Let’s go through the process and explain these points one by one.

Operating environment and basic conditions

Transformers don’t work properly wherever they are placed. The installation location must first meet mechanical requirements such as foundation load, levelness, seismic resistance and vibration isolation, while leaving grounding and maintenance space. For dry-type transformers, room ventilation is particularly critical: the no-load and load losses generated during operation will turn into heat, and hot air must be able to be exhausted. Generally, air enters from a low place and hot air is exhausted from a high place, and the air duct cannot be blocked. Taking common dry-type variable frequency speed rectifier transformers as an example, the standard operating conditions are an altitude of no more than 1,000 meters, a maximum ambient temperature of 40 degrees Celsius, and a minimum ambient temperature of -5 degrees Celsius for household products. There must be no harmful gases and dust in the ambient air that corrode or damage the insulation, and the equipment must not be exposed to water, rain or snow. When conditions exceed these limits, they should be proposed during the ordering stage to be specially designed according to the technical agreement.

There is another point that is easily overlooked in products with forced air cooling: the power supply, steering, interlocking, alarm and backup strategies of the fan must all conform to the design. Once the fan is shut down, the available capacity of the equipment must be reduced according to the nameplate or manufacturer’s data, and it cannot be used at full capacity.

Pre-commissioning inspection and trial operation

What needs to be done before power is often more important than the power itself. After arrival, check the nameplate, packing slip and accessories to see if there is any looseness or damage caused by transportation. Before installation, clean the iron core, coils and air ducts of foreign matter and dust, and check the fasteners, multi-point grounding of the iron core and winding insulation. The core insulation resistance to ground is usually required to be no less than 5 megohms, and the winding insulation resistance to ground must be dried before it falls below the permissible value in the manual. Grounding must also be reliable. The body, housing, fan and temperature control device must all be grounded, and the grounding resistance is generally controlled within 4 euros.

After the fan is connected, the steering and three-phase uniformity must be checked. The three platinum thermal resistors must be inserted into the three-phase temperature measuring tubes and inserted at the same depth. The temperature control device must connect the fan start and stop, over-temperature alarm, and over-temperature trip points. These were all confirmed to be correct before entering trial operation.

The first step in the trial operation is no-load impact closing: with the secondary side disconnected, the gate is closed five times at the rated voltage, with the first power supply lasting no less than 10 minutes, and then closed every 5 minutes thereafter. There is a reason for this caution. The excitation surge when the gate is closed without load can reach 10 to 12 times the rated current. Although it only decays within a few cycles to a few seconds, the protection must be able to withstand it, otherwise it will jump accidentally as soon as the gate is closed. After five impact closures without any abnormalities, the test run is completed after 30 minutes of no-load operation, and then gradually bringing the load to the rated value and running continuously for 24 hours without any abnormalities.

Key points of monitoring during operation

After entering normal operation, the signals to be monitored daily actually follow a pattern: load current, voltage, temperature, cooling equipment, sound, vibration, local overheating, odor, discharge traces and alarm events. For dry transformers, the focus is on ventilation, dust accumulation, condensation and winding surface conditions; for liquid immersion equipment, more attention should be paid to oil temperature, oil level, pressure, leakage and casing, and dissolved gas analysis should be performed if necessary.

Temperature is the most intuitive and most prone to problems. 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, ventilation is blocked, the three-phase current is unbalanced, there are harmonics, and the connection points are hot. These factors are often the root cause. 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.

Infrared temperature measurement is a good auxiliary method, but it is necessary to note the load and environmental conditions at that time, and focus on comparing the temperature difference between the three phases and between each connection point. One thing to note: the absence of hot spots during low loads does not mean that it will be normal during peak loads.

What aspects are covered by Power Transformer Operation and Maintenance

Operation and maintenance are never performed mechanically according to a general periodic table. Maintenance plans should be determined based on product type, importance, environment, load, manufacturer requirements, and historical status. Dry and liquid immersion, general power distribution, and rectifier equipment all have different focuses.

Dry product maintenance typically includes infrared inspection, cleaning, ventilation and fan inspection, connection fastening, insulation appearance, temperature control alarms, and necessary insulation or ratio testing. Liquid-immersed products should cover leak and oil level checks, cooling equipment, casing, tap switches, insulating fluid tests, dissolved gas analysis, and temperature meter checks. Taking dry rectifier transformers as an example, it is recommended to have power outages and maintenance every six months in harsh environments, and the normal environmental maintenance interval 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 should also be checked and cleaned regularly. After cleaning, the dust screen must be dried and then put back in.

How to implement Preventive Maintenance of Power Transformer

The goal of preventative maintenance is to resolve problems before they become malfunctions. The key to this matter is not to list more items in the examination, but to use the diagnostic results. 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 multiple pieces of evidence confirm each other. 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.

Several security boundaries deserve to be emphasized separately. 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, the fault cannot be put back into operation until the cause is clearly found. Moisture-affected products can be dried using infrared lamps, ovens, hot air, or short-circuit methods. 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 deemed acceptable. Before re-energizing after each maintenance, confirm that tools and temporary grounding have been removed, wiring and taping are correct, 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. For example, dust, condensation or conductive contamination on the surface 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.

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 shut down until the cause of the fault is determined. Repeated test deliveries cannot be used to determine whether the equipment is still usable.

Treat operation as a closed loop for management. Check before power supply, monitor during operation, perform regular maintenance, and shut down decisively in case of abnormalities. Many transformer failures can actually be blocked out of the door in advance.