1. Daily inspection of transformers:

    1) The oil temperature and thermometer of the transformer should be normal, the oil level of the oil conservator should correspond to the temperature, and there should be no oil leakage or seepage at any part.

    2) The oil level in the bushing should be normal. The exterior of the bushing should be free from damage, cracks, severe oil stains, discharge marks, and other abnormalities.

    3) The sound of the transformer is normal.

    4) The temperature felt on each cooler should be similar, and the fans and oil pumps should operate normally. The oil flow relay should work properly.

    5) The moisture absorber is intact and the adsorbent is dry.

    6) The lead connectors, cables, and busbars should show no signs of heating.

    7) The pressure relief valve, safety vent, and explosion-proof membrane should be intact and undamaged.

    8) There should be no gas inside the gas relay.

    9) The control box and secondary terminal box should be tightly closed and protected from moisture.

2. Special inspection items for transformers:

    1) Within 75 hours of putting into operation, new equipment or transformers that have undergone maintenance or modification.

    2) When there are serious defects.

    3) When there is a sudden change in weather conditions (such as strong winds, heavy fog, heavy snow, hail, cold waves, etc.).

    4) During thunderstorm season, especially after snow and rain.

    5) During high temperature seasons and peak load periods.

    6) When the transformer is operating under emergency load conditions.

External inspection items for dry-type transformers

  (1) Check whether the outer surface of the bushing and winding resin insulation of the transformer is clean, and whether there are creepage marks and carbonization phenomena;

   (2) The grounding of the high and low voltage bushing leads of the transformer is tight, without heating, cracks, or discharge phenomena;

   (3) Check the fasteners, connectors, conductive parts, and other components for signs of rust or corrosion, and ensure that the conductive parts are in good contact;

   (4) Check whether the cables and buses are normal;

   (5) Check whether the electrical equipment in the temperature chamber of the air cooling system is operating normally and whether there are any abnormalities in the signal system;

   (6) Listen carefully to the sound of the transformer from outside the barrier to determine if there are any abnormalities and if the operation is normal;

   (7) Check whether the grounding of transformer base, railings, and cable grounding wires in the transformer room is reliable and in good condition;

   (8) Use a temperature checker to inspect whether the temperature of the contactor parts and the enclosure exceeds the standard.

Abnormal operation and fault handling of transformers

1. Abnormal state of transformer

   (1) Severe oil spill;

   (2) The oil level cannot be seen inside the oil conservator or the oil level is too low;

   (3) Carbonization of transformer oil;

   (4) Abnormal increase in oil level;

   (5) There is an abnormal sound inside the transformer;

   (6) The porcelain components exhibit abnormal discharge sounds and sparking phenomena;

   (7) The transformer bushing has cracks or severe damage;

   (8) Overheating of the lead clamp of the transformer’s high and low voltage bushing;

   (9) Cooling device malfunction;

   (10) The gas within the Buchholz relay continuously accumulates, triggering continuous signaling actions;

   (11) Under normal load and cooling conditions, the oil temperature rises abnormally.

2. Abnormalities during operation and their handling

phenomenon

   (1) The sound of the transformer has noticeably increased, which is highly abnormal, and there is a cracking sound inside.

   (2) The casing exhibits severe damage and discharge phenomena.

   (3) The transformer is emitting smoke and catching fire.

   Handling: When the above situation occurs, the personnel on duty should immediately shut down the transformer.

3. Handling when the oil temperature exceeds the specified limit

   (1) Check the transformer load and the temperature of the cooling medium, and compare them with the normal temperature under the same load and cooling medium temperature.

   (2) Check the temperature measuring device.

   (3) Check the transformer cooling device.

   (4) If the temperature rise is due to a malfunction in the cooling system and cannot be repaired during operation, the transformer should be shut down for repair. If immediate shutdown for repair is not feasible, the personnel on duty should adjust the transformer’s load to the corresponding capacity at the permissible temperature as per the on-site regulations.

   (5) Under normal load and cooling conditions, if the transformer temperature is abnormal and continues to rise, and the temperature indication is proven to be correct after inspection, it is considered that the transformer has suffered an internal fault, and the transformer should be shut down immediately.

   (6) When the transformer operates under various over-rated current conditions, if the top oil temperature exceeds 105°C, the load should be immediately reduced.

4. Handling of transformer accidents

   (1) Handling of automatic tripping of transformer

    To ensure the safe operation and ease of operation of the transformer, circuit breakers and necessary relay protection devices are installed on each side of the transformer, including the high, medium, and low voltage sides. When the circuit breaker of the transformer (on the high voltage side or on all three sides: high, medium, and low voltage) trips, operators should take the following measures:

    1) If there is a standby transformer, it should be immediately put into operation to restore power supply to the users, and then the cause of the tripping of the faulty transformer should be investigated.

    2) If there is no standby transformer, the load should be transferred and the operation mode changed as soon as possible, while identifying the type of protective action. When checking the cause of the transformer trip, it is necessary to ascertain whether there are any obvious abnormalities in the transformer, such as external short circuits, line faults, overloads, as well as any obvious phenomena such as flames, strange noises, or oil spraying. If it is confirmed that the circuit breaker trips on each side of the transformer are not caused by internal faults but by overloads, external short circuits, or malfunctions in the secondary circuit of the protective device, the transformer can be put back into operation without undergoing internal inspection. If it is not confirmed that the transformer trip is caused by the aforementioned external reasons, an accident analysis should be conducted on the transformer, such as through electrical tests, oil analysis, and comparison with previous data. If it can be determined through the above inspection and analysis that there is no internal fault in the transformer, the gas relay of the protection system should be reset to the tripped position, and the transformer should be put back into operation. The entire operation process should be carried out carefully. If it is determined through inspection that there is an internal fault in the transformer, a shell inspection is required until the fault is identified and resolved.

  (2) Handling after transformer gas protection action

    1) Handling after light gas action. After the light gas action, reset the audible signal, check the signal relay, and distinguish whether it is the light gas action of the transformer body or the light gas action of the on-load tap changer. Do not rush to restore the relay trip indicator. Then check whether the oil level in the transformer body or the oil conservator of the on-load tap changer is normal, and how much gas is charged in the gas relay to determine the cause of the action. After identifying the cause of the action, reset the signal relay trip indicator and the light indicator. At this time, collect gas as soon as possible for cause analysis.

   When refilling or filtering oil in newly installed or overhauled transformers, air is often trapped inside and not promptly expelled. Once the transformer is operational, the oil temperature gradually rises, creating convection currents that facilitate the gradual release of trapped air, triggering the gas relay. The frequency of gas relay activations is related to the amount of gas stored inside the transformer. In such cases, a comprehensive analysis should be conducted based on factors such as the transformer’s sound, temperature, oil level, and the performance of the oil refilling and filtering processes.

    The causes of non-transformer faults, such as excessive air accumulation in the oil of the gas relay, incorrect wiring of the gas relay protection circuit, and short circuits in the secondary cables of the terminal strip, can lead to false activation of the light gas relay.

    ② When the light gas acts frequently, monitor and record each occurrence, paying attention to the gas characteristics. If the oil and gas analysis determines it to be air, continue to vent the gas, and be careful not to accidentally touch the tripping test probe of the gas relay.

    ③ If it is uncertain whether the cause of the action is due to a transformer fault or external factors, and no other abnormalities are detected, the gas protection should be activated in the tripping circuit, and monitoring of the transformer should be intensified to carefully observe its development and changes.

    2) Handling after heavy gas protection action. If a transformer in operation trips due to gas protection action, or if both light gas signals and gas trips occur simultaneously, it should first be considered that there may be an internal fault in the transformer, and such handling of the transformer should be cautious. The gas generated inside the faulty transformer is caused by different overheating forms in different parts of the transformer, or even metal short circuits and discharges. Therefore, determining the nature of the gas in the gas relay, the amount of gas accumulation, and the rate of accumulation is crucial for judging the nature and severity of the transformer fault.

    Generally speaking, the accumulated gas is non-flammable, colorless, and odorless. If it is identified as air through chromatographic analysis, the transformer can continue to operate. If the gas is flammable, further analysis should be conducted to determine the cause of the fault. The main methods include oil analysis (including the widely effective chromatographic three-ratio method analysis), electrical test analysis, and shell inspection, all in accordance with relevant regulations and guidelines.

   If the cause cannot be analyzed immediately, the transformer is not allowed to be put into operation before it has been inspected, treated, and tested to be qualified, in order to prevent the expansion of faults or accidents. It is particularly important to note that the focus of analysis should be on faults related to the tap changer (both no-load and on-load) and coil insulation. It should be emphasized that the action of transformer gas protection is a precursor or reflection of internal faults. Therefore, special care should be taken in the forced switching-on, trial switching-on, and supervised operation of such transformers, and forced switching-on is not allowed before the cause of the accident has been identified.

   (3) Handling after transformer differential protection action

   Differential protection reflects electrical faults within the longitudinal differential range (including short circuits inside the transformer and between the bushing leads). Therefore, whenever the differential protection acts, the circuit breakers on each side of the transformer will trip simultaneously. At this time, the operating personnel should take the following measures:

    1) Report to the dispatcher and superior supervisor*, and reset the accident sound and signal.

    2) Open the disconnectors on each side of the transformer tripping device and inspect the exterior of the transformer (such as oil temperature, oil color, explosion-proof glass, insulating sleeves, etc.) for any abnormalities.

    3) Inspect all primary and secondary equipment within the scope of transformer differential protection, that is, observe whether there are any abnormalities and short-circuit discharge phenomena in all equipment, leads, buses, wall bushings, etc. on the high, medium, and low voltage sides of the transformer, and whether there are any abnormalities in the secondary protection circuit, in order to discover the fault point within the differential protection zone.

    4) Inspect the transformer differential protection circuit, observe whether there is any short-circuit discharge or breakdown at the terminals of the current transformer used for differential protection, and whether there is any open circuit, accidental touch, or incorrect wiring in the secondary circuit.

    5) Measure the insulation resistance of the transformer and inspect for any internal insulation faults.

    6) Check the DC system for any grounding issues.

    After the aforementioned inspections, if it is determined that the differential protection malfunction is caused by non-internal fault reasons (such as protection mis-operation or transient faults), the transformer can be tested with the gas protection in the tripping position. If it cannot be determined as a non-internal fault, further measurements, inspections, and analyses (such as testing DC resistance, simplified oil analysis, or oil chromatography analysis) should be conducted on the transformer to determine the nature of the fault and the reason for the differential protection action. If characteristics of an internal fault are found, the transformer must be lifted or returned to the factory for inspection and repair.

   (4) Handling after simultaneous activation of heavy gas and differential protection

    After the simultaneous tripping of heavy gas and differential protection, the transformer should be taken out of service and placed in maintenance mode. Immediately report to the dispatcher and do not attempt to restore the service until the issue has been inspected and resolved. During this period, it is important to collect gas samples as soon as possible for analysis to determine the cause.

   (5) Handling after current quick-break protection action

    When the current quick-break protection trips, the handling process should refer to that of the differential protection action.

   (6) Handling after tripping due to definite-time overcurrent protection action

   The definite-time overcurrent protection serves as a backup protection, which can be used as a backup for the protection of subordinate lines, or as a backup for the protection of subordinate buses, or as a backup for the main protection of transformers. Therefore, when the overcurrent protection trips, a comprehensive analysis and judgment should be made based on its protection range, the action of the protection signal, the corresponding circuit breaker tripping condition, and the equipment fault condition, and then the corresponding measures should be taken. According to statistical analysis, the most common cause of overcurrent protection tripping is bypass tripping due to the failure of subordinate lines to trip; the second most common cause is tripping caused by equipment faults in subordinate buses (mainly in 110kV and below substations).

   When the transformer trips due to definite-time overcurrent protection action, the accident sound should be reset first, and then a check should be made to determine whether there is a possibility of bypass tripping, that is, to check the operation of each outgoing line switch protection device, whether any signal relays have dropped their tags, and whether any operating mechanisms are jammed.

    Due to the failure of the subordinate line equipment, which was not promptly removed, the corresponding circuit breaker on the main transformer side was tripped bypassing the immediate leadership, resulting in a loss of power to the busbar.

   Check the operation status of protection signals for each line on the de-energized busbar: If there is a bypass caused by the operation of a line protection signal due to the failure of the line fault protection circuit breaker to trip, the line circuit breaker that refused to trip should be opened. After removing the faulty line, the transformer should be put back into operation, and power should be restored to the remaining lines simultaneously.

   After inspection, if there is no line protection action signal, it may be due to a line fault, causing the circuit breaker to fail to trip due to the protection not acting. In this case, all line circuit breakers on the busbar should be opened, the transformer should be put back into operation, and then each line circuit breaker should be tested for power transmission one by one. When closing a certain line circuit breaker causes the main transformer to trip, the line circuit breaker should be switched to cold standby, and then the power transmission of the transformer and other lines should be restored.

  The aforementioned faulty line shall not be energized before the cause is identified and addressed.

    2) Due to a fault in the subordinate busbar equipment, the circuit breaker on the main transformer side tripped, resulting in a loss of power to the busbar.

    The buses of various voltage levels in 110kV and below substations generally do not have separate busbar protection, and overcurrent protection is used as busbar protection. If equipment on the busbar fails, the overcurrent protection will actuate and trip. Therefore, after the overcurrent protection trips, it is necessary to inspect the busbar and its associated equipment. If any obvious fault characteristics are found on one side of the busbar or its associated equipment during inspection, the faulty busbar should be removed before power is restored.

    3) If the overcurrent protection trips and the main protection system of the main transformer, such as gas protection, also responds, the main transformer itself should be inspected. If any obvious fault characteristics are found, power cannot be restored.

   (7) Zero-sequence protection action

    (1) Zero-sequence protection action is generally caused by a single-phase grounding fault occurring in a three-phase system with a directly grounded neutral point. After the accident occurs, it is necessary to immediately contact and report to the dispatcher for further instructions.

    (2) Three-phase voltage imbalance zero-sequence protection action

    ① Unbalanced three-phase load causes neutral displacement, resulting in unbalanced three-phase voltage;

    ② The system experiences ferromagnetic resonance, leading to an imbalance in the three-phase voltage,

③ Partial inter-turn and interlayer short circuits occur in the winding, resulting in unbalanced three-phase voltage.

(8) Handling of transformer on fire

    When a transformer catches fire, regardless of the cause, the circuit breakers on each side should be opened first to cut off the power supply, disable the cooling device, and quickly take effective measures to extinguish the fire. At the same time, report to the fire department, dispatch center, and superior supervisors for assistance in handling. If oil is burning on the top cover of the transformer, the oil level should be lowered from the fault oil drain valve; water should be poured on the shell (be careful not to let the water reach the burning oil) to cool the oil and prevent it from burning easily. If the shell explodes, all the oil must be drained into the oil pit or storage tank. If the fire is caused by an internal fault in the transformer, oil cannot be drained to prevent a serious explosion of the transformer. When the transformer catches fire due to oil spraying, yellow sand should be quickly used to cover, isolate, and control the spread of the fire, while using fire-fighting equipment to extinguish the fire. When extinguishing the transformer fire, use a foam-type fire extinguisher, and if necessary, use dry sand to extinguish the fire.

Activation and deactivation of transformers

Before the transformer is put into operation, the personnel on duty should conduct a thorough inspection to ensure that the transformer and its protective devices are in good condition and ready for live operation. Attention should also be paid to whether there are any foreign objects on the exterior, whether the temporary grounding wires have been removed, whether the tap changer is in the correct position, and whether the valves are opened or closed correctly. When the transformer is put into operation at low temperatures, measures should be taken to prevent the breather from being blocked due to icing.

2. The standby transformer in operation should be ready for immediate use. For transformers that have been shut down for a long time, regular charging should be performed, and the cooling device should be activated simultaneously. If the transformer is forced to operate with no load after charging, some coolers should be activated in turn, with the number not exceeding the operating units specified by the manufacturer for no-load operation.

3. Operating procedures for transformer commissioning and decommissioning:

   (1) When putting the circulating transformers into operation, the coolers should be activated one by one, and the number of coolers activated should be controlled based on the load situation.

   (2) The charging of the transformer should be operated through a circuit breaker on the power side with protective devices. When shutting down, the load side should be stopped first, followed by the power side.