A Comprehensive Guide to Three-Phase Oil-Immersed Transformer

Three-phase oil-immersed transformer is a core equipment in power systems, responsible for voltage conversion and power transmission, widely used in power distribution and transmission links. Understanding their structure, operating rules, and maintenance key points is crucial for ensuring the stable operation of power systems. This article will comprehensively sort out the core knowledge of three-phase oil-immersed transformers from the aspects of structural analysis, operation and maintenance, and abnormal handling.

I. Core Structure of Three-Phase Oil-Immersed Transformers

The performance of a three-phase oil-immersed transformer is determined by its structure, which includes core components such as magnetic circuits, circuits, and auxiliary systems. All parts work together to realize power conversion and safety protection.

1. Magnetic Circuit Core: Iron Core

The iron core is the carrier of the transformer’s magnetic circuit, and its design directly affects losses and efficiency.

• Material: Composed of stacked silicon steel sheets with a thickness of 0.35 mm to 0.5 mm. The surfaces of the silicon steel sheets are coated with insulating paint or use surface oxide films to insulate each other, which can significantly reduce hysteresis and eddy current losses.
• Structure: Divided into upright “core columns” (where windings are sleeved) and horizontal “yokes” (which form a closed magnetic circuit), ensuring efficient closure of the magnetic field.

2. Circuit Core: Windings

Windings are the circuit part for power conversion, divided into primary windings (connected to the power supply) and secondary windings (connected to the load).

• Material: Wound with copper or aluminum wires wrapped in high-strength insulation, balancing conductivity and insulation safety.
• Arrangement: The low-voltage winding (with fewer turns) is sleeved on the outer side of the core column, and the high-voltage winding (with more turns) is sleeved on the outer side of the low-voltage winding.

• Reason: The low-voltage winding has lower insulation requirements for the iron core, making insulation easier to achieve; an oil channel is left between the high-voltage and low-voltage windings to facilitate the circulation and heat dissipation of transformer oil.

3. Shell and Insulation Cooling System: Oil Tank

The oil tank is the shell that houses the iron core and windings, filled with transformer oil, which is the core of insulation and heat dissipation.

• Functions of Transformer Oil:

1. Insulation: Transformer oil has better insulation performance than air. Immersing windings in oil can improve the insulation performance of all parts and prevent windings from getting damp by avoiding contact with air.
2. Heat Dissipation: Through the convection of oil, the heat generated by the iron core and windings is dissipated through the tank wall or radiators.
   • Transformer Oil Types: Classified by pour point, suitable for different scenarios as follows:

4. Oil Level Regulation: Oil Conservator (Oil Pillow)

The oil conservator is a cylindrical container above the oil tank (with a volume of about 10% of the oil tank), connected to the oil tank through a pipeline. Its core functions are “oil replenishment/storage” and “prevention of oil deterioration”.

• Structure: Equipped with an internal capsule to isolate oil from air, reducing moisture absorption and oxidation; a linkage-type ferromagnetic oil level gauge is installed, marking the standard oil levels at -30℃, +20℃, and +40℃.
• Oil Level Standards:

• +40℃: The upper limit of oil level during full load at the highest ambient temperature must not be exceeded.
• +20℃: Normal oil level during full load at the annual average temperature;
• -30℃: The lower limit of oil level during no-load at low ambient temperature must not be lower.

5. Key Auxiliary Components

Comparison Table: On-Load Tap Changing vs. Off-Load Tap Changing

II. Operation and Maintenance of Three-Phase Oil-Immersed Transformers

Reasonable operation and regular maintenance are key to extending the transformer’s service life and avoiding faults, which should follow the principle of “normal operation specifications + timely handling of abnormalities”.

1. Normal Operation Specifications

• Rated Operation:

• The maximum upper oil temperature shall not exceed 95℃; to avoid oil deterioration, it should not often exceed 85℃;
• The applied voltage shall not exceed 105% of the rated value (can reach 110% in special cases with permission), and the secondary side can carry the rated current at this time.
  • Allowable Overload:
• Normal Overload: Determined according to the load curve and cooling conditions, can be used daily;
• Emergency Overload: Only used in emergency situations. For self-cooled oil-immersed transformers without the manufacturer’s regulations, the duration shall be limited according to specifications.

2. Abnormal Situations and Emergency Handling

The transformer must be shut down for maintenance immediately if the following situations occur during operation:

• Uneven sound or explosion sound inside.
• Oil temperature rises continuously under normal cooling.
• Oil spray from the oil conservator or explosion-proof tube;
• Oil level is lower than the lower limit of the oil level gauge, or oil color deteriorates (with carbon deposits);
• Severe damage or discharge of bushings.

(1) Abnormal Rise in Oil Temperature

• First, check the load, cooling medium temperature, verify the accuracy of the thermometer, and check whether the cooling system/ventilation is normal.
• If the oil temperature is more than 10℃ higher under the same load and cooling conditions, or the oil temperature rises continuously, it may be an internal fault (such as iron core ignition, coil short circuit), and the transformer must be shut down immediately.

(2) Action of Gas Relay

The gas relay is an “early warning device” for internal faults. When it acts, it is necessary to distinguish between signal action and tripping action (see the table below):

(3) Oil Leakage Handling

• Weld Seam Leakage: Repair welding after lifting the core and draining the oil.
• Seal Leakage: Adjust the sealing gasket (if misaligned/insufficient pressure) or replace the aging gasket (loss of elasticity, cracking).

3. Daily Patrol Inspection Items

Daily/regular inspections should cover the following 10 core contents:

1. Whether there is abnormal sound (uneven sound, discharge sound);
2. Whether the oil level is normal, and whether there is oil seepage or leakage;
3. Whether the upper oil temperature exceeds 85℃;
4. Whether the bushing is clean, with cracks, or with discharge;
5. Whether the joints are overheating;
6. Whether the explosion-proof film of the explosion-proof tube is intact;
7. Whether the gas relay is leaking oil or full of oil;
8. Whether the breather is unobstructed, and whether the silica gel is saturated with moisture;
9. Whether the cooling system is operating normally.
10. Whether the shell grounding wire is intact.

Conclusion

The stable operation of three-phase oil-immersed transformers depends on understanding their structure, standardized operation management, and timely handling of abnormalities. Mastering the functions of core components such as the iron core and windings, and being familiar with the monitoring points of key indicators such as oil level, oil temperature, and gas relay, can effectively reduce the risk of faults and ensure the safe and efficient operation of the power system.

Through the sorting of this article, it is believed that you have a comprehensive understanding of the structure, operation, and maintenance of three-phase oil-immersed transformers. In daily operations, strictly following the specifications and conducting regular inspections are the core guarantees for extending the service life of the equipment.