In power systems, transformers serve as the core equipment for energy conversion and transmission, and their insulation performance directly determines the stability of power grid operation. Partial Discharge (PD) is an "early warning signal" for transformer insulation degradation-this type of local arc discharge that does not penetrate the insulation will gradually erode the insulation material, and may eventually lead to serious accidents such as insulation breakdown and equipment outage.
Transformers with different insulation structures have significant differences in the generation mechanism and key inducing factors of partial discharge. This article focuses on oil-immersed transformers and dry-type transformers (represented by epoxy resin-cast type), combining industry practice and technical principles to deeply analyze the core causes of partial discharge, providing professional references for equipment selection, operation and maintenance testing.
I. Dry-Type Transformers (Epoxy Resin-Cast Type): Insulation Defects and Process Control Are Core Inducing Factors
Dry-type transformers take epoxy resin and other solid insulation materials as the core, and are widely used in high-rise buildings, data centers and other scenarios due to their advantages such as fire resistance, maintenance-free operation and small size. Their partial discharge problems are mainly concentrated in two dimensions: insulation material defects and casting/winding processes.
The core insulation materials of dry-type transformers include epoxy resin, Nomex paper, insulation cardboard, etc. If the material production or selection is improper, it is easy to mix with bubbles, impurities or produce microcracks:
- Bubbles Inducing Electric Field Concentration: Residual bubbles in solid insulation materials have a dielectric constant much lower than that of the base insulation medium (e.g., the dielectric constant of epoxy resin is about 3.5, while that of air is only 1.0). The electric field will be highly concentrated inside the bubbles. When the electric field strength exceeds the material's tolerance threshold, partial discharge will be triggered. Long-term discharge will gradually expand the bubble volume, forming insulation hazards;
- Impurities Causing Electric Field Distortion: Metal debris, dust and other impurities mixed in the insulation material will form a structure similar to a "tip electrode", disrupting the uniform distribution of the electric field and forming a local high field strength area at the tip of the impurity, inducing corona discharge;
- Insulation Hazards of Microcracks: If the stress release is uneven during material curing, or mechanical impact is encountered during transportation and installation, microcracks that are invisible to the naked eye will be generated. The electric field concentration effect at the cracks will also trigger partial discharge, and the cracks will continue to extend with the discharge, accelerating insulation aging.
Process control is the key to preventing partial discharge in dry-type transformers. Negligence in any link such as winding, wrapping, casting and curing may leave hidden dangers:
- Poor Insulation Wrapping of Windings: Loose insulation wrapping between winding layers and turns, with gaps or wrinkles. These gaps will form local low dielectric constant areas, becoming "high-risk areas" for electric field concentration; at the same time, wrinkles generated during wrapping will lead to uneven insulation thickness, further exacerbating electric field distortion;
- Defects in Conductor Processing: Unremoved burrs, sharp corners or scratches on the conductor surface. Under high-voltage operating conditions, the electric field strength at the tips will increase sharply (in line with the "corona discharge" principle), directly inducing corona discharge. The discharge will gradually erode the conductor insulation layer, expanding the defect range;
- Inadequate Voltage Equalization Treatment: The electric field distribution at the end of the transformer winding is naturally uneven. If no voltage equalizing ring or voltage equalizing plate is installed, or the design of the voltage equalizing structure is unreasonable, the electric field strength at the end will exceed the insulation tolerance limit, leading to excessive partial discharge. This is also a common inducement for partial discharge at the end of dry-type transformers;
- Improper Casting and Curing Processes: The core process defects of epoxy resin-cast transformers are concentrated in two links: "degassing" and "curing". Incomplete degassing will cause residual gas in the epoxy mixture to be unable to be discharged, forming internal bubbles after curing (this is a high-frequency cause of excessive partial discharge in dry-type transformers); while excessively high/low curing temperature or excessively long/short curing time will lead to incomplete insulation curing (residual unreacted epoxy monomers) or stress concentration, which in turn generates microcracks and damages insulation integrity.
II. Oil-Immersed Transformers: Insulating Oil Condition and Interface Defects Are Main Risk Points
Oil-immersed transformers adopt a combined insulation system of insulating oil and solid insulation (cardboard, oil-impregnated paper), and have long occupied the main transformer market in power systems due to their advantages such as high insulation strength and good heat dissipation. Their partial discharge problems are closely related to the state of insulating oil and the characteristics of the oil-paper interface, with core inducing factors including the following three categories:
Insulating oil not only serves as an insulation medium but also undertakes the heat dissipation function. Its purity and state directly affect the risk of partial discharge:
- Moisture and Excessive Gas Content: If the oil-immersed transformer is not tightly sealed, moisture in the air will penetrate into the insulating oil, which will significantly reduce the breakdown voltage of the oil; at the same time, if the dissolved gases in the oil (such as oxygen, hydrogen, methane) are not removed in a timely manner, tiny bubbles will form in the oil. These bubbles are prone to discharge under the action of the electric field, and the gas generated by the discharge will further increase the gas content in the oil, forming a vicious cycle;
- Impurity Contamination: If the insulating oil is mixed with metal particles, fiber impurities, etc. during production, transportation or operation and maintenance, it will form a prototype of a "conductive channel" in the oil-impurities will adsorb charges, move directionally under the action of the electric field, accumulate at the interface between the oil and solid insulation, and induce partial discharge;
- Aging and Deterioration of Oil Quality: After long-term operation, the insulating oil will undergo oxidation reactions under the action of high temperature and electric field, producing aging products such as acids, colloids and sludge. These substances will reduce the insulation performance of the oil, and at the same time chemically react with solid insulation materials, damaging the insulation integrity of the oil-paper interface and providing conditions for partial discharge.
The insulation system of oil-immersed transformers is composed of "insulating oil + solid cardboard", and the interface state between the two is a key sensitive area for partial discharge:
- Interface Bubbles and Gaps: If degassing is incomplete during transformer oil filling, or the insulating oil expands when heated and contracts when cooled during operation, tiny gaps or bubbles will form at the interface between the oil and cardboard. The electric field concentration effect in these areas is very likely to trigger partial discharge;
- Moisture and Aging of Cardboard: As a solid insulation material, if the cardboard absorbs moisture (such as rainwater infiltration due to seal failure), its insulation performance will be significantly reduced. At the same time, moisture will promote the aging and degradation of the cardboard, generating microcracks and fiber debris. These defects will induce discharge at the interface, and the discharge will accelerate the carbonization of the cardboard, forming a conductive channel.
In addition to the problems of insulating oil and interface, improper structural design and processes can also lead to partial discharge in oil-immersed transformers:
- Loose Winding Fixation: If the winding is loosely bound, the electromagnetic vibration during transformer operation will cause winding displacement, resulting in gaps in the insulation between layers and turns, and inducing electric field concentration;
- Protruding Tips of Metal Components: If there are sharp corners, burrs on the metal brackets, leads and other components inside the transformer, or their positions are offset during installation, local high field strength areas will be formed under high voltage, inducing corona discharge;
- Poor Tank Sealing: The failure of tank sealing will not only lead to the infiltration of moisture and impurities but also allow air to enter the tank, forming a bubble layer on the oil surface. These bubbles are important sources of partial discharge.
III. Core Differences and Prevention Core of Partial Discharge Between the Two Types of Transformers
| Comparison Dimension | Dry-Type Transformers (Epoxy Resin-Cast Type) | Oil-Immersed Transformers |
| Core Insulation Medium | Solid insulation such as epoxy resin, Nomex paper | Insulating oil + oil-paper composite insulation |
| Main Discharge Inducing Factors | Internal bubbles in materials, process defects (casting/winding) | Insulating oil deterioration, oil-paper interface defects |
| Sensitive Areas | Winding ends, inside the casting body, conductor tips | Oil-paper interface, bubbles in oil, air gap at the top of the tank |
| Prevention Core | Material purity control, refined processes (degassing/curing/wrapping), optimization of voltage equalization structure | Insulating oil purification (degassing/dehydration/impurity removal), sealing protection, interface state monitoring |
IV. Industry Insights: Early Detection and Operation and Maintenance Suggestions for Partial Discharge
The hazard of partial discharge lies in its "concealment"-the initial discharge intensity is low, with no obvious external characteristics, but long-term accumulation will lead to irreversible insulation degradation. Therefore, both oil-immersed and dry-type transformers should attach importance to early detection and operation and maintenance control:
- Regular Testing: Adopt online partial discharge monitoring systems (such as ultrasonic testing, ultra-high frequency testing) to capture discharge signals in real time and accurately locate defect positions;
- Material and Process Control: Prioritize manufacturers with complete qualifications and mature processes when selecting products, focusing on insulation material purity and process test reports;
- Operation and Maintenance Protection: For oil-immersed transformers, regularly test indicators such as moisture, gas content and dielectric loss of insulating oil, and perform filtration and purification in a timely manner; for dry-type transformers, avoid mechanical impact, regularly clean surface dust, and prevent insulation surface creepage;
- Environmental Control: Avoid operating transformers in environments with high humidity, excessive dust and corrosive gases. Dry-type transformers should ensure good ventilation and heat dissipation to prevent insulation overheating and aging.
Conclusion: Partial discharge is an "invisible killer" of the transformer insulation system, and its causes are closely related to equipment type, insulation structure and process level. Mastering the partial discharge characteristics of oil-immersed and dry-type transformers, and through scientific product selection, refined process control and regular testing, can reduce discharge risks from the source and ensure the safe and stable operation of the power system.
