Moisture ingress or dampness in dry-type transformers can occur due to various factors, such as harsh environmental humidity, improper storage during transportation, temporary seal failures, or accidental water exposure during installation. When moisture accumulates in the insulation layer, it significantly degrades the insulation’s dielectric strength, increasing the risk of internal short circuits, partial discharges, or even catastrophic equipment failures. This not only endangers the transformer itself but also threatens the stability of the entire power grid. Therefore, scientific drying processes are essential to mitigate these risks, but they are not a one-size-fits-all solution—determining whether drying is necessary requires a rigorous assessment based on specific operational and test conditions.
For newly installed dry-type epoxy resin transformers transported with oil, the decision to perform drying must be based on a comprehensive analysis of multiple technical indicators, rather than arbitrary judgment. Key evaluation factors include:
- Insulating oil quality: The electrical breakdown strength of the insulating oil must comply with relevant national standards (e.g., GB/T 7595) and industry specifications. Additionally, trace water content in the oil must meet strict limits—excessive moisture in the oil can permeate the insulation system, compromising its performance.
- Insulation resistance and absorption/polarization indices: The insulation resistance value (measured at specified temperatures) should align with the manufacturer’s technical parameters and national grid requirements. The absorption ratio (R60/R15) or polarization index (R10min/R1min) serves as a critical indicator of insulation moisture levels; values below the standard threshold indicate potential dampness, warranting further drying.
- Dielectric loss tangent (tanδ): For transformers with a voltage rating above 35kV or a capacity exceeding 4000kVA, the tanδ (%) must meet regulatory requirements. This parameter reflects the insulation’s energy loss under alternating current, and an abnormal increase often signals moisture absorption or insulation degradation. Note that this indicator is not mandatory for low-voltage (≤35kV) and small-capacity (≤4000kVA) units.
Dry-type epoxy resin transformers and reactors transported under gas-filled conditions require a distinct set of criteria to determine if drying is needed. The evaluation process must prioritize the integrity of the gas seal and insulation performance, including:
- Positive pressure maintenance: From factory shipment to on-site installation, the transformer’s internal cavity must maintain a continuous positive pressure. This prevents external moisture-laden air from entering the unit, ensuring the insulation system remains dry during transportation and storage.
- Residual oil moisture control: Trace water content in the residual oil within the transformer body must not exceed 30ppm. Even minimal moisture in residual oil can affect the insulation’s long-term stability, so strict adherence to this limit is non-negotiable.
- Post-oiling performance verification: After filling the transformer or reactor with qualified insulating oil, comprehensive tests must be conducted. Both the insulating oil’s electrical strength and trace water content, as well as the overall insulation resistance of the unit, must comply with the latest national standards (e.g., IEC 60076 series) to confirm that no moisture-related issues exist.
In cases where the transformer fails to maintain positive pressure during transportation or storage, but there is no visible damage to the seal structure, a holistic judgment is required. This assessment should integrate multiple sources of information, including:
- Detailed installation records: Review transportation routes, storage conditions (e.g., humidity, temperature), and on-site handling procedures to identify potential risks of moisture ingress.
- Comprehensive test data: Analyze results from insulation resistance tests, dielectric loss measurements, and oil quality inspections. If test data shows slight deviations from standards but no obvious signs of severe dampness (e.g., uniform insulation resistance decline), supplementary drying may be recommended as a preventive measure.
- Expert technical evaluation: Engage professional electrical engineers to assess the transformer’s structural integrity, seal condition, and historical performance data. This subjective judgment, combined with objective test results, ensures the drying decision is both scientific and practical.
Dry-type epoxy resin transformers do not require mandatory drying—each case must be evaluated individually based on factual data and equipment conditions. Blindly performing drying procedures can waste resources, extend downtime, or even cause thermal aging of insulation materials if parameters are not properly controlled. Instead, manufacturers and operators should focus on optimizing drying technologies: adopting advanced methods such as vacuum drying or hot air circulation drying, precisely controlling temperature (typically 60–80°C), humidity, and duration to ensure uniform moisture removal without damaging components.
By adhering to rigorous assessment criteria, implementing refined drying processes, and integrating real-time monitoring during operation, dry-type epoxy resin transformers can achieve long-term safe, stable, and efficient operation. This not only minimizes the risk of unplanned outages but also enhances the overall reliability and efficiency of the power supply network, delivering sustained value to industrial users and infrastructure projects worldwide.
