While dry-type transformers are engineered with inherent moisture resistance, their typical open-frame design leaves them vulnerable to humidity ingress, especially for domestically produced models with lower insulation grades. For optimal reliability, these transformers require an operating environment with relative humidity below 70%; prolonged exposure to higher humidity levels can degrade insulation performance over time. Additionally, long-term shutdowns should be avoided to prevent severe dampness buildup. A critical benchmark: if the insulation resistance drops below 1000V (relative to the operating voltage), it indicates significant moisture contamination, and commissioning must be halted immediately to address the issue.
Dry-type transformers deployed in step-up power stations differ fundamentally from oil-immersed alternatives in one critical aspect: open-circuit operation on the low-voltage side is strictly prohibited. This precaution prevents insulation breakdown, which can occur when grid-side overvoltages or lightning strikes trigger voltage surges that propagate through the transformer. To mitigate overvoltage risks, industry best practices mandate installing a dedicated set of surge arresters (such as metal oxide arresters, MOAs) on the transformer’s voltage bus side. These devices act as a first line of defense, diverting excess voltage to ground and safeguarding the transformer’s internal components.
A systematic commissioning process is essential to verify transformer performance and identify potential issues. Below is a detailed protocol aligned with industry standards:
Transformer Ratio & Connection Group Verification: Conduct tests to confirm the turns ratio and connection group configuration, ensuring they match the manufacturer’s factory specifications. Simultaneously, measure the DC resistance of both high-voltage and low-voltage windings. Compare the results with the original factory test data—any significant deviations may indicate winding damage, poor connections, or manufacturing defects.
Insulation Resistance Testing: Evaluate the insulation resistance between coils and between coils and the ground (earth). If the measured value is substantially lower than the factory-recorded data, it signals moisture absorption or insulation degradation. When resistance falls below 1000V (operating voltage), the transformer must undergo professional drying treatment before further commissioning.
Withstand Voltage Test Compliance: Ensure the test voltage for withstand voltage trials adheres to national and international standards (e.g., IEC, ANSI). For low-voltage withstand voltage tests, remove the TP100 temperature sensor prior to testing to prevent damage. After completing the test, reinstall the sensor promptly and verify its proper functioning.
Auxiliary Fan Functionality Check: If the transformer is equipped with cooling fans, energize the fan system to confirm normal operation. Inspect fan rotation speed, noise levels, and vibration to ensure effective heat dissipation—critical for maintaining transformer efficiency under load.
Phase loss is a serious issue that can lead to overheating and equipment damage. Dry-type transformer manufacturers highlight three primary causes:
- Grid-related phase loss: A temporary or permanent loss of one phase in the power supply, often resulting from grid faults, damaged power lines, or incorrect wiring during installation.
- High-voltage fuse failure: Blown fuses in the transformer’s high-voltage circuit, which may be triggered by overloads, short circuits, or voltage fluctuations.
- Transportation-induced lead damage: Thin high-voltage lead wires can break due to excessive vibration during transportation—especially if the transformer is not properly secured. Importantly, such breaks may not ground the system immediately, making detection more challenging.
When a transformer is energized with an improperly positioned tap changer, it typically emits a loud, shrill chirping sound. In severe cases, this misalignment can cause high-voltage fuses to blow. Additionally, poor tap changer contact can lead to spark discharge, which risks burning the contacts once the transformer is under load. If these symptoms are observed, the following steps are imperative:
- Immediately shut off the power supply to prevent further damage.
- Arrange for professional technicians to inspect the tap changer, clean or replace worn contacts, and recalibrate the mechanism to ensure precise positioning.
- Conduct post-repair tests to verify proper functionality before reintroducing the transformer to service.
Unusual sounds such as rhythmic “ding-ding” knocking, whistling, or squeaking—even when voltage, current, and temperature readings are normal—often stem from the following issues:
- Loose core clamping bolts: Over time, vibration during operation or transportation can loosen the through-core screws that secure the transformer’s iron core.
- Foreign metal debris: Nut fragments, small tools, or other metallic objects left on the core during manufacturing or installation can fall into the transformer.
- Magnetically induced noise: Small spacers or components may be attracted to the core by magnetic force, producing squeaking sounds.
In most cases, these noises do not immediately compromise operational safety, as they do not affect the transformer’s electrical parameters. However, proactive maintenance is recommended: schedule a planned power outage to tighten loose bolts, remove foreign debris, and inspect the core for signs of wear or damage. This preventive measure helps avoid potential issues such as core vibration amplification or short circuits in the long run.
