Why Dry Type Transformers Show Blue Corona: Ungrounded Esd Explained

A visible blue glow inside a dry type transformer indicates corona discharge, an electrical phenomenon tied to high electrical stress and localized ionization of surrounding air. This issue frequently occurs when internal metal components lack proper grounding, allowing static electricity to accumulate until an electrostatic discharge bridges the insulation gap, causing gradual material degradation.

What Causes Blue Glow and Electrostatic Discharge?

Blue corona is a luminous electrical discharge resulting from the ionization of fluid such as air surrounding a high-voltage conductor. Inside a cast resin dry type transformer, ungrounded hardware or floating metal inserts act as capacitors. They store static charge until the dielectric strength of the air fails, producing visible blue light, hissing sounds, and ozone.

Common Sources of Floating Potential

Small structural imperfections often lead to localized voltage potentials across internal distribution systems.

1. Core Lamination Isolation: Small voltage potentials build up across the magnetic core when grounding straps loosen over time, even in compact units like a 15 kva dry type transformer used in light industrial distribution settings.

2. Unbonded Clamps and Fasteners: Floating hardware inside the enclosure accumulates charge from the alternating electromagnetic field. This static buildup eventually discharges to nearby earthed structures, producing continuous electrical noise and surface tracking.

Preventing Corona in Commercial Power Systems

Mitigating surface discharges requires rigorous grounding verification during routine system maintenance. Medium-sized installations, such as a 45kva dry type transformer operating in commercial facilities, must maintain continuous electrical continuity across all structural frames, protective enclosures, and clamping assemblies to prevent static charge buildup and safeguard solid insulation structures.

Maintenance Steps for Static Electricity Control

Implementing structured testing procedures eliminates floating potentials and stabilizes electrostatic fields across all operational distribution equipment.

1. Darkened Room Inspections: Perform visual assessments in total darkness to detect visible ionization or ultraviolet emissions around terminal blocks, resin surfaces, and structural support brackets.

2. Resistance Verification: Measure earth bonding resistance across all non-current-carrying metal parts using a calibrated micro-ohmmeter. Ensuring zero-potential grounding eliminates floating electrical fields effectively.

3. Particulate Removal: Clean accumulated dust and moisture regularly, particularly within a larger 112.5 kva dry type transformer where airborne contaminants frequently create conductive tracking paths for electrical leakage.

Addressing ungrounded components promptly prevents long-term insulation failure and unexpected operational downtime. Consistent bonding inspections, combined with environmental cleanliness, ensure that electrostatic fields remain safely neutralized throughout the operational lifespan of the electrical distribution apparatus.