Composition Requirements and Normal Operation of Annealed Transformer Cores

During the normal operation of an annealed transformer, an electric field exists between the energized windings and the tank, with the core and other metallic components situated within this field. Due to uneven capacitance distribution and varying field strengths, if the core is not reliably grounded, charging and discharging phenomena will occur. This can degrade the solid insulation and reduce the dielectric strength of the oil; therefore, the core must have a reliable single-point ground.

The core of an annealed transformer is composed of silicon steel sheets. To minimize eddy currents, there is a certain level of insulation resistance between the sheets (typically ranging from a few ohms to several tens of ohms). Because the inter-sheet capacitance is extremely large, it can be considered a conductive path in an alternating electric field. Thus, grounding the core at a single point is sufficient to clamp the potential of the entire stacked core laminations to ground potential.

If the core of an annealed transformer or its metallic components are grounded at two or more points, a closed loop will be formed between the grounding points. This loop links a portion of the magnetic flux, inducing an electromotive force (EMF) and creating a circulating current. This leads to localized overheating and can even burn out the core. A single-point ground is the only acceptable and normal grounding condition for a transformer core. In other words, the core must be grounded, and it must be grounded at exactly one point.

Core faults are primarily caused by two factors: poor construction techniques resulting in short circuits, and external factors or accessories causing multi-point grounding.