Composition Requirements and Normal Operation of Annealed Transformer Cores

Grounding Requirements and Normal Operation of Annealed Transformer Cores

During normal operation, an electric field exists between the energized windings and the oil tank in an annealed transformer core. The iron core and other metallic components are situated within this electric field. Due to uneven capacitance distribution, the electric field strength varies across different areas. If the iron core is not properly grounded, charging and discharging phenomena will occur. This can damage both solid insulation and the dielectric strength of the insulating oil; therefore, it is essential that the iron core has a single-point ground connection.

Annealed transformer cores are composed of sili

con steel sheets. To minimize eddy currents, there must be a certain level of insulation resistance between the laminations (typically ranging from just a few ohms to several tens of ohms). Because the interlaminar capacitance is extremely high, these gaps can be considered as electrical pathways under an alternating electric field. Thus, grounding the core at only one point is sufficient to clamp the potential of the entire stacked laminations to ground potential.

If the annealed transformer core or its metallic components are grounded at two or more points (multi-point grounding), a closed loop will form between the grounding points. This loop links part of the magnetic flux, inducing an electromotive force (EMF) and creating a circulating current. This leads to localized overheating and may even burn out the iron core. Therefore, a single-point ground is the only acceptable and normal grounding configuration for a transformer core. In short, the iron core must be grounded, and strictly at one point only.

Iron core failures are primarily caused by two factors: poor construction techniques resulting in short circuits, and multi-point grounding triggered by accessories or external environmental factors.