Normal Operation and Composition Requirements of Transformer Core Laminations

Normal Operation and Grounding Requirements of Transformer Core Laminations

During normal operation, an electric field exists between the energized windings and the oil tank. 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.

Transformer core laminations are composed of silicon 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 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.

A separate grounding lead wire should be installed specifically for the yoke clamps. If the iron core accidentally contacts the upper clamp and causes a multi-point grounding fault, the fault current would otherwise circulate only internally between the core and the clamp, with no current flowing through the main core grounding lead. This could mislead maintenance personnel into believing that no fault has occurred. By installing this dedicated lead wire, any contact between the core and the clamp—regardless of the location—will create a detectable circuit through the two external grounding leads. This ensures that the grounding current can be accurately detected externally.