Operational Requirements and Composition of Transformer Core Laminations

During normal operation of a 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 transformer core laminations are 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 transformer core 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.

A separate grounding lead must be installed for the yoke clamping structure. If the core accidentally contacts the upper clamping structure, causing a multi-point grounding fault, the fault current will circulate only within the core and clamping structure. No current will flow through the core grounding lead, which could mislead personnel into believing that no fault exists. By installing this separate lead, a closed loop is formed via the two external grounding leads regardless of where the core contacts the clamping structure, allowing the grounding current to be accurately detected externally.