Structure of Transformer Cores

The core is composed of silicon steel sheets. To reduce eddy currents, there is a specific insulation resistance between the sheets (generally only a few ohms to tens of ohms). Due to the extremely high inter-laminar capacitance, the sheets can be regarded as conductive paths in an alternating electric field. Therefore, grounding the core at a single point is sufficient to clamp the potential of the entire stack of core laminations to the ground potential. The core is the main magnetic circuit part of the transformer. It is generally made of hot-rolled or cold-rolled silicon steel sheets with a high silicon content, coated with insulating varnish, and stacked together. The core and the coils wound around it form a complete electromagnetic induction system. The power transmission capacity of a power transformer depends on the material and cross-sectional area of the core.

(1) Set up an independent grounding lead for the core yoke clamp. If the core touches the upper clamp and causes a multi-point grounding fault, the grounding current will only circulate inside the core clamp, and no current will flow through the core grounding lead. This may lead personnel to mistakenly believe that the core has not malfunctioned. After setting up this independent lead, no matter where the core touches the clamp, a loop will be formed through the two external grounding leads. In this way, the grounding current can be accurately detected externally.

(2) Place the core grounding tab at the center of the yoke's cross-section. With this placement, regardless of the position of the yoke strap's insulating bolt and the location of the fault grounding point, the maximum induced voltage in the loop is only 1/4 of the turn voltage. At this time, the maximum grounding current is only about a few amperes, which is much smaller than when the core grounding tab is placed in other positions.

(3) If it is indeed necessary to place the core grounding tab in another position due to on-site installation constraints, the insulating bolt of the yoke strap and the grounding tab should be placed diagonally opposite to each other. This can prevent large currents from occurring.