31 January 2019
Weidmann Shield Rings
Shield rings, which are also referred to as static rings or metalized screen rings, are placed at both ends of the winding. In most cases they are used to shape the electrical field with an appropriate radius, rather than leaving the sharp edge created by the conductor. The electric surface stress at the edge of the conductor decreases dramatically because of the larger radii. The resulting optimum radius is the available space between the winding and the yoke, or the adjacent winding, and the mechanical requirements on the contact area for spacer elements.
Often shield rings are used to balance the small axial height of a winding against neighboring windings, or for “filling up” the open space created by a steep helical tap winding. Typically, these rings are distinctive due to their small width compared to their height.
All the rings are exposed to mechanical stress due to the clamping force. This assures the necessary compressive strength in the winding in the event of a short circuit. The pressing force is transmitted locally to the ring by spacers and press blocks. Even with the changing operational temperatures of the transformer, the core of the static ring must guarantee the function and optimal life of the ring. Weidmann provide solutions based on laminated Transformerboard or laminated wood, and offer the most cost effective option when you consider the design of the maximum force during operation.
The electrostatic shield function is provided by either a thin layer of metal fabric tape which minimizes eddy currents created by the varying magnetic flux, or by aluminum crepe paper. This outer layer is insulated with crepe paper wrapped evenly and tightly around the ring. The stretch of the crepe paper allows it to follow the three-dimensional curved shape of the core ring, and in particular the recess of lead outs. Variations are available, such as applying standard crepe paper or thermally stabilized crepe paper.
Weidmann manufacture static rings for a wide range of power transformers, covering all voltages and sizes, including the world’s largest manufactured rings produced for Ultra High Voltage HVDC applications. Our electronic catalogue provides an overview of the standard sizes available, however does not list all the possible variants. We recommend you contact us to discuss your requirements so we can find the best solution for your application.
For more information please visit: https://www.weidmann-electrical.com/insulation-technology/components/cellulose-based/