Litz-Wire Transformer Winding for Electric Field Grading
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Solution Overview
Problem
Medium-frequency transformers face challenges with high operating voltages and low power, leading to large insulation distances and poor filling ratios, which can result in field peaks causing partial discharge and reduced insulation lifetime, while the use of litz wires at elevated frequencies increases skin- and proximity-effect losses.
Innovation Solution
A transformer design with a first and second winding arranged around an axis, where the second winding uses a litz wire with varying cross-sectional curvatures between its end and middle portions to reduce the electrical field peak magnitude, and a method of manufacturing this transformer by forming the litz wire with a smaller curvature at the end portion to minimize field gradients.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If insulation distances are increased to handle high operating voltages, then electrical insulation reliability is improved, but the filling ratio of the core window deteriorates
Solution Approach 1:
The patent applies local quality by implementing different curvature radii at different locations of the litz wire cross-section. Specifically, the quadrant facing the first winding (inner quadrant) has a larger curvature radius than the outer quadrants, creating locally optimized electric field distribution that reduces peak fields at the winding interface while maintaining adequate insulation distances elsewhere.
Solution Approach 2:
The patent changes the geometric parameter of the litz wire cross-section from a uniform curvature to a variable curvature profile. The curvature radius is specifically modified in the inner quadrant to optimize the electric field grading, thereby reducing peak field magnitude without increasing the overall insulation distance or compromising the filling ratio.
2Reliability
If litz wire curvature at end portion is increased to improve field grading, then electrical field peak magnitude is reduced, but manufacturing complexity increases
Solution Approach 1:
The patent applies local quality by implementing different curvature radii at different locations of the litz wire cross-section. Specifically, the quadrant facing the first winding (inner quadrant) has a larger curvature radius than the outer quadrants, creating locally optimized electric field distribution that reduces peak fields at the winding interface while maintaining adequate insulation distances elsewhere.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The design achieves optimal field grading and reduces electrical field peaks, allowing for a compact and economical transformer structure with minimized losses due to skin- and proximity-effect.
Implementation Method 1
the curvature of the first cross section is smaller than the curvature of the second cross section, thereby reducing a peak magnitude of the electrical field between the end portion of the second winding and the first winding
Implementation Method 2
Because of the elevated frequencies, for example 10 kHz at which MFTs operate, the windings are often made from litz wires. This is necessary to keep skin- and proximity-effect losses within acceptable limits.
Implementation Method 3
This is necessary to keep skin- and proximity-effect losses within acceptable limits.
Data Source
AI summary
A transformer includes a first winding arranged around an axis, and a second winding arranged around the axis. The second winding includes a litz wire having an end portion located at an axial end position of the second winding and a middle portion located at an axial middle position of the second winding. The litz wire has a first cross section at the end portion and a second cross section at the middle portion, the cross sections each including in a quadrant between the axial outward direction and the direction pointing towards the first winding a curvature extending between the axial outward direction and the direction pointing towards the first winding. The curvature of the first cross section is smaller than the curvature of the second cross section thereby reducing the peak magnitude of the electrical field between the end portion of the second winding and the first winding.