A magnetic leakage shield offers an alternative path for part of the field. That can reduce exposure at one structural component while increasing flux density in the shield or moving the leakage toward another region. The protected part’s improvement is only one output of the comparison.
Use the actual pancake currents and leakage geometry. Include the shield’s material directions, gaps, mounting and nearby conductive structures. A uniform external field applied to an isolated shield may not represent its installed duty.
Compare three regions together
| Region | Magnetic quantity | Thermal question |
|---|---|---|
| Originally protected component | Field and induced-current change | Do its losses decrease? |
| Shield body and edges | Local flux density and magnetic state | What loss is introduced in the shield? |
| Nearby unprotected components | Redirected leakage exposure | Has the limiting hotspot moved? |
Track field components and spatial maxima over the relevant waveform. A shield’s average flux density can look moderate while a narrow edge or mounting region has a demanding local state.
Check the nonlinear response
Use a material model appropriate to the shield and excitation. If the local magnetic response becomes nonlinear, the assumed low-reluctance path can change with amplitude. A linear permeability chosen from a favorable operating point may overstate protection elsewhere in the cycle.
Assess sensitivity to shield thickness, gaps and placement using realistic variations. A small installation gap or displaced end can change where flux exits. Keep the same source and boundary assumptions in the with-shield and without-shield models so that the difference belongs to the shield.
For conductive losses, use an appropriate eddy-current model and actual electrical contact assumptions. Fasteners and adjoining metal can create paths absent from an isolated-sheet calculation.
Follow losses into temperature
Lower field at one location does not establish lower maximum temperature across the assembly. Map the losses and their cooling paths. A shield near a restricted oil route may have a different thermal response from the component it protects.
Compare the complete thermal result under the same operating condition. Where model uncertainty is significant, use representative verification evidence to challenge the limiting region. A measurement only on the protected surface can miss a new internal hotspot. Record rated quantities and test conditions on the general power-transformer basis of IEC 60076-1 [1]. Assess the relevant measured quantities with a defined measurement model and propagated uncertainty [2].
Inspect mounting and geometry after assembly because the analysis depends on their actual state. A shield positioned correctly in nominal CAD but moved during lead installation describes a different field problem.
The decision should identify the new limiting location and the shield’s margin under the relevant duty. Protection is demonstrated by the complete redistribution assessment, not by one reduced contour near the component that motivated the shield.
Check the new shield hotspot with the complete loss balance
Assume one 50 Hz loading state. Before shielding, a structural region dissipates 400 W. After shielding it dissipates 150 W, while the shield adds 180 W and a nearby region adds 100 W. The selected-region total becomes 430 W, 7.5% higher, even though the protected part improves by 62.5%. If the shield’s assumed thermal resistance to 60°C oil is 0.20 K/W, its estimated temperature is 96°C. Retain the complete loss map and compare this new location with the applicable temperature criterion; reduced field at the original component does not establish a lower maximum temperature.
References
- IEC, IEC 60076-1:2011, Power transformers – Part 1: General.
- JCGM, JCGM 100:2008, Evaluation of measurement data — Guide to the expression of uncertainty in measurement, 2008.