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在核聚变装置中,开关电源广泛应用在控制系统,保障各系统控制保护动作,但在外磁场达到23.5 mT时,该类电源输出失真畸变,不能正常输出,如图2所示。如此失效及误动作现象随核聚变装置容量增大,发生的频次和危害会进一步加大,因此分析和解决复杂电磁环境下的干扰问题,研究磁性元件的故障特征及保护措施,对于保障大型托卡马克聚变装置的可靠运行具有重大意义。
图 2 外磁场环境下开关电源的输出特性分析
Figure 2. Analysis of output characteristics of switch power supply in external magnetic field environment
变压器是开关电源非常重要的部件,起到电源转换、隔离的作用,由磁芯和绕组组成,如图3所示。变压器在运行期间,会产生磁场,同时绕组和磁芯中会产生损耗,当开关频率越高,损耗越大。分析其性能变化,确定此磁场对其的干扰作用。
1)变压器磁芯损耗
磁芯损耗是在交流磁通磁化时形成的能量损耗,会引起发热问题。其损耗与频率、磁感应强度、体积等有关。
磁滞损耗,与开关频率呈正相关,有经验公式为:
$$ P_{\mathrm{h}}=k_{\mathrm{h}}fV{\boldsymbol{B}}_{\mathrm{m}}^{\alpha} $$ (1) 式中:
kh ——磁滞系数;
f ——开关频率(Hz);
V ——磁芯体积(m3);
Bm ——磁感应强度(T);
α ——斯坦梅兹系数。
涡流损耗为:
$$ P_{\mathrm{e}}=k_{\mathrm{e}}f^2V{\boldsymbol{B}}_{\mathrm{m}}^2 $$ (2) 式中:
ke ——涡流损耗系数。
剩余损耗为:
$$ P_{\mathrm{T}}=\eta f^{\delta}V{\boldsymbol{B}}_{\mathrm{m}}^{\beta} $$ (3) 式中:
η ——损耗系数;
δ ——频率的损耗指数;
β ——磁感应的损耗指数。
2)变压器绕组损耗
绕组损耗与趋肤效应有关,其趋肤深度为:
$$ \Delta = \dfrac{1}{{\sqrt {{\text{π}} f\gamma \mu } }} $$ (4) 式中:
γ ——导线的电导率(S/m);
μ ——磁导率(H/m)。
交、直流电阻的关系为:
$$ {R_{{\mathrm{DC}}}} = {K_{\delta} } \times {R_{{\mathrm{AC}}}} $$ (5) 式中:
RDC、RAC ——直流阻抗和交流阻抗(Ω);
Kδ ——二者比值系数。
无外界环境磁场下的变压器(100 kHz)电磁分布如图4所示。磁芯磁场主要集中在中间磁柱上,而电流损耗分布较为合理,计算得到磁芯损耗为1.768 W,绕组铜损耗为5.028 W,漏感为48.22 μH。
图 4 无外界环境磁场下的变压器电磁分布
Figure 4. Electromagnetic distribution of transformers under no external magnetic field
通过计算不同外界环境、不同方向下的变压器铜耗和磁芯损耗的变化关系曲线(如图5 ~ 图7所示),可以发现,随着外界环境磁场的增强,变压器的铜耗和磁芯损耗都在增加。具体来说,当施加在X方向时,变压器的铜耗增大幅度较小,而磁芯损耗则迅速增大;当施加在Y方向时,变压器的磁芯损耗增大幅度较小,而铜耗却迅速增大;当施加在Z方向时,变压器的铜耗和磁芯损耗都有较大幅度的增加,且铜耗的增大幅度大于磁芯损耗。
图 5 变压器损耗随外界磁场(X方向)变化
Figure 5. Relationship between the loss of transformers and the external magnetic field (X-direction)
图 7 变压器的损耗随外界磁场(Z方向)变化
Figure 7. Relationship between the loss of transformers and the external magnetic field (Z-direction)
图 6 变压器损耗随外界磁场(Y方向)变化
Figure 6. Relationship between the loss of transformers and the external magnetic field (Y-direction)
以开关电源的电子变压器器件为对象,研究了外加磁场对磁性器件的特性影响。随着外磁场的不断增大,变压器具有输出特性下降和失效的现象,是实际托卡马克装置的器件失效具有较为典型的案例。
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在确定托卡马克磁性器件失效的根源,需要解决此类问题的发生,即电磁兼容问题。电磁屏蔽是将被保护装置包围起来,衰减和阻断传播途径,抑制外(内)部干扰源对内(外)部设备的影响[20-21]。屏蔽性能由屏蔽效能(单位:dB)表示。
对于电场屏蔽效能为:
$$ \mathrm{SE}\mathrm{_d}=-20\mathrm{lg}\dfrac{E_{\mathrm{s}}}{E_0} $$ (6) 对于磁场屏蔽效能为:
$$ \mathrm{SE}\mathrm{_c}=-20\mathrm{lg}\dfrac{H_{\mathrm{s}}}{H_0} $$ (7) 式中:
Es、Hs ——存在屏蔽时的响应;
E0、H0 ——不存在屏蔽时的响应。
直流磁场主要通过消除或抑制干扰来处理。直流磁场的磁力线通常会优先通过低磁阻的路径。可以选择高磁导率材料(如铁、坡莫合金等)来进行屏蔽设计。低频磁场会随着距离的增加而衰减,如果无法远离磁场源,则需要进行屏蔽。同时,要注意材料的磁饱和问题,磁导率越高的材料越容易发生磁饱和。
如图8所示,方形屏蔽体(带开孔)可以看作是封闭的方形屏蔽体和开孔的完美导电(PEC)方形屏蔽体的叠加,总磁场也是通过叠加计算:B1=B2+B3。
封闭的方形屏蔽体(封闭)尺寸为a×b×c,厚度为Δ,材料的电导率为σ,相对介电常数为εr,相对磁导率为μr,则其屏蔽效能可以通过形状因子进行近似计算:
$$ {\mathrm{SE}}=\left\{\begin{gathered}1+\frac{j\omega\mu_0\sigma V\Delta}{A},\ \Delta\ll\delta \\ \left(1+\frac{\gamma V\Delta}{\mu_{\mathrm{r}}A}\right)\frac{\mathrm{exp}({\gamma\Delta})}{2},\ \Delta\gg\delta\end{gathered}\right. $$ (8) 式中:
V ——屏蔽体的体积(m3),V = abc;
A ——屏蔽体的表面积(m2),A = 2(ab+bc+ca)。
建立边长为1 m的正方体屏蔽体结构,厚度为d,如图9所示。选择铁和铝作为稳态磁场和瞬态磁场的屏蔽材料,在Z方向施加0.5 T的磁感应强度。
使用铝作为屏蔽材料时,在平行于轴的侧面开设一个半径为r的圆孔。通过计算得到不同开孔半径和不同导体厚度下的铝屏蔽体在不同频率的屏蔽效能曲线分布,如图10所示。由此可见,随着频率的增加,铝材料的屏蔽效果越好。当频率达到临界值时,屏蔽效能不再增加,该临界值与屏蔽体厚度有关,与开孔尺寸无关。此外,随着开孔半径的增大,屏蔽效能会降低,频率效果也会变差;而随着屏蔽体厚度的增加,屏蔽效能会提高,频率效果也会更好。然而,随着频率增大,最终的屏蔽效能将趋于一致。
经计算发现,铝材料对直流稳态磁场几乎没有屏蔽作用。因此,在屏蔽稳态磁场时,需要采用高磁导率材料(如铁)进行屏蔽设计。同样构建一个边长为1 m的正方体铁材屏蔽体结构,其屏蔽效能计算结果如图11所示。可以看出,开孔尺寸越小、屏蔽体厚度越大,屏蔽效能就越高,这表明屏蔽效果越好。
图 11 铁屏蔽体中心点的屏蔽效能随频率的变化关系
Figure 11. Relationship between shielding effectiveness of the center point of the iron shielding body and the frequency
超导聚变装置外围稳态磁场最大为200 mT,以开关电源20 mT为安全值,则屏蔽体的屏蔽效能最小需要做到SE = 20 dB。被保护的电力电子器件的尺寸不一,需要根据实际情况来计算,以100 mm的正方体空间为被测安全区域,保证在200 mT的磁场条件下,其内部的磁场不大于20 mT。设计全屏蔽结构如图12所示。
为了确保内部100 mm的正方体空间的磁场最大值Bmax低于20 mT,需要不断增大屏蔽厚度,计算得到磁场最大值Bmax与屏蔽体厚度d的关系曲线如图13所示。随着厚度增大,磁场值越小,当厚度大于20 mm时才能满足低于最大磁场值20 mT的要求。实际情况需要根据器件本身的耐受磁场值,屏蔽材料选择、屏蔽结构的设计、安装位置的要求等条件实时设计。
Failure Analysis and Shielding Effectiveness Study of Magnetic Components in Strong Magnetic Environments for Magnetic Confinement Fusion Devices
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摘要:
目的 文章旨在探究核聚变装置产生的空间磁场对磁性器件的影响,并深入了解复杂电磁环境下的磁场屏蔽特性。 方法 研究过程中,以核聚变内电源系统中的开关电源和电子变压器为具体研究对象,通过详细分析磁场对损耗的影响以及不同方向磁场下的特性变化,同时深入探讨影响磁场屏蔽效能的关键因素以及不同材料的应用情况,来实现研究目的。 结果 结果表明,强磁场环境下磁性元件的损耗明显增加,且不同方向磁场对元件特性的影响程度不同。通过对比分析不同材料的屏蔽效果,发现具有高导电性和磁导率的材料在屏蔽设计中具有更好的应用前景。此外,优化屏蔽结构能够进一步提高屏蔽效能,降低磁场对磁性元件的影响。 结论 通过系统分析核聚变装置强磁场对磁性元件的影响及屏蔽效能,为设备磁场耐受能力的测试和屏蔽设计的验证提供了关键参考。未来工作将聚焦于进一步优化屏蔽材料和结构设计,以提高磁性元件在强磁场环境下的稳定性和可靠性。 Abstract:Introduction The purpose of this study is to explore the impact of the spatial magnetic field generated by nuclear fusion devices on magnetic devices and to gain an in-depth understanding of the magnetic field shielding characteristics in a complex electromagnetic environment. Method In the research process, the switch power supply and electronic transformer in the nuclear fusion internal power supply system were selected as the specific research objects. By analyzing in detail the impact of the magnetic field on the loss and the characteristic changes under different directions of the magnetic field, as well as by exploring the key factors affecting the magnetic field shielding effectiveness and the application of different materials, the research objectives were achieved. Result The results indicate that the loss of magnetic components significantly increases in a strong magnetic field environment, and the degree of influence of different directions of magnetic fields on component characteristics varies. By comparing and analyzing the shielding effects of different materials, it is found that materials with high conductivity and magnetic permeability have better application prospects in shielding design. Additionally, optimizing the shielding structure can further improve shielding effectiveness and reduce the impact of magnetic fields on magnetic components. Conclusion By systematically analyzing the impact of strong magnetic fields on magnetic components and the shielding effectiveness of nuclear fusion devices, key references are provided for testing the equipment's magnetic field tolerance and verifying shielding design. Future work will focus on further optimizing shielding materials and structural design to improve the stability and reliability of magnetic components in strong magnetic field environments. -
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