Magnetic Materials for Current Transformers
Magnetic Materials for Current Transformers
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电流互感器用磁性材料
DOI：
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发表时间：
2013
期刊：
影响因子：
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通讯作者：
P. Ruffieux
中科院分区：
文献类型：
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作者：
S. Aguilera;P. Odier;P. Ruffieux
在 CERN，循环束流测量由两种类型的变压器提供：直流电流互感器 (DCCT) 和快速束流互感器 (FBCT)。每种类型的变压器都需要不同的磁特性，如磁导率、矫顽力和磁化曲线形状等参数。每个变压器都是基于具有这些特性的磁性材料环形磁芯构建的。例如，DCCT 由三个核心组成，其中两个用于测量直流分量，一个用于测量交流分量。为了研究这些参数变化对电流互感器的影响，根据铸态特性选择了几种有趣的原材料，并通过退火工艺来调整其特性，以满足每个变压器的个性化需求。首次退火测试表明，可以修改材料的磁化曲线以及磁导率，从而为构建和研究各种变压器铁芯提供了可能性。 CERN 的电流互感器 自 20 世纪 60 年代以来，CERN 加速器综合体就使用电流互感器来测量电子束电流。目前，共有变压器96台，其中DCCT 22台，FBCT 74台。还存在 6 个 DCCT 备件和 22 个 FBCT 备件。目前，安装最早的 FBCT 可以追溯到 1970 年，DCCT 可以追溯到 1982 年。为了适应不同的真空室尺寸，还有不同尺寸的变压器。变压器铁芯的磁性材料 DCCT 由三个磁芯组成，一个用于信号的交流分量，两个用于直流分量，而 FBCT 通常仅由一个磁芯组成。这些磁芯由缠绕的软磁材料带制成。电流互感器的材料选择及其磁特性会影响互感器的参数，例如 DCCT 情况下的分辨率。欧洲核子研究中心使用的变压器中的磁性材料是为了使仪器尽可能灵敏而指定的。为此，使用了最大相对复磁导率超过 50 000 的软铁磁材料。寻求的其他特性包括低巴克豪森噪声、约 1 A/m 的矫顽场和低磁致伸缩 [1]。通常，软磁材料包括坡莫合金（含有 80% 镍和 20% 铁的合金）、由约 80% 过渡金属（主要是铁和/或钴）和 20% 准金属（硼、硅和碳）组成的非晶合金以及在非晶基体中具有纳米尺寸晶粒的纳米晶合金 [2]。 CERN 开发和制造磁芯的动机是能够在内部制造不同尺寸的磁芯，获得调整磁芯磁性的专业知识以及磁性材料参数对变压器响应的影响，从而提高仪器的性能和分辨率。材料对变压器响应的影响 众所周知，金属带的厚度会影响变压器的响应，因为涡流会随之增加。例如，具有不同叠片厚度的磁芯可以组合在单个变压器中，使得第一微秒内的上升时间和初始衰减取决于具有较薄叠片的磁芯，并且时间常数取决于磁芯的质量[3]。材料的功率损耗与磁滞回线的面积成正比。研究由涡流阻尼产生的损耗随着磁化频率的增加而变化是很有趣的[2]。损耗是需要考虑的一个重要因素，因为磁芯在运行过程中会发热。材料的最高使用温度（材料仍具有其所有特性的极限温度）应高于变压器使用时材料将达到的工作温度。为了限制温度升高和材料中耗散的功率损耗，通常采用层间绝缘。磁芯绝缘可以通过多种方式完成。最常见的是使用 Dupont Kapton® 等绝缘体生产绕带磁芯，或使用陶瓷来绝缘各层。后一个过程可以通过溶胶-凝胶法将带材浸入热处理后变成陶瓷的溶液中来完成[4]。 IBIC2013 会议记录，英国牛津 MOPF24 光束电荷监测器和一般诊断 ISBN 978-3-95450-127-4 263 C op yr ig ht c ○ 20 13 by JA C oW — cc Cre ative Com m on sA ttt ri butio n 3. 0 (CC -B Y3. 0) 塑造磁化曲线 对于不同类型的变压器，其磁化（B-H）曲线的要求是不同的。 DCCT 需要矫顽力场约为 3 A/m 的圆形曲线，而 FBCT 则需要更多平坦且低矫顽力的曲线。为了改变磁化曲线的形状，需要对合金进行热处理（过程定义为退火）。根据所需的最终性能，退火必须在磁场下进行，以获得 FBCT 所需的更平坦的 B-H 曲线。还应考虑到改变这些性能的适当温度应高于居里温度并低于结晶温度以维持其晶体结构。巴克豪森效应 巴克豪森效应是一种物理现象，表现为铁磁材料暴露在变化的磁场中时磁化强度的一系列跳跃。通过次级线圈围绕样品，感应电压可以转化为声学噪声，巴克豪森噪声 (BN) 一词即由此而来 [5]。这种效应通常用作无损测试，以检查材料的微观结构（晶界、位错、不均匀性等）和应力配置的变化。正因为如此，该技术可以很好地概述磁域的变化。很明显，BN 会影响变压器的性能及其分辨率 [6]，因此研究热处理对 BN 的影响以及对仪器响应的影响是一个有趣的特性。所用材料摘要 本研究中所用材料为铁基非晶合金和纳米晶合金以及钴基非晶合金。铁基合金购自秦皇岛延钦纳米科技有限公司（http://www.yanqin.com），钴基合金购自 Nanostructed & Amorphous Materials (Nanoamor), Inc. (http://www.nanoamor.com)、Vacuumschmelze GmbH & Co. KG (http://www.vacuumschmelze.de)，型号为 VC 6025 G40 和 Hitachi Metals Europe GmbH (http://www.metglas.com) 为 2705M。从表1可以看出，铁基合金的居里温度高于钴基合金。表 1：材料汇总 类型 居里温度 [℃] 结晶温度 [℃] 铁基非晶 420 560 铁基纳米晶 560 510
At CERN, the circulating beam current measurement is provided by two types of transformers, the Direct Current Current Transformers (DCCT) and the Fast Beam Current Transformers (FBCT). Each type of transformer requires different magnetic characteristics regarding parameters such as permeability, coercivity and shape of the magnetization curve. Each transformer is built based on toroidal cores of a magnetic material which gives these characteristics. For example, DCCTs consist of three cores, two for the measurement of the DC component and one for the AC component. In order to study the effect of changes in these parameters on the current transformers, several interesting raw materials based on their as-cast properties were selected with the annealing process used to tune their properties for the individual needs of each transformer. First annealing tests show that the magnetization curve, and therefore the permeability, of the material can be modified, opening the possibility for building and studying a variety of transformer cores. CURRENT TRANSFORMERS AT CERN At CERN’s accelerator complex, current transformers have been used to measure the beam’s current since the 1960’s. Nowadays, there are a total of 96 transformers out of which, 22 are DCCTs and 74 are FBCTs. There also exist 6 spares for DCCTs and 22 spares for FBCTs. Currently, the oldest installed FBCT dates back to 1970 and the DCCT to 1982. There are also different sized transformers in order to adapt to the different vacuum chamber dimensions. MAGNETIC MATERIAL FOR TRANSFORMER CORES DCCTs consist of three magnetic cores, one for the AC component of the signal and two for the DC, while FBCTs consist normally of only one magnetic core. These cores are made out of wound ribbon of soft magnetic material. The choice of material and its magnetic characteristics for current transformers affects the parameters of the transformer such as the resolution in the case for the DCCT. The magnetic material in the transformers used at CERN was specified for making the instrument as sensitive as possible. For this, soft ferromagnetic material with a maximum relative complex permeability of more than 50 000 has been used. Other characteristics sought included low Barkhausen Noise, coercive field of around 1 A/m and low magnetostriction [1]. Typically, soft magnetic materials include permalloys (alloys with 80 % nickel and 20 % iron), amorphous alloys composed of about 80 % transition metals (mainly iron and/or cobalt) and 20 % metalloids (boron, silicon and carbon) and nanocrystalline alloys, with nanometer sized grains in an amorphous matrix [2]. The motivation for developing and manufacturing magnetic cores at CERN is driven by the interest of being able to make different sized cores in-house, to acquire the know-how for tuning the cores’ magnetic properties and the influence of the magnetic material’s parameters in the transformer response in order to improve the instrument’s performance and resolution. Influence of Material in Transformer Response The thickness of ribbons is known to affect the response of the transformer, as Eddy currents increase with it. For example, cores with different lamination thicknesses can be combined in a single transformer, making the rise time and initial decay in the first microseconds dependent on the core with the thinner lamination, and the time constant dependent on the mass of the core [3]. The power loss of the material is proportional to the area of the hysteresis loop. It is interesting to study the change in the losses with increasing magnetization frequency, produced by the damping from Eddy currents [2]. Losses are an important factor to take into consideration, as the magnetic core will heat up during operation. The maximum service temperature of the material (the limit temperature at which the material still has all of its characteristic properties) should be higher than the operational temperature the material will reach when the transformer is in use. In order to limit the temperature increase and the power loss dissipated in the material, insulation between layers is usually employed. Insulation in cores can be done in several ways. The most common are producing tape wound cores with an insulator like Dupont Kapton® or using a ceramic to insulate the layers. The latter process can be done by means of the Sol-Gel method by immersing the ribbon into a solution that becomes ceramic after a thermal treatment [4]. Proceedings of IBIC2013, Oxford, UK MOPF24 Beam Charge Monitors and General Diagnostics ISBN 978-3-95450-127-4 263 C op yr ig ht c ○ 20 13 by JA C oW — cc C re at iv e C om m on sA tt ri bu tio n 3. 0 (C C -B Y3. 0) Shaping the Magnetization Curve For the different types of transformers, the requirements of the magnetization (B-H) curve are different. DCCTs require a round-shaped curve with a coercive field of about 3 A/m, whilst the FBCTs require more of a flat-shaped and low coercivity curve. In order to change the shape of the magnetization curve, it is necessary to thermally treat the alloy (process defined as annealing). Depending on the desired final properties, the annealing must be conducted under a magnetic field in order to achieve the flatter B-H curves required by the FBCTs. It also should be taken into consideration that the adequate temperature to modify these properties should be above the Curie temperature and below the crystallization temperature to maintain its crystalline structure. Barkhausen Effect The Barkhausen Effect is a physical phenomenon which is manifested as a series of jumps in magnetization of ferromagnetic material when exposed to a varying magnetic field. Surrounding the sample by a secondary coil, the induced voltage can be transformed into acoustic noise, from which the term Barkhausen Noise (BN) derives [5]. This effect is often used as a non-destructive test to check changes in microstructure (grain boundaries, dislocations, inhomogeneities, etc.) and stress configurations of materials. It is because of this, that this technique offers a good overview of changes in magnetic domains. It is clear that the BN influences the transformer performance and its resolution [6], therefore it is an interesting characteristic to study to see the effect of the thermal treatment on the BN and then on the instrument’s response. Summary of Materials Used For this study, the materials used were iron-based amorphous and nanocrystalline alloys and cobalt-based amorphous alloys. The iron-based alloys were purchased from Qinhuangdao Yanqin Nano Science & Technology Co., Ltd (http://www.yanqin.com), and the Cobalt-based were purchased in Nanostructured & Amorphous Materials (Nanoamor), Inc. (http://www.nanoamor.com), Vacuumschmelze GmbH & Co. KG (http://www.vacuumschmelze.de) as VC 6025 G40 and Hitachi Metals Europe GmbH (http://www.metglas.com) as 2705M. As can be seen from Table 1, iron-based alloys have a higher Curie temperature than cobalt-based alloys. Table 1: Materials Summary Type Curie temperature [ C] Crystallization temperature [ C] Iron-based Amorphous 420 560 Iron-based Nanocrystalline 560 510