Internal dielectric strength of vacuum interrupters (VIs) is primarily determined by the macroscopic electric field strength and effective area on the electrode surfaces. While internal dielectric strength is usually not an issue for medium-voltage VIs ( U_(m) <= 52kVU_{m} \leq 52 \mathrm{kV} ), it requires special consideration when going to higher voltage levels. Generally, the vacuum breakdown is initiated by field emissions from critical enhancement points with higher electric field on a cathode. The most critical point of the macroscopic electric field in a VI is present at the front contact edge between the main contacts. A vapor shield of a VI is one component that determines the electric field strength and distribution. Its floating electrical potential can basically be determined by the geometry of the VI and its surroundings. Since the shield potential can change the maximum electric field strength at the front contact edge in the VI, it seems possible to increase the internal dielectric strength of the VI by giving the vapor shield a well-defined electric potential. The present study focuses on the possibility of electrical potential control of the vapor shield for increasing the internal withstand voltage of the VI. Basically, the potential control of the vapor shield can be realized by connecting a parallel capacitor network. This allows controlling the shield potential arbitrarily between H.V. potential and zero by changing the voltage division ratio. From the field calculation results for a 72.5 kV VI model, the maximum electric field strength can be reduced by up to 23.3%23.3 \% compared to a floating vapor shield. Dielectric withstand tests for the 72.5 kV VI with potential control to determine the internal breakdown voltage with positive impulse voltage were carried out. The experimental results show that the breakdown voltage can be increased by 11.2%11.2 \%, by setting the vapor shield potential to onethird of the applied voltage. The increase in the breakdown voltage of the VI can be explained by the relaxation of the electric field on the critical point as a result of the decrease in the potential difference between cathode and vapor shield. 真空灭弧室 （VI） 的内部介电强度主要取决于电极表面的宏观电场强度和有效面积。虽然内部介电强度通常不是中压 VI （ U_(m) <= 52kVU_{m} \leq 52 \mathrm{kV} ） 的问题，但在达到更高的电压水平时需要特别考虑。通常，真空击穿是由阴极上具有较高电场的关键增强点的场发射引发的。VI 中宏观电场的最关键点位于主触点之间的前触点边缘。VI 的蒸汽屏蔽是决定电场强度和分布的一个组件。其浮动电势基本上可以由 VI 的几何形状及其周围环境决定。由于屏蔽电位可以改变 VI 前接触边缘的最大电场强度，因此似乎可以通过为蒸汽屏蔽提供明确定义的电势来增加 VI 的内部介电强度。本研究的重点是蒸汽屏蔽的电位控制以增加 VI 内部耐压的可能性。基本上，蒸汽屏蔽的电位控制可以通过连接并联电容器网络来实现。这允许通过改变分压比在高压电位和零点之间任意控制屏蔽电位。从 72.5 kV VI 模型的场计算结果来看，与浮动蒸汽护罩 23.3%23.3 \% 相比，最大电场强度最多可降低。对 72.5 kV VI 进行了介电耐压试验，并进行了电位控制，以确定具有正脉冲电压的内部击穿电压。 实验结果表明，通过将蒸汽屏蔽电位设置为外加电压的三分之一，可以通过 增加 11.2%11.2 \% 击穿电压。VI 击穿电压的增加可以用阴极和蒸汽屏蔽之间的电位差减小导致临界点上的电场松弛来解释。
VACUUM circuit breakers (VCBs) have proven to be suitable applications in the medium voltage range ( U_(m) <=U_{m} \leq 52 kV ) through more than half a century of operation experience [1]. As application of SF_(6)\mathrm{SF}_{6} is now subject to increasing restrictions for ecological reasons, VCBs are being more intensely 通过半个多世纪的运行经验，VACUUM 断路器 （VCB） 已被证明适用于中压范围 （ U_(m) <=U_{m} \leq 52 kV） 的应用 [1]。由于生态原因，现在 的应用 SF_(6)\mathrm{SF}_{6} 受到越来越多的限制，VCB 变得更加严格
evaluated as a possible alternative for SF_(6)\mathrm{SF}_{6} circuit breakers [2]. For use in higher voltage levels, different measures can be taken. One is to connect medium voltage vacuum interrupters (VIs) in series, which must be ensured with the exact coordination of switching instants [3], [4]. Another option is to use single-break but larger VIs and to apply additional potential-free shields, which provide capacitive field grading, namely multi-shield VIs [5]. 评估为断路器的可能替代方案 SF_(6)\mathrm{SF}_{6} [2]。对于更高电压级别的使用，可以采取不同的措施。一种是将中压真空灭弧室 （VI） 串联起来，这必须通过开关瞬时 [3]， [4] 的精确协调来确保。另一种选择是使用单断开但更大的 VI 并应用额外的无电位屏蔽，以提供电容场分级，即多屏蔽 VI [5]。
One important aspect for the design of a high voltage VCB is to ensure sufficient electric insulation at open-state in normal operation and during overvoltages e.g., in switching operations [6]. The internal breakdown voltage of a VI which is the main switching component of a VCB, is primarily determined by the electric field on an electrode surface. The most critical point of the macroscopic electric field in a VI is present at the front contact edge between the main contacts. Furthermore, back contact edges, shield edges and the internal triple junctions (vacuum-conductor-insulator) also show a high field enhancement. This can basically be determined by the internal geometry of the VI. A metal-vapor condensation shield in the middle position is also a factor to determine the macroscopic maximum electric field strength on the main contacts and the general field distribution in the VI [7]-[9]. It is fixed on a potential-free ring between the upper and the lower ceramic tube, and its potential is normally maintained capacitively at half-potential. But it can be influenced not only by the capacitance of the VI itself but also by the capacitance between the vapor shield and the external enclosure. Especially when being very closely surrounded by ground potential, such as in a dead tank type VCB, the potential difference between high voltage electrode and vapor shield can become higher [10]. It may increase the probability of breakdown between high voltage electrode and vapor shield. 高压 VCB 设计的一个重要方面是确保在正常工作时和过压期间（例如开关作）在开路状态下有足够的电绝缘[6]。VI 是 VCB 的主要开关元件，其内部击穿电压主要由电极表面的电场决定。VI 中宏观电场的最临界点位于主触点之间的前触点边缘。此外，背接触边缘、屏蔽边缘和内部三重结（真空导体-绝缘体）也显示出高场增强。这基本上可以由 VI 的内部几何形状决定。中间位置的金属-蒸汽冷凝屏蔽层也是决定主触点上宏观最大电场强度和 VI 中一般电场分布的一个因素[7]-[9]。它固定在上下陶瓷管之间的无电位环上，其电位通常电容保持在半电位。但它不仅会受到 VI 本身电容的影响，还会受到防潮罩和外部外壳之间的电容的影响。特别是当被接地电位非常紧密包围时，例如在死罐型 VCB 中，高压电极和蒸汽屏蔽之间的电位差会变得更高[10]。它可能会增加高压电极和防潮罩之间击穿的概率。
The present study focuses on the possibility of potential control of the vapor shield(s) of VIs for increasing the internal withstand voltage. Firstly, a concept of shield potential control through FEM simulation to determine the maximum electric field strength is presented. Secondly, dielectric withstand tests on a 72.5 kV VI with applied vapor shield potential control are performed to determine its internal withstand voltage under impulse voltage stress. 本研究的重点是 VI 的蒸汽屏蔽的电位控制以提高内部耐压的可能性。首先，提出了通过有限元仿真来控制屏蔽电位以确定最大电场强度的概念。其次，对应用蒸汽屏蔽电位控制的 72.5 kV VI 进行介电耐受测试，以确定其在脉冲电压应力下的内部耐受电压。
II. Concept of Vapor Shield Potential Control 二、蒸汽屏蔽电位控制概念
A. FEM Calculation Model A. 有限元计算模型
Fig. 1 shows the field calculation model of a 72.5 kV VI with only one vapor shield, which is assessed by COMSOL with a 2D 图 1 显示了仅具有一个蒸汽屏蔽的 72.5 kV VI 的现场计算模型，该模型由 COMSOL 使用 2D
Fig. 1. Axial symmetrical model of a 72.5 kV vacuum interrupter test setup in oil environment. 图 1.油环境下 72.5 kV 真空灭弧室试验装置的轴向对称模型
axisymmetric model. The geometrical data, relative permittivity epsi_(r)\varepsilon_{\mathrm{r}} of the materials and the boundary conditions of the electric potential on metal parts were input, and an EQS analysis has been performed. The movable contact was set as the high voltage electrode, and the fixed contact was defined with ground potential. The gap distance was kept at 40 mm . The volume inside the VI was modeled as vacuum ( epsi_(r)=1\varepsilon_{\mathrm{r}}=1 ), enclosed by ceramic insulators ( epsi_(r)=10\varepsilon_{\mathrm{r}}=10 ), and the space outside the VI was modelled as insulating oil ( epsi_(r)=2.2\varepsilon_{\mathrm{r}}=2.2 ) to match the simulation model to the laboratory experiment, which will be discussed later on. The vapor shield was defined as being either on floating potential or on a given electric potential. An infinite boundary condition for the model was defined. The field distribution of the simulation model would not be affected by the proximity of the boundary condition allowing more exact calculations. Axisymmetric 模型。输入几何数据、材料的相对介电常数 epsi_(r)\varepsilon_{\mathrm{r}} 和金属部件上电位的边界条件，并进行了 EQS 分析。将可移动触点设置为高压电极，固定触点定义为地电位。间隙距离保持在 40 毫米。VI 内部的体积被建模为真空 （ epsi_(r)=1\varepsilon_{\mathrm{r}}=1 ），由陶瓷绝缘体 （ epsi_(r)=10\varepsilon_{\mathrm{r}}=10 ） 封闭，VI 外部的空间被建模为绝缘油 （ epsi_(r)=2.2\varepsilon_{\mathrm{r}}=2.2 ），以使仿真模型与实验室实验相匹配，这将在后面讨论。蒸汽屏蔽被定义为在浮动电位或给定的电位上。为模型定义了一个无限边界条件。仿真模型的场分布不会受到边界条件接近度的影响，从而可以进行更精确的计算。
In this structure, when positive polarity voltage is applied to the movable contact (anode), the highest electric field strength exists on the edge of fixed contact (cathode). Only the electric field strength on the cathode has to be considered because of the mechanism of vacuum breakdown. Generally, pre-breakdown electron emission originates from certain types of isolated microstructures that adhere to, or are embedded in, the surface layer of an extended cathode surface [6]. 在这种结构中，当向活动触点（阳极）施加正极性电压时，固定触点（阴极）的边缘存在最高的电场强度。由于真空击穿的机制，只需考虑阴极上的电场强度。通常，预击穿电子发射源于某些类型的孤立微观结构，这些微观结构粘附或嵌入扩展阴极表面的表层[6]。
Figs. 2a and 2b show the potential distribution of the VI with floating shield potential. In case (a) the VI is settled in open space or in an insulator (as e.g., in a live tank circuit breaker), and the shield takes a potential of ~~50%\approx 50 \% of the applied voltage. In case (b) the VI is surrounded by a grounded metal enclosure as mentioned in the introduction and here the shield potential gets 27%27 \% of applied voltage. This causes a more stressed electric field on the upper contact edge. 图 2a 和 2b 显示了具有浮动屏蔽电位的 VI 的电位分布。如果 （a） VI 位于开放空间或绝缘体中（例如，在带电的油箱断路器中），并且屏蔽层吸收了外加电压 ~~50%\approx 50 \% 的电位。在情况 （b） 中，VI 被引言中提到的接地金属外壳包围，这里屏蔽电位达到 27%27 \% 施加的电压。这会导致上触点边缘产生更大的应力电场。
Fig. 2. Equi-potential distribution of VIs in cases of (a) installation in open space with resulating shield potential of ~~50%\approx 50 \% of the applied voltage and (b) surrounded by a grounded metal enclosure and a resulting shield potential of 27%27 \%. 图 2.在以下情况下，VI 的等电位分布：（a） 安装在开放空间中，屏蔽电位为 ~~50%\approx 50 \% 外加电压，以及 （b） 被接地金属外壳包围，产生的屏蔽电位为 27%27 \% 。
Fig. 3. Maximum electric field strength with variation of vapor shield potential, normalized to the maximum electric field strength in case that the vapor shield is on floating potential (51.1%). 图3.最大电场强度随蒸汽屏蔽电位的变化而变化，在蒸汽屏蔽处于浮动电位的情况下归一化为最大电场强度 （51.1%）。
B. Maximum Electric Field Variation With Potential Control of Vapor Shield B. 蒸汽屏蔽电位控制的最大电场变化
With the chosen model it can be shown that the electric field strength on the contact edge can be influenced by the vapor shield potential. Fig. 3 represents the relative increase and decrease of electric field strength normalized to the electric field in case that the vapor shield is floating in open space. The horizontal axis shows the ratio of vapor shield potential U_(S)U_{\mathrm{S}} to the applied voltage UU. Here, U_(S)U_{\mathrm{S}} also means the potential difference between cathode and vapor shield. In the case of uncontrolled shield potential, U_(S)U_{\mathrm{S}} is 51.1%51.1 \% of UU. This slight inequality originates from the difference of the capacitances between vapor shield and each contact. 通过所选模型，可以证明接触边缘的电场强度会受到蒸汽屏蔽电位的影响。图 3 表示当蒸汽屏蔽漂浮在开放空间中时，归一化为电场的电场强度的相对增加和减少。横轴显示蒸汽屏蔽电位 U_(S)U_{\mathrm{S}} 与施加电压 UU 的比率。这里， U_(S)U_{\mathrm{S}} 也表示阴极和蒸汽屏蔽之间的电位差。在屏蔽电位不受控制的情况下， U_(S)U_{\mathrm{S}} 为 51.1%51.1 \%UU 。这种轻微的不等式源于蒸汽屏蔽和每个触点之间的电容差异。
As the calculation results of Fig. 3 show, the maximum electric field strength on the cathode is linearly reduced with the 如图3的计算结果所示，阴极上的最大电场强度随着
Fig. 4. Electric field distribution on cathode with and without vapor shield potential control under applied voltage of +100 kV . 图 4.在+100 kV 施加电压下，有和没有蒸汽屏蔽电位控制的阴极上的电场分布。
decrease of the potential difference between cathode and vapor shield. In case of much lower potential difference between cathode and vapor shield, the maximum electric field domain appears on the edge of vapor shield. From the calculation results of the given model, it can be seen, that the optimum case is achieved for U_(S)U_{\mathrm{S}} equal to 33.5%33.5 \% of UU, where the maximum electric field strength is reduced by 23.3%23.3 \%. 减小阴极和蒸汽屏蔽之间的电位差。如果阴极和蒸汽屏蔽层之间的电位差要小得多，则最大电场域出现在蒸汽屏蔽层的边缘。从给定模型的计算结果中可以看出，最佳情况是等于 U_(S)U_{\mathrm{S}}33.5%33.5 \% ， UU 其中最大电场强度减少了 23.3%23.3 \% 。
From above simulation results, it seems plausible to achieve a higher withstand voltage by controlling the electric potential of the vapor shield. 从上述模拟结果来看，通过控制蒸汽屏蔽的电势来实现更高的耐压似乎是合理的。
C. Effective Area on Cathode C. 阴极有效面积
The influence of area effect on the internal breakdown voltage of a VI should also be considered when the shield potential is changed. Generally, the effective area is defined as the field region with a field strength of more than 90%90 \% of the maximum field strength [11]. Fig. 4 shows the electric field distributions on cathode surface with/without vapor shield potential control under the applied voltage of +100 kV . When the contact gap distance is large enough, the contact surface has a considerably non-uniform electric field distribution without being affected by vapor shield potential. Therefore, there is hardly any influence of the potential control of the vapor shield on the effective area. 当屏蔽电位发生变化时，还应考虑面积效应对 VI 内部击穿电压的影响。通常，有效面积定义为场强大于 90%90 \% 最大场强的场区域[11]。图 4 显示了在+100 kV 施加电压下，有/没有蒸汽屏蔽电位控制的阴极表面的电场分布。当接触间隙距离足够大时，接触面具有相当不均匀的电场分布，而不受蒸汽屏蔽电位的影响。因此，防潮罩的电位控制对有效面积几乎没有任何影响。
D. Shield Potential Control by External Circuit D. 通过外部电路控制屏蔽电位
In high voltage applications an electric field coupling may be modeled by capacitances. Fig. 5 shows a simple description of an equivalent circuit of a VI with an external potential control circuit. Inside the VCB there is capacitive coupling between electrodes and the vapor shield. To calculate these capacitances an electrostatic simulation was performed defining every element (contacts and shield) as a terminal. The capacitances between each electrode and vapor shield, namely C_("HV-Shield ")C_{\text {HV-Shield }} and C_("GND-Shield ")C_{\text {GND-Shield }} in the VI represent the sum of the coupling through vacuum, ceramic insulator and the oil for external insulations. The values are about 10 pF in this model. Changing the vapor shield potential can easily be achieved by connecting an external capacitive circuit. Capacitances of control capacitors C_(1),C_(2)C_{1}, C_{2} 在高压应用中，电场耦合可以通过电容进行建模。图 5 显示了带有外部电位控制电路的 VI 等效电路的简单描述。VCB 内部的电极和防潮罩之间存在电容耦合。为了计算这些电容，进行了静电模拟，将每个元件（触点和屏蔽层）定义为端子。每个电极和蒸汽屏蔽之间的电容，即 C_("HV-Shield ")C_{\text {HV-Shield }} VI 中的电 C_("GND-Shield ")C_{\text {GND-Shield }} 容，表示通过真空、陶瓷绝缘体和外部绝缘油的耦合之和。此模型中的值约为 10 pF。通过连接外部电容电路，可以轻松实现改变蒸汽屏蔽电位。控制电容器 C_(1),C_(2)C_{1}, C_{2} 的电容
Fig. 5. Equivalent capacitive circuit of VI with external potential control circuit with capacitive voltage division. 图 5.VI 的等效电容电路，具有电容分压的外部电位控制电路。
Fig. 6. Potential control circuit behavior for different applied polarities. (a) Positive. (b) Negative. 图 6.不同施加极性的电位控制电路行为。（a） 积极。（b） 否定。
and C_(3)C_{3} in the potential control circuit, whose ratio determines the vapor shield potential, should be distinctly higher than the C_("HV-Shield ")C_{\text {HV-Shield }} and C_("GND-Shield ")C_{\text {GND-Shield }} in the VI. C_(3)C_{3} 在电位控制电路中，其比率决定蒸汽屏蔽电位，应明显高于 C_("HV-Shield ")C_{\text {HV-Shield }} VI 中的 和 C_("GND-Shield ")C_{\text {GND-Shield }} 。
Under AC voltage stress the cathode of VI changes periodically between the two contact electrodes. High voltage rectifier diodes in the potential control circuit serve to eliminate the difference in polarities. The reverse voltage for the rectifier diodes needs to be half of applied voltage, which is also necessary for each capacitance in the potential control circuit. The rectifier diodes are represented by equivalent ideal opened or closed switches in Fig. 6. 在交流电压应力下，VI 的阴极在两个接触电极之间周期性变化。电位控制电路中的高压整流二极管用于消除极性差异。整流二极管的反向电压需要是施加电压的一半，这对于电位控制电路中的每个电容也是必要的。整流二极管在图 6 中由等效的理想开路或闭路开关表示。
If positive voltage is applied, see Fig. 6a, the vapor shield has almost the same potential as U_(23)U_{23}, since the current can flow 如果施加正电压，请参见图 6a，防潮罩具有与 U_(23)U_{23} 几乎相同的电位，因为电流可以流动
Manuscript received March 25, 2018; revised May 28, 2018; accepted June 24, 2018. Date of publication June 27, 2018; date of current version November 20, 2018. This work was supported by the Alexander von Humboldt Stiftung/Foundation. Paper no. TPWRD-00301-2018. (Corresponding author: Yusuke Nakano.) 稿件于 2018 年 3 月 25 日收到;2018 年 5 月 28 日修订;2018 年 6 月 24 日接受。出版日期：2018 年 6 月 27 日;当前版本的日期为 2018 年 11 月 20 日。这项工作得到了亚历山大·冯·洪堡基金会/基金会的支持。文件编号 TPWRD-00301-2018。（通讯作者：中野佑介。
