Decay Modes of Anode Surface Temperature After ... - IEEE Xplore

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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 42, NO. 5, MAY 2014

Decay Modes of Anode Surface Temperature After Current Zero in Vacuum Arcs-Part I: Experimental Study Zhenxing Wang, Hui Ma, Guowei Kong, Zhiyuan Liu, Member, IEEE, Yingsan Geng, Member, IEEE, and Jianhua Wang

Abstract— Anode surface temperature has a significant impact on the interruption capacity of a vacuum circuit breaker. The objective of this paper is to experimentally understand the decay process of anode surface temperature after extinguishing a high-vacuum arc with a large contact gap. The anode surface temperature after current zero was measured by a two-color pyrometer and arc modes observation was recorded by a highspeed charge-coupled device. A pair of asymmetric butt type contacts (contact materials: CuCr25 and CuCr50) was subjected to an axial magnetic field in a demountable vacuum chamber. The experimental result shows that there are two modes in decay processes of anode surface temperature: Mode I and Mode II. Mode I describes the anode surface decay process after diffuse low-current extinction and Mode II corresponds to highcurrent extinction. In addition, the decay time of anode surface temperature will last longer, if the proportion of chromium rises from 25% to 50% in anode material. Index Terms— Anode spot, anode surface temperature, contact material, dielectric recovery strength, vacuum arc.

I. I NTRODUCTION

I

N VACUUM interrupters (VIs), anode surface condition after current zero is a significant influential factor for a successful interruption because a hot anode surface that is also the post-arc cathode will evaporate an amount of metal vapor that dominates the dielectric recovery strength of the VIs directly [1], [2]. In addition, metal vapor is expected to be positively correlated with the anode surface temperature. Thus, a dielectric recovery process will be retarded seriously if the anode surface is still high after current zero, especially when the surface remains in a molten state. For this reason, axial magnetic field (AMF) [3] and transverse magnetic field (also Manuscript received January 19, 2013; revised January 5, 2014 and March 19, 2014; accepted April 4, 2014. Date of publication April 22, 2014; date of current version May 6, 2014. This work was supported in part by the National Natural Science Foundation of China under Project 51177122 and Project 51221005, in part by the State Key Laboratory of Electrical Insulation and Power Equipment Fund under Grant EIPE11118 and Grant EIPE14311, and in part by the China Post-Doctoral Science Foundation under Grant 2013M542350. The authors are with the State Key Laboratory of Electrical Insulation and Power Equipment, Department of Electrical Engineering, Xi’an Jiaotong University, Xi’an 710049, China (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2014.2316131

known as radial magnetic field) [4] technologies are adopted to control arc energy input into contact surfaces and make sure that there is no overheating on the surfaces ensuring a successful interruption. The significance of anode surface temperature has drawn a lot of researchers’ interests. Dullni et al. [5] measured the decay of anode surface temperature by adopting a pyrometer and a thermionic current method, respectively. They suggested that the boiling temperature of the anode material could be reached if an anode spot appears during the arcing period. Schellekens and Schulman [6] also measured the anode surface temperature after extinguishing a drawn vacuum arc using an infrared pyrometer. Their results showed that the peak value of anode surface temperature at current zero increased linearly with the peak value of the arc current. In addition, the temperature distribution on the surface was relatively homogeneous after extinguishing a high-current vacuum arc. Watanabe et al. [7] measured the melting time of the anode surface after current interruptions and they calculated the decay process of the anode surface temperature. They found that there was an interruption limit for CuCr50 contact material when anode surface temperature reached 1750 K at current zero. Niwa et al. [8] revealed that anode surface temperature at current zero increased as the proportion of Cr content increased for CuCr contact material. Comparing different contact materials, Ide et al. [9] found that anode surface melting times after current zero were associated with contact material and increased in the order of Cu, CuCr, and AgWC. However, anode surface temperature characteristics are not well understood after extinguishing a high-vacuum current with a large contact gap adopted by a high-voltage level VCB. For instance, the static contact gap of a 126-kV VCB can reach 60 mm which is much larger than that of a medium voltage VCB. In addition, the vacuum arcs become unstable in the large gap condition, which need a comparative strong magnetic field to restrict the arcs to the contact gap. The objective of this paper is to experimentally understand the decay process of anode surface temperature after extinguishing a high-vacuum arc with a large contact gap. The anode surface temperature after current zero was measured by a two-color pyrometer and arc modes observation was recorded by a high-speed charge-coupled device

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WANG et al.: DECAY MODES OF ANODE SURFACE TEMPERATURE

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TABLE I A RRANGEMENT OF E XPERIMENT

Fig. 1. Sketch of a pair of contacts. The upper contact was fixed as an anode and the lower one was able to move as a cathode.

(CCD) camera. A pair of asymmetric butt type contacts was adopted subjected to an AMF. The decay of anode surface temperature can be classified as two modes which correspond to different arc modes and may have a different impact on the dielectric recovery process. II. E XPERIMENTAL S ETUP A. Test Specimen Fig. 1 shows the sketch of contacts used in the experiments. A pair of asymmetrical contacts with different contact diameter was used. The asymmetrical structure tends to make an anode spot appear on an anode surface easier and the experiments were designed to obtain the temperature results after current zero, especially after high-current vacuum arc extinction [10]. Thus, this structure made less difficult to achieve the experiment’s object taking account of the capacity of the power source supply. Accordingly, anodes were assigned with diameters D of 12 and 60 mm, respectively, and were fixed in the upper position to observe the activities of the surfaces. The lower contacts which were able to move were assigned as cathodes with diameters D of 60 and 12 mm, respectively. The thickness of the contacts was 5 mm. Anode surface temperature is affected by the thermal properties of anode materials [11]. The material thermal properties define the melting temperature, boiling temperature, and thermal conductivity. In addition, the input energy comes from the vacuum arc emitted from cathode spots between a contact gaps. Typically, the arc properties are determined by the cathode material [12]. At the same time, the anode plays as a passive collector absorbing particles and energy from the plasma, and is heated up continually [13]. When the surface temperature exceeds the melting temperature of the contact material, the anode may become active and even evaporate metal vapor or metal droplets to the contact gap. Although the evaporation can affect the distribution of the surface temperature, the energy input into the anode mainly depends on the vacuum arc emitted from the cathode spots. Accordingly, CuCr25 (25% Cr) and CuCr50 (50% Cr) were chosen as the anode material, but only CuCr25 was chosen as the cathode material. Table I shows the arrangement of the experiments. B. System Setup Fig. 2 shows the experimental setup for measuring anode surface temperature. The contacts were installed inside

Fig. 2.

Experimental setup for anode surface temperature measurement.

a demountable vacuum chamber which was evacuated and sustained to 10−3 –10−4 Pa. As shown in Fig. 2, the contacts were surrounded by an external Helmholtz coil which generated a uniform AMF by passing through a 380-A dc current. The Helmholtz coil with a radius of 16 cm was installed coaxially with the pair of butt type asymmetrical contacts in the demountable chamber. In addition, the AMF flux density was set at 110 mT with a direction from the upper contact to the lower contact in the chamber. The applied AMF flux density was chose according the parameters of commercial VIs. The AMF flux densities for the commercial ones are in the range of 145–235 mT depending on interruption currents from 20 to 40 kA [14]. But in this case, the vacuum currents were lower than 10 kA because of the asymmetrical structure application, so the AMF flux density was also lower than that used in the commercial VIs. There were two observation windows on the demountable vacuum chamber. A high-speed CCD video camera was used to record the vacuum arc evolution through one window and a two-color pyrometer measured anode surface temperature through the other one. The CCD video camera had a recording speed of 10 000 frames/s. Its aperture was fixed at four and exposure time was set as 2 μs. A L−C discharging circuit was adopted to provide arc currents. The current frequency was 50 Hz and the arc currents can reach 10-kA rms. The applied arc current can be controlled by adjusting the voltage of the capacitor banks. The movable lower contact can be controlled electronically to make sure the

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without knowing the emissivities of measured surfaces. Sometimes temperature measured by the two-color pyrometer is called color temperature. The relation between color temperature TR and true temperature T can be expressed as [15] −1 ln(ε1 /ε2 ) 1 + (1) TR = −1 T C2 (λ−1 2 − λ1 ) where C2 = 1.4388 × 10−2 m·K is Planck’s constant, λ1 and λ2 are the wavelengths and ε1 and ε2 are the emissivities at the two wavelengths. The temperature error E T can be defined as ET =

Fig. 3. Waveforms of arc current, arc voltage, anode surface temperature, and opening displacement. It should be noted that the temperature curve before current zero cannot indicate surface temperature because of arc emission.

arcing time was about 10 ms. The contact separation length l, which was equal to the length of contact separation at current zero, was set as 24 mm that was chose according to the parameters of commercial 126-kV VIs. The average opening velocity used in the experiments was set as 2.4 m/s which were referred as an average velocity during the arcing period. Fig. 3 shows the waveforms of measured signals in the experiments. C. Anode Surface Temperature Measurements A two-color pyrometer, which can measure temperatures from 1073 to 1873 K, was chosen to measure the anode surface temperature. The two-color pyrometer’s measurement wavelengths were 850 and 1000 nm, respectively and its time response that arise from 10% to 90% of the total range was 0.5 ms. A distance from the receiver of two-color pyrometer to anode surface was fixed at 50 cm in the experiments. To aim at the peak value of surface temperature, we have to find out the region of anode surface that was eroded by arcs seriously. Usually, this region was associated with anode spots which can be observed by the CCD camera. Thus, the pyrometer aimed at the area where an anode spot appeared. Fortunately, anode spots reside at the same region for the same contacts if an anode spot has formed on the surface in the previous experiment. Therefore, we did not need to adjust the region at which the pyrometer aimed every time. In addition, it should be clearly noted that only the temperature measured by the pyrometer after current zero can be processed as valid data. This is because glaring arc emission during arcing period overwhelmed the emission from contact surface, so makes the data unreasonable. Fortunately, the experiments were carried out to measure the temperature of contact surfaces after current zero and the data before current zero can be neglected. A two-color pyrometer measures surface temperatures by obtaining the ratio of the spectral radiation energy at one wavelength to that at another wavelength based on Planck’s law. In addition, it has the advantage that the measurement errors can be limited by choosing feasible wavelengths,

|TR − T | × 100(%). T

(2)

To estimate the error, the emissivity has to be obtained in advance. In addition, the emissivity is varied with the wavelength and the emissivity ελ at wavelength λ can be expressed empirically as [16] ελ =

1 1 + a0 λ2

(3)

where a0 is a constant which can be deduced from experimental data. In [6], the emissivity of CuCr contact surface was 0.63 at a wavelength of 1000 nm. Therefore, the constant a0 was 0.58 by substituting the parameters into (3). In our experiments, the contact materials were CuCr and the wavelengths λ1 and λ2 were 850 and 1000 nm, respectively. Then, the emissivity at the wavelength 850 nm was 0.7 based on (3). According to (1) and (2), the measured temperature error was ∼5% at a temperature of 1300 K. III. E XPERIMENTAL R ESULTS A. Arc Voltage Fig. 4(a) and (b) shows the arc voltages for various arc currents. The upper anode was 60 mm in diameter. The lower cathode was 12 mm in diameter. The AMF flux density applied was 110 mT. The arcing period was 10 ms and a closed pair of contacts was opened with a velocity of 2.4 m/s, so the contact separation length was 24 mm at current zero. Fig. 4(a) shows the case in which the anode material was CuCr25 (25% Cr) and the cathode material was CuCr25 (25% Cr). The arc current (rms) was 3.1, 5.8, and 8.5 kA, respectively. Fig. 4(b) shows the case in which the anode material was CuCr50 (50% Cr) and the cathode material was CuCr25 (25% Cr). The arc current (rms) was 2.3, 3.1, and 6.1 kA, respectively. As shown from Fig. 4(a), the arc voltage remained in the range of 15–50 V and appeared smooth during the arcing period. The waveform in Fig. 4(b) was not smooth as shown in Fig. 4(a), but the fluctuations were limited. Fig. 5 also shows the arc voltages for various arc currents. But the upper anode was 12 mm in diameter. The lower cathode was 60 mm in diameter. The anode material was CuCr25 (25% Cr) and the cathode material was CuCr25 (25% Cr). The arc current (rms) was 3.2, 7.8, 8.8, and 9.5 kA, respectively. As shown from Fig. 5, the arc voltage remained in the range of 15–50 V and also appeared smooth during the arcing period.


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Fig. 4. Arc voltage. Anode diameter: 60 mm; cathode diameter: 12 mm; and cathode material: CuCr25 (25% Cr). (a) Anode material: CuCr25 (25% Cr). Arc current (rms) was 3.1, 5.8, and 8.5 kA. (b) Anode material: CuCr50 (50% Cr). Arc current (rms) was 2.3, 3.1, and 6.1 kA.

Fig. 5. Arc voltage. Anode diameter: 12 mm; cathode diameter: 60 mm; cathode material: CuCr25 (25% Cr); and anode material: CuCr25 (25% Cr). Arc current (rms) was 3.2, 7.8, 8.8, and 9.5 kA.

B. Anode Activities Fig. 6 shows the phenomena of vacuum arc including diffuse arcs and constricted arcs. The AMF flux density applied was 110 mT. The arcing period was 10 ms and a closed pair of contacts was opened with a velocity of 2.4 m/s so

Fig. 6. Vacuum arc appearances. Anode diameter: 60 mm and cathode diameter: 12mm. Cathode material: CuCr25 (25% Cr). (a) Anode material: CuCr25 (25% Cr); arc observation at current (rms) 1.4, 3.1, 5.8, and 8.5 kA. (b) Anode material: CuCr50 (50% Cr); arc observation at current (rms) 1.1, 2.3, 3.1, and 6.1 kA.

the contact separation length was 24 mm at current zero. The upper contact was assigned as the anode with a diameter D of 60 mm; the lower contact which was able to move was assigned as the cathode with a diameter D of 12 mm.

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Fig. 7. Vacuum arc appearances. Anode diameter: 12 mm and cathode diameter: 60 mm. Cathode material: CuCr25 (25% Cr); anode material: CuCr25 (25% Cr); arc observation at current (rms) 3.2, 7.8, 8.8, and 9.5 kA.

Fig. 6(a) shows the phenomena of vacuum arc with the anode material of CuCr25. The cathode material was CuCr25. The arc current (rms) was 1.4, 3.1, and 8.5 kA, respectively. For the arc current of 1.4 kA, the arc remained in a diffuse arc mode and threw out a dim light during the whole arcing period. In addition, the anode only played as a collector to absorb the particles produced by the cathode and cannot be observed any activities on it. With the increase of the arc current to 3.1 kA, the arc column emitted luminous light and began to constrict. The anode became active, but there was no obvious anode spot appeared on it. For the arc current of 8.5 kA, it was observed that the anode was active and an anode spot was formed because of the glaring arc column. The anode began to project particles into the contact gap, which was another evidence to confirm that the vacuum arc was in a high-current mode. Fig. 6(b) shows the activities of anode surface with the anode material of CuCr50. The cathode material was CuCr25. The arc current (rms) was 1.1, 2.3, and 6.1 kA, respectively. At the arc current of 1.1 kA, the arc remained in a diffuse arc mode and threw out dim light during the whole arcing period. With the increase of the arc current to 2.3 kA, the arc column emitted luminous light and began to constrict. For the arc current of 6.1 kA, the arc flamed with brighter light, and an anode spot was formed projecting particles into the contact gap.


Fig. 8. Dependence of anode surface temperature decay on arc current. Anode diameter: 60 mm and cathode diameter: 12 mm. Cathode material: CuCr25 (25% Cr). (a) Anode material: CuCr25 (25% Cr). (b) Anode material: CuCr50 (50% Cr).

Fig. 7 shows the phenomena of vacuum arc that has a small anode. But the upper anode was 12 mm in diameter and the lower cathode was 60 mm in diameter. The anode material was CuCr25 (25% Cr) and the cathode material was CuCr25 (25% Cr). The arc current (rms) was 3.2, 7.8, 8.8, and 9.5 kA, respectively. At the arc current of 3.2 kA, the arc remained in a diffuse arc mode. After the arc current was higher than 7.8 kA, the arc column constricted seriously and threw out luminous light. C. Decay Process of Anode Surface Temperature After Current Zero Fig. 8 shows the decay process of anode surface temperature after current zero in the case of large anodes. The cathode material was CuCr25. The diameter of anode was 60 mm and the diameter of cathode was 12 mm. The AMF flux density applied was 110 mT. The arcing period was 10 ms and a closed pair of contacts was opened with a velocity of 2.4 m/s so the contact length was 24 mm at current zero. The anode material was CuCr25 and CuCr50, respectively. Fig. 8(a) shows the decay process of anode surface temperature after current zero. The anode material was CuCr25


and the cathode material was also CuCr25. The arc current (rms) was 3.1, 5.8, and 8.5 kA, respectively. Although we measured the anode surface temperature after extinguishing a diffuse vacuum arc of 1.4 kA (rms), we cannot record its decay process after current zero exactly. This is because that the two-color pyrometer used in the experiments measured temperatures in a range of 1073–1873 K and the anode surface temperature after extinguishing a diffuse arc was far less than the lower limit temperature 1073 K. Therefore, we can deduce that the anode surface temperature was lower than 1073 K after extinguishing a diffuse vacuum arc. At the arc current of 3.1 kA, the anode surface temperature decayed exponentially after current zero and dropped to 1073 K in a relatively brief period of time 0.4 ms. However, the curves of the arc current of 5.8 and 8.5 kA were quite different from the curve of the arc current of 3.1 kA. The curves presented three stages in the decay processes. In the first stage, the temperature decayed exponentially from the initial temperature at current zero to the melting point. In the second stage, the surface temperature remained constant for a while. In addition, the plateau period for the curve of 8.5 kA was longer than that for the curve of 5.8 kA, which indicated the anode surface remained in a molten state longer after extinguishing 8.5-kA current. In the third stage, the temperature also decayed exponentially to the room temperature, which was analogous to the shape of the curve in the first stage. In addition, we defined decay time as a period from current zero to the instant when anode surface temperature dropped to the lower limit of the pyrometer measurement range of 1073 K. The decay time was 0.4, 1.2, and 1.9 ms for the arc current of 3.1, 5.8, and 8.5 kA, respectively. Fig. 8(b) shows other decay processes of anode surface temperature after current zero. The anode material was CuCr50 and the cathode material was CuCr25. The arc current was 2.3, 3.1, and 6.1 kA, respectively. We also measured the anode surface temperature after extinguishing a diffuse vacuum arc 1.1 kA. As we mentioned above, however, the anode surface temperature was lower than 1073 K. At the arc current of 2.3 and 3.1 kA, the anode surface temperature decayed exponentially after current zero, but the decay time of 1.5 ms under arc current of 3.1 kA was longer than that of 0.4 ms under arc current of 2.3 kA. At the arc current of 6.1 kA, the curve also presented three stages in the decay process and the decay time increased to 3.1 ms. Fig. 9 shows the decay process of anode surface temperature after current zero in the case of a small anode. But the upper anode was 12 mm in diameter. The lower cathode was 60 mm in diameter. The anode material was CuCr25 (15% Cr) and the cathode material was CuCr25 (25% Cr). The arc current (rms) was 3.2, 7.8, 8.8, and 9.5 kA, respectively. At the arc current of 3.2 kA, the anode surface temperature decayed exponentially after current zero and dropped to 1073 K in a period of time 0.3 ms. At the arc current of 7.8, 8.8, and 9.5 kA, the curves also presented the three stages in the decay process and the decay times were 1.9, 2.6, and 3.5 ms, respectively. Fig. 10 shows a relation between the measured decay times of anode surface temperature and the arc currents. CuCr25 and CuCr50 were adopted as the anode material, but the cathode

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Fig. 9. Dependence of anode surface temperature decay on arc current. Anode diameter: 12 mm and cathode diameter: 60 mm. Cathode material: CuCr25 (25% Cr); and anode material: CuCr25 (25% Cr).

Fig. 10. Relation between the decay time of anode surface temperature and the arc current in the case of two anode materials CuCr25 and CuCr50.

material was CuCr25. The two curves started closely at a low value of the decay time, which indicated that anode material had no significant impact on the decay time after extinguishing a relatively low current. Then, the two curves increased with the arc current linearly. However, the decay time for anode material CuCr50 grew more rapidly than that for the CuCr25 material. The results suggested that the CuCr50 anode material had a higher decay time of anode surface temperature after current zero than that of CuCr25. D. Anode Surface Erosion of Cu–Cr Materials Fig. 11(a) and (b) shows the erosion condition on anode surfaces after the experiments. The anode materials were CuCr25 and CuCr50, respectively. The cathode material was CuCr25. The anode was 60 mm in diameter and the cathode 12 mm. There was a circle of mountain appearing at the center region of the anodes. However, the degree of erosion cannot be differentiated for the two kinds of anode materials CuCr25 and CuCr50. Fig. 12 also shows the erosion condition on anode surfaces after the experiments. The anode material was CuCr25.

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TABLE II M ELTING P OINT, B OILING P OINT, AND T HERMAL C ONDUCTIVITY OF C ONTACT M ATERIALS

Fig. 11. Erosion of the contact surface after the experiments. Anode diameter: 60 mm. (a) Anode material: CuCr25 (25% Cr). (b) Anode material: CuCr50 (50% Cr).

Fig. 12. Erosion of the contact surface after the experiments. Anode material: CuCr25 (25% Cr); diameter: 12 mm.

Here, Mode I and Mode II are adopted to denote the two patterns of the decay curves, respectively. In Mode I, surface temperature decayed exponentially after current zero and dropped to 1073 K, which was the lower measurement limit of the two-color pyrometer. It is reasonable to infer that the surface temperature finally dropped to the room temperature far below 1073 K. In addition, the whole decay process depended on the initial temperature at current zero. With the decrease of the initial anode surface temperature at current zero, the duration of the decay processes decreased as well. If the initial temperature dropped below 1073 K which cannot be measured by the instrument in our experiments, the temperature may also decay exponentially as mentioned above because of the same physical principle. In general, Mode I presents a relatively simple case of thermal conduction on anode surface. In Mode II, the temperature curve is obviously different with that shown in Mode I. The curve can be divided into three stages according to the turning points. In the first stage, the temperature dropped from the initial temperature at current zero to 1370 K which was close to the melting point of Cu 1356 K as shown in Table II, in the second stage, the temperature remained constant for a period of time; in the third stage, the temperature exponentially dropped from 1370 K to the room temperature. The duration of the decay process was also influenced by the initial temperature at current zero which is higher than 1370 K. B. Correlation Between Arc Modes and Anode Surface Decay Modes

Fig. 13.

Typical modes of anode surface temperature after current zero.

The cathode material was CuCr25. The anode was 12 mm in diameter and the cathode 60 mm. The anode surface was burned by the arc completely but remained in a smooth condition. The cathode surface was also heated uniformly. IV. D ISCUSSION A. Classification of Anode Surface Temperature After Current Zero As shown in Figs. 8 and 9, the decay process of the anode surface temperature presented different curves after extinguishing vacuum arcs. Although the curves were not completely identical to each other, the decay process can be classified into two basic patterns as shown in Fig. 13.

After classifying the decay process of anode surface into the two modes, we would like to relate the decay modes to their corresponding arc modes which are believed to have a major impact on the interruption capacity of a VI. Mode I describes the anode surface decay process after extinguishing a low current. As shown in Fig. 6(a), the vacuum arc remained in a diffuse arc mode at the current of 1.4 kA. Although the anode surface temperature after current zero was lower than 1073 K, which cannot be measured by the instrument, the corresponding surface temperature can also be classified as Mode I according to the analysis of the Section IV-A. With the increase of the arc current to 3.1 kA, the arc column began to constrict, but no obvious anode spot was appeared on the surface. According to Fig. 6(a), the corresponding temperature curve with arc current of 3.1 kA in Fig. 8(a) can also be classified as the Mode I.


Mode II describes the anode surface decay process after extinguishing a high current. As shown in Fig. 6(a), it is obvious that the vacuum arc was in a high-current mode of the current of 8.5 kA. The corresponding temperature in Fig. 8(a) can be classified as the Mode II. Also, this relation can be verified by the temperature curves shown in Figs. 8(b) and 9. Sometimes, it is not clear to judge the boundary between low-current and high-current mode by high-speed camera observation. However, the surface temperature curves after current zero provide further information to assist the judgment. Therefore, the information with a clear physical meaning can make the judgment more reasonable and accurate. The contact surfaces after extinctions shown through Figs. 11 and 12 are other evidences to support this view. It is apparent that the anode surface was ablated by the arc, which resulted in a phase transition process on the surface. However, a melting process may also occur in the Mode I. As shown in Fig. 6(a), the surface has melted during the arcing period at the current of 3.1 kA, but the temperature shown in Fig. 8(a) indicates the surface recovered to a solid state before current zero. This is because the arc returned to a diffuse arc before current zero and the energy into the anode decreased to a relatively low level resulting in a solidification process before current zero. In addition, the asymmetrical contacts made the vacuum arcs easier to constrict which caused the surface melt much easier. The vacuum arcs were forced to constrict on a small region on the surface forming a glaring spot as shown in Figs. 6 and 7. Consequently, the surfaces were damaged by the arcs as shown through Figs. 11 and 12. It should be noted that the threshold current appearing an anode spot for a pair of asymmetrical contacts is lower than that for a normal symmetrical pair of contact. In addition, the destroyed area was more concentrated on a small area resulting in regional over-heat in the low-current conditions. Therefore, an asymmetrical pair of contact may lead to a failure of interruption even if the interrupted current is low. To sum up, decay modes are directly associated with arc modes. Different surface temperature decay modes result from different arc modes and can be classified into Mode I and Mode II. Mode I corresponds to low-current condition when a relative small vacuum current is extinguished causing a slight damage on the anode surface. Meanwhile, Mode II correspond to high-current condition when a high-vacuum arc is extinguished causing a severe damage on the anode surface. This classification can be used to assist judging the level of the currents. In addition, the surface temperature dominates the generation of metal vapor that can be considered as an indicator to the dielectric recovery strength of a VI. Therefore, surface temperature measurement has a definite physical meaning to analyze the interruption capacity of a VI compared with visual observation.

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longer than that of anode material of CuCr25 at the same arc current. These phenomena can be explained by the thermal properties of anode materials. The thermal conductivity of Cu is 394 W/(m·K), but that of Cr is only 67 W/(m·K) shown in Table I and a lower thermal conductivity lead to a slower cooling process. In addition, an anode contact adopting CuCr50 contains a higher proportion of chromium, which makes the heat diffusion from the anode more difficult because of the low-thermal conductivity of chromium. Thus, the decay time of the anode material CuCr50 was longer than that of CuCr25. Our experimental results also supported Li et al. findings [17]. They found that contacts made of CuCr with 30 wt% Cr content can give a best performance on the interruption capacity of a VI. In addition, he suggested that the thermal conductivity of contact materials can be the one of the most significant factors that influences the dielectric recovery process after current zero. There is another evidence to confirm the relation between the thermal property of contact material and the decay time of surface temperature after current zero. As shown in Fig. 8, the anode surface temperature after current zero in the Mode II stayed at a value of 1370 K that is near the melting point of Cu for a period of time. This phenomenon can also be observed in Fig. 9. The reason can be explained by the thermal property of contact surface. As shown in Figs. 11 and 12, the contact surfaces were damaged by the vacuum arcs, indicating that both copper and chromium materials on the contacts melted during the arcing periods. However, chromium material has a higher melting point 2148 K compared with that of copper 1356 K as shown in Table II. Therefore, the chromium material recovered to solid state after current zero, but the copper was still melting because of the lower melting point. In addition, the initial temperature at current zero was lower than the melting point of chromium 2148 K, so the solidification was dominated by the thermal property of copper after current zero. Mode II tends to result in a failure of interruptions. Anode surface temperature has a significant impact on the dielectric recovery strength after interrupting a high current [1]. When a VI interrupts a low current, the dielectric recovery strength after current zero is influenced by the cathode surface on which cathode spots wander and generate plasma, metal vapor [18]. However, the cathode spots heat up the cathode surface homogeneously so it is unlikely to generate too much metal vapor from the local region of the surface. On the contrary, an anode spot which generates a lot of metal vapor appears on the anode surface in the high-current condition [19]. The anode spot resides at the same place and heats the local surface seriously. The over-heating surface will evaporate metal vapor after current zero because the surface will not cool down even the vacuum arc is extinguished. V. C ONCLUSION

C. Effect of Contact Material Contact material has a significant impact on the decay time of surface temperature after current zero. As shown in Fig. 10, the decay time of anode material of CuCr50 was

In this paper, the anode surface temperature decay processes after current zero were measured by a two-color pyrometer and arc activities were recorded by a high-speed CCD. The relation between arc modes and anode surface temperature

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after extinguishing vacuum arcs subjected to an AMF has been studied. The conclusions are summarized as follows. 1) The decay of anode surface temperature after current zero in vacuum arcs can be classified as two modes. Mode I: the anode surface temperature decays exponentially after current zero and Mode II: the temperature drops from the initial temperature at current zero to 1370 K, which is close to the melting point of Cu, then the temperature remains constant for a period of time, and the temperature exponentially drops from 1370 K to the room temperature. 2) The temperature decay Mode I describes the anode surface decay process after extinguishing a low current and the Mode II describes the process after extinguish a high current. 3) By considering the impact of anode surface temperature on the generation of metal vapor, the temperature decay modes are suitable to estimate the dielectric recovery strength after current zero with the aid of metal vapor evaporation estimation. 4) A higher chromium component of 50% in contact material extended the decay time of anode surface temperature than that of chromium component of 25%, which was an evidence to explain CuCr25 contact material has a better interrupting performance than that of CuCr50. ACKNOWLEDGMENT The authors would like to thank Prof. S. Yanabu and Prof. E. Kaneko for providing the two-color pyrometer instrument and their helpful discussions. R EFERENCES [1] E. Schade and E. Dullni, “Recovery of breakdown strength of a vacuum interrupter after extinction of high currents,” IEEE Trans. Dielectr. Electr. Insul., vol. 9, no. 2, pp. 207–215, Apr. 2002. [2] E. Schade, “Physics of high-current interruption of vacuum circuit breakers,” IEEE Trans. Plasma Sci., vol. 33, no. 5, pp. 1564–1575, Oct. 2005. [3] M. B. Schulman and H. Schellekens, “Visualization and characterization of high-current diffuse vacuum arcs on axial magnetic field contacts,” IEEE Trans. Plasma Sci., vol. 28, no. 2, pp. 443–452, Apr. 2000. [4] E. Dullni, E. Schade, and W. K. Shang, “Vacuum arcs driven by crossmagnetic fields (RMF),” IEEE Trans. Plasma Sci., vol. 31, no. 5, pp. 902–908, Oct. 2003. [5] E. Dullni, B. Gellert, and E. Schade, “Electrical and pyrometric measurements of the decay of the anode temperature after interruption of high-current vacuum arcs and comparison with computations,” IEEE Trans. Plasma Sci., vol. 17, no. 5, pp. 644–648, Oct. 1989. [6] H. Schellekens and M. B. Schulman, “Contact temperature and erosion in high-current diffuse vacuum arcs on axial magnetic field contacts,” IEEE Trans. Plasma Sci., vol. 29, no. 3, pp. 452–461, Jun. 2001. [7] K. Watanabe et al., “The anode surface temperature of CuCr contacts at the limit of current interruption,” IEEE Trans. Plasma Sci., vol. 25, no. 4, pp. 637–641, Aug. 1997. [8] Y. Niwa, K. Yokokura, T. Kusano, E. Kaneko, I. Ohshima, and S. Yanabu, “The effect of contact material on temperature and melting of anode surface in the vacuum interrupter,” in Proc. 19th Int. Symp. Discharges Electr. Insul. Vac., Xi’an, China, 2000, pp. 524–527. [9] N. Ide, R. Sakuma, E. Kaneko, and S. Yanabu, “The electrode surface state after current interruption in vacuum circuit breaker,” in Proc. 22nd Int. Symp. Discharges Electr. Insul. Vac., Matsue, Japan, 2006, pp. 396–399. [10] G. Kong, Z. Liu, Y. Geng, H. Ma, and X. Xue, “Anode spot formation threshold current dependent on dynamic solid angle in vacuum subjected to axial magnetic fields,” IEEE Trans. Plasma Sci., vol. 41, no. 8, pp. 2051–2060, Aug. 2013.

[11] M. B. Schulman, P. G. Slade, L. D. Loud, and L. Wangpei, “Influence of contact geometry and current on effective erosion of Cu-Cr, Ag-Wc, and Ag-Cr vacuum contact materials,” IEEE Trans. Compon. Packag. Technol., vol. 22, no. 3, pp. 405–413, Sep. 1999. [12] A. Anders, “Ion charge state distributions of vacuum arc plasmas: The origin of species,” Phys. Rev. E, vol. 55, no. 1, pp. 969–981, 1997. [13] H. C. Miller, “A review of anode phenomena in vacuum arcs,” IEEE Trans. Plasma Sci., vol. 13, no. 5, pp. 242–252, Oct. 1985. [14] Y. Zhang, X. Yao, Z. Liu, Y. Geng, and P. Liu, “Axial magnetic field strength needed for a 126-kV single-break vacuum circuit breaker during asymmetrical current switching,” IEEE Trans. Plasma Sci., vol. 41, no. 8, pp. 1670–1678, Aug. 2013. [15] B. Muller and U. Renz, “Development of a fast fiber-optic two-color pyrometer for the temperature measurement of surfaces with varying emissivities,” Rev. Sci. Instrum., vol. 72, no. 8, pp. 3366–3374, 2001. [16] T. Duvaut, “Comparison between multiwavelength infrared and visible pyrometry: Application to metals,” Infr. Phys. Technol., vol. 51, no. 4, pp. 292–299, 2008. [17] W. P. Li, R. L. Thomas, and R. K. Smith, “Effects of Cr content on the interruption ability of CuCr contact materials,” IEEE Trans. Plasma Sci., vol. 29, no. 5, pp. 744–748, Oct. 2001. [18] G. A. Farrall, “Recovery of dielectric strength after current interruption in vacuum,” IEEE Trans. Plasma Sci., vol. 6, no. 4, pp. 360–369, Dec. 1978. [19] G. Frind, J. J. Carroll, and E. J. Tuohy, “Recovery times of vacuum interrupters which have stationary anode spots,” IEEE Trans. Power Appl. Syst., vol. 101, no. 4, pp. 775–781, Apr. 1982.

Zhenxing Wang was born in 1983. He received the B.S. and Ph.D. degrees in electrical engineering from Xi’an Jiaotong University, Xi’an, China, in 2006 and 2013, respectively. His current research interests include vacuum arc, dielectric recovery after current zero in vacuum interrupters, and operating mechanisms for vacuum circuit breakers.

Hui Ma was born in China in 1987. He received the B.S. degree in electrical engineering from Xi’an Jiaotong University, Xi’an, China, in 2010, where he is currently pursuing the M.S. degree. His current research interests include vacuum arcs and vacuum circuit breakers.

Guowei Kong was born in China in 1985. He received the B.S. and Ph.D. degrees in electrical engineering from the Shenyang University of Technology, Liaoning, China, and Xi’an Jiaotong University, Xi’an, China, in 2008 and 2014, respectively. His current research interests include vacuum arcs and vacuum circuit breakers.


Zhiyuan Liu (M’01) was born in Shenyang, China, in 1971. He received the B.S. and M.S. degrees in electrical engineering from the Shenyang University of Technology, Liaoning, China, and the Ph.D. degree in electrical engineering from Xi’an Jiaotong University, Xi’an, China, in 1994, 1997, and 2001, respectively. He was with the General Electric Company Research and Development Center, Shanghai, China, from 2001 to 2002. He has been with the State Key Laboratory of Electrical Insulation and Power Equipment, Department of Electrical Engineering, Xi’an Jiaotong University, since 2003, where he is currently a Professor. He has authored more than 100 technical papers. He is involved in the research and development of highvoltage vacuum circuit breakers. Dr. Liu is a member of current zero club and the CIGRE Working Group WGA3.27: The impact of the application of vacuum switchgear at transmission voltages.

Yingsan Geng (M’98) was born in China in 1963. He received the B.S., M.S., and Ph.D. degrees in electrical engineering from Xi’an Jiaotong University (XJTU), Xi’an, China, in 1984, 1987, and 1997, respectively. He is currently a Professor with the State Key Laboratory of Electrical Insulation and Power Equipment, Department of Electrical Engineering, XJTU. His current research interests include theory and application of low-voltage circuit breaker and highvoltage vacuum circuit breakers.

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Jianhua Wang received the M.S. and Ph.D. degrees in electrical engineering from Xi’an Jiaotong University (XJTU), Xi’an, China, in 1981 and 1985, respectively. He is currently a Professor with the State Key Laboratory of Electrical Insulation and Power Equipment, Department of Electrical Engineering, XJTU. His current research interests include theory and application of intelligent electrical apparatus system, and CAD/CAE in electrical engineering. Prof. Wang is the Chairman of professional branch committee on intelligent electrical system and its applications of the China Electrotechnical Society, an Associate Council Member of the China Electrotechnical Society, and the Director of the State Key Laboratory of Electrical Insulation and Power Equipment.