Abstract- It is well known that the modification of insulator surface electric field contributes significantly to the surface flashover in vacuum. For the purpose of ...
2013 Annual Report Conference on Electrical Insulation and Dielectric Phenomena
An Electro-optical Measurement on Electric Field of Insulator Surface under HV Pulse in Vacuum W. Liu, Y.Fu, X.Zou, X. Wang Dept. of Electrical Engineering, Tsinghua University, 100084 Beijing, China Abstract- It is well known that the modification of insulator surface electric field contributes significantly to the surface flashover in vacuum. For the purpose of studying the surface electric field, an on-line measurement system based on Kerr electro-optical effect is developed. And the preliminary experiments are performed, in which a modified surface field is observed when insulator is being stressed by a HV nanosecond square pulse.
I. INTRODUCTION In general, the HV withstand capability of system working in vacuum is determined by the surface flashover potential of the insulators employed in system for supporting high voltage conductors, since the flashover voltage of insulators is lower greatly than breakdown voltage of the vacuum gap with same dimension if insulator is removed. It has been proved that prebreakdown process, in which the electric field distribution along insulator surface is modified by surface charging, is an important factor for the occurrence of surface flashover [1, 4, and 6]. This work is for the purpose of investigating the modified electric field distribution of insulator surface electrooptically based on Kerr effect, in which an on-line measurement system is developed, and several preliminary results are given. II. KERR EFFECT Kerr effect is an electro-optical effect in which an external electric field changes the optical isotropy of a dielectric. This effect is usually observed obviously in certain kinds of Kerr liquid, such as nitrobenzene, propylene carbonate, and so on. When stressed by a strong electric field, a Kerr liquid generally exhibits birefringence which is similar to that of a single-axis crystal, with the optical axis aligned to the applied electric field direction [2, 3]. Fig. 1 demonstrates that a linear polarized incident laser owns two orthogonally polarized components when a high voltage being stressed in Kerr liquid, i.e. PĠ and Pĵ, which are optical electric field components oscillating parallel and perpendicular to the applied electric field E, respectively. And the applied electric field E is in the x-y plane, and usually aligned to the x-axis. P Ġ and P ĵ travel in Kerr liquid with different index of refraction. Therefore, when the light emerges from the cell, a relative phase difference exists between PĠ and Pĵ, and the incident linearly polarized light becomes elliptically polarized. The phase difference caused by Kerr effect is described as:
Δϕ = 2πBl E
2
π + θi 2
θi
Fig. 1. Kerr cell is filled with Kerr liquid, in which two electrodes are connected with high voltage to form a strong electric field in Kerr liquid. The z direction denotes the direction of the laser propagation. The applied electric field E is aligned to the x-y plane.
Where B is the specific Kerr electro-optic constant of Kerr liquid, l is optical distance under electric field influence, and E is the magnitude of electric field E mentioned above. It can be seen from equation (1) that the information on electric field can be deduced from the phase difference. And this phase shift can be measured with a linear polarization configuration as shown in Fig.1&Fig.2, which consists of two polariscopes, i.e. polarizer and analyzer, between which the Kerr cell is positioned. Their polarization axes are generally set orthogonal to each other, and usually at 45 to the xaxis, respectively. As a result, the emerging light out of analyzer has intensity I as following [2]:
f e
I = I 0× sin 2(Δϕ 2)
(2)
Where I0 is the light intensity of incident linear polarized laser into the Kerr cell; and the ij in equation (2) is the phase shift integrated along the light path l, which can be described as:
伽
Δϕ = 2πB ³ E dz 2
(3)
l
Accordingly, the magnitude of applied electric field E informed by the light intensity is actually spatially averaged ones. In experiments, an expanded laser beam is usually used to cover the entire sample cross-section for a global observation, as shown in Fig.2. And the intensity distribution of light passing through the analyzer can be imaged on a screen and recorded by a digital camera. III. EXPARIMENTAL APPARATUS Fig. 3 shows that the measurement system includes HV pulse power source, YAG laser, optical detecting system,
(1)
978-1-4799-2597-1/13/$31.00 ©2013 IEEE 1121
synchronous control system, and Kerr cell with insulator. And all of these components are designed to meet specific requirements in nanosecond time-domain on-line measurements.
Fig.2. P1& P2 are two linear polariscopes with orthogonal transmission axes, and the angle between the P1 / P2 polarization and x-axis is set ±45°, respectively. The light path is along the z-axis, and the polarization plates are on the x-y plane.
As shown in Fig.3 and Fig.4, the pulsed laser of wavelength 532nm emitted from YAG laser is divided into two parts. The triggering laser conducts the gas switch shown in Fig.4, while the charging voltage on pulse forming line (PFL, a highvoltage cable of 10 meter long) by Marx reaches the first peak amplitude. Once it happens, a HV square pulse of 100ns in FWHM is stressed onto the insulator specimen in Kerr cell, during which the detecting laser propagates through the cell to detect the Kerr effect with the polariscopes configuration mentioned above. This on-line measurement is realized through a synchronous control system including an optical delay arrangement and certain electronic devices used for control signal generation and propagation [5].
field close to the liquid/insulator interface can be assumed to be equivalent to that close to the vacuum/insulator interface.
Fig.4. In Gas switch vessel, the bottom electrode is connected with the end of pulse forming line, and the upper one is connected with anode in Kerr cell with metal components.
Fig. 5 gives a global view of optical devices arrangement in on-line experiment. The optic delay apparatus is constructed with mirrors M1~M6, which are arranged in different positions of the lab to make detecting laser goes into the Kerr cell exactly when the square pulse stressed on insulator reaches flat-top voltage, after travelling a long enough distance. Due to that the Kerr cell and the gas switch vessel are positioned together vertically, as shown in Fig.4, the mirrors M4&M5 are arranged vertically to alter the propagation direction of detecting laser in order to pass through the Kerr cell. The detecting laser beam is expanded to about 100mm to cover the entire sample cross-section in order to obtain a global observation on light intensity distribution. And a photoelectric detector is arranged close to the incident window of the cell to monitor the laser pulse. The laser detector output and pulsed voltage stressed on insulator are monitored simultaneously to verify the measurement time and the exact value of HV pulse while Kerr effect is being detecting.
Fig. 3. Components of on-line measurement system and their main functions
Fig. 4 also exhibits the configuration of Kerr cell and gas switch chamber in detail. A hollow, rectangular, nylon prism as insulator specimen attached to the electrodes is positioned in the Kerr cell. The upper electrode is connected with a metal airway tube electrically, which is grounded and connected with vacuum pump. As a result, the inner space of insulator specimen is evacuated to form an insulator/vacuum interface. Propylene carbonate is chosen as the Kerr liquid filling the cell because of its safety and nontoxicity. Therefore, the insulator/liquid interfaces measured in experiments are formed while the specimen with electrodes is immersed together in the Kerr liquid. Moreover, because the walls of hollow specimen are designed thin enough (about 1.6mm) for mechanical support and vacuum seal, the measured electric
Fig.5. M1~M7 are mirrors, in which M4 and M5 are arranged vertically. A photoelectric detector denoted by a red dot is positioned close to incident window of Kerr cell. The lens is used to concentrate the energy of triggering laser.
IV. RESULTS AND DISCUSSION
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Fig.6 gives typical waveforms of PFL voltage, HV pulse stressed on insulator, and the laser-detector output signal which are obtained in on-line measurement. It can be seen from Fig.6 that the on-line detection on Kerr effect is performed when the square pulse just reaches the flat-top value. UPFL (A) Upulse (B)
UPFL(Upulse)(kV)
250 200
Laser (C) C A
Cathode
1.5 1.2
100
B
0.3
0
0.0 200
400 600 t(ns)
800
Cathode
0.6
50
0
Anode
(a)
0.9
150
-50
field of insulator surface in vacuum. The preliminary experiments prove that surface charging takes place in the preflashover process, which can modify the surface electric field significantly. And a further work is also need to continue for a more detailed investigation on the distributions of surface field and surface charge.
Anode
(b)
-0.3 300
Fig. 7(a) shows an on-line experiment on electric field along insulator surface, in which the light intensity distribution between two circular electrodes (80mm in diameter) with insulator of 20mm in height is obtained. Correspondingly, the similar measurements with two electrodes of same dimension without insulator are also performed, and the electrode gap distance is kept as same as that of electrodes bridged by insulator. The waveforms obtained in former experiment are also given in Fig.7(c). A chopped waveform for HV pulse stressed on insulator implies that a flashover happens in experiment. Therefore, the surface electric field distribution in preflashover process is measured. Due to circular electrodes are used, the phase shift in equation (3) is integrated approximately along the string of a circle area determined by electrode [2]. As a result, the light intensity exhibits a fringe pattern between the electrodes because of the different light integrating paths. A distortion of fringe in the vicinity of insulator/liquid interface, especially near cathode, can be obviously observed as comparing with the fringe pattern shown in Fig.7 (b) which is obtained in experiment without insulator. This distortion of optical fringe pattern implies that the surface field distribution is modified during the prebreakdown process due to the existence of surface charge. Furthermore, it can be preliminarily supposed that the insulator surface near the cathode is charged negatively at the time when measurement is being performed, according to the relationship between light intensity distribution and phase shift demonstrated in equation (2) and (3).
UPFL(Upulse)(kV)
Fig.6. Waveform A ~ C denote the voltage at the end of pulse forming line, square pulse applied on insulator, and laser signal converted by photoelectric detector, respectively.
Upulse (A) UPFL (B)
Laser(C) B
250
C
200 150 100 A
50 0 -50 850 (c)
900
950 t(ns)
1000
1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 1050
Fig.7. (a). Light intensity distribution between electrodes, the rectangular dark area denotes the insulator; (b) light intensity distribution between electrodes with the same gap as (a); (c). Corresponding waveforms obtained in experiment (a), the HV square pulse shows a chopped waveform. The gap distance is 20mm, the HV voltage is 104kV at the measurement time in both experiment (a) and (b).
V. CONCLUSION In summary, an on-line electro-optical measurement system based on Kerr effect is established to study pulsed electrical
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REFERENCES H. C. Miller, “Surface flashover of insulators,” IEEE Trans. Elect. Insul. , vol. 24, no.5, pp.765-786, October 1989 [2] M. Zahn, “Transform relationship between Kerr-effect optical phase shift and nonuniform electric field distribution,” IEEE Trans. Elect. Insul. , vol. 1, no.2, pp.235-246, April 1994 [3] E. C. Cassidy, H. N. Cones, and S.R.Booker, “Development and evaluation of electrooptical high-voltage pulse measurement techniques,” IEEE Trans. Instrum. Meas., vol. IM-19, no.4, pp.395-402, November 1970 [4] J. E. Thompson, J. Lin, “Electro-optical surface flashover measurements,” Appl. Phys Lett., vol. 37(6), pp.574-576, September, 1980 [5] H.Zhu, W.Liu, X.Zou, X. Wang, and S.Xu, “System for on-line measuring electric field on insulator surface based on Kerr effect”, High Voltage Kerr Engineering, Vol.37, No.3, pp717-724, March 2011; , , Vol.37 (3) pp.7172011 [6] X. Wang, W.Liu, W.Qiang, H.Zhu, and X.Zou, “Measurements of electric field and charge on insulator surface”, High Voltage Engineering, Vol.37, No.3, pp10132-10139, March 2011; , Vol.37 (3), pp731-737, 2011 [1]
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