Calibration methods of carbon nanotube gas sensor for partial discharge detection in SF/sub 6/

The authors proposed a new type of gas sensor for high sensitive detection of decomposition products generated by partial discharge (PD) in SF/sub 6/ gas. The sensor employed carbon nanotubes (CNTs) as gas sensing transducer and was fabricated by electrokinetic manipulation of CNTs using dielectrophoresis. Due to complicated gas decomposition process of SF/sub 6/ gas, calibration of the CNT gas sensor is an essential and challenging subject in order to realize reliable and stable detection of PD under practical conditions. In this paper, two methods for the CNT gas sensor calibration were proposed and tested. The first method was based on initial conductance dependence of the CNT gas sensor response. The CNT gas sensor response to PD increased almost linearly with the amount of CNT trapped onto the sensor electrode, which could be quantified by the initial conductance of the sensor. However, the calibration accuracy of this method was about 50 % and was not high enough for practical use. The second method employed NO/sub 2/ (nitrogen dioxide) as calibration gas because of the similarity in the CNT sensor response to PD decomposition products and NO2. It was found that the accuracy of the NO/sub 2/ calibration was about 10 % and far better than the first method. Finally, effectiveness of the NO/sub 2/ calibration was demonstrated by a PD monitoring test in which two CNT sensors were simultaneously exposed to SF/sub 6/ decomposition products with identical concentration.     

Impulse partial discharge and breakdown characteristics of rod-plane gaps in N/sub 2//SF/sub 6/ gas mixtures

Impulse partial discharge (PD) and breakdown (BD) characteristics of rod-plane gaps in N/sub 2//SF/sub 6/ gas mixtures were investigated for different gap geometry, gas pressure and SF/sub 6/ gas content. Experimental results revealed that the 50% probability breakdown voltage increased with gas pressure, which agreed with the theoretical values with consideration of discharge time lag for impulse voltage application. For the calculation of discharge inception voltage, the volume-time theory was successfully applied to the N/sub 2//SF/sub 6/ gas mixture. Furthermore, impulse PD light emission image was observed together with PD current pulse and light intensity waveforms. The long discharge with stepwise propagation and double-peak PD pulses could be observed, which corresponded to the streamer/leader transition leading to breakdown.     

SF/sub 6/ reclaimer from SF/sub 6//N/sub 2/ mixtures by gas separation with molecular sieving effect

This paper discusses the various methods for sulfur hexafluoride (SF/sub 6/) separation from a mixture of low concentrations of SF/sub 6/ in N/sub 2/ pressure swing adsorption (PSA) with a suitable kind of synthetic zeolite, which has the expected molecular sieving effect. This molecular sieving effect, derived from molecular size difference between SF/sub 6/ and N/sub 2/, is confirmed by the difference between two equivalent volumes filled with SF/sub 6/ and N/sub 2/. Prototype equipment of SF/sub 6/ separation and liquefaction, that is about 1 m cube in size and 150 kg in weight, has been assembled and tested. The ability of gas mixture handling is 13 l/min on average, and the SF/sub 6/ content is reduced to 0.0% (undetectable level) in separated N/sub 2/ to exhaust into the atmosphere.     

Power frequency dependence of space charge behavior and partial discharge characteristics in SF/sub 6/ gas

The partial discharge (PD) and breakdown (BD) characteristics in SF/sub 6/ gas under commercial and higher frequency (/spl sim/600 Hz) ac voltage applications were investigated using high-speed electrical and optical measuring techniques with phase gate control method. Experimental results revealed that 400 Hz BD voltage at a certain gas pressure range was higher than that for 60 Hz and PD characteristics especially at the positive PD inception phase were much influenced by the applied power frequency. From these results, we clarified the dependence of space charge behavior on the applied power frequency and discussed the physical mechanism of PD and BD in SF/sub 6/ gas with consideration of the space charge behavior generated by PD in the previous half cycle of ac voltage.     

不同电极结构中SF_6/N_2混合气体正向流注电晕放电特性

宏观上假定气体放电过程中产生的正、负离子束和电子束为流体,采用二维流体动力学模型对SF6/N2混合气体正向流注电晕放电过程进行建模,利用通量校正传输法求解连续方程,通过求解耦合的泊松方程处理空间电荷畸变电场对放电的作用,仿真过程中假设带电粒子的输运参数是折算电场的函数,对比分析了平行板电极间隙、同轴电极间隙和棒-板电极间隙三种电极结构中正向流注电晕放电特性。结果表明:流注电晕的形成加强了流注头部与阳极间的场强,减弱了流注尾部与阴极的场强。只有流注头部所在位置的初始场强足够大时放电才能继续发展,否则放电转化为稳定的流注电晕放电。     

Impulse partial discharge characteristics and their mechanisms under non-uniform electric field in N/sub 2//SF/sub 6/ gas mixtures

We have investigated impulse partial discharge (PD) and breakdown (BD) characteristics of a needle-plane gap in N/sub 2//SF/sub 6/ gas mixtures under positive lightning impulse voltage application, and discussed their physical mechanisms. The 50% probability PD inception voltage (PDIV/sub 50/), leader discharge onset voltage (LOV) and BD voltage (BDV/sub 50/) were measured and analyzed as a function of gas pressure and SF/sub 6/ content. Experimental results revealed the stepwise propagation process of the impulse PD and enabled us to classify the impulse PD in N/sub 2//SF/sub 6/ gas mixtures into two types, the streamer discharge and the leader discharge. We also discussed the impulse PD propagation mechanisms in terms of PD parameters such as propagation length, time interval and current pulse magnitude, and suggested a sequential relationship in the PD propagation process under non-uniform electric field.     

Use of zeolite filter in portable equipment for recovering SF/sub 6/ in SF/sub 6//N/sub 2/ mixtures

We propose using zeolite filter in portable processing equipment to separate and recover SF/sub 6/ from SF/sub 6//N/sub 2/ mixtures. Testing of a prototype filter has shown that SF/sub 6/ is completely adsorbed by the filter, and the exhaust gas from the filter is pure N/sub 2/ (i.e., at an undetectable level of SF/sub 6/ measured by a gas chromatograph). The SF/sub 6/ fraction of the sorbate phase (solid phase) has been evaluated from the tests, and it is a high fraction as expected. For example, 70% sorbate corresponding to processing a mixture of 5% SF/sub 6/, and 83% sorbate corresponding to processing a mixture containing 10% SF/sub 6/. The process of releasing the SF/sub 6/ from the filter and renewing the filter by thermal swing adsorption (TSA) technique has also been investigated to determine a commercial operating temperature of 150/spl deg/C. The feasibility of applying this new concept in a portable SF/sub 6//N/sub 2/ mixture processing equipment is discussed and compared with an alternative system that the authors developed previously which was based on pressure swing adsorption (PSA) technique and also using zeolite.     

Interruption capability of CF/sub 3/I Gas as a substitution candidate for SF/sub 6/ gas

We studied the interruption performance of CF₃I gas because its environmental effect is smaller than that of SF₆ gas with a model arc-extinguishing chamber. First, we measured the arc time constant and arc power loss coefficient using Mayr's equation. Comparing CF₃I with other gases, the arc time constants are SF₆3I2< H₂2. The arc power loss coefficient is H₂>SF₆ >CO₂>Air>N₂ >CF₃I. Next, we evaluated the short line fault (SLF) interruption capability by measuring the di/dt-dv/dt characteristic. Consequently, the SLF interruption performance of pure CF₃I was about 0.9 times that of SF₆. However, CF₃I application to gas insulated switchgear (GIS) and gas circuit breakers (GCB) is difficult because the liquefying temperature of the gas is high. Therefore, we adopted a countermeasure to obtain a lower liquefying temperature mixing CF₃I with CO₂. The result showed that the SLF interruption performance of the mixture approximated that of pure CF₃I when the ratio of CF₃ I exceeds 20%     

Electric field modeling and field formation mechanism in HVDC SF/sub 6/ gas insulated systems

This paper reports on the electric field behavior in HVDC SF/sub 6/ gas insulated systems. In the field model definitions, the SF/sub 6//epoxy interfaces are assumed as thin slightly conducting layers with non-zero thickness. The boundary conditions involving surface-reduced parameters of these layers are adopted. This allows avoidance of problems of calculation regarding the proximity of interfaces between subdomains. The boundary-value problem describing capacitive-resistive time-varying electric field is formulated. On this basis, the transient HVDC electric field model and time domain-calculating algorithm are constructed. The model accounts simultaneously for both the surface layer properties and bulk conduction parameters of the bounding dielectrics, while surface conductivity may be specified as a non-linear or an arbitrary function. The method employs direct integration of a continuity equation in the time domain with the calculation of an electrostatic field approximation at each integration step. The basic features and applicability of the method are discussed. The model has been applied for analysis of HVDC electric field formation mechanism in SF/sub 6//epoxy insulation. It is demonstrated that charge accumulation phenomena cannot be described by a single time-stable mechanism. Some directions regarding further development of the field model and calculating algorithm are suggested.     

The behavior of N/sub 2/+SF/sub 6/ gas mixtures subjected to orthogonal electric and magnetic fields

The limiting equivalent electric fields in N/sub 2/+SF/sub 6/ binary gas mixtures due to Townsend discharges are evaluated directly from a Monte-Carlo simulation when the mixture is subjected to orthogonal electric and magnetic fields. Along with the limiting equivalent electric fields, transverse and perpendicular drift velocities, electron mean energies and collision frequencies are also determined within the scope of the Monte-Carlo simulation. The equivalent reduced electric field (EREF) concept for the corresponding limiting electric fields is also investigated for the calculated mean energy levels and collision frequencies. The EREF values are found to be in good agreement with the previously published limiting electric field data.     

Simulation of corona discharge. Negative corona in SF6

Negative coronas in SF₆ under three voltage levels are investigated by Monte Carlo simulation. For the purpose of simulation, the gap between a hyperboloidal needle and a plane is divided into two regions: region I where the electric field is very steep and most electrons and ions are accumulated, and region II the rest of the gap. In region I electron motion is simulated by dividing the region into a number of cells and a small cell size improves the accuracy. The magnitude of current pulse increases with increasing voltage and there appears to be more than one peak in each pulse as observed by experiment. The development of electron avalanches is due to ionization and photoionization in the high field region, while the quenching of the avalanches is due to the space charge field suppression. Also the accumulation of positive and negative ions are displayed in detail. The space charge field distortion is studied and found to increase with increasing applied voltage     

Simulation of corona discharge. Positive corona in SF6

Positive coronas in SF₆ under three voltage levels, 3.0, 3.5 and 4.0 kV, are studied for a 0.5 cm gap between a hyperboloid needle and a plane electrode at a gas number density N=2.12×10¹⁸ cm³. The current in the form of a burst of pulses is found to increase with increasing voltage. The initiation and development of successive avalanches are traced as function of time after voltage increases. The development of electron avalanches and streamers are due to ionization and photoionization and quenching of avalanches is due to space charge. The simulation provides a detailed structure of avalanches from which essential differences between positive and negative corona with regard to total field distribution, propagation of successive avalanches and ion distribution can be discerned     

Toepler's spark law in a GIS with compressed SF/sub 6/-N/sub 2/ mixture

This paper deals with the experimental measurements and analysis of the formative time lags to breakdown and an estimation of the Toepler's constant for gas gaps, under the application of 50 Hz AC voltage. The experiments were carried out in a 145 kV gas insulated system (GIS) bus duct with pure N/sub 2/, pure SF/sub 6/ and SF/sub 6/-N/sub 2/ mixture as insulating media. The formative time lags to breakdown in the gas gaps were measured using a fast response capacitive sensor. Toepler's spark law has been used to explain the breakdown phenomenon in the GIS and the values of Toepler's constant (k/sub t/), which gives an estimation of the formative time lags, were determined. Results show that the formative time lags vary inversely with gas pressure and the gas mixture concentrations for two gaps studied (0.46 mm and 0.61 mm). In the case of another gap (0.20 mm), the variation in the formative time lags with pressure as well as SE, concentration in the mixture has been found to be negligibly small between gas mixtures, although significant variation can be seen between pure SF/sub 6/ and pure N/sub 2/. Toepler's constant, k/sub t/, increases with gas pressure as well as SF/sub 6/ concentration in the mixture for the gaps studied. Hence, k/sub t/ is a function of the gas pressure and the concentrations of SF/sub 6/ in the gas mixture for the above-mentioned gaps.     

Analytical approach to SF/sub 6/ breakdown under transient conditions

Under many circumstances, the initiation of breakdown can be viewed as conversion of energy stored in the electric field (capacitive energy) to thermal energy which converts matter in a thin channel from a nonconducting state to a conducting plasma. This paper analyzes this process in the context of breakdown in SF/sub 6/ as a prelude to future papers which treat the subject of breakdown under highly inhomogeneous thermal field, quasihomogeneous electric field conditions.     

Gas-flow Simulation with contact moving in GCB considering high-pressure and high-temperature transport properties of SF/sub 6/ gas

This paper addresses a thermal gas-flow simulation in gas circuit-breaker (GCB) chambers, introducing SF/sub 6/ gas constants up to a pressure of 10 MPa and to a temperature of 30 000 K. In the simulation, moving parts, such as nozzle, movable arcing contact, and operating rod, are moved with the opening motion of GCB to see if different results are produced from the conventional simulation method, in which fixed parts in the real GCB are moved. As a result, as far as the pressure profile in the puffer chamber is concerned, it is confirmed that this simulation method can produce better results than the conventional method for the hybrid-puffer-type chamber.     

Dielectric strength coordination and generalized spacer design rules for HVAC/DC SF/sub 6/ gas insulated systems

The problems of HVAC/DC gas insulated systems (GIS) dielectric strength coordination and support spacers design criteria are discussed in this paper. The most important challenge is to protect the spacers against flashover and consequently exclude them from consideration when coordinating an insulation of GIS. Both experimental and numerical studies demonstrate that electric field formation mechanisms and insulation behavior at SF/sub 6//epoxy interface are substantially different for AC and DC voltage excitations. Nevertheless, in actual power systems, HVDC apparatus are frequently subjected to impulse and time-varying voltages, implying capacitive field grading, and vice versa, HVAC apparatus can actually be subjected to harmful pseudo-DC voltages occurring in certain operating modes. Therefore, the above-mentioned problem is considered in order to find generalized criteria which will provide a reliable spacer performance in both HVAC/DC gas insulated systems. To illustrate the specificity of SF/sub 6//epoxy insulation behavior at HVDC stress, static V-T characteristics for post-type spacer models were studied experimentally. Dielectric strength coordination requirements and constraints arising for AC/DC GIS conditions were discussed. Some design and technological alternatives directed to improve the DC spacer reliability were analyzed. Based on these results, generalized design criteria for AC/DC conical and post type spacers were formulated. For conventional range of GIS operating stress and insulation parameters, it has been shown that proposed design rules can be realizable with no additional enlargement of apparatus dimensions. Such an approach provides unification and interchangeability of support spacers in HVAC/DC SF/sub 6/ apparatus, which could be technologically worthwhile and economically beneficial for manufacturers and power utilities.     

Effects of hot SF/sub 6/ on post-arc circuit breaker design

SF/sub 6/ is blown through the arc during interruption of an SF/sub 6/ puffer circuit breaker. This hot gas flows down an exhaust tube into the chamber of a dead tank circuit breaker where it can lower the dielectric withstand between the exhaust tube and the tank of the circuit breaker, leading to dielectric breakdown during the transient recovery voltage after clearing. This paper presents experimental evidence that any such breakdown is controlled by the hot gas near the exhaust tube, with cold gas in the remainder of gap having little effect on the breakdown voltage. This experimental conclusion is supported by a detailed theory for conditions under which breakdown can occur.     

Barrier effect under nonuniform field in atmospheric pressure SF6 gas

Generally, it is known that insertion of a barrier under a nonuniform field in air increases the flashover voltage. This is called the barrier effect. However, there are many uncertainties that remain to be clarified about the barrier effect in SF₆ gas. Therefore, the influence of the barrier shapes and positions on the flashover voltage for the lightning impulse voltages and voltages (50 Hz) is examined by inserting a barrier between a hemispheric-end rod and a plate in SF₆ gas under near-atmospheric pressure. As a result, the following findings were obtained:. (1) Insertion of a disk-shaped barrier reduces the flashover voltage; a cup-shaped barrier whose surface curves almost along the equipotential line increased the flashover voltage by 30 to 40 percent;. (2) trapped charge on the barrier surface is of the same polarity as the applied voltage and the magnitude of positive charge was greater than that of negative charge. It was found also that there is a correlation between the amount of trapped charge and the flashover voltage; and. (3) application of a barrier under a nonuniform field in the atmospheric pressure SF₆ gas helps reduce the size of, for example, a cubic-type gas-insulated switchgear.