EXPERT SYSTEM TO DIAGNOSE INCIPIENT FAUTLS IN GAS INDSULATED SUBSTATIONS

Transcription

1 EXPERT SYSTEM TO DIAGNOSE INCIPIENT FAUTLS IN GAS INDSULATED SUBSTATIONS 1 ALAMURU VANI, 2 P.S.R MURTHY, 3 S.S.TULASI RAM 1 Associate Professor EEE Vjit 2 DR. Dean School Of Electrical Engineering Snist 2 3 DR. Professor EEE JNTUH 3 Abstact- The increasing application of SF6 as an insulating gas has led to many studies on SF6 decomposition in gasinsulated equipment and the developing GIS (gas insulated substations) systems. In the presence- of an electric arc, spark or corona, SF6 decomposes to a wide variety of chemically active products which possess completely different properties from SF6. The accumulation of these decomposition products in the equipment has caused concerns regarding personnel safety and material compatibility problems. This paper discusses SF6 decomposition relating to the operation of gas insulated switchgears, gas-insulated transmission lines, and electrostatic accelerators, by-products and their formation rates in various modes of electrical discharges. In particular, the influence of discharge energies and impurities on the formation of SOF2and S02F2, the two dominant stable by-products, is discussed. The effects of the by-products on personnel safety and equipment dielectric integrity are presented. The application ofsf6 gas analysis as a tool for diagnosing the internal condition of gasinsulated equipment is assessed. A tool is developed in Matlab for the incipient detection of decomposition to take safety measures Keywords- SF6 Gas, By-Products, Safety Measures, Developed Tool In Matlab I. INTRODUCTION The increased applications of SF6 in gas-insulated switchgear/substation(gis), electrostatic accelerators, x-ray equipment and pulse power apparatus [9,20] have led to growing interest in the mechanisms of SF6 decomposition and the properties of the decomposition products. In the presence of an electric arc, spark or corona, SF6 decomposes into lower fluorides of sulphur which in turn react with the electrodes or gas impurities to form many chemically active products. Although SF6 is chemically inert and environmentally acceptable, the decomposition products of SF6 are known to be toxic and corrosive [23].The motivation behind the studies on SF6 decomposition is the concern for personnel safety and material compatibility. The studies may also lead to improved equipment diagnostic techniques.from a health and safety point of view, the toxicity of SF6 decomposition products must be clearly identified and the quantities generated under all possible circumstances must be determined so that procedures can be established to protect personnel during maintenance and clean-up of faulted equipment [21]. The requirements for long-term equipment reliability impose severe constraints on the material compatibility with the SF6 environment. The effects of SF6 decomposition products on materials used in gas-insulated equipment have to be examined carefully to ensure that no degradation of the materials' insulating, physical and chemical properties would occur, leading to premature aging. In the development of SF6 circuit breakers, the influence of decomposition products on the dielectric recovery after current zero is critical to the breaker's function. On the more positive side, the identification of SF6 decomposition products in electrical equipment offers a means to diagnose the equipment's internal condition. Diagnostics by SF6 gas analysis is very similar to the well known technique of gas-in-oil analysis for detecting abnormal conditions in transformers. An in-depth knowledge of the SF6 environment, decomposition mechanisms, and decomposition rates, properties of the decomposition products and the interaction of the by-products with the equipment are required. The objective of this paper is to discuss the major investigations relating to SF6 decomposition and its detecting tool designed. II. THE SF6 OPERATING ENVIRONMENT 2.1 Conditions for SF6 Decomposition In gas-insulated equipment, decomposition of SF6 by an electrical discharge is the most common mode of dissociation. The discharges can be broadly divided into three types according to the energy dissipated in process: 1)power arc2) spark and 3)corona or partial discharge. Table 1 summarizes the types and characteristics of discharges in gas-insulated equipment. 1

2 under normal operating conditions, SF6 can be decomposed in a power arc during current interruptions in circuit breakers. The energy dissipated in the discharge depends on the arc current which ranges from several ka to 100 ka. The energy can be expressed as: u(t)i(t)dt (1) where I is the arc current, U is the arc supporting voltage in the range of several hundred V and t is the duration of arcing [16]. Typical values of t are in the range of 50 to 150 ms. The energy dissipated is in the order of 10 5 to 10 7 J and the arc temperature at the axis can reach K [5]. In SF6 circuit breakers, the electrode material is usually a Cu-W alloy which is highly arc resistant and the hot plasma gas flows through a Teflon nozzle.. The energy level involved in the discharge in GIS is in the order of 10-1 to 10-2 J per spark. In electrical power equipment, flashovers during equipment testing by a resonant test device, where all the energy is that stored in the equipment's capacitance and inductance, can be classified as a spark. The third type of discharge discussed in this paper is corona or partial discharge. Under abnormal operating conditions, partial discharge or corona discharge can occur in SF6 insulated equipment due to electrically floating components, stress enhancements, metallic particle discharges and voids inside solid insulators. 2.2 Impurities The decomposition of SF6 is greatly influenced by gas impurities. In industrial grade SF6 the typical impurities are CF4, N2, O2, and H2O. TABLE 2. Impurities in SF6 Max. SOF, concentrations for operating circuit breakers Max. SOF, concentrations for operating bus ducts Empirical formulae for the amount of decomposition as a function of the arc current and duration were derived. For Cu and Cu-W electrodes the volume of the by-products is given v = I 2.2 t (with Cu-W).. (2) v = I 2.8 t (with Cu) (3) where v is the absolute volume of the decomposed gas in liters, l is the interruption current in ka rms and t is the duration in s. The gas, decomposed in the presence of aluminum and zinc electrodes, contained 100 times the amount of arc by-products as gas decomposed in the presence of silver or copper electrodes. 2 Fig. 1: SF6 decomposition, decomposition products and dielectric strength as a function of arc time arc current products. Conditions: arc current80 A, atmospheric pressure, arc gap 3.2 mm. The arc by-product concentrations were measured by gas chromatographic (GC) and infrared techniques. A linear relationship was obtained on the production rate of SOF2 and total SF6 decomposition as a function of arc energy. Energy dissipated Per Liter de S F6 ( kj/ t Dissipated energy by Liter of SF (kj/l) Fig. 2: Formation of SOF2, 502F2, SOFa4 under the effect of arcs between, electrodes of cutene and cunitene as a function of total energy dissipated per liter of SF6. O2 content, 4000 ppmv; H2Ocontent 200 ppmv. Curve 1: SOF2, Cutene, Curve 2: SOF2, cunitene, Curve 3: SO2F2, Curve 4:SOF4 [23]. Analysis of the byproducts was made with the GC technique. The major products identified in the highcurrent discharge were SF4 and SOF2.The arc byproducts were separated by gas chromatograph and the species were identified by means of IR spectroscopy for S02F2 and mass spectrometry for CF4 and SOF2. Fig. 3: Formation of SOF2 as a function of arc energy for aluminum electrodes

3 SOF2 and it is suspected that S02 is formed as a hydrolysis product of SOF2 and moisture Formation of Solid Arcing By-products in a Power Arc In addition to gaseous decomposition products, solid arcing by-products have been found in circuit breakers or in faulted equipment [23]. TABLE 9 Limiting Constant Values for SOF4 Production Rates Fig. 4: The relation between electrode consumption and the electrode material for power arc in SF Spark Decomposition-of SF6 TABLE 5 Yields of By-Products Formed in Sparked SF6 The relationship between apparent charge x and the quantity of decomposed gases y(l0-8 ml as HF) is expressed by the following equation: log(y) = 1.5 log(x) -A, A to 6.8 (4) Corona Results In the decomposition products, the concentration of S02F2 is much higher than that found in arcs and to a certain extent, in sparks. This appears to be a unique characteristic of corona or low-energy discharges. a- Maximum yield over the pressure range KPA. b- Based on a least-squares fit to a straight line for the yield versus total spark energy Spark Results SOF2 is still the major by-product observed, Decomposition in Corona or Partial Discharge TABLE 7 Limiting Constant Values for SOF2 Production Rates TABLE 8 Limiting Constant Values for SO2F2 Production Rates IV. EFFECTS OF SF6 DECOMPOSITION PRODUCTS 4.1 Toxicity of SF6 Decomposition Products With the increased concern in personnel health and safety, there is a growing interest in the toxicity of SF6 and its decomposition products in gas-insulated equipment. 4.2 Discussion of Toxicity Since SF6-insulated equipment is a completely sealed system, any decomposition products generated should be contained inside the enclosure unless the enclosure is burnt through or a pressure relief valve operates during a fault, releasing the by-products into the ambient air [16]. The greatest risk for personnel exposure to the by-products is during repair or maintenance of the compartment. In order to minimize the risk, an adequate protection system and handling procedure are required. The most common approach to establishing safe handling procedures is to obtain information about the types of decomposition products formed, their formation rates and toxicities. This information was used for the estimation of levels of decomposition products generated under various situations and energy levels. These levels were then compared to existing allowable exposure levels, e.g. Threshold Limit 3

4 Values to establish procedures and protection systems. 4.3 The Effects of By-Products on GIS Gaseous By-Products During normal or abnormal operation of gas-insulated equipment, SF6 is decomposed, and the accumulation of decomposition products in the enclosed compartment can affect the system performance in two ways. The presence of the gaseous and solid byproducts may lower the dielectric strength of the system. If the by-products are left in the compartment for an extended period of time, corrosion of the system components may ensue. completely regenerate the absorptive power. In such circumstances,it may be necessary to replace the absorbent completely. 4.5 Handling Procedures The handling of SF6 arc by-products is governed by the toxicities of the by-products. A full face mask with external air breathing apparatus was recommended for situations in which the ambient SF6 decomposition levels exceeded the TLV limits. Special vacuum cleaners were recommended for the removal of the solidby-products. V. MATLAB DIAGNOSTIC TOOL SF4 + H20 -> SOF2 + 2HF SOF2 + H20 e S02 + 2HF SiO2 + 4HF -e. SiF4 + 2H20 SiF4 + 2HF + H20 -> H2SiF6 + H20 The formation of blisters which contain highly electrolytic materials such as H2SiF6, and the absorption of HF gas in the epoxy resin greatly modify the surface resistance and ultimately lead to the observed drop in the impulse flashover voltage Effects of Solid By-Products The reaction of the surface adsorbed gaseous byproduct and the solid insulating surface when the solid by-product is in contact with the surface is responsible for the reduction of the solid insulator's dielectric strength. From a practical point of view, the problems caused by the solid by-products are more severe because they cannot be easily removed from the system without opening the compartments. 4.4 Removal of SF6 Arced By-Products Since SF6 arced by-products are corrosive and may affect organic materials in the gas insulated equipment, the accumulation of excessive amounts in the compartment has to be removed. The practice of installing absorbents has been extended to disconnect switch chambers where low energy arcing occurs periodically. The buildup of excess heat during the chemical reactions may rupture the metal container. The possible reactions between the hot alumina and the byproducts can proceed as follows: 6 HF + A A1F3 + 3H20 3 SOF2 + A A1F3 + 3S02 3 SF4 + 2A1203-4A1F3 + 3S02 Due to the chemical reactions between the activated alumina and the by-products, reactivation may not The maximum acceptable impurity levels for used SF6gas (according to IEC 60480)[23]are: SF 6 percentage: < 3% Decomposition products: < 50 ppm Moisture: dew point temperature > -9.4 F (MV) or > F (HV) The Sf6 Analysis is done in regard to the presence of four important components. The Sf6 is analyzed for the presence of SOF 2, SO 2 and SO 2 F 2. The dew point is also analyzed for its permissible limits and possible outcomes. The presence of SO 2 above the permissible is a typical indication of occurrence of overheats faults. Similarly the presence of SOF 2 and SO 2 F 2 indicates the probable occurrence of arc discharges and spark discharges respectively. The below Table summaries the results of SF6 analysis of a sample test case Table : Sf6 Diagnosis of test Samples The screen shots of the tool developed for early diagnosis of decomposition through gas chromatography 4

5 The validity of the expert system is tested against an expert clinical opinion and comparative analysis through a set of data which includes gases to indicate the presence and absence of faults. Five sample sets of data are taken. The data is generalized to indicate the presence and absence of a fault without detailing in to the specific kind and type of fault. The decision making ability of the expert system is compared in analysis to any one of the method succeeding in the correct identification of the fault. The tabular column gives the different sample sets used and cumulative number of faults present and absent in that particular data set. Number of Sample gas concentrations indicating fault and no fault. Different performance parameters (Accuracy, Error, Sensitivity, Specificity, Positive predictive value (PPV), Negative predictive value (NPV), False detection rate (FDR), Matthews correlation coefficient (MCC), False prediction rate, False negative rate, Prediction conditioned fallout, Prediction conditioned miss, Rate of positive prediction, Rate of negative prediction, Odds ratio, Likelihood ratio positive, Likelihood ratio negative, Prevalence, Pretest odd, Posttest odds of outcome for given positive test results, Posttest odds of outcome for given negative test results, and Receiver Operating Characteristics (ROC)) are computed using a built in GUI as described in section 4. Using a built function Tabulate Results in Excel all the results of the above mentioned performance parameters are directly tabulated and plotted in an Excel work book. This enables the user to have a recorded view and analysis of the performance measures. The below figure give the snapshot of the Excel work book created to store the performance measures of the five different data sets being tested When the above data sets were analyzed using the different methods designed in this work, their prediction is presented in the form of results are recorded as dichotomous outcomes (positive/negative results) Dichotomous outcomes (positive/negative results) Figure: Screen Shot of Excel sheet populated automatically for Analysis A sample plot of accuracy as plotted automatically in the Excel work book is given in the below 5

6 CONCLUSION Expert System To Diagnose Incipient Fautls In Gas Indsulated Substations The tool developed diagnoses the incipient faults in gas insulated substations based on the by-products formed by DGA Analysis. REFERENCES [1]. Sf6 Decomposition In Gas-Insulated Equipment by F.Y.Chu IESE Transactions on electrical Insulation Vo;s EI-- l No.5, october l986 [2] P. G. Ashbaugh, D. W. McAdam and M. F. James,"SF6 - Its Properties and Uses as a Gaseous Insulatorin Van der Graaf Accelerators," IEEETrans on Nuclear Science, NS-12, p. 266, June [2]. L. M. Babcock and G. E. Streit, "Ion Molecule Reactions of SF6:Determination of I. P., A. P. and D(SF5-F)," J. Chem. Phys.74 (10), p. 5700, May [3]. A. Baker, R. Dethlefsen, J. Dodds, N. Oswalt and P. Vouros, "Study of Arc By-Products in Gas-Insulated Equipment," EPRI Report No. EL-1646,December [4]. A. Bargigia, et al., "Endurance Test on Solid Insulating Materials Used for GIS Cast Insulators,"Proc. Fourth Int. Symp. on High Voltage Engineering, Athens, September [5]. B. Bartakova, J. Krump and V. Vosahlik, "Effect of Electric Partial Discharge in SF6, " Electrotechnichy Obzor, Prague, 67, p. 230, [6]. W. Becher and J. Massonne, "Contribution to the Study of the Decomposition of SF6 in Electric Arcs and Sparks," ETZ-A, Vol. 91, No. 11. pp , [7]. S. W. Benson and J. Bott, "The Kinetics and Thermochemistry of S2F10 Pyrolysis," Intl. J. Chem. Kinetics, 1, p. 451, [8]. G. Bernard, "Electrical Faults Mastery in High Voltage SF6 Insulated Substations," Revue Generale de l'electricite, No. 4, pp , April [9]. T. R. Blackburn and R. W. Boswell., "InsulatorSurface Damage in SF6 Insulated Systems," Proc.Eighth Intl. Conf on Gas Discharge and theirapplications, September [10]. J. P. Boeuf, P. Segur, M. Yousfi, E. Marode anda. J. Davies, "Calculations of Transport Parameters in SF6 for a Non Equilibrium Medium by Monte-Carlo and Boltzmann Methods," Proc. Seventh International Conference on Gas Discharges and Their Applications, pp , September [11]. S. A. Boggs, F. Y. Chu, and N. Fujimoto, "Disconnect Switch Induced Transients and Trapped Charge in GIS," IEEE Trans. Power and Systems,PAS 101, October [12]. S. A. Boggs, F. Y. Chu, and M. Mashikian, "Gas-Insulated Substation Reliability: Present Status and Future Trends," EPRI Final Report EL-4422, February [13]. P. C. Bclin and M. D. Hopkins, "Constraints on Fault Repair of GIS, " Proc. of International Symposium on Gas Insulated Substations, Toronto,September [14]. J. F. Bott, T. A. Jacobs, "Shock Tube Studies of SF6," J. Chem. Phys., Vol. 50, pp , [15]. C. Boudene, J-L Cluet, G. Keib, and G. Wind,"Identification and Study of Some Properties of Compounds Resulting from the Decomposition of SF6 under the Effect of Electrical Arcing in Circuit-Breakers," Revue Generale Electricite Special Issue, June [16]. K. P. Brand and J. Kopainsky, "Particle Densities in a Decaying SF6 Plasma," Appl. Phys., Vol. 16,pp , [17]. K. P. Brand, "Dielectric Strength, Boiling Point and Toxicity of Gases - Different Aspects of some Molecular Properties," IEEE Trans. Elec. Insul.,Vol. El-17, pp , [18]. J.M. Braun, F. Y. Chu, and N. S. McIntyre, "Characterization of Degraded Epoxy Spacer Surfaces by Electron Spectroscopy," CEIDP Annual Report, [19]. J. M. Braun and F. Y. Chu, "Novel Low-Cost SF6 Arcing By-Product Detectors for Field Use in GIS, "IEEE PES Summer Power Meeting, Vancouber, B.C.,Paper 85 SM369-4, July [20]. R. Brockmann and R. von Olshausen, "Flashover Voltage Dependence on the Tangential Field Strength of Contaminated Insulator Surfaces in SF6,"Seventh International Conference on Gas Discharges and Their Applications, pp , September [21]. G. Bruno, P. Capezzuto, and F. Cramarossa, "Inorganic Volatile Fluorides Obtained from Electrical Decomposition of Sulphur Hexafluoride in a Quartz Tube," Journal of Fluorine Chemistry, Vol. 14, pp , [22]. G. Camilli, G. S. Gordon, and R. E. Plump, "Gaseous Insulation for High-Voltage Transformers,"Trans. of AIEE, pp , January [23]. G. J. Carlson, J. M. Houston, W. D. Davis, M. P.Perry, and T. H. Rautenberg, "Fault Sensors for SF6 Equipment," Proceedings of the American Power Conference, Chicago, IL, Vol. 42, pp , April 21-23, [24]. J. Castonguay, A. Theoret, and R. Gilbert, "Arc Degradation of SF6 in the Presence of Polymeric Insulating Materials," Proc. IEEE Int'l. Symp. Electr. Insul., [25]. J. Castonguay, "Kinetics of the Arc Decomposition of SF6- Gas Mixtures," Proc. IEEE Intl. Symp. On Elect. Insulation, June [26]. J. Castonguay and R. Larocque, "Low Cost Technique for Field Analysis of SF6 Decomposed By-Products,"Proc. of Intl. Symp. on Gas Insulated Substations, September [27]. L. Centurioni, G. Camilli, C. Luccini, and P. Vismara, "Behavior of Polymeric Insulating Materials Exposed to Arced SF6," Proc. Fourth Intl. 6