CHAPTER-I INTRODUCTION. The increasing demand for electricity and the growing energy

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1 1 CHAPTER-I INTRODUCTION 1.1 INTRODUCTION The increasing demand for electricity and the growing energy demand in metropolitan cities have made it necessary to extend the existing high voltage network right up to the consumer. During the last two decades for reliable power supply and economic advantages [1], Gas Insulated Substation (GIS) have found a broad range of application in power systems, because of their high reliability, easy maintenance, small space requirement etc. In our country, a good number of GIS units have been in operation and a large number of units are under various stages of installations. GIS is based on the principal of operation of complete enclosure of all energized or live parts in a metallic encapsulation, which shields them from the external environment. Compressed Sulphur Hexafluoride (SF 6) gas, which has excellent electrical properties, is employed on the insulating medium between the encapsulation and the energized part. GIS have a grounded outer sheath enclosing the high voltage conductor. Gas Insulated Substation comprises the following componentscircuit breakers, isolator, disconnector switch, earthing switch, current transformers, voltage transformers, Busbars and connectors, power transformers, surge arrestors, cable termination, Gas supplying and Gas monitor equipment, Density meters and Local meters.

2 2 It can be observed from the literature that GIS sub-station posses the following advantageous features [2-5] GIS station occupies only about 10% of the space required by conventional air insulated substation. GIS can be installed either underground or indoors and in heavily populated areas. GIS are also conveniently used in coastal areas and industrial and urban locations where space and pollution are the main considerations. These substations are generally located closer to the load centres there by reducing the losses in transmission and distribution network. GIS systems are immune at atmospheric condition and pollution, the outages get reduced and coupled with their measured reliability, and the overall maintenance costs are minimized. High service reliability due to non-exposure of high voltage parts to atmospheric influences. Disadvantages of GIS Although GIS has been in operation for several years, a lot of problems encountered in practice need full understanding. Some of the problems being studied are:

3 3 Switching operation generate Very Fast Transient over Voltages (VFTO). Prolonged arcing may produce corrective/toxic by-products. Partial discharges within the enclosures can cause break downs. Metallic particle contamination. Transient electric field and transient magnetic fields. Field non-uniformities reduce withstanding levels of GIS. In general, spacers are used to isolate different sections. Spacers are either cone (or) disc shaped of large majority of spacers are using alumina filled epoxy. Epoxy support spacers in GIS have been highly reliable. Spacer insulation is the single most critical component for the dielectric performance of GIS units. In modern GIS design, internal stresses below 4kV/mm (rms) are used. The requirements on GIS insulators are many and they must be able to [6-8]. Withstand the high internal and surface electric fields, typically up to 4.0kV/mm (rms) for continuous operation. Withstand forces during transportation. Withstand short circuit forces. They must be made of a non-tracking material, so that no conducting tracks occur during testing. Must be relatively insensitive to surface contamination.

4 4 If it is a gas tight insulator, it must withstand a test pressure of 3.25 times the maximum working pressure and should be leak tight so that not more than 0.5% of gas is lost per year. In a GIS, the insulating media employed are the SF6 gas and the solid insulating supports. The behavior of the insulating system depends on the basic properties of the gas and the surface and volume properties of the solid insulators. Spacers acquire charge from corona sources, ionization in the gas and discharges from metallic particles. Discharges from metallic particles and spacers together give the lowest breakdown voltage. In recent years, there has been an emphasis on the long term reliability of the epoxy spacers. Moving particles give rise to both partial discharges and acoustic signals. Particles can attach to spacer surface and can cause flashover of the spacer. Forced services outages caused by spacer problems are mainly due to excessive operating stresses. The main types of defects that occur in GIS are free particles and protrusions on electrode surfaces in the gas medium, surface contamination by gas impurities and defects within the bulk of the solid material. The above defects when present, give rise to local field enhancements, which cause flashovers. If the field enhancements are small, then there will only be streamer corona without any leader development leading to breakdown. These corona discharges chemically react with solid insulating spacers and modify their

5 5 properties. When the applied voltage is further increased, the steamer corona transforms into a leader discharge and the leader growth will then be influenced by the space charges due to corona stabilization under the application of slowly rising a.c or switching impulse voltages. Breakdown occurs after the time that is necessary for the leader propagation and voltage collapse. The time will be longer under the application of steeply rising impulses (rise times lower than a few microseconds) leading to higher breakdown voltages. Therefore, there is a minimum voltage at which breakdown occurs under the application of such fast rising impulses. The defects in the SF 6 gas used in the GIS include free floating particles [9, 10] and metallic protrusions on electrode surfaces. These particles can give rise to partial discharges in the vicinity of the high voltage conductors. Free metallic particles on the electrodes get charged and can traverse the complete gap in the case of a.c voltage and the extent of this motion depends on the size of the particles, their weight and the magnitude of the a.c field. During the motion of the particles, charge exchange occurs by sparking before they hit the surface. The main defects include particles and flashover tracks on the insulation surfaces as well as surface contamination while cracks and voids within the bulk of the insulation can also cause problems. Metallic and other particles can be sticking to the insulator surface,

6 6 and they behave like protrusions on the electrode surfaces. The breakdown voltage when particles are present will depend on the tangential field on insulator surface, the field enhancement due to the particles themselves, and the surface charges caused by earlier partial discharges. These particles are considered to adhere to the insulator surface at critical areas where they get charged by partial discharges and move into low stress areas until the partial discharges extinguish. These partial discharges give rise to surface tracks and reduce the insulator surface resistance. Due to partial discharges or flashovers during testing, the solid insulating material will be carbonized due to the high temperature of the spark resulting in conducting track. A track can be formed by the energy of single flashover or by successive flashovers at same location. These tracks from leakage paths eroding the insulator surface over a period of time. Gaseous impurities and SF6 products also contaminate insulator surface. Air, N2 and lower compounds of SF6 up to 10% were found to have only a minimum effect on the dielectric properties of SF6 gas. During the operation of GIS, SF6 decomposition products are formed and the spacer should have adequate resistance to these decomposition products. All GIS units in service use cast epoxy spacers. In order to avoid the interface problem, where voids or small gas gaps can initiate volume puncture or surface flashover, the spacers are either cast directly on to the conductor or on to metal inserts[11,12], or have the interface well shielded. Decomposition

7 7 products of SF 6 gas due to the partial discharges [13] and disconnector switching are also observed to have no significant effect but the main contaminant that effects the dielectric integrity is the moisture in the gas. Moisture can be condensed on the insulator surface and at the ambient temperature it can cause dew formation on the insulator surface. A dew point of -5 C is considered safe to ensure that no harmful condensation of moisture in liquid form gets deposited on the insulator surface. Within the GIS, non-uniform fields are always present due to the presence of dust, floating metallic particles, fixed particles in the form of electrode surface roughness, condensation of moisture on the insulator surfaces, etc. The insulation strength of compressed SF6 is greatly decreased by contamination in the form of conducting particles. Electrical insulation performance of GIS systems is adversely affected by metallic particle contamination. The accumulated field experience indicates that sources for such contamination are mechanical abrasions, movement of conductors under load cycling and vibrations during shipment and service [14]. These particles may be free to move in the electrical field or may be fixed to the conductors thus enhancing local surface fields. In a horizontal co-axial system with the particles resting on the inside surface of the enclosure the motion of such particle is random in nature. The dynamics of wire particles in a

8 8 horizontal co-axial system are studied because they approximately correspond to the type of particles encountered in practice. Another important source of field non-uniformity within the GIS is sharp points or mechanical edges. These defects are often of minor importance under normal power frequency voltages. However, steep fronted impulse voltages such as lightning, impulse or very fast transients can significantly decrease the di-electric strength of GIS assembly in the presence of these particles. Good design and the adoption of quality assurance methods at all levels will enable the GIS manufacture to limit the quantity and size of any residual particles in a modern GIS to insignificant levels. Although the likelihood of particle effects in GIS is very small, it still does exists, which is why research is in progress to develop diagnostic and analytical methods for detecting and localizing them. When the shape of the spacer is changed from an annular disc to a conical disc keeping the thickness at the base constant, the field around the insulator also changes. The field on the spacer surface increases when the angle of inclination reduces while the maximum stress occurs on the surface of the inner conductor covered by the insulator. A shielding electrode having a suitable shape is used to avoid this field concentration on the conductor. However, the field concentration was observed to be zero when an annular disc insulator is used.

9 9 In recent years, from the view points of the environment friendly and efficient power transmission, electric power equipment tends to be compact and be operated under higher voltage. In a gaseous insulation system, a solid insulator plays an important role for mechanical support for holding insulation clearance between high voltage and low voltage electrodes. In the insulation design of a gassolid composite insulation system which is typically included in GIS and a Gas Insulated Transmission Line (GIL), etc., the insulation technique in the gas-solid interface heavily becomes important as well as the insulation both in gas gap and inside the spacer. In the insulation of a gas-solid interface, various factors of significance are contamination particles, voids, cracks, E-field intensification at triple junction and charging on the spacer surface, as well as the electric field distribution on the spacer surface with a perfectly pure condition. For these reasons, the spacer insulation in the practical gas insulated switchgears, are made improved by various techniques, for examples, controlling the spacer shape, additional shielding electrodes for relaxation of E-field, and the introduction of thin layer made of a low conductivity material on the spacer surface. In addition, a lower permittivity is being applied to the spacer. However, these techniques lead to the complicated structure of the spacer which limits the flexibility of the spacer design and increases the manufacturing cost. In order to overcome the limitations, it is necessary to propose a new

10 10 concept on the spacers by keeping their simple structure and configurations. With a new concept for spacer insulation an application of a Functionally Graded Material (FGM) which has been developed originally for the structural material under thermal or mechanical severe stress in special environment. In electrical applications for us, the FGM spacer has spatial distribution of dielectric permittivity and can make the E-field distribution in and around the spacer more suitable, thus achieving the efficient E-field control by keeping the simple configuration of the spacer. 1.2 LITERATURE SURVEY Ibrahim A. Metwally [15] in his research reported that, from the last two decades, the evolutionary development of GIS has resulted in higher integration of a number of new technologies to enhance performance and reliability by reducing defects, having more compact designs, and reducing maintenance intervals and costs. Incremental improvements are continuing in interrupter technology, such as selfextinguishing features at Medium Voltage (MV) and resistance interruption at Extra and Ultra-High Voltages (EHV and UHV). In addition, SF 6 gas technology for circuit breakers, zinc oxide (ZnO) for arresters, radio communication for condition monitoring, and a choice of porcelain or polymer composite for the full range of equipment are also some of the technologies integrated or innovated by GIS

11 11 manufacturers in recent years. Recently, ac GIS ratings have reached up to 1,100kV rated voltage and 50kA (rms) rated short-circuit breaking current. In addition, 1,200kV ac GIS are going to be visible very soon. Moreover, 500 kv dc GIS for dc transmission systems have become available [16]. Epoxy or cast resin solid insulators are used as spacers in GIS. They represent the weakest points in GIS systems as the electric field on their surfaces is higher than that in the gas space [17]. Generally, the higher the operating voltage of GIS, the higher is the failure rate due to the higher electric field strength. In particular by means of monitoring and diagnostic systems as about 61% of the Failures could have been detected PD in compressed SF 6 GIS arise from protrusions, free conducting particles[18], floating components, and bulk insulation defects (voids). These defects represent about 53% of the total main failure causes in GIS. Some techniques are used for the mitigation and control of particle contaminations in GIS are particle traps, dielectric coating of the electrodes, the use of SF6 gas mixtures, and the use of FGM as solid spacer with optimizing its profile. The ultra-high frequency and acoustic emission techniques can be used for GIS PD monitoring system, dramatic reduction in failure rates can be achieved when using such systems. G. Schoffner et al [19] discussed that Gas Insulated Transmission Lines (GIL) area means of bulk electric power

12 12 transmission at extra high voltage consists of tubular aluminum conductors encased in a metallic tube that is filled with a mixture of SF6 and Nitrogen gases for electrical insulation. Since the first installation of GIL in 1975, second generation GIL has been developed that is more economically viable and its design optimized both for installation and operation. Where GIL is installed in combination with Gas Insulated Switchgear (GIS), compact solutions can be delivered in order to supply large amounts of electric power to meet the high demand of large cities and industry [20]. These new possibilities can mitigate power flow problems, reduce the risk of failure of electrical transmission systems and enable the installation of optimum solutions regarding technical, economical and environmental aspects. The requirements to installations for high voltage power transmission and distribution have changed. HIS and GIL as innovative products offer new possibilities to cope with these new requirements. Depending on each situation, by a coordinated application of the different techniques of GIS, HIS, AIS, OHL and GIL the optimum solution will be provided regarding technical, economical and environmental aspects [21]. D.I.Yang et al explains that the insulator [22] made in epoxy resin was widely used in SF 6 GCB (Gas Circuit Breaker) and Gas Insulated Switchgear because it s electrical and mechanical property are efficiency. Especially, spacer that was used for supporting the conductor and gas division in GCB and GIS is regarded as of the

13 13 important component affected on the lifetime of the power apparatus. The authors report that two different results obtained in the development of several type spacers for GIS [23]. Firstly, for the threephase spacer of 362kV GIS, they presented the optimal design that was obtained by electric field analysis and mechanical stress analysis using commercial program. N. Giao Trinh et al [24] showed that in Electrostatic-field optimization of the profile of the gas dielectric interface was studied as a means of improving the dielectric performance of epoxy spacers. An optimum disc shaped spacer is defined with a dielectric cone angle of 75 0, assuming a dielectric constant of the epoxy resin of 5 or higher. The dielectric performance of the optimum disc shaped spacer is found to be limited, however, to about 85% of that of the conductor system without spacer. A new composite-shaped spacer was developed which combines the advantage of the long leakage distance of a cone shaped profile with that of the quasi-uniform field distribution of a disc shaped profile. Tests indicate that a dielectric performance comparable to that of a conductor system without spacer [25] is possible with the new composite-shaped spacer. From this profile optimization study on epoxy spacers for use in compressed SF 6insulated cables, the following conclusions are made [26]. For simple cone-shaped interfaces, a range of optimal angles could be defined as a function of the relative dielectric constant

14 14 ε r of epoxy. For practical values of ε r, 5 and higher, the optimal di-electric cone angle ranges from 65 0 to Metal inserts embedded in the epoxy can have a beneficial effect when located near the metal-epoxy-sf6 junction, since they artificially reduce the local field intensity at these junctions. The best ac performance obtained with the optimum discshaped spacer is about 85% of the intrinsic disc dielectric performance of the test conductor without spacer at the nominal operating gas pressure of 0.4 MPa. J.M. Braun [27] stated that the Bulk failure by electrical treeing of the solid dielectric in Gas Insulated Switchgear is relatively, the general deterioration process in a void can be described as follows, A high field at the void location and the low dielectric strength of the contents of voids result in partial discharges in the cavity. This leads over time to erosion and enlargement of the cavity and generation of electrical "tree" channels which eventually bridge the insulation and cause failure. Partial discharges in epoxy insulation occur when a combination of a sufficiently high electric field stress and a dischargeinitiating free electron is present in a void. The process depends, among other parameters, on the gas content within the void [28]. In modeling partial discharge characteristics within spacers and decomposition and pressure of gases are found inside the voids are of prime concern. The gases that could be found are obviously residues

15 15 of the epoxy curing process and include entrapped air, curing byproducts, as well as thermal decomposition byproducts. R. M. Radwan [29] et al describes that the effect of the spacer's dimensions and its relative permittivity on the total electric field distribution. These effects will be also outlined for a practical spacer's shape. The field behavior near the triple Junction has been explained [30]. For the spacer's shape, the spacer's thickness has a considerable effect on the maximum field value on its surface. The optimum value of this thickness is 0.5p.u. The relative permittivity of the spacer's material has a considerable effect on the field distribution especially around and near the high voltage and low vo1tage electrodes. For a practical spacer's shape, the maximum electric field on the convex side is about 20% higher than that on the concave side and they occur at Rx=1.5 and 1.65 p.u. respectively. The electric field distribution near the triple junction has a peculiar behavior. Theoretically, it becomes infinity or zero depending on the spacer's relative permittivity and the spacer's inclination angle "" or in other words the spacer's thickness "z s.". J. Jia [31] et al resulted that In GIS, particles near spacer in GIS tend to cause apparatus faults by leading flashover breakdown

16 16 along spacer surface. Metal inserted spacer as a method is designed in purpose to prevent particles from lifting and adhering to spacer. The author s showed that, the influence of metal inserted spacer on particle motion in non-uniform electrical field under DC voltage is calculated for three type spacers. The results show that metal inserted spacer has good performance in preventing particle from lifting and adhering to spacer for disk and ribbed spacer [32]. Also, metal inserted electrode shows a different influence range on particle motion for different spacers. The results can be used to analysis AC condition [33-34] considering the root-mean-square value of voltage. K. Itaka [35] et al discussed that the Problems concerning local electric field intensification on a cone-type spacer which is fitted between flanges in SF6-gas-insulated apparatuses were investigated. Conventional structures, in which flat surfaces of the spacer come in contact with rounded corners of the flange, sometimes cause flashovers at considerably low voltages because of local field intensification [36]. In the improved structure proposed by the authors, surface shape of the spacer and contact position are slightly changed in order to avoid local field intensification. Field calculations and experiments verified that the improved structure is effective for actual use. Problems concerning local field intensification on a conetype spacer fitted between flanges in SF 6-gas insulated apparatus were investigated. The results are summarized as follows:

17 17 The conventional structure sometimes caused flashover at quite low voltages because of local field intensification. It was made clear quantitatively that the above characteristic is caused since the spacer makes contact with the flange at the interface between rounded surface and flat surface which has increased electric stress, and since the small gas gap near the spacer-flange interface becomes like a wedge. An improved structure was designed avoiding these problems. The effectiveness of the proposed structure is made clear by field calculations and experiments. Since these results can be applied for the design of not only cone-type spacers but also disc-type spacers, they are significant for the insulation design of practical gas-insulated apparatuses. N. Giao Trinhn [37] et al studied that the spacer and a composite-profile cone, were evaluated in a coaxial conductor 2.5 X 7 cm in diameter under the influence on the V-t characteristics [38] of the conductor when subjected to repeated applications of impulse voltages of constant wave shape and increasing magnitude. The results show that an insulating spacer can reduce the critical withstand voltage and yield smaller dispersion in the breakdown voltages. These effects can be minimized by adopting a design that favors breakdown in the gas rather than along the spacer interface [39]. The following conclusions are made.

18 18 The presence of a spacer results in a reduced withstand voltage of the conductor, a shorter delay time to breakdown and less dispersion of the breakdown data. The influence is more pronounced under negative polarity and at higher gas pressures. Proper design of the spacer, aimed at preventing breakdowns developing along the interface, can minimize the effects on the V-t characteristic of the conductor. The influence of the spacer is also more pronounced in a 50% SF 6-50% N 2 mixture than in pure SF 6. Insulating spacers were observed to cause a temporary reduction in the withstand capability of the cable, associated with electrostatic charging of the insulators. V.V-Akimov [40] et al stated that DC electric strength of pure SF6 gaps is almost the same as that for AC ones. However, DC electrical strength of real insulation systems including support epoxy spacers is apparently lower than that with AC. One of the major causes of such phenomena relates with the difference between AC and DC spacer electric field formation mechanisms. There are free electric charge accumulation processes on the spacer surface during longterm DC voltage application [41]. This may lead to substantial distortion of an initial (capacitive) field distribution near the spacer surface and as a result to decrease in flashover voltage. In this

19 19 connection, the design criteria developed for AC spacers are not enough valid for their using with DC insulation. 0. Farish [42] et al discussed that in compressed-gas-insulated equipment, the weakest point in the system is often at the interface between the gas and the solid spacers used to support the conductors. The low dielectric strength is attributed to the effects of surface charges, or to ionization at high-field sites such as the gas electrodespacer "triple junction", there have been some studies of surface charge development under dc stress relatively little is known about the way charge builds up on a surface and how the surface charge influences the breakdown process under impulse conditions [43]. In the present work a study was made of impulse flashover of model spacers under conditions where: Surface charge was allowed to accumulate as a result of repeated impulse stressing. A fault was simulated by introducing an annular gas gap at the triple junction The spacer surface was recharged with a line charge or uniform charge distribution Metal inserts were used to shield the triple junction and move the high-field site to the mid-gap region. An important feature of the study was that, in all cases, the surface charge density was measured before and after each impulse-

20 20 voltage application, the equipment was designed so that the complete surface of the spacer, including both triple-junction regions, could be scanned by a charge probe. For a plain spacer, the response to impulse stress is determined by conditions at the impulse junction. For defects of a few tens of microns, discharge activity begins at about 70% of the limiting field strength but the breakdown level is only slightly affected, with large defects at the cathode triple junction the onset level is considerably reduced and charge can be deposited over most of the spacer surface [44]. If charge is allowed to accumulate the impulse strength can be reduced by as much as 30%. When controlled charging methods are used to create regions of high charge density the strength can be as low as 50% of the gas-only value, even for pressures of 1 bar, with the greatest reduction occurring for deposition of hetero charge. Inserts can provide effective shielding of the triple junction. However, they introduce a normal field component which can attract surface charges. This may be detrimental in a system in which charges are produced as a result of micro discharge activity in the gas. K. Tekletsadik [45] et al discussed about the Breakdown or flashover arcs in Gas Insulated Systems (GIS) that produce a magnetic force which influences the path of the arc and has a decisive effect on the spacer-surface damage experienced during flashovers. A mild-steel flange and aluminum disc piece were made to hold the test epoxyresin spacer and to have an access to take arc photographs in an

21 21 open-to-air configuration at the open end of the GIS [46-48], with special spacer-electrode arrangements to study the arc lift off and push onto the surface during flashover on both sides of the spacer. The principle of arc dynamics is discussed, along with, experimental arrangements, results obtained from arc photographs and the effects of the arc path on spacer-surface damage. Breakdown and flashover arcs in GIS produce a magnetic force which influences the motion of the arc. The arc dynamics are found to have a decisive effect on the spacer-surface damage experienced during flashovers. A flashover arc can be pushed onto the surface of the spacer or lifted off the surface depending on which side of the spacer surface failed. An arc path which is pushed onto the surface causes more severe surface damage. It should be noted by designers that there is, in many cases, a strong probability that the current flow will come from one side of the spacer, and in these cases the spacer geometry can be modified to allow the magnetic-field effect to lift the arc away from the surface of the spacer. Shigemitsu Okabe[49] et al stated that The insulation strength decreased the most when the lighting impulse voltage was applied to internal insulation of the spacer, In the experiment in which alternating current voltage is applied for a long period of time, it was found that there is no decline in the insulation properties even after the voltage is applied for the equivalent of 30 years when the electric

22 22 field intensity is 12kVrms/mm or less although the combination with the multiple lightning impulse application may bring about damages to the spacer insulation. The degradation mechanism caused by generation of micro-pits was also understood through simultaneous microscopic observation of the surface and of the interface between the electrode and epoxy. In order to contribute to high reliability and rational Insulation design of Gas Insulated Switchgear, V-N characteristics [50] (the dielectric breakdown voltage vs. number of repetitions of voltage application characteristics) regarding the internal insulation and creeping insulation of the epoxy spacer, the main insulating element of GIS, were obtained against the lightning impulse voltage and the switching impulse voltage. Further, effects of long-time ac voltage application on spacer degradation were examined and the following results were obtained. Regarding V-N characteristics (internal insulation, creeping insulation) The gradient n of V-N characteristics of the epoxy spacer internal insulation. The proportional decrease of dielectric strength was largest when the lightning impulse was applied to the epoxy spacer internal insulation. Micro discharge traces were observed both on an embedded electrode surface and resin surface on the interface in a spacer to which impulse voltages were applied repeatedly or ac voltage was applied for a long time. The generation of the

23 23 micro-discharge-traces was influenced by the roughness of the electrode surface and flaking on the electrode interface. Spacer insulation degradation caused by impulse and ac voltage application is assumed to lead to erosion due to the spread of discharge traces when the electric field is intensified at the micro protrusions and the small areas of flaking on their tips. T. Nitta et al [51] observed that various factors controlling the flashover of solid insulators in pressurized SF 6, are reviewed and their influences in gas insulated systems are discussed from a practical point of view. Flashover voltage of clean insulator surface is under the influence of the insulator-metal contact as well as the macroscopic electric field distortion due to the high dielectric permittivity of solid insulator. Conducting particles or even fine metal powder can reduce the flashover voltage. Their effects are strongly dependent on the position they are located, the size of the insulator and gas pressure. Humidity of SF6, gas should be strictly governed in SF6, gas insulated apparatuses, and since the condensation of water can decrease flashover voltage considerably. Decomposition products of SF 6, due to the arcing in switchgears are deleterious to epoxy insulators [52] particularly when silica is used as their filler. The decomposition products decrease the leakage resistance on the insulation surface. The field strength near positive electrode is enhanced by the electrolytic effect in the surface conduction layer. In some extreme

24 24 condition, it initiates tracking on the insulator surface. Some of the important factors influencing the flash over characteristics on the surface of solid insulators in compressed SF6 [53]. R. M. Radwan [54] et al stated that the Solid insulating spacers are one of the critical components affecting reliable performance of Gas Insulated Systems. The breakdown strength of GIS is strongly influenced by the roughness of the spacer's surface and defects produced from improper manufacturing. Also, GIS are likely to be contaminated with non-conducting and conducting particles, produced during mechanical abrasion or arcing occurring during operation of the isolating switches and circuit breakers. The presence of a conducting particle in a GIS can strongly influence the dielectric performance of the system. This depends on the type, location and density of the particles. Studies reported on scaled models and on an actual spacer with a particle fixed on its surface have revealed drastic reduction in the system breakdown voltages. Therefore the knowledge of the electric field intensity around the spacer's defects and conducting particles on its surface contributes towards better understanding of its surface flashover phenomenon [55]. The Finite Element Method has been employed to compute the electric field at the dielectric interface. It is an efficient technique for solving field problems. The following conclusions are drawn:

25 25 The electric field on the spacer's surfaces is strongly affected by the presence of surface defects. It may increase to almost 150 % or 185%from its value without protrusion or depression respectively. There is no noticeable effect of changing the defect s position on the electric field intensification; the Defect Field Factor (DFF) is almost constant at 1.5 and 1.85 for protrusion and depression respectively. The electric field on the spacer's surfaces is also strongly affected by the presence of conducting particles. It increases at the particle location on the spacer's surface. It reaches almost 1.3 its normal value for a particle of 2 mm. The Particle Field Factor (PFF) is almost constant at 1.15, wherever the particle is located, for a particle of 1 mm. The electric field at the spacer's surface decreases, at the particle location, with the increase of the vertical elevated distance of the flying particle, and it is almost negligible when the particle elevated distance "hp" is 5 times the particle's diameter. For an adhered particle, the electric field reaches almost 3.6 its normal value for a particle of 2 mm. Also the PFF is almost constant at 3.6, wherever the particle is located for a particle of 2mm.

26 26 Hideo Fujinami [56] et al discussed that the Mechanism and effect of the dc charge accumulation on the surface of solid insulating support (spacer) have been studied in compressed SF6 gas, using various cylindrical model spacers [57-59]. The distribution of surface charge has a close relation with the normal component (gas side) En of electric field on the spacer surface. The maximum charge density can be estimated from the condition of En = 0. When voltage is applied in a polarity opposite to pre-stressed dc, surface charge increases the maximum field strength in the arrangement, thus resulting in the reduction of the insulating ability. It is possible to estimate the lowest flashover voltage due to surface charge only from numerical field calculations [60]. An anti-charging spacer shaped along electric lines of force has been proposed and studied. Mechanism and effect of the dc charge accumulation have been studied in compressed SF6 gas, using various cylindrical model spacers. The main conclusions are as follows. The surface charge distribution on a spacer has a close relation with the normal component (gas side) En of the electrical field on the surface, and is also influenced by the surface roughness. Possible causes of surface charge are (a) micro discharge or field emission from surface projections, (b) motion of dust particles, and (c) natural ionization of SF 6 gas in a prolonged time range. Charge carriers drift through the gas along electric lines of force

27 27 up to the maximum charge density on the spacer surface given by the condition of En = 0. Numerically calculated results of maximum charge density showed good agreement with the experimental values. The flashover voltage of a spacer with surface charge can be estimated by composing the two fields due to the surface charge and due to the applied voltage without charge. It is possible to estimate the lowest flashover value that is in the safest side, only from numerical field calculations. An anti-charging spacer profile which has no normal field component on the surface was proposed and verified experimentally. T. Nitta [61] et al stated that a technique to design, fabricate and test dc gas-insulated switchgear has been developed to apply the advantages of compressed gas insulation to metal-enclosed HVDC equipment [62]. Charge accumulation on solid insulators is one of the fundamental problems which have to be solved in establishing the design stress for HVDC equipment. The present theory is a review of the studies which have been performed in the development of a +125 kv HVDC gas-insulated converter station and +500 kv HVDC-GIS. The properties and mechanisms of surface charging, the optimum design of the spacer and its breakdown characteristics are summarized. Problems associated with capacitive probe measurement of surface charge and a practical solution to obtain the charge

28 28 distribution on the spacer, are presented. Charge accumulation on spacers in HVDC gas insulation has been studied as one of two critical factors in designing the insulators applicable to 550 kv HVDC- GIS. From the results, the following conclusions have been drawn [63-65]. The analytical computation method to evaluate charge distribution on a conical spacer from the capacitive Outputs has been developed. At present the method is the only way to transform the probe measurements into charge densities on the spacer surface. In an industrially clean system, the charge carriers are transported from the surface of the conductor and the sheath to the surface of the spacer through the gas phase. Negative charges due to field emission from micro protrusions and/or micro dust attached to the highly stressed parts of the conductor and the sheath is the source of charged carriers. We should design the physical configuration of the spacer and the electrodes for DC-GIS in such a way that the surface of the spacer intersects the electric field lines in acute angle as possible. We should take care to avoid the local enhancement of the electric stress on the conductor and sheath. This practice is different from avoiding the sharp edges which have influenced the insulation design of AC-GIS.

29 29 The conical and post spacers which are selected as the optimum design for HVDC-GIS exhibit satisfactory results even at dc polarity reversal. M.M. Morcos [66] et al observed that the use of compressed gas as the insulating medium has made it possible to use compact equipment compared to that with air insulation. However, the compact construction increases the operating field intensity. Sulphur hexafluoride (SF 6,) gas insulation is extremely sensitive to local increases in electric field, which results from protrusion on electrode, triple junction (the region where the electrode, insulator and SF 6, gas meet) in compressed gas, the presence of conducting particles in gas insulation, and the shape of spacers supporting the conductor inside its grounded casing. The influence of a metallic particle attached to the spacer is particularly significant in the decrease of the dielectric strength of the SF6, [67] insulated system. Therefore, for development of highly reliable compact gas-insulated systems, it is vital to reduce the effect of metallic particles. The flashover withstand of a gas spacer interface is a limiting factor in the design and operation of a SF6, gas insulated system. The surface flashover shows a strong sensitivity to the metallic particle contamination [68] of the spacer surface. The particle may cause a flashover at a small fraction of the clean gas gap breakdown voltage. The particles initiate spacer flashover at low voltage values, not only for AC and DC voltages, but also for impulse and oscillating impulse voltages. Therefore, it is reasonable when

30 30 commissioning a SF 6 gas-insulated system to carry out tests with a voltage wave form for which the particle-contaminated insulation is more sensitive and/or to use diagnostic measurements in order to detect the presence of particles. H.Maekawa [69] al presented that the detecting of Partial Discharge (PD) in gas insulated switchgear is one of important monitoring terms. Authors studied the behavior of surge due to PD in gas insulated switchgear. PD can be detected by catching the electromagnetic wave radiating from the insulated spacer between enclosures, because coaxial between central conductor and enclosure is not completely in insulating spacer part. Antenna can be caught the electromagnetic wave by catching the several waves from different propagating paths. In the same time, PD location [70] in GIS can be estimated. In this system, they have developed and applied to actual 500kV GIS and well performed. In the study, obtained results are summarized as follows. PD in GIS can be detecting by catching the electromagnetic waves with detecting antenna placed by insulated spacer. By finding the arrival timing of signals obtained by each antenna and calculating the time domain differences among them, the location of PD can be estimated easily. D.A. Mansour [71] et al showed that the high reliability, less maintenance and compact size of Gas Insulated Switchgears have

31 31 made them the primary choice for many utilities. However, sometimes insulation defects inside GIS can be a serious threat to safe operation of GIS and can lead to costly disruption of supply. As insulation failure usually starts with partial discharge (PD) activity, author s investigates the differences in PD characteristics [72] in SF6 gas among different types of defects. The defect types considered in this study are particles in a gas gap; particles adhered on a spacer surface and spacer/electrode detachment. Different experiments were made for sequential PD measurements [73] using the system of PD-Current Pulse Waveform Analysis [74] (PD-CPWA). The PD phase characteristics, PD pulse number and PD current were analyzed for the different defect types. Also the ratio of voltage increment to phase increment at the next PD pulse appearance ( u/ φ pattern) was obtained and compared for each defect type. Experimental results shows that correct identification of defects can be achieved based on considered PD characteristics. Partial discharge characteristics were measured and analyzed with a wideband (4 GHz, 20 GS/s) measuring instrument to identify the type of different defect types inside GIS. Different electrode setups were built for simulating possible defect types in GIS. Three types of defects were examined for spacer/electrode detachment, particles in a gas gap and particles adhered on a spacer surface. Naoki Hayakawa [75] et al stated that A metallic particle appeared in a gas insulated switchgear sometimes adheres on a solid

32 32 spacer surface. If the adhered metallic particle is exposed to a surge high voltage, a breakdown (BD) may be induced. Therefore, it is eagerly demanded to diagnose its risk correctly under the service voltage by partial discharge (PD) measurement. In his research, particle-initiated surface PD characteristics were systematically studied in 0.4 MPa SF6 gas by changing the sizes of particles. PD inception voltage [76], temporal change of PD current and the PD pulse number were analyzed in detail. Furthermore, comparing with PD characteristics of particles in a gas gap, the influence of the solid insulator on the PD characteristics was clarified. It was found out that PD characteristics greatly changed with time owing to electric charges deposited on a spacer surface. PD characteristics of various metallic particles on the epoxy plate were measured and analyzed using the ultra-high speed measurement system. Temporal change of PD characteristics and dependence of PD characteristics on the particle size is analyzed. The following results are obtained. The electric field strength near the metallic particle tip was intensified extremely when a metallic particle was fixed on the epoxy plate. And PDIV decreased by about 20 ~ 30%. PD didn t appear in several cycles after the voltage application even if the applied voltage was much higher than PDIV. Temporal change of PD characteristics was extremely large immediately after the voltage application.

33 33 PD current increased with the particle diameter. PD pulse number depended on the particle diameter and time. Immediately after the voltage application, the PD pulse number decreased with metallic particle diameter. However, after several minutes, the PD pulse number started to increase with the particle diameter. Complex PD characteristics of different sizes of particles were qualitatively explained with the surface charges accumulated on the epoxy plate near the particle tip. Hirotaka Muto [77] et al studied that as a mean of diagnosing partial discharge (PD) signals propagate inside a Gas Insulated Switchgear, a study for the leakage of electromagnetic waves [78-79] (EM-waves) emitted from the insulating spacer was implemented. The EM-waves leaking out from the solid insulator have the resonance frequencies [80] depend on the spacing between adjacent bolts in the direction of the flange circumference, because the leakage portion is the equivalent of a slot antenna. In the present work using an electromagnetic analysis model which has a simulated spacer on a concentrically-shaped GIS tank, the output characteristics of the EMwaves that leaked out from the slit ere analyzed under various conditions such as the spacing between adjacent bolts the width of the spacer, the dielectric constant of the spacer and the form of the flange. Also the actual measurement by the experimental equipment used to simulate the model was implemented for comparison with the

34 34 analytical results. Consequently, the optimal specifications of the sensor and the measurement method used to achieve highly-sensitive detection for practical use are summarized and proposed as well as evaluating the effectiveness of the electromagnetic analysis model are adopted. Katsumi Kato [81] et al describes that the applicability of the FGM spacer to gas insulated power equipment. In the FGM spacer, they gave the spatial distribution of dielectric permittivity to control the E- field distribution inside and outside the spacer. Firstly, E-field simulation results when applying the FGM by a finite element method are presented, in which they show that effective reduction of the maximum field strength by applying the FGM. Next, a fabrication technique of the FGM spacer sample with not only step-by-step but also continuous changes of permittivity [82] is presented by use of centrifugal force. Finally, authors proposed the application of FGM as a spacer material for gas insulated switchgears. The application effectiveness was verified by numerical simulation and experimental results and they made continuously changed distribution of permittivity and controlled it by applying the centrifugal force [83]. They optimized the fabrication condition for the permittivity distribution and these fabrication techniques are expected to be extended to future electric power equipment.

35 35 Masahiro Hanai [84], et al describes that for the size reduction and the enhancing reliability of electric power equipment, the electric field stress around insulators should be considered enough. For the relaxation of field stress, the application of FGM with spatial distribution of dielectric permittivity can be an effective solution. Investigating the applicability of FGM for reducing the electric field stress on the electrode [85] surface with contact to solid dielectrics, this was one of the important factors dominating a long-term reliability of the insulating spacer. At last they implemented the application of FGM for reducing the electric field stress on the electrode surface in contact with solid insulators, which was one of the important factors dominating a long term insulating property [86] of the solid spacer. The FGM application effect was verified by numerical simulation of electric field and life time estimation. They made U-shape permittivity distribution [87] and controlled it by applying the centrifugal force, their application duration, author s made various types of the Y-shape permittivity distribution. These fabrication techniques [88] are expected to be extended to the actual application of FGM to the electric power equipment and estimated a long-term insulation performance for the fabricated FGM sample and found the significant effect for life time extension by the application of FGM. Finally, authors verified that high performance of electrical insulation of solid spacer could be obtained by a permittivity graded FGM application.