Insulation Integrity of. GIS/GITL Systems and. Management of Particle. Contamination. Key Words: Gas-insulated switchgear, particle contamination, spacers, ...
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Insulation Integrity of GIS/GITL Systems and Management of Particle Contamination Key Words: Gas-insulated switchgear, particle contamination, spacers, electrode coating
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he presence of particle contaminants in gas-insulated switchgear (GIS) can significantly impair the integrity of the insulation. The most likely causes for such contamination are particles remaining after assembly or produced by mechanical abrasions, movement of conductors under load cycling, and vibration during shipment. Those particles may be free to move in the electric field, may be in the form of a protrusion on the electrodes, or may be strongly adhering to spacers. Many experimental results have been published involving particle contamination in uniform and coaxial fields. Invariably the experimental results pertain to horizontally mounted electrode systems. The particles studied are of many different shapes and sizes such as spheres, filamentary (wire shaped), and fine dust. Insulating particles do not pose a major threat to the insulation integrity, although in the presence of gaseous discharges they may acquire a thin conducting coating and behave like metallic particles. The withstand voltage of SF6 may be drastically reduced due to the presence of conducting particles in a gas-insulated gap [1]. Figure 1 shows the actual breakdown field at the inner conductor in percent of the theoretical value for SF6, in the presence of conducting particles [2]. The effect of metallic particles on the SF6 breakdown voltage is more pronounced at high gas pressures; the loss of dielectric strength of SF6 in the presence of 0.45/6.4 mm (0.45 mm diameter × 6.4 mm long) wire-shaped free conducting particles in a coaxial system subject to dc voltages is more than 80% compared with clean gap of the same conditions under gas pressures of more than 0.5 MPa [3]. Practically no gain exists in the gas insulation at pressures above 0.5 MPa. Attempts have been made to determine the role of conducting particles in the breakdown process of compressed gas insulation [4]. In general, the gas breakdown voltage decreases with the wire particle length, while the effect of an September/October 2000 — Vol. 16, No. 5
M.M. Morcos Kansas State University S.A. Ward University of Zagazig, Egypt H. Anis Cairo University, Egypt K.D. Srivastava University of British Columbia, Canada S.M. Gubanski Chalmers University of Technology, Sweden
In general, the gas breakdown voltage decreases with the wire particle length, while the effect of an increase in the wire diameter is not a dominant factor. increase in the wire diameter is not a dominant factor. Under the influence of the applied voltage, free conducting particles become charged and oscillate in the interelectrode gap. Particle motion largely depends upon the type of applied voltage. Under ac voltages, for a wire particle of given radius, the activity increases with particle length, since the particle charge-to-mass ratio at lift-off increases with length. This ratio decreases with the spherical particle radius and the particle movement is reduced in this case [5].
0883-7554/00/$10.00©2000IEEE
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Conductor Breakdown Field in Percent of Theoretical (%)
100
PARTICLE INITIATED BREAKDOWN 0.45 mm / 6.4 mm WIRE
80 60 Hz (Particle Free) 60 Hz Ramp
60 40
Typical GITL Operating Pressure Range
20 0
0
0.2
0.4 Gas Pressure (Mpa)
0.6
Fig. 1 Degradation in electrical insulation strength of SF 6 caused by aluminum wire particles for a coaxial system [2].
The effects of fixed conducting particles asperities on ac breakdown compared with those of free particles were examined [6]. Tests show that although free particles initiate breakdown when a particle is close to an electrode, a particle fixed on the electrode yields different ac breakdown voltage/gas pressure characteristics. This difference is associated with the free particle’s location at the instant of breakdown and some statistical effects. Free metallic particles, metallic particles attached to insulators, protrusions on electrodes, and poor contacts are the major causes of partial discharge (PD) in GIS. PDs are indicative of a possible future breakdown in GIS. Several methods have been used in checking for PD activity, the most commonly used are ultrasonic contact probes and the electromagnetic coupling devices [7, 8]. PDs in GIS can be directly detected by measuring the voltage signal using capacitive dividers installed in a GIS. They also can be detected by the measurement of tank potential oscillation, electromagnetic waves from the tank, discharge light, and decomposed gas. For free metallic particles, they can also be detected by measuring tank vibration caused by the bouncing of particles inside the GIS [9-11]. For GIS/GITL systems to be reliable and economic, the problem of particle contamination should be overcome. One philosophy of contamination control is to design the major GIS/GITL system components very conservatively, such that a certain degree of immunity can be achieved against the presence of particles. Another technique is to provide designated low-field areas in the system in the form of “particle traps” where the particles can be safely trapped and con tained [2]. Also, attention has been focused on the technique of immobilizing particles by dielectric coatings on the inside surface of the enclosure to increase the particle lift-off voltages [12, 13].
Particle Contamination Intensive experimental efforts were made to determine the effect of particle size and position in the gap on the breakdown voltage [4-6, 14]. A particle-initiated breakdown may develop only if the particle is at, or near, one of the elec26
trodes. The breakdown caused by a particle fixed on the high-voltage electrode occurs at a higher voltage than that when the particle was free [14]. The conducting particle may be spherical or wire shaped. Wire particles were chosen for most experiments rather than spheres since they simulate particles found in practical equipment. The motion of the particle is different under dc, ac, and impulse voltages. The reduction in breakdown voltage is much greater under dc applied voltages than under ac or impulse voltages.
AC Voltages
Once elevated, the particle does not immediately cross the gap, but its activity increases with increasing voltage so that only at substantially higher voltages will it cross [5, 15]. Under ac, particles can remain in the mid-gap region for long periods and can take several cycles of voltage to cross the gap. Computer simulations of the complex ac motion for spherical and wire-type particles have shown good agreement with experimental measurements of the maximum height reached by the particle as a function of applied voltage in SF6 [5]. Figure 2 shows the lift-off, crossing, and breakdown voltages for a wire particle in a parallel plane gap under applied ac voltage.
Impulse Voltages
A particle may be able under the influence of an impulse voltage to cross the gap and reach a critical breakdown zone. The ability of a particle to cross the gap may be characterized by the corresponding minimum impulse amplitude necessary for that crossing (the crossing voltage). The time to crest (front) of the applied impulse has a minor effect on the crossing voltage as only a negligible fraction of the motion develops during that time. In a computational simulation, for an impulse of a constant wave tail of 10,000 µsec applied to an SF6 coaxial 30 mm/50 mm gap with spherical aluminum particles, the crossing voltage increases by less than 10% as the front is increased from 0 to 5000 µsec. On the other hand, the wave tail has a drastic effect on the crossing voltage. The crossing voltage of a particle may drop by as much as 70% as the tail is increased from 3,000 to 15,000 µsec [16].
DC Voltages
With increasing direct voltage, a particle is first lifted by the field and then crosses the gap to the other electrode. As the field is increased, the particle will move back and forth between electrodes with increasing velocity. However, after the particles have crossed the gap many times, they will sometimes hover at the negative electrode, even at high field stresses, with intense corona and emitting visible light. This phenomenon is known as “fireflies” [17], which can have a marked effect on the breakdown characteristics in coaxial electrode systems [4]. With direct voltage applied to GIS, the electric field is oriented in one direction, causing charges to accumulate in the insulating spacers (acting as supports and insulators of the IEEE Electrical Insulation Magazine
high-voltage conductors), which results in metallic particles being readily deposited on the spacers. This effect, where a reverse-polarity surge voltage is applied, for example, is likely to decrease the dielectric performance of the spacers, which has been the subject of extensive studies on applied dc voltages in SF6 GIS [18-21].
Particle-Initiated Gas Breakdown A metallic particle in contact with an electrode acquires a charge dependent on the applied field and particle size, and at a particular field will be lifted and move towards the opposite electrode [5, 16, 22]. Three mechanisms that explain particle-initiated breakdown are: i) The field at the particle may be sufficient, before the lifting field is reached, to initiate breakdown similar to the case of a fixed particle. ii) The particle may lift, cross the gap to touch the other electrode, and initiate breakdown similar to a fixed particle at that electrode. iii) After the charged particle lifts off it approaches the oppositely charged electrode where a discharge takes place between the charged particle and that electrode. The discharge is facilitated by the field intensification, which is higher on the particle side facing that electrode than on the side facing the similarly charged electrode. The result is a conducting link between the particle and the electrode. This link is assumed to have negligible resistance. A second breakdown may then occur in the remainder of the gap since the applied voltage now appears across that section [4, 5, 22]. The computation of the breakdown voltage for the particle contaminated GIS was based on the streamer formation criterion, which assumes a streamer to form when an electron avalanche, produced from microdischarges on the particle tip, reaches a critical size. That is,
An analytical model was developed to describe the performance of particle-contaminated compressed gas insulation. This model, which is termed Breakdown Voltage Profile (BDVP), should take into account the wide practical variety of particle shapes and sizes, insulation configurations, applied voltage magnitudes and waveforms, and gas pressures [24]. The breakdown voltage profile is the instantaneous applied voltage required to break down a particle-contaminated gas insulation as a function of particle position along its path in the gap. Figure 3(a) shows breakdown voltage profiles of a wire particle in a parallel-plane gap. The authors have extended the concept of a breakdown voltage profile to include corona stabilization [25]. The modification is necessary only for a positive point breakdown [26, 27]. For all pressures less than the critical pressure, the radius of the corona cloud replaces the original particle radius in estimating the breakdown voltage [25].
570 kV 500
Breakdown, 1.6 mm SF6
Extrapolated from 51 mm Gap
400
∫ [α( E, P ) − η( E, P )]dx ≥ k
x1
where x1 and x2 are the avalanche beginning and end positions, and α and η the coefficient of ionization and attachment, respectively, both being functions of local field E and gas pressure P. The constant k is taken for SF6 to be 10.5 [23]. The streamer formation criterion gives reliable results only for particles with sufficiently large diameters and at high pressures. At low gas pressures and with pointed particles, corona stabilization may occur; this would generally raise the breakdown voltages. The location of a particle at the instant of breakdown is very important. Measurements with fixed and free particles suggest that breakdown occurs when the free particles are near but not touching the electrode [6]. In SF6 uniform fields the field strength in which the particle moves is not a function of position; nevertheless the particle is always near or in contact with an electrode at breakdown. In the SF6 coaxial-electrode case a particle fixed on the inner conductor is not equivalent to a free particle at the conductor. This critical distance between the particle and the electrode decreased from ~2 mm to very small values as the SF6 gas pressure is varied from 0.1 to 0.8 MPa, and does not increase further at pressures up to 1.46 MPa [6]. September/October 2000 — Vol. 16, No. 5
Voltage (kV rms, 60 Hz)
x2
300
Crossing, 1.6 mm
200
Breakdown, 6.4 mm
Crossing, 6.4 mm 100 Lift-off
0 0
0.5
1.0 Pressure (MPa)
1.5
Fig. 2 Liftoff, crossing, and breakdown voltages in SF 6 with 0.45 mm diameter copper wires, 1.6 mm (o) and 6.4 mm (∆) long in a 76 mm parallel-plane gap. The scatter in the results is for six measurements at each level [5].
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When the effect of corona stabilization is not considered in the calculations, the profile is denoted as the corona inception voltage profile. Figure 3(b) shows both the breakdown and corona inception voltage profiles at two different gas pressures, namely, 0.3 and 0.7 MPa, for a 0.45/6.4 mm wire particle in a coaxial, SF6 insulated gap [10]. When multiple free particles of the same dimension were involved in the breakdown path, the breakdown voltages were not significantly lower than for the cases where only a single free particle was used. Breakdown is initiated by one particle and the breakdown level is not affected by whether a second particle also happens to be involved later in the breakdown arc process. It is the particle nearest to one of the electrodes, in parallel plane gap or the central conductor in a coaxial gap, that will initiate the discharge process. When particles of different dimensions are present in the co-axil gap, it is the longest particle that will determine the breakdown voltage.
Behavior of Particles with Spacers In all electrical power apparatus there is at least one insulating surface subjected to the full voltage rating of the system. The dielectric performance of insulating gases is adversely affected by the presence of spacers, unless special design precautions are taken. Spacers of different shapes and designs are currently in use and the overall dimensions have largely been determined by system voltage, load current, and possible loss of pressure. Design constraints for higher-voltage systems may make the spacer design more critical [4]. Many designs of GIS/GITL systems incorporate some low field areas to trap these particles and prevent them from reaching the spacers [28, 29]. The spacer field typically has been limited to less than 4 kV/mm rms, based on voltage
Instantaneous B.D. Field (kV/cm)
Sphere
200 300
Wire Radius = 0.2 mm Parallel-plane SF6 Gap 0.5 MPa 150
150 0.5 1.0
100
100
1.5 3.0
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0 Lower Electrode
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1.5 1.0 0.5 Distance from Electrode (mm) (a)
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Instantaneous B.D. Voltage (kV)
Length (mm) 0
200
endurance data. However, with improved materials and processing, high stress designs of 5 and 6 kV/mm rms have been developed [30-33]. The maximum permissible spacer surface field is dependent on the SF6 gas pressure, cleanliness, and the spacer design. Most designs use the criterion of the resultant field at the dielectric surface, equating this to the maximum conductor field in the gas. Generally, there has been good agreement with the calculated flashover voltage under very clean conditions [34-42]. This means the spacers are designed (with electrostatic field plots) for a critical field value either at the spacer surface or at a conductor or adjacent shielding electrode [1]. However, there are some indications that with surfaces contaminated with particles, it is the tangential field that is critical and not the resultant field [43]. Thus, the optimum spacer configuration may involve a compromise between the tangential and perpendicular stresses on a spacer surface. All GIS/GITL systems in service use cast epoxy spacers. In order to avoid the interface problem with spacers, where voids or small gas gaps can initiate volume puncture or surface flashover [34, 44, 45], the spacers are either cast directly onto the conductor or metal inserts, or have the interface well shielded. Many different spacer shapes are in use and have been proposed [4]. Special field computation software is used to optimize the profile of spacers in GIS/GITL. The influence of the impulse voltage wave shape on the flashover voltage of a spacer in a SF6 gas-insulated system has been investigated for both clean and conducting particle contaminated conditions [46]. Under clean conditions, the standard 1.2/50 µsec lightning impulse flashover voltage is the highest. With increasing front time the impulse flashover voltage decreases. The effect is more pronounced at higher
PRESSURE (Mpa) 0.7
0.45 mm / 6.4 mm WIRE 70 mm / 190 mm COAX
250 0.3 0.7
200 150 100
0.3 Breakdown Voltage Profile Corona Inception Voltage Profile
50 0
0
1.5
1.0 Distance (mm) (b)
0.5 0 Inner Electrode
Fig. 3(a) Breakdown voltage profiles of a wire particle of variable length in a parallel-plane SF 6 gap at 0.5 MPa pressure [24]. Fig. 3(b) Breakdown voltage profiles and corona inception voltage profiles of an aluminum wire particle (0.45 mm diameter × 6.4 mm long) in a 70 mm/190 mm coaxial SF 6 gap [10].
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IEEE Electrical Insulation Magazine
pressures. In the presence of a conducting particle on the spacer surface, the standard 1.2/50 µsec lightning impulse flashover voltage is lowest. With increasing front time the impulse flashover voltage increases and reaches a maximum at a front time of about 100 µsec. At a further increase of the front time, the flashover voltage decreases slightly. The reduction of the spacer flashover voltage, when the conducting particle length is increasing, is different for impulse voltages of different waveshapes. For a 14 mm long particle, for example, the standard 1.2/50 µs lightning impulse flashover voltage of the spacer is reduced by 70%, while the standard 250/2500 µs switching impulse flashover voltage is reduced by about 35% [46]. The influence of freely moving wire particles on ac surface flashover of epoxy and acrylic disk spacers in a 76 mm/250 mm diameter coaxial system has been examined in SF6 gas at pressures of 0.3 and 0.5 MPa. The experimental results show that 0.45/6.4 mm aluminum wire particles will lift and reach a nearby spacer at voltages comparable to the system operating voltage. The combination of metallic particles and spacer does not reduce the breakdown voltage below that free particles without a spacer, unless a particle is lodged on the spacer very close (< 2mm) to the center conductor. The frequency of occurrence is much greater for horizontal disk or conical spacers than for vertically mounted disk spacers [47]. It is noted that with the step-by-step method of voltage application the minimum value of the breakdown voltage is significantly lower than when the ramp test voltage is applied. The difference between the average values of the breakdown voltage at 0.3 MPa and 0.5 MPa is insignificant. Further, the spread in breakdown voltages and the corresponding average value were not influenced by the number of free particles used in the experiments at 0.5 MPa gas pressure.
Free Particles
A study was carried out experimentally under ac applied voltages for different shape of spacers [47]. From the experiments conducted, it appeared that free conducting particles near a vertical disk spacer might initiate breakdown through a two-stage process, similar to free particles without a spacer. The ranges of breakdown voltages for the two conditions are also similar. The manner of voltage application has a significant effect on conducting particle dynamics. In the experiments with a ramp and systematic voltage application, the breakdown voltage ranges observed were 209 to 280 kVrms and 170 to 239 kVrms, respectively. The systematic method is usually considered more effective for particle trapping at lower voltages but, as the above voltage ranges show, is detrimental at higher voltage. Free conducting particles on a horizontal disk spacer surface may give rise to very low breakdown voltages if a particle happens to be very close (
