RU2456732C2 - Method for protection of insulation clearances in liquid dielectric against electric breakdown with help of mesh screens with controllable electric potentials - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: method for protection against electric breakdown is based on retardation of pre-breakage phenomena in insulation clearances around the conductor. Supplied through active resistances between the mesh screens and the conductor being insulated are corresponding differential electric potentials that, in case of breakdown occurrence in one insulation layer, prevent its further development in the adjacent layers. Breakdown retardation enables complete preservation of the layer insulation properties, the equipment continuing normal operation. The protection method enables complete usage of liquid dielectric service life.

EFFECT: design and production of new generation equipment that will not become disabled in case of breakdown.

12 cl, 6 dwg

Description

FIELD OF THE INVENTION

The invention relates to applied electrical engineering. More specifically, it relates to methods (methods) of protection against electrical breakdown of bushings and internal conductors (electrodes, parts) in high-voltage transformers filled with liquid dielectric, autotransformers, current transformers, and other electrical equipment.

State of the art

To protect the electrical conductors that are located inside the body of electrical equipment filled with a liquid dielectric, paper-oil insulation (BMI) has been used for over 50 years now [1]. The essence of this insulation is that the surface of the insulated conductor is covered with paper, which is impregnated with a liquid dielectric. To increase the breakdown voltage, BMIs are also used with relatively thin several layers that alternate with electrically conductive layers of metal foil (mesh), the so-called paper-oil capacitor insulation (BMCI). [1]. During operation, breakdowns of sufficiently long insulating gaps often occur in the conductors input devices and inside the case. Such breakdowns of insulation in a liquid dielectric are accompanied by large shock waves, which in all cases lead to the destruction of high-voltage transformers, see photo Fig. 1.

There are no other experimentally justified solutions for protection against breakdown of insulation of internal electrical conductors in a liquid dielectric. Existing methods for monitoring the insulating properties of a liquid dielectric are completely dependent on the human factor, which determines the accident in almost 100% of cases.

Figure 2 [1] shows the electrical tension of the plot in oil in the BMI at Eb = 2.5 kV / mm (curve 2) in comparison with the electrical strength of transformer oil (curve 1). BMI can be applied to individual sections of the electrodes, so-called partial paper-oil insulation (BWMI) is obtained. For example, the thickness of BWMI for Russian current transformers of type TFN is: for voltage 35 kV -16 mm, for 110 kV - 54 mm, for 154 kV - 67 mm, for 220 kV - 100 mm. It is seen that the thickness of the BMI makes the internal conductors massive. While the strength of the whole BWMI is determined by the breakdown strength of the oil layer [1].

The closest to the destination prototype of the present invention is BMKI [1, 7]. It is performed by winding paper onto an insulated part (conductor, electrode) until a layer of a given thickness is obtained and applied over this layer of conductive (foil, mesh) or semiconducting lining. Then again the paper layer is wound and over it a lining, etc. Thus, the thickness of the entire paper insulation is divided by conductive (semi-conductive) plates into separate layers. The last outer lining must be earthed, that is, connected to the housing of electrical equipment. Then drying and impregnation of the insulation layers with oil are carried out. All insulation of the conductor is a system of series-connected capacitors acting as a capacitive voltage divider. The voltage between the housing and the conductor is divided inversely with the capacitance values of the series-connected capacitors of this divider. Moreover, the smaller the BMI layer between the nearest plates, see the graph in figure 2 [1], the greater the voltage is maintained by each individual capacitor. And the greater the voltage can withstand the entire insulation of the conductor as a whole, which, ultimately, can reduce the thickness of the insulation.

From the circuit diagram of a capacitive voltage divider, it is clear that when one of the capacitors is broken, if a breakdown of the paper-oil layer caused a short circuit between the plates, the voltage between the electrode and the housing is redistributed between the non-broken layers (capacitors in the divider). The voltage on each insulating layer of BMKI increases, and then the breakdown of the next weak layer occurs (with the least electric strength of the material). An analysis of the breakdown mechanism in the BMCI layer shows that, due to the dense medium of paper saturated with oil, significant pressures and temperatures arise in the breakdown channel under the influence of an electric discharge. The decomposition products of a liquid dielectric under pressure have the ability to move to a greater extent in the layer only along the breakdown channel, see photo in Figure 3 [2]. During breakdowns of liquid dielectrics in strong local fields of ~ 1-100 MV / cm (for various liquids), anisotropic instability [3] is probably the key mechanism for the nucleation of streamer structures and their rapid propagation (experimentally observed velocities of more than 100 km / s [4] ) in the form of thin branching channels, on average oriented along a local electric field [3]. In the process of gas phase formation, electrical breakdown occurs in one of these channels. It was noted in [4] that at a discharge energy of up to 1.2 · 10 ³ J and a current rise rate in the plasma channel of ~ 10 ¹¹ A / s, after the breakdown of the liquid, the plasma of the discharge channel having an initial temperature of ~ (1-2) · 10 ⁴ K, expands at a speed of the order of (3-5) · 10 ³ m / s (Ushakov V.Ya.). This leads to the nucleation and evolution of shock waves, accompanied by an increase in the pressure drop at the front, at distances of about 3-5 mm from the axis of the plasma channel (K. Vilkov, Yu.A. Nagel).

In a layer of paper insulation impregnated with a liquid dielectric, the density of the medium around the breakdown channel is much higher than in a single liquid dielectric. Consequently, the pressure of the medium in the breakdown channel of the BMKI with the same energy release will be greater. In BMKI, decomposition products in the form of a multiphase flow (a mixture of gases, paper solid particles, liquid droplets, ions) move along the breakdown channel in opposite directions from the pressure center at a high speed. After the breakdown of BMKI, some of the decomposition products of insulation necessarily remain in the breakdown channel, which leads in almost all cases to the closure of the plates. Due to the dynamic pressure of the multiphase flow and inertia forces, the decomposition products, moving along the breakdown channel in opposite directions from the center of the layer, abut against the conductive plates of the BMKI layer. The materials used for gaskets do not have sufficient strength to withstand temperature and pressure that arise at the ends of the breakdown channel. The decomposition products penetrate into the adjacent layers of BMCI through the plates of the punctured layer, and these outputs of the breakdown channel in the neighboring layers, see photo of Figure 3, become the centers of formation of subsequent breakdowns. An electric discharge instantly flashes layer by layer. Thus, a random breakdown in one capacitor (weak layer) creates the conditions for a chain series of breakdowns in the adjacent layers of BMCI until all insulation of the conductor is destroyed.

The analysis of accidents that were caused by breakdowns of high-voltage bushings in transformers confirmed the conclusion that in all cases all insulation of the conductor in the transformer oil is completely destroyed. Breakdown in the BMKI layer is irreversible, that is, the insulating properties are not restored after disconnecting the voltage. Moreover, since the breakdown current is uniquely determined by the operating voltage and resistance in the short circuit, the explosion power during breakdowns of the inputs is extremely large. Cases of input devices are destroyed in 100% of cases of accidents caused by such breakdowns. The design of non-destructible casings of input devices and shock absorbers in transformers solves only part of the problem - fire safety, but does not solve the problem of the reliability of the equipment as a whole. That is, there will be no fire and no release of oil into the atmosphere, but the equipment will fail.

It is known that during breakdowns of a liquid dielectric layer, without any fillers (paper), the insulating properties of the interelectrode gap are restored. The operating experience confirms that during breakdowns of a liquid dielectric between the turns of the transformer windings, which make up about 48% of cases of accidents at high-voltage substations, the insulation gap restored its properties after disconnecting the load. And after switching on, the equipment could continue to work under load.

The dielectric strength of a liquid dielectric is not directly related to conductivity or dielectric loss, but it is highly dependent on the presence of impurities. The relative electric strength of the insulating gap filled with a liquid dielectric increases with decreasing gap gap and, as a rule, decreases with increasing dissolved water, the presence of gas bubbles, ionogenes, ionophores and other oxygen organic compounds arising both from energy exposure and the result of its aging [5] during operation. In BMKI between layers, all these harmful accumulations are preserved and are not washed out during operation into the bulk of the liquid dielectric. It is difficult to establish the dependence of the deterioration in the dielectric strength of a liquid dielectric in a BMKI composition on the operating time. The scatter in determining the dielectric strength of a liquid dielectric even without a filler is large [5]. And a lot depends on the nature of the insulating liquid and paper, and for mineral oil - on its structural-group composition. Due to the ability to form charged micelles and conducting bridges, water has a more significant effect on the breakdown of oil if it is in a colloidal state. It is generally impossible to remove all these harmful accumulations in layers of oil-soaked paper from working electrical equipment, except by using the equipment for repair and complete replacement of the BMKI.

SUMMARY OF THE INVENTION

The method of protection against electrical breakdown of insulating gaps in a liquid dielectric using grid screens with controlled electric potentials is based on the inhibition of pre-breakdown phenomena in the insulation layers (liquid dielectric layers without paper) around the insulated conductor. This is achieved by supplying between the insulated liquid dielectric from each other the grid screens (plates) and the conductor the minimum possible (locking breakdown) difference in electric potentials or, simultaneously with the occurrence of breakdown, supplying such a difference in electric potentials that is less than the electric strength of the insulation gap. The control electric potentials are formed by two resistive voltage dividers, see Figure 4, one of which is connected to the housing and the mesh screens of the outer group, and the other to the conductor and the mesh screens of the inner group around the conductor. The active resistances (resistors) of the dividers are set so that, during normal operation of the insulation (in the absence of breakdown), the differences in electrical potentials between the grid screens in the external and internal groups are minimized (blocking), and the operating voltage is applied almost completely to the loaded (middle) insulation layer (or layers with intermediate grid screens). In the event of pre-breakdown phenomena or breakdown in the loaded (middle) insulation layer, the difference in electric potentials between the grid screens in the groups is determined by the parameters of the resistive dividers. The parameters of resistive dividers are such that when the breakdown current flows through them through the loaded layer, the voltage on the dividers is redistributed and the grid potentials adjacent to the loaded layer receive electric potential differences less than the electrical strength of these gaps than completely breakdown them. The punctured loaded insulation layer is instantly replaced by two fresh adjacent insulation layers with smaller differences in electrical potentials on each of them. Thus, insulation gaps with less electric field strength are created on both sides of the punched layer, which prevents further development of breakdown in neighboring insulation layers (insulation gaps). Resistors of voltage dividers in the chains of the mesh screens at the same time limit the breakdown current, as a result of which they reduce the power of the shock waves from the breakdown of the loaded insulation layer, and its mesh screens are not destroyed. Short circuiting of the layer does not occur. Due to current limitation, the breakdown quickly goes out. After some time, the insulating properties of the gap with the liquid dielectric (loaded layer) are restored and the equipment can continue to work. The current through the loaded layer and voltage dividers stops, and the distribution of electric potentials on all grid screens is restored to the state that was before the breakdown. The protection method eliminates the possibility of accumulation in the insulation gaps (insulation layers) during the operation of the aging products and the decomposition of the liquid dielectric. To do this, use mesh screens and spacing spirals, hydrodynamically transparent for a liquid dielectric, as well as openings in the mesh screens for circulating a liquid dielectric, which in combination allows you to develop the entire service life of a liquid dielectric, which is located in the body of electrical equipment. A liquid dielectric circulates in the insulating gaps (insulation layers) between the mesh screens around the conductor and is freely replaced by a fresh liquid dielectric from the surrounding volume.

The experiments carried out by the authors of the present invention on a test chamber, the appearance and location of the mesh screens in which are shown in FIGS. 5 and 6, confirmed the feasibility of implementing this method of protecting insulation gaps from breakdown.

The method of protection against electrical breakdown of insulating gaps in a liquid dielectric using grid screens with controlled electric potentials includes the following known operations:

- coating the surface of the conductor electrically isolated between itself and the conductor layers of the electrically conductive lining - mesh screens (metal mesh);

- installation of a conductor inside the case of electrical equipment (electrode, parts), its connection to other conductors and parts;

- connection of the external mesh screen with the housing of electrical equipment;

- filling the internal volume of the housing with a liquid dielectric;

- the inclusion of a working electrical voltage between the conductor and the housing; characterized in that:

1. The mesh screen is made in the form of a stocking (without a seam) with a grid cell in the form of a square (rhombus), the diagonal of which is placed along the axis of the insulated conductor, which allows one conventional diameter of the stocking to be used for all insulation layers;

- the conditional diameter of the stocking is chosen based on ensuring a tight fit of the insulated conductor along its entire length, including in its smallest and largest sections (at a bend, a lap weld point, a bolt joint, etc.).

2. The spacing and isolation of the mesh screens between the conductor and between them is performed by spirals made of an elastic solid dielectric that is compatible with a liquid dielectric;

- an elastic, pore-free, non-absorbing gas insulator is used as a material for spacing spirals, as a result of which there is no need for laborious drying of insulated conductors, electrodes and parts for electrical equipment.

3. The nominal diameter of the spacing spiral, its thickness, the pitch of the turns and the length are selected based on ensuring a tight fit over the insulated conductor and all layers of the mesh screens along their entire length to create the required thicknesses of the insulation gaps between the conductor and the mesh screens, taking into account their slack between the turns.

4. The actual diameter of the stocking of the mesh screen for a particular layer is obtained by longitudinal stretching or compression by pulling it on an insulated conductor over spacing spirals, as a result of which stockings of different nominal diameters are not required.

5. Remote spirals are pulled over the conductor and the mesh screens closely with the overlap of the turns of one spiral into adjacent spirals, which ensures the required thickness of the insulation gap between the conductor and the mesh screen and between the mesh screens along the entire length of the conductor.

6. The mesh screen and spacing spirals are made hydrodynamically transparent to the liquid dielectric, in the volume of which the insulated conductor is placed, which ensures the continuous washing out of the aging products and decomposition of the liquid dielectric from the insulation gaps between the mesh screens around the conductor into the main volume of the liquid dielectric inside enclosures of electrical equipment, resulting in a quick recovery of insulation after breakdowns (insulation gaps with a liquid dielectric).

7. The mesh screens around the insulated conductor are connected to at least two resistive voltage dividers in this way, see Figure 4, that one divider is connected to the casing and the mesh screens of the outer group, and the other is connected to the conductor and the mesh screens of the inner group around the conductor;

- in the volume of liquid dielectric between the grid screens of the inner and outer groups, at least one insulating gap with the required electric strength (hereinafter referred to as the loaded layer) is formed, loaded with operating voltage, and insulating gaps adjacent to it, which are not loaded with operating voltage, lying in the directions to the surfaces of the insulated conductor and hulls (hereinafter referred to as unloaded layers);

- the number and thickness of loaded and unloaded insulation layers is determined from calculations of the required strength and the probability of breakdown of the entire insulation.

8. The parameters of the resistive voltage dividers are set so that they limit the breakdown current of the loaded layer to a value that excludes damage to the grid screens and, if there is no current through them from the breakdown of the loaded layer (and / or leakage current of the insulation), control signals are fed to the grid screens of the inner and outer groups electrical potentials that create minimal electric field strengths on unloaded layers are such that almost all of the operating voltage is applied to the loaded insulation layer.

9. The parameters of resistive voltage dividers are set such that, when a current arises from them from the breakdown of the loaded layer, simultaneously with its growth, control electric potentials are generated on the grid screens of the inner and outer groups, creating at least two insulation layers closest to the punctured layer, the difference in electric potentials is much smaller than the breakdown voltage so that almost all of the operating voltage is transferred from it to neighboring layers during the restoration of the insulation of the loaded layer isolation.

10. Simultaneously with the inclusion of voltage between the conductor and the housing, resistive dividers supply grid screens to the electric screens, which, in the absence of breakdown of the loaded layer, create on the unloaded insulation layers the minimum differences in electric potentials, which exclude aging and decomposition of the liquid dielectric;

- in the event of a breakdown of the loaded layer (and / or the occurrence of a leakage current through it), simultaneously with an increase in the breakdown current, reduced (blocking the breakdown) electric field intensities are created at least at the two nearest adjacent layers, which prevents further breakdown development, at the same time, subsequent adjacent layers (not loaded before breakdown) load with all operating voltage;

- the protection method reduces the likelihood of a complete breakdown of the entire insulation to almost zero.

11. In the loaded and unloaded insulation layers, in order to increase their electric strength, passive mesh screens isolated from each other by spacing spirals are installed, which are not connected to resistive voltage dividers;

- all mesh screens are provided with holes, the location of which enhance the circulation of the liquid dielectric in the insulation gaps.

12. The number of mesh screens, insulation gaps, passive mesh screens is determined based on the properties of the liquid dielectric, the operating voltage and the required probability of insulation breakdown.

Industrial applicability

The invention can be applied in the design of bushings of high voltage voltage transformers, current transformers and other electrical equipment in order to eliminate electrical breakdowns in them, leading to failure. The use of the invention improves the operational reliability and safety of electrical substations.

Literature

1. Handbook of high voltage electrical apparatus. I. M. Adoniere, V. V. Afanasyev, et al. Ed. V.V. Afanasyev. - L: Energoatomizdat, Leningrad. Department, 1987, 544 s. with silt.

2. Melekhov A. V. Research of prebreakdown cathode processes in distilled water. / S.M. Korobeinikov, A.V. Melekhov, V.G. Posukh, A.G. Ponomarenko, E.L. Boyarintsev, V.M. Antonov. // Materials 13 Int. school-seminar "Physics of pulsed discharges in condensed matter", Nikolaev, 2007. - S.11-13.

3. Kuperstokh A. L., Medvedev D. A. Electro-hydrodynamic instability of liquid dielectrics in strong electric fields and decay into an anisotropic two-phase liquid-vapor system. Reports of the Russian Academy of Sciences, 2006, volume 411, No. 6, pp. 766-769.

4. Grigoriev A.L. The formation of shock waves by pulsed electric discharges in water and the study of their impact on obstacles. Abstract of dissertation for the degree of candidate of technical sciences Specialty: 02/01/05: Mechanics of liquid, gas and plasma. Moscow. 2007.

5. Arakelyan V.G. Goals, concepts and general principles of diagnostic control of high-voltage electrical equipment // Electrical Engineering. 2002. No5. S.23-27.

6. Korobeinikov S. M., Melikhov A. V., Ganenko K. B. Behavior of bubbles in perfluorotriethylamine under the influence of strong electric fields. Thermophysics of high temperatures. - 2002. - T.39. - No. 6. - S. 885-889.

7. RU 2004114918/28 of 05/18/2004. A device for monitoring the condition of equipment with paper-oil insulation of a capacitor type. Auth. Krylov I.P.

Claims (12)

1. A method of protecting against electrical breakdown of insulating gaps in a liquid dielectric using grid screens with controllable electrical potentials, including the steps of coating a conductor (electrode, part) with layers of a grid screen (electrically conductive grid) electrically isolated between itself and the conductor, installing the conductor inside the case of electrical equipment filled with a liquid dielectric, the inclusion of electrical voltage between the conductor and the housing, characterized in that the mesh screen is they are molded in the form of a stocking (without a seam) with a grid cell in the form of a square (rhombus), the diagonal of which is placed along the axis of the insulated conductor, and the conditional single diameter of the stocking is used for all layers of insulation, which is chosen based on providing a tight fit for the insulated conductor throughout it length. 2. The method of protection against electrical breakdown of the insulation gaps according to claim 1, characterized in that the spacing and isolation of the mesh screens between the conductor and between them is performed by spirals made of an elastic solid dielectric, which is compatible with the liquid dielectric in terms of its insulating properties, is used as a material for distance spirals elastic, pore-free, non-gas-absorbing dielectric. 3. The method of protection against electrical breakdown of the insulation gaps according to claim 1, characterized in that the conditional diameter of the spacing spiral, its thickness, pitch of turns and length are selected based on ensuring tight fit over the insulated conductor and all layers of mesh screens along their entire length to create the required thicknesses of the insulation gaps between the conductor and the mesh screens, taking into account their slack between the turns. 4. The method of protection against electrical breakdown of the insulation gaps according to claim 1, characterized in that the actual diameter of the mesh screen stocking for a particular insulation layer is obtained by longitudinal stretching or compression by pulling it on an insulated conductor over spacer spirals. 5. The method of protection against electrical breakdown of the insulation gaps according to claim 1, characterized in that the spacing spirals are pulled over the conductor and the mesh screens closely with the overlap of the turns of one spiral into adjacent spirals, which ensures the required thickness of the insulation gap between the conductor and the mesh screen, and between mesh screens along the entire length of the conductor. 6. The method of protection against electrical breakdown of the insulation gaps according to claim 1, characterized in that the mesh screen and the spacing spirals are hydrodynamically transparent to a liquid dielectric, in the volume of which an insulated conductor is placed, which ensures the washing out of aging products and decomposition of the liquid dielectric from insulating gaps between the mesh screens and fast recovery of liquid dielectric insulation after breakdown. 7. The method of protection against electrical breakdown of the insulation gaps according to claim 1, characterized in that the mesh screens around the insulated conductor are connected to at least two resistive voltage dividers, so as an alternative, see Figure 4, that one divider is connected to the housing and the grid screens of the outer group, and the other is connected to the conductor and the mesh screens of the inner group around the conductor, which in the volume of liquid dielectric between the mesh screens of the inner and outer groups is formed loaded with operating voltage at least one insulating gap with the required electric strength (hereinafter referred to as the loaded layer) and adjacent insulation gaps not loaded with operating voltage lying in the directions to the surfaces of the insulated conductor and the housing (hereinafter referred to as unloaded layers), which is the number and thickness of the loaded and unloaded insulation layers are determined from calculations of their required strength and the probability of breakdown of the entire insulation of the conductor. 8. The method of protection against electrical breakdown of the insulation gaps according to claim 1, characterized in that the parameters of the resistive voltage dividers are set to limit the breakdown current of the loaded layer to a value that excludes damage to the grid screens and, if there is no current through them from the breakdown of the loaded layer and / or insulation leakage current, control electric potentials are applied to the mesh screens of the inner and outer groups, which create minimal electric field strengths in the unloaded layers, virtually all the operating voltage is applied to the loading layer of insulation. 9. The method of protection against electrical breakdown of the insulation gaps according to claim 1, characterized in that the parameters of the resistive voltage dividers are set such that, when current arises through them from the breakdown of the loaded layer, simultaneously with its increase, they are fed to the mesh screens of the inner and outer groups control electric potentials, creating at least two insulation layers closest to the punctured layer, the difference in electric potentials is much less than the breakdown voltage so that almost all operating voltage e on the recovery of the loaded insulation layer is shifted from it to the adjacent layers of insulation. 10. The method of protection against electrical breakdown of the insulation gaps according to claim 1, characterized in that simultaneously with the inclusion of voltage between the conductor and the housing from resistive dividers, control electric potentials are supplied to the grid screens, which in the absence of breakdown of the loaded layer create minimal differences on the unloaded insulation layers electrical potentials, excluding aging and decomposition of a liquid dielectric, which in case of breakdown of the loaded layer and / or occurrence of leakage current through it, synchronously with When the breakdown current is generated, they create reduced (blocking the breakdown) electric field strengths at least on the two nearest adjacent layers, which prevent further breakdown development, at the same time subsequent adjacent layers (unloaded before the breakdown) load with all operating voltage, thereby reducing the probability of a complete breakdown of the entire conductor insulation almost to zero. 11. The method of protection against electrical breakdown of the insulation gaps according to claim 1, characterized in that in the loaded and unloaded insulation layers, passive grid screens isolated from each other by spacing spirals are installed, which are not connected to resistive voltage dividers, that all grid screens are provided with openings, the location of which enhances the circulation of the liquid dielectric in the insulating gaps. 12. The method of protection against electrical breakdown of the insulation gaps according to claim 1, characterized in that the number of mesh screens, insulation gaps, passive mesh screens is determined based on the properties of the liquid dielectric, the operating voltage and the required probability of insulation breakdown.

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