Method for detecting partial discharges in an electric operating means, and electric operating means

Abstract

The invention relates to a method for detecting partial discharges in an electric operating means, wherein the operating means has a gas-filled, closed housing, wherein electromagnetic or electric pulses are detected by means of a sensor device, the pulses are compared with one or more threshold values, if the threshold values are exceeded a partial discharge is identified, a pressure or partial pressure is determined for at least one gas component in the housing, a correction factor is determined from the pressure or partial pressure and / or the type of gas component, and the pulses or threshold values are corrected with the correction factor.

Description

[0001] 202313564 1 Description Method for detecting partial discharges in electrical equipment and equipment The invention relates to the evaluation of partial discharge signals that occur in the area of ​​a switchgear. Partial discharges are local electrical discharges that are caused, for example, in local inhomogeneities in the insulation material, for example in small air pockets, within the insulation of an electrical cable as a result of strong inhomogeneities in an electric field. Partial discharges can locally damage insulation and lead to its failure in the long term. For this reason, electrical machines and switchgear in particular are regularly inspected for partial discharges. Partial discharges cause short electromagnetic pulses with a typical pulse duration of less than 1 µs and frequency components well into the UHF frequency range.There are a variety of measurement methods for detecting partial discharges, some of which are standardized, for example, according to DIN EN 60270. These include offline measurements, which interrupt the regular operation of a piece of equipment and potentially incur high costs, for example, due to the operational interruptions and the costs of the measuring devices. Such measurements can therefore only be performed occasionally. Online measurements during regular operation enable continuous monitoring of the equipment and, in particular, the detection of trends regarding partial discharge activity. The information obtained in this way can enable a reliable forecast that predicts the period during which an item of equipment affected by partial discharges can still be operated reliably.202313564 2 In order to enable a reliable evaluation of the equipment, not only the measurement but also the interpretation of the measured values ​​is necessary. To enable further methodologies such as condition-based maintenance, it is important that threshold values ​​are neither too low (= false alarm) nor too high (= possible destruction of the equipment). In the area of ​​gas-insulated switchgear, a problem is that there is currently a switch from the conventional insulating gas SF6 to alternative insulating media and that the insulating medium used has a considerable influence on the partial discharge behavior. The object of the invention is to specify an improved method for the detection and evaluation of partial discharges in the area of ​​a switchgear and an improved switchgear, with which in particular increased fault tolerance with regard to the application of the threshold values ​​is achieved.The invention solves this problem by a method according to claim 1. Furthermore, a solution consists in the electrical equipment with the features of claim 10. In the method according to the invention for detecting partial discharges in an electrical equipment having a gas-filled, sealed housing, electromagnetic or electrical pulses are detected by means of a sensor device and the pulses are compared with one or more threshold values, wherein a partial discharge is detected if the threshold values ​​are exceeded. Furthermore, a pressure or partial pressure is determined for at least one gas component in the housing, a correction factor is determined from the pressure or partial pressure and / or the type of gas component, and the pulses or threshold values ​​are corrected with the correction factor.The electrical equipment according to the invention, in particular a gas-insulated switchgear, is configured to detect partial discharges and has a gas-filled, sealed housing. Furthermore, it comprises a sensor device for detecting electromagnetic or electrical pulses and an evaluation device configured to compare the pulses with one or more threshold values ​​and to detect a partial discharge if the threshold values ​​are exceeded. The equipment further comprises a device for determining pressure or partial pressure for at least one gas component in the housing, and the evaluation device is configured to determine a correction factor from the pressure or partial pressure and / or the type of gas component and to correct the pulses or threshold values ​​using the correction factor.The method and the operating means according to the invention advantageously enable detection and recognition of partial discharges, taking into account changes in the state within the housing. This results in improved accuracy in the evaluation of the partial discharge signals. Advantageous embodiments of the invention emerge from the dependent claims. The embodiment of the independent claims can be combined with the features of one of the subclaims or, preferably, with those of several subclaims. Accordingly, the following additional features can also be provided: It is expedient if vectors, each with a plurality of value pairs of frequency and scalar correction factor, are used as correction factors.The frequencies used in the value pairs are preferably in a range between 1 MHz and 2 GHz, with the ratio between a highest and a lowest frequency in a vector preferably being at most 100, in particular at most 10. 202313564 4 The pressure can be determined using a pressure sensor arranged in the housing. Pressure sensors are small and inexpensive, so several pressure sensors can be used to improve safety and accuracy. It is possible to provide an initial gas composition. This is available when evaluating partial discharge signals and can be taken into account. The initial gas composition refers to a gas composition whose structure, including partial pressures or other proportions of the components, is known.The gas composition can be determined, for example, during commissioning, but also at later times, for example when refilling the switchgear. In one embodiment of the invention, a leakage measurement takes place in the closed housing and is used to determine one or more partial pressures for gas components in the housing. In this way, precise information about the gas composition can be obtained and a more precise correction of the determined partial discharge signals can be made. Alternatively or additionally, the gas composition can also be determined during operation by simulating mechanisms of gas leakage (gas loss), for example through seals. Advantageously, no sensor is required for this, and no integration errors occur. In addition, a measurement of at least one absolute concentration of a component can also be carried out.For example, the concentration of a component can be determined by measuring the speed of sound in the housing. For this purpose, it is expedient if a system for sound generation and detection is available. Measurement of the absolute concentration here means a measurement which does not sum up losses, i.e. does not measure a change over time. 202313564 5 It is particularly advantageous if a measurement is carried out over a period of at least one minute, in particular at least one hour, a profile of the air humidity in the immediate vicinity of the housing is provided during this period, a correlation value is determined between the profile of the pulses and the profile of the air humidity, and the correlation value is used to determine whether the partial discharges take place inside the housing or outside the housing.This procedure can be used to determine whether partial discharge signals originate from a source inside or outside the housing. This information, in turn, can be used to limit correction of the signals relating to the pressure prevailing in the housing to those partial discharge signals that also originate from inside the housing, thus avoiding incorrect correction of other partial discharge signals. It is possible to perform a calibration using an initial gas composition and to additionally correct the pulses or threshold values ​​using a correction factor associated with the initial gas composition. This advantageously takes into account the fact that a specific configuration of the equipment is present even during calibration. The sensor device is preferably a Rogowski coil or a high-frequency current transformer.In the following, the invention is described and explained in more detail with reference to the exemplary embodiments shown in the figures. There show: Figure 1 a simplified schematic diagram of a gas-insulated switchgear with partial discharge sources, Figure 2 schematically the sequence of a method for signal correction, 202313564 6 Figure 3 schematically the sequence of a method for determining the location of signals from a partial discharge source. Figure 1 shows an example of a highly simplified gas-insulated switchgear 10. The gas-insulated switchgear 10 comprises a closed and gas-tight housing 11. A mechanical switch 12 is arranged in the housing 11, which is only shown symbolically in Figure 1 since its concrete design is not relevant here. Electrical lines 13, 14 connect to the switch 12 and are led out through the walls of the housing 11. Also shown in Figure 1 are a first and second source 20, 21 for partial discharges.The partial discharge sources 20, 21 are symbolized by current sources. Also shown is a partial discharge sensor 16, which in this case is a Rogowski coil. The first of the two sources 20 is located inside the housing 11, while the second source 21 is located outside the housing. While ambient air is present outside the housing 11, the housing is filled with a specific gas mixture. The gas mixture serving as an insulating gas can be, for example, SF6 (sulfur hexafluoride) or artificial air, i.e., a mixture of 20% O2 and 80% N2. Other gas mixtures can also be used. The term "gas mixture" is not intended to refer exclusively to gases with multiple components, but also to gases with only one component. The switchgear 10 further comprises a pressure sensor 17 arranged in the housing 11.The pressure sensor detects temperature-related fluctuations in the gas pressure in the housing, but also, in the long term, pressure changes caused by gas escaping from the housing (leakage). Calibration is typically performed to evaluate partial discharges. During calibration, a partial discharge is simulated, i.e., artificially introduced into the system. The amount of charge introduced is known; it is derived from the current integral over time. Sensor 16 determines the signal resulting from this artificial partial discharge, and based on the sensor response, a threshold value 101 is set at which a critical condition is assumed; in other words, real partial discharges are considered problematic, and a warning is triggered. The processing of the signals, correction factors, and threshold values ​​101 takes place in an evaluation device 18.The sensor signal is, for example, a current signal which, during processing, is converted into a vector of value pairs which, for example, indicates an intensity at a fixed set of frequencies. The threshold value 101 can be a single value or a similar vector for the same frequencies. A comparison can be made, for example, by comparing the intensity at each frequency from the set of frequencies with a respective threshold value 101 at the same frequency. At the time of calibration, a specific gas mixture is present at a specific pressure. The gas mixture comprises one or more components at respective partial pressures. For example, at the time of calibration, a gas mixture of oxygen and nitrogen can be present in the housing 11 with partial pressures of 660 mbar and 2640 mbar.The partial pressures only represent a quantitative indication of the composition of the gas mixture, and other measured variables can also be used, such as density, volume fractions, weight fractions, or particle number fractions. The conversion between the measured signal and an evaluation variable for comparison with the threshold value 101 results in a correction factor that reflects the influence of the type of gas mixture present. This can always be taken into account, even if changes in the gas mixture occur. 202313564 8 If, however, calibration cannot be carried out in a specific switchgear 10, then a correction factor that reflects the influence of the gas mixture present can be selected from known correction factors for gas mixtures.This adapts the comparison between the sensor signal and threshold value 101 to the gas mixture and avoids excessive or insufficient sensitivity to partial discharges. It goes without saying that such a correction can be applied in a completely equivalent manner to the sensor signals or as an inverse to the threshold value 101 used for the comparison. If a change in the gas mixture subsequently occurs, a further correction factor can be determined based on the change. The further correction factor is also applied to the sensor signal or the threshold value in order to compensate for the influence of the change in the gas mixture on the signal of partial discharges. Such a change can, for example, be a pressure change in the housing 11 of the switchgear 10. Such a pressure change can, for example, be caused by a temperature change or other internal or external circumstances.Correction factors for various pressures of the existing gas mixture are known from appropriately performed simulations, estimates, or measurements and can thus be taken from a database or other storage device during operation when measuring partial discharges. It is advantageous to provide a matrix of correction factors for various pressures and gas mixtures. A gas mixture can then be selected during the provision of the switchgear, and a correction factor can be selected for various pressures occurring during operation. The respective correction factor can be determined from a table of values, for example by linear interpolation from the available values. For example, such a table of values ​​can contain correction factors for pressure values ​​of 1 bar, 1.4 bar, 1.9 bar, 2.4 bar, and 3 bar.The correction factors can be individual values, as already described, or vectors of value pairs, each consisting of a frequency and a scalar correction value. Figure 2 schematically shows the further procedure. If, during operation of the switchgear, a signal 100 of a partial discharge is measured in a first step 31, then in a second step 32, the signal can be corrected using the correction factors K(f) in table 30. For this purpose, a first correction factor K is expediently used. k (f) is determined, which is assigned to the pressure and the gas mixture present during calibration. Furthermore, a second correction factor K a (f) is determined, which is associated with the current pressure and the gas mixture present. The partial discharge signal is then calculated with the ratio K a (f) / K k (f). The correction is conveniently carried out in a frequency-resolved multiplication, ie for all fi the signal TE(f i ) with K a (f i ) / K k (f i). In real applications, it is expedient to only consider a section of the frequency spectrum, which typically lies in the range between 1 MHz and several GHz and whose width is typically significantly smaller than the stated range. For example, a specific sensor can consider the frequency range between 10 MHz and 200 MHz and, for this purpose, consider three individual values ​​at 10 MHz, 50 MHz, and 200 MHz. In this range, the signal is considered for a selection of frequencies, so that the signal TE(f) corresponds to a vector with value pairs of frequency and signal strength. For the correction factors, it is expedient to determine and store a similar vector with the same frequencies. After the correction, the corrected signal 100 is compared with the threshold value 101 in a third step 33.If the corrected signal 100 exceeds the threshold value 101, a critical condition is assumed and, for example, a warning signal is triggered in a fourth step 34. 202313564 10 In other embodiments, a change in the composition of the gas mixture can also be taken into account. The composition manifests itself, for example, as a change in the ratio of the partial pressures or other proportions of the components of the gas mixture present. Such a change can occur, among other things, through a - typically very slow - selective escape of the gases from the housing. This can be, for example, gas- and temperature-dependent permeation through elastomer seals.If a switchgear 10 is designed to detect such a change, then a correction factor can also be determined based on the detected composition of the gas mixture, which is applied to the signals from sensor 16 or the threshold value 101 and thus improves the assessment of the partial discharge. Pre-provided tables of correction factors can also be used for this purpose, with these tables expediently containing correction factors for various combinations of pressure and gas composition. Alternatively, the change in the partial pressures of the gas mixture due to permeation as a function of time can also be simulated using model calculations. The corrections described assume that the source of the partial discharge is located in the housing 11 and is therefore influenced by the gas mixture present there.A partial discharge outside the housing is independent of pressure changes in the housing and should therefore ideally not be subject to pressure-induced correction. During operation, the location of partial discharges that occur cannot be easily determined. For example, a partial discharge that causes signals in sensor 16 cannot be directly assigned to one of the two sources 20, 21. However, a location can advantageously be determined using the correction factors. This requires measuring and storing measurement results over a longer period of time, which may at least be minutes, but in many cases hours. 202313564 11 The signals of the partial discharges, i.e. the sensor signals of sensor 16, are expediently stored over this period. Furthermore, the curve of, for example, the measured pressure in the housing 11 is also stored.Alternatively or additionally, the humidity and / or pressure present outside the housing can also be stored or made available in some other way. While the pressure in the housing 11 does not necessarily change over the course of minutes or hours, this is likely given the air conditions outside the housing. If the source 20 of partial discharge signals is located inside the housing 11, then the curve of the measured signals exhibits a correlation with the pressure curve in the housing 11. However, there is no connection with humidity fluctuations outside the housing 11. If, on the other hand, the source 21 of partial discharge signals is located outside the housing 11, then the curve of the measured signals exhibits a correlation with the humidity curve outside the housing 11, but not with the pressure curve inside the housing 11. The procedure is shown schematically in Figure 3.In a first step 41, a profile of partial discharge signals is recorded over a period of time. These can be the signals from sensor 16 directly or pre-processed signals. For example, for location determination, a simplified scalar partial discharge signal can be determined for each point in time to be stored. In a second step 42, data for the existing air humidity outside the housing 11 during the same period are provided. The first and second steps 41, 42 take place in parallel, provided the second step consists of a measurement. In a third step 43, a correlation value is calculated between the two profiles thus determined.If the correlation value exceeds a predetermined threshold, this means that there is a noticeable connection between the course of the partial discharges and the air humidity outside the housing 11; then, in a fourth step 44, a location outside the housing 11 can be assumed as the location of the partial discharges. In this case, a correction of the measured signals for the pressure present in the housing 11 can be omitted. If, on the other hand, the correlation is too low, a location inside the housing 11 can be assumed as the location of the partial discharges in the fourth step 44. In this case, a correction of the measured signals for the pressure present in the housing 11 or other existing variables should be made.

[0002] 202313564 13 List of reference symbols 10 Switchgear 11 Housing 12 Mechanical switch 13, 14 Electrical lines 16 Sensor for partial discharges 17 Pressure sensor 18 Evaluation device 20, 21 Partial discharge source 30 Value table 31…34, 41…44 Process steps 100 Signal 101 Threshold value

Claims

202313564 14 Patent Claims 1. Method for detecting partial discharges in an electrical device (10) having a gas-filled, sealed housing (11), - electromagnetic or electrical pulses are detected by means of a sensor device (16), - the pulses are compared with one or more threshold values (101), - if the threshold values (101) are exceeded, a partial discharge is detected, characterized in that - a pressure or partial pressure for at least one gas component in the housing (11) is determined, - a correction factor is determined from the pressure or partial pressure and / or the type of gas component, - the pulses or threshold values (101) are corrected with the correction factor.

2. Method according to claim 1, wherein the pressure is determined by means of a pressure sensor (17).

3. Method according to claim 1 or 2, wherein an initial gas composition is provided. 4.Method according to one of the preceding claims, in which one or more partial pressures for gas components are determined by a leakage measurement.

5. Method according to one of the preceding claims, in which one or more partial pressures for gas components are determined by simulating the gas escaping from the housing (11).

6. Method according to one of the preceding claims, in which - a measurement is carried out over a period of at least one hour. 202313564 15 - a profile of the air humidity in the immediate vicinity of the housing (11) is provided during the period, - a correlation value is determined between the profile of the pulses and the profile of the air humidity, - based on the correlation value, it is determined whether the partial discharges occur inside the housing (11) or outside the housing (11).

7. Method according to one of the preceding claims, in which vectors each having a plurality of value pairs of frequency and scalar correction factor are used as correction factors.

8. Method according to claim 7, in which the values for the frequencies in the value pairs originate from a range between 1 MHz and 2 GHz. 9.Method according to one of the preceding claims, in which - a calibration is carried out for an initial gas composition, - the pulses or threshold values are additionally corrected using a correction factor which is assigned to the initial gas composition.

10. Electrical equipment (10) configured to detect partial discharges, comprising - a gas-filled, sealed housing (11), - a sensor device (16) for detecting electromagnetic or electrical pulses, - an evaluation device (18) configured to compare the pulses with one or more threshold values (101) and to detect a partial discharge if the threshold values (101) are exceeded, characterized in that - the equipment (10) comprises a device (17) for determining pressure or partial pressure for at least one gas component in the housing (11). 202313564 16 - the evaluation device (18) is designed to determine a correction factor from the pressure or partial pressure and / or the type of gas component and to correct the pulses or threshold values (101) with the correction factor.

11. Electrical equipment (10) according to claim 10, comprising a pressure sensor (16) arranged in the housing (11).

12. Electrical equipment (10) according to claim 10 or 11, comprising a device for measuring leakage for at least one gas component.

13. Electrical equipment (10) according to one of claims 10 to 12, comprising a sensor for air humidity arranged outside the housing (11).

14. Electrical equipment (10) according to one of claims 10 to 13, wherein the sensor device (16) is a Rogowski coil or a high-frequency current transformer.

15. Gas-insulated switchgear (10) according to one of claims 10 to 14.

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