Method for detecting partial discharges in an item of electrical equipment, and equipment

Abstract

A method for detecting partial discharges in an item of electrical equipment with at least one electrical line is specified, in which a sensor device is used to capture electromagnetic or electrical pulses, the pulses are compared with at least one threshold value, an error signal is triggered if the threshold value is exceeded, and in which a measured value for the temperature in the region of the electrical line is determined, a correction value is determined from the measured value for the temperature, and the pulses or the threshold value is / are corrected with the correction value.

Description

[0001] Description

[0002] Method for detecting partial discharges in an electrical device and device

[0003] The invention relates to the evaluation of partial discharge signals that occur in the area of ​​a switchgear.

[0004] Partial discharges are local electrical discharges caused, for example, by local inhomogeneities in the insulation material, such as 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, in the long term, lead to its failure. Therefore, 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 ps and frequency components extending well into the UHF frequency range.

[0005] For detecting partial discharges, there are a variety of partially standardized measurement methods, 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 operational interruptions and the cost of the measuring devices. Such measurements can therefore only be performed occasionally.

[0006] Online measurements during regular operation enable continuous monitoring of the asset and, in particular, the detection of trends regarding partial discharge activities. The information obtained in this way can enable a reliable forecast that predicts the period in which an asset affected by partial discharges can still be operated reliably. To enable a reliable assessment of the asset, 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 asset).Variations in the load and the ambient conditions adversely affect the behavior of partial discharges and even the occurrence of corona discharges, which can distort the measurement.

[0007] It is an object of the invention to provide an improved method for detecting and evaluating 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.

[0008] The invention solves this problem by a method according to claim 1. Furthermore, a solution consists in the electrical equipment having the features of claim 8.

[0009] In the method according to the invention for detecting partial discharges in an electrical device with at least one electrical line, electromagnetic or electrical pulses are detected by means of a sensor device, the pulses are compared with at least one threshold value and an error signal is triggered if the threshold value is exceeded.

[0010] Furthermore, a measure of the temperature in the area of ​​the electrical line is determined, a correction value is determined from the measure of the temperature and the pulses or the threshold value are corrected with the correction value.

[0011] The electrical device according to the invention is configured to detect partial discharges and has a sensor device for detecting electromagnetic or electrical pulses. It further comprises an evaluation device configured to compare the pulses with at least one threshold value and to trigger an error signal if the threshold value is exceeded.

[0012] The device further comprises a device for determining a temperature measure in the area of ​​the electrical line. Finally, the evaluation device is configured to determine a correction value from the temperature measure and to correct the pulses or threshold values ​​using the correction factor.

[0013] Advantageously, the method and the device according to the invention enable detection and recognition of partial discharges, taking into account changes in the local temperature in the region of the electrical conductor. In particular, the current load, i.e., the current flow and its influence on the temperature in the vicinity of the electrical conductor, is taken into account. This results in improved accuracy in the evaluation of the partial discharge signals.

[0014] The equipment could, for example, be a gas-insulated switchgear. The temperature measurement refers to an electrical signal that directly or indirectly represents a temperature measurement. The measurement does not have to be explicitly output or determined; it can also be an intermediate result that is immediately processed.

[0015] 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 be provided: The measure of the temperature can be determined by means of a temperature sensor. The temperature sensor is expediently arranged close to the electrical conductor for this purpose. Even if it is not, a suitable correction can typically be carried out, since the temperature in a switchgear is essentially influenced by the influence of load fluctuations. Advantageously, it is not necessary to calculate a temperature from other data if a measurement signal from a temperature sensor is available.

[0016] In an advantageous embodiment, a calibration is performed, and a measure of the temperature at the calibration time is determined. This provides a base value for the temperature. If a current temperature corresponds to the base temperature, no adjustment of the threshold value is necessary.

[0017] In an alternative embodiment, the temperature measure can be determined using measured values ​​for the current flow in the electrical conductor. This advantageously eliminates the need for a temperature sensor, and the determined temperature values ​​correspond very closely to the actual temperatures, since electrical losses are the primary influence on the temperature near the electrical conductor.

[0018] The temperature can be determined using a thermal model of at least part of the equipment. At least part of the switchgear is modeled with thermal capacitances and resistances, and the temperature distribution is determined as a result of a simulation as a function of the current flow.

[0019] This simulation can take place during ongoing operation or in advance as part of the configuration or calibration. If the simulation is carried out in advance, the results can be saved in a table of values, for example, in order to determine the temperature measure from this table of values ​​in a relatively simple manner during ongoing operation. It is advisable to take into account not only the current value but also the current profile over a previous period of time, since temperature is not just a function of the instantaneous current value but rather results from previous heat inputs and heat losses through heat removal. The table of values ​​can, for example, contain pairs of values ​​from a current integral / current sum over a previous period of time and an associated temperature.

[0020] The correction value can be determined using a difference between the temperature measurement and a base temperature, which was determined and set during calibration, for example, and a proportionality factor. For this purpose, a base value for the threshold value is conveniently added, which, for example, was also determined during calibration and represents a threshold value for the base temperature.

[0021] The pulses are conveniently summed over a half-wave of the current in the line, and the resulting sum is compared with the threshold value. This allows for an evaluation of the partial discharges without having to consider each of the potentially numerous pulses. Summation can also be performed over several half-waves, with positive and negative half-waves being considered separately.

[0022] The sensor device is preferably a Rogowski coil or a high-frequency current transformer. These have sufficient bandwidth to detect the very high-frequency pulses of partial discharges.

[0023] The invention is described and explained in more detail below with reference to the exemplary embodiments shown in the figures. Figure 1 shows a simplified schematic diagram of a gas-insulated switchgear with partial discharge sources,

[0024] Figure 2 shows schematically the sequence of a method for signal correction with a temperature measurement,

[0025] Figure 3 shows schematically the sequence of a method for signal correction with a temperature determination by current measurement.

[0026] Figure 1 shows an exemplary, highly simplified gas-insulated switchgear 10. The gas-insulated switchgear 10 comprises a sealed and gas-tight housing 11. A mechanical switch 12 is arranged in the housing 11; this switch is only shown symbolically in Figure 1, since its specific 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.

[0027] Also shown in Figure 1 is a source 20 for partial discharges, located inside the housing 11. The source 20 for partial discharges is symbolized by a current source. Also shown is a sensor 16 for partial discharges, which in this case is a HEGT (high frequency current transformer).

[0028] In a first exemplary embodiment, the switchgear 10 further comprises a temperature sensor 17 arranged in the housing 11. The temperature sensor 17 detects the temperature near the electrical conductor 13. This temperature is influenced by the outside temperature, but primarily by the current flowing in the conductor 13 and the resulting electrical losses.

[0029] Typically, a calibration is carried out for the evaluation of partial discharges. During calibration, a partial discharge is simulated, i.e. artificially introduced into the system. The introduced charge quantity is known; it results from the current integral over time. The sensor 16 determines the signal resulting from this artificial partial discharge and, based on the sensor response, a threshold value S is set at which a critical condition is assumed. In other words, real partial discharges are considered problematic and a warning is triggered. Furthermore, a base temperature T B which is present at the time of calibration. The processing of the signals, correction factors and the threshold value S takes place in an evaluation device 18.

[0030] To evaluate partial discharges during operation, the partial discharge signals are recorded. The partial discharge signals are very short pulses and are recorded with a bandwidth of 1 μHz to 80 MHz. Since these occur very frequently, it is advisable not to analyze each individual signal, but to sum their measured values ​​over a half-wave. The result is a charge in coulombs.

[0031] If a subsequent change in temperature occurs, a correction factor can be determined based on the change. The correction factor is applied to the sensor signal or the threshold value to compensate for the influence of the change in the gas mixture on the signal from partial discharges. For example, the temperature change can be multiplied by a proportionality factor and added to the threshold value.

[0032] For the present invention, it was recognized that a local temperature increase in a conductor leads to the formation of a thin layer with a greatly increased insulating gas temperature around the conductor (difusive heat conduction) and above this a hot gas plume of the surrounding insulating gas (convective heat conduction by self-convection).

[0033] Based on the relationship of the relative gas density 5 :

[0034] 5 = p / po o / T where pO is the normal pressure and T o the normal temperature or a base temperature at a calibration time (for example T o = 293 K) , then with increasing local temperatures T c and constant pressure a reduction of the threshold voltage Ud for partial discharges.

[0035] U d / U d0 ~ P / Po • T o / T = 5

[0036] A change in the threshold voltage leads to an increase or decrease in the number of partial discharges occurring over a half-wave compared to the number at the threshold voltage Udo - While it is therefore possible to consider the partial discharges individually and thus also to directly consider a reduction in the threshold voltage, it is also sufficient to consider the sum of the partial discharge signals over a half-wave.

[0037] Figure 2 schematically illustrates the procedure. During operation of the switchgear 10, in a first step 31, signals from partial discharges are recorded by the sensor 16. In a second step 32, these are integrated over a period of time, for example, a half-wave duration of the current, i.e., summed in the actual implementation.

[0038] In a third step 33, a temperature measurement takes place. This step and the first two steps are carried out in parallel and are not dependent on each other. In a fourth step 34, a threshold value is determined for comparison with the summed signal. The threshold value S is determined as the sum of a base threshold value S B , which corresponds to the base temperature T B at the time of calibration and the temperature difference of the current temperature T c to the base temperature T B , multiplied by a proportionality factor k .

[0039] S = S B + k ■ ( T c - T B ) In a fifth step 35, a comparison is made between the summed signal and the threshold value S. If the summed signal exceeds the threshold value S, a critical condition is assumed, and in a sixth step 36, an error signal is generated or a warning is issued.

[0040] In a second embodiment, the switchgear 10 does not comprise a temperature sensor 17 or the temperature sensor 17 is arranged so far away from the electrical lines 13, 14 that the temperature measurement value is not meaningful for the temperature near the electrical lines 13, 14.

[0041] In this case, a value for the temperature can be obtained from the current flowing in the electrical lines 13, 14 and the resulting ohmic losses. For this purpose, for example, a simulation can be carried out using a thermal model of the switchgear 10. In addition to the ohmic losses, an external temperature of the switchgear can also be taken into account. Such a simulation can be carried out in advance and the results, i.e. the relationship between the current and the temperature, can be calculated using a simple function T c = f (M z) can be stored. Alternatively, a table of values ​​can be stored and, for example, by means of a linear interpolation to the temperature T c be closed .

[0042] The temperature is less a function of the current Ic, but rather a function of the current curve in a previous period t s , which in the current current I c ends . This is determined by the current measure M z symbolized . The current curve can be smoothed or summed / integrated for conversion, for example M z = Et = -ts ... o I ( t ) .

[0043] As an alternative to the calculation taking place in advance, the thermal network can also be calculated in real-time in systems where sufficient computing capacity is available, whereby the current curve in the previous period of length t s serves directly as an input variable.

[0044] Figure 3 schematically shows the procedure, which largely corresponds to that of Figure 2. During operation of the switchgear 10, in a first step 41, signals from partial discharges are recorded by the sensor 16. In a second step 42, these are integrated over a period of time, for example, a half-wave duration of the current, i.e., summed up in the actual implementation.

[0045] In a third step 43, a current measurement takes place. This step and the first two steps are carried out in parallel and are not dependent on each other. The measured current signal is stored, as it will be needed at a later time. In a fourth step 44, stored current values ​​for a previous period are fed into a thermal model and a temperature value T M determined .

[0046] In a fifth step 45, a threshold value is determined for comparison with the summed signal. The threshold value S is determined as in the procedure according to Figure 2, only here using the temperature value TM determined by the model, i.e. S = S B + k ■ ( T M - T B ) .

[0047] In a sixth step 36, a comparison is made between the summed signal and the threshold value S. If the summed signal exceeds the threshold value S, a critical condition is assumed, and in a seventh step 37, an error signal is generated or a warning is issued.

[0048] It is understood that the calculation of the temperature value T M and the calculation of the threshold value can be mathematically summarized and the temperature value T M as an explicit result is unnecessary. List of reference symbols

[0049] 10 Switchgear

[0050] 11 Housing 12 mechanical switch

[0051] 13 , 14 electrical cables

[0052] 16 Partial discharge sensor

[0053] 17 Temperature sensor

[0054] 18 Evaluation device 20 Partial discharge source

[0055] 31...36 , 41...47 Process steps

Claims

Patent claims 1. Method for detecting partial discharges in an electrical device (10) with at least one electrical line (13, 14), - electromagnetic or electrical pulses are detected by means of a sensor device (16), - the pulses are compared with at least one threshold value, - if the threshold value is exceeded, an error signal is triggered, characterized in that - a measure of the temperature in the area of ​​the electrical line (13, 14) is determined, - a correction value is determined from the temperature measure, - the pulses or the threshold are corrected with the correction value.

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

3. Method according to claim 1 or 2, in which a calibration is carried out and a measure of the temperature at the calibration time is determined.

4. Method according to one of the preceding claims, in which the measure of the temperature is determined using measured values ​​for the current flow in the electrical line (13, 14).

5. The method according to claim 4, wherein the measure of temperature is determined using a thermal model of at least part of the operating means (10).

6. Method according to one of the preceding claims, in which the correction value is calculated from a difference of the measure for the temperature and a base temperature and a proportionality factor.

7. Method according to one of the preceding claims, in which the pulses are summed over a half-wave of the current in the line (13, 14) and the sum thus determined is compared with the threshold value.

8. Electrical equipment (10) designed to detect partial discharges, comprising - a sensor device (16) for detecting electromagnetic or electrical pulses, - an evaluation device (18) designed to compare the pulses with at least one threshold value and to trigger an error signal if the threshold value is exceeded, characterized in that - the operating means (10) has a device (17) for determining a measure of the temperature in the region of the electrical line (13, 14), - the evaluation device (18) is designed to determine a correction value from the temperature measurement value and to correct the pulses or threshold values ​​with the correction factor.

9. Electrical equipment (10) according to claim 8 with a temperature sensor (17).

10. Electrical equipment (10) according to claim 8 or 9, wherein the sensor device (16) is a Rogowski coil or a high-frequency current transformer.

11. Gas-insulated switchgear (10) according to one of claims 8 to 10.

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