ELECTRICAL CIRCUIT ARRANGEMENT

Description

[0001] The invention relates to an electrical circuit arrangement for connecting an electrical measuring device, which is used to measure partial discharges in a power grid with an operating voltage in the range of 1 kV to 69 kV and has an A / D converter for converting a measured voltage pulse into a digital signal and a microprocessor unit for evaluating the digital signal, with a measuring output of a capacitive voltage testing system of a switchgear for the power grid with the operating voltage in the range of 1 kV to 69 kV.

[0002] Furthermore, the invention relates to a measuring arrangement for measuring partial discharges at a measuring output of a capacitive voltage testing system of a switchgear for a power grid with an operating voltage in the range of 1 kV to 69 kV, wherein the measuring arrangement has an A / D converter for converting a measured voltage pulse into a digital signal and a microprocessor unit for evaluating the digital signal.

[0003] Furthermore, the invention relates to a measuring system for locating the location of a partial discharge in a power grid with an operating voltage in the range of 1 kV to 69 kV.

[0004] Partial discharge measurements are performed on high- and medium-voltage lines for diagnostic purposes. In particular, the occurrence of partial discharges can indicate insulation defects, which can lead to insulation failures over time.

[0005] A preferred method is to measure partial discharges without first disconnecting the cable from the power supply. HFCT (High Frequency Current Transformer) sensors are known for this purpose. These are current transformers comprising two semi-circular sections wound with wire (Rogowski coil) that are closed to form a ring that is placed around an earth connection line leading from the cable shield of the cable being measured. When partial discharges occur, resulting in current pulses flowing along the earth connection line, corresponding voltage pulses are generated in the HFCT sensor that can be detected and evaluated by a connected measuring device. Such a measuring device comprises, among other things, an A / D converter for converting a measured voltage pulse into a digital signal and a microprocessor unit for processing the digital signal.A separate computer is usually connected for further analysis. This analysis filters out, among other things, interference signals to obtain information about any partial discharges that may occur.

[0006] It is also known to generate an additional synchronization signal which is synchronized with the phase of the alternating voltage in order to determine the phase position of detected partial discharge pulses, whereby additional statements can be made about possible causes of the partial discharges.

[0007] An application of HFCT sensors for partial discharge measurement is described, for example, in "Application of HFCT and UHF Sensors in OnLine Partial Discharge Measurements for Insulation Diagnosis of High Voltage Equipment", Fernando Älvarez et al., Sensors 2015, 15, 7360-7387.

[0008] Such online partial discharge measurements are performed in the switchgear area of ​​high- and medium-voltage grids. However, this is only possible in switchgear suitable for this purpose. For example, an online connection of an HFCT sensor is not possible if the cable shield's earth connection is located within the switchgear and is not accessible without isolating the switchgear from voltage, or if the cable shield's earth connection is permanently connected to the switchgear housing. Since the switchgear housing is usually also separately connected to earth, in the latter case, the current pulses caused by partial discharges are not diverted in a defined manner via the cable shield's earth connection. In such cases, the installation of an HFCT sensor requires the first de-energization of the power supply, which is associated with considerable effort. Alternative options for measuring partial discharges would therefore be advantageous.

[0009] For this purpose, measuring devices designed for connection to HFCT sensors have already been connected to a measuring output of a capacitive voltage detecting system (VDS) in a power grid switchgear with an operating voltage in the range of 1 kV to 52 kV. Such capacitive voltage detecting systems in power grid switchgear with an operating voltage in the range of 1 kV to 52 kV are standardized in IEC 61243-5:1997. Such a voltage detecting system has a coupling capacitor connected to the live component to be tested, which is connected to the measuring output, usually designed as a connection socket, via a coaxial connecting cable of a defined length. The shield of the coaxial connecting cable is connected to ground. The coaxial connecting cable thus forms an additional capacitance.If a measuring device designed for connection to an HFCT sensor is now connected to the connection socket, it can only detect partial discharges occurring in the area of ​​the switchgear and in the immediate vicinity of the switchgear, but not partial discharges occurring further away.

[0010] A voltage detection system with the same design can also be used for voltages up to 69 kV. Depending on the definition, such a voltage is still referred to as medium voltage.

[0011] To locate the location on the cable where partial discharges occur, it is known to couple a measuring device for measuring partial discharges to the cable via an HFCT sensor and to record the time interval between a partial discharge pulse and the delayed reflection of the partial discharge pulse from the remote cable end. From the time difference between the partial discharge pulse and the pulse resulting from the reflection at the remote cable end, and with a known propagation velocity of the pulse, the distance of the location of the partial discharge from the cable end (= length of the cable between the point on the cable where the partial discharge occurs and the cable end) can be determined. This method is also referred to as a TDR-based time measurement method.

[0012] With very long cables, however, the pulse reflected at the cable end can be so strongly attenuated that it is not detected or cannot be detected reliably. Especially for such cases, or for cables with complex reflection behavior, such as T-branched or multi-branched cables, it is known to couple measuring devices for measuring partial discharges at two cable ends (or at more than two cable ends in the case of branched cables) to the cable via HFCT sensors. A common, highly accurate time base is created for the measuring devices arranged at the cable ends, i.e. the measuring units have synchronized time recording units. This type of synchronization can be achieved using GPS, for example. The difference in the transit times required for a partial discharge pulse to reach a respective measuring device is then evaluated.Based on the known propagation speed of the partial discharge pulses, the difference in the cable lengths traversed and thus the location of the partial discharge can be determined.

[0013] EP 2 204 660 A1 discloses a measuring device for measuring partial discharges, among other things, on high-voltage cables. A measuring point is connected to the test object via a coupling capacitor and to ground via a measuring impedance. The signal present at the measuring point is amplified by an amplifier and filtered using various bandpass filters. This measuring device is intended to improve the protection against interference signals from external sources and to improve the separation of multiple superimposed partial discharge sources. This document thus discloses a measuring device for measuring partial discharges, but the problem of connecting such a measuring device to a measuring output of a capacitive voltage detection system in a switchgear assembly is not addressed.This also applies analogously to DE 10 2011 006 098 A1, which describes a measuring device for detecting partial discharges in the insulation system of rotating electrical machines. A coupler is connected to each of the stator windings as a high-pass filter. An adapter device with a high-pass filter and a low-pass filter is connected downstream of the coupler.

[0014] The measurement of partial discharges in offline mode is also known. In this case, the operating voltage is decoupled, and the cable is loaded with a high voltage from a separate high-voltage source. The extraction of partial discharge pulses for detection by a measuring device is usually not carried out with HFCT sensors, but with a circuit arrangement according to the IEC 60270 standard, using very large and heavy coupling capacitors in the nF range.

[0015] The object of the invention is to expand the possibilities for measuring partial discharges in medium-voltage networks in online operation, i.e., without prior voltage disconnection. According to the invention, this is achieved by an electrical circuit arrangement having the features of claim 1, by a measuring arrangement having the features of claim 10, or by a measuring system having the features of claim 12.

[0016] An electrical circuit arrangement according to the invention comprises a frequency response matching resistor connected between an input line, in particular a coaxial line, which serves to connect to the live pole of the measurement output of the capacitive voltage detection system, and ground, as well as a high-pass filter connected downstream of the frequency response matching resistor. Such a circuit arrangement can advantageously connect a measuring device, which has an A / D converter for converting a measured voltage pulse into a digital signal and a microprocessor unit for evaluating the digital signal, to a measurement output of a capacitive voltage detection system (VDS) of a switchgear for medium-voltage networks, wherein the measurement output is designed, for example, as a connection socket.

[0017] The frequency response matching resistor can shift the resulting frequency response in conjunction with the capacitive voltage detection system to lower frequencies, allowing partial discharge pulses with lower frequency components to be detected. Such partial discharge pulses with lower frequency components are obtained when partial discharges occur not in the immediate vicinity of the measurement location but further away, whereby the low-pass characteristics of the cable alter or disperse the high frequency components of the partial discharge pulse. Furthermore, the use of the downstream high-pass filter can sufficiently filter out frequencies below a permissible frequency range, thus enabling a meaningful evaluation of the measurement signal.

[0018] A capacitive voltage detection system (VDS) comprises, in particular, a coupling capacitor connected to the live part and a coaxial cable connected to the other terminal of the coupling capacitor, which forms an additional capacitance. The shield of the

[0019] The coaxial cable is connected to ground. The measurement output is located at the end of the coaxial cable remote from the coupling capacitor. This output is designed, for example, as a connector socket, as is often preferred.

[0020] For the purposes of this document, medium voltage refers to the voltage range from 1 kV to 69 kV. Therefore, whenever medium voltage is mentioned below, it refers to a voltage within this voltage range. A "medium-voltage grid" therefore refers to a power grid with an operating voltage within this range.

[0021] A low-pass filter is also advantageously connected downstream of the frequency response matching resistor. The electrical circuitry, in conjunction with the capacitive voltage detection system, allows for an overall frequency response that at least largely corresponds to that obtained when the measuring device is connected directly to an HFCT sensor.

[0022] The circuit arrangement according to the invention thus makes it possible to advantageously use a measuring device intended for use with HFCT sensors at measuring outputs of capacitive voltage testing systems of switchgear for medium-voltage networks, whereby the advantageous extended frequency range for partial discharge measurements is mapped.

[0023] An amplifier is advantageously connected downstream of the high-pass filter and low-pass filter, which allows the signal level to be further adjusted to an input level that is advantageous for the measuring device. An impedance matching resistor is preferably connected downstream of the amplifier to match the impedance to the input resistance of the measuring device in order to perform a power adjustment (=impedance matching).

[0024] A measuring arrangement for measuring partial discharges at a connection socket of a capacitive voltage testing system of a switchgear for medium-voltage networks according to the invention has an electrical circuit arrangement according to the invention and a downstream measuring device comprising an A / D converter for converting a measured voltage pulse into a digital signal and a microprocessor unit for evaluating the digital signal.

[0025] The measuring arrangement according to the invention can be a single device (i.e., with a single housing) or two or more devices in separate housings. In particular, in the latter case, the circuitry (containing the frequency response matching resistor and the downstream high-pass filter) can be housed in a first housing, and the measuring device comprising the A / D converter and the microprocessor unit can be housed in a separate housing, with at least one electrical connecting line running between the two devices.

[0026] The invention also enables measurements in situations that cannot be detected using HFCT sensors. One example is when a cable connected to the operating voltage of the medium-voltage network at one end is open at the other end. In such situations, no signal is received via an HFCT sensor, but the method or device according to the invention can. The same applies to cables whose cable shield is not connected to ground at the end of the switchgear.

[0027] In an advantageous development of a measuring arrangement according to the invention, the measuring arrangement has a time recording unit for recording the time difference between voltage pulses recorded by the measuring arrangement. If the measuring arrangement is connected at one cable end to a measuring output of a capacitive voltage detection system, the time interval between the voltage pulse traveling directly from the point of partial discharge to the measuring arrangement and the voltage pulse traveling from the point of partial discharge to the remote cable end and reflected back at this remote cable end can be recorded. From this time interval and the known propagation speed of the voltage pulse, the location of the partial discharge can be determined.

[0028] Determining the location of the partial discharge from the time difference between the direct partial discharge pulse and the partial discharge pulse reflected at the far end of the cable is known per se. However, the invention enables the simplest connection of the measuring system without interrupting the operating voltage, thereby achieving maximum safety for the measuring personnel. No switching or handling in the high-voltage area is required. Distance measurement is possible without the need to install an additional partial discharge sensor.

[0029] Particularly for measurements on very long cables or on cables with complex reflection behavior (T-branched or multiply branched cables), it is advantageous to provide a measuring system for locating the location of a partial discharge in a power grid with an operating voltage in the range of 1 kV to 69 kV according to the invention, which measuring system has at least two measuring arrangements according to the invention for measuring partial discharges at a respective measuring output of capacitive voltage testing systems, wherein the capacitive voltage testing systems are arranged at a respective end of a cable of the power grid. The measuring arrangements have time recording units that can be synchronized with one another. Using these time recording units, a difference in the propagation times of a voltage pulse caused by a partial discharge can be recorded for the respective measuring arrangement.

[0030] From these propagation time measurements, the location on the cable where a partial discharge occurs can be determined. For T-connected cables, such measuring arrangements can be connected to the measurement outputs of capacitive voltage detection systems at multiple cable ends.

[0031] The invention enables the simplest connection of the measuring arrangements without interrupting the operating voltage, thus ensuring maximum safety for the measuring personnel. No switching or handling in the high-voltage range is required. Online partial discharge fault distance measurement is possible even on long cable systems and T-junction cables. With T-junction cables, the measuring arrangement can also be used for cable ends with an open circuit. An HFCT sensor, on the other hand, would not be applicable with an open circuit.

[0032] Further advantages and details of the invention are explained below with reference to the accompanying drawings, in which: Fig. 1 is a schematic representation of a measuring arrangement containing a circuit arrangement according to the invention; Fig. 2 is a representation of the frequency response; Fig. 3 is a second embodiment of the invention; Fig. 4 is a symbolic image for explaining a third embodiment of the invention; Fig. 5 is a schematic representation of an embodiment of a measuring system according to the invention; Fig. 6 is a schematic representation of a T-branched cable.

[0033] A first embodiment of the invention is described below with reference to Fig. 1 explained.

[0034] In Fig. 1A conductor 1 of a medium-voltage network is shown schematically. A voltage in the range of 1 kV to 69 kV is applied to conductor 1. A common voltage range is 6 kV to 36 kV. A cable shield 2 surrounding conductor 1 is indicated.

[0035] Conductor 1 is located in a switchgear. Thus, other conductors of the medium-voltage network can be connected to conductor 1 via switches (not shown).

[0036] To enable verification of whether or not a voltage is present on conductor 1, a capacitive voltage detection system (VDS), designated overall by reference numeral 3, is provided, which is preferably designed in accordance with the IEC 61243-5:1997 standard. The voltage applied to conductor 1 can be in the range of 1 kV to 52 kV, although higher voltages up to 69 kV are also possible (which are still referred to as medium voltage in this document).

[0037] The capacitive voltage detection system 3 has a coupling capacitor 4 connected to the conductor 1. The capacitance of the coupling capacitor 4 is adapted to the voltage applied to the conductor 1 and can, for example, be in the range from 3 pF to 250 pF. For voltages in the range from 30 kV to 10 kV, the capacitance is, for example, in the range from 5 pF to 30 pF. The coupling capacitor 4 is connected via the conductor 5 of a coaxial cable to a connection socket 7, which represents a measurement output of the capacitive voltage detection system. The shield 6 of the coaxial cable is connected to ground. The capacitance of the coaxial cable can, for example, be in the range from 50 pF to 200 pF, and the length of the coaxial cable can be in the range from 0.5 m to 2 m. Longer lengths, in particular up to 6 m, are possible.

[0038] During normal operation, an indicator lamp can be connected to the connection socket 7 to indicate whether conductor 1 is live.

[0039] A load resistor (not shown in the figures) may be provided between the connection socket 7 and earth in order to limit the voltage at the connection socket 7 when the connection socket 7 is open.

[0040] The voltage detection system can, for example, be connected to a busbar of a switchgear system, which is connected to a medium-voltage cable comprising conductor 1 and cable shield 2. The medium-voltage cable typically has three conductors for the three phases, each surrounded by a cable shield. Medium-voltage cables with a cable shield surrounding all three conductors are also known.

[0041] The capacitive voltage testing system 3 can be high-resistance or low-resistance (HR or LRM system).

[0042] To perform a test of the medium-voltage network in a specific area around the connection point of voltage detection system 3 to the medium-voltage network, a partial discharge measurement is performed. This measurement is performed online, i.e., with a voltage in the range of 1 kV to 69 kV applied to conductor 1. Conductor 1 does not need to be disconnected from the medium voltage supply in preparation for the measurement.

[0043] The area around the connection point over which partial discharges can be measured according to the invention can advantageously be more than 1 km, in particular up to several km.

[0044] For this purpose, an electrical circuit arrangement 8 is connected to the connection socket 7 of the voltage testing system 3, possibly after removing the previously connected voltage indicator or control lamp. This electrical circuit arrangement 8 has a frequency response matching resistor 9, which is connected on the one hand to the input line 10 connected to the connection socket 7, and on the other hand to ground. The resistance value of the frequency response matching resistor 9 is preferably more than 25 kΩ and preferably less than 400 kΩ, with a range between 50 kΩ and 200 kΩ being preferred.

[0045] The frequency response matching resistor 9 is selected such that, in conjunction with the voltage detection system 3, a high-pass filter with a cutoff frequency of less than 1 MHz, preferably less than 200 kHz, is obtained. The cutoff frequency of the high-pass filter thus formed is advantageously more than 20 kHz, preferably more than 50 kHz. An exemplary cutoff frequency for the high-pass filter thus formed is in the range of 100 kHz (+ / - 30%).

[0046] However, the voltage testing system 3 in conjunction with the frequency response matching resistor 9 does not sufficiently filter out frequency components below the desired cutoff frequency. The partial discharge pulses to be measured are very small voltage pulses that are superimposed by various interference signals and are therefore not easily detectable. Among other things, a superposition occurs due to the decoupled component of the medium voltage applied to conductor 1 with the corresponding frequency of this medium voltage (which lies in the range of 16 2 / 3 Hz to 60 Hz). A high-pass filter 11 is therefore connected downstream of the frequency response matching resistor 9. The input of the high-pass filter 11 is connected to the input line 10.The cutoff frequency of the high-pass filter 11 advantageously corresponds at least in terms of magnitude (preferably in the range + / - 30%) to the cutoff frequency of the high-pass filter formed by the voltage testing system 3 in conjunction with the frequency response matching resistor 9.

[0047] The high-pass filter 11 is followed by a low-pass filter 12. This filter filters out frequency components above a desired frequency band, which also largely represent interference. The cutoff frequency of the low-pass filter 12 is preferably in the range between 5 MHz and 30 MHz, for example, approximately in the range of 10 MHz (+ / - 30%).

[0048] The low-pass filter 12 is thus connected with its input to the output of the high-pass filter 11.

[0049] A reversed connection of the filters 11, 12 with the frequency response matching resistor 9 is also conceivable and possible, ie the low-pass filter 12 could be connected to the frequency response matching resistor 9 (i.e. to the input line 10 of the circuit arrangement 8) and the high-pass filter 11 could be connected to the output of the low-pass filter 12.

[0050] An amplifier 13 is connected downstream of the high-pass filter 11 and low-pass filter 12. An impedance matching resistor 14 is connected downstream of the amplifier 13. The input of the impedance matching resistor 14 is thus connected to the output of the amplifier 13, and the output of the impedance matching resistor is connected to the output line 15 of the circuit arrangement 8.

[0051] The amplifier 13 serves to adapt to a favorable input level of the measuring device 16 connected to the output line 15 of the circuit arrangement 8. The impedance matching resistor 14 serves to adapt the output resistance of the circuit arrangement 8 to the input resistance of the measuring device 16 so that they at least substantially match (preferably in the range of + / - 20%).

[0052] Essential components of the measuring device 16 are an analog / digital converter (=A / D converter) 17 and a microprocessor unit 18. The A / D converter 17 serves to convert an analog signal applied to its input into a digital signal. This signal is fed to the microprocessor unit 18 and further processed there.

[0053] In the exemplary embodiment, a capacitor 19, preferably designed as a block capacitor, is connected to the input of the measuring device 16 and serves in particular to decouple any DC voltages that may be present at the input.

[0054] A transformer 20, which serves primarily for potential isolation, can be connected to the capacitor 19. The transformer can, for example, have a resistance of 50 Ω.

[0055] A diode protection circuit 21, which serves in particular to limit the potential level, is advantageously connected to the transformer 20. In the exemplary embodiment, the A / D converter 17 is connected to the diode protection circuit 21.

[0056] The design of the measuring device 16 is known. A measuring device designed in this way can be connected to an HFCT sensor to measure partial discharges.

[0057] Measurement signals output by the microprocessor unit are fed to an output line. In the exemplary embodiment, a separate computer 22 is connected to this line. This computer also further separates the voltage pulses caused by partial discharges from interference voltage components. In particular, pulse shape recognition is performed for this purpose.

[0058] Computer 22 also serves to display the measurement results. Computer 22 also provides a communication interface with a user.

[0059] The connection to the computer 22 can also be made wirelessly via a radio connection or via an optical cable.

[0060] The frequency response formed by the voltage testing system 3 in conjunction with the electrical circuit arrangement 8 is Fig. 2shown as a solid line 23. The attenuation A in dB is plotted against the frequency f in MHz. Thus, a bandpass filter is formed, the cutoff frequencies of which, in the example, are 100 kHz and 10-20 MHz.

[0061] For comparison, the dashed line 24 represents the frequency response that would result from the voltage testing system 3 alone in conjunction with the measuring device 16. Furthermore, the dashed line 25 represents the frequency response that would result from using the measuring device 16 in conjunction with a conventional HFCT sensor. Thus, the circuit arrangement 8, in conjunction with the capacitive voltage testing system 3, imitates the frequency response of a conventional HFCT sensor.

[0062] In Fig. 1The conductor 1 represents only one phase of a medium-voltage cable, to which the voltage detection system 3 is connected. In an analogous manner, voltage detection systems can also be connected to the other phases of a medium-voltage cable (not shown), at whose connection sockets a measurement of partial discharges is carried out in an analogous manner by means of a measuring device 16 with the interposition of a circuit arrangement 8.

[0063] A circuit arrangement 8 and / or measuring device 16 can also be designed to measure all phases simultaneously.

[0064] The frequency response matching resistor 9 is preferably a nonreactive resistor, as shown. It would also be conceivable and possible to implement such a resistor using circuit elements that emulate a nonreactive resistor, e.g., a field-effect transistor with a correspondingly controlled channel resistance. For example, a nonreactive resistor can also be emulated using operational amplifiers.

[0065] The same applies in principle to the impedance matching resistor 14.

[0066] A low-pass filter 12 could also be realized by performing appropriate signal processing of the digital signal after the A / D conversion.

[0067] Preferably, at the input of the circuit arrangement there is a Fig. 1A protective circuit (not shown) is provided to protect the downstream components from voltage spikes. Such protective circuits for absorbing voltage spikes are known. Semiconductor components, such as varistors, diodes, or gas discharge tubes, are particularly useful for this purpose.

[0068] In Fig. 3 A second embodiment of the invention is shown. Differences from the first embodiment are explained below. Otherwise, the statements regarding the first embodiment apply analogously.

[0069] In the embodiment of Fig. 3The circuit arrangement 8 and the measuring device 16 are housed in a common housing, indicated by a dashed line. The components of the circuit arrangement 8 and the measuring device 16 correspond to those described in the first exemplary embodiment. Thus, a measuring arrangement comprising the circuit arrangement 8 and the measuring device 3 is housed in a single housing, which is connected to the connection socket 7 of the capacitive voltage testing system 3.

[0070] In the second embodiment, the functionality of the computer 22 connected in the first embodiment is further integrated directly into this measuring arrangement. The measuring arrangement thus has a screen 26 and an input / output device 27, in particular comprising a keyboard and / or mouse, which are connected to the microprocessor unit 18. The arrangement can be provided in the same housing, or the screen 26 and / or the input / output device 27 can have their own housing. A corresponding writable memory is also integrated into the microprocessor unit 18.

[0071] On the other hand, in the second embodiment, analogous to the first embodiment, a separate computer 22 could also be connected to the measuring arrangement, i.e. to the output of the measuring device 16.

[0072] Conversely, in the first embodiment, the functionality of the computer 22 or a part thereof could be integrated directly into the measuring device 16, analogously to the second embodiment.

[0073] A third embodiment of the invention is described below with reference to Fig. 4 explained. The differences from the previously described embodiments are explained in particular below. Otherwise, the descriptions of the preceding embodiments can be applied analogously.

[0074] This embodiment is particularly suitable for measuring partial discharges in medium-voltage cables where the conductors 1a, 1b, 1c of all three phases are enclosed by a common cable shield 2. In such cables, partial discharges can occur not only between one phase and the cable shield, but also between two phases. All three phases are again equipped with a respective voltage detection system 3. Fig. 4 The voltage detection systems 3 are shown only for two of the phases. At the outputs of two of the voltage detection systems, a circuit arrangement 8 (not shown in Fig. 5). The circuit arrangement is modified here to have two inputs. The difference between these two phases is then calculated. If partial discharges occur between these two phases, the resulting voltage pulses at the outputs of the two associated voltage detection systems 3 are of inverse polarity. By calculating the difference, the voltage pulse of a respective partial discharge is doubled in magnitude, while interference signals that have the same or similar effects on both phases are eliminated or reduced.

[0075] Three such differential measurements can be carried out simultaneously to determine the partial discharges between two of the three lines 1a, 1b, 1c.

[0076] The difference calculation could also be performed in the measuring device 16 instead of in the circuit arrangement 8. Two circuit arrangements 8 would thus be connected to the two outputs of the voltage testing systems 3. The measuring device 16 would then have two inputs, which would be connected to the outputs of the two circuit arrangements 8.

[0077] The circuit arrangement could also have three inputs for all three phases, with the selection of two of the three phases for differential calculation being performed by appropriate switches, including electronic switches. If the differential calculation is performed in the measuring device, this could have three inputs.

[0078] A measuring arrangement according to the invention advantageously further comprises a time recording unit for recording the time difference between voltage pulses recorded by the measuring arrangement. This makes it possible to determine the location of a partial discharge in the cable. For this purpose, the measuring arrangement is connected at one end of the cable to a measuring output, which is designed, for example, as a connection socket, of a capacitive voltage testing system of a switchgear, and the time interval between the voltage pulse running directly from the point of partial discharge to the measuring arrangement and the voltage pulse running from the point of partial discharge to the remote cable end and reflected back at this remote cable end is recorded. From this time interval and the known propagation speed of the voltage pulse, the location of the partial discharge can be determined by an evaluation unit of the measuring arrangement.Statistical methods can be used to evaluate a large number of such events.

[0079] For example, in order to determine the location of the partial discharge even in very long cables (when the partial discharge pulse reflected at the far end of the cable cannot be detected or cannot be reliably detected), the use of measuring arrangements according to the invention in a measuring system for locating the location of a partial discharge (in a power grid with an operating voltage in the range of 1 kV to 69 kV) is schematically shown in Fig. 5Shown are a first and a second switchgear 28, at which a live cable 29 terminates. Each of the switchgears 28 has a capacitive voltage detection system 3 for the cable 29, which is provided to check whether the cable 29 is live. The capacitive voltage detection systems 3 can be designed in a conventional manner, in particular analogously to the Fig. 1 and 3 described. Preferably, the capacitive voltage detecting systems 3 are designed in accordance with the standard IEC 61243-5:1997.

[0080] Further cables 30, which end at each of the switchgear 28, are in Fig. 5 indicated. Switch 37 is also indicated.

[0081] The capacitive voltage testing systems 3 with their respective measuring outputs are thus arranged at the two ends of the cable 29. A measuring arrangement 31 is connected to the measuring output of each of the capacitive voltage testing systems, which measuring arrangement 31 comprises a circuit arrangement 8 which is designed in accordance with the Fig. 1 or 3 described circuit arrangement, and comprises a measuring device 16 which is configured in accordance with the Fig. 1 or 3 described measuring device.

[0082] Each measuring arrangement 31 additionally has a time recording unit 32. The time recording units 32 of the two measuring arrangements 31 are synchronized with each other using a common time base. Such a common time base can be provided, for example, by a GPS timestamp. For this purpose, an antenna is indicated on each time recording unit 32, which also receives the indicated GPS signals. Time synchronization using GPS is known.

[0083] The time recording units 32 determine the respective time at which a respective partial discharge pulse (=voltage pulse due to a partial discharge) is detected. By comparing the times at which the voltage pulses caused by the partial discharges are detected, the differences in the propagation times of these voltage pulses to the two measuring arrangements 31 can be determined.

[0084] To enable communication between the two measuring assemblies 31, the measuring assemblies 31 further comprise communication units 33. These communication units 33 can preferably be used to provide wireless communication between the two measuring assemblies 31, for example, via the mobile radio network. Wired communication (fiber optic cables, internet, intranet) is also conceivable and possible.

[0085] At least one of the measuring assemblies 31 also has an evaluation unit 34, which evaluates the measured values ​​recorded by the two measuring assemblies 31. In the exemplary embodiment, the measuring assemblies 31 are designed identically, so that each of the measuring assemblies 31 has an evaluation unit, with the evaluation being performed by one of the measuring assemblies 31, which is operated as the "master." The other of the measuring assemblies 31 is thus operated as the "slave," although more than one such additional measuring assemblies can also be present, as explained further below.

[0086] The evaluation unit 34 determines the distance of the location of the partial discharge from the relevant cable end based on the difference in the propagation times of a voltage pulse caused by a partial discharge to the two measuring arrangements 31, the length of the cable 29, and the known propagation speed of the voltage pulses. This allows the location of the cable 29 where the partial discharge occurred to be located. Preferably, a plurality of measurements are included in the evaluation.

[0087] The evaluation unit 34 could in principle also be a separate device which communicates with the measuring arrangements 31 or to which the measured values ​​obtained during the measurements are fed after the measurements.

[0088] The invention can also be used for so-called T-branched cables. One such cable is Fig. 6shown schematically as an example. Branch lines 36 lead from a main line 35. A switchgear 28 is arranged at each end of the main line 35 and at each end of the branch lines 36. The switchgear 28 each has a capacitive voltage detection system for the cable (not shown in Fig. 6 ). If, in addition to the arrangement of measuring arrangements, analog Fig. 5 If similar measuring arrangements are also provided at the end of one, several, or all branch lines 36 at both ends of the main line, with all measuring arrangements being time-synchronized, the localization of the location of the partial discharge in the T-branched cable can be improved. Localization in the area of ​​the branch lines 36 is also possible in this case.

[0089] Time synchronization between two or more measuring systems 31 can also be achieved using methods other than GPS, for example, via optical fiber transmission. If the time recording units are sufficiently accurate, it would also be conceivable and possible to first synchronize the measuring systems 31 at a common location and then transport them to their respective deployment sites.

[0090] The measuring devices 31 are each microprocessor systems. The time recording unit, communication unit, and evaluation unit are to be understood as functional units within such a microprocessor system and will generally share physical components, such as a microprocessor, memory modules, etc.

[0091] It can advantageously be provided that a respective one of the measuring arrangements according to Fig. 5It is also designed for individual use in a capacitive voltage detection system at one cable end to determine the location of the partial discharge from the time interval between the partial discharge pulse traveling directly to this cable end and the partial discharge pulse reflected at the remote cable end, as already described. This time interval can then be determined by the time recording unit 32 and evaluated by the evaluation unit 34.

[0092] A capacitive voltage detection system can be coupled to the conductor of the power grid cable in various ways. For example, the coupling capacitor of such a capacitive voltage detection system can be housed in an insulating support or a cable connector, or it can be mounted separately. Although it is advantageous in many situations for a connection socket to be provided as the measurement output of the capacitive voltage detection system, this is not absolutely necessary. The measurement output of the capacitive voltage detection system could also be wired directly to the circuit arrangement according to the invention. legend To the reference numbers 1 Director 25 line 1a Director 26 Screen 1b Director 27 Input / output device 1c Director 28 switchgear 2 Cable shield 29 Cable 3 Voltage testing system 30 Cable 4 Coupling capacitor 31 Measuring arrangement 5 Director 32 Time recording unit 6 screen 33 Communication unit 7 connection socket 34 Evaluation unit 8 Circuit arrangement 35 Main thread 9 Frequency response matching resistor 36 branch line 37 Switch 10 Input line 11 High-pass filter 12 Low-pass filter 13 amplifier 14 Impedance matching resistor 15 Output line 16 measuring device 17 Analog / digital converter 18 microprocessor unit 19 capacitor 20 Transformer 21 Diode protection circuit 22 computer 23 line 24 line

Claims

1. Electrical circuit arrangement, configured to connect an electrical measuring device (16), which is used to measure partial discharges in a power grid having an operating voltage in the range from 1 kV to 69 kV, and which comprises an A / D converter (17) for converting a measured voltage pulse into a digital signal and a microprocessor unit (18) for evaluating the digital signal, to a measurement output of a capacitive voltage testing system (3) of a power switching station for the power grid having the operating voltage in the range from 1 kV to 69 kV, wherein the electrical circuit arrangement (8) comprises a frequency response matching resistor (9), which is connected between ground and an input line (10) for connection to the live pole of the measurement output, and a high-pass filter (11) connected downstream of the frequency response matching resistor (9).

2. Electrical circuit arrangement according to claim 1, characterized in that the measurement output of the capacitive voltage testing system (3) is in the form of a connection socket (7).

3. Electrical circuit arrangement according to claim 1 or 2, characterized in that the rating of the frequency response matching resistor (9) is greater than 25 kΩ, the rating of the frequency response matching resistor (9) preferably being lower than 400 kΩ.

4. Electrical circuit arrangement according to claim 3, characterized in that the rating of the frequency response matching resistor (9) is in the range from 50 kΩ to 200 kΩ.

5. Electrical circuit arrangement according to any of claims 1 to 4, characterized in that the cut-off frequency of the high-pass filter (11) is in the range from 50 kHz and 200 kHz.

6. Electrical circuit arrangement according to any of claims 1 to 5, characterized in that a low-pass filter (12), the cut-off frequency of which is preferably in the range between 5 MHz and 20 MHz, is connected downstream of the frequency response matching resistor (9).

7. Electrical circuit arrangement according to claim 6, characterized in that an amplifier (13) is connected downstream of the high-pass filter (11) and the low-pass filter (12).

8. Electrical circuit arrangement according to claim 7, characterized in that an impedance matching resistor (14) for matching to the input resistor of the measuring device (16), the rating of which is preferably lower than 100 Ω, is connected downstream of the amplifier (13).

9. Measuring assembly for measuring partial discharges at a measurement output of a capacitive voltage testing system of a power switching station for a power grid having an operating voltage in the range from 1 kV to 69 kV, the measuring assembly comprising an A / D converter (17) for converting a measured voltage pulse into a digital signal and a microprocessor unit (18) for evaluating the digital signal, characterized in that the measuring assembly comprises an electrical circuit arrangement (8) according to any of claims 1 to 8 connected upstream of the A / D converter (17).

10. Measuring assembly according to claim 9, characterized in that the measurement output of the capacitive voltage testing system (3) is in the form of a connection socket (7).

11. Measuring assembly according to claim 9 or 10, characterized in that, in order to measure partial discharges between two conductors (1a, 1b, 1c) of a multi-pole cable for voltages in the range from 1 kV to 69 kV, the measurement signals obtained by way of the partial discharge measurements on the two conductors (1a, 1b, 1c) are subtracted from each other.

12. Measuring assembly according to any of claims 9 to 11, characterized in that, in order to pinpoint the location of a partial discharge, the measuring assembly comprises a time detection unit for detecting the time difference between voltage pulses detected by the measuring assembly.

13. Measuring system for pinpointing the location of a partial discharge in a power grid having an operating voltage in the range from 1 kV to 69 kV, characterized in that the measuring system comprises at least two measuring assemblies (31) according to any of claims 9 to 12 for measuring partial discharges at a respective measurement output of capacitive voltage testing systems (3) arranged at a respective end of a cable (29) of the power grid, and in that the measuring assemblies (31) comprise time detection units (32), capable of being synchronized with each other, for detecting a difference in the transit times of a voltage pulse, generated by a partial discharge, to the respective measuring assembly (31).

14. Measuring system according to claim 13, characterized in that the measuring system comprises at least one evaluation unit (34) for evaluating the detected difference in the transit times of the voltage pulse, generated by a partial discharge, in order to determine a location of the partial discharge, wherein preferably at least one of the measuring assemblies (31) comprises such an evaluation unit (34).

15. Measuring system according to any of claims 12 to 14, characterized in that the measuring assemblies (31) comprise communication units (33) for exchanging data with each other, preferably wirelessly.