SELF-TESTING MEASURING DEVICE

Description

[0001] The invention relates to a self-testing measuring device. Furthermore, the invention relates to a method for the self-monitoring of a measuring device. Additionally, the invention relates to a rail vehicle.

[0002] In the operation of an electric rail vehicle, electrical current is supplied via a so-called overhead circuit. This circuit carries high-voltage primary current from the overhead contact line. This primary current is picked up by a pantograph and transformed down to a lower voltage by a main transformer. The lower voltage is then converted by a four-quadrant converter into a DC link voltage for the traction circuit. From this DC link, the traction motors, which are powered by three-phase alternating current, are supplied with three-phase alternating current via a pulse inverter. Such an arrangement is used in FIG 1 depicted.

[0003] The overhead circuit also contains a primary current transformer, which reduces a very high primary current to lower values. The primary current transformer measures the primary current drawn from the railway power grid via the pantograph. It converts this primary current into a measurable, lower current suitable for standard devices. For example, 600 amperes are reduced to 1 ampere for a relay or measuring instrument.

[0004] A secondary-side digital interference current monitoring or current measuring device, as used in FIG 2 This method, as demonstrated, has been used to monitor the mains current of a rail vehicle with regard to low-frequency alternating current components, in order to detect a disturbance in the monitoring of a track section by a track occupancy detection system in the frequency range of a track voltage of a generator of the track occupancy detection system, for example in a frequency range around 42 Hz. Such monitoring of a track section is in FIG 3 bis FIG 5 This illustrates that since the interference current monitoring is absolutely safety-critical, it must function reliably. Therefore, monitoring the functionality of the interference current monitoring system is necessary.

[0005] Previously, this self-monitoring was achieved by exciting a test winding of a current transformer, which is connected upstream of the current monitoring device, with an AC power source carrying a test signal at a specific frequency, for example, 300 Hz. The test signal was detected via a measuring winding of the current transformer and measured by the current monitoring device. This means the test signal must always be part of the measurement signal, or always measured as part of the measurement signal. If the test signal is not detected, a defect is assumed.

[0006] The described self-monitoring requires a variety of components, such as an AC power source, a test winding, an amplifier circuit and wiring.

[0007] US Patent 7,791,353 B2 describes a system for detecting ground loops. The system includes an exciter and a detector. The exciter comprises an AC signal generator, an amplifier, and a converter. The detector includes a Rogowski coil, a filter, and a readout unit. The readout unit indicates the presence of a ground loop in a conductor located between the converter and the Rogowski coil.

[0008] Document US 4,198,595 A discloses a measuring device comprising a sensor, a secondary-side differential amplifier with a first input and a second input, wherein the first input is supplied with a predetermined first electrical potential and the second input with a predetermined second electrical potential different from the first electrical potential, and an output to which an output voltage offset is applied.

[0009] The task is therefore to enable simplified self-monitoring of a measuring device, in particular a disturbance current monitoring device of an electrified rail vehicle.

[0010] This problem is solved by a self-testing measuring device according to claim 1, a method for self-monitoring a measuring device according to claim 8 and a rail vehicle according to claim 12.

[0011] The self-testing measuring device according to the invention, preferably a current measuring device, particularly preferably a disturbance current monitoring device for a disturbance current transformer of a rail vehicle, has a passive current transformer unit with a primary side and a secondary side.

[0012] Furthermore, the self-testing measuring device comprises a secondary-side differential amplifier with a first input and a second input, the first input of which is connected to a predetermined first electrical potential and the second input to a predetermined second electrical potential different from the first, preferably an electrical reference potential. A differential amplifier is an electronic amplifier or an electronic amplifier circuit, preferably an operational amplifier, with two inputs, in which the difference between the two input signals is amplified. The differential amplifier also includes an output at which an output voltage offset is applied, representing the difference between the two input signals, optionally amplified accordingly.Part of the self-testing measuring device according to the invention is also an evaluation unit for comparing the measured output voltage offset with a reference value. This "comparison with a reference value" is intended to include a comparison with different reference value intervals, each representing different functional states of the measuring device, in particular fault-free operation and a defective state. By assigning the measured output voltage offset value to one of the reference value intervals, it can advantageously be determined whether the measuring device is functioning correctly or has a defect.

[0013] If the measuring device according to the invention is used as an interference current monitoring device for a rail vehicle, it preferably has a digital interference current monitoring unit with a frequency filter for a rail vehicle supplied with mains electrical current via a railway power network and a power supply line for measuring a potentially flowing interference current.

[0014] A current measuring device or interference current monitoring device specifically used for monitoring interference currents in rail vehicles preferably includes a control unit for switching off a main switch and contacting the power supply line, disconnecting the pantograph from the power supply line if a fault is detected during the comparison described above between the measured output voltage offset and a reference value. Advantageously, the monitoring of the interference current monitoring device's function is independent of the main switch closing, which is only possible several seconds after the rail vehicle is switched on. The function of the interference current monitoring device can therefore be tested even before the power supply line is contacted, thus increasing safety during rail vehicle operation.The measuring device according to the invention enables monitoring of the measuring device, and in particular of the passive current transformer unit encompassed by the measuring device, in accordance with IEC 61508 Safety Integrity Level (SIL) 2: The fault conditions "short circuit" and "open circuit" are reliably detected. If the measuring device is used as an interference current measuring device in a rail vehicle, the hardware of the existing interference current measuring device can be used for self-testing, whereby the output voltage offset measured when the interference current measuring device is switched on is compared with one or more suitable reference values ​​to determine whether the interference current measuring device is functioning correctly.Furthermore, due to the increased reliability of the passive converter, in the usual redundant design of interference current measuring devices in railway vehicles, a passive converter equipped with two preferably identical secondary windings for the measurement and test signals can be used for two differential amplifiers instead of one differential amplifier, according to the prior art. In addition, the test winding used in conventional interference current measuring devices, as well as the associated lines and resistors, can be eliminated. If the differential amplifier is part of an electronic assembly that typically has one or more outputs for controlling a test winding, these outputs can be used for other applications instead of the test winding, since the test winding and its control are no longer required due to the internal self-test function of the measuring device, preferably an interference current measuring device, according to the invention.

[0015] In the method for self-monitoring a measuring device with a passive converter unit having a primary side and a secondary side and a secondary-side differential amplifier comprising a first input and a second input, wherein the method is preferably applied to a current measuring device, and particularly preferably to a disturbance current monitoring device as a current measuring device in a rail vehicle, the measuring device is first switched on. Subsequently, a predetermined first electrical potential is applied to a first input of the secondary-side differential amplifier of the measuring device, and a predetermined second electrical potential, which differs from the first electrical potential, is applied to a second input of the secondary-side differential amplifier of the measuring device.Furthermore, an output voltage offset is measured at the output of the differential amplifier and compared with a reference value. The inventive method for self-monitoring a measuring device shares the advantages of the inventive self-testing measuring device.

[0016] The converter used for transforming the mains current, preferably a current transformer connected upstream of the differential amplifier, most advantageously includes a so-called Rogowski coil, which can measure the measured quantity, preferably a mains current, with mA accuracy. The coil is ironless and therefore exhibits no saturation effects and has excellent linearity. This current transformer is particularly reliable and suitable for precise measurements. If the measuring device is used for measuring interference current, the input offset (DC voltage) must be isolated from the interference frequencies (e.g., 42 Hz) by suitable frequency filters (low-pass, high-pass). The transient response of these frequency filters, used for frequency selection in the interference current monitoring device, is completely uncritical for monitoring the current transformer. The transient response typically lasts about 200 to 300 ms.In contrast, at least 3 seconds are required to release a main switch when a rail vehicle starts up, ensuring that a digital interference current monitoring device equipped with the self-testing measuring device according to the invention is ready for use in good time. This advantageously achieves early detection of a defect in a passive current transformer. If necessary, the monitoring of the measuring device can be carried out using modified software without modifying the conventional hardware, since a conventional interference current monitoring device is already installed in most rail vehicles and can be reused for the method according to the invention, possibly after appropriate modification.

[0017] It should be noted at this point that the method described above can be used not only for transformer-based current transformers, and in particular for current transformers based on the Rogowski coil principle, but can be used for all sensors that are designed in such a way that an external offset voltage at their output leads to a current through the sensor that (a) is measurable and (b) does not adversely affect the functioning of the sensor; in particular, the method can be used for inductive sensors.

[0018] The rail vehicle according to the invention has a pantograph for contacting the rail vehicle with a power supply line of a railway power network, a high-voltage circuit with a self-testing measuring device according to the invention, preferably a current measuring device according to the invention, a main switch for connecting and disconnecting the high-voltage circuit with or from the railway power network, and a control unit for controlling the main switch depending on a result of a self-test of the current measuring device.

[0019] The rail vehicle also preferably includes additional functional elements, in particular a main transformer and the aforementioned main switch for contacting the main transformer with the railway power network.

[0020] The aforementioned control unit disconnects the main transformer from the power supply line if the measuring device or current measuring device according to the invention detects a defect. The rail vehicle according to the invention shares the advantages of the self-testing measuring device according to the invention.

[0021] A typical fault response is the opening of the main switch; the pantograph is only automatically lowered if a defect in the main switch is detected / suspected.

[0022] Some of the aforementioned components of the measuring device according to the invention can be implemented wholly or partially in the form of software modules in a processor of a corresponding computer system, z.B. from a control unit or an existing computer system of a rail vehicle. A largely software-based implementation has the advantage that even previously used computer systems can be easily retrofitted by a software update to operate in the manner of the invention. In this respect, the problem is also solved by a corresponding computer program product with a computer program that can be directly loaded into a computer system, with program sections to execute the steps of the method according to the invention, at least the steps executable by a computer, in particular the frequency selection step and the step of comparing the measured output voltage offset with a reference value, when the program is executed in the computer system. In addition to the computer program, such a computer program product may optionally include additional components, such as...Documentation and / or additional components, including hardware components such as hardware keys (dongles, etc.) for using the software, may be included.

[0023] For transport to and / or storage on or in the computer system or control unit, a computer-readable medium, such as a memory stick, a hard drive, or other portable or permanently installed data carrier, can be used. This medium stores the program sections of the computer program that can be read and executed by a computer system. The computer system may, for example, include one or more cooperating microprocessors or similar components.

[0024] The dependent claims and the following description each contain particularly advantageous embodiments and further developments of the invention. Furthermore, within the scope of the invention, the various features of different embodiments and claims can also be combined to form new embodiments.

[0025] Preferably, the self-testing interference current monitoring device according to the invention is configured such that the predetermined second potential is lower than the first potential. In this variant, an electric current flows from the first input of the differential amplifier via the secondary winding of the passive current transformer unit to the second input of the differential amplifier if the passive current transformer unit is functioning correctly or is short-circuited.

[0026] A ground potential is particularly preferred as the predetermined second potential. Advantageously, the second potential can be established by a simple electrical ground connection.

[0027] The evaluation unit of the measuring device according to the invention, preferably the current measuring device according to the invention, is also preferably configured to determine one of the following states of the measuring device according to the invention based on the comparison: a short circuit of the two inputs in the event that the output voltage offset has a value in a first predetermined value interval, particularly preferably a value of 0 mV, error-free operation in the event that the output voltage offset has a value in a second predetermined value interval which does not overlap with the first value interval, a cable break in the event that the output voltage offset has a value in a third predetermined value interval with values ​​higher relative to the first predetermined value interval and the second predetermined value interval, which does not overlap with the first and second predetermined value intervals.

[0028] It is advantageous to differentiate between various defects, namely a broken cable or a short circuit. Furthermore, a broken cable and a short circuit can also be distinguished from a correctly functioning state.

[0029] The first predetermined value interval preferably includes a value of 0 mV. The second predetermined value interval preferably lies between 7 and 10 mV, and the third predetermined value interval preferably lies between 70 and 100 mV. Advantageously, the value intervals associated with the correct operating state and a cable break are sufficiently far apart so that the different states can be easily distinguished from one another.

[0030] The evaluation unit of the measuring device according to the invention preferably comprises a first and a second parallel monitoring unit. The first parallel monitoring unit preferably comprises a first filter stage with a bandpass filter having a center frequency of an expected interference current, for example 42 Hz, a second filter stage with a bandpass filter having a frequency higher than the center frequency, for example 100 Hz, and a current amplitude-time monitoring unit. The two filter stages and the current amplitude-time monitoring unit constitute the actual interference current monitoring.

[0031] In parallel, the evaluation unit comprises a second monitoring unit, preferably with a low-pass filter, to separate the output voltage offset of the differential amplifier of the measuring device from the alternating signals of the detected interference currents in the primary current. Furthermore, the second monitoring unit includes a comparator unit for the self-test according to the invention, i.e., for checking, based on the output voltage offset, whether the measuring device is defective. That is, the output voltage offset is preferably checked to determine whether it is in the "normal" range, the "short circuit" range, or the "open circuit" range. By extending the evaluation unit with the second parallel monitoring unit, a conventional interference current measuring device is advantageously enhanced with the self-test function according to the invention.

[0032] The invention is explained in more detail below with reference to the accompanying figures and exemplary embodiments. The figures show: FIG 1 a schematic representation of a conventional rail vehicle, FIG 2 a schematic representation of a conventional interference current monitoring unit, FIG 3 a schematic representation of a track current measurement between a generator and a motor relay, FIG 4 a schematic representation of the detection of a rail vehicle in a track section, FIG 5 a schematic representation of a scenario in which a disturbance current is generated by a rail vehicle, FIG 6 a schematic representation of a block diagram of a disturbance current monitoring device, FIG 7 a schematic representation of a current measuring device according to an embodiment of the invention, FIG 8 a flowchart illustrating a method for self-monitoring a current measuring device according to an embodiment of the invention, FIG 9 a schematic representation of an interference current monitoring device according to an embodiment of the invention, FIG 10 a schematic representation of a rail vehicle according to an embodiment of the invention, FIG 11 a schematic representation of the interaction of the interference current monitoring function with the monitoring of the primary current transformer according to the invention, FIG 12 A diagram illustrating a typical input signal of an interference current measuring device.

[0033] In FIG 1 A schematic representation of an electrified rail vehicle 1 is illustrated. The electrified rail vehicle 1, in this case a rail vehicle exclusively for operation with high voltage in the form of alternating current, comprises a pantograph 2 for supplying electrical energy from a railway power network N, in this case an AC railway power network. The pantograph 2 is electrically connected to a primary current transformer PW via a main switch 3. The primary current transformer PW converts the alternating current into lower currents in the range of 100 to 1200 A. A disturbance current transformer RS ​​of a disturbance current monitoring unit DSU is connected downstream of the primary current transformer PW. Such a disturbance current monitoring unit DSU measures disturbance currents in the range of a few milliamperes. A main transformer 4, which steps down the high voltage, is connected downstream of the disturbance current monitoring unit DSU.Electrically connected to the main transformer 4 are two power converters 5, which convert the alternating current into three-phase current for the traction motors via a DC intermediate circuit.

[0034] In FIG 2 is the in FIG 1 The shown interference current monitoring unit DSU of a rail vehicle 1 with a Rogowski coil RS and a monitoring circuit US is shown in a rough schematic. The Rogowski coil RS comprises on the primary side a current-carrying conductor L through which a mains current IN flows, and on the secondary side coil windings in the form of air coil windings TW, M. A first air coil winding of the coil windings comprises a measuring winding M (in FIG 2 (shown below) and a second air coil winding of the coil windings includes a test winding TW (in FIG 2 (shown above).

[0035] The monitoring circuit US is connected to the two coil windings TW and M on the secondary side and comprises a measuring amplifier circuit V1, a test amplifier circuit V2, an analog-to-digital converter W, and a digital signal processor DSP. The measuring winding M is connected to the measuring amplifier circuit V1, whose output is connected to the digital signal processor DSP via the A / D converter W. The test winding TW is driven by the digital signal processor DSP via the amplifier circuit V2.

[0036] In FIG 3 Figure 1 is a schematic representation of an unoccupied track section 10 with track monitoring using track current measurement between a generator 6 (bottom left in the image) and a motor relay 8 (bottom right in the image). The track current measurement is used to determine whether track section 10 is clear of a rail vehicle or occupied. This is intended to prevent collisions between two rail vehicles traveling on the same track or rails 7a, 7b. The FIG 3 The generator 6 shown on the left below generates two electrical voltages that are 90° out of phase, one of which is between the rails, i.e., the insulated rail 7a (at the top of the image) and the earth rail 7b (drawn between the generator and the insulated rail), of track section 10, and the other is conducted via a power line 6a to the motor relay 8, which is located in FIG 3 The motor relay 8 is held in a rest position by spring force, as shown on the lower right. The two electrical voltages generate a rotating magnetic field and thus a torque. Therefore, when track section 10 is unoccupied, the motor relay 8 rotates into its operating position, and track section 10 is recognized as clear.

[0037] In FIG 4 is the in FIG 3 The previously shown track section 10 is depicted in a situation where a rail vehicle 1 is located on the monitored track section 10. The rail vehicle 1 short-circuits the track voltage between rails 7a and 7b with its chassis, causing the rotating magnetic field in the motor relay 8 to disappear. The spring pulls the motor relay 8 into its rest position, and track section 10 is thus recognized and reported as occupied.

[0038] However, the return current of an electric rail vehicle 1 can interfere with the track occupancy detection if it corresponds at the measuring point to the supplied current, i.e., the current measured in the case of a clear track section. For this to occur, a response threshold must be exceeded at the operating frequency of motor relay 8, and this exceedance must persist long enough for motor relay 8 to activate and for the activation to be registered in the interlocking system (not shown).

[0039] In FIG 5 Figure 1 shows a schematic representation of a scenario in which such a disturbance current is generated by a rail vehicle 1.

[0040] To avoid or detect such safety-critical interference currents, rail vehicles 1 are equipped with an interference current monitoring unit DSU (in FIG 5 between overhead line OL and rail vehicle 1, see also details in FIG 2 This interference current monitoring unit (DSU) measures the reverse current and detects when it exceeds certain limits. For example, within a predetermined frequency range (e.g., 42 Hz), the reverse current must not exceed a predetermined current limit for a period longer than a predetermined duration to ensure the motor relay always operates correctly. Typical values ​​are 42 Hz ± 2 Hz for the frequency range, 2.8 amperes for the current limit, and 0.5 s for the predetermined duration.

[0041] In FIG 6 A schematic representation of a block diagram of a fault current monitoring unit (DSU) of a rail vehicle is shown in detail. As already mentioned in FIG 2 The interference current monitoring unit shown is an air coil or measuring winding M of a Rogowski coil RS (top left in the image). FIG 6 (shown) are electrically connected. In this image, the amplifier units V1 and V2 and the test winding TW are made of FIG 2 For clarity, it is not shown separately and is considered part of the A / D converter W. The measured mains current IN of the rail vehicle 1 is acquired and digitized by an A / D converter W. The measured mains current IN is further processed and evaluated by a digital signal processor DSP. For this purpose, the digital signal processor DSP includes a frequency filter F, which filters the frequency components to be monitored from the input signal. Furthermore, the digital signal processor includes a monitoring unit IT, which compares the amplitude I of the input signal with a current limit value and, if the current limit value is exceeded, the duration T of the exceedance with a time limit value. An exceedance of the required limit values ​​is triggered by opening the main switch 3 (see FIG 1 ) Reacts.

[0042] The current monitoring unit DSU includes a unit K for defining coefficients for generating monitoring channels, which can be parameterized as desired in their filter characteristics and monitoring properties, for monitoring up to 5 frequency bands between 20 Hz and 450 Hz. The parameterization of the frequency filter F is performed via a parameterization file PD, which is stored in a parameterization unit PE, located in FIG 6 The parameter file PD is symbolized by a computer and stored in a database. During the vehicle design phase, the filter coefficients and monitoring parameters are determined offline from this parameter file.

[0043] Furthermore, the current monitoring unit DSU also monitors itself. For this purpose, it includes software self-monitoring EU1 and hardware self-monitoring EU2. In the event of a fault, a main switch trip AL and a fault message STM are sent to the vehicle control system or control unit SE of the rail vehicle. If the fault current reaches a lower threshold, at which the function of the motor relay is not yet impaired, a "softer" reaction, such as a torque reduction, can be triggered via a response message AS.

[0044] In FIG 7 Figure 70 shows a schematic representation of a current measuring device according to an embodiment of the invention.

[0045] The current measuring device 70 comprises a passive current transformer unit RS with a primary winding L and a secondary winding, also called the measuring winding M, and a measuring amplifier circuit in the form of a differential amplifier V1. A first, non-inverting input E1 of the differential amplifier V1 is connected to a reference potential U1 via a first complex resistor Z1, and a second, inverting input E2 of the differential amplifier V1 (feedback to the output of the differential amplifier V1 via a second complex resistor Z2 and via a complex resistor Z3) is connected to a second reference potential U2, preferably ground GND. Due to the asymmetry of the potentials U1 and U2, a DC current is generated that flows through the measuring winding M of the passive current transformer unit RS. A voltage offset UA is measured at the output of the differential amplifier V1, proportional to this DC current.As mentioned previously, this voltage offset UA is 0 volts when the passive current transformer unit RS has a short circuit. If the passive current transformer unit RS is functioning correctly, the voltage offset UA value is in the middle range. If the passive current transformer unit RS has a broken wire, the voltage offset UA value is in the higher range. The specific ranges of the voltage offset UA values ​​for the different states of the passive current transformer unit RS described above also depend on the values ​​of the complex resistances Z1, Z2, and Z3.

[0046] In FIG 8 Figure 800 illustrates a method for self-monitoring a current measuring device 70, in this case a disturbance current monitoring device, with a passive current transformer unit RS having a primary side and a secondary side and a secondary-side differential amplifier V1 with a first input E1 and a second input E2 according to an embodiment of the invention.

[0047] In step 8.I, the interference current monitoring device 70 is switched on before the pantograph of a rail vehicle makes contact with an overhead line and before the main switch of the rail vehicle is switched on. This allows the correct function of the interference current monitoring device 70 to be tested before the mains voltage is applied to the interference current monitoring device 70 and a defect in the interference current monitoring device 70 could potentially cause further damage to the electronics.

[0048] In step 8.II, a predetermined first electrical potential U1 is applied to the first input E1 of the differential amplifier V1 of the interference current monitoring device 70, and a predetermined second electrical potential GND, which differs from the first electrical potential U1, is applied to the second input E2 of the input-side differential amplifier V1 of the interference current monitoring device 70.

[0049] In step 8.III, an output voltage offset UA is measured at the output of the differential amplifier V1.

[0050] In step 8.IV, the measured output voltage offset UA is compared with a reference value or different reference value intervals of the interference current monitoring device 70, which are defined for different states (short circuit, correct function, cable break). If a defect is detected in step 8.IV, a message can be sent to the operator or the driver of the rail vehicle, further operation of the rail vehicle can be prevented, and repair of the interference current monitoring device 70 can be initiated.

[0051] In FIG 9 A schematic representation of an interference current monitoring device 70 according to a second embodiment of the invention is shown. The FIG 9 The arrangement shown essentially corresponds to the one in FIG 7 The arrangement shown is a composite circuit. Instead of the basic circuit of a non-inverting amplifier with an operational amplifier, the basic circuit of a differential amplifier with an operational amplifier was used. In addition to the resistors determining the gain, the capacitors typically required to limit the amplifier bandwidth, as well as a burden resistor RB for matching the source resistance of the current transformer, were also included. The first potential according to the invention is designated U1, and the second potential according to the invention, U2, is designated GND. A resistor RB in parallel with the measuring winding M of the passive current transformer RS ​​symbolizes a burden or load at the inputs of the differential amplifier V1. The burden resistor RB has a low resistance. Typical values ​​for the burden resistor range from 1 to 1000 ohms. The choice of resistor values ​​depends on the type of passive current transformer.Resistors R1, R2a, and R2b determine the gain of the differential amplifier and are chosen so that the amplifier reaches full output at maximum operating signal amplitude, but is not yet overdriven. Capacitors C1 and C2 implement a band limiter, common in amplifiers of this type, to prevent high-frequency oscillations.

[0052] In FIG 10 Figure 1 shows a schematic representation of a rail vehicle 1 according to an embodiment of the invention. The rail vehicle 1 has a pantograph 2 for contacting the rail vehicle 1 with a power supply line OL of a railway power network.

[0053] The rail vehicle 1 also has a high-voltage circuit with a primary current transformer unit PW and a self-testing current measuring device 70 according to the invention, as described in FIG 7 and FIG 9 as shown in detail. Part of the rail vehicle 1 is also a control unit SE for controlling the pantograph 2 depending on the result of a self-test of the current measuring device 70.

[0054] FIG 11 includes a schematic representation of the interaction of the interference current monitoring function with the monitoring of the primary current transformer according to the invention.

[0055] In FIG 11 is shown how a DSU (Disturbance Current Monitoring Unit) is designed according to FIG 6 to extend the monitoring according to the invention. The signal of the primary current transformer (Rogowski coil RS) is amplified by the amplifier V1, which according to FIG 9 The signal is implemented and digitized using the analog-to-digital converter W. The signal is then distributed to the parallel monitoring units. For example, for the DB (Deutsche Bahn) railway network, filter stage F1 is a bandpass filter with a center frequency of 42 Hz, F2 is a bandpass filter with a center frequency of 100 Hz. These two bandpass filters, together with the current amplitude-time monitoring units IT, constitute the actual interference current monitoring. In parallel, a low-pass filter F3 is used to separate the output offset of amplifier V1 from the alternating signals of the detected interference currents in the primary current. The control unit SE can then check whether the output offset is in the "normal" range, the "short circuit" range, or the "open circuit" range.

[0056] In FIG 12Figure 1 shows a diagram illustrating a typical input signal US of an interference current measuring device 70. The input signal US has a DC component UG, which is suppressed during an interference current measurement by applying a high-pass filter. The input signal US also has an AC component, which oscillates, for example, at an interference current frequency of 42 Hz and is evaluated during an interference current measurement. For the self-test according to the invention, however, the DC component UG generated by the offset voltage applied to the input of the differential amplifier V1 is precisely what is determined. The DC component UG can be separated, for example, by a low-pass filter.

[0057] It should be noted that the above-described method can be used not only for transformer-based and Rogowski-coil-based current transformers, as described here, but can be used for all sensors that are designed in such a way that an external offset voltage at their output leads to a current through the sensor that (a) is measurable and (b) does not adversely affect the functioning of the sensor, especially for inductive sensors.

[0058] Finally, it should be noted once again that the methods and devices described above are merely preferred embodiments of the invention and that the invention can be varied by a person skilled in the art without departing from the scope of the invention, insofar as it is defined by the claims. For the sake of completeness, it should also be noted that the use of the indefinite articles "a" or "an" does not preclude the possibility that the features in question may be present multiple times.

Claims

1. Self-testing measuring device (70) comprising: - a sensor having a finite source resistance and a secondary side, - a secondary-side differential amplifier (V1) having - a first input (E1) and a second input (E2), - the first input (E1) carrying a predetermined first electrical potential (U1) and - the second input (E2) carrying a predetermined second electrical potential (U2), which is different from the first electrical potential (U1), - an output to which an output voltage offset (UA) is applied, characterized in that the self-testing measuring device additionally comprises an evaluation unit (DSP) for comparing the measured output voltage offset (UA) with a reference value.

2. Self-testing measuring device according to Claim 1, the sensor comprising a passive current transformer unit (RS) having a primary side and a secondary side.

3. Self-testing measuring device according to Claim 2, the passive current transformer unit (RS) comprising a Rogowski coil.

4. Self-testing measuring device according to one of the preceding claims, the predetermined second potential (U2) being lower than the first potential (U1).

5. Self-testing measuring device according to one of the preceding claims, the predetermined second potential (U2) comprising an earth potential (GND).

6. Self-testing measuring device according to one of the preceding claims, the evaluation unit (DSP) being configured to take the comparison as a basis for determining one of the following states of the measuring device (70): - a short circuit between the two inputs (E1, E2) if the output voltage offset (UA) is a value in a predetermined first value range, - correct operation if the output voltage offset (UA) is a value in a second predetermined value range, which does not overlap the first value range, - a cable fracture if the output voltage offset (UA) is a value in a third predetermined value range, which does not overlap the first and second predetermined value ranges.

7. Self-testing measuring device according to Claim 6, the first predetermined value range being at approximately 0 mV, the second predetermined value range being between 7 and 10 mV, and the third predetermined value range being between 70 and 100 mV.

8. Method for self-monitoring of a measuring device (70) comprising a passive current transformer unit (RS) having a primary side and a secondary side and a secondary-side differential amplifier (V1) having a first input (E1) and a second input (E2), comprising the steps of: - switching on the measuring device (70), - applying a predetermined first electrical potential (U1) to the first input (E1) of the differential amplifier (V1) of the measuring device (70), - applying a predetermined second electrical potential (U2), which differs from the first electrical potential (U1), to the second input (E2) of the input-side differential amplifier (V1) of the measuring device (70), - measuring an output voltage offset (UA) at the output of the differential amplifier (V1), characterized by: - comparison of the measured output voltage offset (UA) with a reference value.

9. Method according to Claim 8, wherein the output voltage offset (UA) is determined from an amplifier output signal of the differential amplifier (V1) using a low-pass filter.

10. Method according to Claim 8 or 9, wherein the self-monitoring, after being switched on, is started before a main switch ON state.

11. Method according to one of Claims 8 to 10, the measuring device (70) comprising an interference current measuring device for a rail vehicle.

12. Rail vehicle (1) comprising: - a pantograph (2) for contact-connecting the rail vehicle (1) to a power supply line (OL) of a railway power supply network (N), - a high-voltage circuit comprising a self-testing measuring device (70) according to one of Claims 1 to 7, - a main switch (3) for connecting and disconnecting the high-voltage circuit to and from the railway power supply network (N), - a control unit (SE) for controlling the main switch (3) on the basis of a result of a self-test of the measuring device (70).

13. Computer program product comprising instructions that, when the program is executed by a computer, cause the computer to carry out the steps of the method for self-monitoring of a measuring device (70) according to one of Claims 8 to 11.

14. Computer-readable storage medium comprising instructions that, when executed by a computer, cause the computer to carry out the steps of the method for self-monitoring of a measuring device (70) according to one of Claims 8 to 11.