Electrical measuring arrangement and measuring method for earth connection monitoring for an insulation monitoring device and insulation monitoring device

Abstract

The invention relates to an electrical measuring arrangement (10) and a measuring method for earth connection monitoring for an insulation monitoring device (4), which is connected to at least one active conductor (L1, L2) of an unearthed power supply system (2) and to an earth connection point (PE) of an electrical installation (3), with an active loop measuring device (20), which can be connected between an earth connection terminal (E) of the insulation monitoring device (4) and the earth connection point (PE) and has an independent excitation voltage source (U gEKE ) for the quantitative determination of an earth resistance (R EKE ) of the earth connection (6) between the insulation monitoring device (4) and the earth connection point (PE). Furthermore, the invention relates to an insulation monitoring device (30) with an electrical measuring arrangement (10) according to the invention.

Description

[0001] The invention relates to an electrical measuring arrangement and a measuring method for earth connection monitoring for an insulation monitoring device which is connected to at least one active conductor of an unearthed power supply system and to an earth connection point of an electrical installation.

[0002] Furthermore, the invention relates to an insulation monitoring device which can be connected to at least one active conductor of the unearthed power supply system and to an earth connection point of an electrical system in order to monitor an insulation resistance of an unearthed power supply system.

[0003] When increased requirements for operational, fire, and contact safety of electrical systems are met, an ungrounded power supply system is used, also known as an isolated network (French: "Isole Terre" - IT) or an IT power supply system. In this type of power supply system, the active parts of the power supply system are isolated from the earth potential—to earth.

[0004] The exposed conductive parts of the connected electrical loads are grounded individually or collectively. The advantage of these systems is that even the first insulation fault does not affect the function of the connected electrical loads, allowing continued operation despite the faulty insulation. Due to the ideally infinite electrical resistance (insulation resistance) between an active conductor of the system and ground potential, a closed circuit cannot form.

[0005] The insulation resistance of the unearthed power system must therefore be constantly monitored, since a possible further fault (insulation fault) on another active conductor would create a fault loop and the resulting fault current, in conjunction with an overcurrent protection device, would result in the system being shut down and operational standstill.

[0006] Insulation monitoring devices (IMDs) are used to monitor insulation resistance. State-of-the-art, actively operating insulation monitoring devices are connected between the active conductors on the one hand and earth on the other, and superimpose a preferably pulse-shaped measuring voltage (measurement pulses) on the network. If an insulation fault occurs, the measuring circuit between the network and earth closes across the insulation fault, resulting in a measuring current proportional to the insulation fault. This measuring current causes a corresponding voltage drop across a measuring resistor in the insulation monitoring device, which is evaluated by electronics and triggers an alarm if a preset limit is exceeded.

[0007] There are also passive monitoring devices (earth fault monitors) which, however, can only detect asymmetrical insulation faults using voltage measurement methods such as the 3-voltmeter method.

[0008] In all cases of insulation monitoring, minimal contact resistance must be maintained to ensure contact between the earth connection via the insulation monitoring device's earth connection terminal and the earth connection point (earth potential) when determining the insulation resistance. To ensure proper functioning of the insulation monitoring device, the low-resistance connection (earth connection) of the insulation monitoring device to the earth potential must be continuously monitored in order to generate a warning in the event of deterioration or complete loss of the earth connection.

[0009] A regulation for earth connection monitoring can be found, for example, in the standard DIN EN 61557-8:2015, Annex Chapter A.4.2.2 "Testing the function of an indicator indicating a loss of connection to the monitored system." Accordingly, the indicator for loss of the earth connection and the loss of connection to the monitored network must be tested. An alarm must be triggered if the earth connection or the mains connection, or both connections together, are interrupted.

[0010] According to the state of the art, insulation monitoring devices are available on the market which, due to their single-pole coupling to the earth potential, are able to detect a drop in insulation resistance or an interruption in the earth connection, but cannot determine whether the resistance value measured by the insulation monitoring device is the actual insulation resistance of the unearthed power system to earth or is due to a faulty earth connection, since the resistors involved in the measuring circuit are connected in series.

[0011] Insulation monitoring devices are also known from the prior art which have a second earth connection terminal and detect an interruption in the earth connection by means of a passive back measurement of the measuring voltage fed in by the insulation monitoring device.

[0012] The disclosure DE 10 2017 209 243 A1 describes a method and a measuring system for detecting insulation faults and cable defects occurring during the manufacturing process of a modular voltage source as early as possible. Electrical parameters such as the leakage resistance and the leakage capacitance are determined synchronously with the assembly progress. In the largely automated manufacturing process, a connection monitoring system also ensures that the measuring system is properly connected to the voltage source. A conductor loop is connected to the ground connection and to an additional ground connection of the insulation monitoring device, which is arranged parallel to the ground connection.

[0013] German Patent Application DE 43 39 946 A1 discloses a method and a device implementing this method for monitoring the insulation of ungrounded direct current and alternating current systems, using a pulsed alternating voltage with alternating pulse voltage values. To shorten measurement times, the temporal progression of the transient response of the measuring current, or a quantity derived therefrom, depending on the current system, is monitored for each pulse voltage value of the measuring voltage until the transient state is reached. The device may include a passive test circuit to monitor for proper earthing.

[0014] However, all known insulation monitoring devices only distinguish between a good and a bad condition of the earth connection in a binary "hard" 1 / 0 decision - a quantitative determination of the earth resistance of the earth connection is not provided, so that the triggering of an insulation resistance alarm message is always subject to an error probability.

[0015] Likewise, in the case of active insulation monitoring devices, it is not known how to determine the earthing resistance independently of the measuring voltage of the insulation monitoring device.

[0016] The present invention is therefore based on the object of designing an electrical measuring arrangement and a method with which a more reliable monitoring of the earth connection is possible for both actively and passively operating insulation monitoring devices and for both active and inactive measuring voltage.

[0017] This task is solved by an active loop measuring device which can be connected between an earth connection terminal of the insulation monitoring device and the earth connection point and has an independent excitation voltage source for the quantitative determination of an earth resistance of the earth connection between the insulation monitoring device and the earth connection point.

[0018] The basic idea of ​​the present invention is to determine the earthing resistance of the earth connection using a standalone measuring system that is independent of the measurement voltage of the insulation monitoring device. For this purpose, the measuring system is designed as an active loop measuring device with a standalone excitation voltage source and is thus capable of reliably monitoring the earth connection even when the measurement pulse of the insulation monitoring device is inactive or in the case of passive insulation monitoring devices.

[0019] The measuring setup is connected to the earth terminal of the insulation monitoring device on the one hand and the earth connection point (PE) on the other, enabling a "soft" decision for a quantitative determination of the earth resistance of the earth connection between the insulation monitoring device and the earth connection point. This allows the quality (continuity) of the earth connection to be assessed in a graded manner, thus ensuring that the insulation monitoring device delivers reliable results, especially at alarm thresholds in the low-resistance measuring range.

[0020] By adjusting the frequency or signal shape, it is also possible to respond to any interference signals more easily and independently of the measurement technology used for the actual insulation monitoring. Another advantage is that quantifying the earth resistance makes it possible to adapt the alarm thresholds for earth connection monitoring to customer needs.

[0021] In addition, the development of insulation monitoring devices overcomes the often occurring difficulty that, due to external factors, the circuit reference potential must be set equal to the earth potential.

[0022] In a further embodiment, the loop measuring device has a series circuit comprising the excitation voltage source for superimposing an excitation signal, a loop measuring resistor, a loop coupling resistor, a measuring and filtering circuit which detects a loop measuring voltage between the loop measuring resistor and the loop coupling resistor, a microcontroller which evaluates the loop measuring voltage to calculate the earth resistance, and a control earth connection terminal for connecting the loop coupling resistor to the earth connection point.

[0023] The active loop measuring device consists of a series connection between the earth terminal of the insulation monitoring device and a control earth terminal which is connected to the earth connection point.

[0024] The series circuit comprises the excitation voltage source, a loop measuring resistor and a loop coupling resistor, a measuring and filtering circuit, and a microcontroller as functional elements.

[0025] Since line and contact resistances (transition resistances) are present both at the earth connection terminal of the insulation monitoring device and at the control earth connection terminal of the loop measuring device provided according to the invention, a resistive current loop forms across the earth connection point and the control earth connection terminal. The current flowing in this loop is driven by the excitation voltage superimposed by the excitation voltage source, independently of the mains voltage and the measurement voltage of the insulation monitoring device. Knowing the loop measurement resistance and the loop coupling resistance, as well as the loop measurement voltage detected by the measurement and filter circuit, the microcontroller calculates the earth resistance of the earth connection between the insulation monitoring device and the earth connection point.

[0026] Preferably, the excitation voltage is a bipolar square wave voltage with a fundamental frequency in the range of 0.1 Hz to 10 Hz and a voltage swing of less than 10 V.

[0027] The frequency range from 0.1 Hz to 10 Hz can be separated with sufficient accuracy in the low-pass filter and measurement circuit. In the case of significant interference from external sources, such as frequency converters, it would be possible to optimize the excitation voltage of the excitation voltage source via the microcontroller through a software change to a sine wave mix of multiple frequencies. Targeted frequency analysis using DFT / FFT then allows only undisturbed signal components to be used for further analysis.

[0028] Due to the influence of the insulation monitoring device's measurement voltage source, which is unsynchronized to the excitation voltage source, or due to external sources, extraneous direct currents may occur on the ground connection, which could interfere with the measurement method according to the invention. To ensure robustness against this, the voltage swing of the excitation voltage and the voltage swing of the loop measurement voltage are used to calculate the ground resistance instead of the absolute levels.

[0029] It is advantageous to connect a TVS diode between the earth terminal of the insulation monitoring device and the control earth terminal of the loop measuring device.

[0030] By placing a TVS diode (transient voltage suppressor or suppressor diode) with a breakdown voltage of less than 10 V between the ground terminal and the control ground terminal, the connection via the control ground terminal can act as a redundant ground connection. The TVS diode also limits the extraneous DC voltage for circuit protection. A TVS diode response is detectable by the measuring and filtering circuitry in conjunction with the microcontroller and can be evaluated as an impermissible condition.

[0031] Furthermore, an insulation monitoring device according to the invention is claimed, in which a known insulation monitoring device relating to the invention is extended with the electrical measuring arrangement according to the invention for earth connection monitoring.

[0032] In addition to an embodiment of the electrical measuring arrangement according to the invention as a separate structural unit separated from an insulation monitoring device, integration into an insulation monitoring device in the form of a common structural unit is also possible.

[0033] The function of the electrical measuring arrangement according to the invention described above is based on the measuring method for earth connection monitoring for an insulation monitoring device described in the independent method claim. In this respect, the aforementioned technical effects and resulting process-related advantages also apply to the method features.

[0034] Further advantageous design features will become apparent from the following description and the drawings, which illustrate a preferred embodiment of the invention using examples. They show: Fig. 1: a state-of-the-art earth connection monitoring system and Fig. 2: an earth connection monitoring system with insulation monitoring device and an electrical measuring arrangement according to the invention.

[0035] Fig. 1 shows a prior art earth connection monitoring device with an insulation monitoring device 4, which is connected to the active conductors L1, L2 of an unearthed power supply system 2 and via an earth connection 6 to an earth connection point PE of an electrical system 3.

[0036] In Fig. 2 shows an earth connection monitoring system with an insulation monitoring device 4 connected to the unearthed power supply system 2 and an electrical measuring arrangement 10 according to the invention.

[0037] The insulation monitoring device 4 is connected via coupling resistors R aIMD coupled to the active conductors L1, L2 of the unearthed power supply system 2 and includes a measuring voltage generator U gIMD for supplying a measuring voltage UgIMD and a measuring resistor R mIMD for measuring a measuring current I corresponding to the insulation resistance m . The insulation monitoring device 4 is connected to the reference potential GND via the earth connection terminal E to the earth connection point PE in order to establish the earth connection 6.

[0038] The electrical measuring arrangement 10 according to the invention is arranged between the earth terminal E of the insulation monitoring device 4 and a control earth terminal KE. The electrical measuring arrangement 10 is designed as an active loop measuring device 20 in the form of a series circuit. The series circuit comprises an excitation voltage source U gEKE , which has an excitation voltage U gEKE The excitation voltage U gEKE drives a current in a current loop via the earth terminal E and the control earth terminal KE with the contact resistances R E and R KE .

[0039] As a result, a loop measuring resistor R mEKE a voltage drop is caused, which is known as the loop measuring voltage U mEKE recorded and used to measure the earth resistance R EKE is used.

[0040] The measuring point M of the loop measuring voltage U mEKE is connected to the control earth terminal KE via a loop coupling resistor R aEKE tied together.

[0041] The contact resistances R E , R KE in the current loop formed via the earth terminal E, the earth connection point PE and the control earth terminal KE, the earth resistance R EKE can be calculated using the current / voltage relationships applicable to linear networks (mesh and node rule as well as Ohm's law) to REKE=RE+RKE=RmEKE(ΔUgEKEΔUmEKE−1)−RaEKE.

[0042] In order to make the method more resistant to interference, the voltage swing Δ of the excitation voltage U gEKE and the voltage swing Δ of the loop measuring voltage U mEKE instead of the absolute values.

[0043] To record the loop measuring voltage U mEKE At the measuring point M there is an electronic measuring and filter circuit 12 with low-pass effect and possible stages for level adjustment.

[0044] The measuring and filtering circuit 12 forwards the measurement result to a microcontroller 14, which generates the loop measuring voltage U mEKE to calculate the earth resistance R EKE according to the equation given above.

[0045] The microcontroller 14 has an evaluation algorithm that calculates the earth resistance R EKE as resistance value of the loop in the range R EKE = 0...2 kΩ. For this purpose, threshold R EKEthbe set via software. Furthermore, the microcontroller 14 can also perform tasks other than loop measurement, e.g., the measurement technology of the insulation monitoring in the insulation monitoring device 4 or an HMI control.

[0046] In Fig. 2, a TVS diode 16 is optionally inserted in a line branch between the earth terminal E of the insulation monitoring device 4 and the control earth terminal KE in order to provide a redundant earth connection.

Claims

[1] Electrical measuring arrangement (10) for earth connection monitoring for an insulation monitoring device (4) which is connected to at least one active conductor (L1, L2) of an unearthed power supply system (2) and to an earth connection point (PE) of an electrical installation (3), characterized by an active loop measuring device (20) which can be connected between an earth terminal (E) of the insulation monitoring device (4) and the earth connection point (PE) and an independent excitation voltage source (U) gEKE ) exhibits for the quantitative determination of an earth resistance (R EKE ) the earth connection (6) between the insulation monitoring device (4) and the earth connection point (PE). [2] Electrical measuring arrangement (10) according to claim 1, characterized by , that the loop measuring device (20) has a series connection comprising the excitation voltage source (U gEKE) for superimposing an excitation voltage (U gEKE ), a loop measuring resistor (R mEKE ), a loop coupling resistor (R aEKE ), a measuring and filtering circuit (12) connected between the loop measuring resistor (R mEKE ) and the loop coupling resistor (R aEKE ) a loop measuring voltage (U mEKE ) recorded, a microcontroller (14) that controls the loop measurement voltage (U mEKE ) for calculating the earth resistance (R EKE ) evaluates, and a control earth terminal (KE) for connecting the loop coupling resistor (R) aEKE ) to the earth connection point (PE). [3] Electrical measuring arrangement (10) according to claim 2, characterized by , that the excitation voltage (U gEKE ) a bipolar rectangular voltage with a fundamental frequency in the range of 0.1 Hz to 10 Hz and a voltage swing of less than 10 V. [4] Electrical measuring arrangement (10) according to claim 2 or 3, characterized by , that a TVS diode (16) is connected between the earth terminal (E) of the insulation monitoring device (4) and the control earth terminal (KE) of the loop measuring device. [5] Insulation monitoring device, (30) which can be connected to at least one live conductor (L1, L2) of the ungrounded power supply system (2) and to an earth connection point (PE) of an electrical installation (3) for monitoring an insulation resistance of an ungrounded power supply system (2), characterized by an electrical measuring arrangement (10) according to one of claims 1 to 4. [6] Measuring method for earth connection monitoring for an insulation monitoring device (4) which is connected to at least one active conductor (L1, L2) of an unearthed power supply system (2) and to an earth connection point (PE) of an electrical installation (3), characterized bya quantitative determination of an earth resistance (R) EKE ) the earth connection (6) between the insulation monitoring device (4) and the earth connection point (PE) by means of an active loop measuring device (20), which can be connected between an earth connection terminal (E) of the insulation monitoring device (4) and the earth connection point (PE) and which has an independent excitation voltage source (U) gEKE ) exhibits. [7] Measuring method according to claim 6, where the quantitative determination of the earth resistance (R) EKE ) in the loop measuring device (20), which has a series connection comprising the excitation voltage source (U gEKE ), which can be connected to the earth terminal (E) of the insulation monitoring device (4), a loop measuring resistor (R mEKE ), a loop coupling resistor (R aEKE ), a measuring and filtering circuit (12) with a voltage tap between the loop measuring resistor (R) mEKE ) and the loop coupling resistor (R aEKE ), a microcontroller (14) and a control earth terminal (KE) for connecting the loop coupling resistor (R) aEKE ) to the earth connection point (PE), characterized by Superposition of an excitation voltage (U gEKE ) by means of the excitation voltage source (U gEKE ), Capturing and filtering a loop measurement voltage (U) mEKE ) using the measuring and filter circuit (12), Evaluating the loop measurement voltage (U) mEKE ) and calculating the earth resistance (R EKE ) using the microcontroller (14). [8] Measuring method according to claim 7, characterized by , that as excitation voltage (U gEKE) a bipolar rectangular voltage with a fundamental frequency in the range of 0.1 Hz to 10 Hz and a voltage swing of less than 10 V is superimposed. [9] Measuring method according to claim 8, characterized by , that the voltage swing of the excitation voltage (U gEKE ) and the loop measuring voltage (U mEKE ) in the measuring and filter circuit (12) and the microcontroller (14) for calculating the ground resistance (R) EKE ) is used.

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